Handbook of Membrane Separations Chemical Pharmaceutical Food and Biotechnological Applications

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Pabby et al./Handbook of Membrane Separations 9549_C000 Final Proof page i 21.5.2008 7:54pm Compositor Name: BMani

Handbook of

Membrane Separations Chemical, Pharmaceutical, Food, and Biotechnological Applications

Pabby et al./Handbook of Membrane Separations 9549_C000 Final Proof page ii 21.5.2008 7:54pm Compositor Name: BMani

Pabby et al./Handbook of Membrane Separations 9549_C000 Final Proof page iii 21.5.2008 7:54pm Compositor Name: BMani

Handbook of

Membrane Separations Chemical, Pharmaceutical, Food, and Biotechnological Applications

Edited by

Anil K. Pabby Syed S. H. Rizvi Ana Maria Sastre

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

Pabby et al./Handbook of Membrane Separations 9549_C000 Final Proof page iv

21.5.2008 7:54pm Compositor Name: BMani

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-0-8493-9549-9 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Handbook of membrane separations : chemical, pharmaceutical, food, and biotechnological applications / editor(s), Anil Kumar Pabby, Syed S.H. Rizvi, and Ana Maria Sastre. p. ; cm. Includes bibliographical references. ISBN-13: 978-0-8493-9549-9 (hardcover : alk. paper) ISBN-10: 0-8493-9549-6 (hardcover : alk. paper) 1. Membrane separation--Handbooks, manuals, etc. I. Pabby, Anil Kumar. II. Rizvi, S. S. H., 1948- III. Sastre, Ana Maria. [DNLM: 1. Membranes, Artificial. 2. Biotechnology--methods. 3. Ultrafiltration. TP 159.M4 H236 2008] TP248.25.M46H35 2008 660’.2842--dc22 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

2008009730

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Contents Foreword ..................................................................................................................................................................................... ix Preface......................................................................................................................................................................................... xi Editors ....................................................................................................................................................................................... xiii Contributors ............................................................................................................................................................................... xv

SECTION I Membrane Applications in Chemical and Pharmaceutical Industries and in Conservation of Natural Resources Chapter 1

Membrane Applications in Chemical and Pharmaceutical Industries and in Conservation of Natural Resources: Introduction....................................................................................................................... 3 Ana Maria Sastre, Anil Kumar Pabby, and Syed S.H. Rizvi

Chapter 2

Application of Membrane Contactors as Mass Transfer Devices........................................................................ 7 A. Sengupta and R.A. Pittman

Chapter 3

Membrane Chromatography ............................................................................................................................... 25 M.E. Avramescu, Z. Borneman, and M. Wessling

Chapter 4

Membranes in Gas Separation............................................................................................................................ 65 May-Britt Hägg

Chapter 5

Pervaporation: Theory, Practice, and Applications in the Chemical and Allied Industries............................. 107 Vishwas G. Pangarkar and Sangita Pal

Chapter 6

Current Status and Prospects for Ceramic Membrane Applications................................................................ 139 Christian Guizard and Pierre Amblard

Chapter 7

Membrane Technologies and Supercritical Fluids: Recent Advances ............................................................. 181 D. Paolucci-Jeanjean, G.M. Rios, and S. Sarrade

Chapter 8

Techniques to Enhance Performance of Membrane Processes ........................................................................ 193 A.G. Fane and S. Chang

Chapter 9

Separation and Removal of Hydrocarbons Using Polymer Membranes ......................................................... 233 S.I. Semenova

Chapter 10

Zeolite Membranes: Synthesis, Characterization, Important Applications, and Recent Advances ................. 269 M. Arruebo, R. Mallada, and M.P. Pina

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Chapter 11

Membrane Fouling: Recent Strategies and Methodologies for Its Minimization............................................ 325 Mattheus F.A. Goosen, S.S. Sablani, and R. Roque-Malherbe

Chapter 12

Membrane Extraction in Preconcentration, Sampling, and Trace Analysis..................................................... 345 Jan Åke Jönsson

Chapter 13

Hybrid Liquid Membrane Processes with Organic Water-Immiscible Carriers (OHLM): Application in Chemical and Biochemical Separations ................................................................................... 371 Vladimir S. Kislik

Chapter 14

Advancements in Membrane Processes for Pharmaceutical Applications....................................................... 409 Ralf Kuriyel, Masatake Fushijima, and Gary W. Jung

Chapter 15

Membranes in Drug Delivery........................................................................................................................... 427 Mario Grassi

Chapter 16

Bio-Responsive Hydrogel Membranes............................................................................................................. 473 John Hubble and Rongsheng Zhang

SECTION II Chapter 17

Membrane Applications in Biotechnology, Food Processing, Life Sciences, and Energy Conversion

Membrane Applications in Biotechnology, Food Processing, Life Sciences, and Energy Conversion: Introduction .................................................................................................................................. 495 Syed S.H. Rizvi

Chapter 18

Ultrafiltration-Based Protein Bioseparation...................................................................................................... 497 Raja Ghosh

Chapter 19

Membrane Distillation in Food Processing ...................................................................................................... 513 Sanjay Nene, Ganapathi Patil, and K.S.M.S. Raghavarao

Chapter 20

Applications of Membrane Separation in the Brewing Industry ..................................................................... 553 Carmen I. Moraru and Ernst Ulrich Schrader

Chapter 21

Developments of Bipolar Membrane Technology in Food and Bio-Industries............................................... 581 Gerald Pourcelly and Laurent Bazinet

Chapter 22

Applications of Membrane Technology in the Dairy Industry ........................................................................ 635 Philipina A. Marcelo and Syed S.H. Rizvi

Chapter 23

Microporous Membrane Blood Oxygenators ................................................................................................... 671 S.R. Wickramasinghe and B. Han

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Chapter 24

Transporting and Separating Molecules Using Tailored Nanotube Membranes ............................................. 693 Punit Kohli and Charles R. Martin

Chapter 25

Use of Emulsion Liquid Membrane Systems in Chemical and Biotechnological Separations ....................... 709 Jilska M. Perera and Geoff W. Stevens

Chapter 26

Membrane Electroporation and Emerging Biomedical Applications............................................................... 741 K.P. Mishra

Chapter 27

Proton-Conducting Membranes for Fuel Cells................................................................................................. 759 Vineet Rao, K. Andreas Friedrich, and Ulrich Stimming

SECTION III

Chapter 28

Membrane Applications in Industrial Waste Management (Including Nuclear), Environmental Engineering, and Future Trends in Membrane Science

Membrane Applications in Industrial Waste Management (Including Nuclear), Environmental Engineering, and Future Trends in Membrane Science: Introduction ............................................................. 823 Ana Maria Sastre and Anil Kumar Pabby

Chapter 29

Treatment of Radioactive Effluents: Introduction, Fundamentals, and Scope of Different Membrane Processes ........................................................................................................................................ 827 B.M. Misra and V. Ramachandhran

Chapter 30

Radioactive Waste Processing: Advancement in Pressure-Driven Processes and Current World Scenario............................................................................................................................. 843 Grazyna Zakrzewska-Trznadel

Chapter 31

Liquid Membrane-Based Separations of Actinides.......................................................................................... 883 P.K. Mohapatra and V.K. Manchanda

Chapter 32

Reverse Osmosis-Based Treatment of Radioactive Liquid Wastes Generated in Hospital Facility and in Steel Industry: Case Studies.................................................................................................................. 919 M. Sancho, J.M. Arnal, G. Verdú, and J. Lora

Chapter 33

Evaluation of Membrane-Based Processing of Radioactive Nuclear Plant Waste: Case Studies ................... 933 Anil Kumar Pabby, S.K. Gupta, S.R. Sawant, N.S. Rathore, P. Janardan, R.D. Changrani, and P.K. Dey

Chapter 34

Application of Donnan Membrane Process for Recovery of Coagulants from Water Treatment Residuals ......................................................................................................................................... 945 Prakhar Prakash and Arup K. SenGupta

Chapter 35

Utilization of Membrane Processes in Treating Various Effluents Generated in Pulp and Paper Industry ............................................................................................................................... 981 Mika Mänttäri and Marianne Nyström

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Chapter 36

Membrane Bioreactors for Wastewater Treatment......................................................................................... 1007 Eoin Casey

Chapter 37

Membrane-Assisted Solvent Extraction for the Recovery of Metallic Pollutants: Process Modeling and Optimization............................................................................................................................ 1023 Inmaculada Ortiz and J. Angel Irabien

Chapter 38

Membrane Contactors for Gaseous Streams Treatments ............................................................................... 1041 Alessandra Criscuoli and Enrico Drioli

Chapter 39

Strip Dispersion Technique: Application for Strategic and Precious Metal Separation and Treatment of Wastewater Streams........................................................................................................... 1057 Anil Kumar Pabby, S.C. Roy, J.V. Sonawane, F.J. Alguacil, and Ana Maria Sastre

Chapter 40

Electrically Enhanced Membrane Separations and Catalysis......................................................................... 1071 V.M. Linkov, B.J. Bladergroen, and A.M. Maluleke

Chapter 41

Membrane Processes for Treatment of Industrial Tannery Effluents: A Case Study .................................... 1087 A. Bódalo, E. Gómez, and A.M. Hidalgo

Chapter 42

New Developments in Nanofiltration Technology: A Case Study on Recovery of Impurity-Free Sodium Thiocyanate for Acrylic Fiber Industry................................................................. 1101 S. Sridhar and B. Smitha

Chapter 43

Future Progresses in Membrane Engineering................................................................................................. 1131 Enrico Drioli and Enrica Fontananova

Index...................................................................................................................................................................................... 1147

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Foreword During the middle of the last century, when the first synthetic membrane with tailor-made separation properties became available, a multitude of technically and commercially interesting applications were identified. Today, 50 years later, membranes and membrane processes have indeed become valuable tools for the separation of molecular mixtures. They are the key components in artificial organs and in devices for the controlled release of active agents, or in energy conversion and storage systems. Seawater and brackish water desalination using reverse osmosis and electrodialysis are energy efficient and highly economic processes for large-scale production of potable water. Micro- and ultrafiltration are used for the production of high-quality industrial water and for the treatment of industrial effluents. Blood detoxification by hemodialysis and hemofiltration improves the quality of life for more than 1.3 million people suffering from acute and chronic renal failure. Membrane processes have found a multitude of applications in chemical and pharmaceutical industries as well as in food processing and biotechnology. They are used on a large scale in gas separation and as tools in analytical laboratories. Today’s membrane-based industry is serving a rapidly growing multibillion euro market with a large number of products and processes. The development of membranes with improved properties will most likely increase the importance of membranes and membrane processes in a growing number of applications for the sustainable growth of modern industrial societies. The term ‘‘membrane’’ refers not to a single item, but covers a large variety of structures and materials with very different properties. The same is true for membrane processes, which can be very different in the way they function. However, all membranes and membrane processes have one feature in common, i.e., they can perform the separation of certain molecular mixtures effectively and economically at ambient temperature, and without any toxic or harmful reaction by-products. In the early days of membrane science and technology, research was mainly concentrated on elucidating the membrane mass transport mechanism and on developing membrane structures with specific mass transport properties. The fundamentals of most membrane processes and membrane preparation procedures are described in great detail in a large number of publications in various scientific journals and in several excellent textbooks. However, the application of membranes and membrane processes is much less comprehensively covered in today’s literature. Only a relatively small number of applications of membrane processes such as reverse osmosis, micro- and ultrafiltration, and gas separation and pervaporation are treated in textbooks and reference books. A large number of interesting membrane applications in the food and drug industry, in chemical and electrochemical synthesis, and in artificial organs are often not adequately treated in the membrane-related literature, but are published in journals specific for certain industries, which are outside of the interest of many membrane scientists. Furthermore, application-oriented membrane studies that are often carried out in industrial enterprises are described only as patents, or are not published at all. Therefore, it is difficult to obtain a reasonably complete overview of the very large and heterogeneous field of membrane applications without reading a number of very different journals and patents where most of the publications are not really membrane related. The aim of Handbook of Membrane Separations: Chemical, Pharmaceutical, Food, and Biotechnological Applications is to fill the gap in the presently available membrane literature by providing a comprehensive discussion of membrane applications in the chemical, food, and pharmaceutical industries, in biotechnology, and in the treatment of toxic industrial effluents. The applications of membranes in different areas are described by scientists and engineers who not only are experts in membrane science and technology but also have extensive experience in the specific field of membrane application. This book is not competitive, but rather complementary to other textbooks and handbooks on membrane science and technology presently available in the market. It provides enough background information on the various membrane components and processes to evaluate their potential applications without a detailed treatment of the fundamental aspects of membrane mass transport theories and membrane structure development. The book should, therefore, be of great value to scientists and engineers who are not necessarily membrane experts but are interested in using membrane processes in solving specific separation and mass transport problems. It is equally suited for the newcomers in the field of membrane science as for engineers and scientists, who do have basic knowledge in membrane technology but are interested in obtaining more information on specific present and potential future membrane applications. It also provides an excellent base for courses and lectures in postgraduate education. Professor Heiner Strathmann University of Stuttgart Germany

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Preface During the past two decades, membrane technology has grown into an accepted unit operation for a wide variety of separations in industrial processes and environmental applications. Tighter environmental legislation calls for equipment that is able to deal with the removal of components across a wide range of concentration levels and that offers considerable flexibility and efficiency. Membrane technology first became important during the 1960s and 1970s in water treatment and in processes such as reverse osmosis, ultrafiltration, dialysis, electrodialysis, and microfiltration. During the 1980s, membrane technology began to be applied on a large scale in the field of gas purification. The successful introduction of membrane technology in these fields was mainly the result of the development of reliable and selective polymeric membranes. There are a number of reference publications in the field of membrane technology, such as handbooks, monographs, and compendia of conference and workshop proceedings. The relative abundance of such works begs the questions, ‘‘Why another?’’ and ‘‘How will this one be different?’’ These questions are probably best answered by considering what the Handbook of Membrane Separations: Chemical, Pharmaceutical, Food, and Biotechnological Applications has to offer. The handbook covers the full spectrum of membrane technology and discusses its advancement and applications in a series of chapters written by experts, prominent researchers, and professionals from all over the world. The handbook is divided into three main sections: The first section deals with membrane applications in chemical and pharmaceutical industries, and in conservation of natural resources; the second section covers membrane applications in biotechnology, food processing, life sciences, and energy conversion. Finally, the third section deals with membrane applications in industrial waste management (including nuclear), environmental engineering, and future trends in membrane science. Each section is divided into chapters that deal with the subject matter in depth and focus on cutting-edge advancements in the field. Several authors were commissioned to write the chapters under the supervision of the editors, and each chapter was peer-reviewed for content and style before it was accepted for publication. The aim was to maintain the perspective of a practical handbook rather than merely a collection of review chapters. The editors would like to acknowledge the contributions of a number of authors and institutions that have played a major role in drafting the handbook from conception to publication. The handbook would not have been possible without their input. These contributors are leading experts in their fields and bring a great wealth of experience to this book. The editors would also like to acknowledge the efforts of the reviewers who devoted their valuable time to revising the chapters before the deadlines and suggested improvements to maintain the high standard of the handbook. Finally, we would like to acknowledge the support of our home institutions at every stage in the handbook’s conception: the Bhabha Atomic Research Centre, Mumbai, India; Cornell University, Ithaca, New York; and the Universitat Politècnica de Catalunya, Barcelona, Spain. Anil Kumar Pabby Syed S.H. Rizvi Ana Maria Sastre

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Editors Anil Kumar Pabby is affiliated with one of the pioneering research centers of India, the Bhabha Atomic Research Centre (Department of Atomic Energy), Tarapur, Mumbai, Maharashtra. He received his PhD from the University of Mumbai and subsequently completed his postdoctoral research at the Universitat Politècnica de Catalunya, Barcelona, Spain. Dr. Pabby has more than 150 publications to his credit including 4 book chapters and a patent on nondispersive membrane technology. He was invited to join the team of associate editors at the Journal of Radioanalytical and Nuclear Chemistry during 2002–2005. He has also served as consultant to the International Atomic Energy Agency (IAEA) for developing a technical document on the application of membrane technologies for liquid radioactive waste processing. Dr. Pabby has been a regular reviewer for several national and international journals and also serves on the editorial board of various journals. His research interest includes pressure-driven membrane processes, nondispersive membrane techniques, extraction chromatography, solvent extraction, and macrocyclic crown compounds. In 2003, Dr. Pabby was elected fellow of the Maharashtra Academy of Sciences (FMASc) for his contribution to membrane science and technology. In 2005, he received the prestigious Tarun Datta Memorial Award (instituted by Indian Association for Nuclear Chemists and Allied Scientists) for his outstanding contribution to nuclear chemistry and radiochemistry. Syed S.H. Rizvi is an international professor of food process engineering and has served as director of graduate studies at the Cornell Institute of Food Science, Cornell University, Ithaca, New York. He has a PhD from Ohio State University, an MEng (chemical engineering) from the University of Toronto, and a BTech from Panjab University, India. Dr. Rizvi teaches courses devoted to engineering and processing aspects of food science and related biomaterials. His laboratory is engaged in research on experimental and theoretical aspects of bioseparation processes using supercritical fluids and membranes, high-pressure extrusion with supercritical carbon dioxide, physical and engineering properties of biomaterials, and novel food processing technologies. An invention of Cornell researchers, and subsequently patented, supercritical fluid extrusion offers several advantages over the conventional high-shear cooking extrusion and is being used to investigate the dynamics of the process and the mechanics of the microcellular extrudates generated for both food and nonfood applications. A major long-term goal is to develop new and improved unit operations for value-added processing of food and biomaterials. Derivative goals include new techniques for measurement and control of processes and properties for industrial applications. Dr. Rizvi has published more than 140 technical papers, coauthored=edited 6 books, served on the editorial board of several journals, and holds 7 patents. Ana Maria Sastre is a professor of chemical engineering at the Universitat Politècnica de Catalunya (Barcelona, Spain), where she has been teaching chemistry for more than 28 years. She received her PhD from the Autonomous University of Barcelona in 1982 and has been working for many years in the field of solvent extraction, solvent impregnated resins, and membrane technology. She was a visiting fellow at the Department of Inorganic Chemistry, the Royal Institute of Technology, Sweden, during 1980–1981 and carried out postdoctoral research from October 1986 to April 1987 at Laboratoire de Chimie Minerale, Ecole Europeenne des Hautes Etudes des Industries Chimiques de Strasbourg, France. Professor Sastre has more than 190 journal publications and more than 80 papers in international conferences. Dr. Sastre also holds four patent applications, guided 11 PhD and 16 master thesis students, and is a

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reviewer of many international journals. In 2003, she was awarded the Narcis Monturiol medal for scientific and technological merits, given by the Generalitat de Catalunya for her outstanding contribution to science and technology. Professor Sastre was the head of the chemical engineering department from 1999 to 2005 and is presently vice rector (vice chancellor) for academic policy at the Universitat Politècnica de Catalunya.

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Contributors F.J. Alguacil Centro Nacional de Investigaciones Metalúrgicas Consejo Superior de Investigaciones Cientificas Ciudad Universitaria Madrid, Spain Pierre Amblard Techno-Membranes Parc Scientifique Agropolis Montpellier, France J.M. Arnal Chemical and Nuclear Engineering Department Polytechnic University of Valencia Valencia, Spain M. Arruebo Department of Chemical and Environmental Engineering University of Zaragoza Zaragoza, Spain M.E. Avramescu Membrane Technology Group Faculty of Science and Technology University of Twente Enschede, the Netherlands Laurent Bazinet Institute of Nutraceuticals and Functional Foods Department of Food Sciences and Nutrition Laval University Laval, Québec, Canada B.J. Bladergroen South African Institute for Advanced Materials Chemistry University of the Western Cape Bellville, South Africa A. Bódalo Departamento de Ingeniería Química Universidad de Murcia, Campus de Espinardo Murcia, Spain Z. Borneman Membrane Technology Group Faculty of Science and Technology University of Twente Enschede, the Netherlands

Eoin Casey School of Chemical and Bioprocess Engineering University College Dublin Dublin, Ireland S. Chang Global Product Development, UF=MBR Technology GE Water & Process Technologies Oakville, Ontario, Canada R.D. Changrani Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India Alessandra Criscuoli Research Institute on Membrane Technology University of Calabria Rende, Cosenza, Italy P.K. Dey Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India Enrico Drioli Research Institute on Membrane Technology University of Calabria Rende, Cosenza, Italy and Department of Chemical Engineering and Materials University of Calabria Rende, Cosenza, Italy A.G. Fane UNESCO Centre for Membrane Science and Technology University of New South Wales Sydney, New South Wales, Australia and Singapore Membrane Technology Centre Nanyang Technological University Singapore Enrica Fontananova Research Institute on Membrane Technology University of Calabria Rende, Cosenza, Italy and Department of Chemical Engineering and Materials University of Calabria Rende, Cosenza, Italy

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K. Andreas Friedrich German Aerospace Center Electrochemical Energy Technology Stuttgart, Germany

John Hubble Department of Chemical Engineering University of Bath Bath, United Kingdom

Masatake Fushijima Crossflow Technology, SLS Pall Corporation Port Washington, New York

J. Angel Irabien Departamento de Ingeniería Química y Química Inorgánica Universidad de Cantabria Santander, Spain

Raja Ghosh Department of Chemical Engineering McMaster University Hamilton, Ontario, Canada E. Gómez Departamento de Ingeniería Química Universidad de Murcia Campus de Espinardo Murcia, Spain Mattheus F.A. Goosen Office of Research Alfaisal University Riyadh, Saudi Arabia Mario Grassi Department of Chemical, Environmental, and Raw Materials Engineering University of Trieste Trieste, Italy Christian Guizard Laboratoire de Synthèse et Fonctionnalisation des Céramiques Saint Gobain CREE Cavaillon, France S.K. Gupta Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India May-Britt Hägg Department of Chemical Engineering Norwegian University of Science and Technology Trondheim, Norway

P. Janardan Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India Jan Åke Jönsson Analytical Chemistry Lund University Lund, Sweden Gary W. Jung Membrane Technology Consultant Daytona Beach, Florida Vladimir S. Kislik Casali Institute of Applied Chemistry The Hebrew University of Jerusalem Jerusalem, Israel Punit Kohli Department of Chemistry and Biochemistry Southern Illinois University Carbondale, Illinois Ralf Kuriyel Biopharm Applications R&D Pall Life Sciences Pall Corporation Northborough, Massachusetts V.M. Linkov South African Institute for Advanced Materials Chemistry University of the Western Cape Bellville, South Africa

B. Han Department of Chemical and Biological Engineering Colorado State University Fort Collins, Colorado

J. Lora Chemical and Nuclear Engineering Department Polytechnic University of Valencia Valencia, Spain

A.M. Hidalgo Departamento de Ingeniería Química Universidad de Murcia Campus de Espinardo Murcia, Spain

R. Mallada Department of Chemical and Environmental Engineering University of Zaragoza Zaragoza, Spain

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A.M. Maluleke South African Institute for Advanced Materials Chemistry University of the Western Cape Bellville, South Africa V.K. Manchanda Radiochemistry Division Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India Mika Mänttäri Laboratory of Membrane Technology and Technical Polymer Chemistry Lappeenranta University of Technology Lappeenranta, Finland Philipina A. Marcelo Department of Chemical Engineering The Research Center for the Natural Sciences University of Santo Tomas Manila, Philippines Charles R. Martin Department of Chemistry Center for Research at the Bio=Nano Interface University of Florida Gainesville, Florida K.P. Mishra Radiation Biology and Health Sciences Division Bhabha Atomic Research Center Tarapur, Mumbai, Maharashtra, India B.M. Misra Nuclear Desalination Unit Division of Nuclear Power Nuclear Power Technology Development Section Department of Nuclear Energy Vienna, Austria P.K. Mohapatra Radiochemistry Division Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India Carmen I. Moraru Department of Food Science Cornell University Ithaca, New York Sanjay Nene Biochemical Engineering Group Chemical Engineering and Process Development Division National Chemical Laboratory Pune, Maharashtra, India

Marianne Nyström Laboratory of Membrane Technology and Technical Polymer Chemistry Lappeenranta University of Technology Lappeenranta, Finland Inmaculada Ortiz Departamento de Ingeniería Química y Química Inorgánica Universidad de Cantabria Santander, Spain Anil Kumar Pabby Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Maharashtra, India Sangita Pal Department of Chemical Engineering Institute of Chemical Technology Mumbai University Mumbai, Maharashtra, India Vishwas G. Pangarkar Department of Chemical Engineering Institute of Chemical Technology Mumbai University Mumbai, Maharashtra, India D. Paolucci-Jeanjean European Membrane Institute Université Montpellier Montpellier, France Ganapathi Patil Department of Food Engineering Central Food Technological Research Institute Mysore, Karnataka, India Jilska M. Perera Department of Chemical and Biomolecular Engineering University of Melbourne Parkville, Victoria, Australia M.P. Pina Department of Chemical and Environmental Engineering University of Zaragoza Zaragoza, Spain R.A. Pittman Membrana-Charlotte Celgard Charlotte, North Carolina

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Gerald Pourcelly European Membrane Institute Université Montpellier 2 Montpellier, France

S.S. Sablani Department of Biological Systems Engineering Washington State University Pullman, Washington

Prakhar Prakash Chevron Energy Technology Company Richmond, California

M. Sancho Chemical and Nuclear Engineering Department Polytechnic University of Valencia Valencia, Spain

K.S.M.S. Raghavarao Department of Food Engineering Central Food Technological Research Institute Mysore, Karnataka, India

S. Sarrade Waste Management Division French Atomic Energy Commission Bagnols sur Ceze, France

V. Ramachandhran Desalination Division Bhabha Atomic Research Centre Tarapur, Mumbai, Maharashtra, India

Ana Maria Sastre Chemical Engineering Department Universitat Politècnica de Catalunya Barcelona, Spain

Vineet Rao Department of Physics Technische Universität München Garching, Germany

S.R. Sawant Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Maharashtra, India

N.S. Rathore Nuclear Recycle Group Bhabha Atomic Research Centre Mumbai, Maharashtra, India

Ernst Ulrich Schrader Beverage Engineering Inc. Concord, Ontario, Canada

G.M. Rios European Membrane Institute Université Montpellier Montpellier, France Syed S.H. Rizvi Food Process Engineering Institute of Food Science Cornell University Ithaca, New York R. Roque-Malherbe School of Science and Technology University of Turabo Gurabo, Puerto Rico and Institute of Chemical and Biological Technology University of Turabo Gurabo, Puerto Rico S.C. Roy Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Maharashtra, India

S.I. Semenova Vladimir State University Vladimir, Russia A. Sengupta Membrana-Charlotte Celgard Charlotte, North Carolina Arup K. SenGupta Department of Civil and Environmental Engineering Fritz Engineering Laboratory Lehigh University Bethlehem, Pennsylvania B. Smitha Membrane Separation Group Chemical Engineering Division Indian Institute of Chemical Technology Hyderabad, Andhra Pradesh, India J.V. Sonawane Nuclear Recycle Group Bhabha Atomic Research Centre Tarapur, Maharashtra, India

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S. Sridhar Membrane Separation Group Chemical Engineering Division Indian Institute of Chemical Technology Hyderabad, Andhra Pradesh, India Geoff W. Stevens Department of Chemical and Biomolecular Engineering University of Melbourne Melbourne, Victoria, Australia Ulrich Stimming Department of Physics Technische Universität München Garching, Germany and ZAE Bayern, Division 1 Garching, Germany G. Verdú Chemical and Nuclear Engineering Department Polytechnic University of Valencia Valencia, Spain

M. Wessling Membrane Technology Group University of Twente Enschede, the Netherlands S.R. Wickramasinghe Department of Chemical and Biological Engineering Colorado State University Fort Collins, Colorado Grazyna Zakrzewska-Trznadel Department of Nuclear Methods in Process Engineering Institute of Nuclear Chemistry and Technology Warszawa, Dorodna, Poland Rongsheng Zhang Department of Chemical Engineering University of Bath Bath, United Kingdom

Pabby et al./Handbook of Membrane Separations 9549_C000 Final Proof page xx 21.5.2008 7:54pm Compositor Name: BMani

Pabby et al./Handbook of Membrane Separations 9549_C001 Final Proof page 1 13.5.2008 12:54pm Compositor Name: BMani

Section I Membrane Applications in Chemical and Pharmaceutical Industries and in Conservation of Natural Resources



Applications in Chemical 1 Membrane and Pharmaceutical Industries and in Conservation of Natural Resources: Introduction Ana Maria Sastre, Anil Kumar Pabby, and Syed S.H. Rizvi CONTENTS References .................................................................................................................................................................................... 5

In the last 40 years, membranes have developed from a research topic to a mature industrial separation technology. This increase in the use of membrane technology is driven by spectacular advances in membrane development, the wider acceptance of the technology in preference to conventional separation processes, increased environmental awareness and, most importantly, strict environmental regulations and legislation. Various membrane processes are currently applied in the chemical (including petrochemicals), pharmaceutical, and food and beverage industries. Particularly, strong development and growth of membrane technology can be observed in the purification of wastewater and the production of drinking water. This statement summarizes the discussions at a conference on the ‘‘Exploration of the potential of membrane technology for sustainable decentralized sanitation’’ held in Italy (at Villa Serbelloni, Bellagio) on 23–26 April 2003 [1].* Due to plummeting costs and dramatically improving performance, water-treatment applications based on membranes are blossoming. In particular, membrane bioreactors (MBRs) are today robust, simple to operate, and ever more affordable. They take up little space, need modest technical support, and can remove many contaminants in one step. These advantages make it practical, for the first time, to protect public health and safely reuse water for non-potable uses. Membranes can also be a component of a multi-barrier approach to supplement potable water resources. Finally, decentralization, which overcomes some of the sustainability limits of centralized systems, becomes more feasible with membrane treatment. Because membrane processes make sanitation, reuse, and decentralization possible, water sustainability can become an achievable goal for the developed and developing worlds.

A membrane can essentially be defined as a barrier that separates two phases and selectively restricts the transport of various chemicals. It can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, and can carry a positive or negative charge, or be neutral or bipolar. Transport across a membrane can take place by convection or by diffusion of individual molecules, or it can be induced by an electric field or concentration, pressure or temperature gradient. The membrane thickness can vary from as little as 100 mm to several millimeters. A membrane separation system separates an influent stream into two effluent streams known as the permeate and the concentrate. The permeate is the portion of the fluid that has passed through the semipermeable membrane, whereas the concentrate stream contains the constituents that have been rejected by the membrane. The correct choice of membrane should be determined by the specific objective, such as the removal of particulates or dissolved solids, the reduction of hardness for the production of ultra pure water or the removal of specific gases=chemicals. The end use may also dictate the selection of membranes in industries such as potable water, effluent treatment, desalination, or water supply for electronic or pharmaceutical manufacturing. Membrane technology covers various chemical technology disciplines, such as material science and technology, mass transport and process design. By manipulating material properties, membranes can be tailor-made for particular separation tasks * From Fane, A.G., Editorial, J. Membr. Sci., 233, 127, 2004. With permission.

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Handbook of Membrane Separations

to be performed under specific separation conditions. Membranes are manufactured as flat sheets, capillaries, or in tubular shapes and are applied in various module configurations. The following membrane modules are commonly used for industrial applications: (a) the plate and frame module; (b) the spiral wound module; (c) the tubular membrane module; (d) the capillary membrane module; and (e) the hollow fiber membrane module. Membrane separation processes have numerous industrial applications and provide the following advantages: They offer appreciable energy savings; they are environmentally benign; the technology is clean and easy to operate; they replace conventional processes like filtration, distillation, and ion exchange; they produce high-quality products; and they offer greater flexibility in system design. Pressure-driven processes such as ultrafiltration, nanofiltration, and microfiltration are already established and various applications have been commercialized in the fields of pharmaceutical and biotechnology. Recently, the development of a means of characterizing, controlling, and preventing membrane fouling has been proved vital. The development of tailored membranes, fouling prevention, and optimization of chemical cleaning will ensure a high level of membrane process performance. In the last five years, the development of new techniques for membrane characterization and the improvement of existing techniques have increased our knowledge of the mechanisms involved in membrane fouling. More advanced techniques, such as environmental scanning electron microscopy (ESEM), have been used to study membrane fouling during the microfiltration of high metal content solutions with aluminum oxide membranes [2]. This will provide not only useful insight into the fouling mechanism but also a better understanding of the factors that affect membrane fouling. The combination of molecular separation with a chemical reaction, or membrane reactors, offers important new opportunities for improving the production efficiency in biotechnology and in the chemical industry. With regard to the future of biotechnology and pharmaceutical processes, the availability of new high-temperature-resistant membrane contactors offers an important tool for the design of alternate production systems appropriate for sustainable growth. Membrane technology has widespread applications in chemical and pharmaceutical industries and its use in various other fields is increasing rapidly. It has established applications in areas such as hydrogen separation, the recovery of organic vapors from process gas streams, and the selective transport of organic solvents, and it is creating new possibilities for catalytic conversion in membrane reactors. It provides a unique solution for industrial waste treatment and for the controlled production of valuable chemicals. Since it deals with the smallest penetrants in the size spectrum, gas separation requires extremely precise discrimination of size and shape—often in the range of 0.2–0.3 Å—between permeated and rejected species. Such demands truly push the state of the art in materials science for these specific applications. In addition to polymeric media, ceramic, carbon, zeolite, and metal membranes are attractive options as they provide both precise separation and robustness. Vision and commitment are required to make the most of the large energy savings (and CO2 emission reductions) offered by membranes when compared with traditional, thermally driven separations and energy conversion. The use of membranes for extraction in analytical chemistry has increased recently. The main aim is to selectively extract and enrich the compounds to be determined (analytes) from samples of varying chemical complexity. In contrast to many technical uses of membranes, in analytical applications it is essential to recover the extracted analytes as efficiently as possible so that they can be transferred to suitable analytical instruments for the final quantitative determination. Similarly, membrane contactors have proved to be efficient contacting devices, due to their high area per unit volume that results in high mass transfer rates. They are not only compact but also eliminate several of the problems faced in conventional processes such as ion exchange, solvent extraction, and precipitation. Membrane contactor processes, in which phase contacting is performed or facilitated by the structure and shape of the porous membrane, provide new dimension to the growth of membrane science and technology and also satisfy the requirements for process intensification. In addition, membrane contactors represent a significant step forward from the initial success of blood oxygenators. Their integration with other membrane systems, including membrane reactors, could lead to the redesign of membrane-based integrated production lines. This introductory section outlines several established applications of membranes in the chemical and pharmaceutical industries, reviews the membranes and membrane processes available in this field, and discusses the huge potential of these technologies. In addition, other important topic dealing with conservation of natural resources (zeolite membranes) is also presented in this section. Each chapter has been written by a leading international expert with extensive industrial experience in the field. Chapter 1 (the current chapter) presents an overview of different membrane processes and a description of all of the chapters presented in Section I. Chapter 2 explains the potential of hollow fiber contactors in the field of chemical technology and how they have changed industrial preferences regarding contacting devices. This chapter gives an introduction to membrane contact technology, its principles of operation, and the benefits obtained from the use of membrane contactors. Important applications, new product development requirements, and future directions are also discussed. Chapter 3 deals with membrane chromatography. This chapter discusses the latest developments in membrane-based stationary phases (affinity membranes and mixed matrix membrane adsorbers) and monolithic separation media (organic and inorganic). It also provides information on new types of chromatographic support, focusing on membrane materials, properties, and preparation. Finally, it considers possible applications of chromatographic membranes in various process conditions. Chapter 4 focuses on the important aspects of membrane application in gas separation. It deals with the subject comprehensively, providing an


Membrane Applications in Chemical and Pharmaceutical Industries

5

introduction and discussing transport mechanisms, different membrane materials for gas separation, module design, current and potential applications, and novel developments in this field. Chapter 5 presents developments in pervaporation (PV). It first gives a brief introduction to the theory of pervaporation and then discusses sorption thermodynamics in polymers, the solution diffusion model, the criteria for membrane polymer selection, and important applications of PV in different cases of aqueous and organic separation. Chapter 6 focuses on advances in the field of ceramic membranes, covering interesting applications in this area. Chapter 7 describes important developments in the fields of supercritical fluids and membrane technology. Chapter 8 presents the various methodologies or techniques for improving the membrane performance of microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. The aim is to present the techniques that attempt to minimize concentration polarization (and fouling) and allow the membrane to perform closer to its intrinsic capability. The methods range from the critical flux approach to the suite of hydrodynamic techniques and other potential strategies. Chapter 9 records important developments in the field of polymeric membranes for the separation and removal of hydrocarbons. It provides an introduction to the subject, discusses the background and physicochemical regularities of hydrocarbon permeation in membrane-based glassy and rubbery polymers, and lists some important applications. Chapter 10 describes some of the main characteristics of the use of zeolite membranes in separation applications. Zeolite membranes separate molecules based on the differences in their adsorption and diffusion properties. They are therefore suitable for separating gas and liquid phase mixtures by gas separation and pervaporation, respectively. This chapter reviews the basic mechanisms of gas separation and pervaporation through zeolite membranes and presents examples of industrial applications. Chapter 11 focuses on membrane fouling and the strategies used to reduce it relative to pressure-driven processes. This chapter highlights recent strategies for minimizing membrane fouling. In particular, it discusses the literature on fouling phenomena in reverse osmosis and ultrafiltration membrane systems, the analytical techniques employed to quantify fouling, preventive methods, and membrane cleaning strategies. Specific recommendations are also made on how scientists, engineers, and technical staff can help to improve the performance of these systems by minimizing membrane fouling phenomena. Chapter 12 describes membrane extraction and its use in preconcentration, sampling, and trace analysis. Chapter 13 presents applications of aqueous hybrid liquid membranes (AHLM) and organic hybrid liquid membranes (OHLM) in the separation of organic and metal species, respectively. Chapter 14 provides an introduction to membrane applications in the pharmaceutical industry, its current status, and future potential in this very important area. Chapter 15 is devoted to membrane applications in the drug delivery field with emphasis on the mechanisms governing mass transport to modulate the release kinetics. Hydrogel membranes, as a derivative construct of hydrogels, have become increasingly attractive for precisely controlling the drug delivery rate via chemical sensing and triggering. Their current status, challenges, and opportunities are highlighted in Chapter 16.

REFERENCES 1. Fane, A.G., Editorial, J. Membr. Sci., 233, 127–128, 2004. 2. Skerlos, S.J., Rajagopalan, N., DeVor, R.E., Kapoor, S.G., and Angspatt, V.D., Microfiltration polyoxyalkylene metalworking fluid lubricant additives using aluminum oxide membranes, J. Man. Sci. Eng. Trans., 123, 692–699, 2001.



of Membrane Contactors 2 Application as Mass Transfer Devices A. Sengupta and R.A. Pittman CONTENTS 2.1 Introduction ...................................................................................................................................................................... 7 2.2 Scope of This Chapter...................................................................................................................................................... 7 2.3 Description of Membrane Contactor................................................................................................................................ 8 2.4 Principle of Operation ...................................................................................................................................................... 8 2.5 Benefits of Membrane Contactor Technology............................................................................................................... 10 2.6 Mass Transfer Process in Membrane Contactor ............................................................................................................ 10 2.7 Literature Review on Membrane Contactor Applications ............................................................................................. 12 2.8 Use of Gas–Liquid or Liquid–Gas–Liquid Contact....................................................................................................... 12 2.9 Use of Liquid–Liquid Contact ....................................................................................................................................... 13 2.10 Review of Membrane Contactor Design Options.......................................................................................................... 14 2.11 Commercial or Precommercial Installations of Large-Scale Membrane Contactors..................................................... 15 References .................................................................................................................................................................................. 20

2.1 INTRODUCTION Membrane contactors as a type of membrane device have been known for quite a few years now [1–2]. They involve a unique class of membrane-based mass transfer and separation technologies, which have grown beyond academic curiosity and found commercial applications across various industries and markets. It has been found to be a cost-effective technology and is therefore used to supplant or replace other technologies that might or might not be based on membranes. In some situations, membrane contacting has emerged as an enabling technology that is filling some previously unmet commercial needs. By the standard of business size, membrane contactor technology is currently a minor player compared to other much better-known membrane separation technologies such as reverse osmosis (RO), membrane filtration, membrane gas separation, diffusion dialysis, and electrodialysis. By its very nature, the membrane contactor does not function or compete with the other membrane devices, and the capability and functionality of contactors are significantly different from the other devices. But membrane contactor technology seems to have the potential to be applicable over a much wider array of industries. Use of membrane contactor devices in various forms is growing continuously. In many applications the contactor is not even called a contactor but is referred to by other names depending on the specific application it is deployed in. Examples include blood oxygenator (the earliest use of membrane contactor), gas transfer membrane, membrane degasifier, membrane deaerator, membrane distillation device, osmotic distillation device, membrane gas absorber, membrane extractor, and membrane humidifier.

2.2 SCOPE OF THIS CHAPTER Considering the wide applications of membrane contactors and the evolving nature of this technology, it is difficult to cover every aspect in a monograph. The intent of this article is to first explain the technology and the principles of operation, with some remarks on the mass transfer process in membrane contactors. This is followed by description of various types of contacting possibilities and review of a wide sampling of literature on the technology to date. Design options of membrane The authors’ references to the various patents mentioned in this article do not constitute a grant of a license to practice any of these technologies, nor do they imply the authors’ acknowledgment of the validity of any of the referenced patents.

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Handbook of Membrane Separations Lumen fluid outlet

Microporous hollow fibers

Shell fluid inlet

Pores Potting Shell fluid outlet

Lumen fluid inlet

FIGURE 2.1

Microporous hollow fiber membrane in a membrane contactor.

contactors are then reviewed. Finally, some current and emerging commercial applications at different stages of development are discussed in detail.

2.3

DESCRIPTION OF MEMBRANE CONTACTOR

From outward appearance membrane contactors look similar to other membrane devices. However, functionally the membranes used in contactors are very different. They are mostly nonselective and microporous. Membrane contactors can be made out of flat sheet membranes and there are some commercial applications. Most common commercial membrane contactors are, however, made from small-diameter microporous hollow fiber (or capillary) membranes with fine pores (illustrated in Figure 2.1) that span the hollow fiber wall from the fiber inside surface to the fiber outside surface. The contactor shown as an example in Figure 2.1 resembles a tube-in-shell configuration with inlet=outlet ports for the shell side and tube side. The membrane is typically made up of hydrophobic materials such as Polypropylene, Polyethylene, PTFE, PFA, and PVDF. The membrane in a contactor acts as a passive barrier and as a means of bringing two immiscible fluid phases (such as gas and liquid, or an aqueous liquid and an organic liquid, etc.) in contact with each other without dispersion. The phase interface is immobilized at the membrane pore surface, with the pore volume occupied by one of the two fluid phases that are in contact. Since it enables the phases to come in direct contact, the membrane contactor functions as a continuous-contact mass transfer device, such as a packed tower. However, there is no need to physically disperse one phase into the other, or to separate the phases after separation is completed. Several conventional chemical engineering separation processes that are based on mass exchange between phases (e.g., gas absorption, gas stripping, liquid–liquid extraction, etc.) can therefore be carried out in membrane contactors.

2.4

PRINCIPLE OF OPERATION

Principle of membrane contactor operation is based on the natural phenomenon of capillary force. When one side of a hydrophobic microporous membrane is brought in contact with water or an aqueous liquid, the membrane is not ‘‘wetted’’ by the liquid, i.e., the liquid is prevented from entering the pores, due to surface tension effect. The interface between a liquid and a solid substrate can be characterized by the parameter ‘‘contact angle’’ (Figure 2.2). The wettability of a solid surface by a liquid surface decreases as the contact angle increases. A contact angle of less than 908 implies that the liquid will tend to wet the substrate (hydrophilic), whereas if contact angle is greater than 908 the liquid will not tend to wet the surface (hydrophobic). Table 2.1 lists the contact angle values for few different materials in water at ambient temperature. If a dry microporous hydrophobic hollow fiber membrane with air-filled pores was surrounded by water there would not be any penetration by water into the pores until the water pressure exceeds a certain critical breakthrough pressure. The magnitude


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Application of Membrane Contactors as Mass Transfer Devices

Vapor

Contact angle (q)

Liquid droplet

Solid surface

FIGURE 2.2 Representation of contact angle.

of this critical breakthrough pressure ‘‘differential’’ (water pressure minus air pressure) DPC has been mathematically derived, and is expressed [3] by the following equation that is often referred to as the Young–Laplace equation: DPC ¼

4l cos u d

(2:1)

where l is the surface tension of water u is the contact angle for the system air–water–membrane in degrees d is the effective diameter of the membrane pore, assuming pores are circular in shape For a hydrophobic porous material with contact angle greater than 908, the DPC is >0 and depends on the liquid surface tension and the membrane pore size. As an example, considering water–air–polypropylene system, one can calculate that for a dry membrane with a pore size of 0.03 mm (30 nm) the critical entry pressure of water is more than 300 psi (>20 bar). Since the liquid phase does not enter the pores, a stable gas–liquid phase interface can be created and maintained (as illustrated in Figure 2.3) as long as the liquid phase pressure is higher than the gas phase pressure and the phase pressure differential DP is between 0 and DPC. The pores remain air filled at this condition. The liquid and the gas phases could be flowing at different flow rates on either side of the membrane wall, but the phase interface remains stable all along the membrane. Thus, by proper control of pressures, the two immiscible phases come in constant contact without a need to disperse one into the other. This allows mass transfer or mass exchange between phases [4–5], such as gas absorption or gas stripping (desorption). The same principle of operation as described above is applicable also to liquid–liquid extraction where an aqueous liquid and an organic liquid contact each other inside the contactor for extraction of a solute selectively from one phase to another [6–8]. The critical breakthrough pressure for liquid–liquid system could be calculated by Equation 2.1, except that the term l would now be the interfacial tension between the two liquids. Further variation of membrane contacting technology is called gas membrane or gas–gap membrane where two different liquid phases flow on either side of the membrane, but the membrane pores remain gas filled [9–10]. In this situation two separate gas–liquid contact interfaces are supported on each side of a single membrane.

TABLE 2.1 Contact Angle for Various Materials in Water at Ambient Temperature Substrate Ordinary glass Platinum Anodized aluminum PMMA Nylon Polyethylene Polypropylene Teflon

Contact Angle (In degrees) 20 40 60 74 79 96 108 112

Pabby et al./Handbook of Membrane Separations 9549_C002 Final Proof page 10


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Handbook of Membrane Separations A single hollow fiber wall

Gas flow

Liquid water phase flowing around hollow fibers; Liquid pressure PLIQUID

Liquid-gas phase interface at stable condition

Gas phase flowing inside hollow fiber, gas pressure PGAS

Pores on hollow fiber wall

Two required conditions for stable phase interface, 1. PLIQUID > PGAS, and 2. 0 < (PLIQUID–PGAS) < ∆PC

FIGURE 2.3

2.5

Liquid–gas interface in a membrane contactor.

BENEFITS OF MEMBRANE CONTACTOR TECHNOLOGY

Primary list of features and resulting benefits for the technology are shown in Table 2.2.

2.6

MASS TRANSFER PROCESS IN MEMBRANE CONTACTOR

In gas–liquid, liquid–liquid, or liquid–gas–liquid contactors there is no convective flow of any phase across the membrane. Mass transfer occurs only by diffusion across the immobilized phase in the pores. The direction of mass transfer of any molecular species depends on the concentration driving force maintained across the membrane for that species. The presence of the stationary phase in the membrane pore creates an extra diffusional mass transfer resistance. However, it can be shown that in many cases the membrane resistance is negligible, and that in most cases the high active mass transfer area created inside a membrane contactor more than compensates for any additional mass transfer resistance [4–5]. Mass transfer resistance in a continuous-contact separation device is the inverse of the mass transfer coefficient. In membrane contactors, the total resistance could be expressed as three resistances in series. These include the individual resistances in each flowing phase and the membrane resistance (Figure 2.4). For a liquid–gas contact system Equation 2.2 could be written for each diffusing species: 1 1 1 1 ¼ þ þ dOUT KTOTAL dOUT kSHELL HdAVG kM HdIN kTUBE

(2:2)

where K is the overall coefficients k is the individual mass transfer coefficients TABLE 2.2 Benefits of Membrane Contactor Technology Features High concentration of active phase contact area Flow rates of phases in contact can be controlled independently Modular in nature Mass transfer does not depend on gravity No need to disperse or coalesce phases Can be operated with high fluid outlet pressures

Benefits Profile or footprint of membrane contactor systems are small; fit into existing building; no additional structure needed No physical limitations such as flooding or loading; contact area constant irrespective of phase flow rates; process more flexible Easier to add system capacity incrementally; can often be retrofitted into existing systems; easier scale-up Contactor can be mounted vertically or horizontally; will also work in microgravity; able to process two fluid phases of same densities Eliminates extra steps; more efficient utilization of device volume Eliminates or reduces need for transfer pumps or booster pumps after contactor



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Application of Membrane Contactors as Mass Transfer Devices Total mass transfer resistance

Membrane resistance Outer phase (shell side) resistance

Inner phase (tube side) resistance

C TUBE

C SHELL

Hollow fiber wall (membrane) For composite membrane, multiple resistances possible

D IN /2

D OUT /2

FIGURE 2.4 Mass transfer resistances in membrane contactor.

Each term on right side of Equation 2.2 represents an individual resistance as depicted in Figure 2.4. Hollow fiber diameters are dOUT and dIN. The term H is the Henry coefficient (liquid–gas equilibrium constant) for the species in question. In the case of liquid–liquid contact, the term H in Equation 2.2 should be replaced by mD, the equilibrium distribution coefficient between tube side liquid and shell side liquid. The membrane transfer coefficient kM is a function of (1) the diffusion coefficient in the phase occupying membrane pores and (2) various membrane geometric parameters. Assuming pure Fickian diffusion in a symmetric microporous membrane, kM can be shown as [5] kM ¼

2D«M t M ðdOUT dIN Þ

(2:3)

where D is diffusivity in the pore phase «M and tM are membrane porosity and tortuosity factors, functions of the membrane morphology In case of complex membrane morphology such as asymmetric or composite membranes, or when Fickian diffusion is not valid, evaluating kM will be more complex. Individual mass transfer coefficients in Equation 2.2 depend on multiple factors such as temperature, pressure, flow rates, and diffusion coefficients and could often be estimated from empirical correlations available in literature [1,2,6]. The rate of mass transfer, R, for each species from shell side to tube side at any point inside the contactor is given as R ¼ KTOTAL A½CSHELL CTUBE

(2:4)

where A is the membrane transfer area based on outside diameter of the hollow fiber CSHELL and CTUBE are bulk concentrations of the species in shell side and tube side, respectively Strictly speaking Equations 2.2 and 2.4 are valid only locally within the contactor. The concentrations in each phase could change continuously inside the contactor. It is also possible for one of the mass transfer coefficients to change within the contactor. In such cases rate of mass transfer will be varying continually within the contactor, and the average overall mass transfer will be obtained by integrating over the entire contactor.



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Useful simplifications are often made in Equation 2.2. We will use gas–liquid contact as an example, and assume gas-filled homogeneous membrane of high porosity, thin wall, and low tortuosity. Since diffusion in gas phase is generally of three orders of magnitude faster than in liquid phase, one can show that kM and kG are quite high in this case compared to kL, and so the controlling resistance to mass transfer is in the liquid phase. This means KTOTAL is essentially the same as kL. If kL is constant within the contactor the total mass transfer rate in Equation 2.4 can be approximated for the entire contactor as R ¼ kL ATOTAL DCLOG---MEAN

(2:5)

DCLOGMEAN is the log mean of the concentration differential (CSHELL CTUBE) from one end of the contactor to the other.

2.7

LITERATURE REVIEW ON MEMBRANE CONTACTOR APPLICATIONS

Over the years many research and development groups, both academic and industrial, have investigated membrane contactor technology and suggested or developed a wide range of possible applications. There is quite a spectrum of patent and published literature on this subject. Markets and industries that benefit from the development of this technology include medical, biotechnology, pharmaceutical, semiconductor and electronics, food and beverage, environmental, and other special process industries that are finding new uses. It is impossible to mention all the work done to date.

2.8

USE OF GAS–LIQUID OR LIQUID–GAS–LIQUID CONTACT

As mentioned earlier, membrane blood oxygenators probably would qualify as the earliest form of membrane contactors. Reference [11] is a good illustration of a hollow fiber device. However, most work on liquid–gas membrane contactor over the years has focused mainly on two categories: (1) separation, purification, and treatment of water or aqueous media and (2) absorption of gaseous species from air either for purification or for recovery, which will be discussed separately. Applications in multiple markets and industries have been investigated in each category. An early example of a patent on membrane contactor for gas transfer is in Ref. [12]. Harvesting of oxygen dissolved in water and discharging of CO2 to the water is presented in Ref. [13]. A membrane device to separate gas bubbles from infusion fluids such as human-body fluids is claimed in Ref. [14]. A hollow fiber membrane device for removal of gas bubbles that dissolve gasses from fluids delivered into a patient during medical procedures is disclosed in Ref. [15]. Membrane contactors have also found application in dissolved gas control in bioreactors discussed in Refs. [16–17]. Application of membrane contactors for water degasification has been thoroughly investigated and reported in Refs. [18–21]. During the last few years this has been one of the most successful applications of membrane contactors on large commercial scale. Specifically, oxygen removal and gas transfer from ultrapure water for semiconductor industry have been discussed in Refs. [22–27]. Deaeration process for beverage water is discussed in Ref. [28]. Oxygen removal from boiler feed water as substitute for steam deaerator or oxygen scavenger is presented in Ref. [29]. Membrane contactors have also been used to carbonate water [30], to nitrogenate beer [31], to simultaneously nitrogenate and decarbonate beer, to control CO2 level in beer, and to control dissolved gas profile in beverages using mixed sweep gases of CO2 and N2 [32–33]. Removal of dissolved volatile organic compounds (VOC) from water in membrane contactors has been the subject of several investigations. VOC can be separated from water by applying a vacuum, the process is often termed vacuum membrane distillation [34–36]. Alternately, air can be used as a sweep gas to strip VOCs from water across the membrane [37]. Air stripping of water in packed or spray columns is a widely accepted process for ground water or process water treatment. If membrane contactors were used broadly for this purpose, the market potentials are certainly high. A variation of membrane air stripping process is discussed in Ref. [38] where the driving force for VOC stripping of water is established using methanotropic bacteria. Total organic carbon (TOC) reduction from ultrapure water during membrane degassing has been reported in Ref. [39]. Removal of tri-halo methane (THM) compounds, a chemical class of undesirable species, from ultrapure water has been discussed in Ref. [40]. Use of microporous membranes in combination with RO to separate dissolved gases from water is disclosed in Ref. [41]. Study on removal and recovery of volatile aroma compounds from water was presented in Ref. [42]. Adding oxygen or other beneficial gas species to water without forming gas bubbles is another application of membrane contactors. This subject has been discussed in Refs. [43–46]. Membranes in module form and hollow fibers in unconfined form have been investigated. Use of membrane contactors for supplying oxygen to a biofilm is claimed in Ref. [47]. A similar process where gaseous hydrogen is added to aqueous liquid without bubble formation is disclosed in Ref. [48]. The purpose for such a process would be to use dissolved hydrogen to biologically or catalytically remove oxygen, nitrite, or nitrate from water. Membrane contactors are also used to add trace quantity of CO2 into ultrapure water to control water resistivity and prevent formation of static electricity [49]. A more recent and significant application of membrane contactors is the addition of gaseous ozone to water for the purpose of disinfection and removal of organic contamination, such a process is disclosed in Ref. [50]. A number of applications of the previously termed gas membrane have also been studied over the years to remove or recover volatile species from water or other aqueous media. The primary drivers for these investigations are the intriguing and




13

creative possibilities of the gas membrane, which in effect combines two gas–liquid contact processes (stripping and absorption) within a single microporous membrane. Some of the early-published studies include recovery of bromine [51], cyanide [52], ammonia [53–55], and ethanol [56]. Applications of this technology for commercial purposes are in various stages of development [57]. Membrane processes termed as osmotic distillation or membrane distillation could be shown to be applications of membrane contactor technology also. Both of these processes are based on gas membranes. Osmotic distillation, sometimes called osmotic evaporation, involves transfer of water vapor across a gas-filled membrane, the process is driven by a difference in water vapor pressure maintained across the membrane [58–59] by separate aqueous liquids. Membrane distillation is a process where water vapor transfer is driven solely by a temperature difference across the gas-filled membrane [60–61]. Water evaporates from a hot aqueous phase and condenses on a cooler surface. This process may be useful in desalinating water or producing pure water if a good natural source of warm water is available, such as in a geothermal process. As mentioned in Table 2.2, one unique feature of membrane contactors is the ability to operate without the aid of gravity. This, along with the advantage of smaller sizes for contactor systems, has led to the interest in possible use of this technology in microgravity and confined spaces such as spacesuits, manned spacecrafts, and space station. Primary applications are (1) separating gas and liquid phases in microgravity and (2) removal of unwanted gas species from liquids [62–64]. We now discuss the second category of applications that focus on treatment and conditioning of air or gas streams. This is done either (1) by capturing (absorbing) gaseous species from air or other gases into water or aqueous liquids or (2) by controlling the properties of air or gas phase by other means of heat and mass transfer across membrane in a contactor. The first detailed investigation of absorption of a gas species (CO2) in a liquid using a membrane contactor was discussed in detail in Refs. [4,5]. The mass transfer analysis in these early papers has been most influential for understanding the technology. Absorption of various gases such as CO2, SO2, NH3, and carbon monoxide in water using membrane contactors was studied by many other research groups and reported in Refs. [65–70]. Removal of CO2 as a greenhouse gas from air and bulk removal of CO2 from air in contactors using conventional absorbents have been reported in Refs. [71–73]. The topic of scrubbing CO2 from air for self-contained breathing systems using microporous membrane is discussed in Ref. [74]. Capturing CO2 from atmosphere using membrane contactors, as part of a hydrogen storage process, was suggested in Ref. [75]. Use of membrane contactors for recovery of VOCs from air was reported in Ref. [76]. A hollow fiber membrane bioreactor, for the purpose of destroying toxic compounds from air, is shown in Ref. [77]. Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78–79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82].

2.9 USE OF LIQUID–LIQUID CONTACT A historical perspective on aqueous–organic extraction using membrane contactor technology is available in Refs. [1,6,83]. The mechanism of phase interface immobilization was first explored in Ref. [84], while application of membrane solvent extraction for a commercial process was first explored in Ref. [85]. Two aspects of liquid–liquid contact in membrane contactors that are different from typical gas–liquid contact are (1) the membrane used could be hydrophobic, hydrophilic, or a composite of both and (2) the membrane mass transfer resistance is not always negligible. Ensuring that the right fluid occupies the membrane pores vis-à-vis the affinity of the solute in the two phases can minimize membrane resistance. These aspects have been discussed in detail in Refs. [6,86,87]. Membrane contactor applications in the liquid–liquid extraction field fall in two categories: (1) removal of unwanted species from water and (2) removal and recovery of valuable species from water. Many investigations have been conducted over the year by academia as well as by industry. Below we are providing some samples from the wide range of applications reported in literature. The examples presented are divided roughly into three sections: (a) biotech and pharmaceutical products, (b) industrial chemicals and VOC, and (c) metals. Processes for production of ethanol and acetone–butanol–ethanol mixture from fermentation products in membrane contactor devices were presented in Refs. [88,89]. Recovery of butanol from fermentation was reported in Ref. [90]. Use of composite membrane in a membrane reactor to separate and recover valuable biotechnology products was discussed in Refs. [91,92]. A case study on using membrane contactor modules to extract small molecular weight compounds of interest to pharmaceutical industry was shown in Ref. [93]. Extraction of protein and separation of racemic protein mixtures were discussed in Refs. [94,95]. Extractions of ethanol and lactic acid by membrane solvent extraction are reported in Refs. [96,97]. A membrane-based solvent extraction and stripping process was discussed in Ref. [98] for recovery of Phenylalanine. Extraction of aroma compounds from aqueous feed solutions into sunflower oil was investigated in Ref. [99].



14


Extraction of phenol from aqueous solution using hollow fiber membrane contactor was first investigated in Ref. [100]. However, the membrane used was not completely microporous. Instead, it was a dialysis-type membrane. A commercial plant to separate phenol from hydrocarbon fraction using microporous membrane contactors was reported in Ref. [101]. Soda lye was used to react with the phenol transferred from the feed phase to create and maintain the driving force for separation. This industrial-scale application enabled the processing of hydrocarbon fraction to a full-value raw material for phenol and acetone synthesis. The first known commercial membrane-based liquid–liquid extraction system involved extraction of by-products from a wastewater stream using an aromatic solvent [102]. Before the membrane system was installed, the entire wastewater stream had to be incinerated leading to high costs for the gas fired incinerator per year. The membrane system lowered the contaminant concentration to adequate levels before the biological wastewater treatment plant, and saved significant operating cost. A process to separate naphthenes from paraffins is claimed in Ref. [103]. It involves the use of a polar solvent for separation in a microporous membrane device. Use of membrane extraction to remove p-nitrophenol in wastewater from dye and pesticide synthesis was investigated in Ref. [104]. Removal of nonvolatile pesticide components from water is presented in Ref. [105]. Removal of several important organic pollutants such as phenol, chlorophenol, nitrobenzene, toluene, and acrylonitrile from wastewater was investigated in Ref. [106]. Removal of VOC contaminants from water was discussed in Ref. [107]. This particular process used sunflower oil to absorb the VOC compounds transferred from water across a gas-filled microporous membrane. However, to prevent any possibility of liquid breakthrough, a plasma-polymerized di-siloxane coating was applied on the oil side of the membrane. Report [108] presents results from a pilot trial where organic pollutants such as chlorinated organic compounds and aromatic organic compounds were removed from plant wastewaters. Various investigators have also explored removal or recovery of metals from aqueous process or waste streams. Liquid– liquid extraction is particularly useful for metal removal since alternate technologies such as distillation are not feasible. A process to separate molybdenum from tungsten leachate using a microporous membrane was disclosed in Ref. [109]. Copper extraction in a membrane contactor using metal chelating agent was presented in Ref. [110]. Other applications suggested in literature include extraction of gold from aqueous solutions [111], removal of copper from edible oil [112], separation of yttrium from heavy rare-earth metals [113], removal of copper and chromium from wastewater [114], and extractions of mercury, copper, and nickel from water [115].

2.10 REVIEW OF MEMBRANE CONTACTOR DESIGN OPTIONS Although membrane is the heart of the membrane contactor technology, appropriate internal design of the contactor device or module is critical for any commercial advancement of the technology. Internal design dictates how the two phases flow inside the contactor and how the hydrodynamics in each phase is managed. As shown in Equation 2.2, the rate of mass transfer is directly dependent on the mass transfer coefficients in each of the phases, which in turn is dependent on the internal hydrodynamics. As the devices become larger to serve large commercial-scale process capacities, dependence on internal flow management becomes more critical. The device design is also important in developing the processes for large-scale manufacturing of the contactors. In the following section, we are reviewing various design options investigated over the years. Designs of membrane contactors with hollow fiber membranes fall in one of the two categories: (1) the primary fluid being treated flows through the inside (lumen) of the hollow fibers and (2) the primary fluid being treated flows on the outside (shell) of the hollow fibers. Another consideration is the flow direction of the fluid in each phase with respect to the axis of the membrane and with respect to each other. In most membrane contactors of early commercial designs, the contactor housing was of cylindrical shape with tube-in-shell configuration (as in tubular heat exchangers) where the primary fluid flows on the lumen side from one end of the fiber to the other and the other fluid flows on the shell side in parallel direction. This design is generally called the parallel-flow design and is illustrated schematically in Figure 2.5a. The contactors of such a design are relatively easy to manufacture. However, the main drawback of the parallel-flow design is the nonuniform spacing of hollow fibers and the resulting poor flow distribution or flow channeling on the shell side, particularly as the contactor diameter increases. A significant improvement over this parallel-flow design is the transverse-flow design where the primary fluid flows on outside of the hollow fiber membrane at a transverse direction to the fiber axis, while the other fluid flows on lumen side of the hollow fibers. The relative merits of the two designs were first analyzed comprehensively in Ref. [116]. It determined that transverse flow on shell side significantly improves the mass transfer coefficient compared to the parallel-flow design. However, it was still difficult to ensure that the transverse flow on shell side is completely uniform along the fiber length. Most investigations on membrane contactors continued to focus on parallel-flow design, since they are easier to fabricate on small scale. The effect of shell side hydrodynamics in parallel-flow contactors was investigated and reported in Ref. [117]. The twin problems of (1) ensuring transverse flow uniformly along the length of fibers and (2) ensuring even flow distribution on shell side were solved largely by adopting the concept of hollow fibers in fabric array form that was wound around a central hollow mandrel with porous wall. The shell side fluid could be introduced in the membrane contactor through the central distribution mandrel. It could then flow radially outward, in a direction transverse to hollow fiber axes. The central



15

Application of Membrane Contactors as Mass Transfer Devices Potting

Hollow fibers

Shell fluid inlet

Lumen fluid inlet

Lumen fluid outlet Shell fluid outlet

(a) Parallel-flow design Lumen fluid outlet

Lumen fluid inlet Hollow fiber

Shell fluid outlet

Shell fluid inlet Porous center tube

Baffle

Center plug

(b) Transverse-flow design

FIGURE 2.5 Primary design options for membrane contactors.

hollow mandrel ensures axial uniform flow distribution whereas the hollow fiber array ensures constant fiber-to-fiber distances and uniform transverse-flow distribution. A further improvement was the use of one or multiple flow-directing baffle inside the shell, just as practiced in commercial heat exchangers, which made the contactor more efficient and facilitated commercial production. Figure 2.5b schematically illustrates a transverse-flow contactor with flow-directing baffle. Detail investigations of this design are shown in Refs. [118–120], and both cylindrical and rectangular contactors are investigated in Ref. [118]. An interesting variation of the contactor design with baffle is disclosed in Ref. [121] for a degassing application. This shows a spiral-wound contactor similar to that shown in Ref. [120], but the baffle was placed on the gas side of the device and the water flow was on lumen side. Since most of the mass transfer resistances in liquid degassing process are essentially in the liquid phase, it is not clear how such a design would improve the hydraulic efficiency of the device. In addition to what was discussed above, there have been many other contactor designs proposed over the years. A membrane contactor of rectangular design is disclosed in Ref. [122] made by laminating hollow fiber fabric sheets, and preventing flow channeling by specifying the densities of the hollow fibers and the warp fiber of the fabric. A similar structure of membrane contactor apparatus is claimed in Ref. [123]. Reference [124] discloses a contactor with multiple frames of square, polygonal, or circular, where the longitudinal directions of the fibers or tubes of adjacent frames are substantially perpendicular to each other. Stackable sub-modules with multiple frames of hollow fiber membranes in each sub-module were suggested in Ref. [125]. A rectangular contactor was also suggested in Ref. [75]. A tubular hollow fiber membrane contactor of parallel-flow design, with special spacers on shell side to reduce flow channeling, was disclosed in Ref. [50]. A radial-flow transverse-flow membrane contactor without any flow-directing baffle was shown in Ref. [126]. In some applications, particularly in degassing processes using deep vacuum, it has been shown that presence of flow-directing baffle such as claimed in Ref. [119] could actually be detrimental to performance because of internal diffusion in the gas phase. A hollow fiber membrane contactor that does not use a shell at all has been disclosed in Ref. [127].

2.11 COMMERCIAL OR PRECOMMERCIAL INSTALLATIONS OF LARGE-SCALE MEMBRANE CONTACTORS Applications of membrane contactor technology in commercial processes are in various stages of development. Early success has come mainly in water degassing or gas addition applications. Membrane contactor systems of a wide range of flow capacities are currently in operation in various parts of the world. Systems with large capacities were possible only after membrane contactors of sufficiently large size and cost competitiveness could be produced commercially on a routine basis. Currently, the largest known commercially produced membrane contactor module has an active contact area of about 220 m2 [128]. Commercial availability of such products has greatly facilitated the large-scale acceptance of this technology. A few examples of various installed contactor systems are provided below.



16


Reverse osmosis (RO)

Pretreatment

Reclaim water

Vacuum tower RO tank O2 removal hybrid or stand alone system

Electro deionization (EDI)

Tank

UV

O2 and CO2 removal to recycle back into loop

CO2 removal to improve EDI performance Total gas control and final polishing of O2

Final filter

Polishing beds

Point of use

UV

O 2 removal and N2 control for megasonic cleaning efficiency

FIGURE 2.6 (See color insert following page 588.)

Part of semiconductor plant ultrapure water production process.

Figure 2.6 schematically illustrates sections of a typical semiconductor ultrapure water (UPW) production process in a semiconductor plant. The water circuit consists of two main sections: (1) makeup (or central) system and (2) polishing loop, which provides water at the point of use. There are multiple locations in such a water process where membrane degassing could be needed as shown in the figure. Reverse osmosis is mostly used in makeup line as the primary purification means in such processes. In the past, large and inflexible vacuum towers were frequently used after RO to remove dissolved gases, such as O2, N2, and CO2. Membrane contactors are the norm today for replacement or supplement to vacuum towers in makeup lines, as shown in Figure 2.6. A second place where membrane contactors are often used is before the electro deionization (EDI) step in UPW plants to remove residual CO2 gas. Removal of CO2 improves efficiency of EDI unit. Membrane contactors allow the opportunity to decarbonate water inline and under pressure just prior to EDI. Contactors are also used in polishing loop for final degassing and to treat recycled or reclaimed water (Figure 2.6). The exact UPW process configuration depends on the specific needs in the plant and on quality of water supply to the plant. But no

64 ft (19.5 m)

Design conditions for both systems Flow: 748 gpm (170 m3 / h) Design spec: Dissolved O2 30 parts per billion 4.5 ft (1.15 m)

6 ft (1.8 m)

5.5 ft (1.65 m)

7.5 ft (2.2 m)

Membrane contactor system Vacuum tower

FIGURE 2.7

Relative system sizes of vacuum tower and membrane contactors.



17


System capacity: 600 gpm (136 m3 / h) System design: 3 Trains of 3 large contactors in series 9-Contactor system expandable to 16 contactors System performance spec: 40 the loss of loading capacity is below 3%. The timescale for axial diffusion is much bigger than that of the radial diffusion indicating mass transfer resistance in the boundary layer. This means that the concentration gradient in radial direction can be neglected when the condition dp2=4D L=v is fulfilled. In general, axial diffusion is neglected when the radial Peclet number (Per) is smaller than 0.04. Per ¼

dp2 v < 0:04 4DL

When individual membranes are stacked into a column, it is advisable to use at least 30 membranes on top of each other to average out the membrane heterogeneities and the channeling effects. The pressure drop over the membrane stack should be as low as possible. Therefore, producers like Sartorius, Millipore, and Pall advise not to apply pressures >4 bar. Fanning equation: where v ¼ viscosity (m2=s) d ¼ pore diameter (m) r ¼ density (kg=m3) u ¼ linear velocity in the pore (m=s) L ¼ thickness of membrane stack (m)

DP ¼

4 24 v L 1 ru d2 2


49

Membrane Chromatography Convective through pores

Diffusive pores

FIGURE 3.19

Visual representation of a stacked column.

For particles under viscous conditions and Re 1 a more simplified form, known as the Kozeny–Carman equation, can be used to calculate the pressure drop within the chromatographic column. Kozeny---Carman equation:

Dp ¼

150 v u L ð1 «Þ2 dP2 «3

where dp ¼ equivalent spherical particle diameter (m) « ¼ void fraction of the bed () L ¼ column length (m) When diffusion inside the adsorbent is the limiting mass transfer step, two different diffusion mechanisms with different driving forces namely pore diffusion and surface diffusion have to be considered [187]. Surface diffusion is often neglectable in case of proteins for which the lumped sum is often described as effective pore diffusivity while it has to be taken into account in case of ions. However, using high surface coverage surface diffusion can really contribute to the mass flux. For high capacity membrane adsorbers the surface diffusion is the dominating factor in the mass transfer thus the shape of the breakthrough curve is strongly dependent on the feed concentration. In membrane chromatography the target solutes are transported by convective bulk flow to the active sites. The solutes experience three different resistances against transport [188].

3.6.3 DISPERSION Membranes act as short wide chromatographic columns with pore sizes up to 3 mm (Figure 3.19) resulting in very low resistance against mass transport and high fluxes of over 1000 cm=h coupled with a low-pressure drop. Longitudinal dispersion and convection of matter by flowing through a packed bed or membrane are coupled. This can be visualized by the maximum when the PeL number is plotted against the Reynolds number [189]. The residence time distribution (RTD), also referred to as the distribution of ages, is based on the assumption that each element traveling through the column takes a different route and will therefore have a different residence time. Different methods are developed to determine the RTD in a module or in a reactor [190]. The RTD of a chromatographic column is defined by a function E (Figure 3.20), such that E dt is the fraction of material in the exit stream with an age between t and t þ dt. The E-curve lies between the extremes of plug Ð 1 flow and continuously stirred tank reactor. The surface below the curve between t ¼ 0 and t ¼ 1 has to be equal to unity 0 E(t) dt ¼ 1, because all elements that enter the module must also exit the module.

3

au

1 = Injection 2 = First tracer molecules in eluate 3 = Maximum tracer concentration 4 = Last tracer molecules in eluate 1

4

2 Total area = 1

0

FIGURE 3.20

Example of an E-curve.

Time

t t + dt


50


The E(t) curve is obtained out of the tracer concentrations. The existing relationship between the E(t) and C(t) is Ð1 . The average residence time can be determined by t ¼ 0 t E(t) dt. This E(t) can be obtained by various E(t) ¼ PC(t) C(t) methods of which the two most common are given below: .

Tracer Impuls Method: In this method a certain amount of tracer is injected (very short time interval) into the system and the concentration is measured at the outlet of the column. The concentration followed in time can directly be converted to the E(t) curve as described above. Heaviside Step Function: This function implies measurements of the integrated function of the distribution curve, cumulative RTD function F(t). This can be established by changing one liquid (usually water) from one steady value to another Ð t with a detectable tracer. The equation that relates this measurement to the tracer impulse method is F(t) ¼ 0 E(t) dt.

.

3.6.4

DETERMINING

THE

AVERAGE RESIDENCE TIME

AND THE

VARIANCE

The recorded F(t) curves can be translated to the E(t) curve by taking the derivative of the recorded curve. The data were F(tnþ1 )F(tn1 ) . translated with E(tn ) ¼ @F(t) @t ¼ Dt First, the area under the curve is determined by the zero moment MO0. In this way the curve can be normalized if necessary. MO0 ¼

ð1

E(t) dt ¼ 1 ¼

0

n¼x X

1 2fE(tn )

þ E(tnþ1 )gDtn

n¼0

¼ 12fE(t0 ) þ E(t1 )gðt1 t0 Þ þ þ 12fE(tx1 ) þ E(tx )gðtx tx1 Þ Second, the average residence can be determined by calculating the first moment MO1 ¼ t ¼

ð1

t E(t) dt ¼

0

n¼x X

1 2ftn E(tn )

þ tnþ1 E(tnþ1 )gDtn

n¼0

¼ 12ft0 E(t0 ) þ t1 E(t1 )gðt1 t0 Þ þ þ 12ftx1 E(tx1 ) þ tx E(tx )gðtx tx1 Þ For the calculations of the variance determined by s2t ¼ MO2 MO21 , it is necessary to determine the second moment 1 ð

MO2 ¼ ¼

t 2 E(t) dt ¼ 0 1 2 2 t0 E(t0 )

n¼x X 1 2

2 tn2 E(tn ) þ tnþ1 E(tnþ1 ) Dtn

n¼0

2 þ t12 E(t1 ) ðt1 t0 Þ þ þ 12 tx1 E(tx1 ) þ tx2 E(tx ) ðtx tx1 Þ

The RTD measurement is a dynamic measurement meaning that the volume in the column measured with the experiments (Vflow-through) can be determined. Static volume measurements can also be done to determine the column hold up, Vhold up. V up The average residence time, tVhold up, based on the column hold up can be determined by t Vhold up ¼ hold Q The following is possible: . . .

t > tVhold up adsorption of the tracer in the column or transport into the porous structure t ¼ tVhold up completely utilized column t < tVhold up presence of dead volume and channeling within the column

When the determined residence time approaches the tVhold up just external flow is obtained whereas for residence time getting closer to the tVaccessible value, internal flow (due to diffusion or convection) is also obtained. It may be seen that a single parameter s2t =ðt Þ2 is sufficient to characterize the sharpness of the peak. The inverse of this parameter is the number of theoretical plate heights of the adsorber, which describes the efficiency of the system.

3.6.5

BROWNIAN MOTION

During the convective transport individual target molecules are dispersed by the presence of small eddies. The random walk motion of small particles suspended in a fluid due to bombardment by molecules obeys the Maxwellian velocity distribution. If a number of particles subject to Brownian motion are present in a given medium and there is no preferred direction for


51

Membrane Chromatography

the random oscillations, then over a period of time the particles will tend to be spread evenly throughout the medium. Thus, if two compartments one containing component A and the other containing component B are brought together then component A will move to compartment B. The speed of this transport is dependent on the concentration difference within the two compartments, the particle size, the viscosity of the medium, and the temperature. This process is called diffusion and is the macroscopic manifestation of Brownian motion on the microscopic level. In a convective fluidum the movement of one component in a certain medium, in a time interval (t) and in a certain direction, from its starting point over the distance (x) is related to the diffusion coefficient (D) of that component.

3.6.6 INTERACTION

WITH THE

MATRIX

AND

ACTIVE SITES

In membrane chromatography the active sites are immobilized inside the membrane pores. For particle-loaded membranes the active sides are immobilized at the outer surface of the imbedded particles or, to increase the total active surface area, they can also be located inside the porous particles. In the latest case if very small pores are involved, the productivity is low even in spite of a high active surface area. This is because the transport of the target molecules to the active sites is by diffusion alone, which is a much slower process than the convection through the big pores. Therefore, the loading step is much more timeconsuming. The interaction between the target molecules and the ligands is often considered instantaneous. Nevertheless, in the case of affinity complex formation the building of the complex can be that slow that the limiting step is not longer the convection or the diffusional transport to the active site but the complex formation.

3.7 ADSORPTIVE MEMBRANE GEOMETRY AND OPERATION MODES Membrane chromatography can be performed on units of various geometry, microporous adsorptive membranes and related systems such as single or staple flat-sheet membranes, hollow-fiber, spiral-wound and cassette devices being already commercially available. An overview of commercially available supports in membrane chromatography is presented in Table 3.3. Few of these systems (Sartobind, Sartorius [Figure 3.21]; MemSep, Millipore; Quik Bind, Sepracor; Biorex, Bio-Rad; Lowprodyne, Pall; AbSorbent, Genex; ActiDisk, FMC Bioproducts; MAC, Amicon) have been already developed for a small number of technical applications [8–10]. Continuous beds including methacrylate-based disks CIM QA from BIA Separations (Figure 3.22) and SWIFT from Isco, acrylamide UNO monoliths from Bio-Rad, Ultimate polystyrene divinylbenzene disks from LC-Packings, cellulosebased Seprasorb monoliths from Sepragen and silica units Conchrom from Conchrom or Chromolith from Merck are also

TABLE 3.3 Examples of Commercially Available Membrane and Monolithic Products Trade Name

Material

Separation Principle

Flat sheets, disks

Regenerated cross-linked cellulose

IEX, AF, IMAC

PVDF, regenerated cellulose, silica, polypropylene PVC Cellulose, polypropylene Nylon, PES

AF, IEX, RP

FMC (USA) Cuno Life (USA) Pall (USA)

Flat sheets, syringe, disks, pipette tips Flat sheets Depth filter Flat sheets, disks Hollow fibers, monolithic rods Particle-loaded membranes, disks Particle-loaded membranes, full=hollow fiber modules Monolithic columns Monolithic rods, disks, tubes

Polyamide, silica PTFE, PS-DVB, silica PES

IEX, RP IEX, AF IEX, AF

Seprasorb CIM disk

Merck (Germany) 3M Mosaic Systems (The Netherlands) Sepragen (California, USA) BIA (Slovenia)

IEX IEX, RP, AF

Swift Conchrom UNO

Isco (USA) Conchrom (Germany) Biorad (California, USA)

Monolithic columns Monolithic rodss Monolithic rods

Regenerated cellulose Polyglycidyl methacrylate-co-ethylene dimethacrylate Styrene, methacrylate Modified silica Polyacrilamide

Sartobind, Vivapure Immobilon, Intercept Q, MemSep, Zip Tip ActiDisk ZetaPlus=PolyNet Biodyne, Mustang (Q, S, and E) Fractoflow, Chromolith Empore Mosaic Systems

Manufactures Sartorius, Vivascience (Germany) Millipore (USA)

Module Type

IEX IEX IEX, AF

IEX, RP IEX, RP IEX

Note: IEX ¼ ion-exchange chromatography; AF ¼ affinity chromatography; IMAC ¼ (metal-)chelate chromatography; RP ¼ reversed-phase chromatography.


52


(a)

(b)

(c)

FIGURE 3.21 (a) Layered membrane adsorbers, (b) labscale membrane adsorber, and (c) higher magnification membrane adsorber. (Courtesy Sartorius, Göttingen, Germany.)

manufactured as large scale. The smaller laboratory scale systems often contain various amounts of stacked flat sheet membranes; single sheet adsorbers are rarely used. The bigger process systems consist of radial feeded layered or layered and pleated sheets [184]. For an optimal design of the membrane chromatographic systems several parameters have to be taken into account. Most commercial systems like Mustang from Pall and Sartobind from Sartorius make use of functionalized microporous membranes. The fibrils reinforced membranes are (pleated) layered around a porous core. The feed is forced to permeate through the membranes in radial direction. This approach results in high area to volume ratio. The 3M and Mosaic Systems approach is different. Instead of functionalization of a porous support they make use of already functionalized beads, which are embedded in a porous support. In this approach, the beads are responsible for the capacity and selectivity where the porous matrix controls the hydrodynamics. The 3M modules consist of stacked flat sheet or pleated membranes, while Mosaic Systems makes use of porous fibers in which the active particles are embedded (Figure 3.23). In processes where large macromolecules with slow associated kinetics are involved, the adsorption kinetics between the ligand and the dissolved molecule in the feed solution is the rate-limiting step. Not often the pressure drop or the membrane mechanical strength is the limiting factor. When thin membranes are applied the axial diffusion also becomes more dominant and requires a lower linear flow rate through the matrix. Besides this in-homogeneities in porosity and thickness affect the performance of the system negatively. To overcome this problem, often a stack of at least 30 membrane sheets are stacked to average out these heterogeneities. By using hollow fiber membranes the ideal module comprises short fibers with a wide bore to avoid a high pressure drop in the flow direction thereby disturbing the uniform radial flow pattern and creating channeling. The membranes should also possess thick porous walls with small pore size and a high ligand density. The hollow fiber modules can be operated in cross flow mode what makes them especially suitable in the treatment of solutions containing particulate material.

(a)

FIGURE 3.22

(b)

(a) A 800 mL continuous bed module and (b) 8, 80, and 800 mL monoliths. (Courtesy BIA Separations, Ljubljana, Slovenia.)


53


(a)

(b)

FIGURE 3.23 (a) Particle-loaded fiber module and (b) SEM picture of a particle loaded fiber. (Courtesy MOSAIC Systems, Breda, The Netherlands.)

3.8 APPLICATIONS OF MEMBRANE CHROMATOGRAPHY The operating interactions within the microporous media involved in chromatographic separation processes are identical to those in the packed columns including affinity interactions, size-exclusion, ion exchange, hydrophobic interactions, and reverse phase (Table 3.4). The functional groups immobilized on the porous membrane surface include, depending on the target application, (i) affinity ligands, e.g., Protein A=G, BSA, biomimetic dyes, immobilized metal, various hydrophobic amino acids; (ii) ion-exchange groups such as carboxylic or sulfonic acid, tertiary or quaternary amines; and (iii) hydrophobic ligands, e.g., octyl and phenyl groups. Several detailed reviews of the various applications of membranes in chromatographic separations have been recently published [8–13]. Therefore, in the following, we will give just a short overview of some of the successful applications of membrane chromatography, classified as a function of the interaction mode involved in the separation process. Readers should also take into account the possible application of membrane chromatography already mentioned in Section 3.3. The different modes of chromatography reports are briefly summarized in Table 3.5. Detailed overview of tables on application in membrane chromatography may be found in Refs. [8,12,13]. The affinity techniques performed on single or stacked membranes, disks, tubes, and rods are based on a biospecific interaction that results in a change of protein properties such that the protein can be separated from other complex biomoleculecontaining mixtures. The ligand molecule is immobilized on the porous surface and the mixture containing the protein of interest is passed through the adsorptive membrane. A specific interaction takes place between ligand and ligate and retains the desired protein within the matrix support, while the other components from the feed solution pass freely through the adsorber. Affinity chromatography allows for purification of biopolymers based on biological functions rather than individual physical or chemical properties. Isolation of a protein or a group of proteins such as g-globulin fractions, human serum albumin, and various clotting factors from body fluids was successfully achieved by using affinity membrane chromatography. Immunoaffinity techniques were widely employed for the analyses and purification of proteins [191–193]. Immobilized antibodies were used, e.g., for industrial scale production of human interferon-a2a, interleukin-2, and interleukin-2 receptor, while protein A and protein G were successfully used in therapeutic applications including purification of human immunoglobulin G from plasma and serum [194–196]. Pseudoaffinity ligands such as dyes, lysine, and histidine are now well established in membrane chromatography. Cibacron Blue F3GA dye membranes (having a specific binding for nicotinamide adenine dinucleotide dependent enzymes) and Procion Red HE-3B (with a specific binding for nicotinamide adenine dinucleotide phosphate dependent enzyme) were successfully employed in purification of several enzymes including human serum albumins from plasma, dehydrogenases

TABLE 3.4 Types of Interactions Involved in Chromatographic Separation Processes Type of Chromatography Affinity chromatography Ion-exchange chromatography (Metal-)chelate chromatography Size-exclusion chromatography Normal=reversed-phase chromatography

Separation Mode=Interaction Type Molecular structure=biospecific adsorption Surface charge=ionic binding Metals complex formation=coordination complex Molecular size and shape=size exclusion Hydrophobicity=hydrophobic complex formation


54


TABLE 3.5 Examples of Membrane Chromatographic Applications Separation Mode Affinity chromatography (immunoaffinity, metal affinity, chiral)

Ion exchange

Hydrophobic interaction=reverse phase

Ligate Type

Ligand Type

References

Human interferon-a2a, interleukin-2 receptor, human IgG, human serum albumin, dehydrogenases glucose-6-phosphate dehydrogenase, formate dehydrogenase, pyruvate decarboxylase, S-oxynitrilase, bovine liver catalase, recombinant protein G, plasmid DNA, human blood coagulation factor VIII, enantiomers of kynurenine, tryptophan, b-blockers, practolol, thiopental Serum proteins, microbial proteins and enzymes, membrane proteins, cytokines or nucleic acids, BSA, HSA, a-chimotrypsinogen, lysozyme, tripsine inhibitor, cytocrom c, ovalbumin, a-lactoalbumin, conalbumin, ferritin, myoglobin, chymotripsin, human recombinant antithrombin, monoclonal and polyclonal antibodies, immunofussion proteins Recombinant tumor necrosis factor, myoglobin, ovalbumin, lysozyme and chimotrypsinogen, chymotrypsinogen A, ribonuclease, myoglobin, trypsinigen and lysozyme, tricyclic antidepressants, antiarrhythmic drugs, amiodarone, desethylamiodarone, mexiletine, flecainide, alcohols, phenols, aldehydes, ketones, esters, polar pesticides from water

Antibodies, protein A and protein G, lysine, histidine, Cibacron Blue F3GA, Procion Red HE-3B, combinatorial peptides, metals

[36,135,191–201, 218–225]

Quaternary amino, diethylaminoethyl groups

[36,58,202–211, 222–224,226]

C4, C6, C8, or C10 linear aliphatic chains

[2,14,205,212–217,226]

glucose-6-phosphate dehydrogenase from a clarified yeast solution, formate dehydrogenase from Candida bodinii and pyruvate decarboxylase from Zymomonas mobilis homogenates [36,197]. Applications of metal affinity chromatography include separation of S-oxynitrilase from Sorghum bicolor on IDA-Cu2þ ion membranes, purification of human serum albumin with an IDA-Ni2þ membranes cartridge and purification of bovine liver catalase (both on an analytical and preparative scale) by IDA-Cu2þ composite cellulose membrane [198,199]. A unique application for affinity purification is the separation of proteins differing by a single histidine molecule in their sequence by immobilized metal affinity chromatography. Kasper et al. [200] proposed an affinity-chromatographic method for a fast, semipreparative isolation of recombinant protein G from Escherichia coli. Rigid, macroporous affinity discs based on a GMA-co-EDMA polymer were used as chromatographic supports. Human immunoglobulin G was immobilized by a single-step reaction. The globular affinity ligands were located directly on the pore wall surface and were therefore freely accessible to target molecules (protein G) passing with the mobile phase through the pores. Examples of bioaffinity separations using monolithic stationary phases based on antigen–antibody, enzyme–substrate, enzyme–inhibitor, receptor–ligand interactions were reviewed recently [201]. Giovannini et al. [202] prepared poly (GMAco-EDMA) membranes for the separation of supercoiled plasmid DNA. Gradient and isocratic elution was investigated and high-performance membrane chromatography experiments were compared with similar ones performed on a conventional packed-bed column. Amatschek et al. [135] developed an affinity-chromatographic method in which peptides, derived from a combinatorial library, were used as immobilized ligands for the purification of human blood coagulation factor VIII. Affinity monoliths based on a copolymer of glycidyl methacrylate and ethylene dimethacrylate using rabbit IgG and anti-FITC antibodies as ligands were recently developed for ultrafast immunoextractions [203]. Ion-exchange chromatography is probably the most widely used chromatographic method for protein separation. The principle of protein separation by ion exchange is the electrostatic interaction between the charges of the macromolecule and the adsorber surface. The protein must displace the counter ion of the exchangers and become attached on the sorbent’s surface. The amount of macromolecule bound per unit volume of adsorptive membrane can be very high. However, for most exchangers, the binding capacity depends on the molecule size of the protein and adsorption conditions (pH, ionic strength, and protein concentration). Ion-exchange membranes can be produced by modification of commercially available microfiltration membranes. Anion-exchange chromatography on membranes, disks, and rods bearing mainly quaternary amino groups or diethylaminoethyl groups as ligands has been used for the separation of serum proteins, microbial proteins and enzymes, membrane



55

proteins, cytokines, or nucleic acids [36,58,204–209]. BSA and HSA, a-chimotrypsinogen, lysozyme, trypsine inhibitor, cytocrom c, ovalbumin, a-lactoalbumin, conalbumin, ferritin, myoglobin, and chymotrypsin are just a few of the compounds isolated by anion-exchange membrane chromatography. The implementation of a CIM DEAE (diethylaminoethyl) monolithbased step into a large-scale plasmid DNA purification process was recently performed [210]. Cation-exchange membranes, despite the fact that they are not as widely investigated as that of anion-exchange membranes, allow recovery of human recombinant antithrombin from cell culture supernatants, purification of monoclonal and polyclonal antibodies, and isolation of immunofusion proteins produced extracellularly by E. coli [211–213]. The separation of similar size proteins such as serum albumin and hemoglobin using adsorber membranes loaded both with cation or anion-exchange particles was recently reported [214]. Li et al. [215] have used a cation-exchange monolith (prepared by polymerization of aqueous solution of appropriate monomers including the desired ligand directly in a fussed-silica tube), as chromatographic supports for separation of four standard proteins (cytocrom c, lysozyme, myoglobine from horse and whale). High adsorption of lysozyme on macroporous chitosan carboxymethyl=cellulose blend membranes was recently reported [216]. The protein separations based on hydrophobic interactions and reverse phase use the interaction between aliphatic chains on the adsorbent and corresponding hydrophobic regions on the protein surface. Typical hydrophobic adsorbents commercially available include C4, C6, C8, or C10 linear aliphatic chains, eventually with a terminal amino group. The main problems in hydrophobic chromatography are the slow association–dissociation processes and protein–protein interactions. Similar proteins can interact with each other as well as with the adsorbent leading to a large degree of overlapping between the elution components. Hydrophobic chromatography has not been used in protein separation as intensive as ion exchange or affinity chromatography since sharp separations are not achieved. Nevertheless, chromatographic units with butyl ligand were used for purification of the recombinant tumor necrosis factor [207]. Tennikova et al. [2] reported separation of a protein mixture containing myoglobin, ovalbumin, lysozyme, and chimotrypsinogen on monolithic columns with C4 or C8 ligands. Zeng et al. [217] reported intensive studies on the preparation of monolithic stationary phases based on acrylic comonomers including piperazine diacrylamide, methacrylamide, and N-isopropylacrylamide for application in hydrophobic interaction chromatography of proteins. Chymotrypsinogen A, ribonuclease, myoglobin, trypsinigen, and lysozyme could be recovered with estimated values ranging from 91% to 99%. PS-co-DVB continuous rods were also investigated for the reversed-phase separation of proteins, alkylbenzenes, and peptides [218,219]. Particle-loaded membranes with embedded C8 hydrophobic adsorbents were intensively investigated for several drug separations. Tricyclic antidepressants, antiarrhythmic drugs, amiodarone and its metabolite desethylamiodarone, and mexiletine and flecainide were extracted from serum using a 11 mm C8 membrane adsorber with recoveries ranging from 82% to 98% [14,220]. Highly hydrophobic sorbents including porous carbon and copolymers of styrene and divinylbenzene (SDB) were widely investigated for environmental applications. The particle-loaded membranes containing modified SDB particles with surface sulfonic acid groups were successfully used for recovering different alcohols, phenols, aldehydes, ketones, or esters from aqueous samples [221]. Carbon-based PLM were also used for isolation of highly polar pesticides from water [222]. Chiral crown ether bonded on negatively charged polyacrylamide gels was prepared by Koide et al. [223] for enantiomeric separation of primary amino compounds with high efficiency and good reproducibility. Monolithic columns prepared by free radical polymerization of piperazine diacrylamide and methacrylamide in the presence of acetylsalycilic acid and L-tryptophan as additives were recently reported [224]. The monoliths, carrying immobilized human serum albumin as chiral selector, showed improved resolution when used for kynurenine and tryptophan enantiomeric separation. Continuous polymeric beds prepared either by entrapment in and simultaneous covalent linkage of allyl cellulase to the polymeric bed during preparation or by covalent immobilization of cellulase on an epoxy-activated continuous bed were investigated for enantiomers separation by Mohammad et al. [225]. Enantiomers of b-blockers and practolol were rapidly separated with good resolution. Wang et al. [226] prepared a microfluidic system containing porous PVDF and PDMS membranes impregnated with bovine serum albumin as chiral chromatographic support. On-line BSA adsorption onto the membranes was employed for the preparation of chiral stationary phase allowing separation of racemic tryptophan and thiopental mixtures with high resolution. Chiral separation of racemic tryptophan, phenylglycine, and phenylalanine was also investigated through immobilized DNA membranes [227]. Multistage chromatography combines different types of chromatographic supports to achieve a higher degree of selectivity. A sequence of three membrane-based separation steps (including cation exchanger, dye-ligand, and anion-exchange membranes) enabled the purification of formate dehydrogenase from C. boidinii [228] in micro- and laboratory scale. Using a sequence of ultrafiltration, diafiltration, Cibacron blue, anion exchanger, and heparin-membrane adsorber, pure recombinant human antithrombin III was obtained from hamster cell culture. Branovic et al. [229] combined affinity and ion-exchange chromatography in a single column used to determine the impurities in immunoglobin preparation. In their method, a monolithic disk bearing immobilized protein G with the leading role of retaining the immunoglobulins was placed on top of an ion-exchange disk, which could capture the impurities. Using specific elution conditions the immunoglobulins could be separated. A strong ion-exchange membrane adsorber carrying


56


TABLE 3.6 Comparison among Conventionally Packed Beds, Membrane and Monoliths Used in Bioseparations Packed Bed Big Beads Capacity Flow rate Productivity Resolution DP Prepurification

þ þ þ þ þþ þ

Small Beads þþ þ þþ

Membrane Adsorber Particle Loaded

Chemical Functionalized

Monolithic Column

þ þþ þþ þ þþ þ

þ þþ þ þ þþ þ

þ þþ þ þ þ þ

sulfonic acid or quaternary ammonium groups and a Cibacron blue immobilized affinity adsorber were also investigated for mixed-mode interactions chromatography of standard protein mixtures as well as for the separation of whey proteins (BSA, IgG, a-lactalbumin, and b-lactoglobulin) and recombinant human antithrombin III from biotechnological culture supernatants [230]. Using a series of four continuous beds each carrying a different affinity ligand, the one-step fractionation of four different antibodies from biological mixtures was achieved [231]. A mixed particle-loaded membrane consisting of a C8 or C18 hydrophobic adsorbents and a strong cation-exchange material can be used for selective extraction of pigments or imiquimod metabolites from urine with up to 95% recovery [232].

3.9

CONCLUSIONS

Packed-bed chromatography is still the standard method involved in capturing and polishing of biomolecules despite some major disadvantageous such as sensitivity to fouling and plugging, bed consolidation and compression, and high pressure drop and long cycle times. Membrane chromatography has obvious advantages over packed bed (Table 3.6). It uses microporous membranes that contain biospecific ligand molecules attached to their inner pore surface, as adsorbents. As a result of the convective flow of the feed solution through the porous structures the mass transfer resistance is tremendously reduced and the binding kinetic is usually dominating the adsorption process. This results in rapid processing and improved adsorption, washing, elution and regeneration steps, and minimize the probability of protein denaturation. The technology is robust, fast and able to process huge volumes containing low concentrations of target molecules. Especially in the area where packed beds have severe shortcomings because of their limited pores size, for example, DNA recovery and virus removal, the membrane chromatography has a good opportunity to expand till market leader. The advantages of membrane chromatography have been confirmed by many successful applications. Nevertheless there is a scope for further improvement of the adsorptive membrane design by increasing the binding capacity, extension of the supplied ligands, and optimization of the hydrodynamics, and to assess the use of membrane chromatography in large-scale processes. In the near future, membrane chromatography may be the synergetic outcome of two mature techniques, chromatography and filtration that are at this moment the standards in biologic processing.

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19.5.2008 7:24am Compositor Name: JGanesan

4 Membranes in Gas Separation May-Britt Hägg CONTENTS 4.1 4.2

4.3

4.4

4.5

Introduction ...................................................................................................................................................................... 66 Transport Mechanisms for Gas through Membranes ...................................................................................................... 68 4.2.1 Solution Diffusion ................................................................................................................................................ 69 4.2.2 Knudsen Diffusion ............................................................................................................................................... 71 4.2.3 Selective Surface Flow......................................................................................................................................... 71 4.2.4 Molecular Sieving ................................................................................................................................................ 72 4.2.5 Ion-Conductive Transport .................................................................................................................................... 73 4.2.5.1 Proton-Conducting Membranes ............................................................................................................. 73 4.2.5.2 Oxygen-Conducting Membranes ........................................................................................................... 73 4.2.6 Facilitated Transport............................................................................................................................................. 74 Membrane Materials Used for Gas Separation ................................................................................................................ 74 4.3.1 Polymeric Membranes.......................................................................................................................................... 74 4.3.1.1 Polymers Receiving Special Interest...................................................................................................... 78 4.3.1.2 Fixed-Site Carrier Polymers................................................................................................................... 79 4.3.2 Carbon Molecular Sieving Membranes ............................................................................................................... 80 4.3.2.1 Separation Properties for CMS Membranes .......................................................................................... 81 4.3.2.2 Regeneration .......................................................................................................................................... 83 4.3.3 Mixed Matrix Membranes=Nanocomposites ....................................................................................................... 83 4.3.3.1 MMM: Polymer with Carbon Molecular Sieves ................................................................................... 84 4.3.3.2 MMM: Polymer with Nonporous Nanoparticles ................................................................................... 84 4.3.4 Inorganic Membranes........................................................................................................................................... 84 4.3.4.1 Modified Inorganic Membranes............................................................................................................. 85 4.3.5 Ion-Conducting Membranes................................................................................................................................. 86 4.3.5.1 Proton-Conducting Pd Membranes........................................................................................................ 86 4.3.5.2 Proton-Conducting Polymeric Membranes............................................................................................ 87 4.3.5.3 Oxygen-Conductive Membranes ........................................................................................................... 87 Module Design ................................................................................................................................................................. 88 4.4.1 Flat Sheet Plate and Frame=Envelope Type ........................................................................................................ 89 4.4.2 Spiral-Wound Membrane ..................................................................................................................................... 89 4.4.3 Hollow Fiber Membranes .................................................................................................................................... 90 4.4.4 Membrane Contactors .......................................................................................................................................... 90 4.4.5 System Design...................................................................................................................................................... 91 Current Applications and Novel Developments .............................................................................................................. 91 4.5.1 Hydrogen Recovery ............................................................................................................................................. 92 4.5.1.1 Novel Applications for Hydrogen: Fuel Cells....................................................................................... 93 4.5.2 CO2 Removal ....................................................................................................................................................... 94 4.5.2.1 CO2 Removal from Natural Gas............................................................................................................ 94 4.5.2.2 CO2 Removal from Biogas .................................................................................................................... 95 4.5.2.3 Novel Applications for CO2 Capture..................................................................................................... 96 4.5.3 Air Separation ...................................................................................................................................................... 98 4.5.3.1 Production of High-Purity Nitrogen ...................................................................................................... 98 4.5.3.2 Oxygen-Enriched Air ............................................................................................................................. 98 4.5.3.3 Novel Developments for High-Purity Oxygen ...................................................................................... 99 4.5.4 Recovery of Volatile Organic Compounds........................................................................................................ 100

65


66



4.5.5 4.5.6

Separation of Hydrocarbons—Novel Membrane Developments ...................................................................... 100 Other Applications ............................................................................................................................................. 101 4.5.6.1 Water Vapor Removal from Air .......................................................................................................... 101 4.5.6.2 Dehydration of Natural Gas................................................................................................................. 101 4.5.6.3 Helium Recovery ................................................................................................................................. 101 4.5.6.4 Recovery of Aggressive Gases: Cl2 and HCl...................................................................................... 101 4.6 Summary ........................................................................................................................................................................ 101 Acknowledgments.................................................................................................................................................................... 101 References ................................................................................................................................................................................ 102

4.1

INTRODUCTION

The application of membranes for gas separation is a fairly young technology compared to the use of membranes for liquid separation. Although the basic theoretical principles were partly understood and date back to the early nineteenth and twentieth century with Fick’s law (1855), osmotic pressure (Van t’Hoff, 1887 and Einstein 1905), and membrane equilibrium (Donnan 1911), it was not until around 1950 that theories for gas transport through a membrane were presented and later further developed (pore model by Schmid in 1950 and Meares in 1956, solution-diffusion model by Lonsdale in 1965) [1]. The breakthrough for industrial membrane applications came with the development of the asymmetric membranes achieved by Loeb and Sourirajan around 1960 [2]. These membranes were developed for reverse osmosis and consisted of a very thin dense top layer (thickness 20 Å [28]. The classical Knudsen equation for diffusion of gas is DKn ¼

dp dp vA ¼ 3 3

rffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffi 8RT T ¼ 48:5 dp pMA MA

(4:10)

where dp ¼ average pore diameter (m) vA ¼ average molecular velocity (m=s) MA ¼ molecular weight of gas component A (g=mol) T ¼ temperature (K) Hence for Knudsen diffusion, the square root of the inverse ratio of the molecular weights will give the separation factor. However, recent findings of Gilron and Soffer [29] indicate that the Knudsen mechanism can be significant for pore sizes as small as dp ~ 5Å. The Knudsen flow in this region takes on a slightly different form as indicated in the following expression derived as transport through a series of constrictions using resistance in series model: Dact, Kn ¼ gd dp

rffiffiffiffiffiffiffiffiffiffi 8RT DE exp pMA RT

(4:11)

Here gd is the probability that a molecule can make a jump in the right direction given the jump length is dp and the velocity is yA.

4.2.3 SELECTIVE SURFACE FLOW Selective surface flow is, as Knudsen diffusion, associated with transport through microporous membranes, usually inorganic materials. The mechanism of surface diffusion is disputed and several different approaches have been proposed in the literature.



72


Theories ranging from viewing the low surface coverage adsorbed gas as a 2D gas through a hopping model and into a more liquid like sliding layer exist. The mechanism dominating the surface diffusion coefficient will be influenced by a number of factors such as homogeneity of the surface, the temperature versus the adsorption enthalpy, and the surface concentration, cs [30]. All three regimes can be described by a 2D analogue of Fick’s law (Equation 4.1, given for a single component, a). The flux, Ja, is now evaluated as molecules crossing a hypothetical line in the surface perpendicular to the direction x. Ds is the surface diffusion coefficient and dcs=dx is the surface concentration gradient in the x-direction. The following expression may be used to determine if the surface transport is dominated by the 2D-gas model [30]: q=RT < 1=a

(4:12)

where q is the adsorption enthalpy (J=mol) a is an energy fraction factor The energy barrier for surface migration, E, is then defined as E ¼ aq

(4:13)

The 2D-gas is characterized by a surface mean free path, ls, inversely proportional to the surface concentration, cs, and this ls value can be much larger than the spacing between adjacent surface sites. If the q=RT part of Equation 4.12 is increased, then ls will no longer be controlled by collisions between adsorbed molecules. As q=RT increases, ls decreases and is approaching the spacing between adjacent sites, and a hopping mechanism is observed. If the cs is low then a random walk diffusion of independent molecules can be expected, and Ds would be given as Ds ¼ 14nl2s

(4:14)

where n is the jump frequency factor, a factor which has a temperature dependence according to Arrhenius’ law, n ¼ n0exp(aq=RT) [1=s]. When cs is increased, the chance of a molecule hitting another molecule increases and this interaction will bear some similarity to diffusion in liquid. Thus, the region of the sliding layer prevails. A more comprehensive discussion on this theory may also be found in Ref. [31]. Selective surface diffusion is governed by a selective adsorption of the larger (nonideal) components on the pore surface. The critical temperature, Tc, of a gas will thus indicate which component in a mixture is more easily condensable. The gas with the highest Tc will most likely be the fastest permeating component where a selective surface flow can take place. For a mixed gas an additional increase in selectivity may be achieved if the adsorbed layer now covering the internal pore walls restricts the free pore entrance so that the smaller nonadsorbed molecules cannot pass through.

4.2.4

MOLECULAR SIEVING

Molecular sieving is the dominating transport mechanism when the pore size is comparable to the molecular dimensions, 3–5 Å; hence the smallest molecule will permeate, and the larger will be retained. The dimensions of a molecule are usually described with either the Lennard-Jones radii or the Van der Waal radii. For separation by molecular sieving, this is not a satisfactory way of stating the molecular size; a shape factor should also be included [9]. The sorption selectivity has little influence on the separation when molecular sieving is considered. An Arrhenius type of equation is still valid for the activated transport, but attention should be drawn to the pre-exponential term, D0 (see Equation 4.7). From transition state theory this factor may be expressed as shown in Equation 4.15 [32]: kT Sa,d exp D0 ¼ el R h 2

where k and h are Boltzmann’s and Planck’s constants, respectively Sa,d is the activation entropy for diffusion e ¼ gddp in Equation 4.11

(4:15)



73

Membranes in Gas Separation

A change in entropy will thus have a significant effect on the selectivity when molecular sieving is considered. This is thoroughly discussed by Singh and Koros [9]. The flux may be described as in Equation 4.16 where Ea,MS is the activation energy for diffusion in the molecular sieving media. Ja ¼

Dp Ea,MS D0 exp RT RTl

(4:16)

The selectivity for separation will normally decrease with increasing temperature because of increased diffusion rate for permeating components, and the sorption will be of minor importance.

4.2.5 ION-CONDUCTIVE TRANSPORT There are two important types of ion-conducting membranes for gas separation: (1) the proton (Hþ)-conducting palladium membranes which are of great interest for combination with fuel cells and (2) the oxygen ion (O2)-conducting inorganic membranes, usually perovskite-type of oxides. Both are suitable for high temperature=high pressure applications, and an interesting feature is the 100% selectivity in favor of H2 and O2, respectively. 4.2.5.1

Proton-Conducting Membranes

Palladium and its alloys are recognized as very efficient proton-conducting membranes, which may be used for hydrogen separation and membrane reactor (MR) applications. The alloys are less apt to hydrogen embrittlement than pure Pd, and alloys with silver or copper represent the least expensive alternative of the alloys. These alloys also seem to produce membranes with enhanced chemical resistance (for instance toward H2S), additionally Pd–Ag alloys have a relatively higher H2 permeability than pure Pd. Efforts to produce economically viable Pd membranes have focused on preparing supported composite membranes with a thin dense Pd or Pd alloy layer. Forming this thin layer from two or more metals is quite challenging. The advantages of palladium membranes are especially the ability to separate out high-purity H2, and that it may be used at high temperatures (3008C and above). Hydrogen is present in many gas streams, being a product from dehydrogenation of hydrocarbons, a component in syngas or byproduct in bioprocesses. The basic flux equation for hydrogen, JH2 (mol=m2 s), is given in the following equation [33]. The flux for hydrogen atoms will be twice that of JH2: JH 2

DM KS ( pnH2 ,ret pnH2 ,perm ) ¼ 2 l

(4:17)

where DM is the diffusion coefficient of a hydrogen atom in the metal (m2=s) KS is the Sievert constant (mol=(m3 Pa0.5)) For bulk transport of hydrogen, n ¼ 0.5 but approaches 1 for transport limited by surface kinetics. The exponent of 0.5 reflects the dissociation of the gaseous hydrogen molecule into two hydrogen atoms (protons) diffusing into the metal where an ideal solution of hydrogen in palladium is formed, then association again as H2 on the other side of the membrane. The hydrogen permeability of the palladium, here denoted as k, corresponds to the constants in Equation 4.17, expressed as in the following equation: k ¼ 12 DM KS (mol=m s Pa0:5 )

(4:18)

Among the proton-conducting membranes Nafion or Nafion-like sulfonated perfluorinated polymers should also be mentioned. These materials are used for polymer electrolyte membrane (PEM) fuel cells, and in addition to being chemically very stable, they exhibit high proton conductivity at temperatures lower than 1008C. It is believed that permeability and thermal stability may be increased if tailor-made lamellar nanoparticles are added to a proton conducting polymer. 4.2.5.2

Oxygen-Conducting Membranes

The zirconia and perovskite membranes may be considered as solid electrolyte membranes containing an oxygen ion conductor (various oxides). Depending on the type of materials used, the oxygen separation may take place according to direct excitation of the oxygen at several hundred degrees (gas separation controlled by electric current) or a mixed conductor method where the gas separation is proportional to log( p1=p2), the ratio of the partial pressures. (The reaction taking place at the electrodes is 2 1 2 O2 þ 2e $ O , at the positive electrode the reaction is shifted to the left, at the negative electrode it is shifted to the right.) Detailed equations for transport can be found elsewhere [34,35].



74

4.2.6


FACILITATED TRANSPORT

Facilitated transport indicates that a carrier is introduced into the membrane matrix, usually a polymer matrix. This carrier will be selective for a certain gas component and enhance the transport of this component through the membrane. The use of facilitated transport membranes for gas separation was first introduced by Ward and Robb [36] by impregnating the pores of a microporous support with a carrier solution, and a separation factor of 1500 was reported for CO2=O2. These membranes or supported liquid membranes (SLM) are discussed by several investigators, and initially very good separation properties are observed [37–40]. They are however known to have serious degradation problems like loss of carrier solution due to evaporation or entrainment with the gas stream, and the complexing agent (carrier) can be deactivated. These problems have restricted further development of SLMs. The use of ion exchange membranes as supports was proposed as an approach to overcome the problems of SLMs, and the application of ion exchange membranes for the facilitated transport of CO2 and C2H4 was first reported by LeBlanc et al. [41]. Since then a number of papers have been published on this type of membrane. Along with the use of ion exchange membranes as supports, another approach to overcome the above mentioned limitations was developed by introducing carriers directly into solid polymer membranes as illustrated in Refs. [42,43]. The FSC membranes have carriers covalently bonded to the polymer backbone, hence the carriers have restricted mobility, but are favorable when stability is considered. It is obvious that the diffusivity (and thus permeability) in an FSC membrane is lower than that of a mobile carrier membrane. The diffusivity of a swollen FSC-membrane should however show diffusivities between that of a mobile and a fixed carrier. Various ways of enhanced carrier transport have later been suggested [15,44,45]. It is suggested by many that CO2 will be transported as carbonate or bicarbonate anions in anion-exchange membranes and as anions of various amines in cation-exchange membranes. The characteristic of a facilitated or carrier-mediated transport is the occurrence of a reversible chemical reaction or complexation process in combination with a diffusion process. This implies that either the diffusion or the reaction is rate limiting: The total flux of a permeant A will thus be the sum of both the Fickian diffusion and the carrier-mediated diffusion as illustrated in Equation 4.19 [46]: JA ¼

DA DAC ðcA,0 cA,l Þ þ ðcAC,0 cAC,l Þ l l

(4:19)

where the first term on the right hand side of the Equation 4.19 is the Fickian diffusion (DA), and the second term represents the carrier-mediated diffusion (DAC). l is the thickness of the membrane, while c (as defined by Henry’s law, Equation 4.6) is the concentration of the component A and its complex AC at the interfaces of the membrane, and 0 and l indicate feed side and permeate side, respectively. The concentration difference of the complex AC in Equation 4.19 must be further expressed by an equilibrium constant of the complexing reaction and a distribution coefficient. This is given in detail by Cussler [46]. Nonpolar gases in a gas mixture will exclusively be transported through the membrane by Fickian diffusion, while by using partial pressures ( pA) instead of concentration cA (inserting Equation 4.6 into Equation 4.19), it can easily be seen that the driving force through the membrane will be the difference in partial pressures for the Fickian diffusion, and that transport also will depend on the solubility coefficient, SA, for the gas in the polymer. For carrier-mediated transport (second term in Equation 4.19), the driving force will be the concentration difference of the complex AC through the membrane. The permeation of the nonpolar gases may additionally be hindered by polar sites introduced into the membrane matrix [15,44,45]. This should then lead to an increased permeance of the carrier-transported gas compared to ideal gases in the mixture (like CH4, N2, and O2), giving high selectivities in favor of the complexed gas (like for instance CO2). A proposed mechanism for facilitated transport for CO2 in an FSC membrane is illustrated in Figure 4.4 [15].

4.3

MEMBRANE MATERIALS USED FOR GAS SEPARATION

To select the right membrane for a given gas separation is very challenging as the criteria are quite complex. The first choice is usually based on favorable flux and selectivity for a given gas mixture. Membrane performance will however have to be evaluated with respect to operating conditions as well as mechanical strength and durability. Finally, separation efficiency will have to be balanced against cost for each case evaluated. Choice of ‘‘the right membrane’’ may therefore have more than one answer. In this section properties for various materials available for gas separation membranes are discussed.

4.3.1

POLYMERIC MEMBRANES

Polymeric materials are still the most widely used membranes for gas separation, and for specific applications the separation technology is well established (see Section 4.6). Producing the membranes either as composites with a selective skin layer on flat sheets or as asymmetric hollow fibers are well-known techniques. Figure 4.5 shows an SEM picture of a typical composite polymeric membrane with a selective, thin skin layer of poly(dimethyl)siloxane (PDMS) on a support structure of polypropylene (PP). The polymeric membrane development today is clearly into more carefully tailored membranes for specific



75


Carriers fixed on polymer backbone

Feed side CO2 CH4

CO2

NH2

Only by diffusion

H2O

Reversible reaction

Polymer layer coated on porous support +

NH3

H2O

Porous support

−

HCO3

CO2

CH4 CO2 Permeate side

FIGURE 4.4 A proposed mechanism of facilitated transport of CO2 in an FSC membrane. (From Kim T.J., Hägg M.B., J. Pol. Sci. Part B Polym. Phys., 42, 4326, 2004. With permission.)

applications. The important material properties defining the separation performance are molecular structure, glass transition temperature (Tg), crystallinity, degree of crosslinking, and as a function of these variables, durability with respect to possible degradation or loss of performance. The basic transport mechanism through a polymeric membrane is the solution diffusion as explained in Section 4.2.1. As noted, there is a fundamental difference in the sorption process of a rubbery polymer and a glassy polymer. Whereas sorption in a rubbery polymer follows Henry’s law and is similar to penetrant sorption in low molecular weight liquids, the sorption in glassy polymers may be described by complex sorption isotherms related to unrelaxed volume locked into these materials when they are quenched below the glass transition temperature, Tg. The various sorption isotherms are illustrated in Figure 4.6 [47]. The solubility in glassy polymers is usually described by the so-called dual-mode model, which implies that there is a need for a more detailed definition of the sorption, c, in the flux Equation 4.1. Equations 4.20 and 4.21 illustrate this and can relate to

FIGURE 4.5 SEM-picture of a typical composite membrane comprising of support structure of PP and a selective skin layer of PDMS.



76

c, Concentration in polymer (cc (STP)/cc polymer)

Handbook of Membrane Separations Flory–Huggins BET III (b)

Henry’s law (a)

Dual mode (c)

cD kD

=

cH

+ cH¢

p, Partial pressure of penetrant (atm)

FIGURE 4.6 Typical gas sorption isotherms for polymers: (a) Henry’s law illustrating ideal sorption as in a rubbery polymer where solubility is independent of concentration, (b) illustrating a highly nonlinear behavior according to Flory-Huggins; as can be expected for interactions between organic liquids or liquids with polymers (swelling results), and (c) illustrating the dual mode sorption (Langmuir) typical for a glassy polymer. (From Koros W.J., Chern R.T., Separation of gaseous mixtures using polymer membranes. In: Rousseau R.W., ed. Handbook of Separation Process Technology. John Wiley & Sons, New York, 862, 1987. With permission.)

Figure 4.6. The term cD accounts for Henry’s law, while cH is the Langmuir term with b being the hole affinity constant (bar1) and c0H b is the saturation constant (cm3 (STP)=cm3): (4:20) c ¼ c D þ cH c ¼ kD p þ

c0H bp 1 þ bp

(4:21)

The dual-mode model has been extensively covered by several authors [25,47–49]. Figure 4.7 illustrates how the available free volume for transport increases with increasing temperature (Vf ¼ VT Vl), and the remarkable change when passing the Tg of a polymer [50]. According to the free volume diffusion model, the diffusion of molecules depends on the available free volume as well as sufficient energy to overcome polymer–polymer attractive forces. The specific volume at a particular temperature can be obtained from the polymer density, whereas the volume occupied at 0 K can be estimated from group contributions. Details on this theory may be found in relevant handbooks, textbooks, and numerous publications [25,48–52]. Nonideal gases dissolve more easily in polymers, and hence the separation factor may easily be in favor of a larger, nonideal gas component, compared to a small ideal gas. At the same time the nonideal component may swell the membrane, hence the net result is a decrease in selectivity. For a polymeric membrane, flux and selectivity are inversely related, hence a high flux usually means low selectivity. Elastomers have higher flux and lower selectivity for a given gas pair than a glassy material. This problem can be addressed by various methods: controlled crosslinking, opening the matrix by inserting carefully designed side groups to the main polymer chain, or functionalizing the polymer. Tables 4.2 and 4.3 illustrate how separation properties may be changed within two families of polymers by the change of side groups: polycarbonates [25,53] and polyimides [54]. Glassy polymers may swell in the presence of plasticizing agents, hence selectivities will be reduced and flux will be increased—the membrane loses performance. One way of avoiding this problem may be to incorporate cross-linkable functional groups in the polymer backbone. The crosslinking will be performed in the posttreatment process, and will hinder uncontrolled swelling. Regions of crystallinity and chemical crosslinking have somewhat similar effects on the transport properties of a rubber by causing restrictions of swelling and suppression of long-chain segmental motion. Transport in semicrystalline polymers is more complex due to tortuosity caused by the presence of the typically impermeable crystalline regions. Studies of gas sorption and transport strongly support the notion of the impenetrability of crystalline domains by even tiny gas molecules. The sorption coefficient seems to be essentially proportional to the volume fraction of amorphous material, while the effect on diffusion is more complex [25].



77

Membranes in Gas Separation Glass

V (cc/gm)

Vg

Rubber

Actual glassy specific volume

Vl

Equilibrium volume of densified glass Tg

Temperature

FIGURE 4.7 Polymeric specific volume as a function of temperature. (From Chern R.T., Koros W.J., Sanders E.S., Chen S.H., Hopfenberg H.B. In: Whyte T., Yon C.M., Wagener E.H., eds. ACS Symposium Series 223 on Industrial Gas Separations. American Chemical Society, Washington DC, 47, 1983. With permission.)

TABLE 4.2 Structures of Characterized Families of Polycarbonates and Polyimides Polycarbonates

Polyimides

CH3 PC

O

O O C

C

PMDA-ODA

TMPC

O

CH3

H3C

CH3

O

TMHFPC

O

CF3

Br O

O

O

O

O C

C

C

Br Br

PMDA-IPDA CH3

O

O

6FDA-MDA

O

O

O

C

C

N

O

O

O

O

O

O

C

C

C

C

C

H

CH3 N

C

C O O

C

C

O

C

C

O C

O

C

6FDA-IPDA

O

F3C

O

6FDA-DAF

CF3

O

CH3

C

C F3C

CF3

C H

N C

N

N

C

C

N

CH3

H N

O

O

PMDA-DAF

CF3

C

O

CH3

C CH3

O

O O

O

C

C

N

N C O

C

C F3C

CF3

O

Br Br O

C Br

C

F3C

O

C

C

C

N

O C

CF3 TBHFPC

H N

N

O C

C

N O

Br

CH3

N

O

CH3 TBPC

6FDA-ODA

O

CH3

C

H3C

O

O

CF3

CF3

C

N

C

PMDA-MDA

C

H3C

O

C

C

CF3 HFPC

O

C

C CH3

C

O

C N

CH3 H3C

O

CF3

O C Br

Source: From Koros W.J., Hellums M.W., Transport properties. In: Kroschwitz, J.I., ed. Encyclopedia of Polymer Science. 2nd ed. Wiley-Interscience Publishers, New York, 1989: Supplement vol. 724–802; Koros W.J., Hellums M.W., Fluid Phase Equilibria, 53, 339, 1989; Kim T.H., Koros W.J., Husk G.R., O’Brien K.C., J. Membr. Sci., 37, 45, 1988. Note: See Table 4.3 for properties.



78


TABLE 4.3 Permeabilities and Selectivities of Polycarbonates and Polyimides Permeabilities at 358C (Barrer) Polymer

PC TMPC HFPC TMHFPC TBPC TBHFPC TB=TBHF-co-PC

He 10 atm

PMDA-ODA PMDA-MDA PMDA-IPDA PMDA-DAF 6FDA-ODA 6FDA-MDA 6FDA-IPDA 6FDA-DAF

CO2 10 atm

O2 2 atm

13 46 60 206 18 100 49

He=CH4 10 atm

Polycarbonates 6.8 18.6 24 111 4.2 32 16

1.6 5.6 6.9 32 1.4 9.7 4.9

8.0 9.4 37.1 1.9 51.5 50 71.2 98.5

Ideal Selectivities at 358C

Polyimides 2.71 4.03 26.8 0.15 23 19.3 30 32.2

0.61 0.98 7.1 — 4.34 4.6 7.53 7.85

O2=N2 2 atm

CO2=CH4 10 atm

35 50 57 44 140 112 110

4.8 5.1 4.1 4.1 7.5 5.4 6.2

19 21 23 24 34 36 34

134.9 94 41.1 921 135.4 117.1 102.1 156.3

6.1 4.9 4.7 — 5.2 5.7 5.6 6.2

45.9 42.9 29.7 71.6 60.5 44.9 42.9 51.1

Source: From Table of units, J. Membr. Sci., 2, 237, 2004; Koros W.J., Hellums M.W., Transport properties. In: Kroschwitz, J.I., ed. Encyclopedia of Polymer Science. 2nd ed. Wiley-Interscience Publishers, New York, Supplement vol. 1989, 724–802; Koros W.J., Hellums M.W., Fluid Phase Equilibria, 53, 339, 1989; Kim T.H., Koros W.J., Husk G.R., O’Brien K.C., J. Membr. Sci., 37, 45, 1988. Note: 1 Barrer ¼ 7.52 1015 m3 (STP)=mm2 s kPa.

4.3.1.1

Polymers Receiving Special Interest

The block copolymers form an interesting group of materials with promising separation properties for selected gas mixtures. These membranes usually combine the flexible phase of an elastomer with a dispersed phase of a glassy or crystalline polymer. The hard domains will act as physical crosslinks, and the temperature should not be raised above the Tg of the glassy polymer. Morphology and properties of the block copolymers are mainly determined by the A=B ratio and the size of the blocks. The separation mechanism in these membranes is typically based on sorption diffusion, but if correctly tailored, the sorption selectivity will be governing the separation. Examples that can be mentioned are copolymers of ethylene and vinyl alcohol. These two components are able to co-crystallize in the same crystal lattice, and the material can be tailored by varying the amount of the highly polar, diffusion-inhibiting vinyl alcohol component without strongly affecting the crystallinity [25]. Another example reported is block copolymers composed of polyamide (PA) and polyethylene glycol (PEG). In this material, the semicrystalline PA blocks will ensure the structural integrity while high molecular weight PEG will control the separation [55]. The permselectivity will typically be reverse, and these membranes may have a great potential for CO2 capture or VOC removal from gas streams. Interesting results for CO2–H2 separation (reverse selective) have been published by several authors [56,57]. The perfluorinated polymers are materials of special interest due to their exceptionally good chemical and thermal stability compared to other polymers. The challenge has however for many years been to prepare these materials with suitable gas separation properties. They used to be either very crystalline or too porous, hence selectivities were low. They may however be prepared as high flux—low selectivity membranes, which is acceptable for certain applications. Materials prepared from tetrafluoroethylene (TFE) OCF3 FC O

O CF2

FIGURE 4.8

Structure of Hyflon AD.

CF2

C

CF2

n



79

Membranes in Gas Separation O N

O

O N

O

O

n

FIGURE 4.9 Example of structure of a cardopolymer based on polyimide.

and cyclic dioxole (TTD) are highly hydrophobic and have good potential for use in gas–liquid membrane contactors. The copolymer known commercially as Hyflon AD is made from TFE and TTD, and is an amorphous perfluoropolymer with glass transition temperature (Tg) higher than room temperature. Hyflon AD 60 shows values of permeability and selectivity for gases that make the material interesting for separation—this was documented by Arcella et al. [58]. Kharitonov et al. [59] have shown that direct fluorination of the polyimide Matrimid is possible, hence the resulting membrane should have a nice potential for use in harsh environment. Perfluorinated materials were also studied by Hägg [60] for chlorine gas purification, and were shown to be exceptionally stable in these harsh environments. The selectivity was however too low. In a later publication on chlorine purification [31] it was suggested to use perfluorinated monomers as surface-modifying compounds for pore tailoring of glass membranes for chlorine gas separation. Hydrophilic perfluoromembranes may be prepared from TFE and copolymerized with perfluorosulfonylfluoridevinylether (SFVE), making a so-called Hyflon ion polymer, which is a rubbery polymer at room temperature. This polymer contains the group –SO2F where F can be exchanged for a metal or hydrogen atom. This makes the material suitable for a wide variety of fields ranging from electrochemical electrolyzers (chloralkali and HCl), proton exchange fuel cells, energy storage, and electrodialysis to membrane catalytic reactors and many more applications [58]. The cardopolymers are polymers containing very bulky aromatic structure in the main chain. This structure can be coupled to a polyimide, polyamide, or polysulfone. Example of a cardopolymer based on polyimide is shown in Figure 4.9. The polymer may be further modified by substituting methyl groups or halogens into the aromatic rings. The bulky structure gives the polymers high gas permeability and high solubility for nonideal gases (like CO2 and hydrocarbons); they can be easily processed and are fairly heat stable. These materials have been extensively studied at Research Institute of Innovative Technology for the Earth (RITE) in Japan, and papers have been published documenting nice separation properties for CO2=N2 (>35) [61,62]. This makes the cardopolymers, especially the polyimide based, interesting for recovery of CO2 in flue gas. Takeuchi et al. [63] evaluated the costs of a global process of CO2 fixation and utilization using catalytic hydrogenation reaction, and converting CO2 to methanol. The recovered CO2 was then based on a membrane process using a cardopolymer. The acetylene-based polymers have received new interest. These glassy polymers are amorphous and characterized by very high glass transition temperature (typically >2008C), high free volume, and very high gas permeabilities. The well-known poly(1trimethylsilyl-1-propyne) (PTMSP) has the highest gas permeability of all known polymers. This polymer and other acetylene-based polymers show higher permeabilities to large condensable organic vapors than to small permanent gases [64], and PTMSP has the highest C2þ=CH4 and C2þ=H2 selectivities of any known polymer [65]. The selectivity of these gases is typically reverse compared to what is expected in a polymeric membrane. This can be understood by the large free volume in these polymers, and the high solubility of the hydrocarbons in the material. The transport may be described in the same way as the selective surface flow through a microporous membrane or a mixed matrix material (see Sections 4.3.2 and 4.3.3). Pinnau et al. [66], among others, have investigated the effects of the side-chain structure of substituted polyacetylenes on their gas permeation properties (see Figure 4.10). 4.3.1.2

Fixed-Site Carrier Polymers

As an alternative to conventional polymeric membranes, facilitated transport membranes have attracted attention due to the potential of achieving both high permeabilities and high selectivities. Facilitated transport membranes may for instance selectively permeate CO2 by means of a reversible reaction of CO2 with incorporated complexing agent (carrier) in the membrane, whereas gases such as H2, N2, and CH4 will permeate exclusively by the solution diffusion mechanism. As pointed out in Section 4.2.6, the ion exchange membranes were introduced as an approach to overcome the problems of SLMs, and the application of ion exchange membranes for the facilitated transport of CO2 and C2H4 was first reported by LeBlanc et al. [41]. Since then a number of papers have been published on this type of membrane [67–70]. Along with the use of ion exchange membranes as supports, yet another approach to overcome the above limitations was developed by introducing carriers directly into polymer membranes [42,71,72]. These FSC membranes have carriers covalently bonded to the polymer backbone, hence the carriers have restricted mobility, but are favorable when stability is considered. It is obvious that the diffusivity (and thus



80

Handbook of Membrane Separations 100

n -C4H10 / CH4 Mixed gas selectivity

PTMSP

PMP PDMS 10

PTPSDPA

Ppt BDPA PTMSDPA

P5M2H P6M2H 1 100

1,000

10,000

100,000

n-Butane permeability (Barrer)

FIGURE 4.10 Relationship between mixed gas n-butane permeability and n-butane=methane selectivity for a series of glassy polyacetylenes and rubbery PDMS. Feed pressure: 10 bar; permeate pressure: 1 bar; temperature: 258C. (From Pinnau I., He Z., Morisato A., J. Membr. Sci., 241, 363, 2004. With permission.)

permeability) in an FSC membrane is lower than that of a mobile carrier membrane. The diffusivity of a swollen FSC membrane should however show diffusivities between that of a mobile and a fixed carrier. The aminated polymeric membranes for facilitated transport of CO2 have been investigated extensively in recent years [69,73–75,15]. The findings of several of these investigators were that the aminated polymeric membranes showed higher permselectivity in water swollen condition than in dry condition—an ideal selectivity as high as 1000 for CO2=CH4 was documented by Kim et al. [15]. Although promising, a mixed gas selectivity is, however, expected to show a significantly lower value. The anticipated mechanism for transport through the membrane is described in Section 4.2.6 and is illustrated in Figure 4.4. There has been a major increase in published papers and patents within the field of facilitated transport membranes during the period between writing and publishing of the current chapter; the author apologize for not having been able to incorporate these references. The biomimetic membranes represent a special group of carrier membranes. They are artificial membranes based on biomembrane mimicking, i.e., imitation of the essential features biomembranes use for separation. Nitrocellulose filters impregnated with fatty acids, their esters, and other lipid-like substances may be used—in other words, an imitation of many nonspecific barrier properties of biomembranes. The transport of gas through these membranes will essentially be according to facilitated transport (see Section 4.2). Biomimetic membranes for CO2 capture will transport the gas as HCO 3 . Development of these materials may be expected for selected applications.

4.3.2

CARBON MOLECULAR SIEVING MEMBRANES

The carbon molecular sieving (CMS) membranes are microporous carbon fibers or flat sheets prepared from carbonization of polymeric precursors under controlled conditions. Depending on the membrane pore size and the process conditions, the separation may take place according to (1) molecular sieving (dp < 5 Å), (2) selective surface flow (5 Å < dp < 12 Å), (3) Knudsen diffusion (dp > 20 Å), or combinations of these [6,29] (see Sections 4.2.2 through 4.2.4). The membranes for gas separation are prepared as hollow fibers or flat sheets. The hollow fibers may have the largest potential for becoming a successful separation unit on an industrial scale due to the possibility of making modules with a high packing density (m2=m3). The production process of these membrane modules is however challenging and expensive, and costs need to be brought down to be interesting for larger gas volume applications. The use of cheap polymeric precursors is favorable. Properties that should place the carbon membranes among the most promising membrane materials are their high temperature resistance and excellent chemical resistance to acids, hot organic solvents, and alkaline baths. The carbon membranes are fairly easy to produce as much is known about how carbonization conditions affect separation properties [6–11]. A carbon membrane can thus be tailored with a pore size giving excellent separation properties for a given gas mixture (high flux for permeating component and high selectivity for gas pairs). The most serious disadvantages that have to be overcome or controlled are brittleness and the vulnerability of membranes to oxidizing agents and water vapor resulting in performance loss over time, hence regeneration is needed at intervals. When these effects are detected in the membrane performance, it is important to know how to address the problem (or preferably how to avoid it). It is important to know how to ‘‘open’’ pores which are blocked, how to regenerate the membrane, or how to optimize



81


separation performance by controlling the process variables. If basic knowledge about these membranes is not known to a user, he or she may very quickly draw the conclusion that CMS membranes are unstable, have too large a decrease in performance over time, and will probably judge them as ‘‘not suitable.’’ This may be very wrong. When used correctly, CMS membranes are promising candidates for gas separation and moderate volume gas streams. The CMS membranes may be prepared in two different ways, and in both cases the pore tailoring is the focus for the final membrane: 1. By careful control of carbonization conditions; this is done by controlling heating rate, heating temperature, and choice of inert gas or vacuum during the process [76]. 2. As explained in the patent of Soffer et al. [10], where microporous cellulose fibers are treated with CVD, and pores are tailored by postoxidation. CMS membranes may also easily be functionalized, i.e., metals (like AgNO3, MgO, Fe2O3 or others) are imbedded in the structure of the precursor, and will enhance the separation for certain gas pairs. A comprehensive review on carbon membranes has been given by Ismail and David [77]. 4.3.2.1

Separation Properties for CMS Membranes

The ability of a microporous carbon fiber to separate gases depends on the pore size of the membrane, the physiochemical properties of the gases, and surface properties of the membrane pore. The pore size of a carbon fiber for gas separation is usually within the range of 3.5–10 Å, depending on the conditions for preparation of the membrane during carbonization or treatment afterward (postoxidation or chemical vapor deposition) [6–11]. With reference to the typical range indicated above for the transport mechanisms, one would expect that the dominating mechanisms will be either molecular sieving or selective surface flow. Figure 4.11 illustrates a carbon membrane with pores in the range suitable for molecular sieving [78]. As expected, there is a clear and indisputable correlation between flux and molecular size. In Figure 4.12, the carbon membrane is more open (pore size in the range 6–10 Å). The gas pair reported is CO2 and CH4, and as can be seen, the selectivity is clearly in favor of CO2 indicating selective surface flow. The critical temperatures, Tc, and Lennard-Jones diameters, dL-J, for the two gases are

CO2 : dL-J ¼ 3:94 A, Tc ¼ 304 K

CH4 : dL-J ¼ 3:74 A, Tc ¼ 190:4 K Single gases at 30°C and 2 bara

100

H2

Permeance (m3 (STP)/(h m2 bar))

10−1

He CO2

10−2 Ar

10−3

N2

10−4

CH4

10−5

10−6 10−7 2.4

Xe

2.6

2.8

3

3.2

3.4

3.6

3.8

4

4.2

Lennard-Jones equilibrium kinetic diameter (Å)

FIGURE 4.11 Permeance as a function of L–J diameter for a sieving carbon membrane. (From Lie J.A., Synthesis, performance and regeneration of carbon membranes, Doctoral thesis NTNU 2005: 152, Trondheim, Norway, 2005. With permission.) Note: 1 [Å] ¼ Ångstrom ¼ 1010 m.



82

Handbook of Membrane Separations Single-gas experiments with CMS module 500 1 Selectivity of CO2 over CH4 [−]

450 400 350 5 300 8

2 250

7

200

4 3

150 100 0

9 1

2

Pressure (bara)

6 3

4

5

6

80

70

60

50

40

30

20

10

0

Temperature (°C)

FIGURE 4.12 Selectivity of CO2=CH4 as a function of pressure and temperature for a CMS membrane. (From Lie J.A., Hägg M.B., Characterization of carbon membranes. Poster at The European Membrane Society’s 20th Summer School, Trondheim, Norway, 2003. With permission.)

The easily condensable CO2 molecule will follow an SSF mechanism and seriously hinder CH4 to permeate, hence high selectivities are obtained. Figure 4.13 illustrates the importance of finding the optimum process conditions (temperature, pressure) for the gas mixture to obtain maximum separation effect [79]. The diagram shows how the flux of propane changes with temperature and pressure. At ~3–4 bar and 320 K, the flow changes from SSF to Knudsen diffusion. This ‘‘window’’ will vary for different gases, hence

303 K 308 K 313 K 315 K 318 K 323 K 328 K

Permeance (m3 (STP)/(m2 bar h)) log scale

100

10−3

10−6 300 5

310 4 320 Temperature (K)

3 330

2

Feed pressure (bar)

FIGURE 4.13 Permeance of propane as a function of pressure and temperature through a CMS membrane. (From Lie J.A., Synthesis, performance and regeneration of carbon membranes, Doctoral thesis NTNU 2005: 152, Trondheim, Norway, 2005. With permission.)



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the optimum separation conditions can be found where the nonideal component will permeate according to SSF, and more ideal gases in the mixture, according to Knudsen flow. 4.3.2.2

Regeneration

A CMS membrane will typically have a flux decline over time and regeneration will be necessary at intervals. Oxygen is one of the most detrimental species for a CMS membrane. When carbon materials are exposed to air at room temperature, irreversible chemisorption of oxygen may take place and C–O surface groups are formed [80]. These groups also provide sites of adsorption for H2O. Both phenomena will slightly reduce the effective size of micropores. The chemically bonded oxygen is only completely removed (as CO and CO2) by heating the sample to temperatures as high as 7008C–8008C with an inert gas. Adsorption of water may result in flux decline as well. At low relative humidity, only active polar sites seem to be involved, and this sorption is so weak that the negative effect can easily be managed. With a high relative humidity (>25%), the negative effect may be substantial, caused by hydrogen bonding by neighboring water molecules forming clusters of adsorbed water [81]. It should be noted though that water uptake from various gases with same humidity level may differ greatly, and will therefore also be more or less easily removed. Adsorption of organics may cause the same type of flux decline. To recover a decreased membrane flux, three main approaches are reported: 1. The membrane may be treated at elevated temperatures, at least 2008C, under vacuum or inert atmosphere. If the flux is only partially restored after regeneration, this could be the result of incomplete removal of C–O surface groups. 2. If exposed to organics, treatment of the membrane with propene may be a good solution. Jones and Koros [82] found propene to be very effective in removing sorbed organics. In some cases the flux was completely restored—this was also confirmed by Hägg et al. [83]. 3. The use of electrothermal regeneration (low voltage direct current) has successfully been tested out online [78].

4.3.3 MIXED MATRIX MEMBRANES=NANOCOMPOSITES Mixed matrix membranes (MMM) consist of a continuous polymeric phase wherein nanoporous or dense inorganic materials such as silica, zeolite, carbon particles=nanotubes are dispersed. The separation efficiency can be ‘‘tuned’’ by judiciously selecting the constituent materials (and their intrinsic properties), as well as the composition of the resultant nanocomposite. Molecular separation through the polymer occurs according to solution diffusion and is combined with surface diffusion or molecular sieving through the inorganic phase when a microporous filler is added (see Sections 4.2.1, 4.2.3, and 4.2.4). When dense fillers are added (silica, carbon nanoparticles), the membrane may become reverse selective, but still separate according to solution diffusion (see Section 4.3.3.2). The choice of polymer governs the application temperature range, and fabrication requires excellent adhesion between the filler and the polymer to avoid the formation of voids and, consequently, undesirable Knudsen diffusion [84]. Methods under current investigation to improve such adhesion rely on polymer softening (at temperatures near Tg or through the use of plasticizing agents) or reactive coupling with or without surface treatment of the filler. The field of polymer nanocomposite membranes is developing rapidly due to the wide range of new properties that can be addressed within this unique class of materials. Resulting properties of these hybrid organic=inorganic materials (which differ significantly in property behavior) rely to a large extent on successful blending. Both enhanced thermomechanical properties and separation properties can be expected. The choice of polymers and inorganic fillers must be based on fundamental knowledge about separation properties as well as miscibility of the materials. For illustration of a mixed matrix membrane with microporous filler, see Figure 4.14 [85].

Selective skin region Porous support layer

Polymer matrix Cross section: Asymmetric composite hollow fiber

FIGURE 4.14

Molecular sieving phase: Zeolite or carbon molecular sieves

Illustration of a mixed matrix membrane. (From Koros W.J., Mahajan R., J. Membr. Sci., 175, 181, 2000. With permission.)


84



Robeson [4] showed that there exists a ‘‘trade-off’’ relationship between selectivity and permeability for dense polymer membranes. This plot was later updated by Singh and Koros [9] (see Figure 4.1). Molecular transport of light gases in such membranes typically occurs by a solution diffusion mechanism (as discussed in Section 4.2.1). For a polymer membrane to be commercially considered for the removal of CO2 from H2, CH4, or air, both the CO2 permeability and selectivity must be competitively high. Since the gases in the mixture with CO2 often are smaller (H2) or about the same size as CO2, they may diffuse more rapidly through the polymers, and it follows that the diffusion selectivity (DCO2=DgasB) will be 1. The only way to increase CO2 permeability and selectivity simultaneously is thus to increase the solubility of CO2 in the membrane. Such solubility enhancement is achieved by the introduction of chemical moieties (nanofillers) into the polymer to promote the permeation flux of CO2 by increasing the free volume, or by using physically modified polymer nanocomposites (molecular sieving materials) suitable for adsorption flow. Both result in a mixed matrix membrane. Examples of porous nanofillers are carbon molecular sieves; examples of nonporous nanofillers are fumed silica and carbon black. 4.3.3.1

MMM: Polymer with Carbon Molecular Sieves

Mixed matrix membranes with molecular sieves incorporated combine the high separation capacity of molecular sieving materials (see Section 4.3.2) with the desirable mechanical properties and economical processing attributes of polymeric materials. Vu et al. [86] incorporated CMS materials into polymers to form mixed matrix membrane films for selective gas separations. The CMS, formed by pyrolysis of a polyimide precursor and exhibiting an intrinsic CO2=CH4 selectivity of 200, was dispersed into a polymer matrix. Pure-gas permeation tests of such MMMs revealed that CO2=CH4 selectivity enhanced by as much as 40%–45% relative to that of the pure polymer. The effective permeabilities of fast-gas penetrants (e.g., O2 and CO2) through these MMMs are also improved relative to the intrinsic permeabilities of the unmodified polymer matrices. For a CO2=H2 gas mixture, the CO2 will serve as the fastest permeating component, and H2 will be retained on the feed side to avoid repressurization, in which case the polymer matrix dictates the minimum membrane performance. Properly selected molecular sieves can only improve membrane performance in the absence of defects. The polymer matrix must be chosen so that comparable permeation occurs in the two phases (to avoid starving the sieves) and so the permeating molecules are directed toward (not around) the dispersed sieve particulates. The molecular sieve must be selected so that its pores can separate the gas molecules of interest according to size. In the case of CMSs, varying the carbonization conditions during fabrication controls the pore size. This is an advantage of CMS membranes over zeolitic molecular sieves, in which the pore size is fixed for a given zeolite type. 4.3.3.2

MMM: Polymer with Nonporous Nanoparticles

Nanoscale inorganic fillers will affect the local chain packing, which affects the local free volume, and hence the molecular transport of permeating species. The polymer may be a rigid-chain polymer or a self-organized block copolymer. If the dispersed filler in MMMs consists of a nonporous material (e.g., carbon nanotubes, fumed or colloidal silica), the nanoscale particulates may disrupt the packing of the polymer chains, if they are rigid, and consequently increase the accessible free volume in the polymer matrix. This increased free volume augments molecular diffusion and weakens the size-sieving nature of the polymer, thereby increasing both permeability and reverses the selectivity. Since permeability depends on diffusivity and solubility, these MMMs will favor permeation of the larger (and more soluble) components through the membrane. The introduced nanoparticles may also alter the mechanism by which a copolymer self-organizes, hence influencing the equilibrium morphology and polymer thermodynamics (and then properties). Merkel et al. [87] mixed a substituted polyacetylene, poly(4-methyl-2-pentyne) (PMP), with fumed silica particles possessing hydrophobic trimethylsilyl surface groups. Dispersion of the particles was achieved by matching the polarity of the polymer and the particle surface groups, as well as by controlling film-drying conditions. Addition of fumed silica (up to 30 vol %) (size ~10 nm) promoted a considerable increase in the permeability of CH4 relative to that of pure PMP. In Figure 4.15 various forms of carbon molecules are illustrated [88]. Fullerenes generally refer to the entire class of closed spheroidal aromatic molecules consisting of only carbon atoms, up to 600 atoms.

4.3.4

INORGANIC MEMBRANES

The inorganic membranes had until the late nineties received fairly little attention for applications in gas separation. This has mainly been due to their porous structure, and therefore lack of ability to separate gas molecules. Within the group of inorganic membranes there are however the dense metallic membranes and the solid oxide electrolytes; these are discussed separately in Section 4.3.5. With reference to Section 4.2, the possible transport mechanisms taking place in a porous membrane may be summarized as in Table 4.4 below, as well as the ability to separate gases (þ) or not (). Recent findings [29] have however documented that activated Knudsen diffusion may take place also in smaller pores than indicated in the table. The increasing interest in inorganic membranes for gas applications is undoubtedly due to their excellent high temperature resistance. Inorganic membrane reactors (including carbon membranes) may thus have a very nice potential for industrial



85


FIGURE 4.15 Schematic illustration of different nanostructured carbons. (a) A C60 fullerene, (b) A single-wall carbon nanotube, and (c) a multi-wall carbon nanotube. (From Kroto H.W., Heath J.R., O’Brian S.C., Curl R.F., Smalley R.E., Nature, 318, 162, 1985. With permission.)

applications. The various configurations of membrane reactors are however not discussed in this chapter. Their separation properties may be understood on the basis of the materials used, kinetics, and process conditions. Porous materials have very complex structure and morphology, and parameters like porosity, pore size distribution, and pore shape are extremely important variables affecting gas separation properties. A schematic of different pores is given in Figure 4.16 [89]. As can be seen, pore constrictions, dead-end pores, and interconnection between pores will contribute to the characterization of the membrane, hence the tortuosity (t) plays an important role. The tortuosity will have a value equal to unity ( ¼ 1) for a cylindrical pore. The inorganic membranes may be symmetric or asymmetric. The symmetric membranes are systems with a homogenous structure throughout the membrane. Capillary glass membranes and anodized alumina membranes are examples within this group. In most cases however the inorganic membranes are asymmetric with a composite structure consisting of several layers with a gradual decrease in pore size to the feed side. Examples are ceramic aluminas synthesized by the solgel technique or carbon–zirconia membranes. For gas separation, surface-modified inorganic membranes have become increasingly important. An introduction to the synthesis and properties of inorganic membranes may be found in comprehensive textbooks [11,90,91]. 4.3.4.1

Modified Inorganic Membranes

As illustrated in Table 4.4, the pore size of a microporous inorganic membrane has to be brought down in order to separate gases. Only for pore sizes in the range below Knudsen flow, the separation may be efficient and follow either selective surface flow or capillary condensation. Separation according to configural diffusion may take place if the pore size is sufficiently small (10 nm 2–100 nm

pffiffiffiffiffi 1= M þ þþ þþþ

20 B2. The influence of the membrane material with varying solubility parameter is investigated

[145]

The effects of different substitution derivatives of cellulosic materials were investigated

[74]

3-D solubility parameter and interaction parameter values used to explain behavior (selectivity and flux). Copolymers of AN showed good selectivity with reasonable flux

[15]

Separation of azeotropic mixtures like water, MeOH, and EtOH from organic solvents Separation of azeotropic organic liquid mixtures

Methanol-benzene and methanol-toluene

1-Methoxy propanol and water

1-Methoxy propanol and water

Polyacrylonitrile composite membrane

N-acetylated chitosan membrane Sulzer Pervap-2255 membrane Cuprophane membranes

Membrane from a 5% NA cellulose sulfate solution and 20%–40% solutions of cationic surfactants e.g., N-dodecylpyridinium chloride or hexadecylpridinium chloride Hybrid PV þ distillation systems

PFSA polymer composite membrane

Regenerated cellulose (cellophane), poly(vinyl alcohol) (PVA), cellulose acetate (CA), cellulose triacetate (CTA), two blends of CTA (B1 and B2) with acrylic acid, poly(dimethylsiloxane) (PDMS), and linear low density polyethylene (LLDPE) membrane Polyimide (PI), cellophane, poly(vinyl alcohol) (PVA), cellulose diacetate (CDA), cellulose triacetate, two separate blends of cellulose acetate and CDA with cellulose acetate propionate, and PVA cross-linked with multifunctional crosslinkers Acrylonitrile-based copolymer membrane

[138] [139]

[141] [142]

[80]

[24]

(continued )



130


TABLE 5.6 (continued) Organic–Organic Separation System IPA=benzene, IPA=toluene

Membrane Used

Methanol-toluene

Regenerated cellulose (cellophane), poly(vinyl alcohol) (PVA), cellulose acetate (CA), cellulose triacetate (CTA), two blends of CTA (B1 and B2) with acrylic acid, poly(dimethylsiloxane) (PDMS), and linear low density polyethylene (LLDPE) membrane Acrylonitile-based copolymer membrane

Benzene and cyclohexane mixtures

Copolymers of acrylonitrile with styrene, methyl methacrylate, and vinyl acetate

Methanol removal from mixtures with toluene, MTBE, IPA, and acetonitrile Separation of organic liquid mixtures

Polypyrrole membrane doped with hexafluorophosphate (I) and p-toluenesulfonate (II)

Separation of the benzene=cyclohexane mixtures

Poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) cross-linked with ethylene glycol dimethacrylate (EGDM)

Liquid crystalline polymer networks

Remark

Reference

The IPA selectivity performance was: cellophane > PVA > CA > CDA > CTA

[26]

The permeation, selectivity, and flux variation with methanol concentration has been explained using the solubility parameter approach Copolymers of acrylonitrile with methyl methacrylate and vinyl acetate showed good selectivity and moderate flux Membrane (I) gave high selectivity and acceptable flux

[15]

[146]

[147]

Change in selectivity from H2O selective to ethanol selective at the nematic-isotropic transition temperature The depression of swelling of membranes with the increase of the cross-linker content in the benzene=cyclohexane mixtures enhanced benzene permselectivity

[148]

Remark

Reference

[149]

Special Applications Petroleum Industries System Design and analysis of combined distillation and pervaporation processes for methanol=DMC, methanol=MTBE, and IPA=water Sulfur removal from gasoline

Membrane Used Cross-linked PVA (PERVAP 1510) and plasma polymerized PERVAP 1137

Hybrid PV þ distillation has better economics particularly in terms of low premium energy requirement

[150]

Copolymers of AN with HEMA=MMA

b ¼ 1.5–15

[151]

General and Review Articles System

Membrane Used

Remark

Reference

Development in membrane technology PV and other membrane separations Production of desalinated water from highly contaminated water Separation of ethanol from a real yeast solution

Polymeric and ceramic membranes

Review (German)

[152]

Polymer grafted membrane

[153]

Solvent recovery Separation of organic mixtures

Polymeric membrane Facilitated transport supported liquid membrane (FTSLM)

Organic–water

Dehydration of organics and organic removal from aqueous streams

An overview of selected membrane techniques for environmental applications Measured fluxes are independent of severe fouling and virtually independent of concentration up to 100 g=L total solids With the increase of glucose concentration from 0 to 100 g=L, the total flux decreases by 25% but the separation factor for ethanol increases from 7.7 to 10 Review (Chinese) Some examples of separation of organic compounds from mixtures ( p-xylenes=m-xylenes, benzene=cyclohexane) were discussed, including separation of organic gases, wastewater, and organic solutions Review (Chinese)

Polyetherimide-based polymer film of 40 mm thickness PDMS composite membrane

[154]

[155]

[156] [157]

[158]



131

Pervaporation: Theory, Practice, and Applications in the Chemical and Allied Industries

TABLE 5.6 (continued) Organic–Organic Separation General and Review Articles System

Membrane Used

Separation of liquid mixture

Application of PV and VP

Esterification=alkylation, deNOx reaction, hydrogenation, and dehydrogenation studied

Zeolite molecular sieve membrane

Remark

Reference

General review, examples of polymers for membrane preparation, as well as performance parameters of pervaporation and vapor permeation membranes, are described It is found that zeolite membranes, disk, and coating show high performance or potential as catalysts compared with conventional catalysts

[159]

[160]

and methanol using vapor permeation unit. VP is the analog of PV wherein the feed is vapor instead of liquid. Another documented application is in the manufacture of MTBE and ETBE. Both these ethers form an azeotrope with the alcohol. In a detailed engineering study Hommerich [133] has shown that PV can be advantageously used in a large-scale plant with significant savings in operating costs. With the rising cost of crude oil and hence that of fuel, distillation may be placed at a greater disadvantage because of its energy intensive nature. PV, with its low-operating costs is most likely to receive a revived interest. Particularly important applications of interest will be separation of organic azeotropes and close boiling mixtures. A concerted effort is needed to develop membranes with higher selectivities than the currently available one to ensure that PV does not remain a small-scale application [59].

NOMENCLATURE ai Aii, Aij, Ajj, Aji, Bij, Bji Ai C3(r, o) C3(r, z) Cbi Cfi Cpi Cmi Cm Cb CT Cz D Di Dio Djo dpore DHf HM Ji K ko Ko kL kdi, kdj L Mc MV n

activity of component i diffusional interaction parameters in Section 5.2 plasticization coefficient (m3=mol)n in Equations 5.15 and 5.17 molar concentration at the pore outlet (mol=m3) molar concentration in the cylindrical pore at a point (mol=m3) concentration of component i in the bulk in Equation 5.6 (mol=m3) concentration of selectively permeating solute i in the feed in Equation 5.28 (mol=m3) membrane phase concentration of ‘i’ in the permeate side in Equation 5.28 (mol=m3) concentration of a component i in the membrane in Equation 5.6 (mol=m3) membrane phase concentration of the solute in Equations 5.7 and 5.8 (wt. fraction) bulk liquid phase concentration of the solute in Equation 5.8 (wt. fraction) total concentration of the solute in the membrane in Equation 5.7 (mol=m3) (wt. fraction) weight fraction of solute in zeolite phase diffusion coefficient (m2=s) diffusion coefficient of component i (m2=s) diffusion coefficient of component i at infinite dilution (m2=s) diffusion coefficient of component j at infinite dilution (m2=s) pore diameter (m) heat of fusion of polymer in Equation 5.13 (kJ=kmol) Henry’s constant in Equation 5.8 () flux of component i (kg=m2s) (mol=m2s) sorption coefficient for the solute in the polymer in Equation 5.6 () overall mass transfer (m=s) overall mass transfer coefficient in Equation 5.29 (m=s) fluid to membrane mass transfer coefficient in Equation 5.29 (m=s) constants in Equations 5.18 through 5.21 thickness of membrane in Equation 5.28 molecular weight between two cross-links molecular weight of repeating unit of polymer (kg=mol) constant in Equation 5.15


132

n 1, n 2 ni PiC Pio p2 p3 R, z R S SA SB T Tm U Vi V1, V2



number of moles of component 1 and 2, respectively, in Equation 5.4 ratio of molar volume of polymer to solute in Section 5.2 critical (partial) vapor pressure of component i in Equation 5.35 partial pressure of component i at equilibrium state in Equation 5.35 effective pressure (atmospheric pressure=RO pressure) of A and B at the pore inlet in Equation 5.1 () effective pressure (atmospheric pressure=RO pressure) of A and B at the pore outlet in Equation 5.1 () cylindrical coordinates (m) universal gas constant (8.314 kJ=kmol K) sorption coefficient in Equation 5.29 () sorption coefficient for A () sorption coefficient for B temperature (K) melting temperature of the crystalline phase (K) solution velocity in pore (m=s) molar volume of component i (cm3=mol) molar volumes of components 1 and 2, respectively, in equations in Section 5.2 (m3=mol)

GREEK LETTERS aS aD ao b xAB, xAM xip d dp dd dh dp Fp h l Dm rp sp

sorption selectivity () diffusional selectivity () overall selectivity () separation factor defined by Equation 5.33 () friction constants between A interaction parameter for component i () overall solubility parameter (MPa)1=2 length of the pore (m) in Equation 5.1 contribution of dispersive force toward solubility parameter (MPa)1=2 contribution of hydrogen bonding toward solubility parameter (MPa)1=2 contribution of polar forces toward solubility parameter (MPa)1=2 volume fraction of the polymer () solution viscosity in the pore (Pa s) fraction of noncrystalline polymer in Equation 5.13 () chemical potential difference between the two phases in Equation 5.10 polymer density (kg=m3) surface tension of liquid in pore (N=m) in Equation 5.35

ABBREVIATIONS A ABE AN B CA CDA COD CP CPI CTA EA EB EGDM EP EPDM ETBE FTSM

solute, A acetone–butanol–ethanol acrylonitrile solute, B cellulose acetate cellulose diacetate chemical oxygen demand concentration polarization chemical process industries cellulose triacetate ethyl acetate ethyl butyrate ethylene glycol dimethacrylate ethyl propionate ethylene propylene diene ethyl tert-butyl ether facilitated transport supported liquid membrane




HEMA IPN LDPE MARS MF MMA MS MTBE NBR NR NRS PAA PCE PDMS PEBA PEMA PFSA PMMA POMS PP PSCF PSI PV PVA PVC RO RTV SBR D

133

hydroxyethyl methacrylate interpenetrating polymer networks low density polyethylene membrane aromatic recovery system microfiltration methyl methacrylate mild steel methyl tert-butyl ether N-butyl rubber natural rubber nonrenewable resources poly(acrylic acid) perchloroethylene poly(dimethylsiloxane) poly(etherimide block polymer) poly(ethyl methacrylate) perfluro sulfonic acid poly(methyl methacrylate) poly(octyl methyl siloxane) poly(propylene) preferential sorption–capillary flow pervaporation separation index pervaporation poly(vinyl alcohol) poly(vinyl chloride) reverse osmosis room temperature vulcanization styrene butadiene rubber signifies difference in two properties

ACKNOWLEDGMENT VGP wishes to gratefully acknowledge the Herculean efforts of Mr. Rajendra Prasad and Ms. C. Sona in editing the manuscript as per CRC format.

REFERENCES 1. 2. 3. 4. 5.

6. 7. 8. 9.

10. 11. 12.

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Status and Prospects 6 Current for Ceramic Membrane Applications Christian Guizard and Pierre Amblard CONTENTS 6.1 6.2

Introduction .................................................................................................................................................................... 140 Ceramic Membrane Technologies ................................................................................................................................. 140 6.2.1 Description of Ceramic Membranes .................................................................................................................. 140 6.2.1.1 Membrane Elements ............................................................................................................................ 140 6.2.1.2 Porous Membrane Structure ................................................................................................................ 142 6.2.1.3 Dense Membrane Structure.................................................................................................................. 145 6.2.2 Mass Transfer and Separation Properties........................................................................................................... 146 6.2.2.1 Liquid Filtration ................................................................................................................................... 147 6.2.2.2 Gas and Vapor Transport across Porous Membranes.......................................................................... 151 6.2.2.3 Ion-Conducting Membranes ................................................................................................................ 152 6.2.3 Commercially Available Ceramic Membranes and Implementation in Devices............................................... 153 6.2.3.1 Membrane Modules ............................................................................................................................. 153 6.2.3.2 Modules Arrangement.......................................................................................................................... 154 6.2.3.3 Competitiveness of Ceramic Membranes ............................................................................................ 156 6.3 Current Applications of Ceramic Membranes ............................................................................................................... 158 6.3.1 Treatment of Waste Liquids and Gases ............................................................................................................. 158 6.3.1.1 Cross-Flow Filtration ........................................................................................................................... 158 6.3.1.2 Membrane Bioreactors ......................................................................................................................... 159 6.3.1.3 Air Purification and Hot Gas Filtration ............................................................................................... 160 6.3.2 Liquid Processing............................................................................................................................................... 160 6.3.2.1 Drinking and Domestic Water ............................................................................................................. 160 6.3.2.2 Food and Beverage Processing ............................................................................................................ 161 6.3.3 Product Recovery ............................................................................................................................................... 162 6.3.3.1 Purification of Pharmaceutics and Bioproducts................................................................................... 162 6.3.3.2 Products Manufacturing and Recycling in Various Industries ............................................................ 163 6.4 Recent Developments and Prospects ............................................................................................................................. 164 6.4.1 Filtration and Separation in Liquid Media......................................................................................................... 164 6.4.1.1 Nanofiltration with Ceramic Membranes............................................................................................. 164 6.4.1.2 Membrane Contactors and Distributors ............................................................................................... 165 6.4.2 Gas and Vapor Separation ................................................................................................................................. 166 6.4.2.1 Gas Separation ..................................................................................................................................... 166 6.4.2.2 Pervaporation and Vapor Permeation .................................................................................................. 167 6.4.3 Catalytic Membrane Reactors ............................................................................................................................ 169 6.4.3.1 Chemical Reactors Based on Gas–Liquid–Solid Membrane Contactors ............................................ 170 6.4.3.2 Biocatalytic Membrane Reactors ......................................................................................................... 171 6.4.3.3 High-Temperature Catalytic Membrane Reactors ............................................................................... 172 6.5 Conclusion...................................................................................................................................................................... 173 References ................................................................................................................................................................................ 174

139


140

6.1



INTRODUCTION

Ceramic membranes offer significant advantages of polymeric membranes in many applications where the last ones cannot withstand operating conditions (pH, temperature, chemicals, etc.). Indeed polymeric membranes are still cheaper to produce than ceramic membranes and are commercially available in a large range of polymeric materials and module geometry. Nevertheless, a technical and economical comparison between different membrane processes must take into account both investment and maintenance costs. Investment involves the cost of equipments for pretreatment and posttreatment of fluids in addition to the cost of the membrane plant. Maintenance includes replacement of membranes, electricity consumption, cleaning products, and labor costs. Accordingly, a comparison of overall costs, including membrane lifetime, cleaning procedures, and pretreatment requirements, may be in favor of inorganic membranes in a number of applications. Moreover, the technical progress made in the fabrication of ceramic membranes with production costs getting closer to those of many polymeric membranes explains why they are entering markets much broader than those accessible to the first generation of ceramic membranes. Porous ceramic membranes are now widely applied in microfiltration and ultrafiltration, and they are getting to enter the nanofiltration domain. In fact, the structure of ceramic membranes readily changed since their first development. Membrane elements have gained higher compaction and are now produced with pore sizes down to the microporous range referring to IUPAC classification. Actual ceramic membrane elements are composed of a macroporous inorganic material (with a flat, tubular, multichannel, or monolithic geometry) supporting a multilayer porous ceramic structure exhibiting a non-deformable porosity with pore sizes ranging from macropores to micropores. Ceramic membrane elements are the basic units in which mass transfer and separation obey different transport mechanisms relating first to membrane physical characteristics (pore size, porosity, and membrane thickness), then to physicochemical interactions between feed liquid and membrane surfaces, and finally to hydrodynamics in the element. Most often, commercial ceramic membrane modules are made of a stainless steel housing containing one or several membrane elements. Beyond the current porous ceramic membranes devoted to liquid filtration, new generations of microporous or dense ceramic membranes are likely to develop for gas and vapor separation or biological and chemical catalytic reactors. In particular, much attention has been paid recently to dense ceramic membranes exhibiting high oxygen or hydrogen separation selectivity in view of applications in new technologies for power generation. Transport mechanisms in these dense ceramic membranes are based on ion conduction in the solid at high temperature. This chapter is divided in three main parts. A current status of ceramic membrane technology is presented in the first part dealing with the description of commercially available ceramic membrane elements and modules as well as a brief overview of basic transport and separation mechanisms of these membranes. The second part looks through the current applications of ceramic membranes that have been classified into three domains: treatment of wastes, processing of liquids, and recovery of products. The third and last part of this chapter presents recent developments and prospects in the segment of ceramic membrane technologies. Part of these new developments anticipate the emergence of new ceramic membranes exhibiting a higher selectivity and able to withstand much severe working conditions than currently available ceramic membranes.

6.2

CERAMIC MEMBRANE TECHNOLOGIES

6.2.1

DESCRIPTION

OF

CERAMIC MEMBRANES

Currently available ceramic membranes are generally made up of a macroporous support, one or several macroporous intermediate layers, and a meso- or microporous top-layer. A comprehensive description of ceramic membrane top-layers can be found in a number of papers, reviews, and books [1–7]. In these publications, data are mainly devoted to the membrane top-layer, but less information is available on ceramic membrane elements. These elements may differ in structural shape and in chemical composition. The production of membrane elements combines the utilization of processing technologies such as ceramic paste extrusion for supports, slip-casting of powder suspensions for microfiltration layers [2], or solgel processing [8] of colloidal suspensions for ultra- or nanofiltration layers. The latest developments of ceramic membranes also include dense ion-conducting materials used for gas separation at high temperature. At the commercial level, ceramic membranes are available as modules made up of a rigid casing that contains one or several membrane elements. 6.2.1.1

Membrane Elements

Although some ceramic membrane elements are proposed with a flat geometry, most of them exhibit a cylindrical shape for a multichannel element (Figure 6.1). The reason for that is the much better mechanical properties obtained for cylindrical-shaped ceramics and the easier sealing of the elements compared to flat shapes. Since their early development, the geometrical and structural characteristics of ceramic membrane elements have readily changed (Figure 6.2). Originally, they were prepared as single tubes with an inside diameter ranging from ~6 to 15 mm and a wall thickness of about 2 mm. These ceramic tubes are still available from some suppliers, but the main handicaps with such



141

Current Status and Prospects for Ceramic Membrane Applications Cylindrical shaped membrane element

Feed flow

Permeate flow Channel

Retentate flow

FIGURE 6.1 Schematic representation of the multichannel structure of a ceramic membrane element.

tubular geometry are the high element volume to membrane surface ratio and the high liquid flow required for module feeding. The problem was solved rapidly by developing multichannel geometries as shown in Figure 6.2a. Actually, channels of a few millimeters in diameter minimize both the volume to membrane surface ratio and the feed flow inside the modules. Later on, the volume to membrane surface ratio was still increased by changing the cylindrical shape of the channels for non-cylindrical flower-like geometries, optimizing both space occupancy of the membrane and channel wall thickness in the element (Figure 6.2b). Recently, monolithic structures (Figure 6.2c) with a high density of cells and thin walls led to much larger hydraulic section and high membrane surface per element. Such an evolution of element geometry has resulted in a significant improvement of the filtration capability of ceramic membranes. All these ceramic membrane elements are currently available on the market. A number of relevant examples showing the evolution of these products are discussed hereafter [9]. Originally, multichannel ceramic membranes have been produced at the industrial scale by SCT-Exekia and Orelis in France with commercial elements registered, respectively, as Membralox and Kerasep. The membrane filtration area in this case can reach 0.35 m2=element depending on channel diameter and the number of channels per element. FILTANIUM elements, representative of flower-like geometries, have been produced more recently by TAMI industries in France (Figure 6.3). These elements with a cross-section diameter of either 10 or 25 mm exhibit a number of channels that vary from 3 to 39 and a membrane filtration area of 0.5 m2 for the largest elements. The increase in membrane surface compared to equivalent cylindrical-shaped channels can be estimated at 30%. The more recent monolithic structure (Figure 6.2c) results in a significant increase in surface to element volume ratio compared to the previous geometries. A typical monolith membrane element has multiple square-shaped parallel passage ways separated by porous walls through which permeate is drained at the exterior of the element. The tight arrangement of cells with a small cross-section area generates an efficient turbulent flow in the channels, a reduced pumping energy for feed flow, and a high filtration surface per element. Manufacturing of these large monoliths, up to 200 mm in diameter and 1.50 m long, has been successfully achieved by a number of companies mentioned in Table 6.1. The image of such full-size 10.7 m2 membrane element from CeraMem (Waltham, Massachusetts) is displayed in Figure 6.4. However, there is a limitation in increasing the diameter of monolith membrane elements. The upper limit is when the hydraulic resistance of the porous walls becomes higher than the hydraulic resistance of the membrane top-layer developing in the element. A solution to this problem was found by introducing permeate conduits within the monolith. An example of such monolith design, patented by CeraMem, is given in Figure 6.5. Permeate conduits are obtained by cutting slots on both ends of the monolith, and sealing the ends of these slots. Then the cells at the opposite end of the monolith, opening into the slots are sealed. After sealing the slots=cell opening at both ends of the monolith, it is coated with membrane.

(a)

(b)

(c)

FIGURE 6.2 Evolution of the geometry of ceramic membrane elements. (a) Conventional cylindrical-shaped channels; (b) flower-like designed channels; and (c) honeycomb-type structure.



142


FIGURE 6.3

Ceramic membrane elements with a flower-like geometry, from TAMI industries.

Hollow fiber (outer diameter 0.5 mm) ceramic membranes constitute the very last development in the field of inorganic membranes. As for polymer hollow fiber membranes, a large surface=volume ratio is expected for the modules as well as the possibility of keeping uppermost properties of ceramics, i.e., high mechanical, chemical, and temperature resistance. Interestingly, ceramic hollow fibers or capillaries with an asymmetric structure (Figure 6.6a) have been obtained in single-stage process and used as support for coating of microporous membrane top-layers [10]. CEPAration, newcomer on the ceramic membrane market, has developed compact and low-cost ceramic filters based on hollow fiber and capillary ceramic membranes. The modules ranging from 0.05 to 1 m2 are claimed to be suitable for micro-, ultra-, and nanofiltration. Certain modules can be specially designed for gas separation and high-temperature applications up to 7008C. Polymeric resin or ceramic potted inserts of such ceramic hollow fibers or capillaries, suitable for plastic or stainless steel housing, are shown in Figure 6.6b and 6.6c. 6.2.1.2

Porous Membrane Structure

Homogeneous ceramic porous structures with pore diameter larger than 1 mm can be used directly for microlfiltration, however, most of ceramic membrane elements are constructed from supported multiple ceramic layers constituting an asymmetric porous structure. Scanning electronic microscopy (SEM) images are shown in Figure 6.7 of asymmetric or homogeneous porous structures encountered in glass, carbon, or ceramic membrane materials. The most common ceramic materials used for ceramic membranes and support fabrication have been alumina, titania, and zirconia. Cordierite, silicon carbide, and silicon nitride have been used more recently, in particular for membrane monoliths. Glass as a homogeneous porous material may also be included in the category of available inorganic membrane materials. To a less extent, metallic or carbon macroporous materials may be mentioned as other types of support elements used for ceramic membrane deposition. The porous structure of ceramic supports and membranes can be first described using the IUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques: extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm < pore diameter 2100 guarantees a turbulent flow in the module and a minimum thickness for the concentration polarization layer. A small value of coefficient a means that n has a weak influence on J, so that a weak fluid velocity or even a dead-end filtration mode can be used. On the contrary, when the value of a is high (>0.5), a cross-flow filtration mode with a high-fluid velocity is necessary. Different module assembling and working conditions exist in the cross-flow filtration mode. For small- and medium-scale applications, the batch configuration is often followed with the possibility to perform diafiltration and concentration operations. As shown in Figure 6.20, retentate can be either concentrated in a recirculation loop or recycled continuously in the feed tank. An intermediate operating mode consists of recycling only a part of the retentate in the feed tank. Moreover, the possibility of adding water in the feed tank to compensate permeate flux results in a diafiltration process used for the elimination of unwanted species from the retentate. Another advantage of this configuration is that permeate or retentate can be easily recovered as the desired product. In short, batch filtration is a very versatile system able to adapt to many operating conditions and is widely used in many applications where the volume of liquid to be treated does not exceed a few tens of cubic meters per day. Often a single-stage process does not result in the desired product quality and for this reason, permeate or retentate stream must be treated in a second stage. Then a combination of stages, called a cascade operation, with a more or less complex design can be imagined depending on whether the final permeate or retentate is the desired product. A simple multistage process consists of assembling a number of same modules in successive stages operating in a simple-pass mode (Figure 6.21). Hence

Permeate Feed Dead-end submerged system Air Retentate

FIGURE 6.19

Schematic representation of a submerged module working in the dead-end filtration mode.



156

Handbook of Membrane Separations Diafiltration

Retentate

Cross-flow batch system

N2 Permeate

N2

Feed tank

Feed Retentate purge

FIGURE 6.20

Schematic representation of a batch filtration system.

the volume of the feed decreases with the number of stages and logically the number of modules per stage is adapted to the loss of feed volume. In this arrangement the cross-flow velocity through the system remains virtually constant, but the pressure drop is important. This kind of system is usually implemented for the treatment of large quantities of liquid when permeate is the desired product. A more complex and costly multistage system is when the feed flow is recycled several times on each stage. Two kinds of pumps are required in this case, one or more for plant feeding and a recirculation pump on each stage (Figure 6.22). Several advantages arise from this system compared to the simple-pass filtration mode. The flow velocity and pressure can be adjusted in every stage. Membranes with different characteristics can be implemented in two successive stages. The feed recycle system is more flexible than the single-pass system and is to be preferred in cases where severe fouling and concentration polarization occur as in microfiltration and ultrafiltration. With regard to the common applications of ceramic membranes (food and beverage industry, wastewater treatment, very turbid fluid pretreatment), the feed recirculation system is more often used than the simple-pass system. 6.2.3.3

Competitiveness of Ceramic Membranes

The design and implementation of a membrane process, whatever the nature of the membrane (polymer or ceramic), is a long procedure starting from membrane testing on a laboratory scale and ending in implementation of an industrial plant. It includes both technical and economical evaluation, which makes the membrane process viable or not compared to alternative separation processes. The overall costs are distributed in investment costs and running costs. As already mentioned ceramic membranes are more expensive than their polymeric counterpart and will result in higher investment costs, at least for the membrane system alone. However, the high investment cost can be balanced by a lower running cost when using ceramic membranes (i.e., less pretreatment, longer membrane lifetime, and more efficient cleaning and sanitizing conditions). Indeed, in many cases the membrane system cannot be used directly and often pretreatment is necessary to facilitate the membrane process. Pretreatment is important and necessary in micro-, ultra-, and nanofiltration, while it is not that important for pervaporation (PV), vapor permeation (VP), or gas separation for which feed streams are usually much cleaner and do not contain many impurities. The cost of pretreatments can contribute appreciably to the overall costs. However, due to the intrinsic

Retentate

To next stage

Cross-flow single-pass system Permeate

Feed

FIGURE 6.21

Schematic representation of a simple-pass modules stage.



157

Current Status and Prospects for Ceramic Membrane Applications Cross-flow recirculation system Permeate Recirculation pump

Feed pump

To next stage Retentate

FIGURE 6.22

Schematic representation of a feed recirculation modules stage.

characteristics of modules and membrane material, ceramic membranes require less feed stream pretreatment and authorize very efficient cleaning and sanitizing procedures. Two phenomena are responsible for flux decline during membrane operation with real fluids [20]. One is related to polarization phenomena (concentration and temperature polarization), which normally are reversible processes. Thus, at a finite time, when steady-state conditions have been attained, the flux stabilizes at a value always less than the original one. Membrane fouling is the second phenomenon responsible for flux decline. It consists of the deposition of retained particles, colloids, emulsions, suspensions, macromolecules, salts, etc. on=in the membrane. Fouling always results in a continuous (ir)reversible decrease of membrane flux with time and constitutes one of the major problem to be managed during filtration plant operation. This is a very complex phenomenon and the methods for reducing fouling must be adapted to the different used membranes and treated fluids. Several aspects such as pretreatment of feed solution, membrane and module characteristics, and operating parameters have to be anticipated during the process design, otherwise heavy cleaning procedures should be carried out on the membrane plant in operation. The utmost advantage of ceramic membranes is to allow in-place chemical cleaning at high temperature, while using caustic, chlorine, hydrogen peroxide, ozone, and strong inorganic acids. Moreover steam sterilization can be used for sanitizing membrane plants. Another specific advantage of ceramic membranes is the possibility to be back-pulsed which is basically a permeate flow reversal technique to reduce fouling and to increase filtration efficiency. As shown in Figure 6.23, back-pulsing consists of periodically reversing the permeate flow by applying pressure to the permeate side of the modules. In this manner, the accumulated matter on the feed side of the membrane is periodically removed from the surface and carried away by the circulating fluid. Other physical methods recently investigated utilize gas bubbling [29] or ultrasound [30] to prevent fouling or for cleaning fouled membranes. One can see that ceramic membranes differ from their polymeric counterparts by intrinsic properties (rigid porous structure, high-temperature resistance, high-chemical resistance to aggressive aqueous and organic media, insensitiveness to biological attack) which are based on the inorganic nature of the membrane material. In current applications mainly devoted to the treatment of liquids, general principles of membrane processes apply equally to ceramic and polymeric membranes. For both membrane categories, the modules are implemented in industrial plants, following the same hydrodynamic laws and mass transfer mechanisms. Most often, the choice of ceramic membranes is driven by economical considerations and despite its inherent qualities ceramic membranes are considered as expensive compared to polymeric ones. Nevertheless, a large number of applications are described in Section 6.3, showing that in many cases ceramic membranes have been considered as competitive compared to alternative processes.

Feed side

Membrane

Permeate side

Back-pulsing Cross-flow direction Back-pulsing

Flow reversal

FIGURE 6.23

Schematic representation of permeate back-pulsing allowing in-place removal of accumulated matter on membrane surface.


158

6.3



CURRENT APPLICATIONS OF CERAMIC MEMBRANES

The number of applications of ceramic membranes has continuously been increased over the past 20 years. Even if data on the extent of installed plants are not directly available, the sales in inorganic membranes are estimated between e250 and e350 million in 2005 with a potential growth of 10%–15% a year. Many references on the use of ceramic membranes can be found in the proceedings of the successive International Conferences on Inorganic Membranes as well as in the books of Bhave [1], and Burggraaf & Cot [3]. The aim of this part is not to provide an exhaustive overview of all possible applications but to focus on relevant examples of current ceramic membrane processes. These applications have been classified into three main groups: (i) the treatment of waste liquids and gases; (ii) liquid processing including drinking water, domestic water, and food beverages; (iii) product recovery in various industries. Due to its robustness, ceramic membranes are developing faster in the treatment of liquid wastes than in other sectors. Actually, the demands of efficient liquid waste treatment are being pushed up both by regulatory and cost pressures to reduce environmental impact of human activity [31,32]. In a general way, there are many water shortage problems currently in the world so that the recycling of wastewater streams is an urgent necessity. In liquid processing, the sector of food beverages is the first one where ceramic membranes have been developed at the industrial scale, in particular in the dairy industry. Actually the demand for more secure membranes is increasing in the food industries as an alternative to polymer membranes that are less amenable to sanitary requirements. Finally ceramic membranes can be used in many industries, directly for the recovery of various products, not only from micrometer-sized species (mineral particles, microorganisms, macromolecules, etc.) down to nanometer-sized species (viruses, colloids, molecules, ions), but also as a filtration pretreatment before other separation techniques including polymer membranes.

6.3.1 6.3.1.1

TREATMENT

OF

WASTE LIQUIDS

AND

GASES

Cross-Flow Filtration

The demand for industrial water in developed countries is increasing continuously in comparison with the limited natural resources. In recent years, recovery and reuse of industrial wastewater has arose as one of the important economical and ecological issues for a sustainable economy, the challenge in the modern technology being to allow industry to move toward the ‘‘zero discharge’’ concept. Membrane technology can afford viable solutions to the problem of water recycling in numerous industrial processes. Beyond the large availability and versatility of polymer membrane processes, the performance of ceramic membranes has considerably improved during the last 10 years. They are now able to provide long and reliable service life in many processes where polymer membranes cannot withstand operating conditions including the cleaning procedures, in terms of mechanical, chemical, and temperatures resistances. Moreover, the presence of abrasive particles, extreme pH conditions, organic solvents, and an elevated temperature in certain industrial wastewater effluents renders compulsory the use of inorganic membranes in the implementation of membrane processes. A number of application areas have been identified in which cross-flow filtration with ceramic membranes revealed competitive compared to other separation processes [33]. Several cases can be mentioned: for example, removal of heavy metals from metal-finishing shop wastewater and treatment of spent chemical baths; heavy metal removal from acid mining drainage; recycling of laundry water by removing dirt, oil, and metals from wash water; treatment of wastewater chemical planarization of semiconducting; treatment and recycling of lubricating liquids used for cutting tools in mechanical industry; and in a general way all sorts of cleaning and rinsing waters from food and beverage processing industries. The following examples are representative of the current developments in the treatment of industrial waste liquid streams with ceramic membranes. Textile printing is a water-consuming industry where membranes and, in particular, ceramic membranes are welladapted techniques for process intensification [34–36]. Ceramic membranes are also used for recycling process water in basic domestic activities like laundries [37] as well as in advanced industrial processes like polishing and washing stages of lens [38]. Ceramic membranes participate also to water and chemicals recovery in the automotive industry [39]. For example, a mean volume of 500 L of water per car body is used in pretreatment before electrophoretic painting. In this process, ceramic nanofilters have been chosen for water recycling due to the capacity of enduring working environment with long-service lifetime. New applications are also arising thank to the tight cut-off characteristics of ceramic nanofilters. Enormous volumes of bleach pulp effluent and black liquor are produced in kraft pulp mills. The bleaching process is highly water–consuming, while black liquor is regarded as a bottleneck by-product, which is usually burned in a recovery boiler after concentration by evaporation. Nano- and ultrafiltration techniques using ceramic membranes appear as efficient methods for water recycling in the bleach pulp process and lignin recovery from black liquor [40,41]. Another domain in which membrane separation technologies can afford viable solutions in the near future is the treatment of domestic wastewater [42]. Here, membranes are considered as too expensive and the technology is too sophisticated for sewage treatment. Nevertheless recent works by the European Space Agency [43] have shown that membrane technology including a ceramic membrane stage can cope with the life-support needs and isolation constraints during long-duration missions in space or in isolated scientific bases like in Antarctica. These space-related technologies including membranes used



159

Current Status and Prospects for Ceramic Membrane Applications

for the treatment of wastewater should benefit to our daily lives by providing innovative and efficient solutions for domestic wastewater treatment. 6.3.1.2

Membrane Bioreactors

The membrane bioreactor (MBR) technology is a real alternative to pure filtration processes for the treatment of wastewater and industrial effluents. An MBR is the combination of an activated-sludge process with a membrane filtration. MBRs are composed of two primary parts: the biological unit responsible for the biodegradation of the waste compounds and the membrane module responsible for the physical separation of the treated water from mixed liquor. These systems are implemented based on two main configurations (Figure 6.24). The first (integrated) configuration consists of outer skin membranes that are internal to the bioreactor (Figure 6.24a). The driving force across the membrane is achieved by pressurizing the bioreactor or creating negative pressure on the permeate side. In the second (external) configuration, the mixed liquor is recirculated through a membrane module that is outside the bioreactor. In this case, the driving force is the pressure created by the high cross-flow velocity along the membrane surface (Figure 6.24b). Current available ceramic membranes having internal membrane top-layers are more adapted to the second MBR configuration. There are a large number of potential applications of the MBR technology for the treatment of wastewater from agricultural and agri-industry sources [44]. Manure and wastewater from livestock can be treated by a technique coupling anaeorobic digestion with an aerobic=anoxic membrane bioreactor, producing water for direct reuse or safe discharge. Wastewater generated from food processing industries such as slaughterhouses, meat, dairy, egg, and potato processing could potentially be treated with MRBs resulting in compact systems producing high-quality reusable water. Also effective removal of nitrates,

Process control O2 /Air Submerged membrane unit

Pump

Influent

Fine screen High-quality effluent for discharge, reuse, or recycle Mixing + aeration + membrane scouring

(a)

Process control

O2 /Air External membrane unit Influent

Fine screen

(b)

FIGURE 6.24

Mixing + aeration

Pump

High-quality effluent for discharge, reuse, or recycle

Membrane bioreactors configurations. (a) Internal outer-skin membranes and (b) outside membrane module.


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herbicides, pesticides, and endocrine disrupting compounds may be achieved by MBRs. In drinking water treatment, either biological (bio-denitrification) or physicochemical (reverse osmosis, ion exchange, electrodialysis) techniques can be used for removal of nitrates. The advantage of coupling biodegradation and membrane separation in the MBR concept is to allow the removal of organic matter and the complete conversion of nitrates into nitrogen gas in a one-step relative easy operation. Another example of MBR utilization is the treatment of acid mine drainage. These drainages typically contain dissolved metals at high concentration and more than 3 g=L sulphate. The use of sulphate reducing bacteria (SRB) has been proposed as an alternative to hydroxide precipitation. Generally, MBRs can afford viable solutions to the treatment of industrial waste streams containing high organic loading, and very specific and difficult to treat compounds. MBR are implemented with MF or UF membranes. Several types and configuration of membranes are used, including tubular, plate and frames, rotary disk, hollow fiber, organic, metallic and ceramic membranes [45]. However, ceramic membranes offer inherent advantages over polymeric membranes (possible high-sludge concentration, very efficient cleaning procedures, resistant to microorganisms) for this technology [46]. Although they have suffered from their high prices in the past, low-cost ceramic cross-flow membrane modules were successfully tested, able to promote a low-cost ceramic air lift MBR [47]. Large (200 mm hydraulic diameter by 1500 mm long), high surface area (15–28 m2 depending on cell size) silicon carbide monolith membranes are envisioned in this application with projected membrane price in the range of e200=m2. The modules could be used in either a submerged MBR or an external airlift configuration. Sewage treatment [48] should be an attractive market for MBRs based on this new generation of ceramic membranes exhibiting low fouling behavior, high cleaning ability, and long durability. 6.3.1.3

Air Purification and Hot Gas Filtration

At the boarder line with ceramic membrane filtration, air purification and hot gas filtration using ceramic filters are technologies attracting significant attention in a wide variety of domestic applications and industrial processes [49–53]. The main difference with membrane technologies is that ceramic gas filters operate in a dead-end filtration mode and often as particle traps. Conversely, a major concern with ceramic membrane and filter technologies is the increasing demand for ceramic porous elements with a high efficiency in the retention of very small species (molecules with membranes or nanoparticles with filters). Ordinarily, ceramic filters for gas application exhibit low-density structure with high porous volume and pore size of several micrometer. Particulate structures, fabric, or fibrous media and more recently foams are commonly used as in-line filters to remove particulate contaminants from process gases. Today, new designs are considered for gas ceramic filters based on ceramic porous structures already developed for cross-filtration with ceramic membranes. The major evolution of these new filters compared to other available filters is the submicronic pore size adapted to the treatment of nanoparticulate aerosols, which will be a big issue in the development of nanoparticle production and its utilization in nanotechnologies. The difficulties involved in operating these new filters include the high pressure drop due the tight porous structure of the filter and the evaluation of filter efficiency by measuring low nanoparticle number concentrations downstream. For fabrication and utilization reasons, decreasing filter media thickness to lower pressure drop is not ever practical, hence an alternative approach is the application of new concepts based on hierarchical asymmetric filters [54]. The development of characterization setup and methodology to evaluate the filtration performance are also important aspects to deal with for the future utilization of these new filters [55].

6.3.2 6.3.2.1

LIQUID PROCESSING Drinking and Domestic Water

Water purification for producing drinking and domestic waters is a quite complicated process, which must be adapted to the multiple water resources available as surface or ground water. Due to considerable pollution of natural waters, more and more stringent requirements are applied to natural water treatment. Contamination cases where traditional technological systems are ineffective are increasing continuously. High selectivity of membrane processes and no pollution charged to the environment make these methods very attractive as alternative or complementary processes. Membrane are effective for the removal of suspended particles, precipitates caused by water hardness or salts, organic compounds, and microorganisms (bacteria, spores, even viruses). The basic problem involving the application membrane processes in the treatment of natural water is the decrease of yield with time due to concentration polarization layer and reversible or irreversible fouling phenomena. Periodical renewal of the membranes and cleaning of the filtration plants considerably increase the price of produced water so that a number of methods have been proposed to prevent fouling or at least to make longer the time between two cleaning operations by removing continuously accumulated matter on membrane surface. For example, to maintain high permeate fluxes over long periods of time, adsorbents or coagulants can be used in conjunction with filtration [56]. In general, current commercial ceramic membranes (MF, UF, and NF) are able to treat turbidity and to remove biocontaminants from natural water [57,58]. The modules accept highly turbid waters, stringent cleaning conditions and can take advantage of in-place back-pulsing.




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Utilization of ceramic membranes for water purification started in the 1980s [59] with the implementation of small size membrane plants (5–100 m3=h). Imeca in France was one of the first membrane suppliers entering this market with inorganic membranes. This membrane technology is now developing at a larger industrial scale with, for example, the ceramic-membrane water-purification systems designed by NGK in Japan. This company trusts systems based on ceramic membranes, because they are producing water that is safer to use than polymer membrane-based systems. Effectively, the elements have superior mechanical strength, which prevents them from being easily damaged, and they have lower running costs. However, the relatively high initial cost for these systems is a major concern in applications where large volume of purified water a day must be delivered. Estimated prices for polymer membranes currently in operation for drinking water treatment are about 30–50 e=m2 for micro- and ultrafiltration, and 15–30 e=m2 for nanofiltration and reverse osmosis. In order to render ceramic membrane elements more cost-effective compared to those based on polymer materials, a number of membrane manufacturers develop large ceramic membrane monoliths offering a large filtration surface per element (see Section 6.2.1.1). Thus a long membrane lifetime, a high membrane surface to volume achieved in the modules as well as a high permeate flow are expected to reduce significantly both investment and running costs of ceramic membrane plants. Several tens of cubic meters per hour of purified water are expected for the larger planned elements, which will render this technology competitive in a near future for large water purification systems. 6.3.2.2

Food and Beverage Processing

Contrary to water treatment, general principles cannot be stated for food and beverage processing with membranes. Each sort of food and beverage is a very complex system with specific components interacting with each other so that membrane processes must be specifically adapted to each case. Accordingly, a more detailed description of membrane processes is provided hereafter for the current major applications, i.e., milk, to some extent beer and wine, and a number of other applications showing the continuous expansion of ceramic membrane technologies in food industries. 6.3.2.2.1 Fractionation of Valuable Milk Components Milk is of the utmost importance for human feeding. It contains about 2000 components, which have been thoroughly studied not only for their nutritional properties, but also for the potential utilization of some of them in pharmaceutics. The milk industry has been the first food sector in which membrane processes have been implemented at an industrial scale. The earliest development dates back to the end of the 1960s with ultrafiltration plants devoted to whey concentration. Since this period, almost all the membrane processes ordinarily used in liquid treatment (micro-, ultra-, and nanofiltration, diafiltration, reverse osmosis, electrodialysis) have been applied to the treatment of milk and its derivatives. Both ceramic and polymer membranes are used in milk industry; however, the ceramic membranes offer specific advantages in terms of cleaning procedure and sanitary conditions [60]. Compared to polymer membranes, they are able to withstand better harsh cleaning conditions used in the milk industry. Indeed, in-place chemical cleaning at high temperatures, while using caustic, strong inorganic acids, chlorine, hydrogen peroxide, ozone, and steam sterilization, are as many procedures used at different stages of milk transformation and conditioning. In some cases, ceramic membranes are an integral part of new processes in milk transformation. One example is the fractionation of globular milk fat by membrane microfiltration [61]. Separation of milk fat in small globules (SG) (diameter lesser than 2 mm) and in large globules (LG) (diameter greater than 2 mm) was realized by a patented process using special ceramic microfiltration membranes. Transformation in drinking milks, yoghourts, sour cream, camembert, Swiss cheese, and butters was realized from milks of which the fat content was adjusted from the SG or the LG fractions. These products exhibit significant differences in texture and organoleptic properties compared to traditionally manufactured products, which make them more attractive for consumers. Ceramic membranes with pore diameters of about 2 mm have been identified as very suitable for this application. Permeate fluxes in this case vary from 200 to 700 L=h m2, which make the process economically viable. In a more general way, milk fractionation will lead to a more efficient and diverse use of milk and membrane separation seems a logical choice for that [62]. In addition to the aforementioned separation and fractionation of milk fat from whole milk, removal of bacteria and spores from skim milk (cold pasteurization), concentration of casein micelles from skim milk, and recovery of serum proteins from cheese whey are as many operations that can be currently carried out with membranes. Thanks to the availability of new ceramic membranes with smaller cut-off and narrow pore size distribution, new milk component-based products are under development such as edible coatings, bioactive peptides, or diary and nutritious beverages. However, fouling is still considered as the limiting factor in milk filtration requiring further work on new membranes, module design, fouling control, and modeling to achieve rational design of milk fractionation processes. 6.3.2.2.2 Beer and Wine Cross-Flow Microfiltration Filtration is an important step in beer production that is traditionally done with filter aids, usually Kieselgurh. The use of Kieselguhr, diatomeaceous earth in a form of silica, is, however, getting more unfavorable in several aspects because the filter aids have to be disposed of or regenerated. Disposal is not environmental friendly and is getting more and more expensive. Moreover dusty atmosphere resulting from Kieselguhr handling is hazardous for the workers. The filterability of beer is an


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important parameter and poor filterability can lead to an increase in production cost and also be a big bottleneck concerning the production volume with a significant quantity of wasted beer and spent yeast. Ceramic cross-flow filtration processes have been proposed as a solution with systems able to recover more than half of previously wasted beer, to eliminate disposal costs and to minimize environmental impact. In addition they have the potential to create new revenue streams, as yeast is concentrated to make it more viable for use as a base in animal feed, human food supplements, or as an additive for pharmaceuticals. Wine manufacturing is still a very traditional process in which filtering is a very tricky subject, especially for prestigious wines. However, one can say that nowadays fining and filtering have been accepted by winemakers as useful tools in building a better wine. The challenge in fining and filtering wines is elimination of unwanted flavors and particles, and preserving of all the components that will create the personality of a good wine [63]. Concerning filtration, winemaking consultants agree to say that wines should be filtered to the minimum necessary. As for beer, filtration of wine has been achieved for years by diatomeaceous earth filters with the same pending problems. In the 1990s, cross-flow microfiltration emerged as a very promising technology for this purpose because of its ability to perform wine clarification=filtration=hygienization in one single step. At that time, microfiltration with ceramic membranes offered real advantages over classical filters. Indeed, ceramic membranes function better under pressure, handling 35%–50% throughput. They can be cleaned by a ‘‘cleaning in-place’’ procedure based on an automatic reverse permeate flush. They can work in a continuous mode 18–20 h=day. Finally they last two or three times longer than organic filters. Most of ceramic membrane equipment suppliers have entered the beer and wine filtration market with membrane products adapted to the specificity of these products [64–66]. The pore diameter of available membranes ranges between 0.1 and 1.5 mm, but the choice of pore size cannot be systematically categorized for each family of products, in particular for wines. In fact filtration requirements are not the same for red wines and white wines. Even, in each of these categories the quality properties of the final product are not exactly predictable and render the choice of the membrane pore size very uncertain. Adsorption of wine compounds on membrane surface also influence filtration effectiveness. Polysaccharides with polyphenols are usually the main responsible compounds of the fouling in wine clarification with membranes [67]. Moreover the retention of these compounds will affect the organoleptic characteristics of the filtrated wine so that their retention and their fouling effect must be reduced. Up to now, filtration tests have shown that 0.1 mm for white wines and 0.2–0.4 mm for red wines are the most recommended pore sizes. The choice of pore size about 0.4 mm for beer filtration looks easier but fouling phenomena are still present [68]. The b-glucans contained in the beer seem to be the major factor for membrane fouling. When they are subjected to high shear stress, high pressure, and rapid cooling, they elongate and form hydrogen bonds resulting in a gel-layer formation on the membrane. In order to improve the filterability many breweries add flocculants before filtration and let them sediment to remove big particles. This flocculation=sedimentation step can impact membrane filtration negatively or positively depending on if membrane modules are sensitive or not to residual particles in the feed stream. 6.3.2.2.3 Other Food Components Many other applications for ceramic membranes exist in the food industry [1,69]. First, a wide range of fruit juices (apple, pear, peach, orange, grape fruit, pineapple, kiwi fruit, strawberry, cranberry, carrot, date, etc.) are now processed using ceramic membrane technologies, in particular, for clarification and pasteurization. However, flux decline due to membrane fouling is a major concern as it has been shown for apple juice [70]. Membrane technology has also shown high promise in starch processing industry for reducing evaporation costs, improving product recovery, and removing solids before wastewater treatment [71]. Effectively, membranes can be used in many operations of corn, potato, and wheat starch processing. For example, one of the most common applications for membranes in corn wet milling is the removal of residual lipids, proteins, and colorants from corn syrup [72]. It was reported [71] that in 1997 approximately 75% of the US-produced corn syrup was clarified using membrane filtration instead of conventional vacuum filtration. Moreover, the use of MF or UF with nominal pore sizes 0.01–0.1 mm can provide pasteurization without use of heat, as most microrganisms are larger than this pore size. Here also, clarification and pasteurization are typical operations for which UF and MF ceramic membranes are of great interest [72]. Another interest in membrane filtration is in the sugar industry for which it has been shown that the conventional purification can be partially or completely replaced by UF and NF [73]. Indeed, ceramic membranes are well adapted to the treatment of highly viscous sugar syrups for which separation of colored matter from a green syrup is a crucial step in sucrose crystallization [74,75]. In other respects, it has been reported that the pretreatment of molasses with MF ceramic membranes can be beneficial to ethanol fermentation [76].

6.3.3 6.3.3.1

PRODUCT RECOVERY Purification of Pharmaceutics and Bioproducts

Membrane filtration technologies are finding applications in pharmaceutics industry for genomics, proteomics, drugs discovery, and generally for laboratory applications in life science, with additionally the possible production of ultra pure process water. In the bioindustry, recovery and purification of products also involve membrane technologies. Although polymer membranes are



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Current Status and Prospects for Ceramic Membrane Applications Cross-flow batch system

P Retentate P

T

Feed tank

Permeate pump

Feed pump P

Permeate tank

Purge Retentate purge

FIGURE 6.25

Back flush monitoring

Configuration of a batch system membrane plant designed for microfiltration of fermentation broth.

largely used in these sectors, the place of ceramic membranes is increasing due to a better capability to be efficiently cleaned and sterilized. In particular, the new generation of microporous ceramic membranes able to work in the nanofiltration domain (see Section 6.4.1.1) is of great interest to extend current applications. The use of ceramic membranes in biotechnology was reviewed some years ago [69] showing that this sector was primarily interested by the microfiltration of fermentation broth to separate the yeast cells or cell fragments. Today the separation of microorganisms is still of interest and microfiltration is becoming more common compared to centrifugation, especially when it is used as cell separator in continuous fermentation or biological wastewater treatment. One problem is that during the filtration of cells a filter cake is formed on the membrane surface that reduces permeate flow. This is an example where back flushing has been successfully used to push the filter cake off the membrane surface back into the feed stream [77]. More recently an original idea was proposed for the same application, consisting of the implementation of a pump to withdraw permeate at a constant rate [78]. The advantage of a permeate pump compared with using a valve on the permeate side is the easiness to control the permeate flux and by the way the connective transport of material toward the membrane, and thus preventing filter cake formation. Process monitoring can be made by controlling the increase in transmembrane pressure, which can be used in turn as an indication of filter cake formation (i.e., the occurrence of critical flux for the membrane) showing when the membrane has to be cleaned [79]. A schematic representation of what could be a batch system process combining controlled permeate flux with sequential back flushing for membrane cleaning is shown in Figure 6.25. Next to microfiltration, the commercial availability of ultrafiltration ceramic membranes with well-defined cut-off characteristics allowed the development of membrane processes dedicated to the separation of proteins and enzymes, not only in biotechnology but also in more established industries like dairy [62], fish meal transformation [80], etc. The central problem to protein separation is the adsorption of proteins on the membrane surface and even inside the porosity for molecular weight fractions smaller than pore sizes. This adsorption phenomenon results in most cases in a significant flux decline responsible for intensive cleaning operations impeding the economical feasibility of such processes. The chemical modification of membrane surface has been studied as a way of improving the separation of proteins either by generating specific interaction between the surface and proteins to be recovered or by preventing adsorption to promote a purely size separation effect [81]. An other way is to select membrane materials with a weak tendency to protein adsorption. For example, silicon carbide membrane could be very promising for protein filtration so far as a very low protein adsorption has been evidenced for this membrane material [82]. For the future, interesting developments are expected from the utilization of hybrid organic–inorganic membranes for the separation of molecules of biological or pharmaceutical interest. Chiral-selective membranes [83] or supramolecular chemistry-based membranes for biomimetic transport [84] are studied on the laboratory scale with the aim to separate a number of valuable molecules from reaction mixtures or natural extracts. These studies are attendant to the research on filtration and separation microdevices, which have high development potential in pharmaceuticals and bio-industry. 6.3.3.2

Products Manufacturing and Recycling in Various Industries

Separation of manufactured solids from process liquids and recycling of these liquids (water or organic solvents) is an interesting way to valorize by-products and to minimize the production of liquid effluents in a number of industries. Microfiltration ceramic membranes have been already used for the recovery of particles in the ceramic industry and in drilling operations, of pigments in paint and ink industries, and have potential applications in a wide variety of liquid–solid separation


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processes. Due to the high inlet cross-flow velocity used in microfiltration, most of these particle suspensions have a strong abrasive effect on the membrane that justifies the use of ceramic membranes with good hardness properties, e.g., a-alumina or silicon carbide. The rheological behavior of particle suspensions under shear stress and the formation of a filtration cake are the two main problems, which must be addressed in these applications [85,86]. Cake formation depends not only on applied hydrodynamics in the module, in particular, the shear rate, but also on the interaction between particles depending on several parameters (pH, zeta potential, presence of surfactants or coagulants, etc.). According to Ref. [85], it is likely that cakes formed under cross-flow conditions are more compact than those formed under dead-end or stagnant conditions. This may explain the low permeate flow encountered in microfiltration of particulate suspensions when the chemistry of these suspensions as well as the applied hydrodynamics are not correctly monitored. Coupling with techniques like flotation can help the microfiltration process in solid–liquid separation [87]. Another important application domain for ceramic microfiltration membranes is the recovery of waste oils from industrial wastewater streams or the cleaning of used mineral oils. For example, to prevent formation of higher soot and ash accruement compared to the utilization of original fuel oil, the cleaning of waste oils used as a substitute for fuel oil can be upgraded using microfiltration [88]. In regard to mineral oil regeneration, successful regeneration treatments have been achieved with ceramic ultrafiltration membranes. Moreover, it has been shown that mixing of supercritical CO2 with oil before the filtration process allows for a significant improvement of permeate flux by reducing oil viscosity [89,90]. Microfiltration can also extend the lifetime of cleaning baths in metal surface treatment or of lubricating fluids in machining industry. These fluids contain dirt particles as well as oil and detergents that must be separated and recycled, while the concentration must be adjusted at the correct level in order to be directly reused in the cleaning or machining process. Here also ceramic membranes offer better durability compared with their polymeric counterpart, because of the presence of abrasive particles and aggressive components. Separation of oil=water suspension using cross-flow filtration is another domain of application for ceramic membranes [91]. Ordinarily, ceramic microfiltration membranes with pore sizes close to 0.2 mm are used for demulsification [92] and product recovery [93]. As for the other applications, there is a flux decline with time and backflushing is necessary to reduce the formation of the gel polarization layer and membrane fouling. The use of hydrophilic membranes is preferred to hydrophobic membranes for which oil is easily wetted on the hydrophobic membrane surface and induces more fouling of the membrane. Moreover, the operating pressure must be lower than capillary pressure to avoid the contamination of permeate with oil. As a conclusion to the aforementioned current applications of ceramic membranes, a considerable change is under way for the utilization of these technologies in industry, in particular, for waste treatment and product recovery. In order to adapt to environmental regulation and to look for a sustainable economy, the tendency is to develop long-lasting solutions for clean and efficient industrial production processes. Actually, splitting between manufacturing and treatment of end-of-pipe effluents before disposal or simple discharge in the rivers or in the sea, even if these effluents are reputed to be non toxic, is expensive and not totally satisfactory on an environmental point of view. The reclamation of by-products and process fluids is one of the ways being currently explored to improve the productivity and to lower the environmental impact of industrial activities. Membrane processes have been identified as one of the suitable technologies for the development of sustainable processes in which product manufacturing and recycling are combined, thus avoiding the discharge of large quantity of wastes. Section 6.4 describes a number of new membrane processes that modify the current status of ceramic membrane technologies. Most often, they are based on new ceramic membrane prototypes not totally tested out at an industrial scale.

6.4

RECENT DEVELOPMENTS AND PROSPECTS

A number of new applications are likely to come in the near future for ceramic membranes, in particular the expected short-term developments for ceramic nanofilters and ceramic membrane contactors applied to liquid treatment, including biological membrane reactors. Gas and vapor separations with ceramic membranes seem to be more distant applications, although microporous silica and zeolite membrane modules are currently in operation in pervaporation or vapor separation processes. High-temperature catalytic membrane reactors (CMRs) constitute a more prospective field of investigation with high potential applications in the chemical and petrochemical industry, in particular, for producing hydrogen from fossil fuels. Most of these developments should be based on new membrane materials exhibiting either carefully controlled pore sizes down to the nanoporous range or dense crystalline structures with ion-conducting properties. Moreover, in most of these new applications, the membranes should have to withstand harsh working environment like corrosive chemical fluids and high process temperatures.

6.4.1 6.4.1.1

FILTRATION

AND

SEPARATION

IN

LIQUID MEDIA

Nanofiltration with Ceramic Membranes

As the newest development of the liquid filtration family, nanofiltration (NF) is capable of retaining small molecules from 200 to 1000 Da, and multivalent ions. The main current applications of NF polymeric membranes are dealing with the production of drinking and process water, the sulphate removal of seawater or the desalination of cheese whey. Ceramic nanofilters were



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commercialized recently and operate in a cut-off domain 500–2000 Da, higher than for polymeric NF membranes [94]. Up to now, several application areas have been identified in which the utilization of ceramic NF membranes is highly suitable, for example, when liquid media contain organic solvents. However, depending on solvent polarity and due to ceramic hydrophilicity, filtration and separation of organic solvents with ceramic nanofilters is not an easy operation. Works from the literature have shown that membrane flux is drastically decreased with apolar solvents compared to water or alcohols in the case of ceramic oxide nanofilters [23]. On the basis of these results, surface-modified ceramic membranes have been recently described, in which the wet ability was changed from hydrophilic to hydrophobic with the aim to develop membranes well adapted to apolar organic solvents [95]. Other relevant examples of application for ceramic nanofilters were recently reported [96]. The first separation problem in which ceramic NF membranes are likely to compete with polymeric membranes is the concentration of pharmaceutical components in their reaction solvents. Ceramic nanofilters are also of interest in the agro-food industry for separation processes working at relatively high temperature and or in the presence of organic solvents. Another potential application is in the production of chemically modified sugars in which the N-methyl-pyrrolidone (NMP), used as the solvent for the reaction, needs to be removed from the product (minimum molecular weight about 1000 Da) down to 0.1%. In this case, diafiltration with a ceramic NF membrane has shown to be an efficient way of decreasing the NMP content down to the intended low NMP residual concentration. Also in the sugar industry, where micro- and ultrafiltration are alreadly used for pretreatment of sugar juices, a potential application of ceramic nanofilters is the purification of clarified or pre-evaporated juices as an alternative to ion exchange resins. Typical fluid parameters (sugar concentration from 7 to 25 Brix, feed temperature of 908C–1008C) justify the use of ceramic membranes instead of polymer membranes. Ceramic nanofilters have also been tested for the treatment of textile wastewater, of alkaline solution from bottle washing machines, or of pickling bath solutions from the metal-working industry [97]. For the future, new applications are expected in the chemical industry. A typical problem for which ceramic NF membranes could afford a viable solution is the recovery of homogeneous catalysts from nonaqueous solvents, in particular the recuperation of transition metal complexes [95]. 6.4.1.2

Membrane Contactors and Distributors

There are many operations in chemistry, biotechnology, or agro-industry in which membranes can serve as contactors and distributors between two phases: liquid=liquid or gas=liquid. The high contact surface displayed by the porosity of a membrane allows highly efficient exchanges between hydrophilic phases and gaseous or lypophilic phases. Moreover a narrow distribution of pore sizes allows very sharp process control in a number of applications like emulsification, micromixing, osmotic distillation, perstraction, ozonation, or oxygenation [98–101]. All these technologies can be equally developed with polymer or ceramic membranes, although the very well-defined porosity and the extended range of operating conditions for ceramic porous materials are of specific interest in many applications of membrane contactors and distributors. Membrane emulsification is a relatively new technique with specific advantages (simplicity, potentially less energy demands, less surfactant, and narrow droplet-size distributions) compared to conventional emulsification techniques [102]. Depending on the membrane hydrophilicity=hydrophobicity and the composition of the two liquid phases, O=W, W=O, or MW emulsions may be produced. Most often used, O=W membrane emulsification consists of the pressurization of oil (dispersed phase) through membrane pores at high pressure (Figure 6.26). The oil jet flows formed in the circulating continuous phase are

Membrane

Feed side

Permeate side

Oil droplet Membrane pore

Oil phase

FIGURE 6.26

Continuous phase

Schematic representation of cross-flow membrane emulsification.


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ruptured to calibrated droplets by action of shearing force. The pore size of membrane and pressure determine the droplet size of the emulsion at the same time. It has been established that for producing O=W emulsions with droplet sizes of about 1 mm, membranes must exhibit a nominal pore size in the range 0.2–0.8 mm with a limitation on applied transmembrane pressure. Therefore there is a strong economical limitation because of the low membrane fluxes (a few liters per hour and square meter) expected for such operating conditions. Recently, a two-stage ceramic membrane jet flow emulsification process has been proposed able to solve the paradox between the flux and droplet size [103]. In this process, a coarse emulsion of oil droplets (size ~50 mm) is prepared on the first membrane stage at a pressure allowing a high membrane flux (J > 100 L=m2 h). Droplets result from thin streamlines of oil formed under jet flow, which are then ruptured by action of shearing force along the membrane. Then the coarse emulsion is pressurized on the second membrane stage (pore size >1 mm). Normally, at low pressure when oil droplets are larger than pore size, the oil phase is separated from water (see Section 6.3.3.2). In the present case, due to the combination of high pressure (250 kPa) and high membrane flux (J 100 L=m2 h), the droplets are pressured through the membrane pores resulting in the formation of a monodispersed emulsion with micron size droplets. In fact, the second stage looks like a multiple emulsification process. A wide range of application areas of membrane emulsification have been developed from simple food spreads to the manufacture of complex colloidal assemblies [99]. This method affords the production of a variety of structured particulate materials by means of sequential secondary reactions=processes in the emulsified droplets, such as polymerization, gelation, evaporation, freeze-drying, solidification, crystallization, and droplet swelling. For example, nonaqueous droplets containing a monomer mixture can be converted to polymer microspheres by heat-induced polymerization. Besides, the oil droplets can be made up of a high melting-point oil and can be transformed into solid particles by cooling. Similarly the hot droplets of a metal solder can be solidified to produce fine solder powder. Derived from membrane emulsification, membrane micromixing and dispersion=precipitation have been described as an innovative method to produce unagglomerated inorganic spherical nanoparticles with a narrow size distribution [104]. In this last example, barium sulphate particles (widely used in pigment, printing ink and medicine) with diameters in the range of sub-200 nm have been produced using a low-energy consumption method developed from the combination of direct precipitation method with an inorganic membrane dispersion technology. Another important application area for membrane contactors consists of gas–liquid absorption or liquid–liquid extraction (LLE) processes. For example, ozonation is applied in several environmental engineering processes, such as disinfection of drinking water, oxidation of micropollutants, or removal of odor, color, and particles. Operational disadvantages of conventional ozonation processes are an inefficient control of mass transfer because of independent contact between gas and liquid phases resulting in flooding, uploading, emulsion, and foaming. The utilization of membranes for ozonation of liquids affords decisive advantages over conventional processes: (1) much higher interfacial area per volume; (2) higher mass transfer coefficient; (3) easy recycling of O2=O3 gas mixture back to ozone generator; (4) easly control of ozone gas concentration at liquid–gas interface [105]. Ceramic membranes are superior to polymer ones in this application because of the much better resistance of ceramic material to ozone, a strong oxidant agent. However, to be competitive, ceramic membranes that are hydrophilic by nature, should exhibit a stable hydrophobic surface in this case, to provide high gas transfer rates [106]. Membrane contactors can be also proposed as an alternative to conventional LLE, with the implementation of techniques such as membrane extraction and in a less extent membrane pertraction. These techniques can avoid specific problems of LLE, e.g., solvent loss, emulsion formation, and complex phenomena resulting from the physicochemical instability of the organic– aqueous phase interface [107]. Two other techniques based on a membrane contactor design should be mentioned: (i) osmotic evaporation, which is based on a water vapor pressure difference induced by the difference in the water activity of two aqueous solutions, separated by a porous hydrophobic membrane [108], (ii) membrane distillation in which transport depends upon the vapor pressure gradient caused by the difference in temperature across an hydrophobic membrane [109]. Nevertheless, the utilization of ceramic membranes as contactors for all these applications will require the development of ceramic membranes with a sable hydrophobic surface and a narrow pore size distribution [110].

6.4.2 6.4.2.1

GAS

AND

VAPOR SEPARATION

Gas Separation

In most of the literature, gas separation using ceramic membranes, including carbon membranes, is considered to be very promising. However, there are still economical and technical limitations to the industrial development of gas separation processes based on inorganic membranes. Most often, the synthesis of ceramic gas separation membranes is a tricky and not reproducible process with the presence of residual defects and pinholes in the membrane top-layers, which induces high production costs. Accordingly, most of R&D projects in this area are still focused on the membrane as a material and much less on the membrane process, so that insufficient data are available on the real possibility of developing large-scale applications. Anyway, the last decade has seen significant progress in microporous and dense ceramic membranes exhibiting fascinating gas permeation and separation properties [7]. These inorganic membranes able to carry out gas separation at elevated temperatures are potentially useful in emerging areas such as catalytic reactors for gas oxidation and hydrogenation reactions, natural gas or




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biogas processing, power generation using fuel cells, and hydrogen production from different fuel sources. There are three main categories of porous inorganic membranes currently investigated for gas molecular separation [111], e.g., silica-based, zeolite, and carbon membranes, while dense gas separation membranes refer to polycrystalline materials in which gas molecules permeate as ionic species [25]. Hybrid organic–inorganic membranes have also been mentioned to having potential application in gas processing, in particular, for natural gas processing and the production of high-density hydrogen without any impurities [112]. Since the very early publications dealing with gas separation using inorganic membranes, a large part of the studies have been based on silica and its derived materials. Silica microporous membranes have been extensively studied and most often synthesized by the solgel process [113–115]. These membranes are quite stable when used in dry atmosphere, but gas permeance decreases drastically in humid conditions due to silica network rearrangement and subsequent formation of a more tight structure. Silica-based inorganic membranes selectively separate hydrogen from other gases, but permselectivity between similar sized molecules, such as oxygen and nitrogen, is not sufficient [116]. Different ways have been explored for increasing performance of silica membranes, for example, by improving the quality of support (smaller pores and smoother surface) as well as coating of silica films under clean-room conditions [116,117], by the templating solgel approach [118–122], or by CVD-based preparation methods [14,15]. The hydrophobic modification of pore surface has also been used to prevent interaction with water [123]. In other respects, we can consider zeolite membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico–aluminate structures with different alumininum=silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeolites are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeolite membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. Besides the aforementioned crystalline or amorphous microporous ceramic membranes, molecular sieve carbon (MSC) membranes have been identified as very promising candidates for gas separations [125,126]. Ordinarily these membranes are prepared by pyrolyzing thermosetting polymers and exhibit pore diameters of 3–5 Å. They have ideal separation factors for various combinations of gases ranging from 4 to greater than 1000. Carbon membranes can be prepared as supported and unsupported membranes. They have been produced under different membrane geometries: flat or tube for the former, and flat, hollow fiber, or capillary for the latter. Carbon membranes have strong application potential for hydrogen recovering from waste gases or for alkenes=alkanes separation. However, MSC membranes are very brittle and fragile, requiring careful handling. When they are operated with non-purified gases, they require a pre-purifier step for removing traces of strongly adsorbing vapors, which can clog up the pores. This problem may be avoided by operating at high temperature, but carbon membranes are not stable at an elevated temperature under oxygen or steam atmospheres. The last category of inorganic membranes used for gas separation is constituted of dense materials including metals and ceramics [127]. Metallic membranes are not considered as belonging to ceramic membranes so that its related applications will not be discussed in detail. Ordinarily these membranes are mainly made up of Pd-based alloys and are used for hydrogen separation and hydrogenation or dehydrogenation reaction control [128–130]. Metallic membranes exhibit high hydrogen separation selectivity but most of the used metal alloys suffer from instability and must be used in controlled atmosphere and temperature range. Many basic research projects on this topic are still in progress [131,132]. However, metallic membranes have been already used at an industrial scale, for example, in semiconductor industry to produce ultra-high purity hydrogen [133]. The second class of dense membranes thoroughly investigated for gas separation involves ceramic oxide materials exhibiting high solid-state diffusion rates of oxygen ions and protons at high temperature [25,134]. At this time, the development of inorganic proton-conductive membranes able to work at intermediate and high temperatures, in absence of water, is very challenging in many industrial sectors such as hydrogen separation in gas processing, hydrogen or direct methanol fuel cells, electrochemical treatment of aqueous solutions, electrosynthesis, or advanced ceramic membrane electrolyzers for hydrogen production [135,136]. For comparison, the characteristics, permeation, and selectivity of these different categories of gas separation membranes are summarized in Table 6.2. By nature, mesoporous membranes exhibit high permeation flux, but low selectivity based on the Knudsen diffusion mechanism. On the contrary, permeate flux for microporous membranes is at one order of magnitude lower with higher separation selectivity than for the mesoporous membranes, but which strongly depends on temperature [137]. In other respects, for defect-free dense ion-conducting membranes the selectivity is infinite as only oxygen ions or protons can migrate through the crystalline structure [25]. 6.4.2.2

Pervaporation and Vapor Permeation

Pervaporation and vapor permeation are typical membrane processes with high application potential in chemical industry due to their high efficiency in the separation or the dehydration of organic solvents. Developed initially with organo-polymeric



168


TABLE 6.2 Gas Separation Characteristics of Inorganic Membranes Membrane Type

Pore Size Range

Mesoporous ceramics Mesoporous glass Microporous oxides Microporous carbons Zeolites Ion conductive ceramics a

~4 nm ~4 nm dpore, the transport mechanism is Knudsen flow and the molar flux Nik from Equation 19.37 can be given as [39,61] Nik ¼

Dki DPi RT d

(19:41)

where Dki is the Knudsen diffusion coefficient, which is given by Dki

2r« 8RT 0:5 ¼ 3t pMi

(19:42)

when dpore is much larger than l, and the stagnant air is trapped within the membrane pores, then the transport of vapor through the membrane is by molecular diffusion. The DGM then gives the well-known diffusion through a stagnant film equation. Ni ¼

K1 PDw-air T b1 Dpi Tb Rd Pam

(19:43)

where Pam is the log mean of air pressures at the feed and permeate vapor–liquid interfaces. The temperature has been separated into two terms Tb1 and Tb and b ¼ 2.334. When the average membrane pore size is comparable to the molecular mean free path length (l), that is dp l, the surface and molecular diffusion can be neglected and in such case the general form of DGM falls into Knudsen–Poiseuille transition region and can be written as [61] Piavg DP 1 Dpi þ B0 K0 v Ni ¼ d m d RTavg

(19:44)

where P is the total pressure Dpi is the gradient in the partial pressure of ith species Tavg is the average temperature Schofield [49] has shown that in DCMD applications, the net flux of air across the membrane is extremely small relative to the flux of water, and viscous flux can be neglected (unless the process solutions have been degassed). With these assumptions Equations 19.36 and 19.37 reduce to the Knudsen–molecular diffusion transition form and can be written as [43] Nw ¼

1 Dpw 1 pa þ RTd K0 v K1 PDw-air

(19:45)

where Dw-air is the ordinary water diffusion coefficient and can be calculated from Equation 19.46 [43,52] PDw-air ¼ 4:46 106 T 2:334

(19:46)

From Equations 19.37 and 19.38 another flux Equation 19.47 can be written for water vapor, where the Knudsen and molecular diffusion, and viscous flow are considered, while surface flow was neglected. N¼

M K0 K1 Dwa v B 0 nw DPT Dnw þ m 1 þ (xa K0 v=K1 Dwa ) d K1 Dwa þ xa K0 v

where nw is the mole concentration of water vapor Dnw is the water vapor mole concentration difference across the membrane xa is the mean mole fraction of air, and other parameters as defined earlier Mass transfer through boundary layer is explained in Section 19.3.5.

(19:47)



525

Membrane Distillation in Food Processing

19.3.1.2

Comparison of the Models

Schofield showed in his model Equation 19.30 (or Equation 19.23) that T / PDP for viscous flux and J / DP for Knudsen flux, and obtained a correlation Jm ¼ aPbr DP that approximates the Knudsen–Poiseuille transition of DGM. However, the Schofield model has the advantage that the exponent b indicates the extent of Knudsen diffusion and Poiseuille flow contributes to the permeability, while such approximations are not possible from K0 and B0 used in the DGM. Schofield tested his model on membranes having different pore sizes ranging from 0.10 to 0.45 mm and estimated the values of b in a range of 0.1–0.6, which suggest that both the mechanisms (Knudsen and viscous) play an important role in MD flux. On the other hand, Schofield’s model has two main disadvantages compared to the DGM model. One is, the components a and b are dependent upon the gas used. The other is, reference pressure Pref is chosen in such a way that the dimensionless pressure becomes close to unity (Pr 1), and a is evaluated at Pref, hence the parameters a and b also depend on the reference pressure chosen. Further, Imdakm and Matsuura [61] have developed a Monte Carlo simulation model to study vapor permeation through membrane pores in association with DCMD, where a three-dimensional network of interconnected cylindrical pores with a pore size distribution represents the porous membrane. The network has 12 nodes (sites) in every direction plus boundary condition sites (feed and permeate). The pore length l is assumed to be of constant length (1.0 mm), however, it could have any value evaluated experimentally or theoretically [62]. The mass balance for each node of the entire network, assuming the transport of pure substance (water vapor only) can be written as [61] X

Cij DPij ¼ 0

(19:48)

j

where Cij is the mass conductance coefficient in the ijth pore, for viscous flow transport Cij is defined as Cijv Ppavg pr 4 8RTpavg ml

(19:49)

0:5 2r 3 8p 3l RTpavg Mi

(19:50)

Cijv ¼ Similarly for Knudsen transport mechanism Cijk Cijk ¼

where m is the fluid viscosity DPij is the pressure drop across the pore Ppavg and Tpavg are the average pore pressure and temperature, respectively Once the pore size and length l are given to the pore network, one can calculate the effective pressure field (by using iteration method), the temperature field through the network, and its effect on the vapor flux through the membrane. This model takes into account all molecular transport mechanisms based on the kinetic gas theory for a single cylindrical tube and could be applied to all forms of membrane distillation process [61]. 19.3.1.3

Process Parameters

The effect of parameters such as concentration, operating temperature, and flow rate on permeate flux are generally considered for the optimization of DCMD. Permeate flux decreases with an increase in feed concentration. This phenomenon can be attributed to the reduction of the driving force due to decrease of the vapor pressure of the feed solution and exponential increase of viscosity of the feed with increasing concentration. The DCMD flux gradually increases with an increase in temperature difference between feed and cooling water. Lagana et al. [63] reported that the viscosity of apple juice at high concentration induces severe temperature polarization. It may be noted that temperature polarization is more important than concentration polarization, which is located mainly on the feed side. In DCMD, increase in flow rate increases the permeate flux. The shear force generated at high-flow rate reduces concentration polarization. Banat [64] found that the flow rate of cooling water had minimal effect on the permeate flux. Ohta [65] has shown that an increase in coolant velocity from 0.02 to 0.08 m s1 resulted in 1.5-fold increase in the permeate flux. In the same study, it was found that an increase in velocity of hot feed increased the flux by twofold.



526


Cold surface

Membrane

Hot solution or feed

FIGURE 19.6

Water condensation

Air gap

Cold water

Schematic representation of AGMD.

19.3.2 AIR GAP MEMBRANE DISTILLATION In air gap membrane distillation (AGMD), an air gap is interposed between the membrane and the condensation surface (Figure 19.6). In this configuration, water evaporates on the feed surface of the membrane, transports through the membrane pores, and condenses on a cold surface, which is separated from the membrane by an air gap. Carlsson [66] was the first to describe the AGMD in 1983, subsequently; more studies were carried out on AGMD and its applications [26,33,67–71]. In AGMD, the presence of air gap acts as a thermal insulation between membrane and condenser wall, and reduces considerably the heat loss caused by conduction and temperature polarization, improving the separation efficiency. The main advantage of AGMD over DCMD arises from the possibility of condensing the permeate vapors directly in a cold surface rather than in a cold liquid. Additionally, in AGMD the permeate boundary layer is absent thus reducing the overall mass transfer resistance. 19.3.2.1

Heat and Mass Transfer

The stages of heat transfer in AGMD (Figure 19.7) include heat flux from the feed boundary layer to the membrane surface, vapor permeation through the membrane, and the diffusion in air gap, then condensation at the cold surface and finally heat transfer to the cooling water. Heat flux from the feed boundary layer to the membrane surface can be written similar to Equation 19.11: Qf ¼ h1 (Tf Tsl )

(19:51)

where h1 is the liquid film heat transfer coefficient that can be calculated from Equation 19.21 for laminar flow and Equation 19.13 for turbulent flow. Tf Ts2

Ts1

Ts4

Ts6

Ts3 Membrane

Ts5 Feed or hot solution

Air gap Cold water

Cooling plate

FIGURE 19.7

Temperature profile in AGMD.



527


The heat transfer through membrane material takes place by conduction and the latent heat transferred by vapor flow through the membrane, which is small and could be neglected. The equation for heat transfer through the membrane can be written as Qm ¼

km (Ts1 Ts2 ) d

(19:52)

The vapors flow from the cold surface of the membrane to the condensation surface through the air gap. Natural convection is very small and could be neglected [72], and only heat transfer (Qa) by conduction across the air gap is considered, which is given by Qa ¼

kg (Ts2 Ts3 ) (dg dc )

(19:53)

where kg is the thermal conductivity of the air dg and dc are the air gap and condensate film thickness, respectively The condensate film thickness can be calculated by [51] dc ¼

3mmc pDt r2f g

0:33 (19:54)

where m is the flow rate of condensate mc is the viscosity of condensate film Dt is the plate thickness rf is the density of condensate film The heat transfer by condensation of a pure vapor on a solid surface at constant temperature is given by Q4 ¼ h1 (Ts3 Ts4 )

(19:55)

where Ts3 is the dew point temperature of the vapor and h1 is the film heat transfer coefficient, which can be obtained from Equation 19.56 (for vertical walls) [52] 3 2 0:33 k r dpg h1 ¼ 1:333 3mm

(19:56)

Heat transferred by conduction through the cooling plate and further by convection to the cooling fluid can be calculated from equations similar to Equations 19.52 and 19.51, respectively. The mass transfer studies in AGMD suggest that, the transport of vapor through the membrane can be modeled by molecular diffusion mechanism [57], since the pore size of the membrane used is much bigger than the molecular mean free path of water vapor. Equations 19.23 and 19.27 can be used to calculate the mass transfer through the membrane. 19.3.2.2

Process Parameters

The process parameters influencing the water vapor flux and energy efficiency are the temperature difference between hot and cold solutions, flow velocities of feed and permeate, air gap pressure, air gap width, and membrane type. The permeate flux is strongly dependent on the temperature. An increase in inlet temperature of the hot solution from 408C to 808C increases the flux by nearly an order of magnitude and the thermal efficiency by 12%, while decreasing the coolant temperature has a lesser effect on the flux increase and even slightly reduces the efficiency [72]. The hot solution temperature has a greater influence because of the exponential increase of the partial pressure of the vapor with the temperature, so the driving force is greater at higher temperature. The permeate flux increases with increasing feed velocity due to the reduction of the boundary layer thickness. Therefore, the temperature and concentration at the liquid–vapor interface are approaching values close to those in the bulk solution. Garcia et al. [71] have observed that the feed pressure measured at the inlet of the feed cell frame increased with an increase in the feed flow rate consequently increasing the risk of membrane wetting.



528


The air=vapor gap width is one of the important parameter that plays a major role in AGMD. The major drawback of the air gap is that it creates an extra resistance to mass transfer, which can lower the flux about eight times (for a 5 mm air gap) compared to DCMD under similar conditions [73]. The literature overview of flux obtained so far in flat sheet AGMD [10,50,70,71,73,74] demonstrates that the flux increases considerably by decreasing the air gap width. Alklaibi and Lior [72] and Jönsson et al. [28] observed 2.6-fold increase in the permeate flux as the thickness of the gap is reduced. The increase in permeate flux can be explained by the fact that the air gap conductivity is very low relative to the other regions between the hot and cold solutions. The resultant high temperature and consequent vapor pressure drop across membrane increase flux. Jönsson et al. [28] estimated the optimal air gap width to be 0.2 mm, resulting in large water vapor flux and low-energy loss. However, the practical value of this air gap width has never been tested (because the available space 0.2 mm air gap is quite small for condensed water vapor). Alternative methods to increase the vapor flux, other than reduction of air gap width are reduction in air gap pressure and use of ultrasound technique. Gostoli et al. [50] have showed that decreasing the air gap pressure from 1000 to 400 mbar has increased the flux by 2.4-fold. Reduction of the air gap total pressure from atmospheric to the saturated water vapor pressure of the hot water flow raises the flux by a factor of 2.5–3 in hollow fiber module [75]. Zhu [70] has shown that the permeate flow rate could be increased up to 25% by ultrasonic stimulation. In practical applications of the membrane distillation processes the most important and crucial issue is heat economy, which is determined by cell design and eventually, the operating parameters. Calculations from the literature [28,69,74] claim theoretical energy efficiencies in the range of 70%–99% for AGMD. In other words 70%–99% of energy drop of the hot water flow is used for evaporation of water and that 1%–30% is lost by heat conduction across the membrane and the air gap. Compared to DCMD, AGMD is expected to be more energy efficient especially at lower feed temperatures. Alklaibi and Lior [72] speculate that air gap width exceeding 2 mm does not raise process efficiency; however, Guijt [75] has shown that the theoretical energy efficiencies of 95%–98% can be achieved in a hollow fiber module of air gap width 3 mm, which could be explained by a small heat loss to the surroundings.

19.3.3 VACUUM MEMBRANE DISTILLATION Vaccum membrane distillation, such as any membrane distillation process, is a thermally driven process in which a feed solution is bought into contact with one side of a microporous membrane and a vacuum is created on the opposite side to create a driving force for transmembrane flux (Figure 19.8). The microporous membrane only acts as a support for a vapor–liquid interface, and does not affect the selectivity associated with the vapor–liquid equilibrium [76]. The experimental procedures are quite similar to and often confused with pervaporation. The main difference between VMD and pervaporation is the nature of the membrane used, which plays an important role in the separations. While VMD uses a porous hydrophobic membrane and the degree of separation is determined by vapor–liquid equilibrium conditions at the membrane–solution interface, pervaporation uses a dense membrane and the separation is based on the relative perm-selectivity and the diffusivity of each component in the membrane material. 19.3.3.1


Feed or hot solution

FIGURE 19.8

Schematic representation of VMD.

Membrane

In VMD, the conductive heat transfer across the membrane is very low, mainly due to the low pressure on the permeate side of the membrane and could be neglected. Thus latent heat of vaporization is the only mode for heat transfer to be considered through the membrane [17,47,77]. Equation 19.3 can be used for calculating the rate of heat transfer across the membrane.

Vacuum



529


The heat transfer across the boundary layer present only on feed side (turbulent flow) can be calculated using Equations 19.13 through 19.21, based on the hydrodynamic conditions. In VMD, the pore size of the membrane falls in the range of 0.1–0.2 mm [17,78]. The mean free path of the diffusing species is usually larger than the pore size of the membrane and under such conditions; Knudsen diffusion is the only mechanism for mass transfer through the membrane and the molecular diffusion resistance can be neglected because the molecular diffusion resistance is proportional to the partial pressure of air in the membrane pores. Since VMD generally operates at total pressures in the order of 10–50 kPa, which is below the vapor pressure of the diffusing species, only trace amounts of air can exist within the membrane pores [77]. Thus, VMD is Knudsen diffusion controlled for the membrane with relatively small pores (r l). In this case, the expression is derived based on the kinetic theory of gases or more precisely the DGM Equations 19.26 and 19.23 can be used to estimate the mass transfer across the membrane. However, VMD membranes typically have pores that are comparable to mean free path (l), consequently the more complete Knudsen– Poiseuille transition Equation 19.44 is more applicable [77]. 19.3.3.2

Process Parameters

Feed temperature, flow rate, and solute (nonvolatile) concentrations are some of the parameters commonly studied in VMD. Mengual et al. have observed an Arrhenius type of dependence of the permeate flux on the feed temperature. An increase in the feed circulation velocity increases the heat transfer coefficient in the liquid boundary layer, which in turn increases the VMD flux due to the reduction in the temperature polarization. Concentration factors increased with a decrease in feed temperature during VMD, and for a decrease of 308C to 108C, increase in concentration factors from 7–15.5 to 21–31 were obtained for a highly volatile black currant aroma ester [17]. One of the benefits of VMD relative to the other MD configurations is that conductive heat loss through the membrane is negligible. This enables estimation of the boundary layer heat transfer coefficient. Care must be taken in VMD to prevent membrane wetting, because DPinterface is typically higher in VMD than in other MD configurations.

19.3.4 SWEEPING GAS MEMBRANE DISTILLATION In this case when the water molecules are collected with the help of a sweeping gas, the process is named SGMD (Figure 19.9). The survey of literature [24,79–82] suggests that very little work has been done with regard to SGMD, although this configuration has the advantages of a relatively low conductive heat loss with a reduced mass transfer resistance. This is probably due to the fact that a very small volume of permeate is vaporized into a large volume of sweep gas and has to be collected in an external condenser, which is not an easy task. Basini et al. [80] were the first to study this configuration for the desalination of water. Later it was employed for the concentration of sucrose solutions and the separation of ethanol–water mixtures [83]. In SGMD, a hot feed solution is circulated on one side of a microporous membrane and cold sweep gas on the other side of the membrane. The temperature difference gives rise to a water vapor pressure difference, and consequently, to a water flux through the membrane.

Membrane

Feed or hot solution

Sweeping gas

FIGURE 19.9

SGMD configuration of MD.



530


The SGMD configuration has a great scope for the future due to the advantages mentioned above. SGMD provides much higher permeate fluxes than AGMD while maintaining high temperature polarization coefficient and evaporation efficiency [80–84]. The apparent advantages of SGMD over DCMD are better selectivity, smaller temperature polarization, and higher evaporation efficiency, albeit a low permeate flux [84]. 19.3.4.1


The SGMD is a temperature driven process, and it involves: (a) evaporation of water at the hot feed side, (b) transport of water vapor through the pores of hydrophobic membrane, (c) collection of the permeating water vapor into an inert cold sweeping gas, and (d) condensation outside the membrane module. A decrease in driving force has been observed due to polarization effects of both temperature and concentration [80,82]. To calculate both heat and mass transfer through microporous hydrophobic membrane as well as the temperature and concentration polarization layer, the theoretical model suggested by Khayet et al. [58] can be written as h i ma ma N 2 þ (win þ 0:622) þ B(P pv (Cf ,Tf )) N þ B [Pwin pv (Ta )(win þ 0:622)] ¼ 0 A A

(19:57)

where N is the mass flux win is the inlet humidity ratio ma is mass flow rate of air A is the membrane area P is the total pressure pv is the vapor pressure Cf is the solute concentration in feed B is the net membrane coefficient Tf and Ta are the feed and air temperatures, respectively In Equation 19.57, the net membrane coefficient B can be calculated by using Equation 19.45, because the combined Knudsen molecular diffusive flux is responsible for the transport in SGMD [58,81,82], while the temperature of the feed Tf at the membrane surface can be obtained from

Tf ¼

Tba þ hhaf Tbf þ hf Tbf NlL h i «k «Kfþ(1«)km m þ hf 1 þ gþ(1«)k d dha

«kg þ(1«)km d

(19:58)

where Tbf is the temperature in the feed (bulk) solution Tf is the temperature at the feed membrane surface Tba is the temperature in the permeate bulk solutions, respectively lL is the latent heat of vaporization kg and km are the thermal conductivities of the gas in the membrane pores and membrane matrix, respectively In Equation 19.58, the heat transfer coefficients hf and ha can be determined using Equations 19.17 and 19.19 (Table 19.1). 19.3.4.2

Process Parameters

In SGMD, the temperature of the liquid feed and the circulation velocity of the sweeping gas are very important parameters. To avoid membrane wetting, it is important that the circulation velocity of the air needs to be varied with caution such that pressure difference between both fluids needs to be lower than the liquid entry pressure of water (LEPw) and the air pressure must be lower than the liquid pressure. The increase in temperature of the feed and the circulation velocity of the sweep gas increases the flux; mainly due to the increase in vapor pressure difference (the driving force). In general, SGMD is the most difficult to describe, since in the downstream compartment, unlike in other MD configurations, none of the variables remain constant along the module. All temperatures, concentrations, as well as heat and mass transfer rates change during passage of gas within the module. Gas entering the cold chamber contains low permeate vapor, with a temperature of cooling water. As it



531


sweeps the membrane, condensable vapors and heat are accumulated in the gas phase changing its temperature, concentration, and flux. Khayet et al. [79] have observed an increase in the sweep air temperature by 208C for a module length of 6.5 cm. The heat transferred through the membrane increases the sweep gas temperature along the module length, which results in a progressive increase in partial pressures of the gas. Consequently the driving force decreases rapidly, making the process impracticable at an industrial scale. In order to overcome these problems, Rivier et al. [84] have developed a modified SGMD configuration called thermostatic sweeping gas membrane distillation (TSGMD), in which, a cold wall is added in a downstream compartment to attenuate the increase in sweep gas temperature. Hence the driving force along the module could be enhanced. In TSGMD, an inert gas circulates at more or less constant temperature in the cold chamber and collects the vapors that permeate through the membrane. Part of these permeating vapors condenses inside the module depending on the operating conditions, and the rest in an external condenser.

19.3.5 OSMOTIC MEMBRANE DISTILLATION Osmotic membrane distillation is one of the membrane distillation variants that operate at ambient temperature and atmospheric pressure. In OMD, a microporous hydrophobic membrane separates the two aqueous solutions, namely, feed and osmotic agent (OA) having different solute concentrations (osmotic pressure). The driving force in OMD is vapor pressure difference across the membrane. Water evaporates from the surface of the feed solution having higher vapor pressure, diffuses through the membrane in the form of vapor, and condenses on the surface of a solution with a lower vapor pressure. This results in the concentration of feed and dilution of osmotic agent solution. Like any other membrane distillation, in OMD the membrane material is also hydrophobic, so that water in liquid form cannot enter the pores unless a hydrostatic pressure exceeds the LEPw [37]. In the absence of such pressure difference, a liquid– vapor interface is formed on either side of the membrane pores. In some aspects membrane distillation and OMD can be considered as closely related, although there are some remarkable differences between them. In both cases, it is necessary to maintain a vapor pressure difference across the membrane pores to get a difference in water chemical potential. However, the means of obtaining this vapor pressure difference is different in both the cases. Whereas it is a temperature difference in the case of membrane distillation, it is a concentration difference in the case of OMD. OMD offers major advantages in comparison with the conventional thermal concentration techniques. The low temperature employed can help avoid chemical or enzymatic reactions associated with heat treatment [85] and prevent degradation of flavor, color, and loss of volatile aroma [38]. The low-operating pressure (atmospheric pressure) results in low investment costs, low risks of fouling, and low limits on compactive strength of the membrane. Since the separation is based on vapor–liquid equilibrium, only volatile compounds which can permeate the membrane and the nonvolatile solutes such as ions, sugars, macromolecules, cells, and colloids are totally retained in the feed. These factors make OMD an attractive alternative to traditional thermal routes currently used for concentration of liquid foods or aqueous solutions of thermally labile pharmaceutical products and biologicals [86]. 19.3.5.1


The water transport in OMD is a simultaneous heat and mass transfer process. Evaporation cools the feed and condensation warms the brine solution. This results in a temperature gradient across the membrane, which adversely affects the driving force and in turn the mass flux. The heat transfer through the membrane can be explained by the convective and the conductive component related to the temperature difference. Thus the expression for rate of heat transfer through the system can be calculated using Equation 19.9. Further, Gostoli [87] attempted to integrate the thermal effect in the mass transfer by introducing an efficiency coefficient, h, which represents the fraction of the driving force really effective for mass transfer through the membrane. This is analogous to MD temperature polarization. The value of h was reported to be 0.85 and the permeability was observed to be 15% lower than the experimental values for NaCl and water system with stirred membrane cell where concentration polarization was neglected. The heat transfer coefficients of the two compartments of the OMD system could be estimated using a friction factor, by combining the experimental pressure loss to the theoretical value that would be obtained in the module having straight shaped circulation channels. The basic model used to describe the water transport in the system that relates the mass flux (J) to the driving force is given by J ¼ K(p1 p2 )

(19:59)

where K is the overall mass transfer coefficient that accounts for all three resistances (feed, membrane, and OA side) for water transport.



532

19.3.5.2


Mass Transfer through the Membrane

Diffusion is the main mechanism involved in the mass transfer during osmotic distillation and the resistance to mass transfer comes from both membrane structure and presence of air trapped within the membrane pores. While the former resistance can be described by Knudsen diffusion Equation 19.26, the latter is described by molecular diffusion Equation 19.27. The DGM equations can also be used to find out the mass transfer across the membrane. Since the DGM was derived for isothermal conditions, it would be appropriate to use it for an isothermal membrane process such as OMD. It has been shown that the DGM could be applied to a non-isothermal system also by the inclusion terms for thermal diffusion and thermal transpiration [39,88]. In DCMD, AGMD, VMD, and SGMD these terms are negligible, hence Tavg has to be used in place of T in the DGM equations. Since OMD is generally carried out at ambient temperature and atmospheric pressure, using membranes with pore size ranging from 0.1 to 1 mm, so it can be assumed that, the main transport mechanisms present in this case is Knudsen and molecular diffusion, and the Equation 19.45 can be used to calculate the mass transfer through the membrane. 19.3.5.3

Mass Transfer through Boundary Layers

Boundary layers of concentrated feed and brine solution are present on either side of the membrane. This results in significant resistance to mass transfer that cannot be neglected. In order to characterize mass transfer in the boundary layers, it is necessary to determine the respective mass transfer coefficients. These coefficients depend on the properties of the solutions and on the hydrodynamic conditions of the system. Such coefficient can either be obtained by experiments or be estimated with the help of empirical correlations of dimensionless numbers. The majority of the correlations referred to in the literature for various hydrodynamic conditions have the same general form. These include Sherwood number (Sh), which contains the mass transfer coefficient, as a function of the Reynolds number (Re) and Schmidt number (Sc) [89–91]. General mass transfer correlation can be written as Sh ¼ aReb Scg Sh ¼

kb rNd 2 m , Sc ¼ , Re ¼ m Dw rDw

(19:60) (19:61)

where Dw is the water diffusion coefficient a, b, and g are constants and are to be selected appropriately for the given hydrodynamic conditions [43,92] In the case of a noncircular flow channel, for instance a square channel, an equivalent diameter is given by de ¼ 4S=LP where S is the cross-sectional area and LP the wetted perimeter length of the flow channel [41]. 19.3.5.4

Process Parameters

Osmotic agent, flow rate, membrane pore size, and feed concentration are some of the key parameters associated with OMD. The salt chosen as osmotic agent is, in general, NaCl because of its low cost and nontoxicity [57]. Different salts can lead to remarkably different fluxes mainly due to the different osmotic activity of the salts. In general osmotic pressure difference up to 700 bar can be achieved, however, some authors have reported data for pressures as high as 1500 bar [57]. The commonly used OA in OMD other than NaCl are CaCl2, MgCl2, MgSO4, K2HPO4, and KH2PO4. Potassium salts of ortho- and pyrophosphoric acid offer several advantages, including low-equivalent weight, high water solubility, steep positive temperature coefficients of solubility and safety in foods and pharmaceuticals [93,94]. It has been observed from the literature that transmembrane flux increases with an increase in flow rate. This could be attributed to reduction in the concentration polarization layer. The work carried out by Sheng et al. [95] showed that the transmembrane flux decreased with increase in juice concentration and exhibited a strong dependence on the osmotic pressure difference between the two aqueous solutions. When the difference in osmotic pressure decreased by 33% (from 416 to 280) atmospheric pressure, a fivefold decline in transmembrane flux was observed. As discussed earlier, the pore size plays an important role in influencing the type of diffusion for water transport through membrane. The effect of membrane pore size on transmembrane flux was studied [37,92] and no significant change in flux was observed in the range 0.05–0.2 mm studied. However, from theoretical calculations it was observed that the mode of diffusion (and in turn transmembrane flux) depends on the membrane pore size employed. Naveen et al. [92] have shown the mode of diffusion to be Knudsen when membrane pore size is 0.05 mm and molecular when pore size is 0.2 mm. Brodard et al. [96] who employed ceramic membranes made of alumina having pore sizes of 0.2 and 0.8 mm observed practically no effect of pore size on water flux.



533


19.4 MEMBRANES Membrane characteristics such as porosity, pore size, tortuosity, and membrane thickness dictate the resistance to mass transfer through microporous membranes used in MD [35,97–99]. Furthermore, studies [100,101] have shown that the relationship between intrinsic membrane properties and operating conditions, e.g., swelling, compaction, and wetting, are important in deciding efficient operations. Therefore, selection of an appropriate membrane that poses the least resistance to mass transfer is crucial. Lefebvre [36] has given some criteria for the choice of membranes. Both the Poiseuille and Knudsen models show a large dependence of membrane flux on pore radius. Therefore, it is inimical that membranes used in MD should have a high porosity (60%–80%). Since flux is inversely proportional to the membrane thickness, in OMD it needs to be as thin as possible (0.1–1.0 mm) and should have high heat conductivity to achieve rapid temperature equilibrium near the boundary, while in other variants of MD, such as DCMD, AGMD, SGMD, VMD where temperature gradient is the driving force, membranes should be thicker and should have low thermal conductivity [57]. However, one should not conclude that thick membranes alone are recommended for MD. In fact, with an increase in membrane thickness, the heat resistance increases so also temperature and vapor pressure differences. Similar to OMD, the permeate flux in the DCMD increases with decreasing membrane thickness [102]. The hydrophobic nature of the membrane plays an important role in the performance of MD; a variety of membranes having these characteristics are available. Table 19.2 lists the membranes used by the various researchers till date. This list includes mainly hydrophobic polymers with low surface

TABLE 19.2 List of Membranes Used by Various Researchers in MD Processes MD Configuration

Membrane Module

Membrane Material

Porosity (%)

—

PP PP PP Zirconia Alumina PVDF PTFE PVDF PTFE PP TF–200 TF–450 TF–1000 PTFE PTFE PVDF PVDF (GVHP) PVDF (GVHZ) PTFE PTFE PVDF PP PTFE PTFE PVDF PTFE PVDF PP PP PVDF PTFE PP PP

80 80 80 — — 75 80 80 — 40 80 80 80 55 57 70 62 66 70 90 89 70 70 70 70 80 75 75 75 75

MD

MD

Tubular

MD MD DCMD DCMD DCMD DCMD

Cross-flow cell Flat membrane Flat sheet Hollow fibre Hollow fibre —

DCMD

Flat sheet

DCMD

—

DCMD

Plate and frame

DCMD DCMD DCMD

Flat sheet Sheet membrane —

DCMD

Stirred cell

70

Nominal Pore Size 0.1 0.45 0.2 50 nm 200 nm 0.45 0.45 0.2 0.45 0.03 0.2 0.45 1.0 0.1 0.3 0.2 0.22 0.45 0.2 0.2 0.45 0.22 0.22 0.45 1.0 0.2 0.45 0.1 0.2 0.45 0.025 0.2 0.05

d (mm)

References

60 60 60 —

[103]

110 60 125 120 31 178 178 178 60 60 60 126 116 70 64 77 150 175 175 175 60 110 100 140 110

[105] [106] [107] [63] [108] [109]

150

[111]

[104]

[46]

[48]

[97]

[110] [34] [32]

(continued )



534


TABLE 19.2 (continued) List of Membranes Used by Various Researchers in MD Processes MD Configuration

Membrane Module

DCMD

Flat sheet

AGMD

Plate and frame

Cylindrical (hollow fiber) AGMD AGMD AGMD AGMD AGMD VMD VMD DCMD VMD

— Flat sheet — Flat sheet Tubular Shell and Tube C capillary Hollow fiber

SGMD

Plate and frame

SGMD

Shell and tube capillary

OMD

Lewis cell

OMD

Flat sheet

Membrane Material

Porosity (%)

Nominal Pore Size

TF-200 GVHP PVDF PTFE PTFE PE VA12 PE FA1 UPE test PTFE PTFE PTFE PVDF PP PP PE PP PP PP TF–200 TF–450 PP GVHP (PVDF) FHLP(PTFE) Gelman TF–1000 Gelman TF–450 Gelman TF–200 Gelgard 2400 Gelgard 2500 Accurel IE –PP Accurel 2E –PP Durapel VVSP–PP

80 75 64 90 44 77 70 57 — 85 60 75 — 70 66 53 50 47 80 80 70 70 70 80 80 80

0.2 0.22 0.22 & 0.45 0.22 & 0.45 0.20 0.18 0.21 0.26 0.23 0.1 0.2 0.45 0.2 0.2 0.087 0.074 0.044 0.056 0.2 0.45 0.2 0.2 0.2 1.0 0.45 0.2

— — — — — — — — —

0.19 0.25 0.48 0.54 0.60 0.81 1.08 1.10 1.29

— 55 — 80 60 75 64 70 45 80 80 60 60 80

0.2 0.1 mm 0.1 0.25 0.2 0.2 0.2 0.2 0.05 0.2 0.2 0.20 0.45 0.25

Durapel GVSP–PP

OMD OMD OMD OMD OMD OMD

Plate and frame Stirred cell Plate and frame — Plate and frame Hollow fibre

OMD

Capillary

OMD

Cylindrical semicell

OMD

Flat sheet

MD þ OD

Lewis cell

Goretex L31189 Simitomo Simitomo PP PP PTFE PTFE PTFE PVDF PVDF PP PP=PE PVDF UHMWPE TF–200 TF–450 PTFE

d (mm) 178 125 120 70 180 90 55 250 80 150 60 110 150 — 50 50 65 42 178 178 — 120 175 178 178 178 25 25 90 150 120 120 50 50 50 — 90 mm 100 175 165 125 125 — 28 108 90 165 178 175

References [112] [71]

[75]

[44] [70] [113] [114] [115] [78] [116]

[81] [58]

[37]

[117]

[11] [91] [95] [118] [119] [120] [121] [4] [41] [122]



535


free energies, i.e., polytetrafluoroethylene (PTFE, 9.1 kN m1), polyvinylidene fluoride (PVDF, 30.3 kN m1), polyethylene (PE, 33.2 kN m1), and polypropylene (PP, 30.0 kN m1 surface energy). The survey of literature suggests that the pore size, porosity, and thickness of most of the membranes used in MD fall in the range of 0.2–1.0 mm, 65%–85%, and 0.06–0.25 mm, respectively. The thermal conductivities of polymers are mainly dependent on temperature and the degree of crystallinity of the polymer, which varies in the range of 0.15–0.45 W m1 K1 [123]. Hydrophilic membranes that have been treated to make their surfaces hydrophobic have also been applied successfully to MD [124,125]. The pressure variable can be used in defining wettability in terms of LEP, as represented by the Laplace (Cantor) equation DPentry ¼

2Bg1 cos u rp, max

(19:62)

where DPentry is the entry pressure difference gl is the surface tension of the solution u is the angle of contact between the solution and the membrane surface B is a geometric factor rp,max is the largest pore size Once the pressure drop across the vapor–liquid interface DPinterface exceeds penetration pressure DPentry, the liquid can penetrate the membrane pores and the membrane is termed as wetted. Contact angle measurement and surface tension are the traditional methods to describe the wettability of the membranes. The contact angle comprised of a liquid droplet deposited onto the surface of a smooth solid will have a value higher than 908 if there is low affinity between liquid and solid, and will be less than 908 in the case of high affinity. Wetting occurs at 08, when the liquid spreads onto the surface. In general the solid– liquid angle of contact for the membrane should be at least 908, while a value of 1308 being the optimal for use in MD [39]. Recently, the use of some copolymers of tetrafluroroethylene (TFE) and 2,2,4-trifluroro–5-trifluoromethoxy–1,3-dioxole (TTD), commercially known as HYFLON AD, for fabricating polymeric membranes have provided an increased reliability to the MD process. They have been used for preparing asymmetric and composite membranes that show a high contact angle (1208) for water, demonstrating a highly hydrophobic character [126]. Brodard et al. [96] successfully employed hydrophobic ceramic tubular membranes in the osmotic evaporation process. These ceramic membranes have been obtained by grafting siloxane compounds on alumina porous supports. Ceramic membranes have the advantage of higher physical and chemical stability as compared with polymeric membranes. Barbe et al. [117] have studied nine types of hydrophobic microporous membranes for their influence on the retention of range of volatile organic species. The results of this investigation suggest that membranes having relatively large surface pores have been shown to retain higher organic volatiles per unit water removed than those with smaller surface porosity. However, pores with larger diameters at the membrane surface allow greater intrusion of the feed and OA solutions, leading to an extended boundary layer. This extended layer offers extra resistance for diffusion of volatile components. Further Barbe et al. [127] have studied changes in the surface morphology in hydrophobic membranes. A change in morphology was observed when the membranes were in contact with water as against no change observed when in contact with 30% w=w CaCl2. This was attributed to a lower degree of intrusion of the CaCl2 solution, relative to that of water because of the higher surface tension of the former. Their results also suggested that the larger pores become larger while smaller pores showed shrinkage. Although fouling seems to be of minor importance for MD compared to other membrane techniques, periodic membrane cleaning is necessary. Durham and Nguyen [128] evaluated the effectiveness of several cleaning agents for microporous PTFE and cross-linked acrylic-fluoroethane copolymer membranes. The membranes were cleaned after 2 h of processing of a 21.5% solution of tomato puree, and the change in flux and membrane hydrophobicity measured before and after cleaning. The authors showed that repetitive cleaning and fouling can rapidly destroy certain membranes, an important result in regard to industrial applications.

19.5 MEMBRANE MODULES The choice of membrane module for MD is mainly determined by functional and economic considerations. A large variety of membrane modules, such as plate and frame, spiral wound, tubular, capillary, and hollow fiber have been designed and tested for MD. In plate and frame modules, the membranes are placed (with or without spacers) between two plates. The packing density of this configuration varies between 100 and 400 m2 m3, depending on the number of membranes or cassettes used. Andersson et al. [31] have used a plate and frame configuration for desalination purposes, where various cassettes were stacked together, each consisting of injection molded plastic frames of two membranes. Naveen et al. [92] used a plate and frame membrane



536

Handbook of Membrane Separations Out

Out Feed

OA

In

In

Stainless steel frames

Hydrophobic membrane

Polyester mesh

FIGURE 19.10

Viton gasket

Plate and frame membrane module.

module having a membrane area of 0.016 m2 for the concentration of pineapple and sweet limejuice. The module (Figure 19.10) consists of a polyester mesh, a Viton gasket, and a hydrophobic microporous membrane of pore size (0.05–0.2 mm) supported in between two stainless steel frames. Feed solution and osmotic agent solution were circulated on either side of the membrane in cocurrent mode using peristaltic pumps. Spiral-wound modules have a packing density of 300–1000 m2 m3, which is greater than that of the plate and frame module. This packing density depends upon height of the channel spacers. Koschikowski et al. [129] have used a spiral-wound MD module for desalination experiments, which is assembled by spiral winding of PTFE membrane and condenser foils. It has been reported to have a specific thermal energy consumption of 140–200 kW h m3 of distillate and gained output ratio of 4–6. The use of spiral-wound modules has been proposed by Gore [27] for industrial scale desalination units. Tubular membranes unlike capillaries and hollow fibers are not self-supporting. Hence, such membranes tubes are placed into porous stainless steel, ceramic, or plastic tubes. The diameter of tubular membranes usually varies from 1 to 2.5 cm, with a packing density of about 300 m2 m3. In MD operations, the tubular modules are used either for highly viscous fluids or while operating under high-flow rates. In some cases, tubular modules are used to reduce fouling and concentration polarization phenomena. In order to make SGMD practicable at industrial scale Rivier et al. [84] have used a tubular membrane module (TSGMD) as shown in Figure 19.11. The module is made of glass and has six tubular membranes, each one surrounded by a glass tube of 13 103 m i.d. (inner diameter). The effective membrane surface of the module is 0.019 m2. The feed and sweeping gas are circulated inside and outside the tubular membrane, respectively. Capillary membrane modules consist of a

Retentate Sweeping gas membrane Cold wall

Cooling water

Cooling water

Turbulence promoter

Permeate + sweeping gas Feed

FIGURE 19.11

Schematic representation of TSGMD module.



537

Membrane Distillation in Food Processing Fluid #2 out

Fluid #2 in Distribution tube Cartridge

Hollow fiber membrane Housing Collection tube Baffle

Fluid #1 in

FIGURE 19.12

Fluid #1 out

Liqui-Cel Extra-Flow membrane contractor from CELGARD LLC.

large number of capillaries, which are arranged in parallel as a bundle in a shell and tube configuration. The capillary membranes are self-supporting, and two kinds of module arrangement are possible based on flow of feed or permeate (through the capillary or outside of the capillaries). The packing density is in the order of 600–1200 m2 m3, which is between the tubular and hollow fiber modules. A hollow fiber module is conceptually similar to the capillary module, but differs in dimensions. In this case the diameter of the tubular membrane varies between 50 and 100 mm and several thousand of fibers can be placed in the vessel. The hollow fiber module is the configuration with the highest packing density (with values up to 30,000 m2 m3). Liqui-Cel Extra-Flow (Figure 19.12) is one of the well studied hollow fiber modules for concentration-driven mass transfer. This module consists of several microporous polypropylene fibers, which are woven into a fabric and wrapped around a central tube feeder that supplies the shell side fluid. The woven fabric allows more uniform fiber spacing, which in turn leads to higher mass transfer coefficients than those obtained with individual fibers. The fiber lumen diameter and wall thickness are 240 and 30 mm, respectively [93,130]. The smallest Liqui-Cel modules are 2.5 in. in diameter and contain 1.4 m2 of contact area, while the largest are 10 in. in diameter and offer 130 m2 of contact area by virtue of 225,000 fibers. The large modules can handle liquid flow rates of several 1000 L=min. Some more module configurations are reported for use in MD. Lawson and Lloyd [77] have designed a laboratory-scale MD module as shown in Figure 19.13, where the membrane was sandwiched between the two half-cells, and several hose clamps held the module together. The total area available is 9.7 cm2 and the smooth transitions at the module entrance as well as exit allow achievement of relatively high Reynolds numbers, whereby conventional boundary layer equations are applicable. The module does not require a support in low pressure-drop applications such as DCMD. Wider permeate channels would require a support for VMD experiments. A porous sintered stainless steel material has been used for the gas permeation experiments.

1/4"

1/4"

FIGURE 19.13

Schematic diagram of the MD module.



538

Handbook of Membrane Separations To thermostat

T

TJ MS

PM

MS

PM

M TJ To reservoirs

From thermostat

FIGURE 19.14 Stirred cell. M: membrane, H: membrane holder, MS: magnetic stirrer, PM: propelling magnet, T: thermometer, TJ: thermostatized jacket.

Mengual et al. [37] have used Lewis cell (stirred cell) for their experiments on MD (Figure 19.14). This mainly consists of two equal cylindrical chambers made of stainless steel having a length of 20.5 cm. The membrane was fixed between the chambers by means of a PVC holder. Three viton O-rings were employed to eliminate leaks in the assembly. The membrane surface area exposed to the flow was 27.5 cm2. To investigate the role of heat transfer in OMD and measure the asymptotic temperature difference, Celere and Gostoli [86] used the flat sheet membrane module shown in Figure 19.15. This contains a few flat sheet membranes placed 1 mm apart and supported by mesh-type spacers of 2 mm thickness, leaning against polypropylene walls. One stream (feed) flows between the membranes and other (extractant) flows cocurrently through the spacers. The mass transfer zone of each membrane is 80 mm in breadth and 200 mm in length.

19.6 ENHANCEMENT OF FLUX IN MEMBRANE DISTILLATION In spite of their advantage and potential over conventional processes, membrane distillation suffers from the drawback of low transmembrane flux when compared to RO. In the past few years, additional innovations in module design and new strategies have been explored to reduce concentration polarization and to enhance mass transfer. Some of these attempts are discussed here. Enhancement of transmembrane flux has been shown in OMD of grape juice pretreated by UF [131]. The increase in flux has been attributed to a reduction in the viscosity of the concentrated juice–membrane boundary layer as the result of removal of high-molecular weight biopolymers present in juice. UF is a powerful method for removing natural polymers (polysaccharides, proteins) from fruit and vegetable juices. Lukanin et al. [12] have improved the concept of (UF þ OMD), by enzymatic pretreatment of the apple juice prior to the UF step. Introduction of an additional enzymatic deproteinization step with the pectinase=amylase treatment of apple juice followed by UF has yielded minimal biopolymer content. Such a treatment is found to enhance transmembrane flux during concentration of clarified juice by MD. As in the case of grape juice this has been

F

E

S

M

C

FIGURE 19.15

E

S

Cross section of the MD module. F: feed inlet, C: feed manifold, E: extractant inlet, S: spacers, M: membranes.



539


Pressure inlet

Pressure relief valve

Transparent body Stirring bar

O-ring Membrane

Membrane support Filtrate outlet

Without acoustic field

Feed

Feed

With acoustic field Membrane

Low transmembrane flux

FIGURE 19.16

High transmembrane flux

Stirred membrane cell with acoustic field.

attributed to reduction in juice viscosity. Lower viscosity improved hydrodynamic conditions in membrane channel thereby decreasing concentration and temperature polarization. An ultrasonic technique was applied to AGMD to enhance the permeability for the concentration of various aqueous solutions. In this study, an ultrasonic stimulation of resonance frequency of 20 kHz and power up to 90 W was applied to a flatplate AGMD system of 1 mm PTFE membrane with a temperature difference of 558C. A 200% improvement in the transmembrane flux was observed with an ultrasonic intensity of 5 W cm2. Experimental investigation of the basic mechanism of ultrasonic enhancement of AGMD suggests that, for a continuous ultrasonic stimulation the dominating mechanisms are microstreaming and cavitation [70,130]. Narayan et al. [111] have tried to enhance the transmembrane flux by using an acoustic field (1.2 MHz), by placing the membrane cell over an ultrasonic transducer (Figure 19.16). Their results show an increase in flux by approximately 35% to 98% when compared with control runs (without acoustic field). The increase in flux is due to the fact that acoustic field induces mild circulation currents, which disturbs the hydrodynamic boundary layers of feed and OA solutions, thereby reducing the effect of concentration polarization. On examining the effect of membrane compaction on the membrane permeability, Lawson et al. [100] concluded that the transmembrane flux in MD could be increased significantly up to 11% with relatively small pressure drops 0.2 mm 2. b-glucans of molecular weight >300,000 Da (which, together with those from the first category, represent about 57% of the total b-glucan amount) 3. b-glucans of molecular weight between 30,000 and 300,000 Da (about 10% of the total) 4. b-glucans of small and medium molecular weight—below 30,000 Da (about 30% of the total) Despite the fact that the data available in literature is somewhat controversial, the amount of b-glucans in beer is generally considered to range between 300 and 700 mg=L, from which about 10% can be found in gel form [8]. An extensive review and TABLE 20.1 Main Nonvolatile Components of Beer Category Minerals Carbohydrates

Organic Nitrogen

Phenols

Lipids Glycerol Nonvolatile acids Vitamins

Compound

Concentration (g=L)

Ca, K, Mg, Na, P, and other metals Mono- and oligosaccharides Dextrins (a-glucans) b-glucans Pentosans and others Total True proteins Polypeptides Oligopeptides Amino acids and others Total Monophenols Polyphenols (monomers) Polyphenols (polymers) and others Total

0.5–2 7–13 25–35 0.07–0.5 1.5–3.5 33–44 Traces 0.06–0.2 0.1–0.5 0.02–0.1 0.3–1 0.02–0.06 0.07–0.1 0.02–0.1 0.15–0.35 0.01–0.02 1–3 0.2–1 0.005

Citric, malic, gluconic, etc. B1, B2, B5, B6, B12

Molecular Weight (Da) 150,000 5,000–70,000 1,500–5,000 100 mA=cm2). Moreover, a welldesigned pretreatment is of importance to reduce scaling and fouling. An example of performance of an EDBM pilot plant is reported in Table 21.2 [79]. It concerns the production of caustic soda from an NaCl waste stream from an amino acid production process. Apart from organic components, this salt stream

TABLE 21.2 Performance of the EDBM Pilot Plant Current density Current efficiency (CE)a Total membrane area Specific energy consumption (SEC)b Throughput Potential drop per repeating unit cell HCl concentration Naþ contamination in HCl product NaOH concentration Cl contamination in NaOH product

1000 A=m2 (100 mA=cm2) 60%–70% with respect to Naþ 1.78 m2 2.8 kW h=kg NaOH 1.5 kg NaCl=(m2 h) 2.7 V 5.1 wt% 1150 ppm 10 wt% 178 ppm

Source: Adapted from Engel, D., Lehmann, T.H., Weissland, G., and Picarri, J., Preprints of the Aachener Membrakolloquium, GVC VDI-Gesellschaft Verfahrenstechnik und Chemieingenierwesen, Aachen, 1993. a The CE is defined for each transported ionic species. It is the ratio of the actual molar transport calculated by Faraday’s law with the actual current density j according to CE ¼ F.zi.ni=j with ni the molar flux density (mol=m2 s) and F the Faraday’s constant. b The SEC is given by the relation SEC ¼ DE.I ¼ DE.j.A ¼ DE.(zi.F.ni=CE).A with I the current (A), A the total membrane area (m2), DE the potential difference (V) applied to the EDBM unit.



Developments of Bipolar Membrane Technology in Food and Bio-Industries

593

TABLE 21.3 Some Applications of EDBM Process Pollution control=resource recovery HF=mixed acid recovery

Sulfate recovery

Nitrate recovery Amine recovery Pulp and papers Flue gas desulfurization Chemical processing Organic acid production recovery Ion-exchanger regeneration Brine acidification in chlor-alkali industry Wastewater treatment from amino acid processing Potassium and sodium mineral processing

Sodium methoxide production Methanesulfonic acid production High-purity water production

Stainless steel pickle liquor recovery HF=NaOH recovery from spent aluminum potlinings Fluorosilicic acid conversion to HF, SiO2 Fluoride emission control in chemical processing KF conversion from alkylation process Battery acid recovery Waste sodium sulfate conversion Sodium sulfate conversion in rayon manufacture Ammonium nitrate conversion from the uranium processing KNO3 conversion Recovery of a catalyst used to cure epoxy resins in Al coating moulds Sodium alkali recycling in pulping and bleaching operations Soxal process SO2 recovery Dry sodium scrubbing alkali recovery Acetic, formic, acetylsalicilic, and organic acids Aminoacids Production of highly generated ion-exchange resins Salt splitting of NaCl KCl conversion Solution mining of trona and subsequent sodium alkali production Sodium alkali production from natural brines and solid trona EDBM in methanolic solutions EDBM of sodium methanesulfonate Continuous electrodeionization for high-purity water production

contains about 500 ppm of Ca2þ, Mg2þ ions. A pretreatment including polishing with ion-exchange resins reduces this amount to less than 0.5 ppm. The waste stream is then processed in a three-compartment EDBM unit designed for both batch and feed and bleed mode. The total membrane area is 1.78 m2 (16 repeating unit cells, each membrane with an area of 0.12 m2).

21.3 MAIN APPLICATIONS OF THE EDBM A large number of applications of EDBM have been identified. A classification under two broad categories of pollution control=resource recovery and chemical processing is reported in Table 21.3 [45]. This chapter deals with developments of EDBM technology in food industries and only a short review is devoted to other application domains. The oldest industrial application of EDBM in the pollution control=resource recovery is the recovery of HF and HNO3 from a stream containing KF and KNO3 generated from a pickling bath in a steel plant. This first commercial use of BPM began operations in 1987 at Washington Steel in Pennsylvania [80]. This application was stopped in 2000. Among the other examples, one can quote the regeneration of sodium sulfate in a rayon plant [23], a gas absorption process for the conversion of amine in air, the treatment of flue gas for removal of SO2 [81], the electrodialytic dissociation of alcohols to produce sodium methoxide [82], or the recovery of methanesulfonic acid from sodium methanesulfonate solutions [45]. While the applications of EDBM in the pollution control=resource recovery seem to reach a stage, those concerning chemical processing and food industries are growing [78].

21.4 APPLICATIONS OF EDBM TO FOOD INDUSTRIES In 1990, Hatzidimitriu [83] patented a process for adjusting the pH of aqueous fluids. This process can be used for adjusting the acidity of an edible fluid (specifically sugar syrups, fruit and vegetable juices, wines, sauces, and tomato paste) by electrodialysis in a cell containing membrane pairs comprising a BPM and an ion-selective membrane. More specifically, Dykalo et al. [84] used bipolar membrane ED to control the acidity of dairy products containing an aqueous phase. To control


594



the pH of dairy products, the solution was circulated on the cationic side of the BPM, where the Hþ ions are generated, to lower the pH and on the anionic side of the BPM, where OH ions are generated, to raise the pH. These applications point the way to new applications for BPMs beyond the production of acids and bases. Recently, EDBM has been applied to protein precipitation and purification, enzymatic browning inhibition, simultaneous recovery of coagulant agent and protein, fruit juice deacidification, and dairy wastewater purification.

21.4.1 PRODUCTION

OF

SOY PROTEIN ISOLATE

A large proportion of the soy protein used in the food industry is in the form of protein isolates (>90% protein). Separation of proteins by isoelectric precipitation at the isoelectric pH range 4.2–4.6 is the recognized industrial process. The isoelectric point is the pH value at which the net global charge of the protein is neutral. At this pH value, the solubility of the protein is minimal and can result in complete precipitation. The conventional precipitation procedure involves five steps: extraction, precipitation, washing, resolubilization, and drying. The defatted soy flakes are dissolved in water at pH 9 2 in a ratio ranging from 6:1 to 20:1 at 200 kDa based on sieve effect [8,11,12]. The advent of ceramic membranes that can withstand pH range from 0.5 to 13.5, or from 0 to 14 for some membranes, such as Membranlox, and temperatures over 1008C provided significant advantages in terms of chemical, thermal, and mechanical stability over the more available polymeric membranes [13]. This and the invention of crossflow microfiltration (CFMF), which is an economic and efficient method for purification of fine particle or proteins, increased the utilization of MF in the dairy industry [14]. Today, CFMF is utilized in the dairy industry in three major applications: (1) removal of bacteria, (2) whey defatting, and (3) micellar casein enrichment of milk for cheesemaking and other applications [10]. A fourth and relatively new application of MF in dairy processing, which is based on the pioneering work of Goudédranche et al. [15], is the recovery and fractionation of native milk fat globule from milk, or even from mixed milk, and whey cream [16,17].

22.2.2 ULTRAFILTRATION Ultrafiltration involves the separation of molecules in solution predominantly according to their sizes and shapes as well as charges and affinity for the membrane [18–20]. It involves the use of membranes with a molecular weight cut-off (MWCO) in



637

Applications of Membrane Technology in the Dairy Industry

the range of 1–300 kDa and a pore size of ~0.01 mm, which operate at a pressure of 30% of whey proteins were retained in the MF of skim milk using 0.1 mm composite membrane consisting of a ZrO2 filtering layer on an a-alumina support at 508C. The process was carried out at a TMP range of 0.05–0.2 bar (5–20 kPa) until the volume reduction ratio (VRR) of 2 was reached and was then continued in a feed and bleed mode at a TMP of 3.00 bar (300 kPa). On the other hand, Samuelsson et al. [98] reported 12% retention of whey proteins in the MF of skim milk at 558C, TMP range of 0.1–1.9 bar (10–190 kPa) and 8 ms1 crossflow velocity using 0.14 mm ZrO2 ceramic membrane in tubular configuration until a VRR of 1.15 was reached. Pierre et al. (cited in Ref. [99]) obtained native phosphocaseinate (PPCN) with casein to total protein ratio of 0.93 by MF of skim milk using 0.14 mm multichannel ceramic membrane under high tangential velocity of >6 ms1 and very low uniform TMP along the hydraulic path of 0.1–0.2 bar (10–20 kPa). On the other hand, while flux declined by 50% at concentration factor of 3, Pouliot et al. [99] reported that with conventional CFMF at 508C, average TMP of 1.9 bar (190 kPa) and 6.9 ms1 crossflow velocity using 0.22 mm pore size membranes, native PPCN may also be produced at a casein to total protein ratio of 0.91. Jost et al. [93] showed that after threefold concentration of milk by MF, the casein-nitrogen (CN) of the retentate increased to 83% of the total nitrogen (TN) in the retentate and after DF using six initial retentate volumes of demineralized water, the casein purity was ~95%. Casein enrichment of cheese milk significantly improves the rennet coagulability and the productivity of cheese plants, especially those producing hard cheeses [100]. For instance, the rennet coagulation time of a 3% native PPCN solution is reduced by 53% compared to that of raw milk, and the gel firmness after 30 min is increased by more than 50% (Pierre et al., 1992, cited in Ref. [100]). Milk coagulation depends on a number of factors, such as the kinetics of the enzyme reaction, the concentration, and the state of the proteins, especially casein, the balance of minerals, especially calcium, and pH [101]. Most of these factors are directly influenced by UF or MF processing. Caron et al. [101] compared the rennet coagulation properties of milk enriched with a regular ultrafiltered retentate powder (RUF) to milk enriched with a diafiltered MF (DMF) retentate powder. RUF was prepared by concentrating skim milk to concentration factor of 5 by UF, while DMF was prepared from skim milk concentrated



644


by MF to concentration factor of 3, diafiltered with deionized water and then concentrated by UF (50 kDa MWCO) to concentration factor of 2 to increase the caseins to whey proteins ratio. They observed that the DMF-enriched milk coagulated faster than the RUF-enriched milk. MF and DF increased the casein content, while reducing whey protein and lactose concentrations in the retentate compared with just UF. Compared with that of control milk (with 3.5% total protein content), they reported an overall improved gel firmness but longer coagulation time for milk enriched to 4.5% and 5.0% total protein, regardless of retentate powder used. The authors reasoned that this may have been due to enzyme diffusion retardation in the casein-enriched milk. Reduction of microorganisms in milk before cheese production must be achieved in such a way that the functionality of the milk proteins is not affected. Heat-resistant spores, such as Clostridium tyrobutyricum and C. sporogenes can cause severe spoilage of the cheese by late fermentation that can result in the production of H2 and CO2 gases, and unpleasant smelling fermentation products [102]. Although heat sterilization reduces these spores, because of the heat-induced complexation between b-Lg and k-casein, UHT milk normally does not form a rennet gel and consequently could not be used efficiently for cheesemaking [87]. The partial removal of whey proteins by MF reduces the detrimental effects of heat treatment on the rennet coagulability of milk [100]. The use of MF followed by UF produces micellar casein-rich powder that increases cheese yield. The removal of b-Lg during MF eliminates the heat-induced b-Lg=k-casein complex typically found in NDM used in cheesemaking [101,103]. The extent of formation of the b-Lg=k-casein complex decreases and the increase in the electronegativity of casein micelles is not high enough to prevent their aggregation when caseinomacropeptide is cleaved even if the milk is UHT-treated [100]. The MF of raw, pasteurized, UHT, and sterilized whole milks (3.5% fat) using 0.45 mm pore size membrane allowed the recovery and quantification of whey proteins and whey protein=k-casein macroaggregates in the milk, which Carbonaro et al. [104] stressed may lead to finding the mechanisms by which these aggregates are formed during heat treatment of milk. This will be useful for process modeling and in understanding how such aggregates affect the nutritional quality of milk proteins. To address the adverse effects of heat treatment on the rennet coagulability of milk during cheesemaking, Quiblier et al. (cited in Ref. [105]) developed a process that involves the partial removal of whey proteins, especially b-Lg, from skim milk followed by mild to moderate heat treatment. The process was adapted by Garem et al. [105] to produce milk powder for cheesemaking that is partially depleted of whey proteins to avoid the formation of b-Lg=k-casein complex during thermal processing. The steps involved in the process are as follows (Figure 22.3): (1) partial or total removal of whey proteins of the milk by 0.1–0.2 mm pore size MF membrane, (2) UF of the MF permeate using membranes with MWCO 20 kDa,

Skim milk

MF 0.1 µm pore size

MF retentate

MF permeate

UF MWCO = 20 kDa UF permeate Blending Deseroproteinized milk

UF retentate

Vacuum evaporation and drying

Vacuum evaporation and drying WPC powder Deseroproteinized skim milk powder

FIGURE 22.3 Schematic description of the process of preparation of the whey protein-depleted skim milk powder. (From Garem, A., Schuck, P., and Maubois, J.-L., Lait, 80, 25, 2000. With permission.)




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(3) blending of the MF retentate with the UF permeate, and (4) vacuum evaporation and spray drying of the blend at moderate heat application to produce the recombined milk powder for mozzarella cheese manufacture [105]. They reported about 31% depletion of whey protein from the fresh milk, which resulted in a firmer gel during rennet coagulation and in a significant cheese yield increase of 7.3%. Their data also showed important decrease in stretch water losses without having to modify the cheesemaking process as with common recombined milk. Moreover, since the MF–UF recombined milk does not require the addition of chemical additives, they suggested that its utilization might be of economic advantage over common recombined milk of higher casein to whey protein ratio. Terré et al. [106] showed that MF membrane of 0.2 mm pore size also allows separation of b-casein when this component, by physicochemical means, is solubilized from the casein micelles [92]. Moreover, at high recirculation velocity of the retentate of at least 6 ms1, efficiency of separation is strongly related to the maintenance of low temperatures (28C–68C) in the MF equipment and adjustment of ionic strength in the caseinate solution. Products obtained on both sides of the MF membrane are suitable for modifying the b-casein to as-casein ratio of cheese milks and consequently, the texture and flavor of resulting cheeses, which may result to new cheese varieties [92]. van Hekken and Holsinger [91] reported that the cold MF (48C) of skim milk using either 0.2 or 0.1 mm pore size ceramic membrane gave a permeate, which when ultrafiltered enabled the production of b-casein-enriched retentates with b-casein to as-casein and b-casein to k-casein ratios that were twofold to fourfold and threefold greater than those of skim milk, respectively. The retentates formed softer gels with greater syneresis and hydration, and lower holding capacities than skim milk gels. They suggested that such gels may be used as starting materials in the production of novel soft cheeses. 22.3.3.3

Vatless Manufacture of Cheese

Caron et al. [101] studied the effect of acidification of milk in the MF and UF processes on the rennet coagulability of milk. Diafiltered MF retentate powder from acidified milk (ADMF) was prepared by acidifying milk to pH 6 using lactic acid solution, concentrating the milk by MF 3x with DF, and then concentrating the MF retentate by UF (50 kDa MWCO) 2x. They observed that ADMF contained ~90% protein, while RUF and DMF retentate contained ~69% and ~83%, respectively. They also noted that ADMF had reduced concentrations of mineral, noncasein nitrogen (NCN) and NPN, and whey proteins. This is because acidification of milk increases soluble minerals and during DF of acidified MF milk more lactose and minerals are removed from the retentate while further reducing whey proteins content. This led to decreased gel firmness for milk enriched with ADMF. In a related study, Brandsma and Rizvi [94] gradually acidified pasteurized skim milk to pH 6.0 using 1.6 g of gluconodelta-lactone (GDL) per kilogram of skim milk to promote solubilization of micelle-bound colloidal minerals into the serum phase of milk. GDL was used in the acidification process for its ability to hydrolyze and form gluconic acid in a temperaturedependent manner that allows highly predictable and controlled method of lowering the MF retentate pH. Using 0.2 mm pore size MF ceramic membrane operated at a TMP of 2.6 bar (260 kPa), 508C, and 7.5 ms1 crossflow velocity, they concentrated the acidified skim milk to concentration factor of 8. They observed an effective change in mineral balances, specifically Ca, while maintaining 87.4% nominal whey protein permeation. They reported 20.1% decrease in Ca concentration and 12.6% increase in whey protein concentration in the MF retentate compared to milk at its natural pH of 6.6. The whey protein content of the MF retentate they observed is about 55% lower than typical UF retentates [94]. The process was utilized to produce MF milk for the manufacture of mozzarella cheese using 80–100 mL rennet per kilogram of MF cheese milk at 328C–368C to give post-coagulation cutting time of 15–20 min. When the same MF retentate and butter oil were utilized in the manufacture of low-moisture part-skim (LMPS) mozzarella cheese, Brandsma and Rizvi [107] observed delay in the rheological and functional development of MF retentate LMPS, which occurred between 30 and 60 days as opposed to 7 and 30 days for commercial LMPS mozzarella. They noted an improvement in rheological and functional behavior in the MF retentate LMPS using starter culture. These indicate that the MF of skim milk with in-process acidification to pH 6 can be potentially developed into a novel cheesemaking process to manufacture cheese of improved textural and functional qualities, along with the recovery of highly functional native whey proteins in the MF permeate, or virgin whey [95]. The virgin whey harvested from the MF, which does not contain cheesemaking remnants, fats, or spores, contained ~5.3% total solids (TS), ~6% of which is whey proteins. Brandsma and Rizvi [94], Punidadas and Rizvi [108], Solanki and Rizvi [109], and Ardisson-Korat and Rizvi [110] showed that the virgin whey’s composition is consistent at mass concentration factor of 8. Brandsma and Rizvi [107] suggested uniform transmembrane pressure (UTMP) operation to improve permeate flux and achieve a calcium to total protein ratio in the MF retentate suitable for cheesemaking. Similarly, Vadi and Rizvi [111] suggested that to achieve practical permeate flux values at concentration factor higher than 6 in the MF of skim milk at pH 6 and 508C using 0.2 mm pore size membrane, operation in the UTMP mode is necessary. Table 22.2 gives the permeate flux achieved at different concentration factor values in the MF of skim milk. Using 0.1 mm pore size MF ceramic membranes in a Tetra Alcross M-38 system operated at UTMP of 1 bar (100 kPa), 508C, and crossflow velocity of 5 ms1, Ardisson-Korat and Rizvi [110] showed that at mass concentration factor of 8, the MF retentate contains up to 30% total solids that can be used for semi-continuous vatless manufacture of LMPS mozzarella cheese.



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TABLE 22.2 Permeate Flux Values in the MF of Skim Milk at Different Processing Conditions under UTMP Mode Permeate Flux (ms1) 2.5 105 1.9 105 1.3 105 3.1 105 2.8 106

Crossflow Velocity (ms1)

Concentration Factor

TMP (kPa)

Membrane

Pore Size (mm)

References

6.9 7.2 7.2 5.4 5.0

3 2 10 2 8

190 193 193 138 90

Ceraflo Membralox Membralox Ceramen Membralox

0.22 0.2 0.2 0.05 0.1

[99] [111] [111] [108] [110]

Source: Adapted from Brans et al., J. Membr. Sci., 243, 263, 2004.

They reported 66%–71% whey proteins depletion and calcium to casein ratio in the MF retentate suitable for LMPS mozzarella cheese with properties comparable to those of commercial products. In a related study, Maubois et al. [112] noted high-protein purity of WPI produced from milk microfiltrate, or ideal whey harvested before cheesemaking, by MF of milk at low concentration factor of 2. The same group suggested the suitability of milk microfiltrate as starting material in obtaining native whey protein fractions. Bacher and Kønigsfeldt [113] showed that because of the absence of glycomacropeptide and the relatively large amount of native whey protein in milk microfiltrate WPI (MWPI), it showed vastly improved functionality compared with WPC-80 from cheese whey. However, with the exception of gelation property, MWPI’s functionality is similar to cheese whey WPI. They reasoned that this was probably due to a pretreatment that removes denatured WP from the cheese whey before WPI manufacture. With the objective of increasing calcium and lactose content of MF retentate in addition to micellar casein while maximizing whey protein depletion, Nelson and Barbano [114] developed a multistage MF process that removed ~95% of whey protein from skim milk. They reported that the MF retentate produced from this process contained soluble minerals, NPN, and lactose similar to the original milk. This was accomplished by using the permeate from the UF of the MF permeate to diafilter the MF retentate after achieving a concentration factor of 3 in the MF. Aside from the recovery of native micellar caseins, they showed that the process enabled the production of whey protein stream (UF retentate) with protein content similar to that of commercial WPC.

22.3.4 REMOVAL

OF

MICROORGANISMS

Since the 1980s, MF has been investigated as an alternative technology to centrifugation for clarification and bacteria removal of milk and whey and is often referred to as a gentle sterilization method [8,68,115,116]. Due to bacterial spoilage, hightemperature, short-time (HTST)-processed milk can last up to 14 days only [117]. Although ultrapasteurization can extend refrigerated fluid milk shelf life to 45 days, it imparts distinct cooked flavor to milk, which may be undesirable to some people [52,118,119]. Unlike UHT pasteurization, MF can reduce the amount of bacteria and spores without affecting the taste of the milk, and provides longer shelf life than pasteurization [6,52,87]. Spore-forming bacteria, which represent the main surviving species after pasteurization, are better retained by MF membrane because of their large cellular volume, giving a decimal reduction of ~4.5 (cited in Ref. [8]). For instance, due to the high retention of Bacillus cereus in MF, which is between 99.95% and 99.98%, the shelf life of milk is improved from 6–8 days by conventional pasteurization to 16–21 days by MF [87,120]. Also, decimal reduction factors for MF are higher than that for bactofugation, which is the continuous reduction of bacteria and spores from milk and skim milk by centrifugal force that results to about 98% reduction in spores [6,87]. The work of Holm et al. [121] and that of Piot et al. (cited in Ref. [8]) led to the development of one of the most important applications of MF in the dairy industry, the Bactocatch process with a patent owned by Tetra Pak [8,121]. In this process, cream is removed from whole milk by centrifugation [119]. Skim milk is then passed through 1.4 mm pore size MF membrane where up to 99.99% bacteria reduction at 508C, UTMP of 0.5 bar, and a high crossflow velocity of 7.2 ms1 is accomplished at a volumetric reduction factor of 20, or 200 in larger membrane modules where the retentate, which represents 5% of the total starting volume, is concentrated 10 times more (Figure 22.4) [6,8,13,87,120]. The MF retentate and cream undergo ultrapasteurization treatment (1158C–1308C for 4–6 s) and are then combined with the nearly bacteria-free MF skim milk [11,119]. Common permeate fluxes obtained in the industry are between 500 and 800 Lh1m2, usually up to 10 h operation, giving average decimal reduction of bacteria above 3.5 while proteins and total solids transmissions are about 99% and 99.5%, respectively [8,52]. Trouvé et al. (cited in Ref. [122]) examined the retention of 7 species of bacteria that typically occur in raw milk using the Bactocatch process and reported 2.6 log reductions during the MF of milk contaminated with initial bacterial levels



647

Applications of Membrane Technology in the Dairy Industry Raw milk

Separator

Cream

Skim milk

MF retentate

MF MF retentate

Sterilization (HTT)

Cream

Pasteurization

Surplus cream

MF milk

FIGURE 22.4 MF process used by Olesen and Jensen in collaboration with Alfa-Laval and Ceraver in the integration of CFMF in a milk processing line for the production of milk with low bacteria content. (From Olesen, N. and Jensen, F., Milchwissenschaft, 44, 476, 1989. With permission.)

Relative distribution

of 102–107 cfumL1, where bacterial retention rates were 99.93%–99.99%. While the efficiency of the process in removing bacteria and spores is widely accepted, it does not guarantee the complete removal of pathogenic bacteria in milk as required in milk pasteurization [11]. As the size distribution of bacteria in milk (0.4–2.0 mm) partly overlaps with that of fat globules (Figure 22.5), MF can be used as a pretreatment of skim milk for the production of raw milk cheeses and reduction of spores in acid cheese milk [6,123]. In 1987, Piot et al. (cited in Ref. [9]) showed that MF process using an inorganic alumina membrane of 1.8 mm pore size could skim 98% of the fat and reduce bacterial counts by 2 orders of magnitude, with no apparent rejection of proteins. Using 1.4 mm ceramic membranes, Pafylias et al. [124] compared the reduction in microbial population in nonfat and 1% fat milk in conventional CFMF operation and UTMP MF operation, which were both run at 508C, TMP of 1.0 bar, and a crossflow velocity of 5.0 ms1. They found that, except for fat and bacteria, the distribution of major milk components in the MF milk did not change in either operation modes. While both MF operation modes gave a microfiltered milk with an average fat content of 0.05%, and a bacterial reduction of 4–5 log cycles, the average flux over a concentration factor of 10 was higher with the UTMP mode. The group, however, cautioned that over long-term (8–16 h per day) continuous operation, a gradual increase in fouling or concentration polarization may lead to undesirable changes in milk composition. Eckner and Zottola (cited in Ref. [124]) reported that sterilizing microfilters, such as the 0.2 mm membranes commonly used in the pharmaceutical industry, would foul too rapidly when operated in the conventional crossflow manner, resulting in low flux and permeates with an undesirable solids profile, making them of little practical value in the dairy industry. Guerra et al. [102] employed the combined benefits of reverse asymmetric 0.87 mm pore size polymeric membranes and backshock technique to control the adverse effects of concentration polarization and fouling in the removal of bacterial spores in skim milk

1

Bacteria

Fat Whey proteins

5 10

Cassein micelles

50 100

500 1000

5000

Size of molecule or particle (nm)

FIGURE 22.5 Size distribution of major milk components. (From James, B.J., Jing, Y., and Chen, X.D., J. Food Eng., 60, 431, 2003. With permission.)



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at low linear velocities and, therefore, low-energy consumption. Reverse asymmetric membranes are those whose porous layer, rather than the skin layer, faces the retentate side, making the fouling layer more loose and consequently result in lower cake resistance as long as the porous layer is not completely filled up [125,126]. The backshock technique involves temporarily forcing the permeate stream backwards through the membrane and into the retentate stream by pressurization of the MF permeate side at short time interval of 5.6% protein concentration, the yogurt becomes excessively firm and has a grainy texture and flat flavor. Ozer et al. [135] compared the gelation properties of unconcentrated reconstituted milk (~16% TS) and milk concentrated by UF, RO, and direct concentration on reconstitution (~23% TS) for yogurt making. They found that yogurt from the UF milk contained the highest amount of protein (~9%) and fat (~8%), and the least amount of lactose (~0.8%). They reported that unlike the other milk systems, the complex modulus (G*) of the UF milk did not reach a plateau within the experimental time frame, but continued to increase over the entire period of incubation. Due to the similarity in the loss tangent (tan d) values for the UF milk and those of other samples, the authors surmised that although the same interactive forces were involved in the gel formation of the milk samples, the flocculation kinetics for the high-protein UF milk may have changed. The system may have entered into a metastable state, in which the strength of the gel can continue to increase. Aside from applications to fermented products, UF-standardized milk may also be used for the manufacture of several other dairy products. For instance, Lee and White [136] reported an increase in the protein value of ice cream when UF retentate rather than NDM was used for solids-not-fat (SNF) source in the formulation at substitution levels of 25%–75%. They also reported that because of the high level of casein in the ice cream made from UF retentate, its water-binding capacity increased, which resulted to an improvement in flavor and stability of body and texture.

22.3.6 RECOVERY OF LACTOSE

AND

MINERALS

Vyas and Tong [137] developed a method for calcium fortified reduced lactose skim milk production, which involves two UF steps (Figure 22.7). The process aimed to maximize calcium recovery without adversely affecting the native state of the milk proteins, while effectively removing lactose from the skim milk. In the first UF step (UF1), skim milk was ultrafiltered using a spiral wound module (10 kDa MWCO) at 2 bar (200 kPa) TMP and 108C to concentration factor of 4. The permeate from UF1 was adjusted to pH 8, heated at 638C for 30 min, to cause the formation of calcium complex or precipitates, and was then ultrafiltered (UF2) at the same condition as UF1. The retentate from UF2, which contained the concentrated calcium was mixed with the retentate from UF1 to produce the calcium fortified reduced lactose skim milk. They reported 70% recovery of the calcium from UF1 permeate during UF2 while enabling the recovery of high-purity lactose in the UF2 permeate.

22.3.7 FRACTIONATION

OF

MILK FAT AND ITS COMPONENTS

Milk fat globules are composed of triglyceride core and are surrounded by a natural biological membrane, the milk fat globule membrane (MFGM), which contains proteins, glycoproteins, enzymes, phospholipids, triacylglycerols, glycolipids, and other minor compounds, renders them compatible with the aqueous environment [138–140]. Because of its origin and amphiphilic nature, MFGM isolates were found to show good emulsifying properties and nutritional advantages such as modulation of cholesterol uptake, protection against bacterial infections in the gut, and inhibitory effect on colon cancer [141]. Although the



650

Handbook of Membrane Separations Skim milk

UF 1 CF4, 10⬚C MWCO = 10 kDa

UF1 permeate

pH adjustment to 8

UF1 retentate

Heat treatment at 63⬚C for 30 min

Blender

Calcium fortified reduced lactose skim milk

UF2 retentate (calcium-rich)

UF MWCO = 10 kDa

UF2 permeate (lactose-rich)

FIGURE 22.7 Production of calcium fortified reduced lactose skim milk. (From Vyas, H.K. and Tong, P.S., J. Dairy Sci., 86, 2761, 2003. With permission.)

MFGM proteins account for only 0.03%–0.04% of the total proteins in whole milk [139,142], many properties of dairy products are influenced by the responses of the MFGM components to various treatments [139,143]. For instance, it is supposed that a large part of the microstructure and consistency of dairy products, such as cheese, is largely affected by interactions between the casein matrix and fat globule, so that if the number of fat globules in the milk were to be modified without altering its total fat content, it may be possible to manipulate these interactions [144]. Since it is thought that fat globule size, which ranges between 0.2 and 15 mm, could be a much better separation criterion between small and large globules than fat globule density, MF is the preferred technology over centrifugation to separate fat globules according to size rather than density without damaging the MFGM [140,144,145]. Goudédranche et al. [15] developed a CFMF process, which permits the selection of native small fat globules (SFG, 1–3 mm) and large fat globules (LFG, >5 mm) in raw milk. Using this process, Goudédranche et al. [144] confirmed that increasing or decreasing the number of native fat globules in milk leads to significant changes in the texture of liquid milk, yogurt, fresh cheese, sour cream, Camembert cheese, and mini Swiss cheese. They reported that products with greater amount of SFG gave creamier, smoother, and finer texture. They suggested the possible differences in triglyceride composition of SFG and LFG to be responsible for their observations. Briard et al. [146] reported that small and large native milk fat globules extracted from the same milk using the MF process developed by Goudédranche et al. [15] exhibited small variations in their total fatty acids. On the other hand, using the same system, Michalski et al. [17] found no major differences in triglyceride structures between native milk SFG and LFG after eliminating thermal history and the influence of cooling rates. Using the milk fat globule MF fractionation process of Goudédranche et al. [144], Michalski et al. [140] observed pronounced difference in physicochemical and functional properties of Emmental cheeses made using native SFG and LFG. They reported lower flexibility and firmness in SFG cheeses, which had greater moisture and underwent greater proteolysis compared to LFG cheeses, 52 days after manufacture. However, lipolysis was threefold lower in SFG cheeses. Results of these studies confirm that the use of native milk fat globules with different sizes, separated using MF, can lead to a range of new dairy products with different physicochemical and functional properties [140]. Even interactions between milk serum proteins and MFGM proteins at different physicochemical conditions can now be characterized to elucidate their influence on dairy products quality because native milk fat globules can now be recovered by MF [139]. In the manufacture of butter, cream undergoes churning, a mechanical process which involves agitation in the presence of air that disrupts the phospholipid-protein membranes of the fat globules as they aggregate, releasing the water-soluble material, along with most of the proteins, lactose, and minerals, into the aqueous phase, or buttermilk [147–149]. Aside from the proteins in the buttermilk, which are mainly caseins and whey proteins, phospholipids render buttermilk as a potential source of ingredients of functional and nutraceutical applications [147–149]. Although native MFGM have been isolated from cream and purified by repeated washing-off of casein and whey proteins using simulated milk ultrafiltrate (SMUF) for characterization such as those done by Ye et al. [139,143], a commercial isolation and fractionation methods for MFGM components from buttermilk are yet to be developed [147].




651

Sachedva and Buchheim (cited in Ref. [147]) removed caseins from reconstituted buttermilk by precipitation of the caseins followed by MF of the phospholipids-rich buttermilk serum phase using 0.2 mm pore size ceramic membranes. The authors reported a recovery of 67% of the total phospholipids, giving a final buttermilk extract containing about 1.5% phospholipids and 9% protein (dry basis) but the yields varied depending on coagulation conditions. Corredig et al. [147] observed retention of skim milk-derived peptides, especially caseins, in the UF (250 and 500 kDa MWCO) of reconstituted commercial buttermilk (8% w=v in deionized water) to a concentration factor of 2 and subsequent DF. When a 0.1 mm pore size hydrophilic polysulfone Supor 100 membrane (Pall Gelman, Ann Arbor, Michigan) was used, greater permeation of the whey proteins was observed, although caseins were retained with the MFGM components, allowing the production of buttermilk concentrate. The addition of sodium citrate to buttermilk before MF disrupted the casein micelles, allowing their improved removal from the retentate to produce the MFGM isolate. However, the retention of whey proteins, especially b-Lg, and to a lesser extent, a-La, by the membrane was still considerable, even at repeated DF. The authors surmised that the retained proteins are probably whey protein aggregates or complexes with caseins or the MFGM components, the amounts of which depend on the heat treatment history of the buttermilk, which is commonly very severe to render the cream sterile before butter manufacture. Nevertheless, the proposed process enabled the production of MFGM isolate with 35% (w=w) lipid and 60% (w=w) protein, which comprised 70% MFGM proteins, 6% caseins, and 24% whey proteins. Roesch et al. [148] showed that emulsions containing MFGM isolates produced from the process, developed by Corredig et al. [147], had small particle distribution with droplets of higher surface coverage showed good stability to creaming compared to buttermilk concentrate. Morin et al. [149] compared the permeate flux through a 0.45 mm pore size ceramic MF membrane during volumetric concentration to 2x and two-diavolume DF of regular buttermilk and whey buttermilk at 8–108C and 0.8–0.95 bar (80–95 kPa) TMP. They reported that due to the lower amount of total solids, especially caseins and minerals, in the whey buttermilk, the permeate flux was significantly higher compared with that for regular buttermilk. They suggested that whey buttermilk has the potential of being a suitable starting material in producing MFGM isolate. However, they also recommended the standardization of whey cream before butter making as they noted variability in its lipid and phospholipids content.

22.4 MEMBRANE PERFORMANCE 22.4.1 FOULING

AND

FLUX PREDICTION

Concentration polarization and fouling are the main phenomena that limit the practical applications of membrane filtration. Both result in the unfavorable time-dependent flux decline and alteration of the membrane rejection behavior that compromise process efficiency [19,150–153]. Concentration polarization occurs at the beginning of the filtration process when a concentration gradient of the retained components is formed on or near the membrane surface [19]. The underlying mechanism behind this practically reversible phenomenon is the unbalanced transport of dissolved components between the bulk phase and the membrane surface. Fouling, on the other hand, is the largely irreversible deposition of solid materials on the membrane surface or within its pores [154]. Both occurrences limit the permeation rate by increasing hydraulic resistance to permeate flow. The osmotic back-pressure developed due to solids accumulation on the membrane surface also reduces driving force to permeation [49]. The hydrodynamic conditions employed in the process, and the physicochemical properties of the membrane and the feed solution are the main factors that influence the extent of concentration polarization and the occurrence of fouling [150]. For instance, in dairy processing applications where milk proteins are involved, protein–protein interactions can contribute to fouling of membranes in a variety of ways, such as inducing aggregation in solution, adding to the adsorbed protein layer, or altering the effective sieving coefficient of the constituent proteins [155]. Protein–membrane interactions, on the other hand, may lead to pore plugging, pore narrowing, and solids deposition that give rise to cake formation on the membrane surface [46]. On the other hand, the hydrodynamic conditions in crossflow filtration may alleviate fouling because, while the permeate flux drags particles toward the membrane, the crossflow induces particle back transport into the bulk phase [156]. One important finding in membrane fouling during dairy processing is that at a pH equal to the isoelectric point of the proteins, a minimum flux caused by severe fouling is observed. Huisman et al. [150] summarized the possible explanations to this observation that has been put forward by many authors as follows: (1) Hydrophobic interactions between hydrophobic membranes and the proteins are enhanced at the isoelectric point of the proteins where they become hydrophobic. (2) At their isoelectric point, proteins tend to aggregate more readily due to reduced electrostatic repulsion and this causes severe fouling. (3) Aside from enhanced transmission through the membrane, the small sizes of proteins at their isoelectric point cause them to form a densely packed fouling layer of low permeability. (4) Adsorption of more proteins through protein–protein interactions on the protein monolayer, which is usually hindered by electrostatic repulsion at all pH values, is enhanced at the isoelectric point of the proteins. This absence of electrostatic repulsion also leads to increased protein transmission through the membrane. Mineral precipitation and complexation with proteins also contribute to fouling considerably. Adsorbed minerals may serve as salt bridges between the protein and the membrane, which aggravates fouling. In physicochemical conditions that promote calcium phosphate precipitation or calcium–protein complexation, membrane fouling in the filtration of milk or whey is severe,



652


Flux

Clean water flux

I

II

III

Time

FIGURE 22.8 Conceptual stages of flux decline in UF at constant-pressure operation: stage I, concentration polarization; stage II, membrane fouling (usually due to protein deposition); stage III, further particle deposition or consolidation of the fouling layer. (Adapted from Marshall, A.D., Munro, P.A., and Trägårdh, G., Desalination, 91, 65, 1993. With permission.)

although owing to the stabilizing influence of the casein micelles, the severity of fouling in milk filtration is less compared to that in whey [56]. Away from the isoelectric point of the proteins, added salts that alter the ionic strength tend to reduce the electrostatic repulsion by shielding charges, causing molecular contraction, thereby decreasing permeability [56]. Fat or lipid materials and calcium–lipid complexes also contribute to fouling and flux decline in membrane processing of milk or whey. The transport properties of the feed stream and the changes they undergo as the concentration process proceeds also affect the rate of permeation. At high concentrations, the increased fluid viscosity near the membrane surface limits backdiffusion of solids from the polarized layer to the bulk phase, thereby, depressing flux rate [46]. Huisman et al. [150] observed two fouling regimes in the UF of BSA using flat-sheet module of polysulfone membranes with MWCO of 30, 100, and 300 kDa: (1) the low-fouling regime at the initial stages of filtration and (2) the high-fouling regime at the later stages of filtration. They found that for the more retentive membrane, such as 30 kDa MWCO membranes, the hydrophobic protein–membrane interactions determine the fouling behavior at the low-fouling regime, while in the highfouling regime, protein–protein interactions determine the overall performance. These were not definitively observed for the less retentive membranes, such as 100 and 300 kDa MWCO membranes, for which they found that the structure of the fouling layer seemed to have a more important influence on the final fouling. Although permeate flux may decline far below the theoretical capacity of membranes, the typical variation of flux with time is that of an initial rapid decrease followed by a long and gradual flux decline (Figure 22.8) [157]. Modeling this flux decline caused by fouling during filtration provides better understanding of membrane fouling and provides predictive tools for successful scale up or scale down of filtration systems [153]. 22.4.1.1

Mechanistic Models for Flux Decline

To understand the flux decline in pressure-driven membrane operations, a number of models were developed. Two of the most widely studied models are the resistance model and the concentration polarization model. The resistance model is the oldest and is based on the cake filtration theory, where it is assumed that a cake layer of rejected particles, which are too large to enter the membrane pores, is formed. The frictional drag due to permeation through these immobile particles leads to additional hydraulic resistance [21]. The cake layer and the membrane are considered as two resistances in series, and the permeate flux is described by Darcy’s Law as J where J is the permeate flux Am is the membrane filtration area V is the total volume of permeate t is the filtration time TMP is the transmembrane pressure m is the viscosity of the permeate Rm is the intrinsic membrane resistance Rc is the cake resistance

1 dV TMP ¼ Am dt m(Rm þ Rc )

(22:1)




653

This model has been successful in describing flux decline during dead-end filtration of particulate suspensions, but is not appropriate for application to crossflow filtration where the feed solution continuously recirculates [158]. Also, neither the occurrence of macromolecules and colloidal particles diffusion nor the influence of solute–solute and solute–membrane interactions on flux decline is considered in this model [42,59,159]. Aimar et al. [19] noted that in the UF of complex liquids, such as cheese whey, which contains proteins, salts and casein fragments, concentration polarization, and adsorption and cake formation play a role in flux behavior during crossflow filtration. They may induce osmotic pressure in the retentate side since the chemical potential of the solute-rich polarized layer is lower than that of the permeate, and therefore at equilibrium, a positive osmotic pressure develops in the retentate to equal that of the permeate. The smaller the solute, the greater is its contribution to the osmotic pressure of the liquid, so that in milk, lactose and the minerals have the biggest contribution to osmotic pressure. In skim milk or whey, the osmotic pressure is around 7 bar (700 kPa) and this must be exceeded in RO to commence permeation; in UF, only the proteins contribute to the osmotic pressure, which increases exponentially with protein concentration [56]. In any case, a TMP greater than the osmotic pressure is required for solvent to flow from the retentate side to the permeate side. This leads to the reduction in the effectiveness of applied TMP as driving force to permeation. Concentration polarization caused by macromolecules, which may induce a reversible osmotic pressure that disappears after the filtration pressure is released, and the adsorption on the membrane pores of solid materials or inside the membrane pores of solid materials, which are rid of by rinsing the membrane after the filtration process, are occurrences that both contribute to the reversible resistance to permeation, Rrev. On the other hand, the solids that are deposited on the membrane surface or inside the pores, which are removed only by chemical cleaning of the membrane, constitute the irreversible fouling, Rirrev. When both Rrev and Rirrev are considerable, Equation 22.1 may be written as J¼

TMPeff m(Rm þ Rrev þ Rirrev )

(22:2)

where TMPeff is the effective transmembrane pressure which is equal to TMPappl DP, wherein TMPappl is the applied transmembrane pressure and DP is the osmotic pressure [19,21]. In systems that involve solutes LMA. They note that the low-molecular weight SPAN 80 with a large hydrophilic group mainly comprised oxygen with high electronegativity will have a higher hydration capacity and larger diffusivity compared to the others with high-molecular weight and their hydrophilic group mainly comprised nitrogen with relatively low electronegativity. LMA has the lowest hydration capacity and diffusivity. Li and Shi [94] observed the following order of increasing swelling: SPAN 80 > E644 > EM301, under a given set of conditions. Yan and Pal [119] observed increasing swelling rates with increasing surfactant (EMSORB2500) concentration up to 5 wt% and then a drop with further addition of surfactant. Background Electrolyte Concentration: Since the chemical potential difference between the external and internal phases is the driving force for osmotic swelling [90,120] an increase in the chemical potential difference will result in an increase in osmotic swelling. Wan and Zhang [120] show that electrolyte concentration differences between external and internal phases have no significant effect on entrainment swelling, while this is not the case for osmotic swelling. Volume Ratio of the Oil (Membrane) Phase to the Internal Phase: This parameter can affect the surfactant concentration at the interface of the membrane=aqueous phases and in the bulk membrane phase, affecting emulsion swelling [120]. Both entrainment and osmotic swelling increase with increasing volume ratio of the oil (membrane) phase to the internal phase. The effect of this parameter is also dependent on the surfactant present [120]. Stirring Speed: The emulsion is generally dispersed into a large excess of the continuous phase. To maximize mass transfer a large interfacial area is required and this requires a high stirring speed. Increasing the stirring speed not only increases the interfacial area but can also lead to membrane swelling [94] and membrane breakage [120]. Therefore, optimization of the stirring speed is required. This process has been found to be dependent on the surfactant type [94,106,120]. Emulsion swelling is one of the major drawbacks in the use of this technology in industrial situations. For example, the economic advantages of ELM, over conventional solvent extraction for copper processing are lost with a swelling ratio in excess of 30–40 [102]. In terms of processing, an emulsion swelling of about 10% is considered acceptable [45]. 25.4.2.3

Membrane Leakage

Membrane leakage=rupture=breakdown where the previously extracted solute as well as the stripping reagent are released into the feed stream impacts on the extraction efficiency of the ELM system. It is generally agreed that the properties of the surfactant, diluent, the internal phase, and its volume fraction have significant effects on membrane leakage [46]. Membrane leakage at the rate of ~0.1% is allowable for a practical process [126]. Leakage in ELM systems is well documented: Bunge et al. [93] studied encapsulated phase leakage during ELM extraction of copper ions. They found that the type of surfactant had an impact in that ECA 4360 showed greater encapsulated phase leakage than ECA 5025. The leakage rate decreased with increasing surfactant concentration with a limit at 3 wt%. Leakage is strongly dependent on the volume fraction of the membrane phase with leakage rates increasing greatly when this volume fraction was >0.5. They also noted that the presence of surface active carriers contributed to leakage. Borwanker et al. [25] found that the rate of leakage by membrane rupture is usually a function of surfactant concentration and agitation speed. Okazaki et al. [106] report that the W=O emulsion droplets were not affected by the apparent viscosity of the W=O emulsion but by the organic phase viscosity. They also observed that leakage increased with increasing volume of the internal aqueous phase, leakage was affected by the osmotic pressure difference and that the type of tracer had no effect. In an effort to quantify the effects of such parameters, Okazaki et al. [106] propose two correlations for the leakage rate constant conditional on the osmotic pressure difference between the inner and outer aqueous phases. Shere and Cheung [116] studied the influence of internal phase volume fraction, surfactant concentration, agitation speed, and emulsifying device on leakage. As expected, leakage was significantly dependent on the volume fraction and surfactant concentration. They also found that the use of a high-speed blender rather than an ultrasonic dispenser, for making the emulsion, causes faster leakage and noted that although the viscosity of the emulsion increases if a high-speed blender is used, the stability acquired is offset by the increase in the size of the microdroplets formed with this device. In general, emulsions prepared with a high energy density input such as the ultrasonic method will have smaller



Use of Emulsion Liquid Membrane Systems in Chemical and Biotechnological Separations

723

droplets. This will enhance the membrane stability, provided the surfactant concentration is high enough. Smaller droplets also give a large area for interfacial mass transfer [46]. Efforts to compute breakage=leakage in W=O=W include the work of Boyadzhiev et al. [82], Kinugasa et al. [98], Okazaki et al. [106], Martin and Davies [127], and Goto et al. [128]. Nakano et al. [129] report on leakage in O=W=O systems.

25.4.3 DEMULSIFICATION As stated earlier, after extraction the membrane must be broken and the solute recovered. The breaking or demulsification of a loaded emulsion is one of the most important steps in the process as the membrane phase must be available for recycling [46]. The demulsification process can be divided into three stages [130]: 1. Droplet coalescence and growth 2. Droplet settling 3. Coalescence of the large water and oil droplets with their respective continuous phases in the coalescer To date, chemical or physical treatment is the method used for demulsification. 25.4.3.1

Chemical Treatment

This treatment involves the addition of a demulsifier. For example acetone [85], n-butanol [130,131], and 2-propanol [132] have been found to be effective demulsifying agents for particular applications. Although this method is effective, it modifies the properties and prevents reuse because additional separation steps for recovery and recycling are required. Further recovery of the demulsifier by distillation is expensive [101,131]. 25.4.3.2

Physical Treatments

Physical treatments include heating, centrifugation, high shear, ultrasonics, solvent dissolution, and the use of high-voltage electrostatic fields [46]. Other nonconventional methods, such as microwave demulsification [134] and the use of porous glass membranes [135], have also been investigated. 25.4.3.2.1 Heat Treatment The advantages of heat treatment [131] are reduction in the density and viscosity of the oil and an increased solubility of the surfactants in both the oil and water phases. This in turn leads to a weakening of the interfacial film. The main disadvantage is that heat treatment by itself is slow demulsification kinetics. 25.4.3.2.2 High Shear Emulsion liquid membranes can be effectively demulsified by high shear. A variation on this is to employ centrifugation as a first step, followed by processing in a high shear device [46]. 25.4.3.2.3 High-Voltage Electrostatic Fields By using high-voltage electrostatic fields not only can faster coalescence be achieved, but also the organic phase can be recycled. The mechanisms of electrostatic demulsification are not completely understood. The general belief is that the electric field can polarize and elongate water droplets, neighboring water droplets, after acquiring induced charge from the electric field, will attract each other and coalesce to form a larger drop. The use of electrostatic fields believed to be the most efficient and economic method of demulsification [46] will be discussed further in terms of equipment used, operating parameters, and proposed models for the process. Equipment: Several types of ac and dc electrostatic coalescers have been developed. Documented equipment for electrostatic demulsification includes the continuous insulated electrode coalescer [130,136], two-phase insulated electrodes in parallel [126], circular coalescer [137], continuous coalescer [138], horizontal insulated electrode [130], box electrostatic demulsifier [139], tubular coalescer [140], and batch cylinder demulsifier [141]. Design criteria for electrostatic demulsifiers have been reported by Draxler and Marr [142] and Draxler et al. [143]. Operating Conditions: Parameters such as applied voltage, frequency, waveform, temperature, stirring, and additives have been shown to have an impact on the demulsification process. Kataoka and Nishiki [144] enhanced demulsification with increasing voltage as did Wang et al. [141], Fujinawa et al. [145], and Hauertmann et al. [146]. However, there are limits to maximize voltage that can be applied before formation of a stable sponge emulsion (a high water content water-in-oil emulsion) as noted by Larson et al. [131]. Feng et al. [138] found that coalescence efficiency could be improved with increasing frequency. Draxler et al. [103] demonstrated that for a given degree of demulsification the voltage could be reduced if the frequency was enhanced. It has



724


been shown that under constant voltage an optimum frequency exists, below which the demulsification rate was enhanced with increasing frequency, but above which the rate decreased with increasing frequency [147–149]. A square waveform has been shown to be more effective than a triangular or sine form [102,148,149]. The proposed explanation of this is that under the same peak voltage, the square waveform offers longer time to act on the water droplets to be coalesced. Both stirring and temperature elevation have been shown to accelerate demulsification [130,138,150]. Hsu and Li [130] report that insulation material having a dielectric constant >4 offered better demulsification performance. Goto et al. [140] report on the effect of surfactant type on demulsification. Modeling: Many researchers, including Goto et al. [140], Wang et al. [141], Fujinawa et al. [145], and Hano et al. [150], have attempted to quantify the rate of demulsification. In a recent development, Ichikawa et al. [151] and Ichikawa and Nakajima [152] proposed a theory for electrostatic demulsification of O=W emulsions (by low-external electric fields) based on the Poisson–Smoluchowski equation. Their theory revealed that the applied field induced demulsification according to the following mechanisms [152]: 1. Applied field induced a steady-state current of ions in the aqueous phase, which in turn generated an electric field in the aqueous phase. 2. Electric field in the aqueous phase rearranged surface charges on the oil droplets to compensate the field gradient on the surface of each droplet, which caused the polarization of the electrostatic surface potential. 3. Polarization lowered repulsive osmotic pressure in the overlapped diffuse electrical double layers of the droplets and accelerated demulsification.

25.5 APPLICATIONS Documented applications of ELMs include [2,46,153–158]: 1. Wastewater treatment, e.g., removal of zinc, removal of radioactive materials and nuclear wastes, recovery of nickel from plating wastes, and phenol removal. 2. Biochemical processing, e.g., recovery of penicillin and erythromycin. A summary of a number of systems studied in both categories is presented in Tables 25.4 through 25.8. While the majority of the work to date has been on W=O=W systems, studies on the use of O=W=O systems for the removal of various oils have been reported [1,216,217,129].

TABLE 25.4 ELM Systems for the Separation of Metal Ions Solute Arsenic

Cadmium Cadmium Cerium

External Feed Phase

Extractant

5.5 mg=L As(III) (as As(OH)3) in 0.4 M H2SO4 Cd(CN)2 4

10 vol% 2-ethylhexanol

1100 ppm Cd pH 4.45 Ce3þ in 6.0 M NaNO3 þ 0.1 M HNO3 0.001 M CsNO3 in 0.01 M HNO3

5 wt% DEHPA

Aliquat 336

0.1 M TOPO

Chromium

0.1% K2Cr2O7

0.01 M 8,80 -dibromo-bis (1,2-dicarbolly) Co(III) (BR2DCC) Alamine 336

Chromium

0:06 g=L Cr2 O2 7

1% TOA

Cesium

Surfactant

Diluent

Internal Phase

Efficiency= Recovery

References

2 vol% ECA 4360 polyamine 0.1% SPAN 80, 3% polyamine 3 wt% ECA 5025 3% SPAN 80=20

88 vol% n-heptane

2 M NaOH

>95%

[159]

86% isoparaffin

EDTA, pH 4–6

99%

[160]

82 wt% tetradecane Cyclohexane

6 N H2SO4

97%

[161]

0.05 M sodium citrate, pH 8

98.5%

[162]

4.0% SPAN 80=85

Nitrobenzene

1.0 M KNO3

92%

[162]

0.1% SPAN 80, 3% polyamine 4% LMS-2

86% isoparaffin

10% NaOH

99.7%

[160]

Kerosene

5% NaOH

99.8%

[101]



725


TABLE 25.4 (continued) ELM Systems for the Separation of Metal Ions Solute Chromium Chromium

Chromium

Cobalt

Cobalt

Cobalt

Cobalt

Copper Copper

Copper Europium

External Feed Phase Cr2 O2 7

Diluent

[163]

89.8 wt% HYVIS 2, 10 wt% Shellsol 2046, 0.2 wt% polyisobutylene 5 vol% decanol, kerosene

0.25 M NaOH

80%

[86]

2% v=v SPAN 80

Cyclohexane

2 M HNO3

~90%

[164]

Kerosene

0.5 M H2SO4

~70%–90%

[113]

LOPS

50–200 g=L H2SO4

~95%

[165]

0.005 M EDTA, pH 7.9 250 g=L H2SO4 26.4% v=v H2SO4

99%

[162]

90%

[127]

99%

[166]

30% w=w H2SO4 4 N HNO3

~50%

[166]

>99%

[167]

1.5 M HCl

>99%

[41]

0.1 M Na2SO3=0.5 M NaOH 1 N HCl

99%

[108]

-

[168]

0.5 vol% Aliquat 336

1.5 vol% Paranox 106

6.3% v=v DEHPA

6.3% v=v DEHPA

1.0 g=L CoSO4, 0.1 M NaAc, pH 5 2 105 M Co2þ pH 7.95

DEHPA

5% v=v SPAN 80 or Paranox 100 or ECA 4360 ECA 4360

0.36% LIX 64 N

2.1% SPAN 80

Toluene

80–2500 ppm CuSO4 pH 1.5–7 1000 ppm CuSO4 pH 6 (buffered with NaAc=HAc) 1000 ppm CuSO4 Unbuffered 1.3 103 M Eu3þ

Shell SME 529

3% wt=wt SPAN 80 6.5% v=v Emery DNP-8

Shellsol T

9.98% w=w Emery DNP-8 2 vol% SPAN 80

79.92% w=w Hexadecane 87.5 vol% kerosene, 10 vol% polybutadiene Kerosene or dodecane

Gold

63.9 ppm Au pH 2.54

Iron

5 mM FeCl3

Lead

75–127 mg=mL Pb2þ (PbNO3) and in the presence of LiNO3 or KNO3 1020 ppm Pb pH 4.66 2.5–190 ppm Hg2þ

0.5 vol% DEHPA

5 vol% 2-ethylhexyl phosphonic acid mono-2ethylhexyl ester (PC-88A) 1% N503

References

>99%

75–100 mg=L K2Cr2O7 pH 5.1–5.4 Co(NO3)2 6H2O, 0.5 M KNO3 pH 3.1 Co(NO3)2 6H2O, pH 4.5

10.1% w=w LIX 860


0.1 N NaOH

4%–5% SPAN 80 —

1.6% w=v benzoylacetone

Internal Phase

n-Hexane

20% tri-n-butyl phosphate (TBP) 0.05 M alamine 336

4.6 103 M Eu3þ pH >2.78

Mercury

Surfactant

in 0.5 N H2SO4 0.000962 N K2Cr2O7 pH 1.6

Europium

Lead

Extractant

4 mol% SPAN 80

65.5% v=v decane

—

0.005–0.01 M NaOH

[40]

4% polyamine E644

Kerosene

5 mM CH3(C8H17)3NCl (methyltrioctylammonium chloride) 0.1 M dicyclohexano-18crown-6

2 wt% SPAN 80

Toluene

1 vol% SPAN 80

70 vol% S-100 N, 30 vol% indopol L-100

0.1 M Na4P2O7

>90%

[169]

5 wt% DEHPA

3 wt% ECA 5025 3.5 vol% Rofetan OM (fatty ester)

82 wt% tetradecane Decane

6N H2SO4

99.95%

[161]

0.1N HCl, 7.6 g=L Thiourea

96%

[39]

2.92 kg=m3 1,1-di-n-butyl3-benzoyl-thiourea (DBBT)

(continued )



726


TABLE 25.4 (continued) ELM Systems for the Separation of Metal Ions Solute

External Feed Phase 2þ

Extractant

6 N H2SO4

~80%

[170]

2% Arlacel C

95:5 w=w tetradecane: mineral oil Normal paraffins

2 N H2SO4

>99%

[171]

0.02M TOA

5% SPAN 80

Kerosene

2 M Na2CO3

99.5%

[172]

5% Di(2-ethylyhexyl) dithiophosphoric acid (DTPA) 5 wt% DTPA

3% ECA 4360, 0.2% SPAN 80

Shellsol T

250 g=L H2SO4

3 wt% ECA 11522 polyamine 3 vol% SPAN 80 SPAN 80

92 wt% Shellsol T (paraffin)

250 g=L H2SO4

>99.8%

[157]

90.6 vol% n-Heptane Toluene

1 M H2SO4

42%

[173]

1 M HNO3

>99%

[174]

0.1 M SPAN 80

n-Heptane

1 M HCl, 0.1 M thiourea

>95%

[175]

3 wt% polyamine or SPAN 80 2% proprietary surfactant 3 wt% SPAN 80

97 wt% toluene or paraffin

0.5 M Na2CO3

>90%

[176]

Kerosene þ 2% n-decanol 97 wt% kerosene

0.5–2.0 M NaCl HCl, pH 1.6

>99.9%

[177]

92%

[178]

Mercuric chloride or nitrate

0.35 M oleic acid

Mercury

Mercury nitrate 0.07–0.35 g=L Hg2þ pH 2.8–3.0 1:06 g=L Mo7 O6 24 , 16 g=L H2SO4 2.2 g=L Ni2þ

10% 2:1 mixture of linoleic and oleic acid

Nickel

400–6000 mg=L Ni2þ in H2SO4

Nickel

100–500 ppm Ni(II)

6.4 vol% DEHPA

Nickel

0.2M Ni (NO3)2 6H2O pH 2.5–3.5

Palladium

63 ppm Pd2þ, 25,000 ppm Fe3þ 1 M HCl

Plutonium

50 mg=L Pu4þ in 3 M HNO3=0.05 M NaNO3 1 mg=L Se (IV) or Se (VI) pH 10.5 100 ppm Sr2þ pH 5.3 5 104 M SrCl2 in acetate buffer pH 5.5 þ 0.025 M CaCl2 TcO4, 0.1 N HNO3 TeCl4, 4.2–4.5 M HCl 0:016 g=L UO2þ 2 , ~6 M H3PO4 1:15 kg=m3 UO2þ 2 , 33 kg=m3 H2SO4 4:2 103 M UO2þ 2 , 0.1 M Na2SO4 pH 2

Di DEHPA þ 5,8-dimethyl7-hydroxydodecane6-oxime (LIX63) Di-oleylphosphoric acid (DOLPA) þ 5,8-dimethyl7-hydroxydodecane6-oxime (LIX63) 0.1 M MSP-8 (di-2 ethylhexyl monothiophosphoric acid) 0.2 M dicyclohexano-18crown-6 (DC18C6)

Strontium

Technetium Tellurium Uranium Uranium Uranium

References [160]

Mercury

Strontium

Efficiency= Recovery >99%

Alamine 336

Selenium

Internal Phase 2% NaOH

1.1 g=L Hg

Nickel

Diluent 86% isoparaffin

Mercury

Molybdenum

Surfactant

2% proprietary complexing agent 0.2 M DEHPA

0.1% SPAN 80, 3% polyamine 3 wt% paramins ECA 5025

—

[126]

1.25 102 M DEHPA

2.5% RADIASURF 7155

Dodecane

1 M HNO3

99.99%

[162]

0.5% Aliquat 336 5% DEHPA

3% SPAN 80 3% SPAN 80

Cyclohexane Kerosene

1 M NaClO4 5.0–6.0 M acid

92% ~85%

[162] [42]

DEHPA=TOPO

Polyamine

LOPS

H3PO4

>90%

[179]

0.02 M TOA

5 wt% SPAN 80

Kerosene

1 M Na2CO3

95%

[180]

0.075 M DEHPA

0.96% SPAN 80

Cyclohexane

1.30 M H2SO4

99.7%

[162]



727


TABLE 25.4 (continued) ELM Systems for the Separation of Metal Ions Solute

External Feed Phase

Extractant

16–604 ppm VO3 200 mg=L Zn2þ in 6 g=L H2SO4

2% TOA

Zinc

0.5 g=L Zn2þ

2–4% DTPA

Zinc

ZnSO4 in 50 g=L Na2SO4 pH 1.4 0–5 g=L, 0–1 M Na2SO4

3.5 wt% DTPA

Vanadium Zinc

Zinc

5 wt% DTPA

0–10 wt% bis-2-ethylhexyl mono thiophosphoric acid (MTP)

Surfactant 4% succimide derivative 3 wt% ECA 11522 polyamine 2% ECA 4360 2 wt% ECA 4360 1–20 wt% ECA 4360

Diluent Kerosene 92 wt% Shellsol T (paraffin) Shellsol T 94.5 wt% Shellsol T C7–C12 n alkane

Internal Phase


References

2.5%–5% Na2CO3 250 g=L H2SO4

98–99.3%

[181]

>99.5%

[126]

250 g=L H2SO4 5 N H2SO4

99.5%

[103]

H2SO4

—

[36]

—

[89]

25.6 PILOT PLANT STUDIES=COMMERCIAL APPLICATIONS As stated earlier, one of the major advantages of ELM technology is the creation of systems with very high surface area to volume ratios. As noted by Kentish and Stevens [218], with such systems, large interfacial areas can be achieved in units that occupy significantly less floor space and cost much less than traditional solvent extraction columns. This has led to the commercialization of this technology for the extraction of zinc [157,219], phenol [219], and cyanide [219] from wastewater. Pilot plant studies have also been conducted on the use of ELMs for the removal of contaminants such as cadmium [220], copper [221], chromium [160], and zinc [157,219] from wastewaters. Details of some pilot plant and commercial studies are given in Table 25.9. Other plants for the removal of zinc have been noted in the literature [157]: Glanzstoff AG, Austria (7000 L=h), CFK Schwarza, Germany (200 L=h), and AKZO=Ede, the Netherlands (200 L=h).

25.7 CONCLUDING REMARKS Nearly 40 years since the initial reporting of the emulsion liquid membrane concept and the many potential applications discussed in the literature, very few commercial plants are operating successfully. This technology offers a number of distinct advantages including reduction in the volume of organic, the large interfacial area, hence transfer rates are compared with traditional solvent extraction. However, issues of osmotic swelling, the stabilization, and destabilization of the emulsion remain

TABLE 25.5 ELM Systems for the Separation of Anions Solute

External Feed Phase

Surfactant

Diluent

2 wt% Lan 113-b (or polyamine)

93 wt% kerosene

0.8 wt% NaOH

99.6%

[182]

2% SPAN 80

S100 N

84%

[183]

SPAN 80 1–2% SPAN 80, polyamine

Kerosene Isoparaffin

50% H2SO4 20% FeSO4 30% H2O Na2CO3 CaClþ 2 NH4OH or Ca(OH)2

94% 91%–98%

[44] [183]

Cyanide

130 mg=L CN as in Au(CN)2 pH 9–9.5

Nitrate

0.22 g=L NaNO3

5 wt% extractant M (protonated cation type) (or Alamine) —

Nitrate Phosphate

500 ppm HNO3 0.27%–0.57% PO3 4

TOA Amines

Internal Phase


Extractant

References



728


TABLE 25.6 ELM Systems for the Separation of Acids and Bases Solute Acetic acid Acetic acid

Ammonia

Aniline

Aniline o-Chloroaniline

2-Chlorophenol Citric acid Ethylaniline

Hydroxy benzoic acid m-Nitroaniline

Nitrophenol Phenol Phenol Phenol

o-Toluidine

External Feed Phase 0.0861 M CH3COOH 300 ppm CH3COOH

7.143 102 M NH3, pH >12 0.5%–1.0% aniline in water 3.5 103 M aniline pH 8 3.5 103 M chloroaniline pH 8 7.778 mol=m3 2-Chlorophenol 0.048 M citric acid 3.5 103 M ethylaniline pH 8 200 mg=L C7H6O3 pH 2.3 3.5 103 M nitroaniline pH 8 3000 ppm p-nitrophenol 1000 mg=L phenol 200 ppm phenol, pH 7 787 ppm phenols, 3487 ppm formaldehyde pH 4.6 3.5 103 M o-toluidine pH 8

Internal Phase


Extractant

Surfactant

Diluent

1 wt% tri-n-butyl phosphate (TBP) —

8 wt% E 644 polyamine 10% v=v ethoxylated dinonyl phenol, Emery DNP-8

72 wt% kerosene, 19 wt% paraffin Decane, dodecane, tetradecane or hexadecane

3 M NaOH

>95%

[33]

0.25 M NaOH

[184]

—

4 wt% SPAN 80

96 wt% paraffin

20 wt% H2SO4

Up to 45% depending on the oil phase 99.5%

—

SPAN 80

Kerosene

3.0–6.0 M HCl

99.5%

[133]

—

8% SPAN 80

0.1–0.5 N HCl

99%

[186]

—

8% SPAN 80

Kerosene and isoalkane fractions Kerosene and isoalkane fractions

0.1–0.5 N HCl

96.7%

[186]

—

2 wt% polyamine ECA 4360 4 vol% C9232

98 wt% Shellsol T

500 mol=m3 NaOH 2.5 M NaOH

>99%

[29]

97%

[43]

0.1–0.5 N HCl

99%

[186]

40 vol% Alamine 336 —

8% SPAN 80

Mineral oil, 8 vol% chloroform Kerosene and isoalkane fractions

References

[185]

—

2% ECA 4360J

96% Shellsol T, 2% isodecanol

0.2 M NaOH

99.3%

[30]

—

8% SPAN 80

Kerosene and isoalkane fractions

0.1–0.5 N HCl

96%

[186]

—

4% v=v SPAN 80

76% v=v kerosene

0.7 M NaOH

95%

[85]

—

89.8 wt% kerosene, 6.7 wt% paraffin Dewaxed solvent 100 Neutral 98 wt% Shellsol T

5 wt% NaOH

99.95%

[187]

10 wt% NaOH

90%

[8]

—

3.5 wt% LMS-2 (anion type) 0.1–20 wt% SPAN 80 2 wt% ECA 4360

500 mol=m3 NaOH

80–95%

[188]

—

8% SPAN 80

Kerosene and isoalkane fractions

0.1–0.5 N HCl

99.2%

[186]

—

a problem. Also, the influence of surface active impurities on the stabilization process is an issue during reuse of these systems. These challenges to the industry implementation of this technology remain as barriers to more wide spread application. At present, however, the technology has found some application in the treatment of relatively low-concentration waste streams where other technologies are not economical. Hence experience in these applications will drive further development and confidence in this technology and so find its niche in the available separation technologies.

820 mg=L Co, 1380 mg=L Mn, 3.6 mg=L Ni, 1.4 mg=L Fe, 1.4 mg=L Cr, 410 mg=L Na, pH 3 820 mg=L Co, 1380 mg=L Mn, 3.6 mg=L Ni, 1.4 mg=L Fe, 1.4 mg=L Cr, 410 mg=L Na, pH 3.1

Cobalt=manganese

0.16 g=L Co, 0.16 g=L Ni, 0.1 M NaAc-HAc buffer, pH 3.5–5.8 1.5 g=L Co, 95.0 g=L Ni, 96.0 g=L citric acid buffer Cu, Mg, Al, Fe, Fe 8 g=L Cu,100g=L Zn

Cobalt=nickel

Copper Copper=zinc

Cobalt=nickel

0.01 M Co(II), 0.01 M Ni (II) pH 5.5

Cobalt=nickel

Cobalt=manganese= chromium=nickel= sodium=iron

Barium

10–6 M Eu(III), Am(III), 1 M lactic acid, 5 103 M diethylenetriamine-N, N,N’,N’,N’-pentaacetic acid 0.01 N BaCl2, MgCl2, CaCl2, SrCl2, pH 8.5

External Feed Phase

Americium= europium

Solute

TABLE 25.7 ELM Systems for the Separation of Mixtures

2.5% LIX 64N 5% Acorga PT 5050

12 mol=m3 2-ethylhexyl phosphonic acid mono-2ethylhexyl ester (PC-88A) 9.5 g=L Cyanex 302

0.01–0.1 M LIX 65N, LIX 84, LIX 860

5.5 vol% DEHPA, 0.5 vol% TBP

2% polyamine 2% PA 18

3 vol% ECA 4360

0.005–0.05 M cationic L-glutamic acid dioleyl ester quaternary ammonium phosphoric acid (2C18D9GEC2QAC2PA) 50 mol=m3 PX-100

3 vol% SPAN 80

3 vol% SPAN 80

5 vol% SPAN 80

0.01 M crown ether carboxylic acid TBP

2% v=v polyethylene glycol dioleate

Surfactant

0.1 M DEHPA

Extractant

S100N Shellsol T

Escaid 110

n-Heptane

n-Heptane

90% kerosene

50 vol% mineral oil=45 vol% toluene Shellsol T

Cyclohexane

Diluent

—

Co: up to 80%

99%

0.05 M H2SO4

20% H2SO4 250 g=L H2SO4

—

Co: 98% Ni: 13%

Cr: 88% Ni: 50%–60% Na: 55% Fe: 60% Co: 99%

Co: 85%; Mn: 99%

Co: 64%; Mn: 98.5%

98%


2.0 M H2SO4

0.5 M H2SO4

2 M H2SO4

H2SO4

0.2 M HCl

1 M HCl

Internal Phase

(continued )

[196] [126] [103]

[195]

[132]

[193,194]

[192]

[191]

[190]

[189]

References


Use of Emulsion Liquid Membrane Systems in Chemical and Biotechnological Separations 729

1460 ppm Cu, 900 ppm Ni, 45 ppm Cr, 40 ppm Fe, 0.05 M NaAc-Hac, pH 3.5 100 ppm Ga, 8715 ppm Fe, 60 ppm Co, 80 ppm Ni, 265 ppm Cd, 24,0202 ppm Zn, 6592 ppm Pb, 315 ppm Cu, 3100 ppm Al 52.8–64.1 ppm Au, Pt, Pd, Ag, Cu, Pb, Fe, pH 3.0 1 104 M La3þ, Eu3þ, Lu3þ (nitrate), pH 6–7

Copper, nickel, chromium, iron

Nitrophenols

Lead

Lanthanum, europium, lutetium

Gold

2 vol% of emulsion SPAN 80

3% v=v SPAN 80

5 vol% SPAN 80

0.01 M 2-(sym-dibenzo-16crown-5-oxy)hexanoic acid 0–0.02 M Crown ether dicyclohexano-18-crown-6 (DC18C6) —

4% polyamine E 644

8% ECA 4360J

5 vol% SPAN 80

3 vol% SPAN 80

20000 MW PEG

Surfactant

1% N 503

6% TOPO

10 vol% DEHPA

6.3 vol% DEHPA

0.2 mM ammonium thiocyanate

Extractant

75.5 vol% of emulsion kerosene, 10 vol% of emulsion paraffin, 2.5 vol% of emulsion cyclohexanone

45 vol% mineral oil, 50 vol% toluene Toluene

Kerosene

Kerosene

80% v=v kerosene þ 5% v=v heptane

Kerosene

1,2-dichloroethane

Diluent

0.7 M NaOH

0.05 M Li4P2O7

>98%

[85]

[200]

[199]

Lu3þ: >99%

—

[108]

>97% 0.5 M Na2SO3 HNO3, pH 2

[112]

[4]

[198]

[197]

References

>97%

Cu(II): 60% Co(II): 45% Ni(II): 45% Cu(II): 60% Co(II): 60% Ni(II): 50% Cu(II): 90% Ni(II): 46%


15% w=w HCl

0.5–1.0 N HCl

2 M HNO3

1 M KOH

Internal Phase

730

Pb2þ, Naþ, Kþ, Rbþ, Csþ, Agþ, Tlþ, Mg2þ, Ca2þ, Sr2þ, Ba2þ, Zn2þ 1000 mg=L p-nitrophenol, 1000 mg=L o-nitrophenol, 500 mg= L 2,4 dinitrophenol

1g=L Cu, Co, Ni

Copper, cobalt, nickel

Gallium

100 ppm Cu, Co, Ni

External Feed Phase

Copper, cobalt, nickel

Solute

TABLE 25.7 (continued) ELM Systems for the Separation of Mixtures



Zinc, cadmium, nickel

Zinc, calcium, lead

Zinc, cadmium, copper, lead

Zinc, cadmium, copper

Sodium

Silver

Platinum, palladium Praseodymium, neodymium, lanthanum

6 g=L Zn, 0.014 g=L Ca, 0.004 g=L Pb 0.31.5 103 M Zn, Cd, Ni, pH 6–8 (acetate or tris buffer), 0.1 M KNO3 3.8 103 M 4-acyl-5pyrazolones

5% DTPA

5 wt% di(2-ethylyhexyl) thiophosphoric acid (MTPA)

3–7% monothio or dithio di (2-ethylhexyl) phosphoric acid

0–0.02 M Crown ether dicyclohexano-18-crown-6 (DC18C6) 0.01 M Crown ether carboxylic acid

Agþ Naþ, Kþ, Rbþ, Csþ, Tlþ, Mg2þ, Sr2þ, Ba2þ, Pb2þ, Zn2þ 0.01 M NaCl KCl, RbCl, LiCl 335 ppm Zn 21.6 ppm Cd, 3.9 ppm Cu, 0.3 ppm Co, 73 ppm Mg, 1.7 ppm Fe, 10 g=L Cl, pH 1.4 230 mg=L Zn2þ 2.7 mg=L Cd2þ 1.1 mg=L Cu2þ 0.5 mg=L Pb2þ pH 3.4

— 0.05 M mono(2-ethylhexyl) 2-ethylhexylphosphonate (PC-88A)

100 ppm Pt, 50 ppm Pd 0.0001 M Pr3þ, 0.00035 M Nd3þ, 0.0008 M La3þ, pH 2

3% ECA 11 523 (Exxon)

2% PX100

3 wt% ECA 115222 polyamine

2% polyamine ECA 4360

5 vol% SPAN 80

3% v=v SPAN 80

0.02 M 2C18D9GEC2Q 0.02 M cationic dioleyl L-glutamate quaternary ammonium chloride (2C18D9GEC2Q)

Shellsol T or Shellsol AB

Shellsol T

92 wt% Shellsol T

50 vol% mineral oil, 45 vol% toluene Shellsol T

Toluene

n-Heptane n-Heptane

0.1 M H2SO4

250 g=L H2SO4

250 g=L H2SO4

2.5 M H2SO4

0.2 M HCl

0.05 M Li2S2O3

1 M HClO4 0.5 M H2SO4

—

~99%, 0.2 mg=L Zn2þ, 0.02 mg=L Cd2þ, 0.007 mg=L Cu2þ, 0.01 mg=L Pb2þ —

[203]

[103]

[126]

[202]

[190]

—

~97% ~10ppm Zn

[200]

[104] [201]

~95% >99% for Pr3þ and Nd3þ (good selectivity vs. La3þ) —


Use of Emulsion Liquid Membrane Systems in Chemical and Biotechnological Separations 731



732


TABLE 25.8 ELM Systems for the Separation of Biochemicals Solute

External Feed Phase

Extractant

[139]

Kerosene 90–95 vol% paraffin oil

~100% Up to 50%

[204] [83]

— 15.5 wt% DEHPA

SPAN 80 5 wt% SPAN 80

80%–85% Up to 80%

[205] [206]

—

0.05 vol fraction SPAN 80

Paraffin 79.5 wt% kerosene Heptane or m-xylene

0.2 N HCl Phosphate buffer pH 7.3 or pH 11 glycine, NaCl, NaOH buffer Saponin 1 M HCl

5 wt% Amberlite LA2 0.01 M Amberlite LA2

5 wt% Paranox 100 5 wt% ECA 4360J polyamine

90 wt% kerosene 95 wt% kerosene

0.02 M Amberlite LA2 secondary amine 0.04 M di-n-octylamine (DOA) 20 wt% DEHPA

8 wt% ECA 4360J polyamine

Berberine or ephedrine 50 hexobarbitol, amobarbital, or phenobarbitol, pH 7.3

— —

Erythromycin

103 M erythromycin, pH 8.5–9.5

Lactic acid

0.1 M lactic acid

Penicillin G

0.02 M penicillin G, 0.408 M citrate buffer, pH 5 0.02 M penicillin G in 0.408 M citrate buffer, pH 5 0.002 M penicillin G, pH 6

Penicillin G

Penicillin G

Phenylalanine

12–35 g=L Lphenylalanine, pH 2.5 (with H2SO4)

Phenylalanine

0–0.12 M L-phenylalanine, pH 3.0 60%

Alkaloid

Blood 2 M peptide, pH 2.7


1.7 M H2SO4

20% v=v DEHPA

Di and tripeptides

Internal Phase

Kerosene

6–7 g=L alanine, pH 6

Cholesterol

Diluent

1.5% v=v SPAN 80 2 vol% SPAN 80 5–10 vol% mannide monooleate or montanide 80

Alanine

Barbiturates

Surfactant

5 wt% Paranox 100 5 wt% Paranox 100 polyamine

0.075–0.225 M DEHPA

4% v=v Paranox 100

4% v=v DEHPA

3% v=v SPAN 80

15% w=w DEHPA

2% w=w alkylbenzosulfonic acid 450–500 mg=L 350 mg=L

Pilot plant: 1,000 L=h Commercial plant: 75,000 L=h

—

Boric acid-sodium phosphate buffer, pH 5.5–6.5 0.6 M Na2CO3

98%

[209]

0.1 M Na2CO3

80–95%

[210,211]

Kerosene or kerosene þ paraffin oil Kerosene þ butyl acetate (ratio: 7:3) 75 wt% S-60NR (50 vol% Olefins, 49 vol% Paraffins, 1 vol% aromatics) 86% v=v Telura 619

0.2 M Na2CO3

~90% continuous process 85%–90% continuous process ~80%

[212]

—

[121]

93% v=v kerosene Carbon tetra chloride

1.5 M HCl

~80%

[26]

1.6 M HCl

~80%

[215]

3 wt% DTPA, 2 wt% Exxon PX100 surfactant, 95 wt% Shellsol T 3 wt% DTPA, 3 wt% Exxon PX100 surfactant, 94 wt% Shellsol T

250 g=L H2SO4 250–300 g=L H2SO4

20 kV, 50 Hz, 5 kW h=m3 emulsion

98%

0.5 M K2CO3 1.5 M H2SO4

1.6 M HCl

[207,208]

[213]

[214]



733


TABLE 25.9 Pilot Plant and Commercial Applications of ELM Systems Plant Size

External Phase

Application

Company

Removal of copper from mine solutions [221]

Bureau of Mines, United States

Pilot plant: 90–159 L=h

0.12–1.4 g=L Cu(II)

Removal of copper from synthetic mine solutions [45] Removal of cadmium [220]

Davy McKee Company, United Kingdom

18 L=h

500 ppm Cu(II), pH 2.5

Delft University of Technology, the Netherlands

Pilot plant: 90 L=h

100 ppm Cd

Removal of phenol [187,219]

Nanchung Plastic Factory Guangzhou, PR China

Pilot plant: 200–250 L=h Commercial plant: 400–500 L=h

350–925 mg=L phenol

Membrane Composition

Internal Phase

5.0–7.5 wt% Acorga M5640, 1.0 wt% Paranox 100, 91.5–94 wt% 50–50 Isopar 2.5 wt% LIX 64N, 2.0 wt% polyamine, 95.5 wt% S100N

8.5–20 g=L Cu(II), 160–165 g=L H2SO4

0.5%–1.0% Alamine 304 in Shellsol D 70, 0.5–1.0% ECA 4360J 3.5 wt% LMS-2, 6.7 wt% liquid paraffin, 89.8 wt% kerosene

30 g=L Cu(II), 150 g=L H2SO4

—

>90%

95%

Electrostatic splitter 3 kV, 4 kHz

95%

5 wt% NaOH

Coalescer EC-1 20 kV, 0.6–25 kHz, 50 mA, capacity 20 L, 32–55 L=h

99.6%

constant constant constant constant constant Biot number Damköhler number effective solute diffusivity in the emulsion mixture Sauter mean diameter of the dispersed phase droplets=globules stirrer or impeller diameter ratio of actual mean drop diameter to diameter at f ¼ 0.1 swelling factor external phase mass transfer coefficient reaction rate constant 1=3 (rc =re )1=2 md « 1=3 d32 =g radius of the emulsion globule Reynolds number power input vessel diameter circulation time volume of the fluids total volume of the external phase total volume of the internal phase total volume of the membrane phase Weber number ¼ v2 ds3 rc =g

Greek Symbols ae aI Dr g

5–8 kV, 60 Hz

Efficiency

1 M HNO3, 10 mM KNO3

SYMBOLS B b b1 B2 B3 Bi Da De d32 ds ff Fsw k k1 Nvi R Re P T tc V Ve Vi Vm We

Demulsifier

the distribution coefficient of the solute between the external and membrane phase the distribution coefficient of the solute between the internal and membrane phase density difference between (W=O) emulsion and the continuous phase interfacial tension between the membrane and the external phase



734

« «av «max mc md yc rc f fav fint v


P=V(1 fav)rc average energy dissipation rate per unit mass turbulence kinetic energy dissipation rate per unit mass dynamic viscosity of the continuous phase dynamic viscosity of the dispersed phase kinematic viscosity of the continuous phase density of the continuous phase volume fraction of the dispersed phase vessel average of f volume fraction of the internal aqueous phase in the emulsion drop stirring rate

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204. Tang H, Ma Z, and Liu L. Alkaloid extraction from plants with liquid membranes. Water Treat 1990; 5: 214–221. 205. Yagodin GY, Lopukhin Y, Yurtov E, Guseva T, and Sergienko V. Extraction of cholesterol from blood using liquid membranes. Proceedings of the International Solvent Extraction Conference (ISEC’83), Denver, CO, 26 August to 2 September, 1983: 385–386. 206. Hano T, Matsumoto M, Kawazu T, and Ohtake T. Separation of di and tripeptides with solvent extraction and an emulsion liquid membrane. J Chem Tech Biotechnol 1995; 62: 60–63. 207. Habaki H, Isobe S, Egashira R, and Kawasaki J. Permeation and concentration of erythromycin by supported and emulsion liquid membranes. J Chem Eng Jpn 1998; 31: 47–54. 208. Habaki H, Egashira R, Stevens GW, and Kawasaki J. A novel method of improving low separation performance for w=o=w ELM permeation of erythromycin. J Membr Sci 2002; 208: 89–103. 209. Mok YS, Lee SC, and Lee WK. Water transport in water-in-oil-in-water liquid emulsion membrane system for the separation of lactic acid. Sep Sci Technol 1994; 29: 743–764. 210. Lee SC and Lee WK. Extraction of penicillin G from simulated media by an emulsion liquid membrane process. J Chem Tech Biotechnol 1992; 55: 251–261. 211. Mok YS, Lee SC, and Lee WK. Synergistic effect of surfactant on transport rate of organic acid in liquid emulsion membranes. Sep Sci Technol 1995; 30: 399–417. 212. Lee SC, Lee KH, Hyun GH, and Lee WK. Continuous extraction of penicillin G by an emulsion liquid membrane in a countercurrent extraction column. J Membr Sci 1997; 124: 43–51. 213. Hano T, Matsumoto M, and Ohtake T. Continuous extraction of penicillin G with liquid surfactant membrane using Vibro Mixer. J Membr Sci 1994; 93: 61–68. 214. Hong SA, Choi HJ, and Nam SW. Concentration of amino acids by a liquid emulsion membrane with a cationic extractant. J Membr Sci 1992; 70: 225–235. 215. Dzygiel P and Wieczorek P. Extraction of amino acids with emulsion liquid membranes using industrial surfactants and lecithin as stabilisers. J Membr Sci 2000; 172: 223–232. 216. Kim SJ, Kim SC, and Kawasaki J. A scale-up of stirred tank contactors for the liquid membrane permeation of hydrocarbons. Sep Sci Technol 2001; 36: 3585–3598. 217. Kim SJ and Kim SC. Separation of valuable bicyclic aromatic components from light cycle oil by emulsion liquid membrane. Sep Sci Technol 2004; 39: 1093–1109. 218. Kentish SE and Stevens GW. Innovations in separations technology for the recycling and re-use of liquid waste streams. Chem Eng J 2001; 84: 149–159. 219. Cahn RP and Li NN. Commercial applications of emulsion liquid membranes. In: Li NN, Calo JM, eds. Separation and Purification Technology, New York: Marcel Dekker, 1992: 195–212. 220. Breembroek GRM, Witkamp GJ, and Van Rosmalen GM. Design and testing of an emulsion liquid membrane pilot plant. Sep Sci Technol 2000; 35: 1539–1571. 221. Nilsen DN, Hundley GL, Galvan GJ, and Wright JB. Field testing of a liquid-emulsion membrane system for copper recovery from mine solutions. In: Bartsch RA, Way JD, eds. Chemical separations with liquid membranes, Washington, DC: American Chemical Society, ACS symposium series 642, 1996: 329–341.



Electroporation 26 Membrane and Emerging Biomedical Applications K.P. Mishra CONTENTS 26.1

26.2

26.3

26.4

26.5

Introduction .................................................................................................................................................................. 742 26.1.1 Cell Plasma Membrane and Biotechnological Potential.............................................................................. 742 26.1.2 Permeability Barrier and Electroporation Phenomenon............................................................................... 742 26.1.3 Scope and Outline ........................................................................................................................................ 743 Physicochemical Aspects of Cell Membrane............................................................................................................... 743 26.2.1 Phospholipid Bilayer .................................................................................................................................... 743 26.2.2 Membrane Proteins ...................................................................................................................................... 744 26.2.3 Fluid Mosaic Membrane Structure .............................................................................................................. 744 26.2.4 Physical Organization .................................................................................................................................. 744 26.2.5 Molecular Transport and Permeability Barrier ............................................................................................ 745 26.2.6 Methods to Overcome Bilayer Barrier......................................................................................................... 745 Cell Membrane Electroporation ................................................................................................................................... 745 26.3.1 Basic Principle ............................................................................................................................................. 745 26.3.2 Resting and Induced Transmembrane Potential........................................................................................... 746 26.3.3 Biophysical Basis of Membrane Electroporation ........................................................................................ 746 26.3.4 Electroporation Threshold............................................................................................................................ 747 26.3.5 Mechanism of Electroporation ..................................................................................................................... 747 26.3.5.1 Pore Model................................................................................................................................... 747 26.3.5.2 Mechanical Breakdown Model .................................................................................................... 747 26.3.6 Resealing of Electroporated Membrane....................................................................................................... 748 26.3.7 Factors That Control Electroporation Outcome........................................................................................... 748 26.3.7.1 Electrical Parameters.................................................................................................................... 748 26.3.7.2 Experimental Conditions.............................................................................................................. 749 26.3.7.3 Biological Factors ........................................................................................................................ 749 Electroporation Devices ............................................................................................................................................... 749 26.4.1 Electric Pulse Generators ............................................................................................................................. 749 26.4.2 Cuvettes and Electrodes ............................................................................................................................... 749 Some Applications of Electroporation ......................................................................................................................... 750 26.5.1 Cell Fusion and Hybridoma Technology..................................................................................................... 750 26.5.2 Electrotransfection........................................................................................................................................ 750 26.5.3 Therapeutic Applications ............................................................................................................................. 750 26.5.4 Potential for Overcoming Tumor Resistance............................................................................................... 750 26.5.5 Electro Cell Manipulation ............................................................................................................................ 751 26.5.6 Electroloading of Cells................................................................................................................................. 751 26.5.7 Electrochemotherapy.................................................................................................................................... 752 26.5.8 Radio-Electrochemotherapy ......................................................................................................................... 752 26.5.9 In Vivo Applications.................................................................................................................................... 753 26.5.10 Electroporative Gene Delivery..................................................................................................................... 753 26.5.11 Electrodelivery of Proteins........................................................................................................................... 753 26.5.12 Incorporation of Radioiodine into Tumor Cells .......................................................................................... 754

741


742



26.5.13

Electroporation in Biotechnology ................................................................................................................ 754 26.5.13.1 Water Sterilization ..................................................................................................................... 754 26.5.13.2 Food Sterilization ....................................................................................................................... 755 26.5.14 Transgenic Plants and Animals.................................................................................................................... 755 26.5.15 Transdermal Drug Delivery ......................................................................................................................... 755 26.5.16 Ex Vivo Applications................................................................................................................................... 755 26.6 Single-Cell Electroporation .......................................................................................................................................... 755 26.7 Future Prospects and Challenges ................................................................................................................................. 756 Acknowledgments.................................................................................................................................................................... 756 References ................................................................................................................................................................................ 756

26.1 INTRODUCTION 26.1.1 CELL PLASMA MEMBRANE

AND

BIOTECHNOLOGICAL POTENTIAL

Cells are the basic building units of living organisms, which are bounded by intricate plasma membrane guarding the cell interior. Life or death of a cell critically depends on the intactness of outer membrane whose structure and function are intense research curiosity and also, a potential source of developing various technologies for biomedical and industrial applications. Over the years, considerable understanding of cell membrane structure and function has been obtained but much remains to be learned. It is fairly well established that plasma membrane of mammalian cell mainly consists of lipids, proteins, and cholesterol, and a bilayer of lipids forms the core structure separating the cell interior from the exterior surroundings. The recent impetus in biotechnology has propelled membrane research to unravel the molecular design and function of cellular membrane with the hope to developing novel products and processes. A few tools and devices based on membrane have already been developed and employed for practical applications in medicine, industry, and biotechnology. Modeled after cell membrane, liposomes have been prepared in laboratory, which have found numerous biological, medical, and industrial applications. The precision, speed, and sensitivity with which real cell membranes regulate molecular transport, sense tiny concentration of substances, carry out intermolecular communications have much to offer in developing strategic and biocompatible materials, sensor devices, and signal processor technologies. Extensive recent research has provided a few physical and chemical methods to overcome natural transport barrier of membrane, opening many new prospects for cellular engineering and membrane biotechnology. For example, methods have been developed to introduce exogenous molecules into a variety of plant, bacterial, and mammalian cells but some limitations are encountered. Obviously, gaining access to the cell interior without affecting cell viability holds enormous potential to basic research in exploring the tiny internal world of living cells and in providing exciting new opportunities to modify cellular composition in controlled fashion with prospects in nanoscience, biomolecular engineering, cell membrane fusion and intermembrane trafficking and communication, signaling, sensing processes, etc. that are at work in living cell function. To cope with the harsh variable surroundings, cells have evolved antenna network and implanted microswitches on their membrane surface, which allow them to sort specific ligands and deal with the external stresses for maintaining their survival. The precise steps involved in these functions have remained intriguing but they may offer a whole new opportunity for developing devices to recognize and sort out ensemble of molecules required for numerous applications in diagnostic medicine, pollution science, signal processing, and cell biotechnology. The key to success in achieving newer practical applications largely lies in delineation of molecular design of membrane architecture and in accomplishing the controlled permeabilization of plasma membrane. Advanced imaging methods have aided enormously in learning the intricate dynamic structure and function of cellular membrane, which have generated a host of new opportunities for biomolecular engineering, medicine, and structural science.

26.1.2 PERMEABILITY BARRIER

AND

ELECTROPORATION PHENOMENON

Biological cell membrane is composed of phospholipid bilayer core of a few nanometers thickness, which controls molecular transport and separates cellular interior from the surrounding. Cells possess an intrinsic electrical potential across their membrane due to concentration gradient of ions between the cytosol and surroundings, which plays vital role in regulation of a variety of their functions [1,2]. During the late 60s and early 70s, it was observed that application of high intensity, short duration electric field across cells beyond a certain threshold value dramatically increased the permeability of their membrane allowing entry of otherwise impermeant exogenous substances (Figure 26.1). This phenomenon is popularly called electroporation [3] (and references therein). A variety of external bioactive substances, such as, enzymes, drugs, genes, signaling molecules, and DNA constructs, can be incorporated into cells, which has opened many new prospects in cell biology, biotechnology, and medicine. Scientists have long aimed to change natural makeup of plant, bacterial, and animal cells by introducing external DNA with defined traits but efforts were hampered due to tight bilayer membrane barrier. Over the years, a number of bacterial, yeast, plant,



743

Membrane Electroporation and Emerging Biomedical Applications

+ Electric field

− − − −

+ + + +

Vm = 1.5 Eextr cosq

Pulse type

Molecules get entry through membrane

Resealing of pores

∗ High intensity ∗ Short duration ∗ Repeatable

−

Plasmid

Gene

Antibody

Enzyme

Internalization of exogenous molecules

Drug

Reseal Intact membrane Electroloading Biocapsule

FIGURE 26.1

Biological cell in electric field and entrapment of exogenous substances.

and mammalian cells have been electroporated for introduction of drugs, genes, enzymes, DNA, and other substances providing a valuable tool and technology for drug loading, cancer treatment, gene therapy, cell fusion, etc. [3].

26.1.3 SCOPE

AND

OUTLINE

The area of membrane science and technology is witnessing rapid progress and several devices and procedures are under development for biomedical and industrial applications. This article is confined to description of cell membrane electroporation=permeabilization with examples of a few biomedical technologies for potential practical applications. The phenomenon of cell electroporation has attracted scientists from both basic science as well as engineering disciplines and, currently, it is viewed as an engineering alternative to biological methods for the genetic engineering of cells. This chapter presents a brief review of basic and applied aspects of cell membrane electroporation with relevance to cell biology, biotechnology, and medicine. The content material is certainly not intended to be exhaustive. To enable the reader better grasp the phenomenon of electroporation, a brief account of physical–chemical aspects of cell membrane and basic mechanisms of membrane electropermeabilization have been included. The factors that affect the electroporation efficiency of cells have been outlined including a brief mention of technical advances in electroporator instrumentation. Some applications based on membrane electroporation have been described which are relevant to cell biology, health care, and industry. Cell fusion, loading of impermeant exogenous molecules into cells, electroporative drug and gene delivery, and introduction of DNA and proteins have briefly been described. Recent progress on industrial applications, such as, sterilization of water and liquid food and single-cell electroporation, and their future potential applications in biotechnology have been described.

26.2 PHYSICOCHEMICAL ASPECTS OF CELL MEMBRANE 26.2.1 PHOSPHOLIPID BILAYER The core structure of cell plasma membrane is made of phospholipids arranged in a bilayer with embedded proteins. The phospholipids are amphipathic in nature having both hydrophilic and hydrophobic moieties. The lipids are packed in the bilayer such that the polar head groups face water and the nonpolar segments form the core, insulating the cell interior from the surroundings (Figure 26.2). The bilayer is a self-organized basic membrane structure (5–10 nm thick). The nature and composition of phospholipids control the physical properties of membrane. Though basic skeleton of the bilayer remains unchanged, the composition of the lipids widely differs in bacteria, plant, and mammalian cells. There are three major membrane lipids, namely, phospholipids, glycolipids, and cholesterol which are prevalent in mammalian cells. Of these, the



744


Extracellular fluid carbohydrate

Transmembrane protein

Cholesterol Cytoplasm

FIGURE 26.2

Glycoprotein

Glycolipid

Peripheral protein

Filaments of cytoskeleton

Schematic fluid mosaic model of cell membrane structure.

phospholipids are the most abundant. A typical phosphoglyceride molecule consists of a glycerol backbone, a phosphorylated alcohol, and two fatty acid chains (Figure 26.2). The fatty acids consist of long hydrocarbon chains (14–24 carbon atoms) and may be either saturated or have one or more unsaturated chemical bonds in their structure. Glycolipids are the sugar-containing lipids and in animal cells, the glycolipids are derived from sphingosine. Cholesterol forms another important class of lipids, which is mostly present in the eukaryotes and rarely in prokaryotes. Eukaryotic cells have an abundance of the cholesterol in the plasma membrane. The phospholipids and the glycolipids can readily form vesicles (liposomes) with one or multiple concentric spherical bilayers in aqueous medium, which can be easily prepared in laboratory and they serve as model of membrane. Lipid bilayers are the backbone of the biological membrane [1].

26.2.2 MEMBRANE PROTEINS The cell plasma membrane consists of a variety of proteins associated with the lipid bilayer and they perform multitasks in cell function. The control of transport of ions and molecules across membrane is accomplished through specialized function of membrane proteins. These proteins are distributed in membrane on the outer surface, some on the inner surface, and some others are transmembrane proteins with external and cytoplasmic domains. The majority of the transmembrane proteins are the ion channels or signaling proteins. Generally, lipid to protein ratio is 60:40 but this ratio is found variable in different cells and types of membranes. Membrane proteins impart the dynamic structure and selectivity to membrane function. Both proteins and lipids show motional and diffusion properties within the bilayer structure.

26.2.3 FLUID MOSAIC MEMBRANE STRUCTURE Jonathan Singer and Garth Nicolson proposed the fluid mosaic model of the plasma membrane in 1972 based on the available experimental results [1]. The model suggested that essentially, the membranes were two-dimensional solutions of lipids with proteins incorporated into them forming a mosaic structure (Figure 26.2). This model of membrane structure is widely accepted and continues to be the central to our present understanding of membrane structure. The model is based on following salient features: (1) most phospholipids and glycolipids are arranged in bilayer structure, which has a dual role of acting as a solvent for the proteins forming structural assembly and conferring a permeability barrier to membrane, (2) a small proportion of the lipids closely interact and are in intimate contact with the membrane proteins, (3) membrane proteins are free to diffuse laterally in the lipid matrix unless restricted but they are not free to rotate from one side to the other, and (4) lipids are able to diffuse laterally and they also exhibit flip-flop across the bilayer. Major functions of membrane, e.g., selectivity, receptivity, transport, etc., are attributed to proteins present in their structure.

26.2.4 PHYSICAL ORGANIZATION The major physical forces, which help the membrane to maintain their structure, consist of hydrophobic and hydrophilic interactions, electrostatic forces, and van der Waals interactions. The main driving force for formation of the bilayer originates from the hydrophobic interactions and van der Waals interaction forces between hydrocarbon chains of the lipid molecules. The hydrophobic forces control the order and packing of lipids and electrostatic interactions between the polar head groups and their interaction with water molecules contribute to bilayer stabilization. The bilayer is continuous and it exhibits semirigid properties. The fluid nature of the membrane is governed by the lipid composition and the nature of the forces that exist between the constituent lipids and proteins. Due to fluid lipid bilayer, the diffusion constant for a phospholipids is ~1 m2=s,




745

which means a lipid molecule can travel from one end of the cell to the other in ~1 s. Nature seems to have chosen the two fatty acid chain phospholipids as basic building block of the membranes because they self-associate in water at very low concentrations and are very hydrophobic, which imparts the internal high lateral tension within the bilayer enabling them exhibit self-healing of structural bilayer alterations including induced transient leaks. Lipids suspended in excess of water tend to form liposomes, which exhibit high degree of stability due the energy minimization principles between the energy needed to bend the bilayer and the energy gained by avoiding the exposure of the hydrophobic regions to water [2]. The self-assembly property of thephospholipids allows formation of liposomes in the laboratory which are magic bags consisting of the bilayer containing an aqueous core, which can be prepared from a variety of lipids and can entrap both hydrophobic and hydrophilic therapeutic drugs with novel prospects in pharmaceutical technology and drug delivery.

26.2.5 MOLECULAR TRANSPORT

AND

PERMEABILITY BARRIER

The plasma membrane carries out the job of specialized and fool-proof security to cell interior with a semipermeable barrier for molecules, allowing some and denying others for entry or exit. The controlled and selective permeability protects cells against unwelcome guest molecules and disallows vital cytosolic molecules to leave and thus maintaining the natural integrity of the cell=organelle. The cellular membranes show high sensitivity and exceptional selective permeability to ions, small solutes, and macromolecules including invading organisms. Channels specific to a particular solute or ion mediate control of the molecular transport across bilayer membrane. Cell membranes are structurally and functionally asymmetric, i.e., the outer and the inner surfaces of biological membranes present different interfaces perhaps an essential component of molecular design for functional sophistication. Mammalian cells have predominance of phosphatidylcholine in the outer layer and phosphatidyl serine in the inner layer. The membrane fluidity is controlled by the nature of the fatty acids of the membrane lipids; the saturated fatty acids tend to make the membrane rigid, whereas the unsaturated acids make them fluid basically due to the bending in the unsaturated hydrocarbon chain. The transport of the molecules across the membranes takes place in highly regulated fashion for both active, i.e., using metabolically useful energy, and passive, i.e., without any energy requirement. The transport of water seems to occur freely through diffusion process. The transmembrane proteins act as gates or channels for specific ions. The ion channel proteins are called so because they allow only unidirectional passage of ions, e.g., Naþ, Kþ, and Ca2þ channels. There are other proteins that allow the passage of other essential biomolecules. Molecules can be transported across membrane through passive diffusion and facilitated or active transport processes. In addition to plasma membrane, mammalian cells have a several intracellular membranous network in subcellular organelles through which they control molecular trafficking within the cell.

26.2.6 METHODS

TO

OVERCOME BILAYER BARRIER

Extensive research on the attempts to overcome the bilayer membrane barrier has resulted in the emergence of several different methodologies for partial and temporary cell membrane permeabilization. The prominent among them are chemical permeabilizers, liposomal interactions, ultrasonication, ionizing radiation, and electroporation. The liposome can be made in such a way that they mimic the cellular plasma membrane and drugs=other potential impairment molecules can be loaded in the aqueous core or bilayer milieu of the liposome. Generally, the liposomal membrane fuses with the cell plasma membrane and releases the contents into the cytosol. Thus, liposomes provide an effective tool for overcoming the bilayer barrier and deliver drugs to targets in cells. A number of chemicals, such as, ammonium chloride, poly(ethylene glycol) (PEG), and calcium salts, have been found to facilitate entry of external molecules into cells, though to a limited extent. Exposure of cells to ionizing radiation, ultrasonic waves, and electric fields has been found to overcome membrane barrier allowing exchange of molecules between cell interior and surroundings. Each method, however, is faced with its own merits and demerits imposing restrictions. In the following sections, the methodology of overcoming the membrane barrier by exposure of cells to external electric field is described with some details.

26.3 CELL MEMBRANE ELECTROPORATION 26.3.1 BASIC PRINCIPLE When cells are placed in external applied electric fields, they experience an electric force. Electroporation involves the use of short, high voltage pulses to overcome barrier of the cell membrane. When a cell is submitted to an external electric field of high intensity and short duration (kV=cm, ms), transient and dramatic increase in the permeability of the plasma membrane occurs beyond a point. This phenomenon is popularly called electroporation or electropermeabilization, which allows entry of otherwise impermeable exogenous molecules into the cell interior. This phenomenon has been an active area of research in biology and bioelectrochemistry for more than three decades [3,4] and has found many applications in cell biology,



746


biotechnology, and medicine. The following sections discuss the fundamentals of membrane electroporation and various factors that affect the cell electroporation efficiency.

26.3.2 RESTING

AND INDUCED

TRANSMEMBRANE POTENTIAL

An intrinsic ionic charge gradient across the membrane exists because of semipermeable nature of membrane, which maintains a difference in the concentration of the ions between the cytosol and the extracellular matrix. This difference results in a definite potential across membrane of the normal cells, which is called the resting potential. Normal plant cells, mammalian muscle cells, and neurons have resting potential values of about 120, 90, and 70 mV, respectively. Along with the resistance to the flow of ions, membrane also exhibits a capacitance, Cm, which is given by Cm ¼

Q Vm

(26:1)

where Q is the net excess positive or negative charge on either side of the membrane Vm is the potential across the membrane The typical value of the membrane capacitance is ~1 mF=cm2 of membrane surface area. A general equation governing the potential difference across the membrane is given by Ek ¼

RT PK [Kþ ]O þ PNa [Naþ ]O þ PCl [Cl ]IN ln ZF PK [Kþ ]IN þ PNa [Naþ ]IN þ PCl [Cl ]O

(26:2)

where R is the ideal gas constant F is the Faraday constant Z is the charge T is the temperature in Kelvin [Kþ ]O and [Kþ ]IN are the concentrations of potassium ions outside and inside the cell, respectively [Naþ ]O and [Naþ ]IN are the concentrations of sodium ions outside and inside the cell, respectively [Cl ]O and [Cl ]IN are the concentrations of chloride ions The P’s stand for the partial pressures of the ionic species A biological cell can be viewed as an electrical entity which can be represented by an equivalent circuit.

26.3.3 BIOPHYSICAL BASIS

OF

MEMBRANE ELECTROPORATION

The cell can be imagined to be a nonconducting sphere with an inner side equipotential. For a spherical cell placed in an externally applied electric field, Ee, the transmembrane potential at any point on the cell membrane due to the applied field is obtained from Laplace’s equation and is given by Vm or wm Fm ¼ 1:5rcell Ee cos u

(26:3)

where rcell is the radius of the cell u is the angle between the radius vector of the point at which the potential is measured in the direction of the electric field Factor 1.5 represents a constant for the spherical geometry of the cell (Figure 26.1) At u ¼ 0, Fm ¼ 1.5rcell Ee and at u ¼ p, Fm ¼ 1.5rcell Ee. The difference in the transmembrane potential between the two diametrically opposite points of the cell surface is 3rcell Ee. Thus, rapid application of the electric field results in the membrane polarization changes that can locally deform the membrane leading to formation of leaks or pores. For a cell of radius rcell ~ 10 mm, to produce a transmembrane field Fm ~ 0.5 V, the required applied field is ~300 V=cm [5–8]. However, this is the effect of the direct field (DC). This equation implies that larger is the radius of a cell, smaller will be the field required to reach threshold value for permeabilization to occur. It is implicit that bacteria will require larger fields than mammalian cells and also, while plasma membrane can be permeabilized at a particular field value, subcellular network of membrane, e.g., mitochondrial, Golgi, lysosomal membranes would largely remain unaffected.




747

When an alternating field (AC) is applied on the cell, Equation 26.3 is modified as follows: FmAC ¼

1:5rcell Ee cos u [(1 þ vtmem )]

(26:4)

where tmem is time v is the angular frequency of the AC field Cm is the membrane capacitance It may be noted that for v ¼ 0, Equation 26.4 gives rise to Equation 26.3. The changes in the transmembrane potential have been experimentally observed and quantitatively measured.

26.3.4 ELECTROPORATION THRESHOLD The resting transmembrane potential is important for threshold of membrane permeabilization. The dependence of the transmembrane potential Fm (Equation 26.1) on the radius and the magnitude of the applied electric field make electroporation a threshold phenomenon. When the field reaches a certain threshold value, a transient reorganization of the phospholipid bilayers takes place, making the membrane temporarily more conducting [4–9]. Evidently, at a constant applied electric field, the electroporation efficiency will depend on the cell size. For a particular cell type, there exists a particular threshold value of the electric field. Thus, electroporation is a threshold process and each type of cell displays its characteristics threshold beyond which membrane permeabilization occurs. The length of the pulse does not seem to alter the threshold voltage for a particular cell. Studies have shown that exposure of cells to multiple pulses of subcritical voltage does not permeabilize membrane [10].

26.3.5 MECHANISM

OF

ELECTROPORATION

The application of the electric field imposes modulations on the intrinsic membrane potential, which gives in to induced field force beyond a critical stage. This enhanced permeability is believed to be due to a lipid bilayer membrane reorganization [11–13], which can under certain situations convert to structural membrane defect leading to the formation of hydrophilic pores [10]. These induced pores allow the molecules and ions, which are otherwise impermeable to diffuse through membrane [11–15]. However in cellular membrane, the pores are transient and the resealing or reversal begins after withdrawal of the electric fields [16,17]. Contrary to this view, some researchers suggest structural change in membrane state and prefer to call it electropermeabilization [18] and others suggested local electrical breakdown of membrane when cells were exposed to fields beyond critical threshold value [13]. 26.3.5.1

Pore Model

Different models have been suggested for observed increase in dramatic membrane permeability under external field. According to one model, permeability increase occurs because of the formation of pore, a hypothetical microstructure of transient nature, in the bilayer portion of a cell membrane. In this model, it has been suggested that rupture of a lipid bilayer underlies the induction of pore formation. As suggested by Chizmadzhev and coworkers [14,15], the initial hydrophobic pores formed can be hypothesized to any size, with radii from zero upward, whereas in contrast, the conversion of hydrophobic pores into hydrophilic pores can take place and they are believed to have a minimum size that is related to the phospholipids head group size and several head groups are needed to involve in formation of hydrophilic pores (Figure 26.3). It is also suggested that possible transient structures for hydrophilic pores may involve a block complex or the cell membranes may involve membrane macromolecules. The concept of pore formation is based on bilayer or planar membrane studies [10–12]. Some scientists believe that electroporation of cellular membrane is peculiar and prefer to call it electropermeabilization and have some reservation about the pore model [18,19]. In actual cell membrane, electroporation process seems to follow complicated mechanism, which is a subject of research and debate. 26.3.5.2

Mechanical Breakdown Model

Another model suggested by researchers consists of the dielectric breakdown of membrane beyond threshold, which largely represents irreversible pores and is believed to be more prevalent in planar bilayer membrane though its occurrence in cells was reported [13]. According to this model, because of action of induced opposite charges across bilayer, thinning of bilayer occurs and at weak locations of bilayer, compression pressure wins resulting in increased thinning of membrane which ruptures beyond a field point due to electromechanical force [8,20]. These concepts are a matter of scientific debate and require



748


H

Hydrophobic transient pore

FIGURE 26.3

H

Hydrophilic transient pore

Schematic model for hydrophobic and hydrophilic pore formation by electroporation.

clarification from more research but currently most widely accepted model is the formation of micropores in membrane by the action of high voltage external electric field.

26.3.6 RESEALING

OF

ELECTROPORATED MEMBRANE

Resealing characteristics determine the viability as well as nature and size of exogenous molecules for incorporation into electroporated cells [21–24]. The composition of the membrane differs from one cell type to another, and consequently, the obtained electroporation efficiency of cells. In addition, it is evident from Equation 26.3 that the magnitude of induced membrane potential depends on the shape and dimension of the cell. Exposure of cells to short duration of pulses usually causes reversible micropore formation, which either under certain conditions slip to irreversible hydrophilic pores or may reseal under favorable experimental conditions. The later results in the survival of electroporated cells, which allows several biomedical applications, such as, loading of drug, incorporation of bioactive molecules, gene transfer, etc. However, electric fields causing higher transmembrane potential changes and longer prevailing permeabilization can produce irreversible membrane damage causing cell death [22–25]. The irreversible membrane electroporation is suggested to arise from rupture of a part of the cell membrane creating a permanent hole or due to secondary effects leading to lysis of cells as a result of chemical imbalances caused by the molecular movement across the pores [14,15]. Use of square wave pulse allows control of amplitude and duration separately. In optimized conditions, it is possible to achieve 95% of cells permeabilized with minimum loss of viability in the treated cell population (~5% cells) [26].

26.3.7 FACTORS THAT CONTROL ELECTROPORATION OUTCOME Over the years, extensive research on cell electroporation has revealed that the efficiency of cell poration is governed by several factors, such as, physical, electrical, biological, and suspension medium [22–25,27]. Therefore, optimization of electroporation parameters for each type of cell and for a particular desired application becomes essential for successful outcome. There are several parameters that should be considered for optimization of electroporation outcome especially for in vitro electroporation experiments. 26.3.7.1

Electrical Parameters

26.3.7.1.1 Field Strength and Pulse Shape External field strength and the shape of electric pulse field play a major role in the electroporation efficiency. The applied electric field is generally DC rectangular or exponentially decaying pulses of variable duration. A rectangular pulse is generated from high voltage power supply, whereas discharging a capacitor through sample containing circuit generates exponential pulse. Research has revealed the dependency of electroporation efficiency on pulse shapes, i.e., shape of the electric pulse being of the type exponential decay, or square wave pulses (Figure 26.1). Use of square wave pulses is suggested preferable over exponential type to eliminate solution conductivity effects on electroporation efficiency. More recently, it has been shown that the electroporation outcome of cells also depends on the type of square wave pulses, namely, monophasic and biphasic [28]. The biphasic square wave pulses have been reported more efficient in causing permeabilization. Although square wave and exponentially decaying pulses were most frequently used, some experiments have used trapezoidal pulses for possible controlled pulse rise and fall time [23]. Fields of a few kilovolts for mammalian cells and pulse durations of microsecond to millisecond ranges have generally been used. When experiments are conducted using population of cells, a trade-off is observed between cell viability and transfection efficiency. Optimization of field strength, pulse length, number of repetitive pulses, and electrode geometry are known to affect the electroporation efficiency.



26.3.7.2


749

Experimental Conditions

26.3.7.2.1 Electrodes and Pulsing Medium Electroporation efficiency depends on the parameters of electric pulses that are delivered to the treated cells using specially designed electrodes and electronic devices. In vitro experiments usually employ parallel plate types of electrodes made of inert metals like stainless steel or platinum but needle types of electrodes are also used for tissue electroporation [24,25,27,28] as well as for tumor treatment applications [29–32]. There are two types of electroporator devices available: devices with voltage output and those with current output. However, a voltage output device seems to be preferable, which is widely used for diverse applications. 26.3.7.2.2 Pulsing and Incubation Medium Composition of electroporation buffer is an important factor affecting electroporation yields. Ionic strength of cell suspension medium needs control, which determines resistance of the cell suspension and resultant RC time constant of the field pulse. Medium supplemented with Ca2þ and Mg2þ in ~mM concentration range is found to promote efficiency of transformation and cell viability. Erythrocytes electroporated in isotonic buffer in the presence of EDTA or membrane specific drugs showed significant modification in hemolysis response to electroporation [33,34]. Use of square wave pulse removes the medium conductivity mediated effects on cell=tissue electroporation outcome. Generally, cells are pulsed in suspensions of sucrose, mannitol, or sorbitol. Electroporation as well as incubation of pulsed cells can be carried out in medium containing usual cell culture recipes. 26.3.7.2.3 Post Electroporation Incubation Temperature The temperature of the samples during pulse application and subsequent incubations decisively affects the cell electroporation outcome. Post pulse incubation temperature determines the rate and magnitude of resealing of the electroporated cell membrane: higher the incubation temperature, greater is the percentage cell recovery [35–40]. Results from author’s laboratory have shown that the rate and extent of resealing of electroporated human erythrocytes was dependent on the temperature of post pulsing incubation [40]. Higher percentage of hemolysis was found in erythrocytes electroporated and incubated at 48C. For DNA transfer goals in cells, it has been shown that cooling at the time of permeabilization and subsequent incubation at higher temperature increased transfer efficiency and cell viability [36–40]. 26.3.7.3

Biological Factors

For the identical experimental conditions, electroporation efficiency depends on the type of cells; the composition of the membrane, shape, and size of cells strongly influences the electroporation efficiency [40–42]. In electroporation of bacteria, the growth phase of cell has significant influence on transformation efficiency, which is higher for cells harvested and electroporated from mid-log phase. However, cells from stationary phase can also be transected with reasonably good efficiency. Mammalian cell can be electroporated at relatively lower fields but pulse length controls the entry of external molecules into cells.

26.4 ELECTROPORATION DEVICES There are many types of electroporation devices capable of delivering electric pulses widely varying in shape, amplitude, and frequency of pulses. Design of sample cuvette and electrode geometry affects the transfection yield. Chambers with parallel electrodes provide uniform fields enabling all cells in the sample exposed to same field. Small sample volumes of 10–100 mL are suitable for parallel plate electrodes. Normally, electrode gap is kept 1–2 mm and field strength across electrode is calculated from voltage divided distance between electrodes (V=d). The major components of electroporator consist of the electronic circuit (pulse generator) with discharge switch, electrodes, and cuvette=petri dish.

26.4.1 ELECTRIC PULSE GENERATORS The electric pulse generator (also called electropulsator) is essentially an electronic circuit that is capable of generating steady pulses of defined shapes, voltages, and frequencies. The type and parameters of the electric pulses are characteristics of a particular machine and the output is defined as per the circuit. Most commonly used electroporators provide for exponentially decaying electric pulses. Some of the new generation medical electroporators are capable to produce square wave electric pulses of high voltage. Biorad Gene Pulser, Progenitor Pulse Controller from Hoefer, and BTX Transfector 100 are designed to produce variable pulses of wide amplitudes and they are most commonly used. More advanced commercial electroporator models (e.g., BTX HT 96-Well Electroporator System, electroporator from Inovio Biomedical Corporation and others) have improved features in terms of field parameters, automation, and larger sample handling capability.

26.4.2 CUVETTES

AND

ELECTRODES

For in vitro studies, specially designed corvettes with parallel plate electrodes are generally employed. The separation between the electrodes ranges (1 to a few millimeters). Electroporation cuvettes are available commercially. Cells suspended in


750



electroporation buffer are filled in the cuvette and subjected to electric pulses. For in vivo studies, parallel plate electrodes with adjustable width are favored. Parallel plates are generally used for application of electric field on solid tumors. For example, solid tumor grown in leg of mice can be easily placed between the electrodes and subjected to electroporation. For the electroporation of deep-rooted tumors, specially designed needle electrodes are used. Sanghvi and Mishra have extensively investigated electroporation of human erythrocytes and have demonstrated the effectiveness of the square wave electroporation over the exponential pulses using the electroporator, which was designed and developed indigenously [28,43] (models: BARC=RB&HSD=01–04, Technology Transfer BARC 2005). Often, vernier caliper types of electrodes have been used for the clinical trials of electrochemotherapy (ECT) in cancer. A variety of sample chambers from spectrophotometer cuvettes to tong type have been used.

26.5 SOME APPLICATIONS OF ELECTROPORATION Electroporation has been demonstrated in a variety of bacteria, yeast, plant, and mammalian cells. Experiments have been carried out using artificial bilayer membranes, cell suspensions, single cells, and biological tissues. A wide range of applications in biotechnology and medicine have been demonstrated using electroporation method, e.g., cell fusion and hybridoma for antibody production, ECT for treatment of cancer, electroloading of drugs for drug delivery, gene transfection for variety of biotechnological goals, gene therapy in treatment of diseases including cancer therapy, food sterilization, etc.

26.5.1 CELL FUSION

AND

HYBRIDOMA TECHNOLOGY

Cell fusion continues to be of great importance in understanding the underlying fundamental processes and for various practical applications through nuclear material transfer in agriculture and medicine because of speed of fusion process, high efficiency of fusion, and possibility of fusing two defined and chosen diverse cells [3,4,35]. Electroporated cell membrane acquires fusogenic state due to surface structural changes, such as, loss of lipid asymmetry, pore formation, etc., and molecular details are under exploration. Usually, cells are brought in contact by dielectrophoresis force using high frequency and low amplitude AC field. Under suitable conditions, pearl chains of cells are formed by application of AC fields and subsequent application of DC pulse causes fusion of permeabilized cells to occur. This technology has been extended to the production of hybrid cells of both plant protoplast and mammalian cells [3,44]. The hybridoma technology involves production of hybrid cells by the fusion of the two or more types of cells into a giant cell. Fusion of lymphocytes with myeloma cell has been used to produce antibodies in therapeutic applications. The phenomenon of electro-cell fusion has been extensively investigated from a fundamental viewpoint and numerous applications such as crop improvement in plant science have been demonstrated. In view of the pharmaceutical and industrial importance, basic and applied aspects of cell fusion technology have been extensively investigated but, for the limitations of space, exhaustive coverage has been avoided in this chapter and readers may refer excellent books and reviews available in this field [3,4,44–48].

26.5.2 ELECTROTRANSFECTION Electroporated cells can be used to transfer DNA in bacterial, plant, and mammalian cells. This method offers rapid and efficient incorporation of plasmid and DNA in cells [49]. The in vivo electroporation has been shown to yield enhanced plasmid delivery to a wide range of tissues including muscle, skin, liver, lung, artery, kidney, retina, cornea, spinal cord, brain, synovium, and tumors. The precise mechanisms involved in electroporation applications in vivo are uncertain and require further studies, but appear to involve both electropore formation and an electrophoretic movement of the plasmid DNA.

26.5.3 THERAPEUTIC APPLICATIONS Temporary permeabilization of the cells by electric fields provides an opportunity to access the cellular interior in a controlled fashion without seriously compromising the cell viability (Figure 26.4). This technique has the capability to enhance the incorporation of the drug into electroporated cells and has been demonstrated to increase uptake of anticancer drug in cancer cells [42,43,49–52] (ECT). Efforts have also been made by researchers to study the potential of transdermal drug delivery by electroporation. Gene therapy is yet another potential application with considerable promise [42]. Enhanced action of therapeutic drugs after gamma irradiation and electroporation has been demonstrated in normal as well as tumor cells author’s laboratory [28,43]. These lines of investigations have generated continued interest and have progressed substantially leading to phase trials for cancer patient treatment in clinics.

26.5.4 POTENTIAL

FOR

OVERCOMING TUMOR RESISTANCE

One of the major problems faced by physicians in clinic is the commonly observed side effects and drug resistance in patients suffering from cancer. The resistance of cancer cells to anticancer drugs, a phenomenon called chemoresistance, is a major



751

Membrane Electroporation and Emerging Biomedical Applications Carbohydrate

Peripheral protein

Integral protein

Lipid bilayer

Theoretical cell membrane before and after electroporation

FIGURE 26.4

Cell electroporator and electrodelivery of therapeutic drug into cells.

hurdle in cancer chemotherapy. The electroporation technique has the potential to overcome the problem of drug toxicity by reducing the dose and making drug-resistant tumor cells to respond. Permeabilization of membrane by electroporation allows incorporation of drugs into cells, which are otherwise impermeant. In vitro studies from our laboratory have shown increased toxicity of resistant tumor cells to doxorubicin when combined with electroporation suggesting possibility of apply this method to treat chemo- and radio-resistance tumors (Table 26.1). Similar results were reported earlier by other investigators using bleomycin and other drugs.

26.5.5 ELECTRO CELL MANIPULATION Fusion of cells by electric pulses is based on electropermeabilization phenomenon and it was first reported by Senda [35]. This technique has become very important in the field of biomedical research. Electroporated membrane acquires fusogenic state due to some structural changes in the membrane surface, which facilitates cell fusion. Usually the cells could be brought in contact with each other by dielectrophoresis using very high frequency and low amplitude AC field. Under suitable conditions, a chain of cells were found to be formed due to the AC field and a subsequent electroporation by DC pulse caused cells to become permeable and fuse. Studies on electric field-induced fusion of fibroblasts and erythrocytes were successfully demonstrated [36]. Using fluorescent probes occurrence of cell fusion involving membranes of participating cells has been reported. Electric field-induced fusion of cells offers advantages in terms of specificity, efficiency, and speed of cell fusion. Author and colleagues have achieved fusion of electroporated human erythrocytes by subsequent centrifugation of cells (unpublished results) [53].

26.5.6 ELECTROLOADING

OF

CELLS

Electroporation methodology offers a novel method to load drugs into cells for controlled release or for targeted delivery of drug. Considerable progress has been made to load enzymes, radioactive molecules, and plasmids in human erythrocytes and

TABLE 26.1 In Vitro Electroporation of Resistant Ehrlich Ascites Tumor Cells (P 388) in Combination with Doxorubicin Treatment Treatment EP (5 kV=cm) DOX (10 mg=mL) DOX (100 mg=mL) EP (2.5 kV=cm) þ DOX (20 mg=mL)

Cytotoxicity (%) 50 10 90 92

Source: From Nanda G.S. PhD Thesis, Mumbai University. Notes: P388 EAT cells were in phosphate-buffered saline containing glucose. Exponentially decaying electropulses (T1=2 ¼ 100 ms) were employed.



752


results have demonstrated the usefulness of loaded drugs in enhancing their lifetime in blood circulation. Among the various systems proposed for delivery of pharmacological substances for improvement of their therapeutic potentials, electroporation has been found to have several merits over other methods. An electroporated cell can be allowed to reseal in the presence of the substance to be loaded into cells. The percentage entrapment can be calculated from the substance remaining in the supernatant to that entrapped in resealed cells. The efficacy of electroporation and membrane resealing was investigated by these investigators by monitoring [14] C-sucrose (sucrose with radiocarbon) uptake by human erythrocytes [40,53–55]. Dextran and human serum albumin protein (MW 65 kDa) were entrapped in electroporated and resealed erythrocytes (Nanda and Mishra, unpublished results).

26.5.7 ELECTROCHEMOTHERAPY Growing demands in anticancer research and biotechnology necessitates research on developing new modalities of treatment. Significant developments have taken place in using electroporation for increasing cytotoxicity of anticancer drugs, a process called ECT. The methodology involves increasing local potentiation of an anticancer drug by permeabilizing electric pulses. These approaches acquire greater significance in clinical situations where the side effect of chemotherapeutic drug is severe or the cost of drug is prohibitive. This has found enormous potential for developing effective anticancer therapy in recent years. Substantial success of ECT has been demonstrated in vitro as well as in vivo experiments leading to clinical trials of this approach [43,49–52]. The protocol is mainly focused on three aspects: (1) electroporation of cells in living tissues, (2) potentiation of cytotoxic drugs that are nonpermeant to cells, and (3) intrinsic response of the body systems, i.e., immune response and blood flow patterns of the patient. Undoubtedly, this is a growing field of research with an immense potential for clinical applications. The results of the ECT experiments involving anticancer drugs, doxorubicin, bleomycin, and cisplatin appear close to success.

26.5.8 RADIO-ELECTROCHEMOTHERAPY Research has progressed to suggest that electroporation can enhance the radiation effects on mammalian cells especially tumor cells. It has also been found by author and his coworkers that combined effects of anticancer drug and ionizing radiation can be significantly enhanced by electroporation (Figure 26.5). The results have suggested that radiocytotoxicity of tumor cells in vitro as well as in vivo was enhanced significantly by electroporation methodology, which may offer a potentially improved treatment of cancer [28,29,43]. The research work author’s laboratory has investigated the phenomenon of radio-electrochemotherapy (RECT), which points to enormous potential to effectively kill cancer cells including chemo- and radio-resistant tumors (Table 26.2). It is hoped that optimization of protocols appropriately and clinical evaluation of this approach in radiotherapy settings may offer unique and efficient treatment of patients suffering from cancer [41–43,49–51].

Tumor radio-toxicity: DOX and EP Human breast cancer cell: MCF-7

80 70

Radio-tumor toxicity substantially increased by EP Dox and radio-toxicity markedly increased by EP

Viability (%)

60

Dox-induced tumor toxicity significantly enhanced by EP

50 40 30 20 10

C

on

tro l EP 2 1 G µg Y 5 /m µ L 1 10 g / m µ µ 5 g/ g/ L 1 1 µg mL mL µg 0 / m + 5 / m µg L EP µ / + 10 g / m L + mL EP µg L EP + E + /m E + 2 P L P G + + y EP 2 + Gy 2 G y

0

Treatment groups

FIGURE 26.5 Effect of electroporation on radiation and drug-induced tumor cell toxicity, MCF 7. (From Shil, P. et al., J. Environ. Pathol. Toxicol. Oncol., 25, 1, 2006. With permission).



753


TABLE 26.2 Effect of Electroporation Combined with Administration of DOX and g Irradiation on Growth Delay of Transplanted Fibrosarcoma Tumor on Hind Leg of Swiss Mouse Sample

DTa (Days)

TGDb (Days)

Control Vehicle control EP Radiation DOX DOX þ radiation DOX þ electroporation Radiation þ electroporation Radiation þ DOX þ EP

1.28 0.62 1.30 0.44 2.00 0.12 1.82 0.80 1.94 0.16 2.48 0.02 2.5 0.60c 2.78 0.22c 3.00 0.26c

— — 0.72 0.54 0.66 1.2 1.22 1.5 1.5

Source: From Shil, P. et al., J. Environ. Pathol. Toxicol. Oncol., 25, 1, 2006. With permission. a DT: Doubling time (days), mean S.D. b TGD: Tumor growth delay (days), mean values. c p < 0.001, versus control.

26.5.9 IN VIVO APPLICATIONS Recent research in the field of ECT had concentrated on the preclinical experiments in animal models. Many laboratories have developed specially designed parallel plate electrodes to carry out in vivo studies. Murine tumors grown in Swiss mice have been considered as suitable system for such studies. In vitro, it has been established that high percentage of permeabilized cells, with less percentage of cell killing, can be achieved by using eight square wave pulses with individual pulse strength of 1.3 kV=cm, duration 0.1 ms. These pulses were also effective in producing electropermeabilization in vivo in tumor tissues. In a recent study, Satkauskas et al. have successfully studied the effectiveness of the ECT as a function of electric pulse strength and duration on C57B1 mice bearing LLC tumors. Their investigations revealed that the largest antitumor efficiency of ECT was attained for 1.5 kV=cm and duration of 1 ms. These pulse conditions used neither significantly suppressed tumor growth nor induced noticeable side effects [38]. Reports also suggested that both square wave and exponential wave electroporations have helped in overcoming tumor drug resistance to bleomycin and cisplatin. After successful experimentations in mouse models, clinical trials have been initiated [49–51]. Effective delivery of beneficial drugs or genes has the potential of greatly improving vascular therapy. Areas of vascular therapy that may benefit the most from improved local delivery technologies include the prevention and treatment of restenosis following angioplasty.

26.5.10 ELECTROPORATIVE GENE DELIVERY Conventional techniques for the gene transfer include DBA precipitation by Ca2þ treatment of cells with DEAE, dextran, liposomes, viral vectors protoplast fusion, or direct microinjection into the target cells. Electroporation has been found to be superior to other available methods of gene transfer. It was first demonstrated by Neumann et al. that thymidine kinase (TK) deficient mouse L cells were transformed to TK positive cells by treatment with suitable DC electric pulses in the presence of the plasmid DNA containing TK gene [56]. More recently, short interfering RNA (siRNA) is widely used in vitro to study gene functions or to identify and validate new drug targets. Electroporation-mediated delivery siRNA has been highly efficient method for selective gene suppression with siRNA in various established cell lines and primary cells. High-efficiency gene transfer and eliminated transfection-related toxicity were achieved for a variety of established cell lines as well as primary cell [19,57–59]. This line of research is making rapid progress.

26.5.11 ELECTRODELIVERY

OF

PROTEINS

One of the striking applications of electroporation is incorporation of externally added protein into plasma membrane. Protein molecules with amphipathic nature can be stably entrapped in electroporated membrane when they reseal. This phenomenon called electroinsertion has been demonstrated in a number of investigations. For example, electroinsertion of transmembrane protein CD4 receptors [60] and glycophorin [61] was demonstrated, which may prove valuable in surface engineering and studies on transmembrane proteins. In addition, a number of exogenous peptides and protein enzymes have been introduced



754


Electroporation: an effective protocol to deliver 125I in thyroid tumor

uptake (%)

Control Pulsed

10

125I

10

125I

uptake (%)

20

Control Pulsed

20

0

0 5 1.5 3 4 Electric field (kV/cm)

1

5 125I

10 20 (Microcurie)

40

FIGURE 26.6 Electroporative incorporation and retention of radioiodine in thyroid tumor cell line. (From Gopal, R. et al., Appl. Radiat. Isot., 59, 305, 2003. With permission.)

into cell to study their degradation and therapeutic effects. With the possibility to introduce large proteins, there seems to exist enormous new research in applications of transcriptomics, proteomics, genomics, and nanoscale separation science, which hold promise for future.

26.5.12 INCORPORATION

OF

RADIOIODINE

INTO

TUMOR CELLS

The efficacy of using the electroporation to cancer has been studied by Gopal et al. [62], who have shown uptake of radioiodine 125 I by human thyroid cancer cells which are otherwise recalcitrant. Electroporator designed and developed indigenously giving exponentially decaying voltage pulse in authors’ laboratory was used for this study. It has been inferred that applications of the electroporation-mediated drug delivery may potentially help in overcoming the drug resistance of thyroid tumors. Loss of ability to concentrate iodide makes thyroid cancer cells refractory to radioiodine therapy. In this study, the focus of research was on the effects of post pulse incubation temperature on cell viability, incorporation of 125I, and its retention in the resealed cells. It was observed that the radioiodine was remarkably retained in the cells for up to 24 h. Effects of pulsing and incubation temperature on 125 I uptake were also investigated. Electroporated cells were found resealing efficiently on incubation at 378C (Figure 26.6). Results have shown promise and future validation in in vivo needs to be investigated.

26.5.13 ELECTROPORATION 26.5.13.1

IN

BIOTECHNOLOGY

Water Sterilization

Electroporation-based sterilization of drinking water, wastewater, and seawater has been considered an attractive technology option. An industrial-scale system that utilizes PEF (pulsed electric field) technology results in killing of bacteria in water purification applications surmounting the consumer concern for harmful effects of heat or chemical methods of sterilization. Microorganisms present in water are retained when it is passed through polarized material and in preparation of injection solutions reinforcing the choice of filtering material to be critical for optimization process. Seawater used in mariculture is the potential carrier of pathogenic bacteria, e.g., Vibrio parahaemolyticus and related species. Use of PEF, a nonthermal and nonchemical method, has been demonstrated to overcome the technical problems faced in heat and antibiotic methods of sterilization [63]. Optimization of electric field parameters and other experimental conditions is the main determinant of sterilization outcome. Technologies are also under development to use electroporation for wastewater treatment during filtration of water. Bacteria can be substantially killed by exposure of water to high voltage electric field by irreversible damage to their membrane. It seems interesting that electroporation-based sterilizer of water at lower field strengths can be exploited to increase the efficiency of chemical disinfectants, e.g., chlorine due to transient changes in membrane of the organisms while electroporation at higher field strengths is capable of inactivating=killing bacteria and other pathogens without use of chemicals. Hence, electroporation is at least a partial alternative to chlorination. A prototype of an electroporation system for sterilizing wastewater or drinking water has been developed [45]. Electroporation-based sterilization technology is being developed by Bioelectromagnetics, Inc. (Elm Grove, Wisconsin) in United States to reduce cost and achieve optimum efficiency of water sterilization.



26.5.13.2


755

Food Sterilization

Electroporation technology has been developed to kill spoilage microorganisms, e.g., E. coli, fungi, etc. providing scope for the development of nonthermal sterilization of food free from the disadvantages commonly encountered with existing physical, chemical, or radiation sterilization. It is hoped that the application of high amplitude PEFs can make food sterilization economical, compact, energy efficient, and environmentally acceptable. Exposing liquid foods to intense electric fields over short periods effectively inactivates spoilage microorganisms, which results due to cell’s membrane damage. The damage is governed by the intensity of the electric field, the treatment time, and on the type of microorganisms. Foods that are free of chemical preservatives and having longer shelf life retaining their natural appearance and flavor would make acceptable technologies. Ionizing radiation-based sterilization liquid food suffers from generation of free-radical and possible undesirable effects on the quality and taste of treated foods. Research results suggest that application of high-field electric pulse technology to kill microorganisms for development of industrial-scale nonthermal sterilization of food seems commercially promising food preservation technique with potential to replace or partially substitute for thermal processes. A recent study has reported that PEFs sterilization technique sterilizes liquid foods effectively while maintaining the food quality and reduced the sterilization cost [46]. The method is claimed to be adaptable to liquid foods, such as, syrup, milk, soup stuff, etc. Electrical sterilization of apple juice by high voltage field pulse has been reported, which overcomes some of the problems faced by sterilization by thermal process [47]. Among others, the pilot plant program on liquid food sterilization of the Ohio State University using high voltage PEF has made considerable progress.

26.5.14 TRANSGENIC PLANTS AND ANIMALS Transgenic technology allows transfer of genes between species and prepared clones. Transgenic organisms carry a foreign gene in their cells, which were introduced by laboratory methods. Genes cloned from microbes, plants, are introduced into plant or animal cells. Transgenic animals and plants (agrobacterium) have been produced by microinjection, embryonic stem cell method, retrovirus but electroporation has been used successfully to produce animal and plant transgenic animals and plants [44,48]. Studies have also shown regeneration of shoot after electrostimulation of hypocotyls of plant [64] and pollen for possible plant transformation [65]. These approaches hold great promise for future applications in plant science.

26.5.15 TRANSDERMAL DRUG DELIVERY Application of external electric field to skin results in large increase in molecular and ionic transport [66]. This method has been applied to several drugs and is called transdermal delivery. The transdermal delivery of biomolecules has several important applications to the practice of dermatology and cosmetics. A prototype for the Inovio’s transdermal delivery device has been developed and tested in various applications from drug delivery to cosmetics. Pharmaceutical companies are testing several drugs in the use of transdermal delivery. Transdermal delivery offers a wide range of applications: delivery of classical drugs, such as many different organic chemical compounds, delivery of proteins, peptides, and macromolecules DNA and oligonucleotides for gene therapy-related treatments DNA vaccines. Mammalian skin owes its remarkable barrier function to its outermost and dead layer, the stratum corneum. Transdermal transport through this region occurs predominantly through intercellular lipids, organized largely in bilayers. Electroporation is the creation of aqueous pores in lipid bilayers by the application of a short (microseconds to milliseconds) electric pulse. Flux increases up to four orders of magnitude were observed with human skin in vitro for three polar molecules having charges between 1 and 4 and MW > 1000. Similar flux increases were observed in vivo with animal skin. These results may have significance for drug delivery and other medical applications.

26.5.16 EX VIVO APPLICATIONS Ex vivo therapy is the transfection of cells outside the body. Typically, a small amount of tissue is removed from the patient and the cells within that tissue are put into the culture, which allows clonal expansion of the cells. The approach simplifies the delivery of the genes and allows for post-transfection manipulation of the cells. The genetically modified cells, typically blood, bone marrow, or others, are then returned back to the patient, usually by blood transfusion or direct engraftment. Ex vivo transfection of cells by electroporation can be done using either a discontinuous or a continuous process. Due to recent trends in cellular therapy, the pharmaceutical companies have put emphasis on the use of ex vivo electroporation for both DNA and drug delivery. Using the flow thru system, employing a pump that moves the cell suspension through an electroporation chamber where cells are porated. The entire operation is aimed to a closed system to minimize contamination and facilitate commercial scale cell-processing operations.

26.6 SINGLE-CELL ELECTROPORATION Most of electroporation methods have adopted total population cell treatments. Total population methods yield information about average electroporative behavior of cells. Since cells are heterogenic cell population, it is important to examine if average


756



behavior represents individual cells. Recently, microelectrodes that can produce extremely localized electric fields, such as solid carbon fiber microelectrodes, electrolyte-filled capillaries and micropipettes as well as chip-based microfabricated electrode arrays enabling to electroporate single cells and subcellular structures, have been designed. Single-cell electroporation opens up a new window of opportunities in manipulating the genetic, metabolic, and synthetic contents of single-targeted cells in tissue slices, cell cultures, microfluidic channels or at specific loci on a chip-based device. Automated single-cell methods have been developed allowing to differentially manipulate the genetic, metabolic, synthetic contents of single-targeted adherent cells in a population [67]. In addition, high-throughput and real-time study of single-cell electroporation has been achieved using microfluidics. Recently, nucleofection method has been introduced to much resistant neuron cells by electroporation [68]. Subsequently, an elegant improvement has been introduced to mass 96-well electroporation 96 plasmids in neurons in a single step [69].

26.7 FUTURE PROSPECTS AND CHALLENGES Extensive research during past decades has generated considerable new knowledge on biophysical basis of cell membrane electroporation but our present understanding of the dynamics involved in electropore formation and molecular transport is far from satisfactory. It is hoped that future research would provide deeper insight on the precise mechanisms involved, which would show new direction for research in membrane science and applications. It is, however, important to recognize that applicability of electroporation has been demonstrated in a variety of bacteria, yeast, and mammalian cells and some applications are ready for exploitation while many new technologies seem potentially possible. Notably, some of the biomedical applications have reached close to commercialization (e.g., food sterilization, cancer treatment modality, hybridoma technology for antibody production). Cell electroporation phenomenon would continue to be a frontier area of fundamental and applied research with promise in biomedicine and biotechnology. Electroporation-based tumor killing, gene therapy, cell fusion, drug loading for controlled release, and targeted delivery are some of the attractive developing new technologies with considerable implications to human health such as cancer electrotherapy, RECT, electroporative drug and gene delivery, etc. However, further research is highly warranted to optimize various factors that control the electroporative efficiency of the cells. Current state of development provides enough basis to be optimistic as some of the research progress has reached the pilot plant scale and clinical level evaluations. Combination of electroporation with radiation and anticancer drug is proving an effective way to kill cancer cells in vitro as well as in vivo. Development of user-friendly clinical electroporator with automation and suitable microelectrode devices are among the greatest challenges for medical biotechnology. Considering the immense potential of electroporation technique in research and applications, greater research efforts are needed to achieve comprehensive understanding of molecular processes involved and to apply the new knowledge in realizing the fruits of electroporation technology for innovative applications in food science, industry, and healthcare.

ACKNOWLEDGMENTS Author thanks the coworkers whose work has been cited in this article. The help and cooperation of Dr. B.N. Pandey, Dr. S.H. Sanghvi, Amit Kumar of Baba Atomic Research Center, Mumbai and professor P.B. Vidyasagar and Dr. Pratip Shil of Pune University, Pune are duly acknowledged. Part of this article was prepared when the author was a visiting professor at Research Institute for Radiation Biology and Medicine, Hiroshima University, Hiroshima, Japan. Facilities and encouragements provided by professors H. Hoshi, department of radiation biophysics, and S. Matsuura, department of radiation biology at the institute are deeply acknowledged. The author is also thankful to professor F. Suzuki, Director, RIRBM for his support and patronage. The help extended by Dr. Deborshi Bhattacharjee in organizing some of the content material of this article at final stage of preparation is gratefully appreciated. Lastly, he thanks his family members especially his wife, Usha deserves his deep gratitude for her silent support and exemplary tolerance for the long hours of working during preparation of this chapter.

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8. Coster, H.G. and Zimmermann, U., Mechanisms of electrical breakdown in membrane of valonia utricularis, J. Memb. Biol., 22, 73, 1975. 9. Gabriel, B. and Teissie, J., Time course of mammalian call electropermeabilization observed by millisecond imaging of membrane property changes during pulse, Biophys. J., 76, 2158, 1999. 10. Coster, H.G. and Zimmerman, U., Dielectric breakdown in the membranes of valonia utricularis. The role of energy dissipation, Biochim. Biophys. Acta, 382, 410, 1975. 11. Hibino, M. et al., Time courses of electroporation as revealed by the submicrosecond imaging of transmembrane potential, Biophys. J., 64, 1789, 1993. 12. Kinosita, K. et al., Electroporation of cell membrane visualized under a pulsed-laser fluorescence microscope, Biophys. J., 53, 1015, 1988. 13. Nanda, G.S. and Mishra, K.P., Electroporation of biological cells: Approaches and emerging applications, Bull. Eelectrochem., 9, 547, 1993. 14. Chernomordik, L.V., Melikyan, G.B., and Chizmadzhev, Y., Biomembrane fusion: A new concept derived from model studies using two interacting planar lipid bilayers, Biochem. Biophys. Acta, 906, 309, 1987. 15. Weaver, J.C., Electroporation of cells and tissues, IEEE Trans. Plasma Sci., 28, 24, 2000. 16. Rols, M.P., Dahhou, F., Mishra, K.P., and Teissie, J., Control of electric field induced cell membrane permeabilization by membrane order, Biochemistry, 29, 2961, 1990. 17. Nanda, G.S. and Mishra, K.P., Studies on electroporation of thermally and chemically treated human erythrocytes, Bioelectrochem. Bioenerg., 34, 189, 1994. 18. Rols, M.P. and Teissie, J., Electropermeabilization of mammalian cells. Quantitative analysis of the phenomenon, Biophys. J., 58, 1089, 1990. 19. Walz, D., Teissie, J., and Millazo, G., Eds. Bioelectrochemistry of Membranes, Birkhauser, Basel=Boston=Berlin, 2004. 20. Zimmermann, U., Vienken, J., and Oilwat, G., Dielectric breakdown of cell membrane, Biophys. J., 14, 881, 1974. 21. Mishra, K.P. et al., Encapsulation and targeting of drugs in electrically hemolysed red cells, Bibl. Haemt., 51, 115, 1985. 22. Mishra, K.P., Bedekar, V.W., and Singh, B.B., Resealing of electrically hemolysed rat and human erythrocytes, Ind. J. Expt. Biol., 21, 641, 1983. 23. Puc, M. et al., Enhancement of biomolecule transport by electroporation: A review, Bioelectrochem. Bioenerg., 64, 113, 2004. 24. Mir, L.M. and Orlowski, S., Mechanisms of electrochemotherapy, Adv. Drug Deliv. Rev., 35, 107, 1999. 25. Nanda, G.S., Bedekar, V.W., and Mishra, K.P., Biomedical potential of cell membrane electroporation. Proceedings of 2nd Biomedical Symposium, BARC, Mumbai, 1994. 26. Mir, L.M., Therapeutic perspectives of in-vivo cell electropermeabilization, Bioelectrochemistry, 53, 1, 2000. 27. Crowley, J.M., Electrical breakdown of bimolecular lipid membranes as an electromechanical instability, Biophys. J., 13, 711, 1973. 28. Shil, P. et al., Enhancement of radiation cytotoxicity in murine cancer cells by electroporation: In vitro and in vivo studies, J. Environ. Pathol. Toxicol. Oncol., 24, 291, 2005. 29. Mir, L.M., Therapeutic perspectives of in vivo cell permeabilization, Bioelectrochemistry, 53, 1, 2001. 30. Pliquett, U. and Gusbeth, C., Surface area involved in transdermal transport of charged species due to skin electroporation, Bioelectrochem. Bioenerg., 65, 27, 2004. 31. Sanghvi, S.H., PhD Thesis, Electrical and magnetic field effects on cellular systems to develop instrument for biomedical applications, Mumbai University, 2004. 32. Rols, M.P. et al., In vivo electrically mediated protein and gene transfer in murine melanoma, Nat. Biotech., 16, 168, 1998. 33. Nanda, G.S. and Mishra, K.P., Modification of electroporative response of erythrocytes by EDTA in isotonic solution, Bioelectrochem. Bioenerg., 34, 189, 1994. 34. Mishra, K.P. et al., Effects of dielectric discharge on drug treated mammalian cells, Ind. J. Exp. Biol., 19, 520, 1981. 35. Donald, C.C., Bruce, M.C., James, A.S., and Sowers, A.E., Eds., Guide to Electroporation and Electrofusion, Academic Press, San Deigo, CA, 1991. 36. Chang, D.C. and Reese, T.S., Changes in membrane structure induced by electroporation as revealed by rapid-freezing electron microscopy, Biophys. J., 58, 641, 1990. 37. Belehradek, J. et al., Electropermeabilization of cells in tissues assessed by the qualitative and quantitative electroloading of bleomycin, Biochem. Biophys. Acta, 1190, 15, 1994. 38. Satkauskas, S., Kersiene, R., and Venslauskas, S., Effectiveness of tumor electrochemotherapy as a function of electric pulse strength and duration, Bioelectrochemistry, 65, 105, 2005. 39. Sersa, G. et al., Improvement of combined modality therapy with cisplatin and radiation using electroporation of tumors, Int. J. Rad. Oncol. Biol. Phys., 46, 1037, 2000. 40. Mishra, K.P. and Singh, B.B., Temperature effects on resealing of electrically hemolysed rabbit erythrocytes, Ind. J. Expt. Biol., 24, 737, 1986. 41. Gregor, S. et al., Anti-tumor effectiveness of electrochemotherapy with bleomycin is increased by TNF-a on SA-1 tumor in mice, Cancer Lett., 116, 8, 1997. 42. Gehl, J., Electroporation: Theory and methods, perspectives for drug delivery, gene therapy and research, Acta Physiol. Scand., 177, 437, 2003. 43. Shil, P. et al., Radiation cytotoxicity enhancement in murine cancer cells by electroporation, J. Environ. Pathol. Toxicol. Oncol., 25(4), 1, 2006. 44. Schnabl, H., Mahaworasilpa, T.L., Coster, H.G.L., and von Keller, A., Production of hybrid cells from single protoplasts of sunflower hypocotyl and broad bean guard cells by electric fusion, Plant Cell Tissue Organ Cult., 55, 59, 1999.


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Handbook of Membrane Separations NASA Tech. Briefs, Lyndon B. Johnson Space Center, Houston, TX, 2005. Jayaram, S.H., Optimization of electroporation waveforms for cell sterilization, J. Am. Sci., 2, 39, 2006. Lee, H.K., Electrical sterilization of juice by discharge HV impulse waveform, Am. J. Appl. Sci., 2, 2076, 2006. Nickoloff, J.A., Animal Cell Electroporation and Electrofusion Protocols: Methods in Molecular Biology, Vol. 48, Humana Press, Totowa, NJ, 1995. Hui, S.W., The application of electroporation to transfect hematopoietic cells and to deliver drugs and vaccines transcutaneously for cancer treatment, Technol. Cancer Res. Treat., 1, 373, 2002. Dietmar, P. et al., Enhancing the effectiveness of drug-based cancer therapy by electroporation, Technol. Cancer Res. Treat., 1, 71, 2002. Sersa, G. et al., Electrochemotherapy with cisplatin: Clinical experience with malignant melanoma patients, Clin. Cancer Res., 6, 863, 2000. Milkavcic, D. et al., A validated model of in vivo electric field distribution in tissue for electrotherapy and for DNA electrotransfer for gene therapy, Biochim. Biophys. Acta, 1523, 73, 2000. Mishra, K.P., Mechanisms of cell electroporation of human erythrocyte for cell fusion by centrifugation. Proceedings of National Symposium on New Methodologies, Indian Institute of Technology, Mumbai, March 7, 1990. Nanda, G.S. and Mishra K.P., Studies on electroporation of thermally and chemically treated human erythrocytes., Bioelectrochem. Bioenerg., 34, 129, 1994. Mishra, K.P., Sarma, H.D., Nanda, G.S., and Singh, B.B., Combined effects of gamma irradiation and electroporation on ascites tumor cells, Prog. Biophys. Mol. Biol., 65, 98, 1996. Neumann, E. et al., Gene transfer into mouse lyoma cells by electroporation in high electric fields, EMBO J., 1, 841, 1982. Mir, L.M. et al., High efficiency gene transfer into skeletal muscle mediated by electric pulses, Proc. Natl. Acad. Sci., USA, 96, 4262, 1999. Dean, D.A. et al., Electroporation as a method for high level nonviral gene transfer to the lung, Gene Ther., 10, 1608, 2003. Kalat, M. et al., In vivo plasmic electroporation induces tumor antigen-specific CD8 þ T-cell responses and delays tumor growth in a syngeneic mouse melanoma model, Cancer Res., 62, 5489, 2002. Mouneimne, Y. et al., Electroinsertion of full length recombinant CD4 into red blood cell membrane, Biochim. Biophys. Acta, 1027, 53, 1990. Raffy, S. and Teissie, J., Electroinsertion of glycophorin A in interdigitation-fusion of giantunilamellar lipid vesicles, J. Biol. Chem., 272, 25524, 1997. Gopal, R. et al., Electroporation: A novel approach to enhance the radioiodine uptake in a human thyroid cancer cell line, Appl. Radiat. Isot., 59, 305, 2003. Jong-Chul, P. et al., Inactivation of Vibrio parahaemolyticus in effluent seawater by alternating-current treatment, Appl. Environ. Microbiol., 69, 2405, 2003. Gill, R., Mishra, K.P., and Rao, P.S., Stimulation of shoot regeneration of Vigna aconitifolia by electrical control, Ann. Bot. Engl., 60, 399, 1987. Mishra, K.P., Joshua, D.C., and Bhatia, C.R., In vitro electroporation of tobacco pollen. Plant Sci., 52, 135, 1987. Prausnitz, M.R., Reversible skin permeabilization for transdermal delivery of macromolecules, Crit. Rev. Ther. Drug Carrier Syst., 14, 455, 1997. Bae, C. and Butler, P.J., Automated single-cell electroporation, BioTechniques, 41, 399, 2006. Hamm, A. et al., Efficient transfection method for primary cells, Tissue Eng., 8, 235, 2002. Buchser, W.J. et al., 96-Well electroporation method for transfection of mammalian central neurons, BioTechniques, 41, 619, 2006.



Membranes 27 Proton-Conducting for Fuel Cells Vineet Rao, K. Andreas Friedrich, and Ulrich Stimming CONTENTS 27.1

Introduction .................................................................................................................................................................. 760 27.1.1 Basic Principle of Operation of Polymer Electrolyte Membrane Fuel Cells ................................................ 760 27.2 Physiochemical Requirements for the Membranes in Fuel Cell Applications ............................................................ 761 27.2.1 Specific Fuel Cell Applications...................................................................................................................... 762 27.2.1.1 Automotive Application................................................................................................................. 762 27.2.1.2 Stationary Application ................................................................................................................... 766 27.2.1.3 Portable Applications (H2-Fuelled PEMFC) ................................................................................. 767 27.2.2 Porous Structure and Permeability Requirements.......................................................................................... 768 27.2.3 Catalyst Utilization and Interfacial Aspects................................................................................................... 770 27.2.4 Membrane Requirements for Direct Methanol Fuel Cells............................................................................. 773 27.2.4.1 Methanol Crossover ....................................................................................................................... 774 27.2.4.2 Water Permeation........................................................................................................................... 774 27.3 Mechanistic Aspects of Proton Conductivity (Nafion and Perfluorinated Sulfonic Acids) ........................................ 774 27.3.1 Microscopic Structure .................................................................................................................................... 775 27.4 Materials: Physiochemical Properties and Fuel Cell Performance .............................................................................. 776 27.4.1 Perfluorinated Sulfonic Acid Membranes ...................................................................................................... 776 27.4.1.1 Properties of Nafion PFSA Membranes ........................................................................................ 777 27.4.1.2 Performance of DuPont Nafion Membranes.................................................................................. 777 27.4.1.3 Synthesis of PFSA Monomers and Polymers................................................................................ 780 27.4.1.4 Short Side Chain Perfluorinated Sulfonic Acid Membranes ......................................................... 782 27.4.2 Mechanically Reinforced Perfluorinated Membranes .................................................................................... 786 27.4.2.1 PFSA Ionomer in Expanded Porous PTFE, Gore-Select Membranes .......................................... 786 27.4.2.2 PFSA Ionomer with PTFE Fibril Reinforcement; Asahi Glass; Flemion ..................................... 789 27.4.2.3 PFSA Ionomer and PTFE Reinforcement at Asahi Kasei; Aciplex Membranes .......................... 794 27.4.3 Partially Fluorinated Ionomers....................................................................................................................... 796 27.4.3.1 FuMA-TECH Membranes ............................................................................................................. 796 27.4.3.2 Poly(a,b,b-Trifluorostyrene) and Copolymers; Ballard Advanced Materials .............................. 797 27.4.3.3 Radiation-Grafted Membranes ....................................................................................................... 799 27.4.4 Inorganic=Organic (Fluorinated) Composite Ionomer Membranes ............................................................... 801 27.4.4.1 Hydrophilic Fillers (SiO2, TiO2, ZrO2) and Ormosil Networks.................................................... 801 27.4.4.2 Properties of Recast Membranes with Inorganic Fillers................................................................ 805 27.4.4.3 Heteropolyacid Additive ................................................................................................................ 806 27.4.4.4 Phosphate and Phosphonate Additives .......................................................................................... 807 27.4.4.5 Proton-Conducting Membranes Based on Electrolyte-Filled Microporous Matrices=Composite Membranes ................................................................................................... 809 27.4.4.6 Other Concepts............................................................................................................................... 810 27.4.5 Polymer Membranes with Inorganic Acid Impregnation .............................................................................. 810 27.4.5.1 PEMEAS (Celanese) Membranes .................................................................................................. 811 27.5 Summary ...................................................................................................................................................................... 814 References ................................................................................................................................................................................ 814

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760



27.1 INTRODUCTION Polymer electrolyte-based fuel cells are emerging as attractive energy conversion systems suitable for use in many industrial applications, starting from a few milliwatts for portables to several kilowatts for stationary and automotive applications. The ability of polymer electrolyte fuel cells to offer high chemical to electrical fuel efficiency and almost zero emissions in comparison to today’s prevailing technology based on internal combustion engines (ICEs) makes them an indispensable option as environmental concerns rise [1–6]. Although the basic principles of fuel cells have been known for at least a century, the introduction of solid polymer electrolyte membranes (PEMs) a few decades ago revolutionized fuel cell technology. Initially, poly(styrenesulfonic acid) (PSSA) and sulfonated phenol-formaldehyde membranes were used, but the useful service-life of these materials was limited because of their tendency to degrade in fuel cell-operating conditions [7,8]. A critical breakthrough was achieved with the introduction of Nafion, a perfluorinated polymer with side chains terminating in sulfonic acid moieties, which was invented in the 1960s for the chlor-alkali industry at DuPont. This material and its close perfluorosulfonic acid (PFSA) relatives are currently the state of the art in polymer electrolyte membrane fuel cells (PEMFCs). PFSA-based membranes have good proton conductivity, high chemical and mechanical stability, high tear resistance, and very low gas permeability in fuel cell-operating conditions [9,10]. But some problems associated with PFSA-based membranes have precluded large-scale market adoption of fuel cells. Their relatively high cost, limits to the range of temperature over which they can be reliably used (the upper limit is considered to be somewhat above 1008C, because the glass transition temperature Tg is around 1208C, at higher temperatures >1008C, membranes have low water content and thus low-proton conductivity), faster oxidative degradation and faster deterioration in mechanical properties at elevated temperatures, and a stringent requirement for external humidification of reactant gases under these conditions make the fuel cell balance of a plant (BOP) more complicated. Additionally for liquid-phase direct methanol fuel cells (DMFCs), the PFSA membrane is permeable to methanol and water, whose presence on the cathode side seriously degrades the DMFC performance. All these drawbacks have led researchers to make more efforts to discover membranes with improved characteristics on all these accounts. Over the past decade, researchers all around the world have reported success in exploring new concepts for improving the properties of proton-conducting membranes. Companies like DuPont, Dow Chemical, W.L. Gore, PolyFuel, Asahi Glass, Asahi Chemical, Ion Power, and Ballard have brought improved membranes onto the market. The main goal of this chapter is to review some of these new ideas in the field of proton-conducting membranes.

27.1.1 BASIC PRINCIPLE

OF

OPERATION

OF

POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

A fuel cell consists of two electrodes sandwiched around an electrolyte. Air (or oxygen) is supplied to the cathode and hydrogen to the anode, generating electricity, water, and heat. The electrocatalyst used is either platinum or a platinum alloy, usually supported on high-surface area carbon. The hydrogen atom splits into a proton and an electron, which takes different paths to the cathode. The proton passes through the electrolyte, while the electron passes through the external circuit. At the cathode catalyst, oxygen reduction takes place to produce water molecules. The electrons passing through the external load are available for useful work before they return to the cathode, to be reunited with the proton and oxygen in a molecule of water. The theoretical open-circuit potential for a H2=O2 fuel cell is 1.23 V at 258C and unit activity, but because of kinetic losses in the oxygen reduction process at the cathode and ohmic losses in the electrolyte membrane, the workable potential available from this fuel cell is usually around 0.7 V. The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25–100 mm thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper=cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. Existing polymer membranes such as PFSA-based membranes operate most effectively within a limited temperature range and require that the membrane must remain constantly hydrated with water, resulting in complex and expensive engineering solutions (see Section 27.2.1.1). More efficient and better performing polymer membranes are needed for continued advancement of PEMFCs. An additional challenge in developing materials for proton-exchange membranes is that these materials need to endure prolonged exposure to the fuel cell environment. Electrolyte membrane materials must resist oxidation, reduction, and hydrolysis. A further challenge is that the material should be affordable. Finally, it is desirable that the material will permit operation at a higher temperature (>1208C). Carbon monoxide is formed as a by-product when organic fuels are thermally reformed to produce hydrogen (H2), which can then be used in a fuel cell. For such reformate gas-supplied fuel cell systems, high-temperature membranes offer an



761

Proton-Conducting Membranes for Fuel Cells 4e−

Load

2H2O Anode reaction 2H2

4H+ + 4e−

4H+

Cathode reaction

4H+ + O2 + 4e−

2H2O

Electrolyte Membrane

O2

2H2

Catalyst Gas diffusion layer Bipolar plate with flow field

FIGURE 27.1

Diagram of a typical fuel cell.

important advantage because the MEAs based on high-temperature membranes are less susceptible to carbon monoxide (CO) poisoning. Better CO tolerance of high-temperature MEAs results in relatively less stringent demand on purification of the reformate gas to hydrogen. This results in easier and more cost-effective BOP for the fuel cell system. Fuel cells with hightemperature PEMs need smaller and less-expensive cooling systems. Over the last few years, membrane development has intensified and numerous new developments have been reported [10]. This increase in the interest in novel proton-conducting membranes for fuel cell applications has resulted in several studies and review publications on the overall subject and also on some related topics (e.g., nonfluorinated membranes). The content of these reviews has been used and is cited in the appropriate sections. Fuel cells, especially PEMFCs, can be used for various applications ranging from portable power supply for use in consumer electronic devices to stationary deployment for combined heat and power generation. Another potential application is transportation, in which fuel cell systems are developed for the propulsion of cars. The performance, operating conditions, costs, and durability requirements differ depending on the application. Transportation applications demand stringent requirements on fuel cell systems. Only the durability requirement in the transportation field is not as rigorous as the stationary application, although cyclic durability is necessary.

27.2 PHYSIOCHEMICAL REQUIREMENTS FOR THE MEMBRANES IN FUEL CELL APPLICATIONS The fuel cell principle is based on the spatial separation of the reaction between hydrogen and oxygen by an electrolyte. An electrolyte needs to conduct either positively charged hydrogen ions (protons) or negatively charged oxygen (or hydroxide or carbonate) ions. For a technical realization, the specific ionic conductivity of the electrolyte has to be in the range of 50–200 mS cm1 and the electronic conductivity of the electrolyte should be minimal. It is obvious from the principle of fuel cells that the electrolyte should be mostly gas impermeable to effectively separate the reaction volumes. Furthermore, a high chemical stability is required in oxidizing and reducing atmospheres. Often the MEA made from electrolyte membranes and catalysts has to be pressed against the flowfield=bipolar plates to minimize contact resistance or for sealing purposes. This necessitates good mechanical stability for the membrane. Because of these requirements, only a few systems are suitable for technical applications. The main requirement—a high specific conductivity of the electrolyte—is illustrated in Figure 27.2, which shows the conductivity of selected electrolytes used in fuel cells. As can be seen in Figure 27.2, suitable materials are available for different operating temperatures and are also quite different ranging from solid-state ceramics to molten salts and aqueous electrolytes. Interestingly, the specific conductivities differ considerably, being higher for the liquids. It should be noted, however, that the important value is the area-specific resistance with a target value of 30 Å). Nanofiltration has the advantage of very low solute rejection for monovalent species, probably due to their very small hydrated radii and higher rejection for multivalent species which are large enough. The decontamination of ADUF was also investigated in a pilot plant study using nanofiltration membranes [12]. Because of the poor separation of ammonium nitrate, NF membranes have the potential to achieve high VRFs with high decontamination factors for radionuclides, which are multivalent. The studies carried out indicate that nanofiltration is better suited for the decontamination of ammonium diuranate filtrate effluents due to their near constant fluxes, high decontamination factors, and low ammonium nitrate solute rejections. Complexation has been used in conjunction with nanofiltration process for selective removal of trace toxic metal ions [14].

29.3.7 ULTRAFILTRATION Ultrafiltration process is basically a physical filtration process on a very fine scale. It can retain large molecules, suspended, and colloidal particles, but can allow dissolved salts to pass through the bulk of the water. To keep the concentrate mostly in a uniform suspension requires high flows of liquid along the membrane surface. The technique uses similar configurations of equipment layouts as are used in reverse osmosis processes. Ultrafiltration process is used in industries for the treatment of wastes and the associated recovery of useful products in the concentrate. Examples include the fractionation of milk whey and the treatment of cutting oil emulsions. Ultrafiltration process utilizes membranes having pore sizes in the range of 0.01–0.1 mm. The operating pressure is in the range of 3–5 bar. The mechanism of separation is essentially based on the size-based retention of the components. Cellulose esters, polyamide class of polymers, polyacrylonitrile, polyvinylidene difluoride, and polysulfones are largely used as membrane materials. It has application in the concentration of radioactivity, which is or can be converted to a colloidal or suspended state or possible included in large complex molecules. It has the potential advantage over reverse osmosis of being selective in its concentrating



834


action because soluble salts may pass through the membranes and be discharged with the bulk of the water, provided they are nonradioactive. It also operates at lower pressures (95% and >82% transport of Pu(IV) when operated for 7 h. Shailesh et al. [120] have used a sterically modified amide, di-2-ethylhexyl isobutyramide (D2EHIBA) in n-dodecane as the carrier for the selective transport of trace U(VI) in the presence of large excess of Th(IV). The decontamination factor obtained in this method is attributed to lower nitric acid transport rates as compared to TBP=n-dodecane system, which has much higher permeability coefficient as compared to D2EHIBA=n-dodecane system. This work is particularly interesting as it offers an alternative membrane-based separation method to the conventional TBP extraction-based separation methods. Sriram et al. carried out SLM studies on the transport of Am3þ from nitric acid medium using dimethyl dibutyl tetradecy malonamide (DMDBTDMA) in dodecane as the carrier into 0.01 M nitric acid [121]. Significant transport of acid into the strip side resulted in relatively slower transport rate. It was observed that a mixture of 0.1 M DTPA, 0.4 M hydrazine, and 0.4 M formic acid led to highest transport rate (>60% in 8 h) when used as the stripping solution [122]. The effect of organic diluents on the extraction and hence the transport rates of Am3þ were quantified by an empirical correlation using several diluents, namely, n-dodecane, toluene, n-hexane, cyclohexane, 1,2-dichloroethane, nitrobenzene, decalin, and diethyl benzene [123]. The extraction equilibria were not affected and the tri-solvate species were transported in all the cases as per the following extraction equilibrium: Am3þ þ 3NO 3 þ 3A(o) Ð Am(NO3 )3 3A(o)

(31:10)

The extraction (and hence the transport) efficiency depends on several diluent factors such as Schmidt empirical diluent parameter [124,125], the Swain’s acity and basity parameters along with the Dimroth and Reichardt polarity indices [126], dielectric constant [127], refractive index [127] and viscosity [127], and the Hildebrand’s solubility parameter [128]. The permeability coefficients (PAm) were computed from the Wlke-Chang, Scheibel, and Ratcliff [129,130] equations, which compared reasonably well with the experimentally determined values as shown in Table 31.10. Elhassadi and Do [131] have, on the other hand, taken into account only the viscosity and solubility effect of the diluent and the carrier immobilized in SLM. They have reported that these two factors influenced the transport rates significantly.

Pabby et al./Handbook of Membrane Separations 9549_C031 Final Proof page 901 15.5.2008 2:45pm Compositor Name: VAmoudavally

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Liquid Membrane-Based Separations of Actinides

TABLE 31.10 Diluent Effect in the Permeation of Am3þ through an SLM Containing DMDBTDMA in n-dodecane as the Carrier Permeability Coefficient (PAm) in cm=s (104) Diluent n-Dodecane Toluene n-Hexane Cyclohexane Dichloroethane Nitrobenzene Decalin di-Ethylbenzene

PWilke-Chang

PScheibel

PRatcliff

PExpt

1.80 0.73 1.80 0.54 1.99 2.60 0.50 0.65

1.87 0.78 2.12 0.62 1.93 2.48 0.50 0.66

1.75 0.84 2.20 0.66 2.14 2.62 0.50 0.66

1.80 0.90 1.63 0.76 1.80 2.41 0.62 0.71

Source: Data from Sriram, S. and Manchanda, V.K., Solv. Extr. Ion Exch., 20, 97, 2002. With permission.

Though tetraalkyl di-glycolamides (TODGA) are far superior as extractant as compared to CMPO or diamides, to our knowledge very little work on their use as carrier in SLM studies is reported so far. Ansari et al. [132] have studied the transport of Am3þ from nitric acid medium into a receiving solution of distilled water using an SLM made from N,N,N0 ,N0 -tetraoctyl-3oxapentane diamide (TODGA) in n-dodecane as the carrier inside microporous PTFE membranes. The transport of feed acid to the receiver phase was increased with the feed acidity, which influenced the transport rate of the metal ion at higher acidity (>2 M HNO3). Maximum transport was observed with a carrier concentration of 0.1 M and a feed acidity of 2 M HNO3. The membrane stability was remarkably good when tested for more than 20 days of continuous operation. The use of TODGA as a carrier in the polymer inclusion membrane (PIM) is discussed in Section 31.3 [133]. 31.2.3.2.4 Macrocyclic Carriers in Supported Liquid Membrane Macrocyclic carriers such as crown ethers, cryptands, and calixarenes have been used as carriers for the transport studies involving alkali and alkaline earth metal ions [134–136]. There are a few reports on the use of these ligands as the carriers for the transport of lanthanide=actinides as well. Shukla et al. [137] studied the uranyl ion permeation fluxes, physical stability (loss of carrier), and chemical stability of SLMs using crown ethers in various aromatic and aliphatic diluents. They have concluded that the chlorinated hydrocarbons such as chloroform, 1,2-dichloroethane, dichloromethane afford much poor permeability compared to aromatic diluents such as toluene and o-dichlorobenzene. Plutonium transport could be achieved using 0.2 M DC18C6=toluene as carrier and sodium carbonate as strippant. By a combination of both BLM and SLM (using polypropylene filters), >90% transport of Pu was achieved using 0.2 M DC18C6 in toluene as the carrier into dilute sodium carbonate strippant in about 7–8 h without any contamination from fission products such as Ru-106, Cs-137, and Sb-125 [138]. The proven extractability and profound selectivity of dicyclohexano-18-crown-6 (DC18C6) have been exploited by selecting this crown ether as the ionophore in liquid membrane transport studies. Macrocycle-facilitated transport of Pu(IV) and U(VI) against their concentration gradient from aqueous nitric acid solutions across SLM containing DC18C6 in toluene has also been investigated [139]. In the presence of 0.2 M DC18C6 in toluene as the carrier, uphill transport was observed when both the feed and receiver comprised of 0.8 mg=mL of Pu. The effect of radiation on the transport rates has indicated that the permeation of U(VI) and Pu(IV) was not affected when the membranes containing the carrier were subjected to a total dose of 80 kGy. On the other hand, dry membranes (Accurel HF-PP) became somewhat fragile beyond a total dose of 50 kGy though the transport behavior was not very much affected. The chemical stability of the membrane containing DC18C6 in toluene was satisfactory up to 12 h of continuous operation, beyond which a drop in the permeability coefficient was ascribed to the loss of carrier [140]. In another study involving the transport of Pu(IV), Shukla et al. have investigated the role of polymer support upon the performance and stability of an SLM using several solid supports such as polytetrafluroethylene (TE-35, TE-36, TE-37), polypropylene (HF-PP), cellulose nitrate (BA-S-83) of different pore sizes and varying thickness, and DC18C6 in toluene as the mobile carrier [141]. Selectivity of plutonium transport from fission product contaminants such as Cs-137, Ru-103, and Ce-144 was found to be poor with large pore size (0.45 mm) support. Polytetrafluroethylene supports, such as TE-35, TE-36, TE-37, and polypropylene HF-PP membranes, did not show any marked deterioration with nitric acid and organic solvents while cellulose nitrate membrane such as BA-S-83 was attacked by organic solvent. On the other hand, these membranes were found to have good radiolytic stability [142]. Almost no report is available on the membrane transport studies of trivalent actinide ions with macrocyclic carriers. However, there are some literatures available on trivalent lanthanide ions, which can be used as guidelines for any future studies. Brown et al. [143] and Zhu and Izatt [144] have used bis(1-hydroxylheptyl)DC18C6 as the membrane carrier and


902


attempted separation of Eu from trivalent lanthanide cations by reducing it to the þ2 oxidation state. The online reduction was carried out by modifying the transport cell in such a way that the feed compartment contained a mercury pool that acted as the cathode, while a Pt coil worked as the anode. After reduction, Eu2þ behaved similar to Sr2þ and selective transport was observed as compared to other lanthanides such as Nd3þ (4 times lower flux than Eu2þ) and Gd3þ (8 times lower flux than Eu2þ). Tang and Wai [145], on the other hand, have studied the transport behavior of trivalent lanthanide ions using a lipophilic crown ether carboxylic acid, 2-(sym-dibenzo-16-crown-5-oxy) hexanoic acid using a surfactant membrane. The activation energies for the transport of lanthanides in the system were determined to be 3–4 kJ mol1 suggesting that the process is diffusion controlled. This surfactant membrane also shows a selectivity of Lu3þ=La3þ by 1.25 in the transport process. Macrocyclic ligands, such as calixarenes, have the flexibility and favorable conformations to form stable complexes with uranyl ion [146–148]. The special structural features of the calixarenes make them selective ionophores for many lanthanide= actinide ions. Uranyl ion, in an aqueous sodium hydrogen carbonate solution of pH 4–8 (source phase), was simultaneously and selectively transported into a dilute sulfuric acid solution (receiving phase) through a membrane (chloroform, BLM) containing a lipophilic ion-associate of methyltrioctylammonium ion and hydroxycalix[n]arene-p-sulfonate ion, 2n (n ¼ 6, 8), MTAþ 2n, as a metal carrier [149]. None of the other metal ions were transported, or obstructed the transport of uranyl ion, while the presence of large amounts of sodium hydrogen carbonate and sodium chloride in the source phase interfered by causing a delay in the start of transport. Ramakumar et al. [33] have used several calixarenes (Figure 31.8) along with neutral donors such as TOPO and 18 crown 6 for the selective transport of uranium from seawater. Calixarene II and IV were found to show very good transport behavior when used as the carrier. Using calixarene II and TOPO mixture in chloroform, they recovered >95% of uranium from seawater while the permeation of ions such as Naþ, Mg2þ, Ca2þ, Kþ, and Fe3þ was negligible. A significant increase in the transport rates of alkali=alkaline earth ions was observed when 18 crown 6 was used as the synergist. In the minor actinide partitioning by the TRUEX process, CMPO is used as the extractant for the recovery of trivalent actinides from nitric acid medium. A resorcinarene cavitand-based ligand, functionalized with CMPO (carbamoylmethylphosphoryl)-ligating moieties in 2-nitrophenyloctyl ether (NPOE), was utilized for the transport studies of Eu(III) by Boerrigter et al. [150]. The steric pre-organization of four CMPO moieties on the macrocyclic resorcinarene cavitand frame improved the efficiency and selectivity of the metal extraction processes as Eu(III) could be quantitatively removed from acidic solutions using SLM in about 8 h. They had also used the CMPO-functionalized resorcinarene cavitands for the transport studies of UO22þ [151]. Calix-4-arenes mono=bis-crown-6 bound to heteropolysiloxane matrix were used for the selective transport of radionuclides such as Cs-137 from a waste containing Am-241 [152]. Dozol et al. [153] synthesized several grafted membranes containing calixarenes linked to the polypropylene matrix, which were selective for the lanthanides and trivalent actinides. The data with SLM were compared with those obtained with the grafted membranes. 31.2.3.3

Basic Extractants as Carriers

Amines have been used as the extractants for the actinide-bearing anionic species. This is usually from high-acidic or high-ionic strength medium, which generally lead to very high decontamination factors. The precondition of their use in SLM-based separation methods is the inertness of the membranes toward high acidic or salt medium. As membranes made from polycarbonate or polyamide may not be very stable toward these medium, most of the reported work in this area is on PTFE or PP membranes. Chaudry and Mohammad [154] had used tri-n-octyl amine (TOA) dissolved in xylene as the SLM in Celgard 2400 polypropylene filter for the transport of uranyl ions from nitric acid solutions to an alkaline aqueous phase. About 10 h were needed for the quantitative transport of uranyl ion from an aqueous feed of 6 M HNO3. Several other tertiary amines along with TOA have been used as carriers in SLM studies by Sonawane et al. [155]. The diffusion-limited and amine-facilitated Pu(IV) cation permeation in nitric acid media across an SLM was investigated to quantify the membrane carrier type effects on its transport using basic extractants, namely, Primene JM-T (JMT) as primary, Amberlite LA-2 (Amb LA-2) as secondary, trilaurylamine (TLA) and triiso-octyl amine (TIOA) as tertiary, and Adogen-464 and Aliquat-336 as quaternary amines. Recovery of Pu steadily increased from primary to quaternary amines as it follows the order quaternary > tertiary > secondary > primary, similar to that generally observed in liquid–liquid distribution experiments. More than 95% pertraction of Pu(IV) was easily accomplished using tertiary or quaternary amine as carrier employing a feed solution containing about 5 mg=L Pu in 4 M nitric acid solution, while the receiving phase was 0.1 M NH2OH HCl (in 0.3 M HNO3). On the other hand, plutonium permeation was found to plummet to about 49% on employing secondary amine, Amb LA-2, as the carrier [156]. Lakshmi et al. had investigated the transport of uranyl ion from HCl medium using 30% Aliquat-336 in CHCl3 as the carrier [157]. Significant amount of acid transport (about 15%) was observed which affected the metal ion transport though >90% transport of U was observed in about 3 h. In another study involving Alamine 336, the same authors had shown that aromatic solvents such as toluene or t-butyl benzene effect higher transport rates than solvents such as chloroform or hexane [158]. Subba Rao et al. had investigated the transportation of uranium from HCl medium by an SLM made from Aliquat-336 in xylene as carrier and observed that the flux values for HCl or for uranium were higher at lower carrier concentration, which was


903


explained based on the aggregation behavior of the carrier molecules in membrane phase [159]. Effect of phase modifier on uranium transportation from hydrochloric acid medium across SLM using Aliquat-336 as the carrier in xylene medium was investigated by them to understand the effect of degree of aggregation of carrier molecules [160]. In this study, reduction of uranium flux was observed at carrier concentration where viscosity effects were not dominant.

31.2.4 HOLLOW FIBER SUPPORTED LIQUID MEMBRANE Several applications are envisaged in nuclear industry for the preconcentration of very lean solution of radioactive material, low- and intermediate-level wastes. There are, of course, reports on the application of HFSLM technique for highlevel waste processing. Researchers at the plutonium facility at Los Alamos National Laboratory have studied the separation of actinides from waste solutions [161] using water soluble metal binding polymers (phosphonic acid derivative of polyethyleneimine). Kathios et al. [162] have investigated the use of HFSLM modules for process scale metal separations such as radioactive waste stream clean up and environmental remediation under simulated conditions (Nd was used for Am) using CMPO in di-iso-propyl benzene as the carrier. The major challenge in such case is to develop radiation resistant polymers. Rathore et al. have studied the permeation of U(VI) and Pu(IV) in the presence of fission products from medium active acidic waste employing 30% TBP as a carrier through HFSLM and observed that recovery could be enhanced by using fresh strippant each time [163]. They have also used TBP as the carrier for the quantitative separation and recovery of Pu from nuclear waste solutions [164] using an HFSLM system. In the presence of various fission products, the selective permeation of Pu through the HFSLM was observed to be more than 90% into a stripping phase consisting of 0.1 M NH2OH HCl in 0.3 M HNO3. The membrane radiation stability was found to be good as shown by the scanning electron microscope (SEM) pictures of the irradiated and the nonirradiated membranes (Figure 31.11) even after exposure to 1 M Rad dose. A commercial tri-alkyl phosphine oxide, namely, Cyanex-923 was found to be far superior as the carrier as compared to TBP, as >94% transport was reported in 10 h using 0.1 M concentration of the carrier [97]. The permeation efficiency was found to increase from 75% to 90% by increasing the flow rate from 3.0108 to 1.4 107 m3 s1. Patil et al. have used dihexyl octanamide (DHOA) in normal paraffinic hydrocarbon diluent as the carrier in their HFSLM (operated in the nondispersive extraction mode) study [165,166]. From a feed solution comprising 8 g=L of U in 4 M HNO3, transport was more facile into a strippant solution of 0.01 M HNO3 as compared to 1 M sodium carbonate. Transport of Pu(IV) from 3 M HNO3 solutions across Aliquat-336= Solvesso-100 by HFSLM was studied. Permeability of Pu(IV) through a bundle of hollow fibers made up with 20 lumens, of 67 cm2 surface area, 9 cm length, and operated at a flow rate of 108 m3=s on recycle mode was examined. More than 80% Pu from oxalate bearing wastes generated during reconversion process could be transported through 10% Aliquat-336=Solvesso-100 into hydroxylamine hydrochloride strippant in about three runs [167]. HFSLM studies employing dialkyl phosphinic acid (Cyanex-272) were reported by Chiarizia et al. [80] for the decontamination of the severely contaminated Hanford site ground water. Their HFSLM experiments involved a hollow fiber module containing a total membrane area of 9.8 cm2 and the feed contained contaminated Hanford ground water at pH 2 (synthetic samples) circulated through the lumen side, while the strip containing 0.1 M HEDPA was circulated through the shell side. The U(VI) can be concentrated in the strip solution to about 108 times though significant amount of Fe(III) also gets cotransported. Lanthanide–actinide separation was also attempted by HFSLM (operated in the nondispersive extraction mode) method using diphenyldithiophosphinic acid derivatives. Geist et al. have employed a synergistic mixture of bis(chlorophenyl)dithiophosphinic acid and TOPO in a hollow fiber module for the lanthanide–actinide separation [168]. About 99.99% Am

(a)

(b)

FIGURE 31.11 SEM pictures of the hollow fiber lumens used in an HFSLM study involving radioactive solutions. (a) Prior to irradiation; (b) after irradiation. (Reproduced from Rathore, N.S., Sonawane, J.V., Gupta, S.K., Pabby, A.K., Venugopalan, A.K., Changrani, R.D., and Dey, P.K., Sep. Sci. Tech., 39, 1295, 2004. With permission.)


904


(a)

(b)

FIGURE 31.12 Typical representation of the holes formed in a PVDF nuclear track filter after irradiation in accelerator and the subsequent etching with 6 M KOH and 0.1 Fw KMNO4. (Reproduced from Gopalani, D., Kumar, S., Jodha, A.S., Singh, R., Khatri, P.K., and Gopal, R., J. Membr. Sci., 178, 93, 2000. With permission.)

(III) could be extracted by the contactor from 0.5 M nitric acid with about 33% of lanthanides such as Y, La, Ce, Pr, Nd, Sm, Eu, and Gd being extracted.

31.2.5 TRACK-ETCHED MEMBRANES Track-etched membranes (TEM) are, in general, polymeric membranes with artificially made pores made after irradiating with a projectile of heavy ions followed by chemical etching. TEMs, which show a more precise distribution of pores with a known pore size, are used for the transport of ions [169]. Fleischer et al. [170] have used thin sheets of plastics, usually 10–20 mm polycarbonates, perforated with collimated and diffused beams of heavy energetic ions. There is however a limitation on the thickness of the polymeric materials, which can be used for making membranes in view of the limitation of the projectile energy. As a consequence, these thin membranes are not rugged. These membranes with pore sizes in proportion to the energies of the charged particles are used for the investigations involving nonlinear theory in ionic transport [171]. It was reported that track-etched membranes with porosity up to 20% can be prepared using accelerated heavy ions. Pandey et al. [172] had used Makrofol track-etch membranes (after irradiating with 13 MeV Dy-161 ions and etching with 3 M NaOH) for the transport of Am(III) from a feed comprising 1.5 M ammonium nitrate and 0.5 M nitric acid to a strip phase consisting of 1 M oxalic acid using a mixture of 1.2 M TBP and 0.2 M CMPO as the carrier. Both the inner as well as outer diameters of the pores varied linearly with the etching time. The inner pore diameter as well as the porosity were found to govern the metal transport rates. The permeation across the SLM formed by using TEM with 17% porosity was found to be comparable to those SLMs formed by using commercial membranes with much higher porosities. The comparison of P (permeability coefficient) obtained by using TEMs with 29–30 mm and 10 mm thickness indicated that P decreased linearly with thickness and hence transport of Am(III) is diffusion controlled, and the extraction and stripping kinetics are fast. Removal of actinide ions from acidic solutions by carrier-mediated facilitated transport across mesoporous substrates with nanoengineered surfaces has been reported very recently [173]. The thiol self-assembled monolayer (SAM) modified with di(tert-butyl-phenyl) di(iso-butyl)CMPO was used for the selective transport of 239Pu and 241 Am from a mixture containing fission products such as 137Cs and 90Sr, which are not transported to any significant extent. Nuclear track membranes with other materials, such as mica, oscillating quartz, PVDF, Kapton, and even polyster, have been found to be resistant to chemicals and acid. Figure 31.12 shows a typical representation of the holes formed in a polyster nuclear track filter after irradiation in accelerator and the subsequent etching with 6 M KOH and 0.1 Fw KMnO4 [174]. Vater has used track-etched mica filters for collecting the aerosols carrying radioactivity from the environmental air inside a nuclear reactor fuel elements production plant [175]. Ganz et al. [176] have, on the other hand, used Kapton track-etched microfilters (pore size: 3.8–13.4 mm and porosity: 4%–12%) for the separation of 237Np from a feed containg 2 M HNO3. About 0.5 M HTTA in xylene was used as the extractant, which formed emulsion with the aqueous phase and the Kapton membrane selectively allowed the organic phase to pass through.

31.3 POLYMER INCLUSION MEMBRANES Although FSSLM- and HFSLM-based separation methods are getting increasingly popular, their only disadvantage is the stability, which is primarily due to the irreversible loss of carrier from the pores of the filters=lumens. As an alternative, polymeric films have been prepared along with the ligand and the diluent. These membranes termed as polymer inclusion membranes (PIM) have better membrane stability, although they are not suitable under high concentration of acid or alkali [177,178]. In general, the PIMs have relatively lower transport rates compared to the SLMs. Kim et al. [179] have shown that


905

Liquid Membrane-Based Separations of Actinides NO2

BuO BuO

P

O

BuO

O

Tri-n -butyl phosphate (TBP)

2-Nitrophenyl octyl ether (NPOE)

O P

O

O

O O

O

Bis-(2-ethylhexyl) phthalate (BEHP)

Tris-(2-ethyl-hexyl) phosphate (TEHP) O O (CH2)8 O O

Di-n -butyl sebacate (DBS)

FIGURE 31.13

Structures of some plasticizers being used in the casting of PIM.

the flux in case of PIM is as high as 15 times higher than the corresponding SLM system. The carrier molecules crystallize inside the membrane and no new chemical bonds are formed within the polymer matrix [180]. CTA forms bulk of these types of membranes and is vulnerable to acid=alkali attack due to the possibility of hydrolysis of ester groups. It is understood that by increasing the chain length of the substituents the membranes could be made more acid=alkali resistant. Gardner et al. [181] attempted to vary the type of monomer by using cellulose acetate propionate (CAP), cellulose acetate butyrate (CAB), and cellulose tributyrate (CTB) and concluded that the flux decreases when the membranes were made more hydrophobic. The role of plasticizer is found to be extremely important in deciding the membrane stability and the overall flux of these membranes [182]. Figure 31.13 shows the structures of some of the plasticizers being used conventionally in PIMs. Sugiura [183] has used such membranes made from cellulose triacetate (CTA) with plasticizers such as 2-nitrophenyl octyl ether (NPOE) and tris(2-butoxyethyl) phosphate (TBEP), which enhance the membrane elasticity. He [184] has used b-isoproyl tropolone (HIPT) as the carrier trapped in these membranes for the transportation of lanthanide ions. Though there are many reports available [133,185–188] on the sorption=transport behavior of main group elements, very limited work is reported on the actinides using PIMs. Matsuoka et al. [189] have studied the uphill transport of uranium from nitric acid medium using a PIM containing CTA and TBP. In this case, TBP acted as both the carrier and the plasticizer, while Na2CO3 was used as the stripping solution. An increase in U permeation (measured as D1 ¼ S([UO2 Xi]TBP=[UO2 Xi]HNO3)) was observed with increasing aqueous phase acidity. On the other hand, the U release into the receiver phase (measured as D2 ¼ S([UO2 Xi]TBP=[UO2 Xi]Na2CO3)) decreased with increasing Na2CO3 concentration. Significant loss of TBP into the aqueous phase was observed, which was similar to those observed in the case of SLMs. The loss of TBEP was also reported by an energy dispersive x-ray spectroscopic analysis [188]. No such loss of plasticizer is reported in the case of membranes made using NPOE as the plasticizer [190]. There are very limited reports available on the use of PIMs for the uptake=transport studies of trivalent actinides. There are some early reports, by Sugiura [191], on the transport behavior of trivalent of lanthanides, which are considered as surrogates of the trivalent actinides. They have used NPOE as the plasticizer and investigated the effect of the alkyl chain length of some of polyoxyethylene n-alkyl ether on transport behavior of lanthanide ions and concluded that the flux decreased with an increase in the chain length. Bhattacharyya et al. have used a PIM containing TBP as the plasticizer and Cyanex-301 as the


906


Permeability coefficient ⫻ 105 (cm/s)

6

5

4

3

2

1 0

2

4

6

8

10

Time (days)

FIGURE 31.14

Effect of radiation dose (200 kGy) on the subsequent decrease in the Am3þ pertraction as a function of time.

carrier for the transport of Am3þ from an aqueous feed containing 1 M NaNO3 [192]. They have observed insignificant transport of Eu3þ thereby indicating the feasibility of the selective separation of trivalent actinides from lanthanides. However, the major problem of the PIMs is their stability against radiation. The sharp decline in Am3þ transport rates (Figure 31.14) is a consequence of the significant damaging of the plasticized membranes by gamma radiation (Figure 31.15). Kusumocahyo et al. have demonstrated facile transport of Ce3þ across a PIM containing TODGA, which was found to be far more superior as a carrier compared to CMPO [133]. Sodaye et al. [193] have developed a scintillating PIM (S-PIM) containing HDEHP as the extractant, and PPO (diphenyl oxazole) and MSB (1,4-bis(2-methylstyryl)benzene) as fluors in plasticized CTA matrix for the assaying of actinide elements from aqueous samples (tap water and seawater). It was observed that the S-PIM could be made selective toward the tetravalent and hexavalent actinides at higher acidity (>0.25 M), while the trivalent actinides could be trapped from pH solutions. The S-PIM could be made alpha-selective by pulse height discrimination. A linear response in a relatively wide range of radioactivity even in the presence of beta-emitting radionuclides such as 154Eu and 147Pm (Figure 31.16) underlines the applicability of this analytical method for the assaying of actinides.

31.4 SEPARATION OF RADIONUCLIDES FROM WASTE STREAMS The use of membrane-based separation methods for gas separation is being routinely done in the industry. Reverse osmosis (RO) and ultrafiltration (UF) technologies are also very popular in treatment of industrial wastewater. Nevertheless, ultrafil-

(a)

(b)

FIGURE 31.15 Effect of radiation dose (200 kGy) on polymer inclusion membrane stability. (a) SEM pictures of unirradiated PIM; (b) SEM pictures of irradiated PIM.


907

Liquid Membrane-Based Separations of Actinides 1000 900

Scintillation (counts/s)

800 700 600 500 400 300 200 100 0 0

500

1000

1500

2000

2500

Radioactivity (Bq)

FIGURE 31.16 Variation of a-scintillation as a function of a-activity of 241Am sorbed in the samples of S-PIM. The symbols (D), (), and () represent the S-PIM samples loaded with 241Am, 241Amþ147Pm, and 241Amþ154Eu, respectively. b-activity ¼ 1670 Bq; LLD ¼ 4.5 V; amplifier gain ¼ 50. Slope of the fit ¼ 0.386 0.013; correlation coefficient ¼ 0.994; S.D. ¼ 29.44; number of data points ¼ 13 (for 241 Amþ147Pm and 241Amþ154Eu). (Reproduced from Sodaye, S., Tripathi, R., Pandey, A.K., and Reddy, A.V.R., Anal. Chim. Acta, 514, 159, 2004. With permission.)

tration and reverse osmosis membrane technologies for treatment of radioactive liquid effluents are being applied in various nuclear plants [194]. These include the Nine Mile Point nuclear power plant, New York (RO), Comanche Peak nuclear power plant, Glen Rose, Texas (both RO and UF), Dresden nuclear power plant, Morris, Illinois (both RO and UF), Salem nuclear power plant, New Jersey (UF), AECL Chalk River Laboratory, Ontario, Canada (RO), etc. However, the application of liquid membrane-based separation methods for the removal of radionuclides from waste streams is at infancy stage and only a few reports are available. Americium removal from nuclear waste streams has been carried out at the Rocky Flats Plant (RFP) using SLM containing DHDECMP (dihehexyl-N,N-diethyl carbamoylmethyl phosphonate) supported on Accurel polypropylene hollow fibers [195,196]. The waste from the Pu-purification cycle contained HNO3 concentration in the excess of 7 M, which was neutralized partially to yield >7 M nitrate and ~0.1 M hydrogen ion concentration. More than 95% transport was observed into a stripping solution containing 0.25 M oxalic acid. The permeability coefficients were maximum up to 1 103 cm=s. Interference from Fe(III) was minimized by selective complexation. Teramoto et al. [197] have used octyl(phenyl)-N,N-diisobutylcarbamoyl methyl phosphine oxide (CMPO) in diethyl benzene as a carrier and investigated the transport behavior to examine the feasibility of treating low-level radioactive wastewater using a stirred permeation cell. The transport rates were positively influenced by the addition of sodium citrate to the strip solution. For a simulated radioactive waste solution containing ~550 ppm of Ce(III), at a treatment rate of 1 m3 per day (for >99.9% Ce removal) a membrane of 3.3 m2 surface area was recommended. Contaminated reactor pool water was reported to have been treated by SLM method, though the pertraction rates were very low [198]. Chiariazia has used liquid membranes containing three different commercially available long-chain aliphatic amines, namely, Primene JM-T (primary), Amberlite LA-2 (secondary), and trilaurylamine (TLA, tertiary) for the treatment of synthetic Hanford site groundwater, where the strip solution was 1 M NaOH and n-dodecane was the diluent [199].

31.5 GRAFTED MEMBRANES FOR ACTINIDE SEPARATION Uranium is present in the seawater as an extreme dilute solution. However, the total amount, 4 109 tons, is ca. 1000-fold of the amount existing in the mines all over the world. Recovery of uranium from seawater is one of the most challenging problems in separation chemistry [200]. The separation methods using chelating resin [201] methods are limited by the low concentration of uranium (~3 ppb) and high concentration of other interfering ions. Kobuke et al. [202] have used a membranebased separation method using a macrocyclic polycarboxylate ligand as the carrier. In the trial experiment, the feed contained uranyl acetate (3.3 105 M) at pH 4–5, while the receiver phase contained 1.0 M NaHCO3. The carrier (Figure 31.17) when modified with tri-n-octylmethyl ammonium chloride resulted in a significant enhancement in the transport rates. Selective transport rates of uranyl ion were observed in the presence of 4.68 101 M Na, 5.3 102 M Mg, 1.0 102 M K,


908

Handbook of Membrane Separations CONHR COO−

UO2+ 2 N+ OOC

UO2(CO3) 2−

COO− −COC CONHR CONHR COO− UO2

CO2− 3

COO− −OOC N+OOC

CONHR

(a) COO− COO− + – + (C8H17)3N CH3 Cl

COO− H37C18HNOC

−OOC

CONHC18H37

COO−N−(C6H17)3(CH3) COO− K COO− (b)

H37C18HNOC

−OOC

CONHC18H37

FIGURE 31.17 Schemes of the highly selective ligand for uranium recovery from seawater. (a) The hexacarboxylate ligand; (b) after reaction with tri-octyl methyl ammonium chloride. (Reproduced from Tabushi, I., Kobuke, Y., Nakayama, N., Aoki, T., and Yashizawa, A., Ind. Eng. Chem. Prod. Res. Dev., 23, 445, 1984. With permission.)

1.0 102 M Ca along with other ions. Out of these ions, Ca2þ was found to compete with the uranyl ion. However, due to favorable kinetic factors, a 300-fold increase in Ca2þ concentration decreased the uranyl ion transport rate only by 2.5 times. Recently, the use of amidoxime grafted membranes for the recovery of uranium from seawater has shown great promise [203]. A research group at JAERI has proposed a method for the preparation of amidoxime (AO) grafted polyethylene and poly propylene membranes. The schematic of the grafting process is shown in Figure 31.18 wherein methacrylic acid (MAA) and 2-hydroxyethyl methacrylate were cografted with acetonitrile onto polyethylene fibers by radiation-induced graft polymerization. The amidoxime group density and water content determined the U sorption onto the membranes. The AN (acrylonitrile)= MAA ratio of 60:40 was found to be optimum and over a period of 20 days contact time showed a maximum uptake of 0.9 g of U=kg. The preconcentration of uranium from dilute solution is extremely slow and is possibly due to kinetic factors, which decide the slow diffusion of uranyl ion into the polymer matrix [204]. Sriram et al. [205] have also investigated the transport of uranyl ion across amidoxime incorporated polypropylene membranes. The feed consisted of 0.5 M Na2CO3 solution at pH ~ 8, while the receiver phase was 0.1 M HCl. Using a 38% grafted membrane, they have observed 70% U sorption onto the PP-AN membrane. However, it was relatively difficult to leach out the sorbed U from the membrane (~20% with even 10 M HCl). On decreasing the percentage grafting to about 22%, 66% sorption was possible in 6 h and 70% stripping was possible using only 2 M HCl.

31.6 STABILITY OF LIQUID MEMBRANES Despite the many advantages of liquid membranes, which include selectivity and ligand economy, they are not yet applied in industrial scale that is ascribed to their low stability [206]. The common reasons for the instability of liquid membranes are


909


CCH3–COOH CH2 CHC N CH2 Electron beam

CCH3–COOH CH2 CH C NOH CH2 NH2

NH2OH

AN/MAA CH2 CH CH2 CCH3 C N

COOH

PE

CCH3–COOCH2CH2OH CH2

Polyethylene fiber

CHC N CH2

NH2OH

NA/HEMA CH2 CH

CH2 CCH3

C N Irradiation

CCH3–COOCH2CH2OH CH2 CH C NOH CH2 NH2

COOCH2CH2OH

Cografting

Amidoximation

FIGURE 31.18 Radiation graft polymerization of the AO membrane for Uranium recovery from seawater. (Reproduced from Tabushi, I., Kobuke, Y., Nakayama, N., Aoki, T., and Yashizawa, A., Ind. Eng. Chem. Prod. Res. Dev., 23, 445, 1984. With permission.)

breaking of emulsion (in case of ELM) and loss of membrane organic phase due to dissolution, formation of emulsion droplets, differential pressure across the membrane or evaporation, support chemical composition, and preparation methods (in case SLM).

31.6.1 STABILITY

OF

EMULSION LIQUID MEMBRANE

In (W=O)=W dispersions as a liquid surfactant membrane system, the membrane stability depends on the surfactant concentration per unit interfacial area below a critical value. Internal-phase leakage is generally affected by emulsion globule viscosity, internal-phase volume fraction, and surfactant concentration [207]. Takahashi et al. [208] have reported that leakage rates are greatly influenced by the salt concentration in the internal aqueous phase. Takeuchi and others [209] have found that a stable liquid membrane consisting of n-dodecane can be formed from about two layers of surfactant molecules adsorbed at the oil–water interface. The viscosity of the organic solvent had a significant effect on membrane stability. The permeation coefficients of water through the liquid membranes could be correlated with surfactant concentration. Further, entrainment of the external aqueous phase in the (W=O) emulsion droplets was influenced remarkably by the presence of electrolyte in the external solution.

31.6.2 STABILITY

OF

SUPPORTED LIQUID MEMBRANE

The permeability coefficients of some metal species through selected SLMs were measured by Danesi et al. [210] over extended periods of time, in the presence and absence of osmotic pressure gradients. The composition of the SLMs was varied in such a way to obtain membranes with variable interfacial properties. The interfacial tension, viscosities, contact angles, and water solubilities of the organic phases of several SLMs were also measured. Their conclusions suggest that the SLMs are relatively stable when osmotic pressures of feed and strip solutions are approximately equal. In the presence of an osmotic pressure difference, water tends to flow through the organic filled pores of the SLM, providing the organic phase appreciably solubilize water, which affect the metal transport properties. Apart from pressure difference, the other overwhelming factor for the instability of SLMs is the aqueous solubility of the carrier [211]. Apparently, the lipophilicity of the carrier has a significant role in the stability of the membrane. As shown by Hill et al. [212] carriers such as calix-crowns with lipophilic pendent groups have shown better stability as compared to similar compounds with much lower partition coefficient. They have also reported improved stabilities at higher carrier concentrations in a few cases. This is ascribed to the presence of free carrier at given metal ion concentration, which increases with carrier concentration. In another study, Shinbo et al. [213] reported that the membrane solvent must have both a high dielectric


910


constant and low solubility in water for the SLMs to be highly stable and permeable. Chiarizia [199] has investigated membrane stability using flat-sheet supports containing long-chain aliphatic amines as the carriers. The following order of stability was measured: tertiary > secondary > primary. This is the reverse order of the interfacial tension lowering at a water–n-dodecane interface. The water solubility of the amines also seems to play an important role in determining the membrane stability. This can be improved significantly by presaturating the aqueous phases with organic phase [214]. The performance of the SLM has also been reported to be affected by pore-blocking or fouling. Chiarizia [215] and Mohapatra et al. [63] observed such membrane fouling in their studies using amine and beta-diketone carriers, respectively. Nitrate trilauryl amine salts and TOPOHNO3 complex are the causes for precipitation and subsequent blocking of pores of the membranes in these cases. There are several other mechanisms of membrane degradation and will not be discussed here. In general, the stability of SLM can be increased by increasing the membrane thickness, decreasing the pore size, and enhancing the lipophilicity of the carrier. Polymer inclusion membranes are reported to have better stability as compared to the SLMs [177]. However, PIMs made from plasticizing agents such as TBP or TBEP (tris-butoxy ethyl phosphate), which have significant solubility in water can degrade with time. Salazar-Alvarez et al. [188] have shown using energy dispersive X-ray scans (EDS) that significant degradation of the membranes containing TBEP as the plasticizer do occur which could be suppressed by the addition of a small quantity of ethanol to the monomer plasticizer mixture prior to the membrane casting.

31.7 FUTURE PERSPECTIVES Membranes have been used extensively for the separation of radionuclides from liquid effluents generated in different nuclear facilities such as fuel fabrication plant, fuel reprocessing plant, and nuclear reactors. These separations, based on size exclusion, are invariably pressure driven and are not useful for the recovery and purification of a specific element present in a mixture where solvent extraction has a distinct advantage. However, liquid membrane techniques offer the advantages of both solvent extraction as well as membrane separations. Liquid membrane separations provide an efficient and excellent opportunity to explore novel and exotic extractants such as calixarenes, crown ethers, thiophosphinic acids, diamides, and carbamoyl methyl phosphine oxides which are of vital interest to the separation scientists engaged in the development of new strategies for the safe management of high-level radioactive waste. Low inventory of the extractants needed for membrane-based techniques makes them attractive not only from cost considerations but also from the ease of disposal of secondary wastes. Although ELM technique is quite efficient essentially due to the thinness of the membrane, large-scale application of this technique is limited in view of the difficulties encountered in the demulsification step needed for the recovery of the trapped metal ion. On the other hand, promise of the SLM technique has been demonstrated in the lab scale experiments. Large-scale applications of SLM require additional work in the area of stability=reusability of the membranes. Apart from the selective extraction, there is a need to develop the membranes that are compatible with the diluent=solvent mixture with respect to physical properties such as surface tension and viscosity. In addition, chemical=radiation environment of the feed=strip solution to which these membranes are subjected over long duration is an area of particular concern. Additional stability can be obtained by developing chemically grafted membranes. Recently, emphasis has been shifted to hollow fiber contactors as well as hollow fiber nondispersive solvent extraction technique, which can have surface to volume ratio as high as 10,000 m2=m3. Hollow fiber modules that offer a continuous means for large-scale separation applications need to be designed and developed keeping in mind the nature of solvent, feed, and strippant. Apart from the design parameters, such as lumen length=diameter, shell length=diameter, and number of lumens, one needs to ensure the compatability of materials used for making these modules. Nature of adhesives used and microstructures of the polymers used for making lumens also need attention.

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Osmosis-Based Treatment 32 Reverse of Radioactive Liquid Wastes Generated in Hospital Facility and in Steel Industry: Case Studies M. Sancho, J.M. Arnal, G. Verdú, and J. Lora CONTENTS 32.1 32.2

Introduction .................................................................................................................................................................. 919 Origin of Radioactive Wastes ...................................................................................................................................... 920 32.2.1 Wastes from the Electric Power Generation Cycle ....................................................................................... 920 32.2.2 Wastes from Medicine, Industry, and Research ............................................................................................ 920 32.2.3 Wastes from the Dismantling of Nuclear and Radioactive Installations ....................................................... 920 32.3 Case Studies ................................................................................................................................................................. 920 32.3.1 Radiation Influence on Membrane Performance ........................................................................................... 920 32.3.2 Treatment of 125I Liquid Wastes by Ultrafiltration and Reverse Osmosis.................................................... 924 32.3.2.1 Description of the Problem ............................................................................................................ 924 32.3.2.2 Treatment Description.................................................................................................................... 924 32.3.2.3 Experimental Test .......................................................................................................................... 924 32.3.2.4 Radioactivity Removal by Membranes in the Treatment of 125I Wastes...................................... 925 32.3.2.5 Treatment Procedure ...................................................................................................................... 926 32.3.3 Treatment of 137Cs-Contaminated Water by Reverse Osmosis..................................................................... 927 32.3.3.1 Description of the Problem ............................................................................................................ 927 32.3.3.2 Treatment Description.................................................................................................................... 927 32.3.3.3 Reverse Osmosis Performance in the Treatment of 137Cs Liquid Wastes .................................... 927 32.3.4 Radiological Protection in the Treatment of Radioactive Liquid Wastes ..................................................... 928 32.4 Future Trends in Radioactive Liquid Waste Disposal................................................................................................. 931 Acknowledgments.................................................................................................................................................................... 931 References ................................................................................................................................................................................ 931

32.1 INTRODUCTION The applications of membrane technology have grown significantly in the last decade. Some advantages of membrane technologies are continuous performance, low energy consumption, modular structure, and easy installation or automation. Due to this, nowadays membrane processes have become an ideal complement or even a suitable replacement of conventional separation processes (evaporation, chemical precipitation, ion exchange, etc.). Some areas of application are the nuclear industry and the treatment of radioactive liquid wastes, with two main purposes: reduction in the waste volume for further disposal, and reuse of decontaminated water. Pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis [RO]) are widely used for the treatment of radioactive waste. Some research groups worldwide are currently working on the application of membrane technology to the treatment of radioactive liquid wastes with different levels of activity, from low to high activity waste. Research is mainly focused on wastes from the nuclear industry. However, the nuclear industry is not the only source of radioactive wastes; medical and research applications of radioisotopes also generate radioactive wastes.

919


920



Since 1995, the Chemical and Nuclear Engineering Department of the Polytechnic University of Valencia in Spain has been working on the application of membrane technology to the treatment of low and medium level radioactive liquid wastes. In this chapter, we describe two practical cases of radioactive liquid waste treatment using membrane processes: the treatment of 137 Cs-contaminated water generated after a radioactive incident in a stainless steel production factory, and the treatment of 125I liquid wastes from nuclear medicine services at hospitals.

32.2 ORIGIN OF RADIOACTIVE WASTES Radioactive wastes are generated in the following activities: generation of electric power from nuclear fuel; applications of radioisotopes in medicine, industry, and research; and dismantling of nuclear and radioactive installations. With respect to waste volume and specific activity, the activities that generate the highest amounts of waste are those related to electric power generation.

32.2.1 WASTES

FROM THE

ELECTRIC POWER GENERATION CYCLE

This group includes the wastes generated during all the stages of the power production cycle in a nuclear power plant. First, the wastes from uranium mining, concentrate manufacturing, conversion to uranium hexafluoride, uranium enrichment, and fuel manufacturing. Second, the wastes generated during the operation of the nuclear power plant. This includes equipment and devices used for the purification and cleaning of the cooling systems, and other radioactive wastes like tools, working cloths, laundry wastes, papers, etc. Finally, the spent fuel rods that are produced on the completion of the energy production cycle; these elements are temporarily stored in swimming pools or in dry sites in the nuclear power station itself.

32.2.2 WASTES

FROM

MEDICINE, INDUSTRY, AND RESEARCH

In medical installations, the use of radioactive isotopes for diagnosis and therapy has significantly increased in the past years. Nonencapsulated radioactive elements are used for different purposes such as in diagnosis by tracers, treatment of thyroid or blood disorder, and in medical research. These activities produce some solid radioactive wastes like cotton, rubber gloves, syringes, etc., as well as liquid wastes, mainly scintillation liquids. Another type of waste is the encapsulated sources that are used for cancer treatment; these elements must be changed when their activity decays below a certain level. In industry, radioisotopes are used in encapsulated sources, generally of low activity, for the continuous measurement of level, humidity, density, or thickness. Gamma radiation-encapsulated sources are also used in nondestructive tests in metallic constructions (gammagraphies) and in industrial sterilization. In the nuclear research centers, the wastes are obtained from research and experimental reactors, hot cells, pilot plants, and decontamination services. These wastes have varying physical, chemical, and radioactive nature due to the great diversity of isotopes used and the wide range of processes in which they are applied.

32.2.3 WASTES

FROM THE

DISMANTLING

OF

NUCLEAR

AND

RADIOACTIVE INSTALLATIONS

The dismantling of a nuclear or radioactive installation, when its useful life is finished, generates the following radioactive wastes: reactor vessel and the components inside biological shield, external parts of the biological shield (heat exchangers, circulation pumps, pipes, etc.), activated and contaminated concrete, and auxiliary systems and building structures. Furthermore, some secondary wastes such as decontamination liquids and gas filters are also generated.

32.3 CASE STUDIES This section describes some case studies carried out by the authors, which are related to radioactive liquid waste treatment using membrane processes: a study about the influence of radiation on reverse osmosis membranes, a practical case about concentration of 125I radioactive liquid wastes by ultrafiltration and reverse osmosis processes, and a practical experience about decontamination of 137Cs liquid waste by reverse osmosis, including a study of radiological protection during the treatment.

32.3.1 RADIATION INFLUENCE

ON

MEMBRANE PERFORMANCE

The influence of radiation on the transport properties of different reverse osmosis composite membranes was studied by Chmielewski and Harasimowicz [1]. Reverse osmosis membranes were irradiated with a 60Co source, with a linear accelerator, and were also immersed in a 137Cs high activity solution, reaching absorbed dose values around 40 kGy. The results of some tests with the irradiated membranes showed that aromatic polyamide composite membranes are highly radiation resistant. This allows these membranes to be used in radioactive solutions with an activity below 3.7 1012 Bq=m3, which corresponds to absorbed dose values around 30 kGy.



921

Reverse Osmosis-Based Treatment

TABLE 32.1 Permselective Performance of Gamma-Irradiated Membranes Absorbed Dose (Gy) 0.25 0.50 1 2 5 10 20 Nonirradiated

Jv (L=[m2 h])

R (%)

28.0 26.7 30.5 31.0 28.5 24.0 20.0 21.6

97.0 98.5 98.6 98.7 98.9 98.9 97.7 97.5

Source: Courtesy of M. Balaban, Desalination, Italy. With permission.

Similar studies were carried out by Nakase, who determined the effect of radiation on some mechanical and thermal properties of the polysulphone film of composite membranes. After a complete experimental and microscopic study, the author concluded that polysulphone film almost keeps transport properties and stability up to dose values of 2 MGy [2]. Following these studies, the authors assessed the influence of different kinds of radiation on reverse osmosis composite membranes, applying radiation doses within the range of values that are expected to be absorbed by membranes in the treatment of low and medium radioactive wastes. Samples of reverse osmosis composite membranes were irradiated with gamma radiation using a 60Co source, in an absorbed dose range between 0.25 and 20 kGy; and with electron radiation using a linear accelerator, in an absorbed dose range between 5 and 25 Gy [3]. The irradiated membranes were then tested in a pilot plant with a plate-and-frame module with capacity for two membranes with an effective area of 90 cm2 each. Permselective results of irradiated membranes were compared to those of nonirradiated membranes [3]. It was observed that the performance of the samples irradiated with electronic or gamma radiation was very similar to that of the nonirradiated membranes. Tables 32.1 and 32.2 show the results of permeate flux and retention index for gamma- and electron-irradiated membranes, respectively. The differences in permselective performance between some irradiated and nonirradiated samples cannot be associated with radiation effects, but with membrane structural dispersion. This is because the membrane samples were taken from a spiral wound module; therefore, the differences observed can be due to the lack of homogeneity in the membrane structure and its characteristics along the module. This fact can be illustrated by the dispersion of the experimental results of the nonirradiated samples. Figure 32.1 shows the permeability obtained for all the nonirradiated samples. The average value of all these samples is Jv av ¼ 21.6 L=m2 h. The highest value of the statistical dispersion is Jv av þ 2s ¼ 38.8 L=m2 h, and the lowest value is Jv av 2s ¼ 4.4 L=m2 h. On the other hand, Figures 32.2 and 32.3 show the permeability of the gamma- and electron-irradiated samples, respectively. As it can be seen, in both cases the dispersion values of permeability are within the limit values of the nonirradiated dispersion, proving that the variability in the results is not the consequence of radiation, but of membrane nonhomogeneity.

TABLE 32.2 Permselective Performance of Electron-Irradiated Membranes Absorbed Dose (Gy) 5 10 15 20 25 Nonirradiated

Jv (L=[m2 h])

R (%)

20.0 26.7 18.7 14.0 20.0 21.6

97.2 97.5 97.4 97.0 97.4 97.5

Source: Courtesy of M. Balaban, Desalination, Italy. With permission.



922

Handbook of Membrane Separations Jv )av + 2s

40 38.8

Jv (L/m2 h)

30 Jv )av

21.6 20

10

Jv )av ⫺ 2s

4.4 0

Nonirradiated

FIGURE 32.1

Permeate flux of nonirradiated membranes.

Gamma radiation 40 Jv )av + 2s Jv (L /m2 h )

30 Jv )av 20 Jv )av ⫺ 2s

10 0 0.25

0.50

1

2

5

10

20

Absorbed dose (Gy)

FIGURE 32.2

Permeate flux of gamma-irradiated membranes.

Electron radiation 40 Jv )av + 2s Jv (L /m2 h )

30 Jv )av 20 10

Jv )av ⫺ 2s

0 5

10

15

20

25

Absorbed dose (Gy)

FIGURE 32.3

Permeate flux of electron-irradiated membranes.

With regard to selectivity, Figure 32.4 shows the results of the retention index for all the nonirradiated samples. The average value of all these samples is R)av ¼ 97.6%. The highest value of the statistical dispersion is R)av þ 2s ¼ 98.1%, and the lowest value is: R)av 2s ¼ 97%. On the other hand, Figures 32.5 and 32.6 show the selectivity of the gamma- and electron-irradiated samples, respectively. As it can be seen, the dispersion of selectivity in the electron-irradiated membranes is within the limit values of the nonirradiated dispersion, proving that the variation in the results is not caused by radiation.



923

Reverse Osmosis-Based Treatment 100

R (%)

99 R )av + 2s

98.1 98

R )av

97.6 97.0

R )av − 2s 96 95 Nonirradiated

FIGURE 32.4

Retention index of nonirradiated membranes.

Gamma radiation 100

R (%)

99

R )av + 2s

98.1 98 97.6

R )av

97.0

R )av − 2s 96 95

FIGURE 32.5

0.25

0.50

1 2 5 Absorbed dose (Gy)

10

20

Retention index of electron-irradiated membranes.

Electron radiation

R (%)

100 R )av + 2s

99 98.1 98 97.6 97.0

R )av R )av − 2s

96 95 5

FIGURE 32.6

10

15 20 Absorbed dose (Gy)

25

Retention index of gamma-irradiated membranes.

For gamma-irradiated membranes, the values of experimental selectivity are higher than the upper limit of the nonirradiated dispersion, so it can be said that gamma radiation can even have a beneficial effect on membrane selectivity. As a conclusion of this study about the influence of electron and gamma radiation on reverse osmosis composite membranes, it can be stated that radiation does not affect membrane performance in a range of absorbed dose values up to 25 Gy, approximately. Therefore, reverse osmosis composite membranes are suitable for use in the treatment of low and medium activity levels, as the values of the radiation absorbed by the membranes will be lower than those assessed in this case study.



924


32.3.2 TREATMENT 32.3.2.1

OF

125

I LIQUID WASTES

BY

ULTRAFILTRATION

AND

REVERSE OSMOSIS

Description of the Problem

Radioimmunoassay (RIA) is used in nuclear medicine for measuring very low levels (concentrations below 1011 g=mL) of some biological compounds in corporal fluids, by means of the combination of radioisotopes and antibodies. The most common radioisotope used in this technique is 125I, a gamma-emitting radioisotope with a half-life of 60 days, which is used for labelling compounds of interest. RIA techniques generate great amounts of radioactive liquid wastes whose composition depends on the specific assays that are carried out in each laboratory. Liquid wastes from RIA techniques are usually composed of different types of proteins (some of them radioactively labelled), preservative solutions, several low-molecular-weight organic compounds, and inorganic salts— all in aqueous solutions. These wastes are classified as low and medium radioactive, but they are also potentially infectious since they can contain pathogens from patient’s blood [4]. Sometimes, the infectious risks of these wastes can be much more dangerous than the risks associated with radioactivity. At present, these radioactive wastes are temporarily stored in the sanitary center for partial activity decay. Then, the wastes are disposed of by an officially designated company depending on their radioactivity levels and other biological and chemical risks. Reduction of RIA waste volume would mean an important decrease in waste disposal costs, and would diminish the problems of waste storage in hospitals. Furthermore, an appropriate treatment of RIA liquid wastes would remove pathogens and chemicals that otherwise would remain in the wastes. 32.3.2.2

Treatment Description

Figure 32.7 shows the scheme of the solution proposed by the authors for treating radioactive liquid wastes from RIA laboratories. Since part of the radioisotope is in ionic form [5], reverse osmosis can be used for radioisotope removal. But before the application of reverse osmosis, a pretreatment is required. It consists of the following two stages: . .

Filtration in cartridges for removal of suspension solids that could damage the membranes. Ultrafiltration membranes are suitable for the treatment of radioactive liquid wastes, especially as reverse osmosis pretreatment [6]. Therefore, ultrafiltration is used for the removal of the radioactivity associated with the proteins and high-molecular-weight organic compounds, as these species can reduce reverse osmosis performance.

The purpose of the treatment is to reduce waste volume for further disposal by an authorized company, and to obtain an organic matter and pathogens-free permeate with an activity level below the legal discharge limit. 32.3.2.3

Experimental Test

Some experiments were carried out with wastes from nuclear medicine services (RIA laboratories). Wastes with different storing periods were used, with activity values between 10 and 1000 kBq=L, depending on their storage time. The experiments consisted of a first stage of ultrafiltration, after which the UF permeate was treated by reverse osmosis, according to the scheme presented in Figure 32.7. If the RO permeate showed an activity higher than the environmental background level in the treatment area, it was again treated by RO, and so on until reaching an activity level in the permeate close to background levels.

Liquid waste

Filtration

Ultrafiltration

UF concentrate

UF permeate Reverse osmosis

Final permeate (discharge)

Final concentrate (disposal by an authorized company)

FIGURE 32.7 Scheme of the RIA liquid waste treatment. (Courtesy of Radiation Protection Dosimetry, Oxford University Press, Oxford. With permission).



925

Reverse Osmosis-Based Treatment 100

R 125I (%)

80 Exp 1 Exp 2 Exp 3

60 40 20 0 0

0.2

0.4 t (h)

0.6

0.8

FIGURE 32.8 Radioactivity removal by UF membranes in the RIA waste treatment. (From Arnal, J.M., Campayo, J.M., Lora, J., Sancho, M., Iborra, I., and Alcaina, I., Desalination, 129, 101, 2000.)

During the treatment, periodical measurements of flow and conductivity of the permeate and feed were taken for calculating the permselective parameters of the membranes (Jv, R). In addition, samples of the feed and permeate were periodically taken for measuring radioactivity. Then, the retention index to 125I was calculated using the following expression: R125 I (%) ¼

AF AP 100 AF

(32:1)

where AF and AP are the activities of feed and permeate (Bq=L), respectively. 32.3.2.4

Radioactivity Removal by Membranes in the Treatment of

125

I Wastes

In the experiments performed on real wastes, ultrafiltration played an important role in the partial reduction of radioactivity. Figure 32.8 shows the evolution of the retention index to 125I with time in some of the UF experiments. Most of the time, radioisotope rejection by UF membrane was higher than 50%, even reaching maximum values of about 80%. This fact proves that part of the radioisotope remains labelled to proteins and other organic compounds, most of which are removed in the UF stage [7]. The rest of the radioactivity, associated with low-molecular-weight organic compounds and ionic solutes, will be removed in the RO stages. In reverse osmosis, radioactivity removal gives two types of results: the results of the first RO application after the UF stage, in which the ultrafiltrate was treated; and the results of the subsequent RO applications, in which an RO permeate of an earlier stage was treated. Figures 32.9 and 32.10 show the evolution with time of the retention index to 125I in the first RO application carried out in three experiments, and in the third RO application performed on experiments 2 and 3, respectively. It can be observed that in the first RO application, the retention index to 125I decreased progressively during the treatment due to an increase of feed activity as a result of concentrate recirculation to the feed tank. Anyway, in this application R125I was higher than 90% most of

RO first application 100

R 125I (%)

95 90 85

Exp 1

80

Exp 2

75

Exp 3

70 0

2

4

6

t (h)

FIGURE 32.9 Radioactivity removal by RO membranes in the treatment of UF permeate. (From Arnal, J.M., Campayo, J.M., Lora, J., Sancho, M., Iborra, I., and Alcaina, I., Desalination, 129, 101, 2000.)



926

Handbook of Membrane Separations RO third application 100

R 125I (%)

95 90 Exp 2 85

Exp 3

80 0

0.2

0.4

0.6

0.8

1

t (h)

FIGURE 32.10 Radioactivity removal by RO in the treatment of an earlier RO permeate. (From Arnal, J.M., Campayo, J.M., Lora, J., Sancho, M., Iborra, I., and Alcaina, I., Desalination, 129, 101, 2000.)

the time. On the other hand, in the subsequent RO applications (Figure 32.10), the retention index to 125I remained quite constant with values above 95% in all the experiments. This was due to lower feed activity values and lower feed activity variation. Table 32.3 shows the values of feed and permeate activity evolution in one of the experiments, representative of a typical treatment process. It can be seen how it is possible to obtain a permeate with a level of activity below the legal discharge limits by means of successive reverse osmosis applications. 32.3.2.5

Treatment Procedure

According to the experimental results, the most suitable procedure for RIA waste treatment was defined. This procedure consists of the following stages: . . . .

Ultrafiltration. In this stage, high-molecular-weight organic compounds as well as biological contaminants are removed, and the activity is reduced to an average of 50%–60%. First Stage of Reverse Osmosis. Almost all the organic and biological compounds that were not eliminated previously by ultrafiltration are removed in this stage. Second Stage of Reverse Osmosis. Salt concentration is reduced below running water levels (lower than 1 g=L). Third Stage of Reverse Osmosis. After this stage, a permeate with an activity below 100 Bq=L is obtained.

The following options are suitable for the disposal of the final permeate: . .

Direct discharge to the sink by dilution, if authorized by the appropriate official institution. Temporary storage and further discharge to the sink after checking that the activity is below the legal limit. It must always be authorized by the appropriate entity.

The final concentrate, which includes UF and RO concentrates, should be disposed of by an authorized company, considering not only waste radioactivity levels but also chemical and biological contaminations, specially in the ultrafiltration concentrate since it contains most of the pathogens of the original waste.

TABLE 32.3 Radioactivity Removal in RO Experiments RO Stage RO1 RO2 RO3

AF (Bq=L)

AP (Bq=L)

117,400 15,360 720

15,360 720 70

Source: Courtesy of Radiation Protection Dosimetry, Oxford University Press, Oxford. With permission.



927


32.3.3 TREATMENT 32.3.3.1

OF

137

CS-CONTAMINATED WATER

BY

REVERSE OSMOSIS

Description of the Problem

In 1998, a 137Cs source was accidentally melted in a stainless steel production factory in Spain. As a consequence of this accident, the cooling system of one of the ovens became radioactively contaminated. The decontamination process produced 40 m3 of 137Cs contaminated water, with an average activity above 300 kBq=L. The authors, in collaboration with LAINSA company, developed the project for decontaminating the radioactive liquids, by means of a reverse osmosis plant. The aim of the treatment was to remove the 137Cs radioisotope from the treated liquid and to reduce the volume of the solution for further immobilization. 32.3.3.2

Treatment Description

Figure 32.11 shows a scheme of the RO plant for the treatment of the 137Cs-contaminated liquids. It was equipped with two kinds of spiral wound reverse osmosis membranes: high-pressure (HP) and low-pressure (LP) membranes. Before flowing through the membranes, the feed was pretreated using 5 mm sleeve filters and 0.45 mm cartridge filters. The treatment was divided into two stages. At the first stage, both types of membranes were used, recirculating all the concentrates to the feed tank. The second stage began when the permeate resulting from the first stage was out of the specifications, and then the two reverse osmosis lines were separated. The low-pressure membranes treated the first-stage permeate, while the high-pressure membranes continued treating the concentrate. The decontamination process by reverse osmosis took about 1 month. After this period, 36 m3 of liquid was decontaminated from the original 40 m3. Later, evaporation was applied to get an additional concentration of the final waste; the volume of concentrate liquid was reduced from 4 to 1 m3. 32.3.3.3

Reverse Osmosis Performance in the Treatment of

137

Cs Liquid Wastes

During the treatment, samples of the permeate, the concentrate, and the feed were taken periodically for measuring radioactivity. The retention index to 137Cs was calculated by an expression that is analogous to Equation 32.1. Figure 32.12 shows the results of the retention index to 137Cs obtained at the two RO treatment stages. From these results, it can be stated that . . .

At the first RO treatment stage, the retention index to 137Cs was higher than 98% in most cases. At the second RO treatment stage, the retention index values in the high-pressure membranes were slightly lower, but always above 96%. At the second RO treatment stage, the retention index to 137Cs in the low-pressure membranes was always higher than 98%, and above 99.5% in most cases.

2

5

3

1

4 P P

8 LP P

6 P HP 7

9 1 2 3 4 5

FIGURE 32.11

Feed tank Sleeve filters Cartridge filters Intermediate tank Pressure pump

Scheme of the 137Cs liquid waste treatment plant.

6 7 8 9

LP RO membranes HP RO membranes Regulation valve Permeate tank



928


R 137Cs (%)

98

96

94 First stage (HP+LP) 92

Second-stage HP (concentrate first stage) Second-stage LP (permeate first stage)

90 0

100

200

300

400

500

600

t (h)

FIGURE 32.12

Retention index to 137Cs of the RO membranes. (Courtesy of M. Balaban, Desalination, Italy. With permission.)

The progressive decrease of the retention index values at the first stage of the RO treatment and in the high-pressure membranes during the second stage is due to two main factors. On one hand, to the increase in feed concentration as a result of concentrate recirculation, and on the other hand, to the increase in membrane fouling. However, in the low-pressure membranes in the second RO treatment stage high values of retention index were obtained because the permeate of the first stage was treated, so the activity values were much lower (below 100 kBq=L in comparison with values higher than 1500 kBq=L in the other two situations). In spite of the successful 137Cs selectivity results, permeability values were not as good as expected; in fact, the low permeability caused the RO treatment to last more than planned. This was mainly due to a lack of knowledge about the characteristics of the solution to be treated; as a result, it was not possible to design a specific pretreatment, and filtration was not enough to avoid severe fouling in the RO membranes. As a summary, the following conclusions can be drawn from the treatment of 137Cs-contaminated liquids [8]: . . .

The application of reverse osmosis was highly successful, decontaminating more than 90% of the original volume. The selectivity values confirm the potential of reverse osmosis composite membranes in the treatment of effluents that contain radioactive isotopes. The decrease in membrane permeability was caused by membrane fouling. This could have been partially avoided with an appropriate pretreatment design.

32.3.4 RADIOLOGICAL PROTECTION 137

IN THE

TREATMENT

OF

RADIOACTIVE LIQUID WASTES

During the RO treatment of Cs-contaminated liquid, dose levels were measured at different sites in the working area with the purpose of following dose evolution and taking the appropriate radiological protection measures [9]. Figure 32.13 shows a scheme of the treatment area, in which the measuring points are indicated by numbers whose description is presented in Table 32.4. Environmental doses were measured at boundary points (1, 2, and 3); in all other points contact, doses were measured. Measures were taken approximately every 12 h in both cases. The following figures show the dose evolution with time of the points indicated in the scheme shown by Figure 32.13. Figure 32.14 shows radiation evolution at the boundary points. The three points show no significant radiation values, although point 3 has slightly higher values than points 1 and 2 because it is closer to the tanks containing radioactive liquid. Figure 32.15 shows the radiation evolution in the surroundings of the permeate and feed tanks (points 4 and 5). Near the permeate tank (point 4), dose variation is not very significant since the permeate has a low activity level [10]. However, the radiation evolution near the feed tank (point 5) shows quite high dose values. This is because the liquid inside this tank has much higher activity than the liquid inside the permeate tank. Dose increase around the feed tank is due to progressive concentration of the liquid feed as a result of membrane concentrate recirculation to this tank [11]. Figure 32.16 shows the radiation evolution of the main elements of the membrane treatment plant. Among these elements, the filtration unit showed the highest dose values, particularly the sleeve filters (point 7) with top values around 140 mSv=h. This high dose is because these filters are the first element through which the feed flows, so they are in contact with a high activity liquid. The dose values at the cartridge filters (point 8) are lower than the values of the sleeve filters, though they are also fairly high, between 10 and 30 mSv=h, approximately.



929


Feed tank

⫻4

⫻5

Permeate tank ⫻3 ⫻7 ⫻8

⫻2

⫻6

9⫻

Membrane plant ⫻1

FIGURE 32.13 Diagram of the working area in the treatment of 137Cs liquid wastes. (From Arnal, J.M. Tratamiento de Residuos Radiactivos Líquidos y Sólidos Contaminados con Cs-137 Mediante Sistemas Combinados de Separación por Membranas, Lixiviación y Precipitación Química, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2003.)

TABLE 32.4 Identification of Dose Measuring Points in the Treatment Area Point

Area or Element Represented

1 2 3 4 5 6 7 8 9

Boundary Boundary Boundary Permeate tank (treated liquid) Feed tank Intermediate tank and pump Sleeve filter Cartridge filter Membrane modules

Source:

From Arnal, J.M. Tratamiento de Residuos Radiactivos Líquidos y Sólidos Contaminados con Cs-137 Mediante Sistemas Combinados de Separación por Membranas, Lixiviación y Precipitación Química, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2003.

Point 1

Point 2

Point 3

Dose (µSv/h)

5 4 3 2 1 0 0

50

100

150

200

250

t (h)

FIGURE 32.14 Radiation level evolution at the boundary points of the treatment area. (From Arnal, J.M. Tratamiento de Residuos Radiactivos Líquidos y Sólidos Contaminados con Cs-137 Mediante Sistemas Combinados de Separación por Membranas, Lixiviación y Precipitación Química, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2003.)



930

Handbook of Membrane Separations Permeate tank

Feed tank

30 Dose (µSv/ h)

25 20 15 10 5 0 0

50

100

150

200

250

t (h)

FIGURE 32.15 Radiation level evolution around the permeate and feed tanks. (From Arnal, J.M. Tratamiento de Residuos Radiactivos Líquidos y Sólidos Contaminados con Cs-137 Mediante Sistemas Combinados de Separación por Membranas, Lixiviación y Precipitación Química, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2003.)

Another important point with regard to radiation level at the treatment plant is the intermediate tank in which the liquid is stored after filtering, and the pump placed beside that tank (point 6). The high values at this point, around 40 mSv=h, are due to the high activity values of the liquid stored in the intermediate tank. Finally, the point that represents membrane modules (point 9) shows quite significant dose values, which increase progressively as the feed concentrates. From the analysis of radiation evolution in the treatment area, the following conclusions can be stated: . . . .

.

Most relevant points from the viewpoint of radiological protection are the filtration unit and the intermediate tank, with the highest dose values. Filtration unit is a point of necessary and frequent access to change filters. In future treatment plants, it should be isolated to facilitate a safe access to the other elements of the plant. Intermediate tank should not be placed in the surroundings of any element that needs frequent access. The pump also has to be placed far from the elements with high dose values, if it requires regular maintenance. Membrane modules reach dose values that cannot be ignored. Although they are not as high as the doses reached at the elements mentioned above, membrane modules are points of continuous manipulation, so they have to be considered from the point of view of radiological protection. Dose values at the area around the feed tank are fairly high, but it is not an important problem since this tank is not of frequent access.

Pump

Sleeve filter

Cartridge filter

Membranes

150 Dose (µSv/h)

125 100 75 50 25 0 0

50

100

150

200

250

t (h)

FIGURE 32.16 Radiation level evolution at the membrane plant. (From Arnal, J.M. Tratamiento de Residuos Radiactivos Líquidos y Sólidos Contaminados con Cs-137 Mediante Sistemas Combinados de Separación por Membranas, Lixiviación y Precipitación Química, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2003.)




931

32.4 FUTURE TRENDS IN RADIOACTIVE LIQUID WASTE DISPOSAL With regard to future trends in the radioactive and nuclear fields, an increasing use of radioisotopes for medical, industrial, and research purposes is expected soon. As a result of this, there will be an increase in the amount of low and medium radioactive wastes. The benefits of these activities are well known, but design of waste disposal strategies for volume reduction and liquid decontamination is essential, to optimize waste storage installations and reduce waste disposal costs. Among these strategies, the application of an appropriate treatment is essential for suitable waste disposal. Membrane technologies have a great potential in the treatment of radioactive liquid wastes, as it has been proved throughout this chapter. In this sense, it is expected a growing use of the membrane processes in the radioactive field, with different possibilities: alone, combined between them (microfiltration or ultrafiltration and reverse osmosis) or combined with other conventional processes like evaporation or ion exchange. Furthermore, some special membrane processes, like membrane distillation or liquid membranes, could be applied for the specific treatment of radioactive wastes. In relation to future trends of the membrane treatments described in this chapter, they are expected to be applied for medical and research wastes of low-medium activity contaminated with 125I, 137Cs, or other radioisotopes with similar characteristics. Furthermore, membrane treatment has been proved as an efficient and quick treatment in an incident that generates low-medium radioactive wastes.

ACKNOWLEDGMENTS Authors would like to express their gratitude to the people from La Fe Hospital and LAINSA company of Valencia for their contribution to the research described in this chapter. We would also like to thank the R & D & I Linguistic Assistance Office at the Universidad Politécnica of Valencia for their help in revising this paper.

REFERENCES 1. Chmielewski AG and Harasimowicz M. Influence of gamma and electron irradiation on transport properties of nanofiltration and hyperfiltration membranes. Nukleonika 1997; 42(4):857–862. 2. Nakase Y. Irradiation effects on properties of reverse osmosis membrane based on crosslinked aromatic polyamide. J Nucl Sci Technol 1994; 31:1214–1221. 3. Arnal JM, Sancho M, Verdú G, Campayo JM, and Villaescusa JI, Treatment of 137Cs liquid wastes by reverse osmosis. Part I. Preliminary tests. Desalination 2003; 154:27–33. 4. Peyrin JO. La gestion des déchets radioactifs hospitaliers. Radioprotection 1992; 27:47–53. 5. Arnal JM, Campayo JM, Lora J, Sancho M, Iborra I, and Alcaina I. Declassification of radioactive waste solutions of iodine (I125) from radioimmune analysis (RIA) using membrane techniques. Desalination 2000; 129(1):101–105. 6. Chmielewski AG and Harasimowicz M. Influence of gamma and electron irradiation on transport properties of ultrafiltration membranes. Nukleonika 1992; 37:61–70. 7. Sancho M. Tratamiento de Residuos Liquidos Hospitalarios Procedentes de RIA (Radioinmunoanálisis) Mediante Técnicas de Membrana, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2002. 8. Arnal JM, Sancho M, Verdú G, Campayo JM, and Gozálvez JM. Treatment of 137Cs liquid wastes by reverse osmosis. Part II. Real application. Desalination 2003; 154:35–42. 9. Arnal JM. Tratamiento de Residuos Radiactivos Líquidos y Sólidos Contaminados con Cs-137 Mediante Sistemas Combinados de Separación por Membranas, Lixiviación y Precipitación Química, 1st ed. Valencia, Spain: Servicio de Publicaciones UPV, 2003. 10. Arnal JM, Sancho M, Campayo JM, and Verdú G. Comportamiento de las Membranas Poliméricas en el Tratamiento de Agua Contaminada Con Cs-137. 8th Mediterranean Congress on Chemical Engineering, Barcelona, November 10–12, 1999. 11. Arnal JM, Sancho M, Campayo JM, Villaescusa JI, and Verdú G. Cs-137 Radioactive Liquid Wastes Treatment using Membrane Techniques. 10th International Congress of the International Radiation Protection Association, Hiroshima, May 14–19, 2000.





of Membrane-Based 33 Evaluation Processing of Radioactive Nuclear Plant Waste: Case Studies Anil Kumar Pabby, S.K. Gupta, S.R. Sawant, N.S. Rathore, P. Janardan, R.D. Changrani, and P.K. Dey CONTENTS 33.1 33.2

Introduction .................................................................................................................................................................. 933 Removal of Activity from Various Waste Streams Using Ultrafiltration and Reverse Osmosis................................ 933 33.2.1 Activity Removal from Fuel Pond Water ...................................................................................................... 933 33.2.2 Removal of Activity from Delay Tank Solutions Using an RO Unit ........................................................... 934 33.3 Electrodialysis for the Decontamination and Concentration of Low-Active Waste ................................................... 937 33.4 Application of a Perfluorinated Membrane in Fuel Reprocessing .............................................................................. 938 33.5 Nondispersive Solvent Extraction for the Separation, Removal, and Concentration of Actinides ............................. 939 33.5.1 Separation of Uranium and Plutonium from Oxalate Supernatant ................................................................ 940 33.5.2 Extraction of Macro-Quantities of Uranium .................................................................................................. 941 33.5.3 Extraction of Americium from Acidic Media................................................................................................ 941 References ................................................................................................................................................................................ 943

33.1 INTRODUCTION Membrane-based processing of nuclear plant radioactive waste and radioactive liquids in general is rapidly gaining acceptance within the nuclear industry. Whereas early membrane systems were limited by unreliable hardware and inconsistent performance; modern systems have proven to be both cost effective and superior to conventional processing technologies. In specific installations, the conversion to membrane-based processing has produced significantly pure water, with less secondary waste generation and lower operator radiation exposure [1]. The possible applications of membranes in nuclear programmes are (1) treatment of radioactive liquid effluents, (2) separation and concentration of useful radionuclides, and (3) separation of gaseous streams containing radioactive species. Membrane processes have been introduced on pilot plant scale in the reprocessing plant of the Bhabha Atomic Research Centre (BARC) to treat various low-active streams. The following case studies are discussed in brief.

33.2 REMOVAL OF ACTIVITY FROM VARIOUS WASTE STREAMS USING ULTRAFILTRATION AND REVERSE OSMOSIS 33.2.1 ACTIVITY REMOVAL

FROM

FUEL POND WATER

The primary function of ultrafiltration (UF) systems is to remove colloids and other particulate foulants from feed streams. Activity from fuel pond water is currently removed via ion-exchange columns. Among the drawbacks of ion-exchange purification is the production of substantial amounts of secondary waste during regeneration of the ion-exchange bed. Hence, ultrafiltration was thought to be an attractive alternative for treating fuel pond water [2]. We undertook a pilot study at our reprocessing plant. The typical composition of fuel pond and delay tank waters for the plant is shown in Table 33.1. The spiral-wound module used for the UF and reverse osmosis (RO) studies is detailed in Table 33.2, and the basic flow diagram through the spiral-wound membrane is shown in Figure 33.1. The setup of the UF membrane pilot plant is shown in Figure 33.2.

933



934


TABLE 33.1 Composition of Fuel Pond and Delay Tank Solutions Fuel Pond Water pH Total dissolved salts (TDS) Turbidity Gross alpha Gross beta=gamma activity

Delay Tank Water

6.8 to 7.8 28 ppm 0.3 NTU 4.6 10 6 mCi=L 3.2 10 4 mCi=L

pH TDS Gross alpha Gross beta=gamma activity

6 to 8 1700 ppm 2.8 10 5 mCi=L 3.5 10 4 mCi=L

Note: mCi=L stands for millicuries per liter.

TABLE 33.2 Details of UF and RO Module Details of Membrane Module Diameter of UF module Length of module Membrane area Centrifugal pump (flow rate) Membrane material Membrane pore size Operating pressure

UF

RO

2.5 in. 12 in. 0.5 m2 180 lph Polysulfone 100 Å 1 bar

6 in. 30 in. 3 430 lph Polyamide TFC (thin-film composite) 5–10 Å 15 bar

trate

cen

Con

d

Fee

ate

entr

c Con

uct

Prod

trate

cen

Con

d

Fee

Feed channel d Fee

Membrane composite Product channel

FIGURE 33.1

Spiral wound module.

The results of the pilot plant study, summarized in Table 33.3, are very positive: 12,000 L of fuel pond water were successfully treated, providing decontamination factor (DF) (Chapter 29) values for alpha and beta of 4 and 5, respectively. The final product obtained from UF had a turbidity of ca. 0.1 NTU (feed turbidity: 0.3 NTU). The flux value was reduced by 15%, which was attributed to possible membrane fouling. The UF module was backflushed with pH 2 solution (HNO3), and the original flux value of the permeate was restored. A long-term performance evaluation study was conducted by monitoring the permeate quality (DF value) and radiation dose on the module. No hot spot generation was observed during a pilot run of UF with fuel handling area (FHA) water.

33.2.2 REMOVAL

OF

ACTIVITY

FROM

DELAY TANK SOLUTIONS USING

AN

RO UNIT

Adapting RO technology will help to achieve the ALARA concept, which will be beneficial for the environment. In recent years, RO systems have been used to replace or augment existing evaporation or ion-exchange technology due to their lower



935

Evaluation of Membrane-Based Processing of Radioactive Nuclear Plant Waste: Case Studies

FIGURE 33.2

UF membrane setup utilized in fuel handing area.

operating costs [1,3]. The final discharge of low-level active waste (i.e., a mixture of evaporator condensate, ammonium diuranate filtrate, hand washings, etc. from the reprocessing plant) is carried out from the delay tank, which temporarily holds low-active waste. We thus evaluated RO for the removal of alpha activity, beta activity, nitrates, and total dissolved salts (TDS) from delay tank water at the pilot plant. The specifications of the membrane module, which was supplied by the Desalination Division of the BARC (Mumbai), are listed in Table 33.2. The RO setup installed at the delay tank site is shown in Figure 33.3. The delay tank water composition is given in Table 33.1. The module was operated at a pressure of 15 bar, which was adjusted by a throttle valve placed on the reject line. The experiments were performed both in once-through and recirculation modes. The alpha and beta activities, and nitrate and TDS levels were periodically monitored by sampling the feed, permeate, and reject solutions using standard analytical methods. The results of the RO studies are summarized in Table 33.4. The DF values for alpha range from 6–10, and for beta, from 6–15. Nitrates and TDS were also strongly rejected by RO. The results from the permeate solution show that after passing 25,000 L, the performance of the pilot plant was quite promising. Furthermore, the same RO setup was used for a delay tank solution containing moderate alpha and beta activity. The DF values (Table 33.4) obtained for alpha are 12–15, which are slightly higher than those obtained for solutions with lower activity. This is because the feed input alpha activity was higher. In recirculation mode, good permeate quality was maintained, whereas alpha and beta activities of the concentrate reached 10 4 and 10 3 mCi=L, respectively, and TDS levels surpassed 15,000 ppm. The results of the RO studies are summarized in Table 33.5.

TABLE 33.3 Typical Analytical Results of UF Pilot Plant Studies Conducted for Fuel Pond Water Feed Activity (mCi=L) Sample at 500 L 2000 L 4000 L 8000 L 12000 L

Alpha 2.8 10 3.8 10 1.4 10 2.4 10 5.2 10

Beta=Gamma 6 6 6 6 6

3.5 10 4.5 10 5.8 10 3.9 10 3.4 10

TDS (ppm)a

DF 4 4 4 4 4

Alpha

Beta=Gamma

Permeate NTU

Feed

Permeate

5 4 4 4 4

4 3 4 4 4

0.05 0.1 0.1 0.1 0.1

28 22 22 35 35

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