Industrial Gases Processing

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Industrial Gases Processing Edited by Heinz-Wolfgang Häring

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Further Reading

A. Züttel, A. Borgschulte, L. Schlapbach (Eds.)

Hydrogen as a Future Energy Carrier 2008 ISBN 978-3-527-30817-0

G. A. Olah, A. Goeppert, G. K. S. Prakash

Beyond Oil and Gas: The Methanol Economy 2006 ISBN 978-3-527-31275-7

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Industrial Gases Processing Edited by Heinz-Wolfgang Häring

Translated by Christine Ahner

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The Editor Dr. Heinz-Wolfgang Häring Lommelstrasse 6 81479 München Germany

Cover Illustration: Hydrogen plant, Oberhausen, Germany, with kind permission of Linde AG

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de. ¤ 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany Printed on acid-free paper Composition Manuela Treindl, Laaber Printing betz-druck GmbH, Darmstadt Bookbinding Litges & Dopf GmbH, Heppenheim ISBN 978-3-527-31685-4

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V

Foreword Industrial gases have become over a period of more than a century ubiquitous ingredients of our daily activities, e.g. metal fabrication, metallurgy, petrochemicals, food processing, healthcare, and many more. In 2006, the business generated globally with industrial gases exceeded the 50 billion US dollar volume. At The Linde Group, generations of scientists and engineers have been working in this field, determining the physical and chemical properties, developing processes for the production, purification and application of industrial gases, as well as their safe handling, storage and transportation. This book, written and compiled by numerous experts and edited by Dr. Wolfgang Häring, is an authoritative, accurate, and useful single-source reference for those who work in this industry, for students or simply for the users of industrial gases. Details are also offered concerning the historical background of these molecules. The term “industrial gases” is herewith intended to include also the category of “medical gases”, which, while being produced by means of “industrial” processes, have meanwhile become real drugs and are subject to Good Manufacturing Practices. It is my pleasure to commend the editor as well as the authors of this outstanding piece of technical literature for their relentless and professional quest for precision and completeness, which for sure will be highly appreciated by all readers. Pullach, October 2007

Dr. Aldo Belloni Linde AG

Industrial Gases Processing. Edited by Heinz-Wolfgang Häring Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31685-4

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VII

Contents Foreword V List of Contributors XIII 1

Introduction 1 References 7

2

The Air Gases Nitrogen, Oxygen and Argon 9 History, Occurrence and Properties 9 Nitrogen 9 History 9 Occurrence 9 Physical and Chemical Properties 10 Oxygen 11 History 11 Occurrence 11 Physical and Chemical Properties 11 Argon 12 History 12 Occurrence 13 Physical and Chemical Properties 13 Recovery of Nitrogen, Oxygen and Argon 13 Introduction 13 Application Range of Membrane Separation, Pressure Swing Adsorption and Cryogenic Rectification 14 Nitrogen Recovery with Membranes 16 Physical Principle 16 Membrane Technology 16 Design 17 Nitrogen and Oxygen Recovery by Means of Pressure Swing Adsorption 18 Physical Principle 18

2.1 2.1.1 2.1.1.1 2.1.1.2 2.1.1.3 2.1.2 2.1.2.1 2.1.2.2 2.1.2.3 2.1.3 2.1.3.1 2.1.3.2 2.1.3.3 2.2 2.2.1 2.2.2 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.2.4 2.2.4.1


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VIII

Contents

2.2.4.2 2.2.4.3 2.2.4.4 2.2.5 2.2.5.1 2.2.5.2 2.2.5.3 2.2.5.4 2.2.5.5 2.2.5.6 2.2.5.7 2.3 2.3.1 2.3.3 2.3.4 2.3.5 2.4 2.5 2.5.1 2.5.1.1 2.5.1.2 2.5.2 2.5.3

3

3.1 3.2 3.3 3.3.1 3.3.2 3.3.2.1 3.3.2.2 3.4 3.4.1 3.4.2 3.5 3.6 3.6.1 3.6.2 3.6.3

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Properties of Molecular Sieves 18 Nitrogen Recovery 19 Oxygen Recovery 20 Cryogenic Rectification 20 Process with Air Booster and Medium-Pressure Turbine for the Recovery of Compressed Oxygen, Nitrogen and Argon 21 Internal Compression 32 Nitrogen Generators 36 Liquefiers 37 High-purity Plants 38 Apparatus 42 Design, Assembly and Transport of the Coldbox 57 Safety Aspects 59 Introduction 59 Air Pollution 61 Ignition in Reboilers 63 Other Hazards in Air Separation Units 64 Process Analysis Air Separation Units 64 Applications of the Air Gases 67 Applications of Nitrogen 67 Applications of Nitrogen for Inerting and Purging 67 Applications of Nitrogen for Cooling, Preserving and Deep-Freezing 74 Applications of Oxygen 83 Applications of Argon 104 References 108 The Noble Gases Neon, Krypton and Xenon 111 History and Occurrence 111 Physical and Chemical Properties 111 Recovery of Krypton and Xenon 112 Pre-enrichment in the Air Separator 113 Recovery of Pure Kr and Xe 115 Catalytic Combustion of Hydrocarbons 115 Cryogenic Separation 116 Recovery of Neon 118 Pre-enrichment 118 Fine Purification 119 Industrial Product Purities and Analytics 120 Applications of the Noble Gases Neon, Krypton and Xenon 121 Applications of Neon 121 Applications of Krypton 121 Applications of Xenon 122 References 124

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4

4.1 4.1.1 4.1.2 4.1.3 4.2 4.3

5

5.1 5.1.1 5.1.2 5.1.3 5.1.3.1 5.1.3.2 5.1.4 5.1.4.1 5.1.4.2 5.2 5.2.1 5.2.2 5.2.2.1 5.2.2.2 5.2.2.3 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4 5.2.3.5 5.2.3.6 5.2.4 5.2.4.1 5.2.4.2 5.2.4.3 5.3 5.4 5.4.1 5.4.1.1 5.4.1.2 5.4.1.3

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IX

The Noble Gas Helium 125 History, Occurrence and Properties 125 History 125 Occurrence 125 Physical and Chemical Properties 127 Recovery 127 Applications 131 References 134 Hydrogen and Carbon Monoxide: Synthesis Gases 135 History, Occurrence and Properties 135 Introduction 135 History of Synthesis Gas 136 Hydrogen 136 History and Occurrence 136 Physical and Chemical Properties 137 Carbon Monoxide 141 History and Occurrence 141 Physical and Chemical Properties 141 Production of Synthesis Gas 143 Production of Hydrogen by Electrolysis 143 Production of Synthesis Gas from Hydrocarbons 144 Generation of Synthesis Gas by Steam Reforming 145 Synthesis Gas Generation by Partial Oxidation (PO) 146 Generation of Synthesis Gas by Autothermal Reforming (ATR) 148 Synthesis Gas Processing 150 Water–Gas Shift Reactor 150 Removal of Carbon Dioxide and Acid Gases 150 Methanation 151 Pressure Swing Adsorption (PSA) 151 Membrane Processes 152 Cryogenic Separation Processes 153 Processes for the Production of Synthesis Gas from Hydrocarbons 156 Reformer Plant for the Production of Hydrogen 157 Reformer Units for the Generation of CO and H2 158 PO-plant for the Production of CO and H2 159 Process Analytics 161 Applications of Hydrogen, Carbon Monoxide and Syngas 164 Applications of Hydrogen 164 Hydrogen Use in the Chemical Industry 165 Hydrogen as an Energy Carrier 166 Fuel Cells 177

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X

Contents

5.4.2 5.4.3

Applications of Carbon Monoxide 182 Applications of Synthesis Gas (Mixtures of CO and H2) References 182

6

Carbon Dioxide 185 History, Occurrence, Properties and Safety 185 History 185 Occurrence 185 Physical and Chemical Properties 186 Safety Issues 188 Recovery of Carbon Dioxide 189 Sources of Carbon Dioxide Recovery 190 Pre-purification, Enrichment, Extraction, Capture 191 Standard Process for the Liquefaction of Carbon Dioxide 193 Compression and Water Separation 194 Adsorber Station 194 Liquefaction and Stripping of Lighter Components 194 Refrigerating Unit 194 Process Steps to Obtain High Product Purity and Recovery Rate 195 Scrubbing 196 Adsorption and Chemisorption 197 Catalytic Combustion 198 Improvement of the Carbon Dioxide Recovery Rate 198 Carbon Dioxide Recovery from Flue Gas 198 Production of Dry Ice 200 Applications 201 References 215

6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.2 6.2.1 6.2.2 6.2.3 6.2.3.1 6.2.3.2 6.2.3.3 6.2.3.4 6.2.4 6.2.4.1 6.2.4.2 6.2.4.3 6.2.4.4 6.2.5 6.2.6 6.3

7

7.1 7.2 7.3 7.4 7.5 7.6 7.6.1 7.6.2 7.6.3 7.6.4 7.6.5 7.7

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182

Natural Gas 217 History 217 Occurrence 218 Consumption 220 Natural Gas Trade 220 Composition 223 Process of Natural Gas Treatment 224 Dew-point Adjustment 224 Separation of Liquefied Petroleum Gas 225 Ethane Separation 229 Liquefaction 231 Nitrogen Separation 236 Applications 238

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8

8.1 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.2.6 8.2.7 8.2.8 8.2.8.1 8.2.8.2 8.2.9 8.2.9.1 8.2.9.2 8.2.10 8.2.11 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5 8.4 8.5

9

9.1 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.3 9.3.1 9.3.2 9.3.2.1 9.3.2.2 9.3.2.3 9.3.2.4 9.3.3

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XI

Fuel Gases 239 Introduction 239 Acetylene C2H2 240 Acetylene and the Beginnings of Welding Engineering 240 Physical Properties 241 Acetylene Decomposition – Deflagration 243 Ignitable Mixtures 243 Liquefaction of Acetylene – Acetylene Hydrate 243 Acetylene Hydrate 244 Acetylides 244 Extraction Processes 246 Acetylene Generated via Carbide 246 Petrochemically Generated Acetylene 246 Gas Supply 246 Storage of Dissolved Acetylene in Cylinders 246 Design of a Gas Supply System 247 Autogenous Engineering Applications 247 Regulations 248 Ethene C2H4 249 Physical Properties 249 Production Processes 249 Application and Use 249 Gas Supply and Safety 249 Regulations 251 Other Fuel Gases 251 Applications 252 References 253 Specialty Gases 255 Introduction 255 Pure Gases 256 Definitions 256 Quality Criteria 256 Sources/Production 257 Purification/Processing 257 Application Examples 258 Gas Mixtures/Calibration Gas Mixtures 261 Definitions 261 Production [9.9] 263 Technical Feasibility 263 Pretreatment of Containers 263 Preparation Methods 264 Analytical Quality Assurance 267 Application Examples 269

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Contents

9.4 9.4.1 9.4.2 9.5 9.6 9.6.1 9.6.2 9.6.3 9.6.4

Electronic Gases 269 Definition/Special Demands 269 Application Examples 271 Disposal 271 Transfer of Gases 272 Selection of the Materials 272 Physical Interaction Forces 272 Tightness of the Gas Supply System 273 Purging of the Gas Supply System 273 References 275

10

Gases in Medicine 277 Introduction 277 Medicinal Oxygen 278 Home-therapy 278 Hospitals and Other Fields of Application 280 Gases for Anaesthesia 280 Medical Nitrous Oxide (Laughing Gas) 280 Xenon 281 Medical Carbonic Acid (Carbon Dioxide) 281 Medical Air 282 References 282

10.1 10.2 10.2.1 10.2.2 10.3 10.3.1 10.3.2 10.4 10.5

11

11.1 11.2 11.2.1 11.2.2 11.2.3 11.3 11.3.1 11.3.2 11.3.3 11.4

Logistics of Industrial Gas Supply 283 Introduction 283 Storage and Transport of Compressed Gases 284 Fundamentals 284 Kinds of Transport and Storage for Compressed Gases 285 Efficiency of Compressed Air Gas Transport 286 Storage and Transport of Liquefied Compressed Gases 286 Fundamentals 286 Forms of Transport and Storage of Liquefied Gases 287 Efficiency of the Transport of Liquefied Gases 288 Special Forms of Supply 289 References 289 Subject Index 291

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XIII

List of Contributors Dr. Heinz Bauer Linde AG Geschäftsbereich Linde Engineering LE-GD Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Sections 7.1–7.6

Dr. Harald Klein Linde AG Geschäftsbereich Linde Engineering LE-HDV Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Section 5.2

Dr. Michael Berger Linde AG Geschäftsbereich Linde Gas Carl-von-Linde-Strasse 25 85716 Unterschleißheim Germany Sections 2.5, 3.6, 4.3, 5.4, 6.3, 7.7, 8.5

Dr. Matthias Meilinger Linde AG Geschäftsbereich Linde Engineering LE-EC Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Sections 2.1, 2.3, 2.4, 3.1, 3.2, 3.5, 5.3, 6.1

Dr. Matthias Duisberg Umicore AG & Co. KG Bereich HC Postfach 1351 63403 Hanau Germany Section 5.4.1.3 Dr. Heinz-Wolfgang Häring Lommelstrasse 6 81479 München Germany Chapter 1

Johann Raab Linde AG Geschäftsbereich Linde Gas LG-THA Seitnerstrasse 70 82049 Pullach Germany Sections 8.1–8.4


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List of Contributors

Dr. Harald Ranke Linde AG Geschäftsbereich Linde Engineering LE-HPP Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Section 5.1

Dieter Tillmann Linde AG Geschäftsbereich Linde Gas LG-SEP Seitnerstrasse 70 82049 Pullach Germany Sections 6.1, 6.2

Jaco Reijerkerk Linde AG Geschäftsbereich Linde Gas Hydrogen Solutions Seitnerstrasse 70 82049 Pullach Germany Section 5.4.1.2

Bernhard Valentin Linde AG Geschäftsbereich Linde Engineering LE-BE Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Chapter 11

Dr. Hans Schmidt Linde AG Geschäftsbereich Linde Engineering LE-GDV Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Sections 4.1, 4.2

Dr. Kurt Wilde Sommerstrasse 1 82234 Weßling Germany Sections 2.5, 3.6, 4.3, 5.4, 6.3, 7.7, 8.5, Chapter 9

Dr. Dirk Schwenk Linde AG Geschäftsbereich Linde Engineering LE-LDV Dr.-Carl-von-Linde-Strasse 6–14 82049 Pullach Germany Sections 2.2, 3.3–3.5

Dr. Joachim Wolf Linde AG Geschäftsbereich Linde Gas Hydrogen Solutions Seitnerstrasse 70 82049 Pullach Germany Section 5.4.1.2

Dr. Hermann Stenger Linde Gas Therapeutics GmbH & Co. KG Gas Solutions Hospital Care Edisonstrasse 2 85716 Unterschleißheim Germany Chapter 10

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1

1 Introduction The history of industrial gases is inextricably linked to the rapid pace of industrialisation that marked the nineteenth century. The large-scale generation of certain gases opened the door for new types of technologies and production processes. Acetylene, for example, was discovered by E. Davy in 1836. A significant landmark followed in 1862, when F. Wöhler succeeded in producing acetylene from the reaction between calcium carbide and water. Then, in 1892, T. L. Wilson and H. Moissan discovered a process for generating calcium carbide in an electric furnace. This paved the way for industrial-scale production of acetylene in 1895 (see also Section 8.2). Initially, acetylene was mainly used for lighting purposes due to its bright flame. Later, its high combustion temperature in oxygen prompted development of autogenous cutting and welding technology, starting in 1901. An even more important step from today’s perspective was the liquefaction of air by Carl von Linde, marking the birth of an entirely new industry. C. v. Linde employed the Joule–Thomson effect, decreasing the temperature of the gas by adiabatic expansion. In 1895, he achieved continuous generation of liquid air at a yield of three litres per hour using a laboratory plant [1.1]. The following years saw the construction and delivery of the first small commercial air liquefaction plants. Figure 1.1 shows a typical early air liquefier (ca. 1899).

Fig. 1.1 Typical assembly of a Linde air liquefier (ca. 1899). Industrial Gases Processing. Edited by Heinz-Wolfgang Häring Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31685-4

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1 Introduction

Fig. 1.2 Linde air separation unit for simultaneous production of 200 m3 h–1 oxygen and 1,800 m3 h–1 nitrogen (ca. 1919).

In 1902, C. v. Linde started using a rectification process to separate liquid air for continuous oxygen production at a purity above 99%. High-purity nitrogen was first recovered in 1905. And five years later, in 1910, simultaneous production of oxygen and nitrogen became possible with C. v. Linde’s invention of the doublecolumn rectifier. Figure 1.2 shows one of these plants (ca. 1919). During this period, there was particularly strong competition between C. v. Linde and G. Claude, one of the founders of L’Air Liquide S.A in 1902, thus spurring further development of air separation technology and resulting in important improvements [1.2]. A century on, innovations in air separation technology have spawned some impressively large plants: Air Liquide installed an air separation unit in 2004 to feed pressurised gaseous oxygen (GOX) to a Sasol partial oxidation plant in South Africa at a rate of 3,500 td–1, for instance. In 2006, Linde received a construction order from Shell for the biggest air separation facility ever built, with eight units producing a total of 30,000 td–1 GOX to feed Pearl, the world’s largest gas-to-liquid (GTL) plant in Qatar. And since 2000, the four units of the Linde air separation facility at Cantarell, Mexico have been producing a total of 40,000 td–1 pressurised gaseous nitrogen (GAN), which is injected into the well to enhance oil recovery (see Figure 1.3). A fifth unit is now also in operation. The industrial-scale availability of nitrogen and hydrogen at the turn of the 19th to the 20th century enabled a host of new applications. The BASF company, for example, succeeded in developing an ammonia synthesis from nitrogen and hydrogen in 1913. This paved the way for mass production of fertilisers. A good overview of the historical development and pioneers of industrial gases may be found in [1.3–1.5].

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1 Introduction

3

Fig. 1.3 Air separation unit at Cantarell, Mexico (2000).

The following table sets out some of the chronological milestones in the history of industrial gases: 1766 1783

Production of pure hydrogen by H. Cavendish First flight of a hydrogen-filled balloon (J. Charles) using hydrogen generated from the reaction between iron and sulphuric acid 1853 J. P. Joule and W. Thomson observe a temperature decrease caused by the adiabatic expansion of compressed gases (Joule–Thomson effect) 1868 First operation under nitrous oxide (“laughing gas”)/oxygen anaesthetic performed by Andrews 1892 H. Moissan and Th. L. Wilson discover a method for generating calcium carbide in an electric furnace, enabling industrial production of acetylene in 1895 1895 C. v. Linde builds the first technical apparatus for the liquefaction of air 1898 Liquefaction of hydrogen by J. Dewar 1898 Discovery of the noble gases neon (Ne), krypton (Kr) and xenon (Xe) (1868: helium, He, 1894: argon, Ar) 1900 First flight of a hydrogen-filled Zeppelin airship 1901 onwards The high combustion temperature of acetylene in oxygen inspires development of autogenous welding technology 1902 C. v. Linde employs a rectification process for technical production of liquid oxygen 1902 G. Claude invents the piston expansion machine for air liquefaction 1908 Liquefaction of helium by H. Kamerlingh-Onnes

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1 Introduction

1910 1913 1917 1922 1925

1936 onwards 1940s onwards 1942 1950s onwards 1960s onwards 1961 1961 1962 1963 1965 1980s onwards

1980s onwards 1999

C. v. Linde invents the double-column rectifier for simultaneous production of oxygen and nitrogen Technical ammonia synthesis from nitrogen and hydrogen by F. Haber and C. Bosch (BASF) First extraction of helium from natural gas in Hamilton, Canada Technical methanol synthesis from synthesis gas by G. Patart (BASF) Development of Fischer–Tropsch synthesis, i.e. catalytic synthesis of hydrocarbons using synthesis gas (mixture of hydrogen and carbon monoxide) by F. Fischer and H. Tropsch. Industrial application since 1932 Commercial use of the Lurgi process to generate synthesis gas from carbon using oxygen and steam Use of Ar and He in tungsten inert gas (TIG) welding Use of liquid oxygen for a V2 missile Use of carbon dioxide in metal active gas (MAG) welding Use of high-purity electronic gases in manufacturing semiconductor elements (contaminations in lower ppb range) First continuous helium/neon laser Linz-Donawitz (LD) process for steel manufacture by injecting oxygen into the converter First use of liquid nitrogen for cryogenic (shock) freezing of food Use of liquid hydrogen and liquid oxygen as fuel for space travel (USA) Commercial use of argon-oxygen decarburization (AOD) process to produce austenitic stainless steel Use of liquid helium for superconducting magnets in nuclear magnetic resonance tomography, for particle accelerators and fusion reactors CO2 laser for cutting metal First public hydrogen fuelling station for cars and buses at Munich Airport, Germany

Which gases are classified as industrial today? According to [1.6], the term “industrial gases” is “a collective term for combustible and non-combustible gases generated on an industrial scale, such as hydrogen, oxygen, nitrogen, carbon dioxide, acetylene, ethylene, noble gases, ammonia, water gas, generator gas, city gas, synthesis gas, etc.”. Taking global market share (percentage of sales), [1.9] identifies the major candidates here as oxygen (29%), nitrogen (17%), argon (10%), carbon dioxide (9%), acetylene (7%), hydrogen (5%) and helium (1%). The total share of all other industrial gases together is 22% of the global gas market. This includes carbon monoxide, nitrous oxide (“laughing gas”), the noble gases krypton, xenon and neon, and a large number of specialty gases and gas mixtures for different applications. Some of the most common specialty gases here are

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5

nitrogen trifluoride, silane, arsine, phosphane (phosphine), tungsten hexafluoride and sulphur hexafluoride (see Chapter 9). Who are the main suppliers? Companies that supply industrial gases can be divided into two tiers according to sales. Three different supply models are used – on-site (including pipeline), bulk and cylinder delivery. In the first tier, with sales exceeding USD 1 billion, there are seven major companies whose combined gas-related revenue accounted for over 75% of the global market at the end of 2005: AL: BOC: AP: Praxair: Linde: TNS: Airgas:

Air Liquide (French gas company) BOC Gases (UK gas company) Air Products and Chemicals, Inc. (US gas company) Praxair, Inc. (US gas company) Linde Gas (German gas company) Taiyo Nippon Sanso Co. (Japanese gas company) Airgas, Inc. (major US distributor)

Figure 1.4 [1.7] shows the global market shares of the first-tier companies in 2005. This reflects the market situation prior to the acquisition of the BOC Group by Linde AG to form a leading gas and engineering company under the name of The Linde Group in 2006. The second tier, with sales below USD 1 billion, contains a larger number of companies such as Iwatani (Japan), Messer (Germany), Air Water (Japan), Sapio (Italy), Cryoinfra (Mexico) and Indura (South America). In addition, there are numerous smaller gas companies active at national or even regional levels. The strength of local gas companies often lies in the high costs entailed in transporting compressed gas in steel cylinders and cryogenically liquefied gas in tank trucks. Production in the customers’ vicinity is therefore a more economical alternative. The value of the global industrial gas business reached USD 49 billion (EUR 39 billion) in 2005, an increase of 9% from 2004. Indeed, for the seven first-tier

Fig. 1.4 Global market shares of industrial gas companies, 2005.

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1 Introduction

Fig. 1.5 Global gas business by end-use sector, 2005.

companies, the increase was as high as 12.1% [1.7]. The leading markets are still North America (33%) and Western Europe (29%), which remain far ahead of the rest of the world. The forecast for 2005 to 2010 anticipates a compound annual growth rate of 7.8% for the industrial gas business, with the highest rates expected in the Middle East, Eastern Europe and Asia [1.7] (see also [1.8]). Finally, who are the end-users? The applications of industrial gases span medicine, food, metallurgy, glass, ceramics and other minerals, rubbers and plastics, paints, environmental protection, water treatment, chemicals, cutting and welding, safety, semiconductors and aerospace, to name just a few. Figure 1.5 provides an overview of the main industries and market sectors supplied by gas companies, together with a growth forecast for 2005 to 2010 [1.7] (see also [1.9]). The chemical, healthcare and electronics industries are set to be the main growth drivers. Various associations have been set up to cater for the common interests of industrial gas companies worldwide. These span all fields of activity from gas production through storage, transport and delivery to the actual application, not forgetting equipment manufacture [1.10]. Two of the main associations are x the Compressed Gas Association (CGA) and x the European Industrial Gases Association (EIGA) The North-American CGA [1.11] was established as far back as 1913. Its European counterpart is the EIGA [1.12]. The EIGA was preceded by the “Commission Permanente Internationale (CPI) de l’acétylène, de la soudure autogène et des industries qui s’y rattachent”, founded in Paris in 1923, which merged with the EDIA, the European Dry Ice Association in 1989, maintaining the name CPI. The institution started operating as the European Industrial Gases Association (EIGA) in 1990 and is currently headquartered in Brussels. These associations were founded with a view to self-regulation and to enable joint solutions to safety issues. Right from the beginning, the main task of

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References

7

the associations was to define and introduce safety standards and implement regulations. This was in response to various accidents – some serious – that occurred in the early days of industrial gas production, involving oxygen and combustible gases such as hydrogen and acetylene. Even now, safety issues still constitute an essential part of the work of these associations. The CGA, EIGA and JIGA, the Japanese Industrial Gas Association, are in particularly close collaboration. Efforts are currently underway to harmonise safety standards worldwide, defining common standards over and above those of individual associations. These safety recommendations are primarily based on the analysis of accidents reported to the association by the companies involved. CGA’s goal is to adapt existing standards every five years to reflect the latest knowledge. Section 2.3 also contains information about the safety requirements of air separation plants. Apart from those mentioned above, there are a number of other industrial gas associations [1.10], including x x x x

China Industrial Gases Industry Association (CIGIA) International Oxygen Manufacturers Association (IOMA) Asia Industrial Gases Association (AIGA) Gases and Welding Distributors Association (GAWDA)

Almost all German companies producing, filling or selling industrial gases are members of the German association Industriegaseverband e.V. (IGV). The IGV [1.13] is a member of the EIGA through its membership of the chemical industry trade association, Verband der Chemischen Industrie e.V. (VCI). This book focuses on the industrial gases of greatest commercial importance. It describes their history and properties, the processes involved in generating or separating them and their industrial and consumer applications, as well as their distribution logistics. It also discusses the future of hydrogen technology. At the end of each gas type chapter, the typical gas applications are listed. For reasons of clarity they are divided into industry segments (e.g. metallurgy, chemistry). The most important applications are described in more detail in concrete application examples. These are indicated in the text by capital letters in bold type (e.g. Example A).

References [1.1] C. Linde: Aus meinem Leben und von meiner Arbeit (1916), reprint Oldenbourg Verlag, Munich, 1979, p. 87. [1.2] W. Foerg: The History of Air Separation, MUST ’96, Munich Meeting on Air Separation Technology, 1996, pp. 1–12. [1.3] E. Almqvist: History of Industrial Gases, Kluwer Academic/Plenum Publishers, New York, 2003.

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1 Introduction [1.4] Winnacker-Küchler, Chemische Technologie, Vol. 3: Anorganische Technologie, 4th edition, Carl Hanser Verlag, 1983, pp. 566–650. [1.5] W. Linde et al.: The Invisible Industries/The Story of the Industrial Gas Industry, International Oxygen Manufacturers Association (IOMA), Cleveland, OH 1997. [1.6] Römpp, 10th edition, Thieme Verlag, Stuttgart, 1997, p. 1915. [1.7] Spiritus Consulting Ltd.: Annual Report 2005, Cornwall, UK, 2006, www.spiritusgroup.com. [1.8] Datamonitor: Global Industrial Gases Research Report, 2005. [1.9] E. Gobina: C-237 – The World Industrial Gas Business, Business Communications Co., Inc., Norwalk, CT, Oct. 2003. [1.10] CryoGas Staff Report: A Look at the Various Industrial Gas Associations, Jan. 2003, pp. 28–35. [1.11] Compressed Gas Association (CGA): www.cganet.com. [1.12] European Industrial Gases Association (EIGA): www.eiga.org. [1.13] Industriegaseverband e.V. (IGV): www.industriegaseverband.de.

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2 The Air Gases Nitrogen, Oxygen and Argon 2.1 History, Occurrence and Properties 2.1.1 Nitrogen 2.1.1.1

History

Owing to its inertness, nitrogen as an element was discovered relatively late. Still in the 17th century, air was supposed to be a homogeneous substance. Only in the year 1770, Scheele and Priestley discovered nitrogen (dinitrogen, N2) as a component of air that does not feed combustion. The chemical symbol N derives from the Latin word “nitrogenium” (nitrum generating). In 1784, Cavendish obtained nitrogen oxides and nitric acid through electric discharge in the air. Only at the beginning of the 20th century, atmospheric nitrogen was used for the large-scale production of calcium nitrate (Frank and Caro, 1902), nitric acid (Birkeland and Eyde, 1905) and ammonia (Haber and Bosch, 1913). One prerequisite was the large-scale availability of N2 through the rectification of liquefied air, successfully performed by Carl von Linde as of 1902. N2 was liquefied by Cailletet for the first time in the year 1877. Today about 85% of the N2-output are used for the production of fertilizers for farming, e.g. ammonia salts, nitrates, lime nitrogen (calcium cyanamide, CaCN2), lime ammonium nitrate and urea [2.1]. 2.1.1.2

Occurrence

The content of nitrogen in the upper 16 km thick earth’s crust is assessed at a mass fraction of about 0.03%. Thus, it belongs to the more frequently found elements. The atmosphere with a N2-content of 78.1% volume fraction or 75.51% mass fraction contains the largest quantities with 3.9 · 1015 t [2.2]. Smaller quantities of N2 are found dissolved in gases of springs and rock inclusions. Bound nitrogen occurs, for example, in nitrates and ammonium compounds. So since 1825, natural Chile saltpetre mainly consisting of sodium nitrate, has been exploited on a large scale as fertilizer. Today however, this happens mostly to recover the iodine contained within. Bound nitrogen, too, exists in the proteins in all organisms and is returned into the N2-cycle through decomposition reactions. Nitrogen oxides are Industrial Gases Processing. Edited by Heinz-Wolfgang Häring Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31685-4

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2 The Air Gases Nitrogen, Oxygen and Argon

formed by the reaction of the airborne oxygen and nitrogen in flashes of lightning, subsequently washed out by rain and deposited on the soil as nitrates (about 0.1 g of nitrate-nitrogen on 1 m2 of soil in one year). 2.1.1.3

Physical and Chemical Properties

At atmospheric pressure and room temperature, nitrogen is a colourless, odourless and non-flammable gas. At 0 °C and 1.013 bar, 1 L of nitrogen weighs 1.2505 g. N2 condenses at –195.8 °C to a colourless liquid with a density of 0.812 kg L–1 that gets solid at –209.86 °C in the form of white crystals. The solubility of N2 in water amounts to 23.2 mL per kilogram of water at 0 °C and 1 bar, at 25 °C it is only 13.8 mL kg–1. Consequently, N2 is less soluble in water than O2. The inversion temperature of N2 is 850 K. Inversion temperature is the temperature below which a gas cools down by adiabatic expansion (Joule-Thomson-Effect). Therefore, N2 can be liquefied from room temperature by means of counter-cooling of previously expanded cold gas, in contrast to H2 und He. Nitrogen is an element of the 5th main group of the periodic system and occurs in compounds in the oxidation stages –3 (e.g. NH3) to +5 (e.g. HNO3). In the N2molecule, both atoms are linked with a homopolar triple bond which is the reason for the stability and the inert character of the molecule. Therefore, high activation energy has to be supplied for the reaction of N2 with other substances, e.g. through electric discharge or high temperature. Even at 3000 °C, there is no noticeable dissociation into the atoms (K = c2(N) / c(N2) | 10–6). Nitrogen as molecule only reacts with lithium at room temperature and with calcium and magnesium at higher temperatures to the respective metal nitrides. Other metals, such as Al, Ti, V and Cr form nitrides only at red heat. An important product from the reaction with boron is boric nitride which is used as grinding material. Owing to its inert character, N2 is often used as shielding gas, e.g. in chip production. The nitrogen atom in compounds is often threefold coordinated and has a tetragonal structure with the free electron pair in one corner of the tetrahedron. Nitrogen occurs in varied forms in organic molecules, e.g. in amines (R–NH2), amides (R–C(=O)–NH2), nitriles (R–C{N), oximes (R2C=N–OH) and nitrogenheterocycles (e.g. pyridine) [2.3]. One of the research aims of the past years was the nitrogen fixation under mild conditions, especially from the aspect of the synthesis of plant-based protein through transformation of atmospheric nitrogen into ammonium ions by microorganisms (atmospheric fertilization). This biological nitrogen fixation is catalyzed by the enzyme nitrogenase [2.4]. In addition to that, there are numerous attempts to carry out nitrogen-fixation by chemical means. However, in contrast to the Haber-Bosch-Synthesis of ammonia which requires high pressures and temperatures (475–600 °C, 200 bar), the N2-fixation should occur under mild conditions. Today, coordination compounds of nitrogen with molybdenum, chrome, rhenium, tungsten, cobalt, nickel, titanium, manganese and all platinumgroup metals are known.

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2.1 History, Occurrence and Properties

11

2.1.2 Oxygen 2.1.2.1

History

Both Scheele (1772) and Priestley (1774) discovered oxygen quite independently of each other. Scheele collected the developing gas of the thermal decomposition of potassium nitrate and mercury oxide as O2 (dioxygen), whereas Priestley generated oxygen and mercury from heated mercury oxide and demonstrated that at low temperature, this reaction is running reverse to the metal oxide again. The Latin name “oxygenium” (= acid generator) is said to come from the French chemist Lavoisier who assumed that oxygen is contained in each acid and determines decisively its properties. Lavoisier was the first to recognize combustion as combination of oxygen with a fuel gas. 2.1.2.2

Occurrence

Oxygen is the most frequent element in our living space (atmosphere, hydrosphere and earth’s crust). The weight proportion of oxygen in the upper 16 km of the earth’s crust is assessed at 48.9%; oxygen occurs mainly in the form of compounds. The earth’s atmosphere contains an average of 20.95% of O2 (23.1% mass fraction), altogether about 1015 t. Up to a height of 90 km, the oxygen content of the air is almost constant. At larger heights, O2 and N2-molecules are split into atoms due to the ultraviolet portion of the sunlight. The concentration of the atmospheric oxygen is in dynamic balance: Respiration and weathering consume oxygen while oxygen is produced through assimilation (photo synthesis).Via photo synthesis, glucose is generated from CO2 and H2O with the help of sunlight simultaneously releasing O2. About 2.7 · 1011 t of O2 arise annually by photo synthesis. Apart from this, oxygen results from the decomposition of water vapour in a height of 70–80 km and from the decay of CO2 in a height of approx. 115 km, however in considerably smaller quantities. The major oxygen consumer is the sea, with the respiration of sea organisms and the oxidation of organic material as the biggest consumers [2.5]. 2.1.2.3


At atmospheric pressure and room temperature, oxygen is a colourless and odourless gas. At 0 °C and 1.013 bar, 1 L of oxygen weighs 1.429 g. O2 condenses at –182.96 °C to a light-blue liquid with a density of 1.141 kg L–1 that solidifies at –218.78 °C in the form of light-blue crystals. The solubility of O2 in water is 49.1 mL O2 per litre at 0 °C and 1 bar, dropping to 31.1 mL at 20 °C [2.6]. The inversion temperature of O2 is 767 °C. The average relative atomic mass is 15.9994. O2 is paramagnetic in gaseous, liquid and solid state. Oxygen is an element of the 6th main group (chalcogens) of the periodic table of elements occurring in compounds mainly in the oxidation stage –2 (e.g. H2O). Besides the biatomic form, even a triatomic form of the elementary oxygen occurs under natural conditions, i.e. ozone (O3). Between –160 and –196 °C, O2 dimerizes to unstable (O2)2-aggregates. With the supply of ignition energy, molecular oxygen

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turns into an extremely reactive gas, which, under energy release, reacts with a number of substances (e.g. carbon, hydrogen, hydrocarbons, sulphur, phosphorus, magnesium, iron powder) to the corresponding oxides. These oxidation reactions often happen under fire and are described as combustion. The reactions in pure oxygen proceed much more oxidizing and are considerably more intensive than in air (atmospheric oxygen) where O2 is diluted with N2 in a ratio of approx. 1 : 4. Therefore, in plants with O2-concentrations above 21 vol.%, a series of safety guidelines have to be obeyed (e.g. IGC Doc 13/02, Oxygen Pipeline Systems,) that lay down, for instance, the range of materials and the pressure depending maximum gas velocities for piping in oxygen service. The oxidation effect of O2 is utilized in steel production by blowing the oxygen into molten steel which reduces the C-content in unalloyed steel from initially 4% to . 2% (oxygen refining). In addition, manganese, silicon and phosphorus, also contained in pig iron, are combusted as well and removed as slag from the top. With the temperature ranging under the ignition temperature, oxidation often occurs very slowly, for instance, during rusting of iron and rotting of wood. The reason is the stability of the double bond in the O2-molecule (binding energy –490.7 kJ mol–1). In the paramagnetic normal state of the molecule (triplet state) two electrons are arranged in two antibonding S*-orbitals according molecular orbital theory [2.7]. With this biradical structure, dehydrogenation reactions proceeding over an intermediate peroxide radical (R–O–Ox) are easily explained. The diamagnetic singlet oxygen (two electrons with antiparallel spin are in an antibonding S*molecule orbital) formed photochemically is an effective oxidizing agent and in contrast to triplet oxygen, it adds to many organic double bond systems via [2+2] and [2+4] cycloaddition reactions. Thus, selective oxidations can be carried out on a ton scale in the odorant industry. O2 forms complexes with a lot of metals, the most important of them is haemoglobin (iron complex), responsible for the oxygen transport in blood due to the reversible O2 uptake. 2.1.3 Argon 2.1.3.1

History

In 1894, argon (Ar) was discovered by W. Ramsay and Lord John William Rayleigh, who noticed that “nitrogen” isolated from the air had a higher density (1.2567 g L–1 under normal conditions) than nitrogen recovered from ammonia nitrite (1.2505 g L–1). Therefore, apart from nitrogen, they derived the presence of another inert gas in the atmospheric air which is heavier than nitrogen. However, the first to isolate argon was Cavendish 100 years before, who obtained a not further reducible gas bubble of 1/120 of the original volume after the removal of O2 and N2 from an air volume. Due to its inert chemical behaviour, the gas was named argon (argos = Greek for inert). In the year 1938, fluorescent tubes filled with argon were more and more used in the lighting industry. However, only its application as shielding gas, e.g. for welding purposes, triggered off a high worldwide demand for argon after 1950.

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2.2 Recovery of Nitrogen, Oxygen and Argon

2.1.3.2

13

Occurrence

Argon is the most frequent noble gas by far. Air contains an average volume fraction of 0.93%. Furthermore water springs in larger depth (Geysire) contain plenty of solved argon. According to the state of the art, argon is obtained by cryogenic air separation. 2.1.3.3


At atmospheric pressure and room temperature, argon is a colourless, odourless and non-flammable gas. At 20 °C and 1.013 bar, 1 L of Ar weighs 1.664 g. Argon condenses at –185.88 °C to a colourless liquid with a density of 1.40 kg L–1 and solidifies at –189.2 °C. Thus, the boiling point of argon ranges between that of N2 and O2. At 0 °C and 1 bar, the solubility of Ar is 51.5 mL per litre of water. Argon has an average atomic mass of 39.948 and is diamagnetic in gaseous, liquid and solid state. Argon is a noble gas and thus appears in the 8th main group of the periodic system. This monoatomic gas is completely inert and therefore technically used as shielding gas against oxidation, e.g. in welding. In discharge tubes together with other noble gases, certain colour effects can be obtained. Attempts to generate real argon compounds such as ArF2 respect. ArF+ have failed [2.9]. Together with water, argon forms a hydrate with a dissociation pressure of 106 bar at 0 °C. At 0 °C and above 106 bar, this hydrate is stable. Moreover, with hydroquinone an inclusion compound is known (clathrates) however containing no real chemical bonds [2.8].

2.2 Recovery of Nitrogen, Oxygen and Argon 2.2.1 Introduction

Nitrogen, oxygen and argon are almost exclusively recovered from atmospheric air. Table 2.1 shows the concentration of these gases as well as of further components relevant for air separation. Three separation methods are predominant, namely membrane separation, pressure swing adsorption and low-temperature rectification. These methods will be described in the following. As the cryogenic rectification has a share of far more than 90% on the worldwide production, it will be presented in greater detail. A typical cryogenic process for the recovery of compressed oxygen, nitrogen and argon will be introduced. This process will be used to classify the essential process steps, to characterize the key components and to show how the development of these components has also evolved the processes. Subsequent sections on nitrogen generators, high-purity plants and liquefiers will give an impression of the creativity, by which the cryogenic separation technology has been adapted to meet even special demands of the market.

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2 The Air Gases Nitrogen, Oxygen and Argon Table 2.1 Composition of the dry atmosphere. Volume fraction in the air N2

78.12%

Kr

1.138 ppm

O2

20.95%

Xe

0.086 ppm

Ar

0.932%

Ne

18 ppm

CO2

~ 400 ppm

He

5.2 ppm

CO

~ 0.1 ppm

H2

0.5 ppm

N2O

0.31 ppm

2.2.2 Application Range of Membrane Separation, Pressure Swing Adsorption and Cryogenic Rectification

The three separation techniques have different process properties, investment and operating costs. A dedicated segment can be defined for each method, in which it allows for the most economic gas production. Table 2.2 characterizes these segments by their production capacity and gas purity. Of course, the numbers given therein are no sharp limits but indicate reasonable application ranges. Table 2.2 Application range of membrane separation, pressure swing adsorption and cryogenic rectification. Gas

Capacity (mN3 h–1)

Typical purities

Preferred separation method

Load range

N2

1–1000

< 99.5% 1)

Membrane

30–100%

5–5000

< 99.99% 1)

Pressure swing adsorption

30–100%

200–400 000

any with residual concentrations down to ppb range

Cryogenic rectification

60–100%

100–5000

< 95%

Vacuum pressure swing adsorption

30–100%

1000–150 000

any with residual concentrations down to ppb range O2 content mostly > 95%


60–100%

O2

Ar 1)

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Cryogenic rectification Including argon.

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Conclusions: x Oxygen is not recovered with membranes. x Cryogenic separation is applied whenever high purity, large quantities, liquid products or argon is required. x Membrane and adsorption plants have a high load range and can be started and powered up to full production within a few minutes. Especially when the gas consumption fluctuates strongly, the flexibility of these plants reduces the overcapacity, which has to be provided, and allows to save energy by fast load matching. A cryogenic plant needs about 2 hours for the start from cold condition until the beginning of production of oxygen and nitrogen. x Membrane and adsorption plants are suitable to cover the demands of small and medium-sized gas consumers on site. This on-site supply competes with the delivery of liquid N2, O2 and Ar by trucks. Here the gas recovery and liquefaction is performed in a large central cryogenic air separation unit, cf. also Chapter 11. The operating costs of the separation units are mainly determined by their energy consumption. Tables 2.3 and 2.4 show the specific energy demand for the production of N2 and O2 by the three separation methods. The figures are only guidelines. The actual values depend on the detailed process design. Cryogenic separation requires the smallest work, which, however, is still significantly larger than the minimal separation work needed for a completely reversible process [2.10]. Table 2.3 Energy needed for the production of one standard cubic meter of nitrogen at 8 bar (kWh). O2 content in nitrogen

2%

0.5%

Membrane

0.43

0.65


0.26

0.34

0.1%

1 ppm

0.45


0.15–0.25

Theoretical minimal separation work

0.08

Table 2.4 Energy needed for the production of one standard cubic meter of unpressurized oxygen (kWh). O2 purity

90%

93%


0.36

0.39

Cryogenic rectification Theoretical minimal separation work

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0.32

99.5%

0.35 0.07

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The economic significance of the specific energy consumption is demonstrated by the example of a cryogenic plant for the production of 50 000 mN3 h–1 of unpressurized oxygen, which is a common plant size. With a specific energy requirement of 0.35 kWh per standard cubic meter of O2 the plant needs annually 1.5 · 105 MWh, which, assuming an electricity price of 0.05 € kW–1 h–1, corresponds to operating cost of 7.7 Mio. € per year. 2.2.3 Nitrogen Recovery with Membranes 2.2.3.1

Physical Principle

Gases penetrate a dense non porous membrane differently well according to the following model: The surface of the membrane absorbs the gas on the highpressure side. This “solution process” is followed by the diffusion through the membrane to the low-pressure side with ensuing desorption. The permeability coefficient Pi describes, how good the gas “i” is transferred through the membrane. The coefficient is the product of the solubility coefficient of Henry’s Law and the diffusion constant of Fick’s Law [2.11, 2.12]. The flow Ji of the component i through the membrane, having the surface A and the thickness l, is A Ji = Pi (2.1) dpi l with dpi being the partial pressure gradient across the membrane. The permeability is usually given in Barrer, with m 1 Barrer = 2.664 ⋅ 10 −9 m3N/h ⋅ 2 m ⋅ bar The ability of the membrane to separate two components i and j from each other is described by the ratio of their permeabilities D = Pi / Pj, which is also called selectivity or separation factor. Nearly all membranes are most permeable for O2. Therefore, in membrane separation nitrogen is recovered as retentate on the pressurized side. 2.2.3.2

Membrane Technology

The efficiency of membrane separation increases with the permeability and the selectivity. Thin membranes are economic, since according to Equation (2.1) the gas flow is inverse proportional to the layer thickness. However thin polymeric films, which have favorable permeability and selectivity, are too weak to withstand the high pressure difference between permeate and retentate side. The economic breakthrough set in with the production of ultrathin compound polymeric membranes. These are designed as hollow fibres with a thick porous back-up layer for mechanical stability and a thin dense non porous membrane layer for gas separation. The porous layer only has a slight influence on gas separation. These hollow fibres are combined in a bundle, which is arranged in a cylindrical container [2.13]. Several of these bundles, also called modules, can be added to

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form larger units. The bundle is being stretched and stuck into a resin layer at its end caps. Feed and retentate usually flow through the interior of the fibres, the permeate escapes through the exterior. Typical parameters of an industrial module are [2.14]: x O2-permeability = 2 Barrers, O2/N2 selectivity = 6 x Feed pressure = 11 bar, permeate unpressurized x Outer diameter of the hollow fibre < 500 µm, inside diameter < 300 µm, thickness of the non-porous membrane layer ~ 50 nm x Length of module = 2 m, module diameter = 300 mm, packing density < 5000 m2 m–3 Carbon molecular sieve membranes, which are currently developed (2003) on different porous carriers in various geometries [2.15, 2.16], are an interesting alternative to the hollow fibre membranes. They promise higher permeability with higher selectivity at the same time. Oxygen ion-conducting Perowskit membranes are also subject of industrial research. In conjunction with a suitable catalyst layer on the permeate side, these membranes enable the oxygen separation and its reaction with methane to synthesis gas [2.32] in one step via 1/2 O2 + CH4 o CO + 2 H2. The oxygen-ion transport through the membrane occurs at high temperatures in the range of 800–1000 °C and is maintained either over a partial pressure gradient or via an electric potential. 2.2.3.3

Design

Fig. 2.1 shows the principal design of a membrane process. The air is compressed to about 6–15 bar, dried by cooling and cleaned with filters. Before entering the membrane, the air is heated to prevent water condensation within the hollow membrane. The membrane modules that remove not only O2 but also CO2 and H2O by means of permeation are the heart of the plant. N2 remains inside the hollow fibres, accumulates there and escapes at the end of the fibres.

Fig. 2.1 Flowsheet of a simple membrane process for the N2-production: (1) compressor, (2) filter, (3) heater, (4) membrane.

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The fibres cannot be cleaned owing to their small inside diameter. Therefore a thorough air pre-purification with fine and coarse filters as well as activated carbon adsorbers is necessary to provide a membrane lifetime of more than 10 years. Membrane plants require little space, are robust, easy to operate and low-priced. They are available for capacities in the range of ~ 1–1000 mN3 h–1, for purities of 95–99.5% and pressures of up to 13 bar. When the N2-purity is high, the product yield is low. With 0.5% of oxygen in the nitrogen product, the N2-yield is typically 22%. With 8% of oxygen in the nitrogen product, the N2-yield increases to 64%. N2-yield = (Conc.N2 in the product · FlowProduct) / (Conc.N2 in the air · FlowAir), where Conc. is given in % volume fraction, flow in mN3 h–1. There are numerous modifications of the basic process shown in Fig. 2.1, which allow a more efficient operation with higher product purity and capacity: x Multi-stage separations: The permeate of a membrane stage is fed back into the feed of the previous stage. x Combination of membrane separation with downstream catalytic combustion of the residual oxygen. 2.2.4 Nitrogen and Oxygen Recovery by Means of Pressure Swing Adsorption 2.2.4.1

Physical Principle

Pressure Swing Adsorption (PSA) makes use of the fact that the amount of adsorbate, which can be deposited on the adsorbent, increases with increasing pressure. Adsorption occurs at high pressure, desorption at low pressure. This technique is applied for the adsorptive recovery of O2 and N2 from air. With Temperature Swing Adsorption (TSA) the adsorbent is regenerated by heat supply. This technique is not used for N2/O2-separation. It is preferably applied, when the component to be separated has a low partial pressure in the feed gas. Section 2.2.5 describes how H2O and CO2 in the feed air to a cryogenic separator are adsorbed by means of a TSA. The cycle time of PSA lies in the range of minutes, the one of TSA in the range of hours. 2.2.4.2

Properties of Molecular Sieves

N2-recovery by PSA is performed predominantly on carbon molecular sieves (CMS), the O2-production on zeolitic molecular sieves [2.17, 2.18]. x CMS: The binding forces of oxygen and nitrogen on the CMS surfaces do not differ particularly, but the material has pores and cavities into which oxygen diffuses faster than nitrogen. x Zeolites: The recovery of oxygen on zeolites uses the fact, that nitrogen is bound more strongly on the surface. The binding of the molecules on the surface is characterized by adsorption isotherms measured in the laboratory. These isotherms indicate the equilibrium loading of the component for different partial pressures above the adsorbent. The adsorbents are designed on the basis of these isotherms.

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Both adsorbents have high specific surfaces of ~ 800–1500 m2 g–1. The molesieve material is filled into horizontally or vertically orientated cylindrical vessels and the gas passes through the material either in axial or radial direction. Design criteria are pressure drop, the so-called lifting velocity above which the bed fluidizes, and the homogeneous distribution of the gas flow. 2.2.4.3

Nitrogen Recovery

Figure 2.2 shows a typical process for nitrogen recovery by means of PSA. Feed air is compressed to 5–12 bar, cooled, and after removal of the condensate fed to the adsorbers (3, 3c). When adsorber (3) is in adsorption mode, its inlet and product valve (a) and (c) are opened and the expansion valve (b) to the atmosphere is closed. H2O, CO2 and O2 are preferably adsorbed on the CMS. The non-adsorbed nitrogen passes through the adsorber (3) and is collected in a buffer tank (4), which is operated at a somewhat lower pressure. The product is finally withdrawn from this tank. At the same time, the adsorber (3c) is regenerated. While its inlet and product valve (ac) and (cc) are shut, the pressurized gas in the adsorber is released into the atmosphere via expansion valve (bc). An adsorption phase lasts about 40–60 s. Then the adsorbers change roles: After equalizing the pressure of the two vessels within 1–2 s, adsorber (3c) is pressurized within about 5 s to take over the adsorption. The buffer tank (4) attenuates the product-pressure fluctuations during the adsorber switching. x Nitrogen purities are within about 98% to 99.99%. The capacity ranges between few standard cubic meters and 5000 mN3 per hour and depends strongly on the residual oxygen content in the product. For instance the product capacity increases by about 30%, when the oxygen content increases from 0.5–2%.

Fig. 2.2 N2-recovery with PSA: (1) compressor, (2) filter, (3) (3c) adsorber, (4) N2-buffer, (5) silencer, (a) (ac), (b) (bc), (c) (cc) switching valves.

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x With elaborate CMS even higher purities with O2-contents down to below 10 ppm may be obtained. Alternatively, very pure nitrogen can be produced by combining the PSA with a downstream catalytic conversion stage. x 1 m3 of CMS material is sufficient to produce about 100 mN3 h–1 of nitrogen with an O2-content of 0.5%. x With the standard PSA-process the product is delivered at ~ 7 bar. The product pressure may be increased by raising the adsorption pressure to about 12 bar. For even higher pressures a separate compressor is applied. x PSA allows higher purities than membrane separation. 2.2.4.4

Oxygen Recovery

O2-generators use zeolitic molecular sieves. Here N2 is adsorbed and oxygenenriched air is obtained as product at the adsorber outlet. During regeneration the adsorbed nitrogen is blown into the atmosphere. Towards the end of the desorption phase the regenerated adsorber may be purged with oxygen to avoid nitrogen contamination of the product at the beginning of the following adsorption cycle. In high capacity plants, adsorption and regeneration are often performed at reduced pressure (VPSA = Vacuum Pressure Swing Adsorption) [2.19]: The air compressor is replaced by a blower with a discharge pressure of about 1.5 bar, whereas a vacuum pump generates a vacuum of typically 35–50 kPa for the regeneration. An adsorption cycle takes about 40–60 s. VPSA-plants require less energy than PSA-plants, since the specific capacity factor increases with decreasing pressure. This factor is the load change dQ between adsorption and desorption phase, related to the load Q of the adsorption phase, i.e. dQ/Q. Although nitrogen is well adsorbed, the achievable oxygen purities are limited to the range between 90 and 93% (at most 95%). This is due to argon, which has a similar adsorption behaviour as oxygen, and which is concentrated in the same proportion as oxygen to a content of about 4.5%. The product capacity of VPSA plants ranges between 100 mN3 h–1 and 5000 mN3 h–1. O2–PSA plants are often operated at an adsorption pressure of 3–4 bar and a desorption pressure of 1 bar. The product capacity ranges between 5–300 mN3 h–1. 2.2.5 Cryogenic Rectification

One of the most important milestones in the history of industrial air separation was the introduction of the so-called double column for the distillative separation of air into its components under cryogenic conditions. Even today this principle is still applied in numerous variations in most of the cryogenic air separators. A frequently applied process is going to be introduced in the following by example. It is built this way or similarly by all commercial vendors of air separation units. Elementary concepts of process technique will be applied to explain the “key features” of the air separator, such as concentration profiles in the columns, argon production or refrigeration. A separate section describes so-called internal

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compression, where oxygen is pressurized without using an oxygen compressor. This technique has been increasingly applied since 1980. Other applications of the cryogenic separation, namely, nitrogen generators and high purity plants as well as liquefiers will be introduced in subsequent sections. The central components molecular sieve, compressor, turbine, heat exchanger and column will be characterized and the technical challenges, that have to be met in design, manufacturing and assembly of these components, will be described. 2.2.5.1 Process with Air Booster and Medium-Pressure Turbine for the Recovery of Compressed Oxygen, Nitrogen and Argon

The process (Fig. 2.3A) is designed for delivering products as described in Table 2.5. A plant based on such a process with a production capacity of 45 000 mN3 h–1 of oxygen typically looks like the one shown in Fig. 2.3B. Compressed gaseous oxygen is often supplied exclusively to a bulk consumer, for example a steel mill. The plant’s liquid production is stored in tanks. During a plant stop this liquid is evaporated to guarantee the continuous gas supply of the bulk consumer and it is also distributed by trucks to smaller consumers in the local area. The plant consists of a warm section including compression, pre-cooling, drying and pre-purification of the air, and a cold section with heat exchange and rectification. The cold section with temperatures down to 80 K like at the top of the low-pressure column is housed inside a “coldbox”. This is a “container” made up of steel panels and filled with insulating material to protect the cold equipment from cold losses. Table 2.5 Product spectrum of the process with air booster and medium pressure turbine (cf. also Fig. 2.3). Product

Purity/Residual impurity

Yield (= mass flow of component in the product/mass flow of component in the feed air)

Compressed gaseous oxygen, pressure between 6 and about 100 bar

> 99.5%

O2-yield between approx. 90% and nearly 100%

Unpressurized gaseous nitrogen

Typically 1 ppm O2 and 100 ppm Ar

N2-yield up to approx. 60%

Compressed gaseous nitrogen at almost 5 bar

Typically 1 ppm O2 and 100 ppm Ar

Up to 20% of the processed air

Argon

1 ppm O2 and 1 ppm N2

Ar-yield between 60% and 95%

Liquid products

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All three products can be delivered partly as liquid

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Fig. 2.3A (legend see p. 23)

22

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Warm Section (Fig. 2.3A) Compressing and Cooling Air is filtered (2) and compressed to 5.7 bar in a compressor (3), leaving the compressor’s last stage with about 100 °C. Subsequently it is cooled first with cooling water (o) and then with chilled water (p) to temperatures between 5 and 20 °C in the direct contact cooler (4). This cooling reduces the moisture content of the saturated air, thus reducing the expenditure for the ensuing H2O removal in the zeolite adsorbers (6/6c). The “chilled” water is supplied by the evaporative

Fig. 2.3B Cryogenic air separator, Linde AG, Linz, Austria. (1) Cold Box; (2) Tank; (3) Machine house; (4) Direct contact cooler; (5) Evaporation cooler.

Fig. 2.3A Cryogenic air separator with pure argon production and internal compression. (1) Coldbox and battery limit for enthalpy balance; (2) Filter; (3) Main air compressor; (4) Direct contact cooler; (5) Evaporation cooler; (6/6c) Adsorber; (7) Regeneration gas heater; (8) Air booster; (10) Heat exchanger; (11) Turbine; (12) Rectification column (pressure section); (13), (16), (19) combined condenser–evaporator unit; (14) Rectification column (low-pressure section); (15) Crude argon column; (17) Pure argon column; (18) Reboiler; (20) Ar – process pump; (21) Internal compression pump; (22) Throttle valve.

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(a, b, c, i, j, k, l, o, p, r) Internal process streams; (d) N2 gaseous; (e) Waste gas; Pressurized – N2 gaseous; (g) Pressurized – O2 gaseous; (h) N2 liquid; (m) Ar liquid; (q) O2 liquid. For simplicity, the subcooler and the purge flows of crude and the pure argon condenser 16, 19 are omitted. GAN = Gaseous nitrogen; PGAN = Pressurized gaseous nitrogen; PGOX = Pressurized gaseous oxygen; LAR = Liquid argon; LIN = Liquid nitrogen; LOX = Liquid oxygen.

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cooler (5). Here it is cooled in counter-current flow with the nitrogen-rich residual gas from the separation. This dry residual gas saturates with moisture. The thereby required evaporation heat is withdrawn from the water and cools it. The heat and mass transfer in the direct contact and in the evaporative cooler takes place on irregular or structured packings often made of plastic. Purification The cold air still contains H2O and about 400 ppm of CO2. In the adsorber, the air is purified such that during the ensuing cooling in the heat exchanger (10) to about 100 K, no ice or CO2-snow is formed by desublimation. This would gradually block the heat exchanger. The vapour pressure PCO2 of solid CO2 at 100 K is about PCO2 = 2.2 · 10–7 bar. Thus the average CO2 concentration xCO2 of air at 5.5 bar has to be smaller than xCO2 < 2.2 · 10–7 / 5.5 = 40 ppb, to avoid CO2-snow even at 100 K. The ambient air is diluted with numerous hydrocarbon components. Some of these molecules are fully or at least partly retained by the molesieve. This is important for the safety of the air separation unit, since all hydrocarbons are less volatile than nitrogen and oxygen and accumulate in the liquid oxygen formed in the bath of the main evaporator (13). Here their concentration must remain far below the solubility and explosion limit. In particular, the molesieve adsorbs acetylene, which has a solubility limit of only ~ 6 ppm, completely. The adsorbers (6/6c) are arranged pairwise and are automatically switched between adsorption and regeneration mode. Cold Section (Fig. 2.3A) Heat Exchange Three fractions of air (a, b, c) with different pressures enter the heat exchanger (10). Here, they are cooled in counter-current flow against gaseous nitrogen (d) from the top of the low-pressure column (14), gaseous residual gas (e), compressed nitrogen (f) from the top of the pressure column (12) and compressed oxygen (g). The compressed oxygen enters the exchanger in liquid form, is evaporated therein and is discharged as pressurized gas. The first air fraction (a) passes into the heat exchanger without further boosting and is cooled down close to its liquefaction temperature. The second fraction (b) is further compressed in a booster (8), partly cooled in the heat exchanger and expanded via the turbine (11) into the bottom of the pressure column. The turbine performs work, which is transformed into electric energy by a generator. The turbine controls the refrigeration balance of the plant, as described in more detail further below. The more liquid products are produced, the higher is the requirement for refrigeration. This is satisfied by increasing expansion flow through the turbine. The third fraction (c) enables the so-called “internal compression” of the oxygen: Fraction (c) is boosted in compressor (9) and liquefied in the heat exchanger. The

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thereby released condensation heat vaporizes the liquid compressed oxygen which is delivered by the pump (21). The liquefied high-pressure air is expanded into the pressure column at a suitable tray via a throttle valve. The expanded air is a two phase flow, being mostly liquid with only a small vapour fraction. The amount of the liquid phase is direct proportional to the amount of internally compressed oxygen. Typically, for one part of internally compressed oxygen about 1.3 parts of liquid air must be fed into the columns. In Section 2.2.5.2 the principle and benefit of internal compression will be explained in detail. Rectification in the Pressure Column In the pressure column (12) the gaseous and liquid air is pre-separated at a pressure between 5 and 6 bar. The rectification is crucially determined by the volatilities of the components, which goes hand in hand with their boiling temperatures, see Table 2.6. Nitrogen, which is more volatile, accumulates at the top of the pressure column. It has a typical residual oxygen content of 1 ppm. At the bottom of the column an oxygen-enriched liquid with an O2-content between 35 and 40% is formed. Table 2.6 Boiling temperatures of N2, O2 and Ar at atmospheric pressure. Boiling temperature at atmospheric pressure N2

77.4 K

Ar

87.3 K

O2

90.2 K

Only in the low-pressure column (14), typically operated at 1.3–1.5 bar, the final separation into pure oxygen as sump product, pure nitrogen (d) as top product and residual gas (e) withdrawn from an intermediate stage, takes place. The residual gas is mainly nitrogen with a small oxygen content in the range of < 1%. The gaseous nitrogen at the top of the pressure column is liquefied in the main condenser (13). This condenser is cooled by evaporating liquid oxygen from the sump of the low-pressure column. Condenser and evaporator are designed as a combined heat transfer unit, see also Section 2.2.5.6. Part of the condensate serves as reflux for the pressure column, the rest is expanded and fed as reflux onto the top of the low-pressure column. The product nitrogen is alternatively withdrawn either x in gaseous form and almost unpressurized from the low-pressure column (d), or x in gaseous form and pressurized from the pressure column (f), or x in liquid form from the pressure or low-pressure column (h)

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The oxygen-enriched liquid from the sump of the pressure column is also expanded and then, on the one hand, fed as liquid into the low-pressure column (i) and, on the other hand, evaporated in the two combined evaporator-condenser units (16), (19) on top of the crude argon column (15) and the pure argon column (17). This cools the two argon columns’ head condensers, thereby providing the necessary reflux for these columns. The evaporated oxygen – enriched liquid is fed into the low-pressure column. The sensible heat of the oxygen-enriched liquid from the pressure column is also used for the heating of the sump of the pure argon column (18). Finally, in addition to the liquid nitrogen and liquid oxygen – enriched streams from the pressure to the low-pressure column, there is a third liquid air fraction (k) from the pressure to the low-pressure side. This flow is adjusted such that the operating line and the equilibrium curve, as shown in the McCabe–Thiele diagram of the low-pressure column, move closer together. This reduces the “irreversibility” of the rectification and increases the O2- and Ar-yield. Rectification in the Low-pressure, Crude and Pure Argon Column Just as in the pressure column, also in the low-pressure column oxygen is rectified downwards and nitrogen upwards. Argon accumulates both above and below the feeding of the evaporated crude oxygen (j) in the form of an argon concentration bulge. Liquid oxygen is withdrawn from the column’s sump and compressed to product pressure by means of a pump (21). Close to the tray with maximal argon concentration, vapour with an argon concentration between 5% and 15% is withdrawn (l) and fed to the crude argon column (15). The feed gas to the crude argon column typically has an N2-content of 100 ppm and an oxygen concentration of 85–95%. The crude argon column separates oxygen from argon. The composition at the top of the argon column amounts to about 1 ppm of O2 and 0.5% of N2. Approx. 1/30 of the gas, which ascends in the crude argon column, is passed on to the pure argon column (17). It is called crude argon (f). In the pure argon column the residual nitrogen is rectified towards the top and ejected into the atmosphere by blowing off a small amount of waste gas (n) with a typical N2-content of 40%. The liquid argon product with 1 ppm of N2 and 1 ppm of O2 is withdrawn from the column’s sump. The head condenser (19) ensures the reflux for the pure argon column. With a double-column, the vapour and liquid flows within the individual column sections are not freely adjustable, unless additional enhancement recycles are applied. This is a specific characteristic of the double-column principle. As a consequence, the product yields are also more or less fixed. In particular, it is not possible to withdraw all the gas ascending to the top of the low-pressure column as pure nitrogen. However, a fraction of up to 50% of the processed air can be obtained as pure nitrogen (d) with an O2-content of ~ 1 ppm, if residual gas with an O2-concentration ranging between 0.1–3% is withdrawn some trays below the top. Thereby the liquid/vapour-ratio in the top section, the so called “nitrogen section”, increases such, that the oxygen can be rectified downwards in this section.

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The withdrawn residual gas is warmed in the heat exchanger and is used for the regeneration of the molesieve adsorber and for evaporative water cooling. The Average Temperature Difference at the Main Condenser Determines the Process Air Pressure The main condenser (13) of the process shown in Fig. 2.3A determines significantly the pressure of the process air after the main air compressor (3) and thus the energy consumption: The pressure in the low-pressure column is selected to compensate the frictional pressure drop of the residual gas (e) on its way from the low-pressure column to the atmosphere. For example, when the low-pressure column is of the packed type, a pressure of about 1.4 bar in the column sump is sufficient. In order to maintain the heat flow in the main condenser – evaporator unit, a driving temperature difference of usually 1–2 K has to be maintained between evaporator and condenser. Therefore, the pressure in the pressure column must be adjusted such that the temperature of the condensing nitrogen at the top of the pressure column exceeds the temperature of the boiling oxygen at the bottom of the lowpressure column by this temperature difference. Cryogenic Losses are Mainly Covered by the Turbine The rectification occurs at low temperatures between 80 K and 100 K. Therefore air must be cooled down prior to rectification and the gaseous products must be warmed up afterwards. This process is associated with cryogenic losses which have to be compensated by performing additional work on the system. Losses occur owing to: x x x x

insufficient warming up of the products in the heat exchanger discharge of liquid products heat flux into the coldbox work performed on the process pumps

On the other hand, an enthalpy balance along the battery limit (1) shown in Fig. 2.3A, reveals two sources of “cryogenic gains”, for which according to the first law of thermodynamics 6 gains = 6 losses holds: x Work performed by the turbine. x Throttling: Air enters the system with overpressure and leaves it nearly unpressurized in form of the separated gaseous products. If the air and its separation products would behave strictly like an ideal gas, whose enthalpy does not depend upon the pressure, the throttling of the process air would not lead to any cold production. However, they are real gases and their enthalpy decreases with increasing pressure. This gives rise to a gain of enthalpy. This gain gets more significant, when the pressure difference between the incoming air and the discharged separation products is large. The cryogenic losses of the process shown in Fig. 2.3A are mainly covered by the turbine.

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The original air liquefier built by C. v. Linde did not use a turbine. The cryogenic losses were exclusively compensated by throttling the processed air, which had been compressed up to 200 bar. This is energetically unfavorable since throttling is an irreversible process, while the expansion in an ideal turbine, having a 100% efficiency (efficiencies of up to 90% are achieved with modern turbines), is a reversible process occurring without loss of exergy. The Concentration Profile in the Low-Pressure Column is Characterized by Two Argon Bulges The triangular diagram, see Fig. 2.4 [2.20], visualizes the concentration profile of the ternary system oxygen/nitrogen/argon in the low-pressure column. The curve within the triangle indicates the composition of the three components in the liquid on each theoretical tray within the column.

Fig. 2.4 Ternary concentration diagram for the down-flowing liquid in the low-pressure rectification column. (1) Bottom of the low-pressure column. (2) Withdrawal of the argon-rich gaseous feed to the crude argon column.

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(3) Feed of oxygen enriched liquid from the pressure column. (4) Feed of liquid air from the pressure column. (a) Connects states of uniform Ar-concentration; (b) connects states of uniform N2-concentration; (c) connects states of uniform O2-concentration.

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The diagram reveals that although the argon concentration of the ambient air is quite small with less than one percent, it has a strong impact on the concentration profile of the low-pressure column. It would not be adequate to describe the separation in the low-pressure column as a binary O2–N2 rectification, in which the presence of argon only represents a minor disturbance: In the lowest, the socalled oxygen section with about 30–40 theoretical trays, a pure binary separation between O2 and Ar takes place. At the upper end of this section, where the argon content reaches its maximum and where the feed gas to the crude argon column is withdrawn, the binary O2–Ar rectification transforms within a few trays into a ternary rectification. The diagram also shows the second, more weakly developed argon bulge above the feed of the oxygen enriched liquid. Cryogenic Production of Pure Argon Until about 1990 the production of pure argon, having a contamination of only a few ppm of O2 and N2, was accomplished by a combination of rectification and catalytic combustion. The O2-impurities of the argon were catalytically burnt by addition of hydrogen to form water. Since then, the pure argon recovery solely by means of rectification, as it is shown in Fig. 2.3A, has been established [2.21] and become the industrial standard. Here the O2–Ar separation is performed in the so called crude argon column. From the top of this column almost O2-free crude argon is withdrawn and forwarded to the pure argon column, where the remaining N2 is separated from the argon. This section describes the special demands on the crude argon column and the plant control, resulting from the close integration of the argon rectification into the air separation process. The rectification in the crude argon column is almost a pure binary separation between O2 and Ar and can be best visualized and understood in the McCabe– Thiele Diagram, shown in Fig. 2.5. This diagram indicates for each tray the Arconcentration y of the vapour directed towards the tray and the concentration x

Fig. 2.5 McCabe–Thiele diagram for the binary O2/Ar-mixture of the crude argon column.

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of the liquid leaving this tray. These points lie on a common straight line, the so-called operating line. Its gradient is defined by the ratio L/V = Liquid/Vapour. The so-called phase equilibrium curve is plotted as well. This curve shows for a given argon concentration x in the liquid the corresponding equilibrium concentration y* of the vapour. The number of theoretical separation stages required is determined by the well known staircase construction, to be arranged between the equilibrium and operating line. The closer the two lines lie together, the more separation stages are needed. The top of the column corresponds to the upper right point in the diagram. The crude argon at the top of the column is rectified to an O2-purity of typically 1 ppm. Since the boiling temperatures of O2 and Ar are close together (see Table 2.6), Ar is only slightly more volatile than O2. Therefore the rectification requires a large number of theoretical separation stages, typically 180 stages. This is visualized by the small gap between the operating and the phase equilibrium curve. The minimal number of theoretical separation stages Nmin, which is required for the crude argon column to accomplish the O2–Ar separation, is estimated with the Fenske formula. This formula holds for infinite reflux, L/V ~ 1. This condition is almost fulfilled since typically only 1 part out of 30 parts of ascending vapour is withdrawn as crude argon from the top of the crude argon column, whereas 29 parts of liquid remain as reflux. log S where log D x DO2 − Ar = K O2 / K Ar is the relative volatility of the oxygen-argon mixture, defined by the component equilibrium factor K O2 = y O* 2 / x O2 for oxygen and the corresponding factor for argon. Here, y* is the concentration of vapour, which is in equilibrium with its liquid, having the concentration x. The relative volatility varies between 0.66 in the column’s sump and 0.91 in the column’s top, i.e. the argon-richer the mixture, the more difficult the separation will be. x O2 , top (1 − x O2 , sump ) . Inserting the x S is the so-called separation factor S = (1 − x O2 , top ) x O2 , sump oxygen purity at the top, x O2 , top = 1 ppm , and the sump oxygen purity of typically x O2 , sump ~ 90% , yields S ~ 1.11 · 10–7.

The equation is N min + 1 =

Together with the relative volatility evaluated at the top of the column, the Fenkse equation results in Nmin ~ 159 theoretical trays. The separation of oxygen from argon by means of rectification became only possible after the introduction of packed columns, in which the mass transfer between vapour and liquid phase occurs on the surface of regularly structured packings. Here the pressure drop of roughly 0.07 kPa per theoretical stage is significantly lower than in a sieve tray column with about 0.35 kPa per stage. A sieve tray column with 180 sieve trays would have a total pressure drop of about 60 kPa and would result in a low condensation pressure of the argon at the top of the column. Due to this low pressure the condensation temperature of the argon would fall below the boiling temperature of the cooling oxygen-rich liquid,

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coming from the pressure column. Thus, there would be no driving temperature difference to ensure the heat transfer at the crude argon condenser. In a packed column the pressure drop amounts only to about 15 kPa and a driving temperature difference of 1–2 K remains. The packed crude argon column with 180 theoretical trays gets high: The height of the packed beds sums up to approx. 40 m. To this, the height of numerous liquid collectors and distributors has to be added. These are arranged between the packing beds to guarantee a uniform distribution of the liquid over the packings. Hence the crude argon column may be split up into two towers standing side by side, the towers being connected with a pump. The large volume of the crude argon column requires a long start-up time for the argon production: X Example: The crude argon column of an air separator, having an argon production of 2000 mN3 h–1 and a process air input of 240 000 mN3 h–1, has a diameter of about 3.3 m. Approx. 36 000 mN3 of argon are stored within the column in form of a liquid argon film on the packing surface, liquid in the distributors and liquid in the sump of the column. Since 2000 mN3 h–1 of pure argon are transferred into the crude argon column, the time passing until the complete column holdup is built up and the argon production can be started, amounts to tholdup = 36 000 mN3 / 2000 mN3 h–1 = 18 h. Argon Yield The argon yield rises, when the argon concentration of the feed gas to the crude argon column [(l) in Fig. 2.3A] is increased. However, the plant operator must be careful, when he increases, in view of a high argon production, the argon concentration in this feed gas by suitable adjustment of the process parameters. This renders the process more sensitive to disturbances. The ternary concentration diagram in Fig. 2.4 explains this: The maximum of the argon concentration in the lower part of the low-pressure column lies at the point where the binary O2–Arseparation transforms into a ternary separation, and where the N2-concentration rises from typically 100 ppm to a percent value within few stages. Operating the plant close to this point bears the risk, that a small disturbance may provoke a strong increase of the N2-concentration in the feed gas to the crude argon column. There, owing to its high volatility, nitrogen is driven to the column’s top. Here it accumulates due to the small top product extraction of about only 1/30 of the column feed. The increasing N2-content of the vapour at the column’s top reduces its temperature, and the driving temperature difference at the crude argon condenser drops, until the vapour flow in the crude argon column collapses. Argon gets lost via x the oxygen product: For example, when the oxygen purity is 99.7% the remaining 0.3% are almost exclusively argon, corresponding to an argon loss of 6.7% x pure and impure nitrogen withdrawn from the top of the low-pressure column

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Fig. 2.6 Argon and oxygen yield: (1) argon yield; (2) oxygen yield.

The argon loss via the pure and impure nitrogen depends strongly upon the amount of reflux onto the top of the low-pressure column. The loss is low when the reflux is high and at the same time has low argon content. These are two contradictory requirements setting up a total limit for the argon yield: High reflux onto the low-pressure column induces a low reflux onto the pressure column with correspondingly high argon content at the top of this column. The optimum is a suitable compromise. The argon yield reacts particularly sensitive to a withdrawal of product nitrogen from the top of the pressure column, (f) in Fig. 2.3A. This discharge is often advantageous, because it reduces the investment for a further nitrogen compressor or makes it even obsolete. However the nitrogen withdrawn in this way is missing as reflux onto the top of the low-pressure column. This reduces the argon yield and to a certain extent also the oxygen yield (cf. Fig. 2.6). 2.2.5.2

Internal Compression

Industrial applications mostly require pressurized oxygen. X Example: Linz-Donawitz-Process for steelmaking: Here oxygen at about 15 bar is blown through a lance into the pig iron melt. With “external compression” (Fig. 2.7), the gaseous oxygen is withdrawn from the low-pressure column, warmed in the heat exchanger and brought to the discharge pressure by means of a compressor. The safe operation of an oxygen compressor requires elaborate and expensive safety measures to avoid heat development caused by mechanical friction. In an oxygen-rich environment this could lead to a sudden exothermal combustion of the material in contact with oxygen. The higher the O2-pressure, the lower the amount of heat required for the ignition (see also Section 2.3).

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Fig. 2.7 Internal and external compression. (1) Heat exchanger; (2) Turbine; (3) Rectification column (pressure section); (4) Rectification column (low-pressure section); (5) Combined condenser–evaporator unit; (6) Booster air compressor; (7) Internal compression pump; (8) Oxygen compressor. (a, b) Internal process flows; (c) Gaseous pressurized oxygen; (d) Low-pressure products.

33

In contrast to the exemplary process shown in Fig. 2.3A, the turbine (2) is operated here with unboostered process air and expanded into the low-pressure column (4). This modification is applied in units with low liquid production. GAN = Gaseous low pressure nitrogen PGOX = Pressurized gaseous oxygen

The alternative for “external compression” is “internal compression” shown in Fig. 2.7. It has gained broad acceptance since about 1980 owing to its safe operation, availability, easy maintenance and low investment costs. With internal compression, the product oxygen is not evaporated in the main condenser of the double column, but it is pressurized as liquid and is evaporated in the heat exchanger against condensing high-pressure air. Q-T-Diagram The so-called Q-T-Diagram [2.22] for external and internal compression (Fig. 2.8) x explains the principle of internal compression

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x enables to calculate the relation between the pressure of the condensing highpressure air and the pressure of the evaporating liquid oxygen, and x illustrates why internal compression needs slightly more energy than external compression The Q-T-Diagram characterizes the heat transfer in the heat exchanger in a similarly distinctive way as the McCabe–Thiele diagram characterizes the mass transfer in the columns: The upper curve of the diagram indicates the amount of heat that has been transferred to all cold streams on the way from their entry point at the cold end of the exchanger to an arbitrary position within the exchanger, at which the streams have been warmed up to a temperature T. This temperature T is shown on the horizontal axis. Correspondingly the lower curve shows the amount of heat that

Fig. 2.8 Q-T-Diagram for internal (a) and external compression (b). (1) cold streams; (2) warm streams. In this example O2 pressure = 14 bar and pressure of boostered air = 31 bar.

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is removed from all warm streams on their way from a position, where they take on the temperature T down to the cold end of the exchanger. The horizontal distance dT of both curves at a given value of the transferred heat Q is the driving temperature difference, maintaining the heat flow from warm to cold streams. Therefore, the two curves may not intersect. The closer they lie together, the smaller the dT and the more exchanging surface is required in this area for the heat exchange. The Q-T-Diagram allows to judge the quality of the heat transfer: A transfer of heat dQ over a finite temperature difference dT is irreversible. That means exergy, i.e. technical available work, is lost [2.32]. This exergy loss must be compensated by extra work performed on one of the compressors. The exergy loss dE caused by a heat flow dQ over the temperature gradient dT = Tc – T is dE = Ta dQ

dT T T′

(2.2)

Here, Ta is the ambient temperature to which the technical available work refers. From this equation one concludes that x the loss increases with the growing temperature difference dT x the loss is larger at low temperatures, owing to the factor T · T c in the denominator Comparison of Internal and External Compression The warm and cold branches of the Q-T-Diagram for internal compression, Fig. 2.8, are almost vertical at the temperature around 126 K. This corresponds to the region in the heat exchanger, where the 14 bar oxygen evaporates and thereby condenses the 31 bar high-pressure air. The warm and cold branch of the Q-T-Diagram may not intersect. This is guaranteed if the boiling temperature of the high-pressure air is slightly higher, typically 1–2 K, than the boiling temperature of the oxygen. From this requirement, the relation between the oxygen pressure and the optimal pressure of the boostered air is constructed. In the example, the oxygen pressure is 14 bar and is lower than the critical pressure Pcr = 50.8 bar of oxygen, and similarly the air pressure is 31 bar being also lower than the critical pressure Pcr = 37.7 bar of air. However, internal compression is also applied at overcritical pressures. The sharp bends in the Q-T-Diagram will get “rounder” then. The Q-T-curves for internal and external compression visualize, that the advantage of the internal compression is paid for by higher energy consumption: For the internal compression the temperature difference between the warm and cold streams is slightly larger in the region around the zone of the boiling oxygen. Therefore the heat transfer in this area induces a higher loss of exergy according to Equation (2.2). As this increased temperature difference occurs at lower temperatures, the exergy loss will be more pronounced, as explained above.

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2.2.5.3

Nitrogen Generators

The double-column process shown in Fig. 2.3A produces oxygen and nitrogen. There are industrial applications requiring exclusively nitrogen. For these, numerous modifications of the double-column system have been developed, two of which are introduced here: The first process is a one-column apparatus. It can be viewed as classical doublecolumn system, the low-pressure column of which is shrunk to a vessel with an evaporator inside and no separation stages above. A detailed process description is given in [2.24]. This apparatus is typically applied to produce nitrogen with an oxygen concentration between 1–100 ppm and pressures up to 10 bar. The process is designed to optimize investment cost and is suitable for small-scale consumers with a nitrogen demand of up to about 7000 mN3 h–1. The second process is energy-optimized and suitable for the production of large quantities of compressed nitrogen. Since 1999, this process was realized by Linde AG in Cantarell/Mexico as five identical plants for the production of a total of 1 675 000 mN3 h–1 of compressed nitrogen at 120 bar. It is recovered from 2 375 000 mN3 h–1 of process air with a power consumption of 450 MW. The nitrogen is pressed via an 80 km long pipeline into an offshore- oil well to increase the oil production rate. The specific energy demand related to one standard cubic meter of pure nitrogen at 8 bar is 0.25 kWh mN–3 for the first and 0.15 kWh mN–3 for the second process. The specific energy consumption of industrial cryogenic nitrogen generators typically lies within the range spanned by these two values. Energy-Optimized Two-Column Nitrogen Generator The two-column process (Fig. 2.9) is based on the classical double column supplemented by a further condenser/evaporator unit (6) at the top of the lowpressure column (5). The column pressures are chosen such, that there is a driving temperature difference to maintain the heat flow at the two condenser/evaporator units (6) and (4): x The pressure on the evaporation side of the heat transfer unit (6) amounts to about 1.3 bar, which is just high enough to compensate the frictional pressure drop of the residual gas (a) on its way from the evaporator to the atmosphere. x A pressure of about 4 bar in the low-pressure column (5) guarantees a driving temperature difference at the transfer unit (6). x A pressure of about 9 bar in the pressure column (3) guarantees a driving temperature difference at the transfer unit (4). The additional condenser (6) produces reflux for the low-pressure column. Consequently the need for reflux (b) from the pressure column is reduced and more reflux is available for the pressure column. This allows to withdraw about 48% of the total nitrogen product from the top of the pressure column. The rest is discharged from the low-pressure column (d). Both nitrogen fractions are finally compressed by (7) and (8) to the product pressure in the warm section.

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Fig. 2.9 Double-column nitrogen generator. (1) Heat exchanger; (2) Turbine; (3) Rectification column (pressure section); (4, 6) combined condenser–evaporator unit; (5) Rectification column (low-pressure section); (7, 8) Compressor. (a) Residual gas; (b–e) Internal process flows; (f) Pressurized – gaseous N2.

The nitrogen content of the residual gas (a) is only approx. 25%. The cryogenic losses are covered by turbine expansion (2) of part of the process air (e) into the low-pressure column. 2.2.5.4

Liquefiers

There is a class of air separation units, specialized to deliver the products exclusively in liquid form. The liquid products are supplied to smaller gas consumers within the regional market via tank trucks. Usual capacities of such plants lie in the range of 3000–20 000 mN3 h–1 of cryogenic liquids. The liquefying unit is either

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integrated into the air separation process or modularly designed as “stand alone unit”. A typical process for liquefying unpressurized gaseous nitrogen, equipped with a nitrogen cycle and two turbines, is described in [2.24]. The specific energy requirement for the liquefaction is typically 0.6–0.7 kWh mN–3, and is significantly lower than the ~ 2.7 kWh mN–3 of the first air liquefiers built after 1895. Nevertheless, the 0.6–0.7 kWh mN–3 are still far away from the theoretical minimum value of about 0.2 kWh mN–3 [2.25], which a reversible process would require. 2.2.5.5

High-purity Plants

There is a growing market for high-purity gases with total impurities ranging from < 1 ppm down to 1 ppb. Especially the semiconductor industry has a demand for highly purified nitrogen, which is used for inertizing. Typical capacities are in the range of 3000–50 000 mN3 h–1. Since the semiconductor surfaces are sensitive to undesired reactions with oxygen, hydrogen and carbon monoxide concentrations of O2, H2 and CO lower than 1 ppb are required. Table 2.7 shows typical purities of liquid high-purity oxygen, nitrogen and argon, which are required by industrial users and which are achieved with cryogenic plants. The purities are usually indicated in terms of degree. The degree states, how often the figure “9” occurs in the specified purity. X Example: Nitrogen with a purity of 99.9999% has the degree 6.0, corresponding to 1 ppm of residual impurities (cf. also Section 9.2.2).

Table 2.7 Typical concentrations of high-pure products. Element

Concentration in the air

Degree

5.0–7.0

N2

Ar

6.0–9.01)

6.0

< 0.1 ppb

< 10 ppb

O2

20.95%

N2

78.08%

< 50 ppb

Ar

0.93%

< 5 ppb

< 5 ppb

H2

0.5 ppm

< 50 ppb

< 5 ppb

< 5 ppb

CO

~ < 1 ppm

< 50 ppb

< 1 ppb

< 50 ppb

CO2

~ 400 ppm

< 50 ppb

< 1 ppb

< 50 ppb

CH4

~ < 5 ppm

< 50 ppb

< 10 ppb

< 50 ppb

< 50 ppb

< 50 ppb

< 0.05 ppb

< 50 ppb

CnHm 1)

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O2

< 200 ppb

Argon as inert noble gas excluded.

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Fig. 2.10 High-purity plant. (1) Rectification column (pressure section); (2, 4) Combined condenser – evaporator unit; (3) Stripping column. (a) Internal process flows; (b) Ultra high purity pressurized gaseous N2; (c) Inert gas exhaust.

High purity plants have specific solutions in design and construction, to be described here by the example of a nitrogen generator (Fig. 2.10). The apparatus is derived from the one-column nitrogen generator described in [2.24]. A central component is the pressure column (1). Contamination with Oxygen Theoretically the oxygen contamination of the nitrogen at the top of the column (a) can be reduced to any arbitrary small value, provided the column has a correspondingly high number of trays and the amount of withdrawn nitrogen product is sufficiently low.

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The oxygen concentration changes from one theoretical tray to the next one above by the separation factor S S=K⋅

V L

(2.3)

Here K = y*/x, the component equilibrium factor, describes the ratio between the O2-concentration of the vapour phase y* and of the liquid phase x, with both phases being in equilibrium. L is the molar downwards liquid and V the molar upwards vapour flow. A depletion of oxygen towards the top of the column occurs, if S < 1, i.e. if the amount of nitrogen, withdrawn from the column’s top is small enough to ensure that L/V > K holds. When the column is designed to be operated with a separation factor close to 1, the nitrogen yield is high (low energy consumption) but a large number of trays is required (high investment costs). X Example: The K-value of oxygen in almost pure nitrogen at 9 bar is K = 0.49 (Table 2.8). With L/V = 0.65 a separation factor S = 0.75 results. Then the O2-concentration depletes over 70 theoretical separation stages by the factor S70 = 2.6 · 10–9, while with 100 separation stages the depletion factor is already S100 = 5.3 · 10–13. Table 2.8 Component equilibrium factors in pure nitrogen at 9 bar. Component

O2

Ar

CO

Ne

H2

He

K = y*/x

0.49

0.58

0.82

26.1

27.5

114.6

All components which are less volatile than oxygen do not accumulate significantly at the top of the pressure column and do not contaminate the product. This holds for all hydrocarbon compounds. Contamination with Argon and Carbon Monoxide The volatilities of argon and carbon monoxide lie between those of oxygen and nitrogen and their separation requires special measures: They are only depleted towards the column top if the liquid to vapour ration L/V according to Equation (2.3) is high enough. For a pressure column of 9 bar, the liquid/vapour-ratio has to be L/V > 0.58 for Ar to be depleted and L/V > 0.82 to enable the depletion of CO. Theoretically, arbitrary small Ar- and CO-concentrations in nitrogen could be acquired this way. However, the energy consumption would be high, because a large L/V ratio close to 1 allows only for a small withdrawal of nitrogen product with a resulting large amount of waste gas. Moreover, owing to the larger amount of air to be processed, the dimension of the apparatus and thereby its cost would increase. If, despite its inertness, the noble gas argon is regarded as an impurity

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and has to be removed from the nitrogen product, it is nevertheless separated in the pressure column. For example, the nitrogen withdrawal from the column is reduced to match the specified Ar concentration. CO, however, is removed in a more economical way from the compressed air before it is passed to the cryogenic separation. This is performed by means of the catalytic reaction 2 CO + O2 o 2 CO2 taking place at ambient air temperature on Cu/Mn-oxide (Hopcalite). The CO2 formed is removed in a downstream adsorber. Contamination with Hydrogen, Helium and Neon Hydrogen can also be removed in the warm section by the catalytic reaction 2 H2 + O2 o 2 H2O on a Pd-surface with ensuing adsorption of the water. In this “front-end-purification”, the correct arrangement of adsorptive and catalytic layers has a great influence on the quality of the purification and the protection of the catalyst from being poisoned by undesired chemical reactions. The catalytic process for H2 removal competes with a cryogenic separation: This cryogenic rectification removes not only hydrogen, which is an undesired impurity in wafer factories, but also the two other highly volatile components He and Ne. This requires a further stripping column (3) equipped with a few theoretical separation stages (see Fig. 2.10). This column is operated at a slightly lower pressure than the pressure-column, in order to ensure the heat flow at the heat transfer unit (2). Gaseous nitrogen product is withdrawn from the bottom of the stripping column (b). By this way the hydrogen content of the nitrogen can be reduced theoretically to any desired small concentration. The vapour ascending in the pressure column (1) transports the hydrogen to the top of the pressure column. It has more or less the H2-concentration of air, approx. 500 ppb. The component equilibrium factor of hydrogen in nitrogen is K = 27.5, see Table 2.8. Therefore the down-flowing liquid, which is in equilibrium with the vapour, has an H2-concentration of 500 ppb/27.5 = 18 ppb. This concentration can not be reduced by increasing the purge flow (c) of the condenser (2). Only in the stripping column (3) the H2-content depletes downwards below the nitrogen feed (a) by the factor (1/S) per tray. Here the separation factor is defined in Equation (2.3). For the reflux ratio L/V = 0.65 and the K-value = 27.5 the depletion factor is 0.024. Thus a depletion by the factor 0.0245 = 7.4 · 10–9 occurs over five trays. Apart from front-end purification by means of adsorption and catalysis and purification by means of rectification, also the “downstream”-purification via chemisorption, adsorption and catalysis is applied. This is preferably done to convert technical nitrogen from standard to high purity.

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X Examples: x Chemisorption of oxygen over activated Cu through oxidation O2 + 2 Cu o 2 CuO. Regeneration with H2, CuO + H2 o Cu + H2O. x Reaction of CO over activated Cu through CO + 2 CuO o CO2 + Cu with ensuing adsorption of the CO2. Regeneration with O2, O2 + 2 Cu o 2 CuO. x Chemisorption of oxygen over Ni through oxidation O2 + 2 Ni o 2 NiO. x For smaller quantities of up to about 50 mN3 h–1 so-called getters are an economic solution. They cover a large spectrum of impurities and are easy to operate. Irreversible chemical reactions on the surface of the getter material form stable components. The getter material must be exchanged periodically. The getter process guarantees high purities, when the inlet impurity exceeds the desired final concentration at best by a factor 100. Concerning the constructive details of high-purity plants, numerous design rules have to be followed, in order to achieve the theoretically calculated product purities. For example: x High demands on the leakproofness of all components, such as plate fin heat exchanger passages and packing edges of packed columns. x The partial pressure gradient determines the flow of impurities. For instance, oxygen can diffuse from the atmosphere against the flow direction of the carrying medium through a leak into a pressurized pipeline. x Welded instead of flanged joints, as well as X-Ray testing of the welds. x Special cleaning of all components after manufacturing. x Short pipes, no dead zones with zero flow. 2.2.5.6

Apparatus

Molsieve Adsorber An important milestone in the history of air separation was the introduction of zeolitic molecular sieve adsorbers for the removal of H2O, CO2 and acetylene from atmospheric air since 1968. This simplified the operation of the plants, which by then had been equipped with so called reversible heat exchangers and liquid oxygen adsorbers for the removal of acetylene. Meanwhile, molsieve adsorbers have reached a high degree of functionality and reliability. In the following, specific design and operating parameters will be presented. The molsieve station consists of two adsorbers which are alternately in adsorption or regeneration mode. One cycle includes x an adsorption phase, which typically lasts between 1.5 h and up to 6 h and which is performed at the pressure of the process air, lying in the range of 5–20 bar x a depressurization to ambient pressure within about 10 min x the regeneration period with dry waste nitrogen in the countercurrent direction, divided into a heating phase with warm regeneration gas and an ensuing cooling phase with cold regeneration gas, as well as x a pressure build-up of about 20 minutes

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The adsorber vessel is filled with zeolitic crystals [2.26] which are bound with ceramic material and formed to spheres of about 3 mm diameter or rods. The crystalline components have pores with diameters in the range of approx. 1 nm. H2O as well as other polar or easily polarizable impurities such as CO2 and various hydrocarbons are retained in the pores. Frequently a thin layer of activated aluminium oxide for H2O adsorption is arranged in front of the zeolitic layer. The so called mass transfer zone of H2O and CO2 in the adsorbent is short. This is the zone where the components are adsorbed and which is moving forward with ongoing time. Due to this compact adsorption front the bed height can be reduced to about 1 m with a correspondingly small pressure drop ranging between 2 and 5 kPa. The molsieve material is housed in horizontally or vertically orientated cylindrical vessels with axial or radial orientated gas flows being possible. X Example: Typical amount of molsieve material needed for a plant, producing 60 000 mN3 h–1 of oxygen from 300 000 mN3 h–1 of process air is about 75 t per adsorber. Regeneration Regeneration is performed by heat, which is transferred into the molsieve via the warm regeneration gas. The amount of regeneration heat needs to be only slightly higher than the heat released during adsorption. Most of the heat is contributed by the adsorbed water, whereas the CO2-adsorption heat hardly counts owing to the low CO2 content. X Example: 300 000 mN3 h–1 of process air being purified in the molsieve at 15 °C and 5.7 bar within an adsorption cycle of 4 h: Typically the heat for adsorption of H2O on a zeolit is 1.2–2 times higher than the H2O condensation heat, which is 1808 kJ mN–3. For this exemplary calculation an adsorption heat of 2680 kJ mN–3 is assumed. The adsorption heat of CO2 is ~ 2050 kJ mN–3. The water content of the saturated air at the molsieve inlet amounts to about 3000 ppm, the CO2-content to ~ 400 ppm. Thus the desorption heat for the water is ~ 2.7 MWh while the desorption heat for CO2 amounts to only 0.27 MWh. The temperature profile of the regeneration gas leaving the molsieve adsorber (Fig. 2.11) and allows the plant operator to judge the quality of regeneration [2.27]: There are two temperatures of about 30 and 40 °C, the so-called holding temperatures, at which the exit temperature of the regeneration gas pauses for a certain time. Only afterwards during the cooling phase, where cold regeneration gas removes the remaining heat out of the adsorber, the temperature reaches a maximum. The first small temperature plateau is induced by the CO2-desorption, the second more developed one has its origin in the water desorption: When the dry and warm regeneration gas, having the temperature Treg, meets the water-

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Fig. 2.11 Regeneration gas temperature at adsorber inlet and outlet. ADS = Adsorbing; D = Depressurizing; H = Heating; C = Cooling; P = Pressurizing.

loaded zone, it desorbs the water molecules until it takes on the temperature dependent saturation water load y*(T). In doing so, the gas cools down to provide the required desorption heat Had. However, owing to this the water adsorption capacity of the regeneration gas decreases. Thus, a steady plateau temperature T* is developed. Only after the water has been completely desorbed, the temperature can rise again. By an enthalpy balance one derives an implicit equation, the socalled psychrometric equation, from which the plateau temperature T* can be calculated y * (T * ) H ad = (TReg − T * ) c p Here, cp is the specific heat of the regeneration gas. The CO2-desorption is subjected to the same mechanism. The excess heat, which is transported by the regeneration gas into the adsorber and which is not needed for the desorption, will only be transported to the outlet of the molsieve, after all components have been desorbed at their respective plateau temperatures. This leads to the aforesaid temperature maximum. A maximal temperature value of about 100 °C guarantees a complete desorption. Numerous detailed constructive solutions have contributed to the successful introduction of the molecular sieves: x The adsorber is subject to temperature changes of up to 200 °C occurring periodically over years. The apparatus is constructed to withstand this alternating thermal stress. x Reliable switching valves have been developed even for large units with diameters of up to 2 m.

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x A uniform gas flow through the molsieve material is accomplished. Maldistribution and by-pass flow weaken the adsorptive purification. x The molsieve material at the outer edge zones is protected from heat loss to the atmosphere. If the regeneration temperature in the wall areas remains significantly below 50 °C, the adsorbens cannot be fully regenerated. Compressors Compressors are cost-intensive components: x The investment for compressors including drive amounts to typically 15–20% of the total investment of an air separation unit. x Approx. 90% of the energy consumption is charged to the compressors. Thus, their efficiency determines considerably the operating costs of the plant. Compressors are designed as turbo or positive displacement machines. The two types have different operational behaviour. With turbo machines the amount of compressed gas decreases with increasing pressure, while the machines of the second group deliver, owing to their displacement principle, nearly a constant mass flow independent from the discharge pressure. Piston and screw compressors are the most prominent displacement machines. Multiple-stage turbo compressors are by far the most frequently used machines in cryogenic air separation. Positive displacement machines are found in niche applications, such as piston compressors for oxygen compression and screw compressors for small plants processing less than 4000 mN3 h–1 of air. Turbo compressors can be of the radial or the axial type. They differ from each other in the direction by which the compressed gas leaves the impeller. If the amount of process air is below about 400 000 mN3 h–1, radial turbo compressors are preferably applied in two particular designs, i.e. the integrally geared turbo compressor and the single-shaft compressor with integrated cooling devices. Their development has decisively pushed the evolution of industrial air separation. The specific thermodynamic and mechanical properties of these compressors will be introduced in the following. Principle of the Radial Turbo Compressor The compressor is made up of several stages, which are arranged on one or more shafts [2.33]. These are driven via a gear either by an electric motor or a steam turbine. The gas enters the impeller in axial direction. Here, the energy transfer takes place. The impeller blades accelerate the gas and forward it in radial direction into a diffuser. The cross section of the diffuser expands along the flow direction and the speed of the compressed gas decreases accordingly. Thus the kinetic energy of the gas is converted into pressure according to Bernoulli’s law. The gas, which is heated by the compression to about 100 °C, flows from the diffuser outlet to a shell and tube heat exchanger, where it is cooled with water before entering the next stage. Since the gas volume decreases from stage to stage due to the increasing pressure, all dimensions shrink accordingly.

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The advantage of the radial turbo compressors is, apart from their reliability and compact design, their high efficiency: The compression process comes quite close to an isothermal state change, because the gas is cooled between subsequent compression stages. And the ideal isothermal compression is known P  to have the lowest possible specific energy consumption R T ln  out  , where  Pin  R = 8.3143 J mol–1 K–1 is the general gas constant. The heat removed in the water cooler is, according to the first law of thermodynamics, almost equal to the work performed in the compression stage. X Example: A compressor for 300 000 mN3 h–1 of process air with a stage pressure ratio of Pout/Pin ~ 1.8 has a power input of about 7 MW per stage. This amount of heat determines the heat transfer surface to be provided in the intercoolers. Therefore, it is not surprising, that the appearance of the compressors is dominated by the large intercoolers and that these can make up almost 30% of the compressor costs. Design Parameters of Radial Turbo Compressors x Diameter D of the impellers between 200 and 1600 mm. x The speed of rotation N is limited by the strength of the impeller material. The peripheral speed u, u = N · S · D of the impellers may not exceed a critical value of typically 380–450 m s–1. With a peripheral speed of 450 m s–1, the maximum speed N for an impeller with a diameter of 1.6 m is N = 5400 r min–1 and for an impeller with a diameter of 0.2 m the maximum speed is N = 43 000 r min–1. x Pressure ratio per compressor stage about 1.4–2.0. With an eight-stage machine, gas can be compressed from atmospheric pressure to about 100 bar. x Isothermal efficiencies for large compressors lie between 73–79% and for small ones in the range of 60–70%. Numerous technical detail solutions have contributed to the reliability and long lifetime of turbo compressors, i.e.: x Non-contact seals designed as labyrinth seals or “Carbon Ring Seals”. x A special starting system in case of an electric motor drive enables a quick compressor start within about one minute such that the voltage drop in the supplying net does not get larger than the admissible value of 2–15%. Thus even for a large scale plant an electrical load of some MW can be switched on rapidly. Underload Capacity and Flow Control by Means of Guide Vanes In times of lower product demand, the plant shall be operated in underload mode with reduced power consumption. This will minimize the operating costs. The underload flexibility is accomplished by the flow control of the compressor, allowing a partial load of typically 70%. Below this value, the compressor falls into a non-stationary so called surging mode, in which it cannot be operated. The majority of compressors are driven by an electric motor with fixed rotational speed.

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Here the flow control is performed by means of a so-called inlet guide vane in front of the impeller. This guide vane consists of a number of stationary blades, which can be rotated, to modify the angle, by which the gas to be compressed hits the impeller blades. This influences the amount of kinetic energy transferred from the impeller to the gas. Turbines Expansion turbines have been developed to have high efficiencies of up to 90%. In the beginning of cryogenic air separation, energy intensive high-pressure recycles were applied to satisfy the refrigeration demand. With the introduction of efficient turbines, these have become obsolete. The enthalpy-entropy diagram (see Fig. 2.12) visualizes the expansion of gaseous air in an ideal turbine, a real turbine and a throttle. The entropy increase dS during the expansion is accompanied by a loss of technical available work dE = Tamb · dS. Here Tamb is the ambient temperature introduced in the exergy definition and the technical available work refers to this temperature. The loss of exergy must be compensated by extra work performed on the compressors of the process. Therefore, a process where expansion occurs in an ideal turbine with dS = 0 requires less specific energy than a process with throttle expansion, which has the highest increase of entropy. A real turbine lies between the ideal turbine and the throttle. In air separation, radial turbines are applied. The gas to be expanded enters the turbine in a radial direction and is directed via a ring of static nozzles to the impeller wheel inside of the ring. On flowing through the nozzles, the gas velocity increases and thus the static pressure is already reduced by about 50% and converted into kinetic energy. Then the gas molecules hit the impeller blades and are ejected from the impeller’s eye in an axial direction. Since there is a static

Fig. 2.12 Turbine expansion in an enthalpy–entropy diagram. (1) Ideal turbine (100% efficiency); (2) Turbine with 80% efficiency; (3) Throttle expansion.

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pressure gradient between impeller inlet and outlet, these turbines are classified as so called back pressure or reaction turbines. The turbine power is controlled by the opening angle of the nozzles which are designed as swivel-mounted blades. Thus, the angle, by which the gas leaves the nozzle ring and also the free diameter of this ring are varied. This allows controlling the throughput of gas continuously. The expanding gas performs work on the impeller. In small turbines with few kilowatts of power, this work is rejected by transforming it into heat via an oil brake or a brake blower. In larger turbines the released work is used for driving either a generator or a single compressor stage arranged on the turbine shaft. This compressor stage often serves as booster for the process gas flowing to the turbine. The technical solutions for bearings and contactless seals are similar to the ones of radial turbo compressors. However special attention is paid to the sealing and insulation: While the turbine’s temperature is significantly below 200 K, the oil lubricated shaft bearing has ambient temperature. For insulation purposes, labyrinths with three chambers are arranged on the shaft between the warm bearing and the cold impeller wheel. So-called sealing gas is blown into the middle chamber which escapes through each neighbouring chamber, on one side towards the bearing, on the other towards the impeller. This prevents the oil or oil mist from creeping along the shaft towards the cold process part and, vice versa, the cold process gas from reaching the bearing. Design Parameters of Expansion Turbines x Impeller diameter between about 80–600 mm. x The speed of rotation is limited by the maximal admissible peripheral speed of the impeller which, for reasons of stability, ranges at about 250 m s–1. With the diameters mentioned above, typical rotational speeds of large turbines lie in the range of 10 000 r/min–1 and speeds of small turbines in the range of 50 000 r/min–1. x Pressure ratio Pin/Pout between 2 and 15. x The higher the volume flow, the higher the efficiency. The best efficiencies achievable are just below 90%. x Gas flow between about 300 and 100 000 mN3 h–1, released power between about 3 and 3000 kW. Liquid Turbines In the air separation process (see Fig. 2.3A) liquid high pressure air (c), resulting from the internal compression of oxygen, is expanded via a throttle valve (22) into the pressure column (12). Alternatively this expansion can be performed in a so called “dense fluid expander”. This is a turbine for the expansion of a liquid or very dense supercritical cold fluid. A turbine expansion produces less exergy loss than a throttle expansion. Owing to this the use of a dense fluid expander reduces the work for gas separation or liquefaction.

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X Example: The application of a liquid turbine instead of throttle 22 (see Fig. 2.3A) reduces the work for the production of 50 000 mN3 oxygen at 30 bar by about 510 kWh: For 50 000 mN3 of oxygen to be internally compressed at 30 bar, about 65 000 mN3 of high-pressure air at 60 bar and 100 K must be expanded to 5 bar via the throttle respectively the liquid turbine. With an efficiency of the liquid turbine of 80%, the entropy increase in the turbine is dSturbine = 0.34 kWh K–1 and in the throttle dSthrottle = 1.7 kWh K–1 and the enthalpy change in the turbine dHturbine = –132 kWh. The change of exergy due to the expansion in the turbine is dEturbine = dHturbine – Tu · dSturbine = –132 – 300 · 0.34 = –231 kWh The change of exergy due to the throttle expansion is dEthrottle = dHthrottle – Tu · dSthrottle = 0 – 300 · 1.7 = –513 kWh Thus the application of the turbine reduces the loss of technical available work by the amount of dEturbine – dEthrottle = –213 + 531 = 282 kWh. This work is saved at the air booster (8, 9 in Fig. 2.3A), which now has to compress a smaller amount of air. Since the booster compression itself is also not a reversible process, but has an isothermal efficiency of typically 72%, the work saving at the shaft of the compressor then amounts to 282 kW/0.72 = 390 kW. If in addition the turbine drives a generator with an efficiency of 91%, then the gain of electrical energy amounts to 0.91 · 132 kWh = 120 kWh. Then, the total savings amount to 390 + 120 = 510 kWh. Heat Exchangers and Condensers The heat exchangers in the cryogenic section of an air separator are almost exclusively of the aluminium plate fin type. Their advantages compared to tubular exchangers are in particular: x High specific surface in the range of 500 to 1800 m2 m–3. x Several process streams can be passed through one block for mutual heat exchange. This allows the design of complex processes. x Lower costs. Figure 2.13A shows the structure of a plate fin heat exchanger module: The process streams are led through passages. Up to 200 of these passages are stapled one on top of the other. This large number of passages makes it possible to bring several streams into thermal contact within one unit. The outer frame is formed by 10–25 mm side bars (5, 6), which are only interrupted for passage inlets and outlets (7). A fluid enters the passage via this inlet. Beginning from here the flow is distributed with special fins over the entire cross-section of the passage and is then passed over to the section of the heat transfer fins (3). The individual passages

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Fig. 2.13A Plate fin heat exchanger, schematically. (1) Cove plate; (2) Partition plate; (3) Heat transfer fin; (4) Distributor fin; (5) Side bar; (6) End bar; (7) Passage outlet.

Fig. 2.13B Plate fin heat exchanger assembled from three modules. (1) Header; (2) Module; (3) Stub pipe.

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are closed on top and bottom against each other by partition plates (2), which have a width of 0.8–2 mm. The arrangement of the individual passages as well as the fin types are selected according to the process requirements. The complex arrangement of passages is possible owing to a particular manufacturing process [2.28]: The partition and fin plates with widths of up to about 1.5 m and lengths of about 8 m consist of aluminium alloy. They are covered with solder the melting point of which is lower than that of the base metal. The partition and fin plates are stacked alternately on top of each other to from a module with a height of up to 1.2 m. In a vacuum soldering furnace, this module is heated and soldered by means of radiant heat applied to the module surface and by ensuing heat conduction into the core. Fluxing agents are not required for this soldering process, thus the finished modules are free of (corrosive) residues. Due to the heating the solder melts and connects fins, partition plates and side bars, so that after cooling these layers are intimately connected to each other. As shown in Fig. 2.13B, several such modules can be welded one on top of the other to form a large core. The collectors or so-called headers (1) are manufactured from semi cylinders and welded onto the module. Cores with dimensions of up to 1.5 m u 2.5 m u 8.0 m are manufactured this way. A leakage between neighbouring passages will contaminate the products. Therefore a high leakproofness of the passages is demanded. This is tested by means of helium which can be detected even in small concentrations. Typically leakage flows between the pressure and low-pressure passages in the range of typically 10–3 mN3 h–1 are tolerated. Exchanger Volumes The exchanger volume V [2.29] depends upon the transferred heat Q and the average temperature difference ¢dT ² between the warm and cold streams. This temperature difference is evaluated from the Q-T-Diagram introduced in Section 2.2.5.2, and lies typically in the range between 3–10 K. The mean temperature difference and the amount of transferred heat are related by Q = Ueff Aeff ¢dT². Here Ueff (W m–2 K–1) is the mean heat transfer coefficient and Aeff (m2) the effective heat transfer surface. The surface density of the plate fin heat exchanger is in the range of Aeff /V ~ 500 m2 m–3, and the effective heat transfer coefficient is in the range of 100 W m–2 K–1, which leads to the following volume estimation formula Q dT V ~ C with the constant C ~ 50 000 W K–1 m–3.

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X Example: Air separation unit for 300 000 mN3 h–1 of process air at 6 bar. The air is cooled down from 300 K to 100 K, thereby warming up the separated products and the residual gas to ambient temperature. The specific heat of air is cp ~ 1300 J K–1 mN–3. Thus the transferred heat is Q = 300 000 mN3 h · 200 K · 1300 J K–1 mN–3 = 21.7 MW. The average temperature difference is typically ¢dT² = 4.0 K. From this, an exchanger volume of about 109 m3 is estimated. There are numerous types of fins (3, 4). The selection of the proper type decides on a good design. The fins must ensure good heat flow, provide the mechanical stability of the passages and induce only a small pressure drop. The fin plates are shaped from 0.15 to 0.6 mm thick aluminium sheets by periodical distortion. The fin height, i.e. the clearance between the neighbouring partition plates, ranges between 3.8 and 12 mm. The distortions form channels with “diameters” ranging between 1.2 and 4.5 mm. The Reynolds number calculated on the basis of this diameter lies between about 500–10 000, i.e. the flow is in the transition range between laminar and turbulent. In order to improve the turbulence and thus the heat transfer, additional “obstacles” are installed. To this end, fins are either perforated or are serrated and periodically displaced against each other with a certain clearance. A typical pressure drop of the low-pressure gaseous products in the exchanger is about 10 kPa. Safe operation and a long lifetime of the plate-fin exchanger require the adherence to some rules: x Maximum operating pressure within the range of 100 bar, maximum design temperature < 65 °C. x To confine the thermal stress, the maximum temperature difference between the different streams is limited. Rapid temperature changes are inadmissible. In the start up phase of the air separation unit the cooling down must be performed sufficiently slowly. x The exchanger passages cannot be cleaned mechanically. Therefore the entering gases shall be free of any particles. Combined Evaporator/Condenser – Heat Transfer Units The combined evaporator – condenser units, such as the “main condenser” (13) or the crude argon condenser (16) in Fig. 2.3A are special applications of plate fin heat exchangers. Figure 2.14 on the left shows this heat transfer unit designed as bath evaporator based on the thermosyphon principle. This is a plate fin exchanger with passages for the condensation of gaseous nitrogen from the top of the pressure column and passages for the evaporation of liquid oxygen from the bottom of the low-pressure column. The gaseous nitrogen is directed to the core via a header at the top (header not shown in the figure) and exhausted as liquid over a header at the bottom. The oxygen passages are open both at the bottom and the top of the block. The core is immersed into a bath of liquid oxygen in the sump of the low-pressure column. The liquid oxygen enters the block via the

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Fig. 2.14 Thermosiphon and downflow evaporator. (1) Rectification column (pressure section); (2) Rectification column (low-pressure section); (3) Process pump. LOX = liquid oxygen; GOX = gaseous oxygen; GAN = gaseous pressurized nitrogen; LIN = liquid nitrogen.

open passages at the bottom and is partially evaporated inside. The two-phase mixture in the oxygen passages communicates with the liquid oxygen bath. The two-phase mixture has a lower density than the liquid oxygen and is therefore discharged according to the thermosyphon effect to the top of the block. A phase separation takes place above the block. The evaporated oxygen ascends upwards in the column while the remaining liquid falls back into the bath and mixes with the fresh liquid coming from the trays above. Typical core heights range between 1–2 m. Larger heights become more and more ineffective since the additional heat exchange surface is paid for with a reduced driving temperature difference between the evaporator and the condenser side. This is because the pressure of the liquid oxygen in the bottom area of the evaporator increases due to the hydrostatic pressure. This gives rise to a higher boiling temperature of the liquid oxygen and thereby reduces the driving temperature difference. X Example: With a core of 2 m height, fully immersed into the liquid bath, the hydrostatic pressure at the bottom of the evaporator amounts to 22 kPa. This corresponds to an increase of the boiling temperature by ~ 1.5 K and reduces the driving temperature difference in the lower area of the block by this amount. With a core height of 3 m, the hydrostatic pressure rises to 33 kPa, corresponding to a reduction of the driving temperature difference by already 2.2 K in the lower area.

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An alternative to the thermosyphon evaporator is the falling film evaporator, shown in the right part of Fig. 2.14. Here the liquid oxygen is fed onto the head of the core by means of a process pump (3) and flows downwards co-current to the condensing nitrogen. A thin liquid oxygen film develops on the passage walls and partially evaporates there. At the bottom outlet of the block the evaporated fraction is separated. The remaining liquid portion is mixed with the fresh liquid oxygen from the bottom tray of the low-pressure column and is fed back to the top of the block via the pump. There is no hydrostatic pressure to reduce the driving temperature gradient. Thus core lengths of about 6 m can be chosen and the required heat exchange surface can be housed compactly within few large cores. The liquid oxygen film, formed on the passage walls, may not dry out within the block. Otherwise traces of hydrocarbon components would be enriched to a dangerous concentrate. This is avoided by a sufficiently high flow rate of the process pump and other measurements. Columns Since the beginning of industrial air separation in 1902 by Carl von Linde, rectification columns have been the central element of the cryogenic separation units. But even nowadays their technical development has not come to a standstill. For example at the end of the 1980’s columns with structured packings were introduced as alternative for tray columns. Packings cause lower pressure drop and have a larger loading range than trays. Columns are the highest equipment within the coldbox (Table 2.9) and determine its vertical dimension. Table 2.9 Typical column heights. Typical theoretical number of trays

Typical height

Pressure column, sieve tray

45

14–25 m

Packed low-pressure column

80

25–40 m

Pressure and low-pressure column on top of each other

45 + 80

< 70 m

Crude argon column divided into two

2 · 100

2 · (23–32 m)

Packed Columns Figure 2.15 shows the essential elements of a packed column: vapour flows upwards through the regular structured packings (2) in counterflow to the liquid, wetting the packing surface. The mass and heat transfer occurs on the surface of the packing elements (1) [2.30, 2.31]. A packing element is about 200 mm high and consists of thin corrugated aluminium sheets with a thickness of less than a millimetre. Several packing elements are stacked on top of each other to form a packing bed. The corrugation of the sheets is inclined by about 45–60°

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Fig. 2.15 Assembly of a packed column. (1) Packing element; (2) Packing bed; (3) Liquid distributor; (4) Liquid collector; (5) Liquid feed; (6) Liquid discharge; (7) Gas discharge.

with respect to the horizontal and the inclination within one element alternates between ascending and descending from sheet to sheet. The gas flows along the channels formed by the corrugations, while the liquid runs downwards along the direction of the steepest descent. Thus the liquid distributes along the plane of the sheet. To guarantee a distribution also in a plane vertical to this, the orientation of the packing elements lying on top of each other is twisted by 90°. Special seals between the packing elements and the column wall prevent a by-pass flow of the gas and liquid along the wall. A uniform distribution of the liquid and vapour phase over the entire crosssection (3) is crucial for the efficiency of the separation. Even if the liquid is initially distributed in a uniform way, its maldistribution increases with the covered distance. Therefore, the height of a packing bed is limited to about 3–6 m. The liquid is collected at the bottom of the packing bed, is mixed and is uniformly redistributed by means of a distributor onto the top of the next packing bed. The specific surface of the packing ranges between 350 and 800 m2 m–3. Despite this high surface density, the packing material occupies only about 10% of the volume. The HETP-value, i.e. the packing height equivalent to one theoretical tray, is between about 170 and 500 mm. Packing columns are manufactured with diameters of up to approximately 6 m corresponding to an oxygen production of about 100 000 mN3 h–1. Larger diameters are possible. However then transportation may get impossible and the column must be assembled on site instead of being assembled in a workshop. Sieve Tray Columns Sieve trays are well-established in air separation. Due to their comparatively simple assembly they are an economical alternative to the packings. A sieve tray consists

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of a perforated plate, an inlet and an outlet downcomer. The gas flows upwards through the perforation and gets into a crossflow contact with the liquid on the perforated plate. This liquid comes from the inlet downcomer via a weir onto the plate and leaves the plate by another weir and outlet downcomer. The mass transfer takes place in the bubbling zone on the perforated plate. In the downcomer a liquid level is formed. It compensates the pressure drop of the vapour on its way through the tray and prevents the vapour from bypassing the perforated plate via the downcomer. For columns with large diameters several downcomers must be arranged on one tray, since the specific weir length, that is the length of one weir divided by the liquid flow, decreases with increasing column diameter. The area of the holes amounts to about 5–16% of the total area of the perforated plate. The distance between neighboured trays lies between 80 and 300 mm. It increases with the column’s diameter and load range. The tray efficiency factor, i.e. the separation efficiency referred to one theoretical tray, ranges between 60 and 90%. Sieve tray columns are manufactured with diameters of up to about 6 m. Pressure Drop of Sieve Tray and Packed Columns The pressure drop over a sieve tray is 0.3–0.5 kPa, while the pressure drop in a packing element corresponding to one theoretical tray is only about 0.08 kPa. The small pressure drop of a packed low-pressure column significantly reduces the energy needed for the separation. X Example: Low-pressure column with 70 theoretical trays: Then the sieve tray column is made up of about 85 sieve trays with a total pressure drop of 85 · 0.35 kPa = 29.8 kPa, while the packed column has a pressure drop of only about 70 · 0.08 kPa = 56 kPa. Owing to the coupling of the low-pressure column and the pressure column via the condenser – evaporator unit (see Section 2.2.5.1), an additional pressure drop dP in the low-pressure column induces an increase of the pressure on the pressure side by an amount of ~ 3 · dP. Thus, for the case of a sieve tray column, the pressure on the pressure side is by about 3 · (29.8 – 5.6) kPa = 72.6 kPa higher than for the case of a packed column. If the discharge pressure in case of a packed low-pressure column is 5.70 bar, it increases to 6.42 bar in case of a sieve tray column, causing an increase in the power demand of this compressor by about 6%. Also due to the low pressure drop of a packed crude argon column, it became possible to recover pure argon only by means of cryogenic distillation. Load Range of Sieve Tray and Packed Columns The efficiency of a plant increases, when the columns can be operated with sufficient overload or underload in order to cover either an additional or a reduced demand of the gas consumers. The specific over- and under-load behavior of the two column types is summarized as follows:

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2.2 Recovery of Nitrogen, Oxygen and Argon Sieve tray column

57

Packed column

Upper load range: x Flooding in the downcomer: Liquid load, at which the level in the downcomer rises close to the tray above. x Jet flooding: Vapour load, at which the entrainment of liquid above the tray gets too large. x Abrupt transition to flooding with a sharp increase of the pressure drop, stationary operation of column no longer possible.

x Above a certain load, the liquid develops a “bubble layer” at the intersection between adjacent packing elements. With increasing load, the bubble layer gets larger until it fills the packing element completely and flooding sets in. x Smooth transition to flooding accompanied by a continuous reduction of the separation efficiency.

Lower load range: x ~ 50% x Weeping: Vapour flow so small, that the liquid weeps through the perforation of the trays, exchange effect gets lost.

2.2.5.7

x < 40% x Tolerant towards low vapour load. x Minimal liquid load such that dewetting of packing surface does not occur. Underload capacity depends strongly upon the quality of the liquid distribution. Low surface tension of the cryogenic liquid is of benefit to the wettability.

Design, Assembly and Transport of the Coldbox

All cryogenic components of an air separator are arranged in a so called coldbox, which is a container with a height of up to about 70 m filled with insulating material. Coldboxes for the processing of up to about 50 000 mN3 h–1 of air are assembled completely in a workshop and are transported as “packaged units” to the erection site. With increasing plant size only submoduls are shop-assembled. Their size is chosen according to the restrictions of the means of transport. Figure 2.16 shows the transport of the two columns (at the back) and the two condensers (at the front) of the nitrogen generator introduced before. It gives an impression of how challenging transportation can be. Equipment and pipes within the coldbox are made of aluminium alloy or 18/10CrNi-steel. These materials are suitable for the application at low temperatures and dispose high ductility even at 100 K, i.e. no tendency to propagation of cracks, originating from notches, sharp edges or abrupt discontinuities of the wall thickness. The equipment and its interconnecting piping is arranged in a compact way within the coldbox in order to reduce space requirement and to minimize the heat flow from outside into the box. When the plant is taken into operation, the material cools down by about 200 °C. The associated contraction of the material has to be taken into account by suitable pipe routing and supports. The longitudinal contraction of aluminium due to this temperature difference amounts to 4 mm m–1, i.e. a 30 m long pipe shortens by 12 cm during the transition from the warm to the cold operating state. These demands on the piping design are

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Fig. 2.16 Transport of shop-assembled components.

Fig. 2.17 3D-view of the Coldbox. (1) Rectification column (pressure section); (2) Second section of the crude argon column; (3) Main condenser/evaporator; (4) Rectification column (low-pressure section); (5) Crude argon condenser; (6) Coldbox shell.

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economically met by using a CAD-system with integrated pipe stress analysis. The design review session, within which the coldbox, as for example shown in Fig. 2.17, is virtually inspected via the CAD-system, is a quality-assuring measure. A good insulation of the coldbox is required since the heat penetrating into the cryogenic area from outside must be removed again at the expense of additional energy: The interior of the coldbox is insulated with perlite. This is a bulk material with grain sizes of about 0.5 mm, which is filled into the coldbox from above and which flows into all cavities of the box like a viscose liquid. Perlite is made from pulverized volcanic rock, from which the crystalline water is expelled at 1100 °C. Thus a “cell-like glass structure” with numerous inclusions of air – filled pores develops, having a low heat conductivity O ~ 0.03 W m–1 K–1. Heat bridges are formed, if areas remain non-insulated after filling of perlites into the box. These are recognized by the icing on the outer coldbox shell and cause a loss of liquid production.

2.3 Safety Aspects 2.3.1 Introduction

Apart from the main air components N2, O2, and Ar a number of trace impurities of the air enter the cryogenic section of an air separation unit. The entry of combustible trace components, like methane, ethane, ethylene and propane, represents a safety hazard even for today’s air separation plants, unless suitable precautions are applied. The hazard results from the simultaneous presence of pure oxygen which may act as an oxidizing agent. Thus compounds like hydrocarbons could combust heavily with oxygen, ignition provided. In principal, it is possible that, as a consecutive reaction of a hydrocarbon fire, even the metallic column internals installed in the immediate vicinity of the fire source will ignite. Typical internals of a rectification column of an air separation unit are e.g. packing or sieve trays of aluminium. The aluminothermic reaction resulting in a violent energy release can destroy the plant and injure persons. Actually the air separation industry has experienced only very few major accidents of this kind, mainly caused by maloperation of the plant. Therefore, the following measures for the safe operation of air separation plants should be observed: x An accumulation of combustible substances in terms of a precipitation of a pure hydrocarbon phase should be avoided (e.g. liquid propane or solid acetylene in liquid oxygen (LOX)). An ignition close to a hydrocarbon phase with a subsequent combustion can transfer a lot of energy to the adjoining metal which might catch fire. x The hydrocarbon concentration should be far away from its explosion range (e.g. methane in O2). Regarding hydrocarbon hazard in LOX of air separation units the lower explosion limit is of interest.

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To exclude a precipitation of a solid or liquid phase, the hydrocarbon is only allowed to enrich below its solubility limit in the LOX-sump. In addition, during the operation of a plant a sufficiently large safety margin to the solubility limit of a combustible component should be kept. Thus in large air separation plants, a value of typically 500 vppm (ppm volume fraction, mL m–3) of methane (actually methane equivalents) in the liquid oxygen would cause a shut down of the plant and a following blow off of the sump. The lower explosion limit of volatile hydrocarbons relevant for air separation units (ASU) is in the range of some percent. Thus only methane, ethane and ethylene reach their lower explosion limits in the LOX without precipitating. Heavy components in gas-producing air separation units are being 500–2000 times enriched compared to their outside air concentration, if the components pass the front-end coolers and molsieve adsorbers completely (e.g. Kr, Xe, CH4). In Kr/Xe-plants the enrichment is enhanced further up to a factor of 3000–5000. To exclude any hazard pertaining hydrocarbon accumulation combustible compounds are burnt to water and CO2 at 500 °C. This is performed at the inlet of Kr/Xe-plants on a noble metal catalyst, simultaneously converting N2O to N2 and O2. In contrast to the mentioned hydrocarbon removal route an alternative Kr/Xe-producing process exists where only the oxygen as main component of the crude Kr/Xe-feed is completely substituted by N2. In order to avoid exceeding a certain enrichment in an ASU, in places with the highest enrichment, i.e. mostly in the sump of the low-pressure column (upper column) or after a falling film evaporator, a certain quantity of LOX per time unit is discharged. In order not to exceed arithmetically an enrichment of 1000, for example, 0.5% of the generated liquid oxygen have to be extracted continuously. To avoid product losses, this liquid is discontinuously being flushed into the warm O2-product pipe. Possible ignition sources within an air separation unit are: Combustion of a hydrocarbon (promoted combustion), particle impact, friction, adiabatic compression, resonance and electric arc, fraction, respectively generation of fresh surfaces, shock waves and autoignition when heating to melting temperature. Investigations pertaining incidents in air separation units elucidated the important role of ignitions caused by preceding hydrocarbon enrichment. Of course, first the ignition of the hydrocarbon phase itself has to be induced. In the past, acetylene was considered to be the most dangerous hydrocarbon, due to its tendency to self-decomposition and low solubility in LOX (5 vppm at –183 °C). In today’s air separation units equipped with an air prepurification by molsieve adsorbers, however, acetylene is completely removed by adsorption upstream the cryogenic section. To ignite aluminium, a material preferably used in the manufacturing of air separation units, temperatures above 2000 °C have to be reached [2.35]. The ignition of metals in oxygen is facilitated, the less susceptible to oxidation the metals are, the smaller the material thickness, the higher the O2-concentration, the higher the ignition energy and in general the higher the pressure. With the help of ignition tests it can be investigated under which conditions a sample of a certain material exhibits a propagation of combustion. Thus numerous publications of tests exist, dealing with the ignition of package, rods, tubes and

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Fig. 2.18 Impingement velocity curve for carbon steel (IGC Doc 13/02/E).

reboilers made of different alloys with varying O2-purity and pressure as the most important parameters. This results in a so-called O2-index, which describes the limiting conditions for a propagation of combustion of metallic material. Promoted Combustion-Tests were often carried out with thermite ignitions (thermite pill: ferric oxide/aluminium powder: 3 : 1) [2.36–2.38]. Furthermore that, for the safety assessment regarding the oxygen tolerance, the gas velocity is of vital importance. Background is the particle impact as ignition mechanism which becomes more likely at high gas velocities. Therefore, international rules and regulations stipulate the compliance with maximal gas velocities for C-steels and stainless steel, which additionally depend on the O2pressure. For C-steel in the pressure range of 3–15 bar, for instance, the flow velocity must not exceed 30 m s–1; above 100 up to 200 bar, 4.5 m s–1 must not be exceeded, as shown in Fig. 2.18. These limitations are applicable to the transfer of gases in C-steel pipes with an O2-concentration above 35%, a temperature up to 150 °C and a probability of an impingement. Generally, ignitions by particle impact at lower pressure with a subsequent combustion of a metal are very unlikely for flow velocities below 50 m s–1. 2.3.3 Air Pollution

Apart from methane, in the sump of the low-pressure column, ethane and propane are often to be found, even if only in traces in the lower vppb-range (µL m–3), since these components are not completely retained by the upstream molsieve adsorbers. In industrial areas the LOX-sumps may also contain ethylene in trace amounts. In addition, a number of inert compounds like nitrous oxide enter the cryogenic part of an air separation plant, which could lead to the blockage of pump filters, passages of reboilers and analyzing lines. The following Table 2.10

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2 The Air Gases Nitrogen, Oxygen and Argon Table 2.10 Air components and impurities. Components

Concentration in clean, dry air (vppm)

Concentration in industrial air (vppm)

Retention in molecular sieve adsorbers (%)

Solubility in LOX at –183 °C

Lower explosion limit (%volume fraction) [2.40]

CH4

1.7–2.0

3–10

1

totally soluble

4.4

C2H2

< 0.001

0.01

100

4–6 vppm

2.3

C2H4

< 0.001

0.05–2

85

1.3–3% volume frac.

2.3

C2H6

0.005

0.05–1

10

12% volume frac.

2.7

C3H6

< 0.001

0.05–2

> 99

0.36–0.67% volume frac.

2.0

C3H8

0.003

0.05–2

70

1.0% volume frac.

1.7

n-C4H10

< 0.001

0.05–5

> 99.99

45 vppm

1.4

Oil

0.01–2 mg Nm–3

partly oil (oil as aerosol)

few vppm

0.8 (naphtha)

Acetone

0.01–0.05 mg Nm–3

100

8 vppm

2.5

Methanol

0.02–0.1

100

12 vppm

5.5

0.3–3

0

mixable

10.9

> 99.99

4.1 vppm

incombustible

70–99

140–160 vppm

CO

0.1

CO2

370

N2O

0.3

He

5.2

0

incombustible

Ne

18

0

incombustible

Kr

1.1

0

> 30% volume frac.

incombustible

Xe

0.086

0

2.7% volume frac.

incombustible

H2

0.4

O3

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0.4–1

0.8–10

0

0.06–0.25 mg Nm–3

100 (decomposition on molecular sieve)

4.0 11% volume frac.

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states air pollutions with their typical concentrations, their solubility in liquid oxygen at –183 °C and their lower explosion limit in the atmosphere. At the same pressure and temperature, the lower explosion limit of a flammable compound is almost independent from the concentration of the inert gas, provided the O2concentration is not falling below a lower limit where no combustion can occur. Thus the lower explosion limit of a hydrocarbon in air or pure oxygen is nearly equal in contrast to the upper limit. In cryogenic liquid oxygen, explosion limits are not known, so that these obtained at higher temperatures (e.g. 20 °C) are also applied for cryogenic temperatures. Apart from this, components such as SO2, NO2, NH3, Cl2 and HCl occur as traces in air. However, they are almost completely retained by the spray cooler and molecular sieve adsorbers. Partly, SF6, CF4 and C2F6 break through as non-flammable compounds which are recovered in highly enriched sumps of Kr/Xe-plants. The component oil given in Table 2.10, is mainly generated by oil-lubricated compressors that are only used in very small plants nowadays. This undesired oil-content can be reduced by means of suitable downstream filters (e.g. activated carbon filters). An oil-load of the compressed air of 0.005 mg of oil per standard cubic meter of air should not be exceeded at the inlet of the molecular sieve adsorbers. In addition, even externally generated organic aerosols, e.g. in areas with extensive forest fires, can enter the cryogenic part of a conventional ASU. Most of the particles with high organic content have a size of 0.1–1 µm. Effective retention of these particles is only possible with ultrafilters or aerosol filters. Once low volatile oil drops or aerosols on organic basis have reached the cryogenic part, they cannot be removed by warming up the ASU, in contrast to the highly volatile C1–C3-hydrocarbons. 2.3.4 Ignition in Reboilers

Up to now, to the air separator industry a number of incidents caused by minor hydrocarbon combustions is known. Commonly they occurred in reboilers leading to only slightly damaged passages. These passages were bulged or started to leak detectible by O2-contamination from adjoining N2-passages. Only in few cases serious aluminium combustion happened arising from the reboiler where the fire could also spread to the packing above resulting in a total damage of the plant and environment. With a bath reboiler (see Fig. 2.14), a safe operation is achieved by a sufficiently deep immersion in the LOX-bath (e.g. 100% immersion depth) and by the high liquid circulation. In the case of a downflow reboiler with cryogenic oxygen being fed from above and gas and liquid discharging at the bottom, dry evaporation and thus deposition of LOX-impurities such as N2O and CO2 might locally occur, even though the solubility limit of the impurity in the fed LOX has not been reached by far. The more liquid is discharged at the outlet of the downflow reboiler, the less dry evaporation accompanied with the deposition of heavy components takes place. A typical liquid/gas mass-ratio at the outlet of the downflow reboiler is 3 : 1, since here the reboiler passages are sufficiently flushed.

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Another measure for the safe operation of a reboiler is the reduction of the concentration of impurities dissolved in LOX. Here, not only hydrocarbon concentrations need to be taken into consideration, but also the concentration of compounds inert towards O2, such as N2O and CO2 which can be depleted in LOX by a cryogenic adsorption. The inert substances involve an indirect danger, since, deposited as solids, they block passages in the reboilers and create cavities in which LOX evaporates and heavy hydrocarbons can accumulate. Since conventional molecular sieve adsorbers are designed for the retention of CO2, usually more N2O than CO2 is found in the analyzed deriming air when air separation plants are being defrosted every 2 or 3 years. In case of simultaneous presence in LOX, both compounds do no longer exhibit their original solubilities (see Table 2.10), but precipitate at lower concentrations. The reason is the formation of a common mixed crystal [2.41, 2.42], which is possible due to the almost identical molecule dimensions of the two compounds. Thus, for instance, maximal solubilities of 3 vppm of CO2 were found at simultaneous presence of 60 vppm of N2O or 2 vppm of CO2 with 100 vppm of N2O [2.43]. Due to this influence on the solubility, a solid deposition is reached earlier in LOX when the two components are simultaneous present. 2.3.5 Other Hazards in Air Separation Units

Apart from the increased combustion hazard in the presence of pure oxygen cold burning due to skin contact during the discharge of liquid gases can injure involved personnel. The symptoms on the skin are very similar to burns on hot surfaces. Moreover, in case of nitrogen leakages in air separation units, there is the danger of suffocation.

2.4 Process Analysis Air Separation Units

Depending on concentration, kind of component to be determined and the gas balance, a couple of process analyzers based on different measuring principles are used in air separation plants producing O2, Ar and N2. The analytical control ensures a reasonable and safe pant operation and monitors the product specification. For a not continuously required analysis commonly 2–6 analyzing points share one instrument. If necessary the analyzing points can be switched one by one by pressing a button or in some cases automatically for an alternate analysis. Apart from the production itself, the adjoining tanks of the liquid products and the filling of the certified liquid gases into tank trucks are analytically monitored. In air separation plants, personnel experienced in analytics is usually entrusted with the maintenance and monitoring of the process and product analysis. The analyzers are accommodated in an air-conditioned analysis room and are recalibrated with certified calibration gas in determined intervals. The analysis

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65

room itself is equipped with powerful ventilation to avoid a concentration of e.g. N2 or O2 in case of leakage however, additional monitoring of the indoor air for O2 is customary. In contrast to process analytics, the product control during the tanking is carried out at another place with different instruments, i.e. directly at the tanks. Table 2.11 sets out important measuring points in air separation units together with the applied analytics. In addition, the water content of the regeneration gas entering the molecular sieve adsorber is analyzed. Due to this dew point analysis the leak-proofness of the pressurized tubes of the regeneration gas heat exchanger conducting superheated steam is guaranteed. In normal operation, a dew point of –70 °C is indicated. In principle, only gaseous samples and those heated to room temperatures can be measured in the analyzer unit. If impurities in cryogenic liquids are to be determined, the liquids have to be evaporated first. In order to avoid faulty measuring results due to an inadequate complete evaporation leading to a reduced content of heavy components in the gas, cryogenic liquids should be heated with a high temperature gradient as quickly as possible. This is guaranteed via evaporation in thin, electrically heated capillaries. All process analyzer were periodically calibrated with certified calibration gases, commonly stored in 10 L steel or aluminium cylinders. If, in addition, krypton, xenon or neon are produced, analytics will extend considerably (see Section 3.5). Due to the presently increasing size of O2-producing air separation units a krypton/xenon recovery is more and more of economical interest. Actually many Linde plants are equipped with such krypton/xenonenrichment columns which are integrated in the main cold box and thus belonging to the main air separation unit. The enriched noble gas containing liquid is continuously directed to a tank and from there often shipped by road tank cars to a Kr/Xe-purification unit for further refurbishment. In this sump a 4000fold enrichment pertaining to the outside air concentrations of Kr and Xe is realized. Subsequently all heavy air impurities which are able to pass the air purification accumulate at this location. Common sump concentrations are: 4000 vppm Kr, 350 vppm Xe, 2000 vppm CH4, 10–150 vppm N2O, 0–2 vppm CO2, balance: O2/N2. Due to plant safety reasons the methane content must be checked. The goal is to be far away from the lower explosion limit of 4 V%. Other hydrocarbons like ethane and propane are of minor interest because of their low air concentrations. For the hydrocarbon sump control often a total hydrocarbon analyzer is applied. To avoid any blocking of the heat exchanger within the Kr/Xe enrichment column also the content of N2O and CO2 must be monitored. Their combined solubility limit in the sump should not be exceeded.

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2 The Air Gases Nitrogen, Oxygen and Argon Table 2.11 Process analytics in air separation plants. Measuring point

Component Expectation value1)

Measuring principle, Measuring range1)

Cause

After air prepurification

CO2 in air < 1 ppm

IR-spectrometry, 0–10 ppm

Performance monitoring of air prepurification

Middle section pressure column

O2 in N2 a few %

Paramagnetism 0–25%

Monitoring of pressure column

Argon transition low-pressure column

O2 in Ar about 90%


Monitoring of lowpressure column

Argon transition low-pressure column

N2 in O2/Ar 100–1000 ppm

Plasma cell 0–1000 ppm

Monitoring of lowpressure column

Condenser sump of low-pressure column

Hydrocarbons 10–100 ppm Methane equivalents

FID – total hydrocarbon analyzer 0–500 ppm with column: C1–C3-hydrocarbons

Safety low-pressure column

Sump Kr/Xeenrichment column

CH4 in O2 2000 ppm

Flame ionization detector

Safety Kr/Xeenrichment column

Sump Kr/Xeenrichment column

N2O/CO2 in O2 0–150 vppm

IR-spectrometry

Safety + monitoring Kr/Xe-enrichment column

N2-product (highly pure) Ar-product (highly pure)

O2 in N2 resp. Ar 0–10 ppm/0–50 ppb

Electrochemical

Verification Product specification

Ar-product

N2 in Ar 0–10 ppm

Plasma cell 0–100 ppm


Ar-product

H2, O2, N2, CO, CO2 in Ar ppm-range

Luminescence by high-frequency argon discharge


O2-, Ar-, N2-product

Hydrocarbons in O2 resp. Ar resp. N2 0.05–100 ppm

FID – total hydrocarbon analyser 0–100 ppm


O2-product

Ar in O2 1 ppm

Ionization through high-frequency discharge of helium (helium detector) 0–100 ppm


O2-product, purity for steel mills

O2 99.5–99.8%



1)

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All concentrations indicated in mol%, mol ppm or mol ppb.

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2.5 Applications of the Air Gases

67

2.5 Applications of the Air Gases 2.5.1 Applications of Nitrogen

Gaseous nitrogen (GAN) is used as a raw material or inert gas, while liquid nitrogen (LIN) is used for refrigeration. The versatility of nitrogen means it has a vast number of applications. These are divided in the following into two main categories of “inerting and purging” and “cooling, preserving and deep-freezing”. 2.5.1.1

Applications of Nitrogen for Inerting and Purging

In the processing industry, nitrogen is used to: x fill and pressurise gas shock absorbers and hydraulic springs (oil conservation) x perform leak tests (e.g. for containers) and protect against corrosion (e.g. for electronic components) x purge stainless steel pipes or containers before welding to perform root shielding (mostly together with hydrogen) x protect and inert electric parts during manufacturing and storage (e.g. lamps, vacuum tubes and magnetic devices) x create a high-purity inert environment for the manufacture of semiconductors and as a carrier gas for epitaxy, diffusion and chemical vapour deposition (CVD) In metallurgy, foundry technology and the steel industry, nitrogen is used to: x activate shut-off switches in burners and furnaces (with possible subsequent N2 purging) x purge and stir metal melts by bubbling them through porous bed stones or lances (recirculation, discharge of gases and slag) x feed powdered alloy components into steel melts (via N2 jet for alloying) x spray metal melts and gain high-quality metal powders (nozzle atomization, powder metallurgy) x sinter metallurgical powders under shielding gas x purge non-ferrous metal melts to reduce the hydrogen content (e.g. aluminium melts) x stabilise the austenitic structure of stainless steel (with N2 as a reactive component) x shield metal parts during thermal treatment (e.g. annealing, sintering, hardening) x adjust carbon transfer in heat-treatment furnaces for gas carburization (e.g. with methanol through the formation of CO and H2) (see Example A, below) x reduce the carbon content of electric sheet metal in decarbonising annealing (e.g. with humidified nitrogen and H2, CO and CH4 formation)

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x maintain the carbon content of steel parts by carbon-neutral annealing (e.g. with cracked hydrocarbon gases before quenching and hardening) In the chemistry/petrochemical industries, nitrogen is used to: x make products such as ammonia, calcium cyanamide and metal nitrides (with N2 as a reactive component) x produce foamed plastic (with N2 as blowing agent) x prevent oxygen and humidity accessing production processes (i.e. when producing graphite, phosphor, sodium, plastics, rubber and synthetic fibres) x inert and handle storage tanks holding combustible liquids (e.g. filling, covering, emptying) (see Example B, below) x inert gasometers before revision or scrapping x inert tankers during emptying and filling at ports (direct LIN feed for LNG tankers) x inert and clean oil pipelines (pipeline scraping or “pigging”) (see Example C, below) x discharge stone dust in oil and natural gas drilling (high-pressure N2 application) x accelerate oil and natural gas recovery (enhanced oil and gas recovery (EOR)) x dry technical equipment (e.g. following repairs, during standstills or before re-starts) In food technology, nitrogen is used to: x ensure pest control and fire safety in silos (e.g. for corn, powdered food) x preserve fatty or powdered food during filling, packing and storing (N2 or mixtures with CO2 and/or O2) (see Example D, below) x protect liquid food from oxidation during storing and filling (e.g. beer, wine, juices) x ensure controlled ripening of stored fruit in combination with “ripening gases” (e.g. N2/ethylene mixtures for bananas) x stabilise the internal pressure of thin-walled beverage cans (administering drops of LIN before closing) In other industries, nitrogen is used to: x avoid self-ignition of coal during handling (especially when transporting coal dust) x prevent mine fires (e.g. inerting of mine drifts in coal-mining) x regenerate adsorbers for combustible materials (e.g. by purging with heated GAN) x reduce the oxygen content in cooling and storage rooms (fire protection, while still allowing access to stores) x shield perishable and combustible goods in receptacles and silos (general inerting)

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x inert pre-stressed steel in pre-cast concrete elements before grouting with mortar (e.g. in bridge construction) x prevent any kind of oxidation, including fire and explosion protection x prevent problems during the assembly of electronic devices (see Example E, below) Example A: CARBOFLEX® Nitrogen/Methanol Atmosphere System

Carburising and carbonitriding are widely used in the automotive industry, mainly to enhance the load performance of transmission and engine parts in heat treatment furnaces. The major advantage of these processes is that they enable the production of parts with very hard surfaces and a ductile and tough core. During carburising and carbonitriding, carbon and nitrogen diffuse into the surface of a steel component to produce a hard martensitic surface layer after quenching. The reactive carbon originates from the decomposition of carbon monoxide, the reactive nitrogen is generated when ammonia decomposes. The carrier gas transports the reactive components to the surface of the steel parts. Carbon and nitrogen potential can be measured and controlled by analysing the furnace atmosphere. Traditionally, this carrier gas is generated by understoichiometric combustion of either natural gas or propane via a catalyst in an endogas generator. The disadvantages of this method include limited gas purity, restricted flexibility and high maintenance effort. The CARBOFLEX® system can provide carrier gas efficiently, cost effectively and reliably, while meeting all safety requirements. The process involves cracking

Fig. 2.19 CARBOFLEX® – continuous carbon control and optimising system on a pusher furnace.

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methanol in a hot furnace to produce hydrogen and carbon monoxide, which, together with injected N2, forms an appropriate carburising atmosphere. The benefits of the CARBOFLEX® process include: x x x x x

Highly versatile range of application (carburising, carbonitriding) Increased process flexibility and productivity High purity gas supply (direct injection and formation in furnaces) Lower costs (investment, cost of operation) Safe operation and reliable performance (e.g. continuous monitoring, see Fig. 2.19) x Capacity for future growth In combination with CARBOJET® high-speed gas injection technology, the CARBOFLEX® technique can deliver higher levels of atmosphere homogeneity and product quality as well as cut costs even further. Example B: Inerting

For safety reasons, the inert gas nitrogen is used to displace atmospheric oxygen to prevent explosions during processing, storage and transport of materials that tend to oxidize in strong exothermal reactions (gases, liquids and powders). This inerting process can be carried out using various methods such as dilution purging (see Fig. 2.20) or blanketing. Based on the same initial and final oxygen content, all of these processes use different quantities of inert gas and are applied for different periods of time. The method used mainly depends on the size and shape of the plant or the container. The residual oxygen content depends on the product and the safety requirements. As a rule of thumb, the oxygen content should be below 8 vol.% for gases and flammable liquids, below 4 vol.% for metal powders and below 10 vol.% for all other powders.

Fig. 2.20 Dilution purging.

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Nitrogen for larger inerting projects is generally supplied in liquid form. Stationary storage tanks and evaporation units are used for long-term inerting and require some time to be installed. Mobile high-performance evaporators and road tankers are available for temporary applications and can be operative within a short space of time. Special inert gas sluices are used for inerting containers that have to be manually or automatically filled via manholes or valves. The sluices create an inert gas mantle over the filling aperture and prevent gas exchange between the container and its surroundings. Example C: Pigging

When products are transported through pipelines (such as oil pipelines or heat exchangers in refineries), impurities or materials can be deposited in the pipelines as a result of physical or chemical reactions. These deposits have a negative effect on throughput, pump efficiency and heat transfer, and are therefore undesirable. Pigging is used to remove relatively soft or pasty deposits that accumulate in pipelines (see Fig. 2.21). During this process, a pipeline inspection gauge (pig) is inserted into the pipeline through a lock system or pig trap. The pig is pushed through the pipeline or pipe network either by the usual product flow or by an inert gas, generally nitrogen. It scrapes along the pipe walls, pushing away any deposits in front of it. Pig traps are regularly spaced along pipeline systems, so that the pipeline can be pigged in sections. Although the simplest pigs are made of foam balls or plastic materials, different types of pigs are available, including those shaped like dumbbells with sealing elements attached to the plates. Pigs are generally deployed by pipeline operators and industrial services companies.

Fig. 2.21 Pipeline purging.

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Nitrogen is used in almost every case as the pressure medium because it can be supplied in large quantities to any location, and is dry and inert. If the pipeline or network is not filled immediately afterwards, nitrogen also offers effective protection against corrosion. Example D: Modified-Atmosphere Packaging (MAP) for Foodstuffs

MAP is used to maintain the quality of foodstuffs and increase the shelf-life of packaged products. The key to this technology lies in applying the gases (generally CO2, N2 and O2) and gas mixtures required for conserving the product in question. The most important prerequisites for successful MAP treatment are: x High-quality products and raw materials x Appropriate temperature control x Strict hygiene standards throughout processes x Appropriate quality and safety systems (such as hazard analysis and critical control points or HACCP) x Gas mixtures suited to the product x Appropriate packaging materials The last point, in particular, is a decisive factor in ensuring the efficiency of MAP. Packaging should generally have low oxygen/gas permeability as well as tight sealings. If this is not the case, too much gas can be exchanged or lost via the packaging, invalidating the benefits of the initial MAP atmosphere. To avoid adverse effects resulting from oxygenation, the concentration of residual oxygen in each package should be below 0.5 vol.%. MAP provides an ideal atmosphere through evacuation and replenishment, or by purging using oxygen-free gas mixtures. Special MAP atmospheres with high concentrations of oxygen (such as those used for fresh meat) are exceptions to this rule. If carbon dioxide is used in a modified atmosphere, it should have a minimum concentration of 20 vol.% in order to capitalise on its bacteriostatic effect (see Figs. 2.22 and 2.23).

Fig. 2.22 Bacterial growth on pork in different atmospheres at 4 °C.

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Fig. 2.23 Microbial growth based on pH levels.

Example E: Inerting During the Manufacture of Electronic Devices

The electronics industry is currently the largest market in terms of revenue, positioned even ahead of the food industry. Its products are used in almost all areas of daily life including mechanical engineering, consumer electronics and the automotive and aerospace industries. When electronic products are constructed, individual components such as chips, condensers, or conductor boards must be connected to fully functional electronic devices. This crucial step is usually performed using reflow or wave soldering processes. Both processes can be performed either in air or in controlled and inert atmospheres (see Fig. 2.24). Using a controlled atmosphere, however, provides the following substantial benefits: x Improved wetting results x Increased soldering quality

Fig. 2.24 Gas installation for soldering machines. 1 Shut-off valve, main 2 Other gas using source 3 Shut-off valve, soldering machine 4 Pressure regulator 5 Pressure gauge 6 Solenoid valve

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x x x x

Reduced solder consumption, especially in wave soldering Reduced environmental impact (e.g. by using milder flux material) Larger process window, greater flexibility without adjustment Better evaporation of dissolvers from flux material, printed surfaces and devices x Less contamination of printed circuit boards and frames x Fewer and smaller voids in soldering points x Easier handling of consecutive soldering processes 2.5.1.2

Applications of Nitrogen for Cooling, Preserving and Deep-Freezing

In the processing industry, nitrogen is used to: x join components by cold-shrinking and positive grouting with LIN (e.g. shafts, gear wheels, valve seats) x extrude aluminium (cooling the extruder head and/or the running strand with GAN or LIN) In metallurgy, foundry technology and the steel industry, nitrogen is used to: x temper and harden metal parts through high-pressure (HP) gas quenching after annealing furnaces (e.g. substitution of oil baths) x embrittle cast-iron parts and knock off lugs (e.g. open risers on ductile cast iron) x remove residual austenite in hardened steel parts using cooling baths (the low temperatures are generated by passing LIN through heat exchangers in the bath) In the chemistry and plastics industries, nitrogen is used to: x cool rubber, pigments and plastics for cryogenic grinding (e.g. for homogenization and recycling) (see Example F below) x cool rubber and plastic moulds for cryogenic deflashing (e.g. in mechanical shakers, rotary drums and jet machines) (see Example G below) x cool rubber and plastic hoses to produce reinforced, high-pressure qualities x cool plastic films internally during blow moulding for manufacturing plastic containers (e.g. to enhance production performance and product quality) (see Example H below) x cool coated metal parts for improved mechanical debonding and decoating (e.g. recycling of used steel belt tyres) x generate a high vacuum using vacuum pumps and cold traps (e.g. for applying aluminium coating to foils) x control the temperature of chemical reactions (e.g. LIN/GAN as a cooling medium for chemical cryogenic syntheses) (see Example I below) x cool extrusion tools for chemical manufacturing in general (compare with the processing industry entries listed above)

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In food technology, biology and medicine, nitrogen is used to: x support or substitute cryogenic plants in cold stores (e.g. emergency cooling) or in refrigerated trucks (transport cooling) x refrigerate feed and mills for gentle cryogenic grinding of spices (with benefits including aroma preservation and improved feed control) x freeze food such as meat, fish or bakery products (e.g. quick freezing in tunnel, spiral or immersion freezers, see Example J below) x cool food during mixing or chopping (e.g. meat emulsions in cutters during sausage production, see Example J below) x cool food during the tumbling process (e.g. treatment of ham, coating with sauces) x preserve biological material (e.g. cryopreservation of cells and tissue, “cryobanks”) x perform cryosurgery for the specific destruction/removal of tissue (e.g. warts) x support the cooling of magnetic resonance imaging scanners (see also applications of helium) In other applications, nitrogen is used to: x condense solvent vapours from exhaust gas flows (over heat exchangers or by direct GAN/LIN injection, see Example K below) x freeze soil for sealing and stabilising purposes in civil engineering (e.g. soil icing, bulkheading groundwater, see Example L below) x cool fresh concrete for security-critical buildings (preventing stress cracks while setting) x cool new asphalt road surfaces (making roads ready to bear traffic more quickly, reduced down-time on building sites) x freeze liquid-carrying pipes (blockages using freeze plugs, e.g. for assembly work) x simulate space conditions in cryochambers and climatic chambers (e.g. high vacuum using cryopumps) x operate cooled wind tunnels for high-speed testing of aircraft and missiles (mostly models) x cool and inert electronic components during operation and storage x cool coated electrodes in glass production (oxidation protection) x reduce air temperature to optimise cooling during glass manufacture (e.g. with LIN heat exchangers in the air intake) Example F: Cryogenic Grinding

Grinding is one of the oldest known technical procedures. Grinding heat-sensitive materials can prove particularly difficult as many types of mills generate large amounts of heat. Using cryogenic industrial gases such as carbon dioxide or nitrogen guarantees reliable cooling, which in turn reduces the amount of energy required for grinding

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Fig. 2.25 Diagram of a cryogenic grinding system.

and normally increases production rates (in comparison to a plant that does not use cryogenic grinding). These gases also provide an inert atmosphere to prevent explosions and fire hazards during cryogenic grinding (see Fig. 2.25). In the food industry, it is common practice to cool mills that grind spices as this enables delicate flavours to be retained. Similarly, many rubber and (thermo-) plastic materials can only be reduced in size if they have been embrittled prior to entering a mill. And the recycling industry also uses embrittlement and different thermal expansion of components to break down composite materials. Example G: Deflashing Rubber

Pressed or injection-moulded rubber parts often have to be deflashed following vulcanization. Previously, this residual material was removed by hand, using knives or scissors, for instance. Later, CO2-based cryogenic machines were used to perform this task. Today, we often employ automatic cryogenic tumblers or cryogenic shot blast deflashing machines. Liquid nitrogen (LIN) is normally used as the cryogenic fluid. Cryogenic deflashing is mainly used in companies that produce rubber parts in a wide range of shapes and sizes. This may include products such as sealings, O-rings, bellows or stoppers (see Fig. 2.26). It is also a common procedure in companies that provide deflashing services for original manufacturers (OMs). A shot blast deflasher system uses small deflashing plastic granules and is a

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Fig. 2.26 Parts before and after deflashing.

particularly efficient, high-quality option for removing flash. Residual flash is removed by granules that are propelled against the LIN-embrittled flashes by a throw wheel. The deflashing temperature, retention time and energy input (throw wheel speed, etc.) are controlled automatically. Different types and sizes of deflasher systems are available on the market, including special machines for large components or tyres. Example H: Gas-assisted Injection Moulding (GIM)

GIM technology is a special injection moulding method for thermoplastic polymers. It is used to manufacture parts with cavities or thick walls. As this process also overcomes sinking, it enables thick-walled parts to be manufactured without sink marks. GIM can be used to cost-effectively produce complex shapes that require highquality surfaces, offering designers a high level of freedom. Examples here include door handles for cars or other plastic car parts such as mirror housings, bumpers or door modules. GIM parts are not restricted to the automotive industry, however. They can also be found in other areas, for example as handles in various household appliances (e.g. refrigerators), TV cabinets or crates for carrying bottles. In order to create a cavity, nitrogen is injected into the melted polymer at high pressure (typically between 80 and 350 bar). The gas forces the polymer towards the walls of the mould, creating the hollow section. After the plastic has solidified, the nitrogen is released from the cavity. Different variants of this technology are available on the market. The short-shot process and the process with overflow cavities are the two most important. The benefits of GIM technology include: x Improved product quality as a result of superior surfaces (elimination of sink marks), reduced warp and improved dimensional accuracy

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Fig. 2.27 Diagram of a GIM installation: Combined with a high-pressure evaporator, Desy® is a practical nitrogen supply system for gas-assisted injection moulding (GIM).

x Reduced cycle time due to faster cooling and enhanced productivity x Energy savings resulting from the lower clamping force required from the injection moulding machine x Reduced resin feed stock, which in turn results in lighter products In addition to the injection moulding machine, a nitrogen supply, appropriate high-pressure compressor and pressure-control module are required for deploying GIM technology (see Fig. 2.27). The pressure-control module is equipped with a precise, highly dynamic pressure valve that allows defined pressure time profiles to be created. To ensure high product quality (e.g. no oxidation reactions) and reliable operation with low levels of maintenance, high-purity nitrogen with a low oxygen content must be used. Example I: Cooling and Heating Systems for the Chemical Industry

Many reactions in fine chemistry and the pharmaceutical industry require precise temperature control. This is increasingly assured using multi-step syntheses across a wide temperature range from –100 °C to + 100 °C. Low-temperature synthesis is gaining in importance, especially for the production of active agents and special chemical substances. During this process, liquid nitrogen cools a heat carrier which circulates in a secondary cycle (see Fig. 2.28). The temperature of the heat carrier is raised either electrically or by using a heating circuit. Linde uses a special hydrocarbon as the heat-carrier medium. This process also allows standard heat-carrier oils (e.g. Syltherm XLT) to be used at temperatures as low as –80 °C. The nitrogen applied for the process can be reintegrated into the inert gas network and used again. The various elements of the system are standardised. Depending on the heat exchanger device, cooling and heating can be performed at 5 kW to 50 kW, within a temperature range of –110 °C to +130 °C.

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Fig. 2.28 Cooling/heating system with secondary heating circuit.

Example J: Food Freezing Solutions

Cryogenic Bottom Injection System (LIX-Shooter®) LIX-Shooter® is a cryogenic bottom injection system that works with either liquid nitrogen or liquid carbon dioxide (“LIX”). This system is very useful in situations when fast and efficient cooling is required without additional process equipment. It can be installed in new or existing process machines, requires a minimum amount of space and cools directly into the product. The resulting low phase separation, low product stress and reduced aroma loss means that the end product has a very high quality. The system works by injecting a discrete amount of coolant directly into the product mass. The coolant evaporates when it comes into contact with the product. It then absorbs heat from the product and continues to have a cooling effect as it passes through to the top of the process container. The system can be used for a wide range of products that require rapid cooling, including meat and vegetable products (e.g. prior to forming), soups, sauces, paste and pulp products, baby food and purées. The Tunnel Freezer The tunnel freezer is designed to meet the exacting standards of the modern food industry (see Fig. 2.29). It combines the highest level of hygiene with the best available control systems. This freezer is not only highly efficient in refrigerant consumption, but also quick and easy to clean, thus keeping costs at a minimum. With specially designed fans for a very high heat transfer and efficient refrigerant spraying, the tunnel freezer has a large capacity for freezing or cooling a wide

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Fig. 2.29 CRYOLINE® MT tunnel freezer (Linde).

range of products despite its small footprint. These products include meat, fish, seafood, and bakery foodstuffs, ready meals and other convenience food. The very low operating temperatures of liquid nitrogen (LIN) and liquid carbon dioxide (LIC) ensure extremely fast freezing. This helps maintain the quality and shape of food products and keeps losses at a minimum. The tunnel freezer can be used with either LIN or LIC as a refrigerant, depending on local availability and product requirements. Manufacturers can select the refrigerant that best meets their specific demands. A fully compatible industrial PC can be used to monitor trends and events during the production cycle and download these to an external computer, where they can be stored as traceable records. The Cryogenic Contact Freezer The cryogenic contact freezer is designed to process products that are delicate, sticky or difficult to handle in an efficient and hygienic way. The freezer uses a disposable plastic film that travels through a conventional freezing tunnel where it comes into contact with cold plates. These plates are cooled by vaporizing liquid nitrogen at –196 °C, which in turn quickly and effectively freezes the contact surface of the product. This ensures that the product is free from belt marks and wrinkles and can be easily handled for further processing. The film ensures that soft, wet or sticky products can be readily handled without deforming or sticking, and even liquids can be easily frozen. As the film is disposable, the freezer can be quickly and efficiently cleaned at the end of production, ready for use the following day. This also means that product changes do not cause expensive delays. The system acts as a contact freezer, where heat is removed from the product following contact with the cold plates. High-speed fans circulate the cold gas atmosphere inside the tunnel and help to freeze the upper surface of the product. The gas generated by vaporizing the liquid nitrogen is exhausted into the atmosphere. As the temperature of the exhaust is controlled, the amount of cold extracted from the nitrogen can be optimised, depending on production requirements.

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Fig. 2.30 CRYOLINE® CS spiral freezer (Linde).

High-capacity Spiral Freezer The cryogenic high-capacity spiral freezer is self-stacking and has the smallest possible footprint (see Fig. 2.30). It is based on a new technology designed to create a more efficient and cost-effective freezer. The eight-sided shape is manufactured with a minimum of space around the belt, ensuring that the cold gas moves at high speeds and removes the heat as quickly as possible. The nitrogen supply and gas balance are controlled by a state-of-the-art automatic system, resulting in less idle consumption compared with existing spiral freezers. The system is suitable for freezing a wide range of products, including meat patties, whole fish or fish fillets, pies, ice cream, pastries, pizza and ready-made dishes. The spiral can also be used as a cooling unit. Liquid nitrogen is used to maintain a very low operating temperature, ensuring rapid freezing that preserves the quality and shape of the product and keeps weight loss to a minimum. Cryogenic freezing is normally most economical with low to medium volumes. This spiral freezer, however, is also suitable for large volumes where a high level of quality is required or the special preservation properties of cryogenic freezing are critical. Example K: Solvent Recovery (Cryocondensation)

Almost all painting, gluing and coating processes produce solvent vapours. Solvents are recovered for reasons of cost and safety. On the one hand, recovering solvents saves energy and process materials; on the other, it prevents environmental and health problems as well as fire hazards. Nitrogen is commonly used as an inert carrier gas in drying units linked to condensation recovery units. If solvents are recovered using adsorbers (e.g. activated carbon), the adsorbent can be regenerated using nitrogen. This is useful if flammable or explosive materials are being recovered, or if the adsorbent adsorbs unwanted moisture from the air and therefore needs to be dried. Nitrogen is the preferred refrigerant for solvent recovery through condensation when very low temperatures (i.e. below approx. –40 °C) are required for recovery

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Fig. 2.31 The CRYOCON™ solvent recovery process (Linde).

or if the volumes for cleaning are very small (up to approx. 500 m³/h) and highly loaded with solvents or other pollutants. Waste gas emission limits as set out by national regulations (see e.g. the German air regulations “TA-Luft”) are easier to maintain if waste gas is continuously emitted. Cryogenic solvent recovery plants become more cost-effective if larger amounts of nitrogen can be used as an inert gas for other operations. The CRYOCON™ process from Linde (see Fig. 2.31) is a typical example of such an installation. Example L: Ground Freezing

Ground freezing is designed to make unstable, soft and water-logged soils stable and watertight (see Fig. 2.32). The process is applied in tunnels, pits, shafts and other special ground construction works when problems arise and conventional stabilising and sealing methods cannot be successfully applied. In some cases, it may make sense to combine different methods, e.g. grouting, drainage and ground freezing. Electrical brine freezing plants were originally used for this process and are still common today. For smaller projects and short-term freezing, however, liquid nitrogen ground freezing is more suitable as it provides a more flexible freezing capacity and reduces the time needed for freezing. Equipment costs are low in comparison to brine freezing, but energy costs are higher. Ground freezing usually involves creating frozen soil walls in soft ground or temporary ice barriers in existing constructions such as bore or sheet-pile walls. The ice acts as a sealant and fulfils a temporary, static function. In this process, ice is created by drilling or pushing freeze pipes into the ground. These pipes can be inserted to form a closed ice wall. A distribution, circulation and controlling system must be installed to transport liquid nitrogen (LIN) into the freeze pipes. The LIN

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Fig. 2.32 Ground freezing: Icing a gap in a sheet pile wall.

evaporates in the freeze-pipe system as it absorbs heat from the surrounding soil, then flows into the exhaust pipe system more or less warm. This method can create a one-metre frozen wall in approximately three days. It is very quick and safe and is often used both in emergencies and as part of scheduled small and medium-sized projects. 2.5.2 Applications of Oxygen

Many industrial processes are using air for combustion and chemical oxidation. The benefits of using oxygen by enriching or replacing air include: x Higher reaction temperatures x Faster reaction through higher partial pressure of reaction partners (by eliminating N2) x Energy savings, especially at high temperatures and pressures (no N2 ballast) x Smaller plant dimensions or higher plant performance (no N2 ballast) x Higher yield and selectivity of reaction x Prevention of undesired secondary reactions with N2 (e.g. formation of nitride) x Few or no valuable products lost with off-gas x Reduced off-gas volumes, emissions and off-gas treatment costs

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In the processing industry and autogenous technology, oxygen is used to: x operate high-performance burners (e.g. for gas welding, flame cutting, flame soldering, flame heating, flame straightening or flame gouging), in particular with the fuel gases acetylene, natural gas and hydrogen (see Examples A and B below; see also Chapter 8) x coat work pieces using oxy-fuel gas flame spraying (e.g. with metals or ceramics, see Example C below; see also Chapter 8) x derust steel sheets or rework concrete surfaces by flame blasting x remove flash from metal die castings by explosion deburring (in a pressure chamber with an O2/H2 or O2/CH4 mixture) In metallurgy, foundry technology and the steel industry, oxygen is used to: x refine pig iron in converters (decarburization: using O2 lances in the LinzDonawitz (LD) method, using O2 bottom blowing in the Oxygen Bottom Maxhütte (OBM) method) x treat scrap in electric arc furnaces (melting, refining and alloying with O2 lances and O2 burners) x increase the performance of induction furnaces in the manufacture of cast iron (using O2 burners) x increase the performance of shaft or cupola furnaces in the manufacture of cast iron (O2 enrichment with wind nozzles, HIGHJET® method) x increase the performance of blast furnaces by injecting O2 (using lances) and fuel (e.g. oil, coal dust) x preheat casting ladles (prevents melting baths from cooling, reduces cycle times) x increase the performance of furnaces in non-ferrous metallurgy (e.g. aluminium or copper hearth-type furnaces) with additional oxy-fuel burners (see Examples D and E below) x boost reheating and annealing furnaces in rolling mills, processing lines and forges (see Example F below) In chemistry, energy engineering and environmental protection, oxygen is used to: x produce chemical products through oxidation of inorganic raw materials (e.g. hydrogen peroxide from hydrogen) and organic raw materials (e.g. ethylene oxide from ethylene) x generate synthesis gases (e.g. mixtures of CO/H2 with possible further reaction to alkanes and alcohols) through partial oxidation of hydrocarbons, e.g. oxidation of heavy oil residues with O2 (see Section 5.2) x generate synthesis gases through coal gasification, e.g. by gasification of coal dust with O2/H2O (see Section 5.2) x increase the performance of Claus plants (extraction of sulphur from process gases containing H2S, production of sulphur-free fuels, see Example G below)

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x regenerate catalysts (e.g. burn off adsorbed coke in the Fluid Catalytic Cracking (FCC) process, see Example H below) x operate catalytic oxidations in the gas/liquid phase (e.g. production of propionic acid) or in the heterogeneous gas phase (e.g. production of vinyl acetate monomer – VAM) x operate power stations with low emissions, e.g. through coal gasification with O2 and subsequent removal/recycling/sequestration of CO2 x treat waste water in biological sewage works, (e.g. by injecting O2 into activated sludge tanks to improve performance and minimise odours, see Example I below) x prevent anaerobic digestion processes in pressurised sewers (e.g. by injecting O2 into pump stations to suppress odours and corrosion) x treat solid waste products in gasification plants (e.g. through partial oxidation with O2 and further off-gas treatment as a low-emission alternative to waste incineration) x treat flame-resistant, harmful liquids (e.g. for environmentally friendly disposal through high-temperature combustion with O2) x produce chemical pulp from wood chips in paper manufacturing (e.g. for delignification, bleaching and black liquor oxidation, see Example J below) x enable oxidative treatment of drinking water (e.g. to remove iron, manganese, ammonia and organic substances, see Example K below) x produce ozone for hygiene and chemical purposes (e.g. for purifying drinking water, bleaching pulp, sterilisation, deodorisation or chemical synthesis) In engineering, food treatment and medicine, oxygen is used to: x substitute chemicals in meat and sausage preparation (e.g. O2 maintains a fresh, red colour) x preserve packaged food (e.g. O2 mixed with CO2/N2 for packaged lettuce) x oxygenate water in fish-farming basins or containers for transporting fish (e.g. O2 for aquaculture or emergency supply) x provide respiratory gas in aircrafts (breathing oxygen) x support and enable various medical applications (see Chapter 10) In other industry segments, oxygen is used to: x recover energy and raw materials (e.g. recycling in the pulp and plastics industry) x improve burnout, slag liquefaction and emission values (e.g. in waste incineration sites) x increase the performance of melting devices and emission control units (e.g. application of oxy-fuel burners or oxygen lances in the glass industry, see Example L below) x improve finish on industrial products (e.g. by fire-polishing glassware, see Example M below) x manufacture semiconductor components (e.g. highest purity O2 for thermal oxidation of silicon, in combination with a carrier gas for gaseous diffusion)

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x impel rockets and missiles in the aerospace industry (O2 as an oxidant for spacecraft fuels) Example A: Oxy-fuel Cutting

In industrial applications, oxy-fuel cutting is the predominant flame process for cutting mild- and low-alloy steels (see Fig. 2.33). With the right gases, a good torch and a steady hand, this process enables an experienced operator to cut steel anywhere, as it does not require electricity or complicated equipment. Although developed at the beginning of the last century, the basic processes still apply today. Oxy-fuel cutting is an intensive combustion process that involves steel being preheated to ignition temperature. This is achieved using an oxy-fuel gas flame. An oxygen jet then burns a narrow section of the metal at the point where the operator wants to make the cut. The jet also removes the molten combustion products (slag) from the cut. The purity of oxygen is a key factor in determining cutting speed. High-purity oxygen increases productivity. The initial quality of the O2 must therefore be maintained along the pipes and metering device until the point of use. The fuel gas influences the quality of the cutting process, the preheating time and the thickness of the material that can be cut effectively. Flame temperature is an important factor when a work piece has to undergo rapid, concentrated heating and preheating (see Fig. 2.34). In this case, the higher the flame temperature, the more heat is transferred to the work piece. In comparison with other fuel gases, the use of acetylene allows the fastest preheating of steel for cutting holes, and the highest cutting speed even for rusty, scaled or primered sheets.

Fig. 2.33 Oxy-fuel cutting.

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Fig. 2.34 Flame temperatures: Advantages of the oxy-acetylene flame.

Oxy-fuel cutting is a versatile process that can be deployed for a wide range of applications, including straight cuts, bevelling or weld-edge preparation using multiple torches simultaneously. Another benefit of this method is that it can easily be automated. Although the process itself looks very easy, handling fuel gases and oxygen actually requires considerable expertise and familiarity with the equipment and relevant safety requirements. Example B: Soldering and Brazing

Soldering and brazing are used to join two metal parts by means of a filler metal that has a lower melting point than the base metal. The joint may consist of one or more metals. The filler metal is normally distributed in the joint by capillary action, with additional flux dissolving unwanted oxides. The necessary heat can be provided from a number of sources, including furnaces or fuel gas torches. In soldering, metals are joined by a filler metal with a melting point below 450 °C. Soldering is used for copper and its alloys, zinc, steel, aluminium and its alloys.

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Fig. 2.35 Mechanised brazing process.

The most common filler metals are tin-based, although special filler metals are required for aluminium. Soldering produces weaker joints than brazing. However, alloying filler metals with silver considerably increases joint strength. Due to the relatively low temperature of the flame, soldering can be performed using simple propane/air burners. The oxy-acetylene process, however, may be a better option for more demanding soldering tasks. In brazing, filler metals have melting points between 450 and 1000 °C. So more powerful torches tend to produce better results here. The oxygen source (O2, compressed air or aspirated air) affects the heating method for the work piece. Acetylene flames cause the surface temperature of a piece to increase more quickly than flames generated by other fuel gases. The reducing effect of acetylene-compressed air flames helps dissolve oxides, supporting the flux applied to the surface of the brazing zone. Braze welding (see Fig. 2.35) uses filler metals such as brass or bronze, which are not distributed in the joint by capillary action. The benefits of the flexible oxyacetylene process make this a frequent choice here. Braze welding produces very strong joints in steel and copper. It is widely used to repair cast parts. Example C: Flame Spraying

Spraying is an interesting surface technology that is growing steadily. During this process, a gas flame and coating material are used to create an optimal surface on a substrate (see Fig. 2.36). It is a simple, fast and profitable procedure that can increase a substrate’s resistance to wear, corrosion and heat. It is also possible to increase or decrease friction or change a surface’s electrical properties. Damaged surfaces can be repaired and faulty parts corrected.

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Fig. 2.36 Flame spraying with metal wire.

In flame spraying, the material used for coating (a wire, rod or powder, for instance) is heated in a gas flame until it melts. The molten particles are then transferred to the substrate by the gas jet, and the new surface is generated. In most cases, the surface can be post-treated without difficulty. Any material can be used to spray a substrate in flame spraying, and any solid substrate can be coated. The fuel gas used depends on the melting temperature of the coating material. This gas also influences the speed at which the gas jet propels the molten coating particles and, as a result, how they adhere to the work piece. Oxygen is the second combustion component in flame spraying. Only an oxy-acetylene flame can provide the required temperature and efficiency (at a neutral flame setting) to coat high-melting materials such as molybdenum. Other fuel gases, including ethylene, hydrogen, propylene, or propane, are suitable alternatives for low-melting materials. In the case of powder flame spraying, a propellant gas (e.g. air, nitrogen, argon) can be used to further accelerate the powder particles. Example D: Recycling of Contaminated Scrap (Oxygen Lancing)

Melting contaminated scrap is a challenging task for an air-fuel combustion system. The metals recycling industry (e.g. aluminium, copper, lead) is forced to recycle even highly contaminated scrap to be competitive and protect the environment. Oxygen lancing improves the combustion efficiency of air-fuel systems and reduces atmospheric emissions. This technology injects oxygen primarily to burn off feedstock contamination (e.g. oil, paint, organics, hydrocarbons).

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Fig. 2.37 Oxygen lances convert feedstock combustibles into energy.

Fig. 2.38 Oxygen lances and oxy-fuel burners in a double-pass rotary furnace.

The benefits of oxygen lancing include: x x x x x

Fuel savings Reduced and controlled emissions Enhanced feedstock variations (contaminated scrap, low-grade scrap) Simple installation Increased production

While oxygen lancing (see Fig. 2.37) achieves elimination rates for feedstock combustibles of up to 99%, air-fuel combustion can only achieve rates between 50 and 80%. Oxy-fuel burners combined with oxygen lancing (see Fig. 2.38) are also very efficient at reducing volatile organic combustibles (VOCs) in the flue gas stream. Example E: Universal Rotary Tiltable Furnace (URTF)

Aluminium and dross recycling is a challenging task with regard to productivity and metal yield. Fixed-angle rotary furnaces require specific charging, tapping and cleaning times. Although fluxes are needed to maximise metal recovery, they create environmental issues and increase costs. The URTF (see Fig. 2.39) has been developed to set new, highest standards in recycling aluminium and dross.

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It provides the following benefits: x x x x x x

Improved metal yields Reduced process-cycle times Higher productivity Fuel savings Reduced feedstock grades Reduced costs for fluxes and disposal (see Fig. 2.40)

The URTF employs the dry salt process and therefore optimises the energy required by the melting cycle. Oxy-fuel combustion can also be combined with oxygen lancing.

Fig. 2.39 Diagram of a URTF with oxygen lancing technology: The URTF delivers unique process technology for recycling contaminated metals.

Fig. 2.40 URTF at Stena Aluminium, Sweden: The URTF reduces costs for fluxes and disposal.

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Example F: Reheating and Annealing Furnaces

A wide range of oxy-fuel solutions are used in reheating and annealing furnaces. These are integrated in rolling mills, processing lines and forges, for example. When industrial-grade oxygen is combusted with either gaseous, liquid or solid fuels in a furnace, it generally optimises thermal efficiency and fuel burnout. Thanks to the higher partial pressure of O2 and the exclusion of nitrogen ballast, reactivity in furnaces improves and flue gas volume decreases (see Fig. 2.41). At the same time, the partial pressure of H2O and CO2 in the flame rises, which in turn causes radiation and heat transfer to increase. Typical improvements compared with air/fuel systems include: x up to 50% more furnace capacity x up to 50% less specific fuel consumption x up to 20% less flue gas volume

Fig. 2.41 Comparison of burner systems for reheating and annealing. Top: Oxy-fuel flame: Optimum combustion and heat transfer, focussed flame. Centre: Air-fuel flame: Dilution and cooling by N2 ballast, reduced efficiency. Bottom: Flameless oxy-fuel: Dilution with furnace flue gases, reduced NOx emissions.

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Fig. 2.42 Flameless oxy-fuel at Outokumpu Stainless, Degerfors, Sweden. The Outokumpu plate mill implemented oxy-fuel technology in its existing walking beam furnace. The stipulated performance guarantee resulted in a 40–50% rise in furnace heating capacity, a 25% drop in specific fuel consumption, more uniform temperatures for correct rolling and a drop in NOx emissions to below 70 mg/MJ.

This also enhances process flexibility and reduces CO2, NOx and SO2 emissions, depending on the equipment and type of fuel used. Industrial gas companies have over 40 years of experience in the field of industrial combustion and heating. They provide technical solutions for reheating and annealing furnaces that cover: x x x x

in-depth analysis of customer processes process and equipment engineering furnace revamping (including combustion technology and equipment) installation, startup, monitoring and optimisation

The technology deployed is usually patent protected, as is the case with REBOX® solutions (see Fig. 2.42). This cutting-edge technology comprises “Direct Flame Impingement” and “Flameless Oxy Fuel”. Example G: Oxygen Enrichment in Claus Plants

Oxygen enrichment helps refineries produce clean fuels. Claus plants use oxidation to remove hydrogen sulphide (H2S) and ammonia (NH3), for instance. Increased levels of ammonia are a by-product of producing clean fuels. Ammonia is created when crude oil is severely hydrogenated to convert organically bound sulphur into H2S. During this process, some of the nitrogen reacts to form ammonia. H2S and NH3 are transported to a refinery’s Claus plant, which then processes H2S into elemental sulphur. A Claus plant can also remove pollutants, in particular, by decomposing ammonia. When a Claus plant’s H2S and NH3 load increases,

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Fig. 2.43 The OXYMIX® oxygen injector (Linde) and the O2 concentration: O2 concentration in the process air is calculated using the computational fluid dynamics (CFD) method.

Fig. 2.44 The FLOWTRAIN® control system (Linde): The system is used to meter O2 feed into Claus plants and fulfils safety requirements.

bottlenecks may occur. Oxygen enrichment can be applied to overcome this problem. Oxygen can be injected into the pipe that carries combustion air to the Claus furnace, for example. This simple method of applying oxygen is mainly used for applications requiring oxygen enrichment levels of up to 28 vol.% (see Fig. 2.43). For higher O2 levels, oxygen is usually injected directly into the burner. Another crucial piece of equipment is the control and safety cabinet (see Fig. 2.44). This is required to regulate the oxygen injected into the combustion air or burner. Figure 2.45 shows the capacity increases that can be achieved in Claus plants through oxygen enrichment. Given a constant sulphur load, the total gas flow decreases as oxygen enrichment increases, due to reduced levels of N2 in the process air. The resulting drop in pressure can be compensated by increasing the amount of feed gas, which in turn increases the Claus plant’s capacity.

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Fig. 2.45 Claus plant capacity as a function of oxygen enrichment.

Example H: Fluid Catalytic Cracking (FCC) with Oxygen

FCC plants are used to convert vacuum gas oil (often mixed with residues from atmospheric distillation, vacuum distillation and visbreaking) into lighter hydrocarbon fractions. The products are a gas fraction (primarily C3/C4), a liquid fraction (primarily gasoline) and solid coke. The coke is deposited on the catalyst and burnt off during catalyst regeneration (see Fig. 2.46).

Fig. 2.46 Overview of an FCC process.

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Fig. 2.47 Test results from an experimental FCC plant.

Oxygen enrichment boosts the regeneration stage and offers the following benefits: x Increased capacity of entire plant x Greater flexibility, particularly with regard to selecting feedstock x Possibility to use heavier feedstock with a greater tendency to form coke x Improved conversion rate and gasoline yield (see Fig. 2.47) x Reduction in by-products x Less catalyst abrasion and erosion of cyclones during catalyst separation thanks to smaller gas streams, resulting in lower gas velocity Example I: Waste-Water Treatment – Aerobic Purification

Many problems in waste-water treatment – both in municipal and industrial plants – are caused by oxygen deficiency. The consequences include inadequate purification or even anaerobic decomposition processes, causing offensive odours. The systematic input of pure oxygen at critical points of the waste-water chain has provided a lasting solution in many cases. SOLVOX® processes (see Fig. 2.48) rely on various O2 transfer principles that input oxygen fast, efficiently, precisely and flexibly. SOLVOX® supports the performance of permanent aeration devices and serves as a preliminary treatment process (e.g. for seasonal campaigns). A high oxygen concentration can be achieved safely and maintained according to individual requirements. The benefits of pure oxygen and SOLVOX® processes include: x Low investment and maintenance costs x Optimum use of O2 and maintenance of ideal O2 concentrations x Systematic and flexible O2 input with low operational costs x Prevention of extensive construction work and downtime x Improved plant performance through implementation of rapid, cost-effective measures

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Fig. 2.48 SOLVOX®-V process (Linde).

Fig. 2.49 SOLVOX®-V oxygenation unit (Linde).

The SOLVOX®-V process, for example, relies on a venturi system (see Fig. 2.49) and is used for pre-treating and oxygenating waste water. It is very easy to install and can even be lowered into full tanks with no drainage required. Example J: Oxygen Delignification – Paper Manufacture

Oxygen delignification can be regarded partly as a continuation of the pulp cooking process and partly as the first stage in bleaching (see Fig. 2.50). It has become a standard step in the production of bleached chemical pulp during which lignin, the coloured substance in kraft pulp, is removed. Using oxygen delignification not only cuts the cost of chemical pulp production, but also reduces environmental impact. During oxygen delignification, pulp is treated with oxygen in a pressurised vessel at high temperatures in an alkaline environment. Delignification may vary from 40 to 70%, depending on the wood used as a raw material and whether one or two reactors are used in series.

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Fig. 2.50 Oxygen delignification installation.

Unbleached kraft (sulphate) pulp has a lignin content of 3 to 5% which, after oxygen delignification, can be decreased to approximately 1.5%, or a kappa value of 8 to 10 (indicator of residual lignin). The benefits of oxygen delignification include: x Additional delignification after cooking with less destruction cellulose x Reduced emissions from bleach plants and lower consumption of bleaching chemicals x Higher brightness ceiling in a given bleaching sequence x Lower shives and extractives content x Consistent pulp strength x Easier system closure Example K: Drinking Water – Oxidative Conditioning

High-quality drinking water is one of life’s essentials, which is why it is subject to stringent statutory requirements. However, low-quality raw water increasingly has to be used as a basis for recovering this high-quality product. In Europe, for instance, strict EU directives determine the parameters and limits for many possible substances in water. The following concentrations of the heavy metal ions iron and manganese as well as unhygienic ammonium ions must not be exceeded: x Iron (II): 0.20 mg/l x Manganese (II): 0.05 mg/l x Ammonium: 0.50 mg/l Raw water usually has to be conditioned to ensure compliance with these statutory limits (see Fig. 2.51). Industrial gases provide a range of different treatments

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Fig. 2.51 Process diagram: Pure oxygen in drinking water treatment.

for this. Applying oxygen either allows precipitation and filtering of unwanted substances or converts them into harmless products. The benefits of oxygen (in comparison with air) include: x Increased plant capacity and chemical reaction rate (grade of contaminant elimination) x Prevention of clogged filters by degassing nitrogen (extended uptime, lower operating costs) x Closed systems with one-step pressurising (hygiene and energy aspects) Example L: Oxygen-Enhanced Combustion – Glass Manufacture

Glass manufacturers are constantly under pressure to enhance production, extend furnace life, improve glass quality and meet increasingly strict emission regulations. Oxygen has been used for many years to successfully overcome these challenges. All oxygen-enhanced combustion processes are based on the full or partial replacement of air through oxygen. They increase flame temperature by eliminating nitrogen and increasing oxygen concentration. At the same time, they raise concentrations of CO2 and H2O in the vicinity of the flame for upgraded thermal radiation. Both effects boost heat transfer in the furnace and significantly improve glass quality and furnace performance. Other effects are described below. Oxygen enrichment: This is the most basic way of using oxygen in glass melting applications. Enrichment is typically used in furnaces nearing the end of their campaigns and suffering from regenerator plugging or impending collapse.

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Fig. 2.52 Flames from vertical crown burners (CGM™ system).

Oxygen lancing: This is also used for enrichment, but is typically associated with more severe or complex conditions. Lancing involves the precise injection of oxygen at the point where it is most needed. Enrichment is not as accurate and is less efficient than selective lancing, which injects more total oxygen. Oxy-fuel boosting: In contrast to enrichment and lancing, boosting involves the addition of an oxidant and a fuel. Boosting technologies can be used to recover glass furnaces and extend furnace life. They are also designed to increase furnace throughput and/or improve quality. CGM™ boost technology (see Fig. 2.52) is an example of a boosting solution.

Fig. 2.53 Energy consumption of air fuel and oxy fuel installations as a function of the flue gas temperature and air preheating temperature.

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All oxy-fuel (AOF) fired melting: As one of the most popular oxygen applications, AOF fired melting eliminates the need for a combustion air preheater or heat recovery device. This process is one of the most efficient ways of reducing nitrogen oxides (NOx) and particulate emissions from glass furnaces. It also reduces energy consumption significantly. Figure 2.53 shows that increased air preheating boosts fuel savings (conventional technology). However, fuel costs can be reduced further by using advanced oxygen systems (AOF technology). Figure 2.54 shows fuel on AOF installation with two types of burners. Burner technology: Customers can choose from a large variety of burners (see Fig. 2.55). These are based on the ‘tube-in-tube’ principle, whereby the fuel jet is conically shielded by an oxygen beam. The flame can be adapted to furnace and melting requirements by changing the velocity of each partner. Where necessary, oxygen can be injected in stages to increase flame length (‘flame staging’). The benefits of oxygen/AOF systems in glass manufacture include: x Reduced emissions (e.g. NOx and particles) x Improved heat transfer (with enhanced glass quality) x Lower capital costs (no air preheating necessary, smaller filter unit, smaller furnace) x Increased productivity (faster melting, higher throughput, extended furnace life) x More stable process (better control over operations, smaller process variations) x Extended useful life (e.g. replacement of combustion air at the end of a process cycle) x Energy savings (see graph above)

Fig. 2.54 Typical All Oxy Fuel installation including combustion equipment.

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Fig. 2.55 Typical oxy-fuel burners used for Oxy Fuel Boosting and All Oxy Fuel installations (Linde).

Application M: Fire-Polishing Glass

Tableware and flacon manufacturing is an important part of the glass industry. In addition to cost reduction, modern glass production must also: x x x x

Deliver optimum glass quality (particularly surface quality) Make machined glass resemble hand-made glass Eliminate acid-polishing Eliminate further mechanical processing

Advanced fire-polishing and fusing can fulfil these requirements. Flames from specially designed burners impinge the glass directly to remelt a thin surface layer. This process can be used to eliminate or reduce the need for etching, thus decreasing the environmental impact and paving the way for cleaner, safer working environments. Traditional post-mixing burners have at least two inlets, one for a fuel gas and one for oxygen and/or air. The fuel and oxidant are fed separately through the burner and only mixed once they have passed their respective exit nozzles. Combustion takes place in a second step. Modern premixing burners function differently. The fuel gas and oxygen/air are mixed in an upstream section. This gas mixture is then delivered directly to the burner, which has just one inlet. Combustion takes place immediately once the mixture has passed the joint nozzle system. Figure 2.56 shows the difference in performance between the two systems. The pre-mixing option provides a number of benefits regardless of the gas used. Furthermore, the heat-transfer rate of traditional air-fuel burners is approximately 50% of the rate associated with comparable oxy-fuel burners. Figure 2.57 shows different pre-mixing burners for oxy-fuel application. Figures 2.58 and 2.59 show typical fire-polishing installations.

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Fig. 2.56 Axial distribution of heat transfer for oxy-hydrogen and oxy-methane flames.

Fig. 2.57 Typical designs for pre-mixing HYDROPOX® burners.

The benefits of pre-mixing oxy-fuel burners for fire-polishing glassware include: x Removal of all pressing burrs and sharp edges x Significantly higher brilliance compared with acid polishing x Removal of cold waves x No deformation of glassware due to short polishing times and precise heating points x Ability to polish decorative surfaces with deep relieves x Ability to polish thick-walled glass parts internally and externally

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Fig. 2.58 Fire polishing with HYDROPOX® following the forming process.

Fig. 2.59 Total removal of burrs and other consumer-relevant flaws with HYDROPOX®.

2.5.3 Applications of Argon

Although the noble gas argon is mainly used for its low reactivity (as is the case with nitrogen), it can provide higher inertness than nitrogen as well as lower thermal conductivity and improved solubility in water and oils. Argon is more common and cost-effective than the noble gases helium, neon, krypton and xenon. Argon is usually applied in its gaseous state.

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In the processing industry, welding and engineering, argon is used to: x perform tungsten inert gas (TIG) welding with a non-consumable tungsten electrode and an additional filler metal (e.g. with the shielding gases argon (Ar), argon/helium (Ar/He), argon/hydrogen (Ar/H2), see Example A below) x perform metal inert gas (MIG) welding of non-ferrous metals with a consumable electrode (e.g. aluminium with inert gases such as Ar, Ar/He, see Example B below) x perform metal active gas (MAG) welding of construction steel or galvanized sheet metal with a consumable electrode and active gases such as argon/carbon dioxide (Ar/CO2) or argon/oxygen (Ar/O2) (see Example B below) x perform MAG welding of stainless steel with admixtures of CO2, O2 and He (to improve welding speed and weld appearance, see Example B below) x perform MAG high-performance welding with shielding gas mixtures based on Ar/He with admixtures of CO2 or O2 (see Example B below) x perform laser cutting with pure argon (e.g. to cut metals such as titanium that would otherwise react with N2/O2) x purge receptacles of stainless steel or reactive metals (e.g. titanium, tantalum, zirconium) before welding (mostly in mixture with hydrogen, compare root shielding and use of nitrogen) In metallurgy, foundry technology and the steel industry, argon is used to: x perform large-scale metallurgical processes with additional oxygen injection (e.g. for controlled decarburisation of cast iron and stainless steel melts with the argon-oxygen-decarburisation (AOD) process) x purge and stir metal melts by bubbling them through porous bed stones or lances (recirculation, discharge of gases and slag) x feed powdered alloy components into steel and other metal melts (injection of calcium or magnesium compounds with an argon jet for desulfurisation) x atomise molten metals with high-pressure jets for producing high-quality metal powders (powder metallurgy) x sinter metallurgical powders under high pressures and temperatures (e.g. hot isostatic pressing (HIP)) x heat or anneal stainless steel (e.g. in furnaces, to avoid nitrogen uptake) x prevent oxidation of hot, light metal alloys (e.g. when die casting aluminium or magnesium) In the lighting and electronic industry, argon is used: x to fill incandescent light bulbs (partly mixed with nitrogen to protect the tungsten filament and prolong bulb life) x to fill fluorescent lamps and luminous electric discharge tubes (with a small amount of neon to create the characteristic blue light) x to protect electric equipment during storage, operation and shutdowns (e.g. rectifiers and overload protection switches) x as a base gas in detectors that measure radioactivity (e.g. Geiger counters)

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In the semiconductor industry, high-purity argon is used to: x grow monocrystalline silicon (e.g. for manufacturing wafers or photovoltaic cells) x manufacture semiconductor components (e.g. by supporting processes such as diffusion, epitaxy, chemical vapour deposition (CVD) or ion implantation) In other industries, argon is used to: x insulate energy-efficient windows (e.g. double glazing, see low thermal conductivity) x protect foodstuffs and beverages during treatment and storage (e.g. modified atmospheres) Example A: Tungsten Inert Gas (TIG) Welding

In addition to aluminium and magnesium, TIG welding is used to weld stainless steel as well as carbon- and low-alloy steel. It is mainly used to weld thin metals (under 6 mm in thickness). In TIG welding, an electric arc is used to heat and melt the material. The electric arc burns between the burner electrode and the work piece (see Fig. 2.60). The shielding gas flows through a gas nozzle positioned concentrically around the electrode. The shielding gas is primarily used in TIG welding to protect the hot and molten parts of the work piece, the filler metal and the electrode from the harmful influence of the surrounding air. Shielding gas also affects the characteristics of the arc and the appearance of the weld. Argon and helium (or mixtures of these gases) are typical shielding gases used here. Hydrogen or nitrogen may also be beneficial under certain conditions. TIG welding is typically used for welding pipes, pressure vessels and heat exchangers. Since it can be used to weld thin metals and small objects, the process is also used in the electronics industry. TIG welding ensures very high weld quality, with-

Fig. 2.60 TIG welding.

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out slag and with very little spatter. It is an extremely versatile method, suitable for most weldable materials and any welding position and joint configuration. Example B: Metal Inert Gas (MIG)/Metal Active Gas (MAG) Welding

MIG and MAG are the most common welding methods. The high levels of productivity offered by these methods and the fact that they are simple to automate have contributed to their popularity. The methods together are often referred to as gas metal arc welding (GMAW). During MIG/MAG welding, a metallic wire is fed through the welding gun and melted in an electric arc (see Fig. 2.62). The wire acts as both the current-carrying electrode and the weld metal filler wire. Electrical energy is supplied from a welding power source. A shielding gas that flows through the gas nozzle protects the arc and the pool of molten material. The shielding gas is either inert (MIG) or active (MAG). An inert gas such as argon or helium does not react with the molten material. Active gases, on the other hand, participate in the process between the arc and the molten material and can stabilise and broaden the arc. An example of an active gas here is argon containing a small proportion of carbon dioxide or oxygen. Ionisation energy is an important property of gases (see Fig. 2.61). It primarily determines the temperature of the electric arc and, therefore, the attainable heat transfer and welding speed. Helium has considerably higher ionisation energy than argon, which is why helium is often added to argon to improve the process performance. Helium also affects the weld pool and improves the penetration profile (bead profile) of the weld.

Fig. 2.61 Dissociation (dark grey bar) and ionisation (light grey bar) energy of gases.

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Fig. 2.62 MIG/MAG welding.

References [2.1] [2.2] [2.3] [2.4] [2.5] [2.6] [2.7] [2.8] [2.9] [2.10] [2.11] [2.12] [2.13] [2.14] [2.15] [2.16] [2.17] [2.18] [2.19] [2.20] [2.21] [2.22] [2.23] [2.24] [2.25] [2.26] [2.27]

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Ullmann’s, 6th edition, 23, p. 175, Wiley-VCH, Weinheim, 2003. Römpp, 10th edition, Keyword: Stickstoff, Thieme Verlag, Stuttgart, 1996. Ullmann’s, 6th edition, 23, p. 178, Wiley-VCH, Weinheim, 2003. Römpp, 10th edition, Keyword: Stickstoff-Fixierung, Thieme Verlag, Stuttgart, 1996. Römpp, 10th edition, Keyword: Sauerstoff, Thieme Verlag, Stuttgart, 1996. Ullmann’s, 6th edition, 24, p. 562, Wiley-VCH, Weinheim, 2003. Hollemann-Wiberg: Lehrbuch der Anorganischen Chemie, 101st edition, W. de Gruyter, Berlin, 1995, pp. 508–510. Römpp, 10th edition, Keyword: Argon, Thieme Verlag, Stuttgart, 1996. Ullmann’s, 6th edition, 23, p. 227, Wiley-VCH, Weinheim, 2003. H. Hausen: Zeitschr. tech. Phys. 1932, 13 (6), 271–277. D. R. Paul, Y. P. Yampolskii (Eds.): Monography on polymeric gas separation membranes: Polymeric Gas Separation Membranes, CRC Press, Boca Raton, Fla., 1994. Ullmann’s, 6th edition, 21, p. 243, Wiley-VCH, Weinheim, 2003. E. Staude: Membranen und Membranprozess, VCH, Weinheim, 1992. State of the art 1990 are membrane fibres with properties as listed: P. S. Puri, Gas. Sep. Purif. 1990, 4, 29. J. Membrane Sci. 2001, 193, 1–18. Sep. Purif. Technol. 2002, 28, 29–41. M. Grahl, P. Leitgeb: Oxygen Production by Pressure Swing Adsorption, MUST‚ 96, Munich Meeting on Air Separation Technology, p. 135. Nitrogen Production Based on Pressure Swing Adsorption, 96 Munich Meeting on Air Separation Technology, p. 185. G. Beysel, P. Leitgeb, G. Scholz: LINDE Reports on Science and Technology 1988, 44, 3–7. J. G. Stichlmair, J. R. Fair: Distillation, Wiley-VCH, New York, 1998. Kryogene Argongewinnung, EP 377117 B2, US 5019145, Inventors: H. Corduan, W. Rohde, LINDE AG. E. Blass: Entwicklung verfahrenstechnischer Prozesse, Springer-Verlag, Berlin, 1997. K. Stephan, F. Mayinger: Thermodynamik, Vol. 1, 14th edition, Springer-Verlag, Berlin, 1992. Ullmann’s, 6th edition, 23, p. 175, Wiley-VCH, Weinheim, 2003. H. Hausen, H. Linde: Tieftemperaturtechnik, 2nd edition, Springer-Verlag, Berlin, 1985. W. Kast: Adsorption aus der Gasphase, VCH, Weinheim, 1988. S. Weiß (Hrsg.): Verfahrenstechnische Berechnungsmethoden, Deutscher Verlag für Grundstoffindustrie, Stuttgart, 1996.

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References

109

[2.28] W. Diery, LINDE Reports on Science and Technology 1988, 44, 72–82. [2.29] The standards of the brazed aluminium plate-fin heat exchanger manufacturers’ association. www.alpema.org. [2.30] K. Sattler: Thermische Trennverfahren, 2nd edition, VCH, Weinheim, 1995. [2.31] Ullmann’s, 6th edition, Wiley-VCH, Weinheim, 2003. [2.32] D. Igkesia, J. Spivey, H. Fleisch (Eds.): Studies in Surface Science and Catalysis 136: Natural Gas Conversion VI, Proceedings of the 6th Natural Gas Conversion Symposium, Elsevier, Amsterdam, 2001, pp. 45–56. [2.33] Borsig Taschenbuch, Deutsche Babcock- Borsig Aktiengesellschaft, Berlin, 1994. [2.34] R. Smith: Chemical Process Design, McGraw-Hill. Inc. New York, 1995. [2.35] Compressed Gas Association, CGA G – 4.8.2000: Safe use of aluminium-structured packing for oxygen distillation, 2nd edition, Chap. 7.2, 2000, CGA, Arlington, VA, USA. [2.36] B. R. Dunbobbin, J. G. Hansel, B. L. Werley: Oxygen Compatability of High Surface Area Materials, in Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Vol. 5, ASTM STP 1111, 1991, pp. 338–351. [2.37] R. Zawierucha, J. F. Million, S. L. Cooper, K. McIlroy, J. R. Martin: Compatibility of Aluminium Packing with Oxygen Environments Under Simulated Operating Conditions, in Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Vol. 6, ASTM STP 1197, 1993, pp. 255–275. [2.38] R. Zawierucha, J. F. Million: Promoted Ignition-Combustion Tests of Brazed Aluminum Heat Exchanger Samples in Gaseous and Liquid Oxygen Environments, in Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Vol. 9, ASTM STP 1395, 2000, pp. 373–383. [2.39] Oxygen Pipeline Systems, EIGA, IGC Doc 13/02/E. [2.40] K. Nabert, G. Schön: Sicherheitstechnische Kennzahlen brennbarer Gase und Dämpfe, 2nd edition 1963, 6th suppl. 1990, Deutscher Eichverlag, Braunschweig. [2.41] E. J. Miller, S. R. Auvil, N. F. Giles, G. M. Wilson: Air Products – Presentation for the 12th Intersociety Meeting, March 5–9, 2000, Atlanta, GA. [2.42] L. Vegard: Naturwissenschaften 1931, 19, 443. [2.43] Linde in-house report, 2000.

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3 The Noble Gases Neon, Krypton and Xenon 3.1 History and Occurrence

In 1897/1898 W. Ramsay found neon, krypton and xenon (Ne, Kr, Xe) by liquefying and fractionating air according to the C. v. Linde method. Neon discovered due to its red spectral lines derives its name from the Greek word “neos” = new, krypton from the Greek word “kryptos” = hidden and xenon from “xenos” = guest, stranger [3.1]. The first neon tubes filled with a helium/neon mixture were manufactured by the “Chemische Fabrik Griesheim-Elektron” in 1913. Up to the middle of the last century the demand for pure neon was quite low, demonstrated by a quantity of 10 L liquid neon shipped in 1960 for the first time. Worldwide, the same concentrations are found in the air: neon is contained with 18 vppm, krypton with 1.14 vppm and xenon with 0.086 vppm (see Table 2.1). Except helium, which occurs enriched in some natural gas sources, all other noble gases are extracted from air. In view of the low concentrations of neon, krypton and xenon large quantities of air have to be processed in order to produce these noble gases on an industrial scale. Radon (Rn) as the heaviest and radioactive noble gas develops from radioactive decay processes and occurs in extremely small traces in the air (6 · 10–14 vppm). In medicine, radon serves as D-source in cancer therapy.

3.2 Physical and Chemical Properties

Neon, krypton and xenon together with He, Ar and Rn belong to the 8th main group of the periodic table of elements (noble gases) and at atmospheric pressure they are colourless, odourless, non-combustible, monoatomic gases. The outer electron shell is completely filled (noble gas configuration), responsible for the inert character of noble gases. Due to their inertness noble gases are used as filling gases for light bulbs. The larger the atom, the more easily polarizable is the electron shell, resulting in stronger interatomic forces (van-der-Waals-forces).


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3 The Noble Gases Neon, Krypton and Xenon

Neon boils at –246.1 °C, krypton at –153.4 °C and the heavy xenon at –108.2 °C at 1 bar [3.2]. Solid xenon at –241 °C and 330 000 bar conducts the electric current like a metal. 1 L of water dissolve 10.5 mL of neon at 20 °C. For a long time it was believed that noble gases are not able to form compounds. In 1962, the American N. Bartlett was the first to synthesize a compound of the composition “XePtF6” by a reaction of xenon with PtF6. In the same year, Hoppe succeeded in synthesizing the first binary noble gas compound XeF2. Noble gas compounds can be subdivided into three types: (1) short-living noble gas containing molecules, (2) molecules with covalent bonds, (3) inclusion compounds and clathrates (e.g. noble gas hydrates). The thermodynamic stability of the noble gas halogenides complies with the following rule: The larger the atomic mass of the noble gas and the smaller the halogen, the more stable is the compound: XeF2 > KrF2 and XeF2 > XeCl2 > XeBr2. Additionally the stability of noble gas compounds increases with the decreasing oxidation stage of the noble gas atom. Apart from covalent xenon fluorides and oxides, krypton fluorides are known, too. Except for the xenon fluorides, all other noble gas compounds are endothermal, thus decompose into their elements under release of energy. Chemical compounds of neon as for the next heavier noble gas argon are not known [3.3].

3.3 Recovery of Krypton and Xenon

Krypton and xenon, which have a concentration of 1.138 ppm and 0.086 ppm in the atmospheric air, are mainly recovered as secondary products from air separation. The worldwide demand increases annually. In 2001, the global annual Xe-production amounted to about 6800 mN3, the Kr-production to about 67 000 mN3. Owing to its low concentration in the air the xenon price is high with typical 4000–8000 € per mN3 and is strongly fluctuating. The price for krypton, which occurs about 13 times more frequently in the air than xenon, is lower by about this factor. Typically the noble gases Kr and Xe are recovered in two steps: In the first step, a pre-enrichment of about 4000 ppm of Kr and 400 ppm of Xe in liquid oxygen is achieved in the sump of an additional column (4), (Fig. 3.1). This column is integrated into the air separation unit. In a second step, the products are extracted from this pre-concentrate in a unit, operating independently from the air separator. The production rate of this preenriched liquid is low. X Example: Separation of 300 000 mN3 h–1 of process air. The Kr-yield shall be 85% and the Kr-purity in the pre-enriched liquid oxygen shall be 4000 ppm. Then the amount of withdrawn pre-concentrate is: 300 000 mN3 h–1 · 0.85 · (1.138/4000) = 73 mN3 h–1. Due to this low production rate, the pre-product is often stored in a tank, which is periodically transported to a central fine-purification plant. In order to keep

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transportation cost low, efforts are made to obtain high Kr and Xe concentration in the pre-product. 3.3.1 Pre-enrichment in the Air Separator

Figure 3.1 shows a typical process for Kr/Xe-pre-enrichment: Since the boiling temperatures of Kr and Xe are higher than that of oxygen (Table 3.1), these two components accumulate at the point of the highest oxygen concentration, i.e. at the bottom of the low-pressure column (2). From here liquid oxygen, containing most of the Kr/Xe from the processed air, is fed (a) to an enrichment column (4) and is evaporated in the sump of this column by means of a suitable heating medium (b), for example by means of gaseous air cooled down close to its dew temperature. Gaseous product oxygen (c) is withdrawn from the Table 3.1 Boiling temperatures (K) of components relevant for the Kr/Xe-extraction at a pressure of 1.013 bar. N2

Ar

O2

77.3

87.3

90.2

CH4

Kr

CF4

Xe

C2H4

N2O

C2H6

C2F6

SF6

C3H8

111.6 119.8 145.2 165.1 169.4 184.6 184.5 194.2 209.9 231.1

Fig. 3.1 Diagram of a two-column air separator with noble gas production. (1) Rectification column (pressure section); (2) Rectification column (low-pressure section); (3, 5, 7) Combined condenser – evaporator unit; (4) Kr/Xe enrichment column; (6) He/Ne enrichment column. (a, b, e, f, g, h, j) Internal process flows; (c) Product O2 gaseous; (d) Kr–Xe Primary Product liquid; (i) He–Ne Primary product gaseous. GOX = gaseous oxygen; LIN = liquid nitrogen.

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top and liquid oxygen (d), enriched with Kr/Xe, is withdrawn from the sump of this column. A second liquid oxygen fraction (e) is fed to the top of the enrichment column in order to reduce the Kr – content of the oxygen product (c) and thereby the krypton loss. This second fraction is extracted a few theoretical trays above the sump of the low-pressure column, where it has only a low concentration of Kr/Xe and CH4. The pre-enriched product (d) typically contains 85% of the krypton, which is fed to the air separation unit via the process air and about 95% of the xenon. The krypton yield is lower than the xenon yield, because Kr is more volatile than Xe (see Table 3.2). The Kr/Xe concentration in the sump of the enrichment column is controlled by the amount of withdrawn sump product (d). The smaller the flow, the higher will be the concentration. Thus the Kr/Xe-concentration could be theoretically increased up to the solubility limit of krypton (30%) and xenon (2%). The maximal admissible concentration is however determined by the solubility and the explosion limit of the hydrocarbon components, which accumulate in the sump product as well. The most prominent component is CH4, which is contained in ambient air with up to 5 ppm. Owing to its low volatility (Table 3.2), CH4 accumulates together with Kr/Xe in the liquid bath. The methane concentration in the Kr/Xe pre-product must remain well below its lower explosion limit of about 4.4%. In order to reduce the CH4 content in the Kr/Xe-pre-product, the enrichment column (4) is divided into an upper and lower section. This allows the discharge of a part of the methane, which enters the enrichment column via stream (a), over the top of the column together with the oxygen product. This is achieved by suitable adjustment of the liquid and vapour flow L und V in the two sections. A component is driven upwards in the column, whenever the vapour flow V is so large, that for the separation factor S the inequality S=K⋅

V >1 L

(3.1)

(see Section 2.2.5.5) holds. Here K is the component equilibrium factor K-value (see Table 3.2). The reflux (e) to the head of the enrichment column, which is poor of CH4 and Kr/Xe, is adjusted such, that methane is driven upwards and krypton Table 3.2 Component equilibrium factor of Kr, Xe and CH4 in liquid oxygen at 1.4 bar. CH4

Kr

Xe

0.29

0.11

0.0028

The component equilibrium factor K = y*/x of a component describes the mass transfer equilibrium between the liquid phase with the concentration x and the vapor phase with the concentration y* (see Table 2.8, Section 2.2.5.5).

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downwards. According to Equation (3.1), this is guaranteed if the liquid/vapourL ratio (L/V)top in the upper section lies in the interval K Kr <   < K CH4 , i.e.  V  top ranging within the limits of 0.11 to 0.29 according to Table 3.2. 3.3.2 Recovery of Pure Kr and Xe

Kr/Xe-fine-purification plants are available in numerous designs, combining the separation techniques cryogenic rectification, catalysis, adsorption, chemisorption or membrane separation. They may be operated continuously or, due to the small capacities, in batch mode. In the following, a continuous process will be described, the process steps of which are realized this way or similarly in a number of plants. The feed to the purification unit is liquid oxygen enriched with about 400 ppm of Xe and 4000 ppm of Kr. It contains numerous additional impurities (cf. Table 2.1), e.g. x hydrocarbons < 5000 ppm (concentration in units of the so-called CH4-equivalent) x Nitrous oxide (laughing gas), N2O < 200 ppm x Greenhouse gases CF4 (tetrafluoromethane), C2F6 (hexafluoroethane) and SF6 (sulphur hexafluoride) < 500 ppb High concentrations of CF4, C2F6 and SF6 in krypton and xenon are not tolerable for applications in the lighting industry and reduce the Kr/Xe-price. Table 3.4 summarizes typical admissible contaminations. The concentration of these greenhouse gases in the air is low and typically ranges between 0.1–100 ppt (1 ppt = 1 part/1012 parts). However, since the boiling temperatures of the above mentioned three components are similarly high as those of krypton and xenon (Table 3.1), they accumulate together with krypton and xenon. The potential accumulation of Kr and Xe with these greenhouse gases can be estimated from the ratio between the concentration of Kr/Xe in ambient air and the concentration of these greenhouse gases in ambient air. X Example: 100 ppt of CF4 and 1.138 ppm of krypton in the ambient air. With a cryogenic separation of krypton and xenon, the CF4 preferably accumulates in krypton owing to its volatility. Thus, the CF4-contamination of the krypton product amounts to 100 ppt/1.138 ppm = 88 · 10–6 = 88 ppm. According to the specification in Table 3.4 this concentration is too high. 3.3.2.1

Catalytic Combustion of Hydrocarbons

Since in the Kr/Xe enrichment steps, the hydrocarbons would enrich as well and would form an explosive mixture, they have to be removed first. This is done by combustion to water and CO2 in an exothermal reaction with oxygen on a Pd- or Pt-catalyst at a temperature of about 400–500 °C. To this end the cryogenic liquid

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crude product (d), Fig. 3.1, is pressurized to a supercritical state at 55 bar by means of a pump, heated to ambient temperature and then expanded to about 6 bar. The compression to the supercritical state prior to heating is required for safety reasons. If the evaporation would be performed at an undercritical pressure, an inadmissible hydrocarbon accumulation would occur in the evaporating liquid phase. The residual concentration of hydrocarbons after the combustion is small and typically < 1 ppm. However in subsequent steps, hydrocarbons will be enriched again. Thus, depending on the purities required, hydrocarbons may have to be further separated in the downstream rectification. N2O is also converted on the catalyst into oxygen and nitrogen, whereas the thermally stable greenhouse gases CF4, C2F6 and SF6 are only partially decomposed. The hot gas from the catalyst’s outlet is being cooled and its water and CO2 content, resulting from the combustion, is removed in an adsorber. An alternative to the combustion of hydrocarbons in the oxygen-rich crude product is to replace oxygen by nitrogen in this product. Hydrocarbons, embedded into nitrogen can be concentrated higher without exceeding safety limits. The exchange of oxygen against nitrogen takes place in a cryogenic stripping column. This process is especially advantageous in connection with an ensuing, purely cryogenic rectification, as the heating necessary for the catalysis and the following cooling down to cryogenic temperatures is not required. Such a plant was built by the Linde AG in Duisburg in 2001 [3.4]. 3.3.2.2

Cryogenic Separation

In the following a process will be described that purifies krypton and xenon by means of cryogenic rectification in five columns. The feed gas is cooled before it is fed to the first column. It contains mostly oxygen with typical 4000 ppm of Kr and 400 ppm of Xe and additional impurities of hydrocarbons and greenhouse gases with concentrations below 1 ppm. Figure 3.2 shows schematically the sequence of the five columns for the isolation of Kr/Xe. In this figure, the multi-component mixture to be separated is characterized in a simplified way as a six-component mixture (A, Kr, B, C, Xe, D). The pseudo component A combines all components that are more volatile than Kr, i.e. mainly O2 und CH4. B and C represent the components with volatilities in between those of the key components Kr and Xe and D summarizes the components less volatile than Xe. In Column “1” oxygen and CH4 is separated to the top of the column, while Kr/Xe together with all other low volatile components collect in the bottom fraction. A strong enrichment of all low volatile components occurs in this bottom fraction. With the flows indicated in Fig. 3.2, the enrichment factor amounts to 3000/14 = 124. Column “2” splits krypton from xenon. Column “3” separates the less volatile components from krypton and isolates the latter as a pure product at the column’s top. Column “4” segregates xenon from its less volatile companions and column “5” isolates xenon as pure sump product.

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Fig. 3.2 Diagram of a rectifying Kr/Xe fine scrubbing. (1–5) Diagram of columns, the figures on the arrows are flows in arbitrary units.

The flow to the Xe-fine-purification column “5” is by about a factor 10–7 smaller than the flows involved with the columns of the air separation unit. The rectification on such a small scale requires special solutions: x Electrical heaters in the sump of the columns guarantee sufficient vapour and liquid load to maintain the rectification. x The pressure in the columns is controlled by condensing the vapour at the column heads with liquid or cold gaseous nitrogen. x Measurement and controlling of the small flows is largely avoided. The gas flow exhausted from the top of the columns is controlled via the temperature profile developing in the column. x Pure xenon solidifies at about 161 K and tends to freeze in the cold parts of the plant. This is avoided by operating the columns at elevated pressure and thereby at elevated temperature, by wrapping the column shells with electrical heating bands and by careful pipe routing. The Kr and Xe product obtained in the cryogenic plant is warmed up and stored and supplied to the consumers in cylinders at pressures of about 60 bar for Xe and about 140 bar for Kr. The pressure of a Xe-cylinder must be lower, because Xe is near its critical state (Tcrit = 289.8 K, Pcrit = 58.4 bar). Thus when a Xe-cylinder is heated, its pressure will increase more than the pressure in a Kr-cylinder (Tcrit = 209.4 K, Pcrit = 55.0 bar). A preceding cleaning of the cylinders by evacuation and purging is necessary to obtain the product purities as specified in Table 3.4.

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Numerous modifications of the process shown in Fig. 3.2 exist: x A second catalytic combustion of hydrocarbons downstream of the krypton and xenon enrichment x Chemisorption of fluorocarbons, SF6 and other components x Adsorption

3.4 Recovery of Neon

The industry’s demand for neon is exclusively covered from air separation plants, where neon is a by-product. The neon content of the atmospheric air is 18 ppm (see Table 2.1). In contrast the recovery of helium from atmospheric air is of minor importance. The most important helium source is natural gas (cf. Chapters 4 and 7). The global annual production of neon amounts to about 250 000 mN3 and its market price is subject to strong fluctuations, ranging typically between 30 and 200 € mN–3. Neon production is performed in two steps. In the first step, a neon enriched gas with about 50% of Ne, 3% of H2, 16% of He and 31% of N2 (see Fig. 3.1) is obtained in a small column (6) integrated into the air separator. The neon enriched gas is often stored in pressure cylinders and transported to a central fine-purification plant. There, in the second step, neon is recovered with the required purity. 3.4.1 Pre-enrichment

Since the boiling temperatures of neon, helium and hydrogen are significantly lower than those of nitrogen (see Table 3.3), they accumulate at the head condenser (3) of the pressure column (see Fig. 3.1) in a non-condensing gas phase, which is extracted. The amount of this extracted inert gas (f) is typically 1/1000 of the processed air. Thus the Ne-concentration increases by a factor of 1000 from 18 ppm in the ambient air to ~ 1.8% in the inert gas. This inert gas is fed into the Ne-concentration column (6). Neon further enriches in its head condenser (7), which is cooled with about 80 K cold liquid nitrogen (g) and (h). The non-condensing proportion (i) with a Ne content of about 50% is withdrawn from the head condenser and is discharged as pre-enriched product after being warmed up in an exchanger. Table 3.3 Boiling temperatures (K) of the components involved in the Ne-extraction (at a pressure of 1.013 bar).

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He

H2

Ne

N2

4.3

20.4

27.1

77.3

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3.4 Recovery of Neon

119

In order to avoid Ne-losses, the liquid nitrogen (j), which is fed to the lowpressure column as reflux, is withdrawn a few trays below the top of the pressure column. For such processes the Ne-yield is significantly over 90%, provided that none of the process air bypasses the pressure column in some way. 3.4.2 Fine Purification

Even with large air separation plants the amount of neon pre-enriched product is small, so that the final purification is often operated in a batch mode. X Example: Separation unit processing 200 000 mN3 h–1 of air. Pre-enriched product with 50% (= 500 000 ppm) of neon. Then the production rate of this pre-product is = (18 ppm/500 000 ppm) · 200 000 = 7 mN3 h–1. Various processes exist, combining adsorption, catalysis, partial condensation and rectification for the isolation of neon. For example, the process applied by the Linde AG either in batch or continuous mode combines these unit operations in the following way [3.5]: x Catalytic oxidation of the hydrogen in the pre-product. For this oxidation oxygen is added. The arising water is removed by ensuing adsorption. x Pre-separation of nitrogen by condensation at 66 K. The non condensed part has a residual N2 content < 2%. x After adsorption of the remaining nitrogen on a silica gel adsorber a mixture of 76% of Ne and 24% of He remains. x Compression of this mixture to 180 bar and cooling down to 50 K. x Throttle expansion down to 25 bar into a separator. The condensate in the separator has a neon content of about 97%. x The condensate is fed to the top of a rectification column, operated at 1.3 bar and about 28 K. From its sump pure neon is withdrawn, warmed up and filled into gas cylinders at a pressure of about 150 bar. The cryogenic part of the apparatus, designed for the production of a few standard cubic meters per hour, is housed in two containers with a volume of about 1 m3. Due to the vacuum- (~ 1 Pa) and the super-insulation, the heat flow into the equipment is small and the low temperatures can be obtained via the Joule Thomson effect.

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3.5 Industrial Product Purities and Analytics

Table 3.4 shows typical demands of industrial consumers on the product purity of inert gases. The control of the Kr/Xe-purification process and of the product require a complex analyzing equipment. The following compounds represent impurities and should be detectible in the range of trace amounts: Kr in Xe, Xe in Kr, CO2, N2, O2, N2O, C1–C4-hydrocarbons, SF6, CF4, C2F6. In case of Ne-production also components like He, N2 and H2 must be analyzed. The gas chromatographs used for process and product control are bearing FID(Flame Ionisation Detector), TCD- (Thermal Conductivity) and PDD-detectors (Pulse Discharge Detectors = ionization of the compounds to detect by means of a high-frequency discharge of helium). Owing to the demand on the detection of many impurities and thus on great variability, these analytical appliances resemble more conventional laboratory gas chromatographs. They require more analytical expert knowledge and maintenance than automatically running process analyzers. Owing to the high enrichment in the production of xenon – xenon is contained in the air with only 0.086 mol ppm – nearly all imaginable air impurities are found Table 3.4 Typical Kr, Xe and Ne purities. Components

Kr

Xe

Kr

99.995–99.9998%

0.1–35 ppm

Xe

0.1–35 ppm

99.995–99.9995%

Ne

99.996–99.999%

He

8–35 ppm

N2

0.1–5 ppm

1–5 ppm

1–4 ppm

O2

0.1–1 ppm

< 1 ppm

0.5–1 ppm

CO2

0.1–1 ppm

0.1–1 ppm

0.5–1 ppm

CO

0.5–1 ppm

CH4

0.1–1 ppm

0.1–1 ppm

CF4

0.1–0.5 ppm for lamp industry

< 1 ppm for lighting industry

C2F6

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Ne

0.5–1 ppm

< 1 ppm for lighting industry

SF6

< 1 ppm

< 1 ppm for lighting industry no requirements for insulating glass

H2O

1 ppm

1 ppm

0.5–1 ppm

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121

in the process, provided they are not eliminated by upstream purging stages like spray coolers, adsorbers and catalytic reactors. The analytics of a Kr/Xe purification process starts with the control of the CH4and N2O-abatement after the catalytic reactor. This sampling point is crucial for the plant safety. Downstream in the first column where the light components N2 and O2 are withdrawn overhead, the O2-content in the sump and the Kr-content in the top of the column are periodically checked. In the following columns all of the above mentioned impurities must be analyzed applying in some different separation cases so called heart-cut methods on GC. Analyzing the tiny pure xenon column causes sometimes problems: An unreduced sample flow may exceed the continuous xenon production and thus interferes with the rectification process. Commonly the pure krypton and xenon is filled in 50 L cylinders. An analytical check of the cylinders requires a good mixing of the content in advance by rolling the cylinders over several hours.

3.6 Applications of the Noble Gases Neon, Krypton and Xenon

Compared to the relatively widespread noble gases helium and argon, the noble gases neon, krypton and xenon are less common and harder to obtain. Nevertheless, they are used in a wide range of modern technologies, in particular lighting, optics and electronics, on account of their special properties. They are preferably used in a gaseous state and in mixtures. 3.6.1 Applications of Neon

Neon is used: x with helium in helium-neon lasers (e.g. laser pointers) x as a refrigerant for special cooling devices x as a filling gas for gas discharge lamps, low-consumption glow lamps (night lights) and stroboscopic lamps x as a filling gas for plasma display panels (PDP) (see Example A below) x as a filling gas for over-voltage protection and lightning protection devices x as a filling gas for thyratron tubes (similar to amplifier tubes/triodes) 3.6.2 Applications of Krypton

Krypton is used: x as a filling gas for incandescent lamps and flash bulbs x as a filling gas for halogen lamps (with halogen components, see Example B below) x with fluorine, neon and helium for eximer lasers (see Example C below)

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x as a filling gas for insulating glass panes (see Example D below) x as a filling gas for detectors designed to measure radiation (e.g. Geiger counters) 3.6.3 Applications of Xenon

Xenon is used: x as a filling gas for plasma display panels (PDP) (see Example A below) x as a filling gas for flash bulbs and gas discharge lamps x as a filling gas for halogen lamps (with halogen components, see Example B below) x as a filling gas for thyratron tubes (similar to amplifier tubes/triodes) x as a filling gas for xenon high-pressure lamps (e.g. lamps for floodlight units and cinema projectors) x with hydrogen chloride and helium for eximer lasers (see Example C below) x as a propellant for small thrusters used to position satellites in orbit (Solar Electric Propulsion (SEP), see Example E below) x as an anaesthetic gas x with oxygen in Computer Aided Tomography (CAT) scanners for mapping blood flow x as a contrast agent in Computer Aided Tomography to obtain pictures of the brain o enzephalography Example A: Plasma Display Panels

Plasma televisions comprise two glass plates, one with vertical conductive lines and one with horizontal lines, and a neon-xenon gas mixture positioned between these plates. When placed together, the two plates form a grid. Electric currents are passed through the horizontal and vertical lines, causing the gas to emit ultraviolet light, which excites fluorescent materials and create the picture (see Fig. 3.3).

Fig. 3.3 A mixture of neon and xenon is used for plasma TV screens.

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123

Example B: Halogen Lamps

Krypton and xenon bulbs are used in automotive headlights. The noble gases provide an inert medium that allows to reduce the size of the bulbs. When mixed with halogen components, the bulbs burn brighter and longer, use less power and generate more light. In xenon headlights (discharge lamps), xenon gas creates a bright, blue-tinted light. Example C: Excimer Lasers

Excimer lasers work with an ultraviolet light beam. The gas mixture in a laser tube contains fluorine gas (F2) and hydrogen chloride (HCl) as halogen donors as well as argon, krypton and xenon as active noble gases, and neon as a buffer gas. The excited monohalides ArF (193 nm), KrF (248 nm) and XeCl (308 nm) are the active laser molecules. Excimer lasers are primarily used for vision correction (cold beam) (see Fig. 3.4), in the lithography process for manufacturing computer chips (short wavelength) and for microstructuring.

Fig. 3.4 Eximer lasers are used for correcting vision (cold beam).

Example D: Insulating Glass Panes

Making buildings as energy efficient as possible cuts costs. Windows with good insulating properties make an important contribution to saving energy. Filling the airspace between window panes with noble gas (argon or, even better, krypton) greatly improves insulating capabilities by reducing circulation between the panes and minimizing heat conduction. This is caused by heavy-atom krypton which does not move around as quickly as nitrogen and oxygen in air. In addition, krypton is monatomic and does not oscillate. This limited atomic mobility reduces heat loss from buildings. Example E: Solar Electric Propulsion

Ion beams are used in one of several types of spacecraft propulsion. In this particular process, xenon flows into the ion engine where it is electrically charged and pushed around by an electrical voltage. Two grids electrified to almost

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1300 volts accelerate xenon ions to very high velocities and shoot them out of the engine. As the ions race away from the engine, they push back on the spacecraft, propelling it in the opposite direction. As a heavy ion, xenon provides ten times more push than chemical propellants.

References [3.1] [3.2] [3.3] [3.4] [3.5]

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Römpp, 10. Aufl., Stichworte: Neon, Krypton, Xenon, Thieme Verlag, Stuttgart, 1996. Ullmann’s, 6. Aufl., 23, S. 218, Wiley-VCH, Weinheim, 2003. Ullmann’s, 6. Aufl., 23, S. 227, Wiley-VCH, Weinheim, 2003. Kryogene Kr-Xe-Gewinnung: EP 1082577 B1, US 6351970 B1. Ullmann’s, 6. Aufl., 23, S. 215, Wiley-VCH, Weinheim, 2003.

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4 The Noble Gas Helium 4.1 History, Occurrence and Properties 4.1.1 History

In 1868, helium (He; from Greek “helios”, the sun) was discovered due to a pale yellow line at 587.6 nm during the spectroanalytical examination of the solar prominences. In 1882, it was also detected in the spectral analysis of lava from the Vesuvius and as gas occlusion in the uranium mineral uraninite in 1889. Only in 1895, helium could be produced in its pure form in larger quantities from the mineral cleveite. And in 1908, H. Kamerlingh-Onnes at Leiden succeeded for the first time in liquefying helium [4.1]. 4.1.2 Occurrence

Helium is produced in radioactive decay processes and emitted as D-radiation. Therefore it is found in all uranium minerals. The helium-method for the age determination of minerals is also based on the formation of D-particles. Consequently, as a decay product of radioactive processes, helium is also found in many natural gases. The largest deposits of He-rich natural gases are located in the USA, Siberia, Algeria and Canada (see Table 4.1). In Europe, helium occurs in Polish natural gas with a molar fraction of about 0.4% and in gas from the North Sea with a molar fraction of up to 0.12% [4.2]. Table 4.2 gives an overview of the worldwide production of He and its reserves. The helium content of ambient air amounts to 5.24 ppm. The total quantity of helium in the atmosphere is in a stationary equilibrium between the gas escaping into space on the one hand and the helium supplied by radioactive minerals and the solar wind on the other hand [4.2]. In the sun, helium is represented with a molar fraction of about 8%, whereas hydrogen amounts to about 92%. In the universe, helium is the second most abundant element with a molar fraction of more than 25%. Helium is an end product of the hydrogen-nuclear fusion in fixed stars [4.1]. Industrial Gases Processing. Edited by Heinz-Wolfgang Häring Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31685-4

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4 The Noble Gas Helium Table 4.1 Helium content of some natural gas sources and assessed reserves (1984) [4.2]. Source Location, field name

Composition, % molar fraction He

CmHn

N2

Reserves, 106 mN3

CO2

USA/Wyoming, Tip Top Field

0.4–0.8

1200

USA/New Mexico, Beautiful Mountain

4.05

49

45

0.9

USA/Alaska, South Barroweast

2.54

90.2

6.8

0.3

USA/Texas, Young Regular

1.17

66.2

31.1

0.1

USA/Kansas, Hugoton

0.44

81.8

17.6

Canada/Alberta, Worsley

0.53

93

6.0

0.5

Canada/Ontario, Norfolk

0.36

91.5

8.1

0.1

Netherlands, Groningen

0.05

83

14

0.9

Netherlands, De Wijk

0.05

94.3

5.6

Poland, Ostrow

0.4

56

43

0.3

North Sea, Indefatigable

0.05

96

3.3

0.5

Germany/Niedersachsen, Apeldorn

0.12

25.9

73.3

0.2

Russia, Urengoi

0.055

94.2

5

0.03

Russia, Orenburg

1270

6000 3400

Algeria, Hassi R’Mel

0.19

93.6

5.8

0.2

2400

Australia, Palm Valley

0.21

97.5

2.3

0.1

900

Table 4.2 Helium – Global production and reserves (in 106 mN3 ) [4.3]. Country

Production

Exploitable reserves

2001

2002

USA

92

90

4300

9400

Algeria

15

15

2100

3200

Canada

2100

China

1200

Poland

1

1

42

300

Russia

4

4

1800

7000

Other countries Total

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Total reserves

3000 112

110

26 000

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4.1.3 Physical and Chemical Properties

At atmospheric pressure and room temperature, helium is a colorless, odorless, nonflammable monoatomic gas of the group of noble gases with the atomic number 2. As natural isotopes, 3He and 4He occur, with 3He amounting to a portion of only 1.38 · 10–6. Gaseous He is characterized by high thermal conductivity (0.143 W m–1 K–1), low density (0.1785 kg m–3, values at 0 °C and 1.013 bar), low solubility in metals and high diffusivity. The critical point of 4He is at 5.20 K and 2.275 bar and at 3.31 K and 1.146 bar for 3He. The liquefied helium is subdivided into two states: He I and He II with a sharp transition point of 2.18 K at 5.04 kPa, the so-called O-point. He I behaves like a normal liquid, whereas He II exhibits interesting properties of a superfluid or quantum fluid. During expansion of liquid He I below this pressure, the previously even surface forms a sharp meniscus at the wall of the container since at the O-point the viscosity decreases by the factor 106 and the thermal conductivity rises by the same factor. The thermal conductivity of He II is about 200 times higher than that of copper at 20 °C. Close to the absolute zero point, the viscosity turns zero and He II becomes an inviscid superfluid. He II flows over obstacles, which lie higher than the surface of the liquid, to reach the lowest level. If two containers of different temperatures are filled with He II and connected to each other by a capillary or another He II-film, He II flows from the cold container into the warmer one. Helium is the only substance that remains liquid in close proximity of the absolute zero point at atmospheric pressure. For the production of solid helium the liquid has to be compressed to about 30 bar at 1 K, at 24 °C to about 117 000 bar. Solid helium is the softest solid known; slight pressure fluctuations lead to changes of the interatomic distances in the crystal lattice and thus to changes of the thermodynamic properties. It is the only known example of a so-called quantum solid. Helium is the lightes of the noble gases. The monoatomic gas is absolutely inert and in contrast to the heavy noble gases, it does not form any chemical compound.

4.2 Recovery

Owing to its very low portion in the air (5.24 ppm), helium is mainly recovered from natural gases (cf. also Chapter 7). In general, helium recovery plants are economic at He concentrations of about 0.2% molar fraction and more in the natural gas. In a pre-cleaning step, H2O, CO2, H2S and other trace components which would solidify during cool down are removed. Then the gas is partially condensed and the remaining gaseous helium is enriched again. The helium content of this so-called “raw helium” usually ranges between 50 and 90% molar

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4 The Noble Gas Helium

Fig. 4.1 Nitrogen removal and crude helium extraction from natural gas. (a) Heat exchanger; (b) High-pressure column; (c) Condenser; (d) Low-pressure column; (e) Crude He-separator; (f) Heat exchanger; (g) Pump.

fraction. A simple but typical modern process for the recovery of raw helium as part of nitrogen removal from natural gas is shown in Fig. 4.1 [4.2]: The pre-cleaned natural gas is cooled against cold product flows in heat exchanger (a) and fed into the sump of the high-pressure column (b). The condenser (c) supplies both the high-pressure column (b) and the low-pressure column (d) with reflux. The non-condensable portion contains the helium which is drawn from the raw helium separator (e) and discharged at the battery limit after preheating. After further cooling down in the heat exchanger (f), the sump product of the high-pressure column (b) is fed into the low-pressure column (d), where the final separation into nitrogen and methane takes place. Nitrogen leaves the column overhead and is heated against the cooled reflux in the heat exchanger (h) and discharged at the battery limit after further heating in (f) and (a). The reboiler (c) of the low-pressure column (d), which serves also as condenser for the high-pressure column (b), depletes the nitrogen. The methane-rich fraction is pumped to higher pressure (g) and is also discharged to battery limit after heating in (f) and (a). Until recently, in the USA raw helium with a molar fraction of about 70% was stored as strategic reserve for future demand in exploited natural gas deposits. For the production of high purity helium from raw helium, pressure swing adsorbers are used, since helium is only poorly adsorbable, whereas methane and nitrogen are relatively well adsorbed. Thus, with zeolitic molecular sieves and fine-pored activated carbons, purities of 99.999–99.9999% molar fraction of helium can be adjusted without additional purification steps. A typical Pressure Swing Adsorber Unit (PSA) is shown in Fig. 4.2 [4.2]:

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Fig. 4.2 Diagram of a four-bed pressure swing adsorber station. FC = Flow control; FT = Volumetric flow measurement; PC = Pressure control; PT = Pressure measurement; G1–G4 = Adsorbers 1 to 4. b = Buffer tank for residual gas.

After volumetric flow measurement (FT) the crude helium gas is fed to the station. At the outlet of each individual adsorber G1–G4 the pressure is measured (PT) and maintained in the pure helium flow (PC). The desorption of impurities from the adsorbent respectively the regeneration of the loaded adsorber is controlled via purging steps with pure helium and via flow respectively pressure control of the residual gas (FC). Pressure or flow fluctuations of the residual gas are dampened by a buffer tank (b). The individual adsorbers go through the following steps cyclically, with the whole cycle taking a few minutes: 1. Adsorption 2. Pressure balance with another adsorber 3. Pressure reduction cocurrent to adsorption to provide purge gas for another adsorber 4. Desorption through further pressure reduction to the residual gas system countercurrent to adsorption 5. Purging with pressure relief gas of another adsorber 6. Pressure build-up by pressure balance with another adsorber 7. Final pressure build-up with pure helium to adsorption pressure For PSA cf. also Section 2.2.4. If hydrogen and neon are contained in the crude gas to the helium recovery plant, the separation of these components in a pressure swing adsorber station is not possible, since they behave similar to helium. Hydrogen can then be

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Fig. 4.3 Diagram of a helium fine scrubbing and liquefaction plant. (a) He-recycle compressor; (b) Cooler; (c) Plate fin heat exchanger; (d) Expansion turbines; (e) Adsorbers; (f) Throttle valve; (g) Liquid-He-tank.

combusted to water, for instance, via a de-oxo-plant, adding atmospheric oxygen. The separation of neon is only possible in another cryogenic step which is also used for the liquefaction of helium in order to allow to transport larger volumes. Such a process is shown in Fig. 4.3 [4.2]. The large-scale liquefaction of helium is carried out with the help of a helium cycle with a compressor (a) and cooler (b), in which cold is supplied at about 80 K by liquid nitrogen (c), and below by expansion turbines (d). Depending on the size of the plant, more or less expansion turbines are used. At about 80 K, traces of N2, O2, CO2 and CH4 are removed via a first adsorber (e), traces of H2 and Ne in a second adsorber (f) at about 20 K. The process heat exchangers (c) are usually aluminium plate fin heat exchangers with very narrow temperature differences in order to minimize exergy losses. At the inlet of the throttle valve (g) the fluid is in a supercritical state. Only after throttling, the helium can be stored as liquid in tank (h) at pressures slightly above atmospheric pressure. The flash gas and the He evaporated due to heat leaks is heated in the heat exchangers (c) and recirculated to the suction side of the compressor (a).

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Fig. 4.4 Diagram of a helium collecting tank. (a) Cooling pipes; (b) Vacuum-superinsulation; (c) Outer casing; (d) Heat-radiation shield; (e) Container for liquid helium; (f) Support.

Cryogenic tanks (h) are built up to a volume of 100 m3. Their insulation loss amounts to about 1% of the design liquid inventory per day. These low heat losses at storage temperatures of about 4 K are possible due to superinsulation, thermal radiation shields and deep vacuum in the clearances of the double-walled container. The heat radiation shields transport the heat to the pipes in which either nitrogen or helium itself evaporates. A typical layout is shown in Fig. 4.4 [4.2].

4.3 Applications

On the one hand, gaseous helium (GHe) is particularly popular for applications that require an inert gas offering high thermal conductivity and low density. Liquid helium (LHe), on the other, is used when extremely low temperatures are essential. It is used in the processing industry x to increase the quality and performance of shielding gas welding processes such as – tungsten inert gas/TIG – metal inert gas/MIG – metal active gas/MAG (GHe combined with Ar/CO2) (see Example A below) x to weld aluminium alloys and stainless steel (GHe combined with Ar/CO2) x to operate CO2 lasers (GHe mixed with N2 and CO2)

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x to operate Excimer lasers (GHe mixed with F2 or HCl) x to purge and shield semiconductors during fabrication (GHe in high purities) x to protect and cool optical fibres during manufacture (with purification and recycling of GHe) x to provide a carrier gas in thermal spraying and related coating technologies (e.g. Cold Spray) It is used in other industries x to temper and harden steel parts by quenching after an annealing furnace (e.g. instead of using an oil bath) x to perform quality control and research on pure metals and superconductive wires at very low temperatures (cf. boiling point of LHe at 4.2 K) x to cool superconductive magnet coils in – magnetic resonance imaging (MRI) scanners used in medical diagnostic equipment (see Example B below) – nuclear magnetic resonance (NMR) spectrometers used for chemical analysis – particle accelerators used for nuclear research x to cool the primary coolant circuit of pebble bed reactors (in nuclear power stations) x to run leak tests on pipelines, heat exchangers, receptacles and food packages by means of thermal conductivity detectors or mass spectrometers x to lift toys, air balloons and air ships x to purge and pressurise rocket fuel systems (see Example C below) x to create respiratory mixtures for deep-sea diving and medical care x to inflate automotive airbags (as passenger safety devices) Example A: Welding with Helium Gas Mixtures

Helium is valued not only for its inertness and high thermal conductivity, but also for its high ionisation energy. These properties make it a popular shielding gas for electric arc welding. It enables higher electric arc temperatures and improves heat transfer to the work piece. It also reduces the formation of pores and fusion faults, at the same time improving gap bridging. Unlike argon, helium is rarely used as a pure shielding gas here. Helium mixtures, however, are proving extremely useful results in many electric arc welding applications using materials such as steel, aluminium, stainless steel and copper. Nowadays, shielding gases with helium are widely used to increase the productivity of many arc welding processes. In the case of laser welding, pure helium can be used as a shielding gas for the laser beam. Helium has an advantage over argon here. It does not absorb the laser radiation with the result that it does not weaken the laser beam.

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133

Fig. 4.5 Gaseous helium as shielding gas for laser welding stainless steel.

But helium mixtures are also used for many kinds of cutting-edge laser technologies. The correct composition of the gas mixtures creates the basis for increased quality and productivity also in laser welding. Example B: Superconductivity, Magnetic Resonance Imaging/MRI

The discovery of superconductive materials has led to the development of some of the most remarkable diagnostic and research equipment ever. Such materials have zero resistance for electricity when they fall below a specific critical temperature. Cryogenic gases play an important role to reach and maintain these temperatures of superconductivity. Liquid helium helps to cool certain metal alloys below this critical temperature. These alloys are used to build the superconductive coils that are needed to create the strong magnetic field for the operation of MRI scanners.

Fig. 4.6 Liquid helium cools superconductive magnets in MRI scanners.

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MRI scanners give doctors a detailed view inside the body of a patient, allowing the diagnosis of many diseases without invasive surgery. Example C: Purging and Pressurizing of Rocket Fuel Systems

In rocket science, helium is used to boost liquid hydrogen and liquid oxygen out of the tanks into the engines during the take off. It is also applied to purge liquid hydrogen systems in space launch vehicles. For example, the Saturn V booster used in the Apollo program has consumed vast quantities of helium for this pressurizing purpose. The helium was extracted from natural gas and partly stored in an exploited gas field in Texas. (Please note: In 1903, helium was found in natural gas, which has remained the main source of commercial helium supply ever since.)

Fig. 4.7 Saturn V rocket launch (Apollo 16 program): Helium is used for fuelling and pressurising liquid hydrogen and liquid oxygen tanks in space rockets.

References [4.1] Römpp, 10th edition, Keyword: Helium, Thieme Verlag, Stuttgart, 1996. [4.2] Ullmann’s, 6th edition, Vol. 23, pp. 215–273, Wiley-VCH, Weinheim, 2003. [4.3] US Geological Survey, Mineral Commodity Summaries, January 2003.

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5 Hydrogen and Carbon Monoxide: Synthesis Gases 5.1 History, Occurrence and Properties 5.1.1 Introduction

Synthesis gas is the term for gas mixtures mainly composed of carbon monoxide (CO) and hydrogen (H2). In the chemical industry it is applied as raw material for the production (“synthesis”) of various products, for example, for the production of methanol or even ammonia (by synthesis of N2 and H2). Synthesis gases are obtained in many ways (see Section 5.2). Fossil fuels such as carbon, petroleum, petroleum residues, natural gas as well as wood, peat and biomass reacting with water vapour, air, O2 or CO2 serve as feed materials. Apart from CO and H2, synthesis gas may contain CO2, H2O, N2, methane and higher-boiling hydrocarbons. Terminologically as well as regarding the practical production process, the boundaries to other industrial gases such as water gas and generator gas are fluid. Table 5.1 Examples of H2, CO and synthesis gas products (see also Section 5.4). H2

Various hydrogenations, as e.g. the fat hydrogenation in the food industry and the desulphurization in refineries, energy carrier for fuel cells and a future hydrogen infrastructure

H2 and N2

Ammonia

CO

Phosgene, polycarbonate, formic acid

H2 and CO in split flows

Acetic acid, intermediate products for the production of polyurethane foams

Mixtures of H2 and CO

Oxo-alcohols, reduction gas for the steel industry, fuel for gas turbines, synthetic fuels from natural gas (Fischer-Tropsch-Synthesis: Gas to Liquids: GTL)

Mixtures of H2, CO and CO2

Methanol


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5 Hydrogen and Carbon Monoxide: Synthesis Gases

In the past, synthesis gas was almost exclusively produced from coal and then called water gas. Generator gas is understood as the gas mixture with high nitrogen content developing during the coal gasification with air mixed with water vapour. Today natural gas is the most important basic feed for the production of synthesis gas. Meanwhile, first small plants for the “CO2-neutral” generation of synthesis gas or pure hydrogen through the gasification of waste products and biomass are operated worldwide. Table. 5.1 gives a first overview of the main fields of application of synthesis gas (cf. also Section 5.4.3). 5.1.2 History of Synthesis Gas

In 1780, Felice Fontana [5.1] discovered that combustible gas develops if water vapour is passed over carbon at temperatures over 500 °C. This CO and H2 containing gas was called water gas and mainly used for lighting purposes in the 19th century. A more detailed historical overview of this epoch is given in [5.2]. As of the beginning of the 20th century, H2 /CO-mixtures were used for syntheses of hydrocarbons and then, as a consequence, also called synthesis gas. In 1921, Patart reported for the first time on the synthesis of methanol, after he had carried out the reaction of H2 /CO-mixtures under pressure and at 400 °C on ZnO-contacts [5.3]. The systematic examination of the reaction conditions of the synthol-process developed by Fischer and Tropsch in 1922 led to the direct hydrocarbon synthesis on ferric catalysts in 1926 [5.4], the so-called Fischer-Tropsch- or FT-synthesis [5.5]. In 1943/44, this was applied for large-scale production of artificial fuels from synthesis gas in Germany, with coal as a basic feed. Nowadays, synthesis gas is mainly used for production of the products listed in Table 5.1 and increasingly in energy engineering again. With the gasification of heavy hydrocarbons and the combustion of the generated synthesis gas in gas turbines for generation of electric energy the original application of “water gas” in the energy sector has been reached again. 5.1.3 Hydrogen 5.1.3.1

History and Occurrence

In the second half of the 17th century, Boyle produced hydrogen for the first time through dissolving iron in sulphuric acid. In 1766, Cavendish published precise values for the specific weight and the density of hydrogen. Owing to its very low density, hydrogen was already used as filling gas for balloons by C. Charles in 1783. Also in the year 1783, Lavoisier termed the combustible product of water separation “hydrogenium” (= water generator), from which H is derived as the symbol of the element. In 1898, Dewar applied the Linde-Process for the first liquefaction of hydrogen. A sad peak of the degree of fame was the fire of the hydrogen-filled airship “Hindenburg” in Lake Hurst in 1937. In 1963, the USA

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started a rocket driven by hydrogen and oxygen in Cape Kennedy for the first time ever. In order to avoid greenhouse gases and other atmospheric contaminants, at least in conurbations, hydrogen has been considered as an energy source for a modern traffic infrastructure for about 20–30 years, cf. also Section 5.4.1.2. Molecular hydrogen is contained in traces in the ground-near atmospheric air with 0.5 vppm. However, with increasing height its content increases as well, until in some 100 km the ultrathin gas layer consists almost exclusively of hydrogen [5.6]. In bound state, hydrogen is a component of water (mass portion of hydrogen 11.2%) and many other compounds, such as hydrocarbons. While the hydrogen content in the upper 16 km of the earth’s crust, including hydrosphere and atmosphere, is assessed at a mass portion of 0.88% (thus hydrogen is ninth with regard to its frequency on earth) H represents the most frequent element of the whole planetary system inside and outside the Milky Way. The sun consists of a mass portion of hydrogen (protons) of 84%. The fusion of these hydrogen nuclei to helium forms its energy source [5.7]. 5.1.3.2


With only one proton and one electron, hydrogen is the lightest of all chemical elements. At ambient temperature, molecular hydrogen, H2, is a colourless and odourless gas. At 0 °C and 1 bar, one litre weighs 0.0899 g. Consequently, H2 is 14 times lighter than air and with this, a suitable filling gas for balloons. The lift of 1 m3 of H2 in air amounts to 1.2 kg. Disadvantageous for this application is its high combustibility, since hydrogen in a concentration range of 4–77% of volume fraction in air forms an explosive mixture. At –252.78 °C (1 bar), hydrogen condenses to a colourless liquid, it freezes at –259.15 °C. Its high diffusion capability owing to its low molar mass respectively its solubility in a number of metals has to be considered in the handling of hydrogen and selection of materials, e.g. regarding the embrittlement of steels [5.50, 5.51]. On the other hand, the good solubility of H2 in metallic platinum and palladium respectively its extraordinarily high diffusion selectivity due to dense membranes of these noble metals can be utilized for the separation of H2. Two other isotopes of hydrogen exist. On earth, heavy hydrogen (D, deuterium), which is stable, occurs in the natural isotopic distribution with an atomic fraction of 0.015%; tritium (T) is a weak E-radiator with a half-life of about 12.3 years, on earth represented in the natural isotopic distribution with an atomic fraction of only about 10–12–10–13%. The Diagrams 5.1 and 5.2 show a comparison of the energy content of different energy sources. From this, it can be construed that the mass-related energy content of H2 is very high, the volume-related energy content for gaseous hydrogen, however, is relatively low. Therefore, an important goal of development is the discovery of a competitive storage medium respectively storage method for hydrogen. With ortho-hydrogen (o-H2: parallel nuclear spin) and para-hydrogen (p-H2: anti-parallel nuclear spin) hydrogen disposes of two nuclear spin isomers. At room temperature, the equilibrium fraction of p-H2 is 25%. This mixture is

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Diagram 5.1 Mass-related energy content of different energy sources in kWh kg–1.

Diagram 5.2 Volume-related energy content of different energy sources in kWh m–3.

called normal-hydrogen (n-H2). At low temperatures, however, the p-portion increases strongly whereas the equilibrium is reached only very slowly. Since the conversion from ortho to para-form is exothermal, further cooling has to be provided after liquefaction of hydrogen to reach the equilibrium. If the hydrogen is in equilibrium, it is also called e-H2. Figure 5.1 shows the p-content of the e-hydrogen in dependency on the temperature. Further important physical properties of n- and e-hydrogen are listed in Table 5.2A and B [5.8, 5.9].

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Table 5.2A Physical properties of n- and e-hydrogen. Unit

n-hydrogen (o- 75%, p- 25%)

e-hydrogen

g mol–1

2.016

2.016

kg m–3 J mol–1 K–1 W m–1 K–1

0.0899 Cp = 28.6, Cv = 20.2 0.1645

0.0899 Cp = 28.6, Cv = 20.2 0.1645

x Temperature x Density (liquid) x Density (gas)

K kg m–3 kg m–3

20.37 70.00 1.319

20.43 70.81 1.316

Heat of evaporation

J mol–1

898

916

Liquid at boiling point (101.3 kPa) x Molar heat x Enthalpy1) x Thermal conductivity

J mol–1 K–1 J mol–1 W m–1 K–1

Cp = 22.0, Cv = 6.51 –7918 0.117

Cp = 23.5, Cv = 7.97 –7932 0.123

Gas at boiling point (101.3 kPa) x Specific heat capacity x Enthalpy1) x Thermal conductivity

J mol–1 K–1 J mol–1 W m–1 K–1

Cp = 23.49, Cv = 12.8 Cp = 22.4, Cv = 12.1 –7020 –7016 0.0185 0.0180

K kPa kg m–3

33.00 1339 30.09

32.98 1310 40.16

K kPa kg m–3 kg m–3 kg m–3

13.81 6.14 86.7 76.4 0.12

13.95 7.03 86.7 77.2 0.13

Molar mass Properties at 273.15 K, 101.3 kPa x Density x Molar heat x Thermal conductivity Boiling point (101.3 kPa)

Critical point x Temperature x Pressure x Density Triple point x x x x x

Temperature Pressure Density (solid) Density (liquid) Density (gas)

1)

The reference point for the enthalpy (incl. transformation heat) is zero for the ideal gas at 0 K.

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5 Hydrogen and Carbon Monoxide: Synthesis Gases Table 5.2B Physical properties of hydrogen in combustion. Combustion (with air at 101.3 kPa) Ignition limits

% volume fraction

4–77

Ignition temperature

K

858

Minimal ignition energy

mJ

0.02

Flame temperature

K

2591 –1

Laminar combustion velocity

ms

Lower calorific value (volumetric)

kWh m–3

Lower calorific value (gravimetric)

2.75

–1

kWh kg

3.0 33.3

Fig. 5.1 p-fraction of e-hydrogen depending on the temperature.

Chemical Properties In air, H2 combusts to water with a hardly visible, weakly bluish flame (detonating gas reaction). The chemical bond between hydrogen and oxygen is very strong and can only be ruptured by adding considerable energy ('Hf (H2O): –286 kJ mol–1). The hydrogen bridge bonds occurring between the hydroxyl groups (-OH) of two water-molecules are important intermolecular bonds that sharply increase the melting and boiling points of light molecules. Hydrogen combines with almost any other element. Metal compounds with negatively charged hydrogen are called metal hydrides (e.g. CaH2, NaH, LiH). With water, they form metal hydroxides and gaseous hydrogen. Through thermal cracking of ethane or naphtha (dehydration), for example, ethylene, propene and H2 are obtained. In addition to that, the hydration of unsaturated hydrocarbons, as for instance in fat hardening, plays an important role. Hydrogen has a reducing effect on a lot of metal oxides when heated. Thus CuO with H2, for example, reacts

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to Cu and H2O. In the 1H-NMR-spectroscopy the H-atom with the nuclear spin of 1/2 is used as a probe for the structural analysis of organic molecules as well as in the medical nuclear spin tomography. 5.1.4 Carbon Monoxide 5.1.4.1

History and Occurrence

Carbon monoxide CO, is a gas that develops in each incomplete combustion of carbon-containing compounds (e.g. coal, petroleum, natural gas). Carbon monoxide was discovered by Lasonne in 1776 when glowing coal with zinc oxide, and in 1796 by Priestley when glowing coal with hammer blow (Fe3O4) [5.10]. Originally, the gas had been interpreted as hydrocarbon, in 1801 Clement and Desormes determined the chemical composition. In 1877, CO was liquefied for the first time by L. Cailletet. The carbon monoxide amounts of the earth’s atmosphere are for the most part based on the bacterial emission of CO in the soil and the sea. In addition, CO is found in volcanic gases. Anthropogenic CO-emissions result from the incomplete combustion of fossil fuels. Typically, industrial air contains about 0.3 vppm of carbon monoxide. Near busy roads the mean value of CO amounts to about 0.6 vppm with peak concentrations of some vppm of CO occurring (immission values). In the exhaust gas of Ottoengines without catalytic converters, CO is contained with a mass portion of about 1.4% (emission value). The reduction of this CO-quantity by means of a catalytic converter installed in the exhaust system amounts to about 90%. 5.1.4.2


Carbon monoxide with the molar mass of 28.01 g mol–1 is colourless, odourless and tasteless and does not irritate the respiratory tracts. It is highly toxic, slightly lighter than air, poorly soluble in water (solubility: 23 mL L–1 at 20 °C and 1 bar) and combustible. Together with air, carbon monoxide forms explosive mixtures in the concentration range of a CO-volume fraction of 10.9–76%. It combusts in air with a bluish, very hot flame. The calorific value is 12.69 kJ m–3 resp. 12.93 kJ kg–1. Carbon monoxide liquefies at 1 bar and –191.5 °C and solidifies at –199 °C. In the laboratory, carbon monoxide (= anhydride of formic acid) is obtained by dripping concentrated sulphuric acid into formic acid at temperatures over 100 °C [5.11]. In engineering, it is obtained by separation from synthesis gas. In steel production, CO occurs as reducing agent of ferric oxide, where it exhausts as blast-furnace gas in large quantities. There are numerous complex bonds of transition metals with CO called carbonyl complexes. Examples are Ni(CO)4 and Fe(CO)5. The possibility of a carbonyl formation at high CO-partial pressure has to be taken into consideration when choosing the material of equipment and piping. The lifespan of CO in the atmosphere is assessed at 1–2 months, with the reaction of CO with OH-radicals to CO2 and Hx being regarded as decomposition reaction [5.7]. CO is a highly toxic gas with a threshold limit value (TLV) of 30 vppm. The reason for its toxicity is its property to displace the oxygen from the haemoglobin-complex

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of blood, since the affinity of haemoglobin (Hb) to CO is about 300 times higher than to O2. A concentration of 660 vppm of CO is enough to block about half of the Hb for the oxygen transport. The bond of CO to haemoglobin is reversible, the CO-decomposition however occurs very slowly. In CO-free air the whole adsorbed CO can be replaced by O2 again, thus re-establishing the functional capability of the haemoglobin. Through ventilation with pure O2, this process is accelerated considerably. The symptoms of a CO-intoxication mainly arise through oxygen deficiency in the tissue. The haemoglobin of a heavy smoker of cigarettes can reach a CO-saturation of up to 15% in the course of a day. Other important properties of carbon monoxide are listed in Table 5.3 [5.12–5.15]. Table 5.3 Physical properties of carbon monoxide. Unit

Carbon monoxide

g mol–1

28.010

kg m–3 J mol–1 K–1 W m–1 K–1

1.250 Cp = 29.05, Cv = 20.68 0.02324

K

81.65

K

74.15

K kPa kg m–3

132.29 3496 301

x Temperature x Pressure

K kPa

68.05 15.25

Explosion range (in air at 101.3 kPa)

% volume fraction

10.9–76

Ignition temperature (in air at 101.3 kPa)

°C

605

Threshold limit value (TLV)

ppm

30

Molar Mass Properties at 273.15 K, 101.3 kPa x Density x Molar heat x Thermal conductivity Boiling point (101.3 kPa) x Temperature Melting point (101.3 kPa) x Temperature Critical point x Temperature x Pressure x Density Triple point

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5.2 Production of Synthesis Gas

143


The term synthesis gas stands for a multitude of different gas mixtures consisting of H2, CO and N2, partially with traces of hydrocarbons, CO2 and Ar. Similarly high as the number of the different and technically relevant synthesis gases is the number of the practically applied production processes [5.16–5.18]. For some years, central importance has been attributed to hydrogen as a future energy carrier. However, the use of this energy carrier represents a solution to the problem of carbon dioxide emissions only if hydrogen is generated from regenerative energy sources like sun, wind or renewable organic raw materials, without carbon dioxide as a by-product [5.19]. All production processes from regenerative energy sources are not yet on a technical level that would enable to meet the current demand for hydrogen, aside from the demand that would arise with the utilization as an energy carrier of the future (cf. also Section 5.4.1.2). Nevertheless, the development regarding the use of hydrogen as energy carrier is progressing rapidly, although at present the conventional hydrogen production from hydrocarbons is still to prevailing [5.20]. These production processes from hydrocarbons as well as the electrolysis of water for the production of hydrogen are explained in detail. 5.2.1 Production of Hydrogen by Electrolysis

A technically far-developed process is the electrolysis of water for the production of hydrogen, although only about 5% of the hydrogen is produced by means of electrolytic processes worldwide [5.21]. The efficiency is limited to smaller hydrogen capacities up to about 5000 mN3 h–1 and the availability of cheap electric energy, for example from water power. Core element is the electrolyser consisting of two electrodes: the negatively charged cathode and the positively charged anode immersed in an electrolyte solution. Since pure water is a poor ionic conductor, alkaline media such as potassiumhydroxide (KOH)-solutions are predominantly used in technical electrolysers. Figure 5.2 shows the structure of an alkaline electrolyser [5.22]. The diaphragm, which is permeable to OH–-ions but separates the generated gases, is arranged between the electrodes. The following electrochemical reactions occur at the electrodes after the application of voltage: Cathode:

2 H2O + 2 e– o H2 + 2 OH–

Anode:

2 OH– o ½ O2 + H2O + 2 e–

The complete process running in the electrolyser results from the sum of these two reactions: Electrolyser:

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H2O + electrical energy o H2 + ½ O2

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Fig. 5.2 Alkaline electrolysis cell.

The concentration of the electrolyte in the solution amounts to a mass fraction of usually 25–30%, the temperature is about 80 °C and the pressure depending on the required product pressure is 1–30 bar. The electrodes are corrosion-resistant, good electric conductors and support the formation of the gases by means of structured surface. 5.2.2 Production of Synthesis Gas from Hydrocarbons

In the production of synthesis gases from hydrocarbons, the components hydrogen and carbon monoxide usually appear as complementary products, carbon dioxide can be obtained as a by-product as well. Apart from hydrocarbons and steam (steam reforming), some processes require carbon dioxide (CO2 reforming) as well as oxygen or air (partial oxidation and autothermal reforming) as feedstock. Usually, the process selection depends on two factors: 1. The desired product composition, usually characterized by the so-called H2/COratio in the raw synthesis gas. 2. The feedstock available (natural gas, residual gases from refineries, LPG (Liquefied Petroleum Gas), naphtha, heavy oils, distillation residues, pitch, coal, carbon dioxide, oxygen) and process utilities (steam, cooling water, …). For a systematic structure, it is practical to distinguish between the actual synthesis gas generation and the synthesis gas processing. In processes requiring more or less pure oxygen (partial oxidation, autothermal reforming), the air separation plant that may have to be installed has to be considered with regard to the selection of the process and the economic evaluation [5.23].

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5.2.2.1

145

Generation of Synthesis Gas by Steam Reforming

In this process, light feedstock like natural gas, LPG, refinery residual gas (ROG) or naphtha are converted into carbon monoxide and hydrogen under the addition of water vapour. Based on the reaction of methane (CH4), representing the main component of natural gas, with water vapour the most important aspects of the reaction process will be explained: CH4 + H2O CO + 3 H2

'HR = 206 kJ mol–1

(5.1)

This so-called reforming reaction is strongly endothermal, and due to the chemical equilibrium the conversion is limited. According to the Le Chatelier principle, a reaction system with heat supply at high temperatures and low pressure is best for the realization of high conversion rates. In the so-called steam reformer, high-temperature resistant tubes filled with nickel catalyst are heated by means of an external furnace. Apart from the reforming reaction, the weakly exothermal water–gas shift reaction, also determined by the chemical equilibrium, takes place in the reforming tubes parallel to the reaction (5.1): CO + H2O CO2 + H2

'HR = –41 kJ mol–1

(5.2)

For higher hydrocarbons, the reforming proceeds according to the general scheme: CnHm + n H2O n CO + (n + m/2) H2

(5.3)

Due to the sensibility of the nickel catalyst for poisoning, the feedstock has to be purified from catalyst poisons like sulphur and chlorides before the reforming. First with the addition of hydrogen the organic sulphur components like mercaptanes and thiophenes are hydrogenated to hydrogen sulphide (H2S). The hydrogen required is taken from the product flow and compressed for recirculation. After the hydrogenation over a cobalt/molybdenum catalyst (CoMo), the hydrogen sulphide can be easily removed with zinc oxide (ZnO) adsorbents by means of chemisorption: H2S + ZnO o ZnS + H2O Especially for higher hydrocarbons, an over-stoichiometric steam to carbon ratio S/C has to be adjusted at the inlet of the reforming tube to avoid soot formation on the nickel catalyst. The higher the steam flow, the higher the methane conversion due to the chemical equilibrium, however, at the expense of a higher firing duty. Efficient values for the S/C ratio in the generation of hydrogen-rich synthesis gases range between 2.5–3.5, for the reformer outlet temperature values of 850 to 920 °C are common which are limited, however, due to the metallurgy of the reformer tubes, especially at higher pressures. A low reformer pressure is advantageous for a high methane conversion. Nevertheless, usually a high natural gas pressure

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Fig. 5.3 Basic types of radiation zones in steam reformers.

should be utilized to avoid energy-intensive product compression of synthesis gas. Nowadays steam reformers with pressures of up to 40 bar are feasible due to the enhancement of the mechanical stability of the materials at high temperatures. For the generation of a CO-rich synthesis gas, the S/C-ratio has to be reduced because of the water–gas shift equilibrium, however, due to the risk of soot formation a minimum value, depending on the selected catalyst, must not be fallen below. Downstream the process, carbon dioxide is scrubbed out of the raw synthesis gas, which is then to be compressed and recycled to the reformer. With this scheme and the import of additional carbon dioxide, the CO-content in the synthesis gas can be further increased [5.24]. Up to a synthesis gas capacity equivalent to about 6000 mN3 h–1, so-called cantype reformers are the most economical alternative for the technical realization of a steam reformer; for capacities exceeding this value, box reformers with the reforming tubes arranged in rows are more economical. Figure 5.3 shows the so-called radiation zone of reformer types used in practice which differ from each other in the arrangement of the catalyst tubes and burners in the furnace box. Due to the high outlet temperatures of the reformer, only about 50% of the fired heat is absorbed by the reformer tubes. The so-called convection zone follows the radiation zone. Here, the energy of the flue gas, which is still up to 1000 °C hot, is used for the preheating of feedstock, for the generation of steam and optionally for the preheating of combustion air. Nowadays, steam reformers with up to 1000 tubes and a hydrogen capacity of up to 300 000 mN3 h–1 are used in technical plants [5.21]. 5.2.2.2

Synthesis Gas Generation by Partial Oxidation (PO)

Contrary to the catalytic steam reforming, the non-catalytic partial oxidation process is also suitable for heavier hydrocarbons such as heavy oils, pitch or coal. Usually, oxidation takes place with pure oxygen. If air is used, the effort for the separation of nitrogen from the synthesis gas would be higher than for the air separation. The reaction is strongly exothermal and is realized uncooled in the

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so-called PO-reactor (or gasification reactor, as the process itself is often called gasification) under oxygen deficiency. Special developed burners ensure a good mixing and complete oxygen conversion: CnHm + n/2 O2 o n CO + m/2 H2

(5.4)

For the atomisation of liquid feedstock, a certain quantity of steam is added to the hydrocarbon stream. Natural gas can be used as feed for the partial oxidation as well, especially if cheap oxygen is available and/or H2/CO-ratios are required smaller than those achievable with the steam reforming process: CH4 + ½ O2 o CO + 2 H2

'HR = –36 kJ mol–1

(5.5)

In the case of oxygen excess, too much of the feed is fully oxidized to undesired CO2 under large development of heat. CH4 + 2 O2 o CO2 + 2 H2O

'HR = –803 kJ mol–1

(5.6)

Insufficient oxygen results in a too low outlet temperature with a high soot formation rate: CH4 o C + 2 H2

'HR = 75 kJ mol–1

(5.7)

2 CO o C + CO2

'HR = –173 kJ mol–1

(5.8)

The oxygen quantity determines the adiabatic reactor outlet temperature. Due to the high temperature, the reactor has to be brick lined inside with heat-resistant materials. After the actual oxidation in the combustion zone, the gas mixture passes through the so-called reaction zone, in which mainly the reforming reaction (5.1) described in the section on steam reforming and the water–gas shift reaction (5.2) occur. Having passed this zone, the gas has a temperature of typically 1200–1400 °C and requires cooling. Figure 5.4 shows a PO-reactor with combined quench cooling and synthesis gas cooler. In the quench chamber, the gas is cooled by quenching with water. Thereby, the temperature of the water dew point of the synthesis gas is reached. In the synthesis gas cooler, the process heat of the hot gases is used for the generation of high-pressure steam. Downstream, the cooled or quenched synthesis gas passes the so-called scrubber, where the possibly formed soot is scrubbed out with water. In process plants, two PO-technologies are applied, which differ from each other, amongst others, in their way of soot-recirculation: the Texaco-method and the Shell-method. Soot-recirculation is essential, especially when heavy hydrocarbons are used [5.25]. Contrary to steam reforming, these two processes can be applied at significantly higher pressures (Texaco: 85 bar, Shell: 65 bar). Since it is a non-

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Fig. 5.4 Texaco PO-reactor with quench and heat recovery boiler.

catalytic process, the pre-treatment of the feedstock for the removal of sulphur from the hydrocarbons is not required, a sophisticated acid gas removal from the synthesis gas is necessary instead. 5.2.2.3

Generation of Synthesis Gas by Autothermal Reforming (ATR)

Autothermal reforming is often regarded as a combination of partial oxidation and steam reforming. The energy required for endothermal steam reforming is generated through partial oxidation of the hydrocarbons in presence of steam in the upper part of the reactor (reactions (5.4) to (5.6)). Figure 5.5 shows a typical autothermal reformer with burner and adiabatic catalyst fixed bed in the lower part. As in the case of steam reforming with catalyst tubes fired from outside the endothermal methane reaction occurs on a nickel catalyst. Due to the lack of heat supply, the gas mixture cools down and reaches almost the value of the methane and water–gas shift equilibrium according to reactions (5.1) and (5.2) at the outlet. Typical values for the outlet temperature vary from 900 °C to 1100 °C [5.26] depending on the application. Since the autothermal reformer has to be brick lined similar to the PO-reactor, there is no limitation to pressures of max. 40 bar compared to steam reformers with catalyst tubes. Besides hydrocarbons, autothermal reformers are also fed with pre-reformed H2and CO-rich gas mixtures from a steam reformer (the so-called primary reformer). In this case the ATR is also called secondary reformer. For the generation of synthesis gases for the ammonia synthesis, compressed air instead of pure oxygen is used.

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149

Fig. 5.5 Autothermal reactor with burner and catalyst bed.

Besides the oxygen required for the partial oxidation, the nitrogen required for the ammonia synthesis is fed as well. In more recent reactor concepts, the hot synthesis gas at the outlet of the autothermal reformer is used for the heating of the catalyst tubes in the primary reformer. In general, this system is called convective reformer. Figure 5.6 shows an example of the so-called tandem reformer for the generation of methanol synthesis gas.

Fig. 5.6 Tandem reformer.

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5.2.3 Synthesis Gas Processing

Usually, the gas mixture from the synthesis gas generation section has not the composition and purity required for the respective application. Therefore, the synthesis gas generation is followed by the raw synthesis gas processing. The process steps presented here can also be applied for the processing of the desired synthesis gas mixture from so-called residual gases of refineries and petrochemical plants. In this case, the synthesis gas plant only consists of the synthesis gas processing section, while the generation part is not required; this represents the most economical variant for the generation of synthesis gas. 5.2.3.1

Water–Gas Shift Reactor

This reactor used for the conversion of CO according to the water–gas shift reaction (5.2) is found in each plant for the production of hydrogen and ammonia synthesis gas. Furthermore, it is used to increase the H2/CO-ratio in H2/COplants. Depending on the temperature range, a distinction is made between hightemperature shift, medium-temperature shift and low-temperature shift (HTS, MTS and LTS). Since the water–gas shift reaction is an exothermal equilibrium reaction, the residual-CO content is the lowest at low temperatures. However, the reaction rate at the catalyst as well as the dew point of the synthesis gas determine the limits of the technical feasibility [5.27]. Usually, the reactors are designed as adiabatic fixed beds with the temperature rising during the reaction. The mediumtemperature shift reaction can be carried out in isothermal reactors with steam generation [5.28]. For sulphur containing synthesis gases from partial oxidations, stable and sulphur-resistant catalysts are available for the so-called Dirty Shift. 5.2.3.2

Removal of Carbon Dioxide and Acid Gases

Usually, carbon dioxide and acid sulphuric components like H2S and COS are removed using chemical or physical scrubbing processes. There is a variety of solvents applied. Representative of the chemical solvents, the amine-containing detergents MEA (monoethanolamines) and the activated aMDEA (methyldiethanolamines) from BASF shall be mentioned. Besides that, the so-called Benfield process with hot potash (K2CO3) as solvent is of technical importance. In the scrubbing column of a chemical wash process the carbon dioxide reacts with the solvent; in the so-called regeneration column the bound CO2 is stripped out again through steam generated by energy supply in the bottom. The energy is provided by the cooling of synthesis gas in the process or by condensing lowpressure steam. The process flow diagram of an aMDEA-scrubbing process according to the BASF process can be found in Section 6.2.2. In the case of sulphur containing synthesis gases from a partial oxidation, amine scrubbing processes can be applied as well in which the sulphuric components H2S and COS are scrubbed out together with CO2. In the case of higher sulphur contents (coal and heavy oil gasification) the sulphur components have to be removed selectively. A physical scrubbing process such as the so-called Rectisol

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Fig. 5.7 Rectisol scrubbing with selective CO2- and H2S-removal.

process is applicable to these conditions. Here, the acid gas components are scrubbed out in the scrubbing column with cold methanol. With an adapted process flow arrangement in the regeneration part, a sulphur-rich fraction can be obtained which is applicable for processing to elemental sulphur in a Claus reactor. Contrary to chemical scrubbing, physical scrubbing is applicable for feedstocks at high pressure and high content of acid gas components to be scrubbed. The process flow chart of a Rectisol-scrubbing with scrubbing columns, stripper, hot regeneration and methanol/water separation is depicted in Fig. 5.7. The refrigeration plant required for this process is not shown. 5.2.3.3

Methanation

After a water–gas shift reaction at low temperatures (LTS) and the following CO2removal, the synthesis gas can be purified in a so-called methanation to remove smaller traces of CO and CO2. Due to the exothermal reversed steam reforming reaction (5.1) smaller quantities of methane are formed which can be accepted as inert component, for example, in the ammonia synthesis, however, only at the expense of a so-called purge gas flow from the synthesis loop [5.18]. Usually, this last purification step is not suitable for the generation of high-purity hydrogen. 5.2.3.4

Pressure Swing Adsorption (PSA)

This adsorptive purification method is suitable for the production of high-purity hydrogen from raw synthesis gas and residual gases from refineries. From the crude hydrogen at high pressure with CO, CO2, H2O, CH4 (in residual gases higher hydrocarbons as well) all components are adsorbed, however, the adsorption of hydrogen is only very low. On this selectivity the separation effect of the pressure

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Fig. 5.8 Hydrogen yield as function of the residual and adsorption pressure.

swing adsorption is based. The unadsorbed, passing hydrogen is highly-pure and under high pressure. After the adsorption phase, the adsorbed components are desorbed by means of pressure decrease, finally accumulating at low pressure in the so-called tail or purge gas. Usually, this tail gas is combustible and can be compressed, for instance, to be fed into the fuel gas system. In case higher tail gas pressure and desorption pressure is chosen for the PSA-design, compression is not required, however, at the expense of the hydrogen yield, which decreases rapidly with increasing tail gas pressure. This effect is shown in Fig. 5.8. A very common practice in steam reformer plants is the utilisation of the tail gas of the PSA as fuel gas for reformer firing. Based on a suitable combination of pressure compensation and purging steps, the hydrogen loss in the tail gas can be minimized to the effect that large plants with up to 16 adsorbers reach H2-yields of more than 90% (for PSA see also Sections 2.2.4 and 2.2.5.6). 5.2.3.5

Membrane Processes

When processing synthesis gases with a membrane process, the light component hydrogen passes from the synthesis gas mixture through a membrane. This permeation is a combination of diffusion and solubility of the gas, e.g. in a polymer membrane. Due to the required pressure difference between the feedstock side and the permeate, up to 98% of pure hydrogen is obtained at low pressure, with the yield decreasing with increasing purity [5.29]. This effect is depicted in Fig. 5.9 for different pressure conditions between raw synthesis gas and permeate. The heavier components such as carbon monoxide and hydrocarbons are retained and accumulate in the retentate. Apart from the hydrogen recovery from purge gases, membrane methods are suitable for the hydrogen recovery from H2-containing residual gases from refineries and for the adjustment of a desired H2/CO-ratio in combination with other processes like a pressure swing adsorption or a cryogenic separation (hybrid process). For example, from the raw synthesis gas from a partial

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Fig. 5.9 Hydrogen yield and purity in membrane processes.

oxidation of natural gas (H2/CO ~ 1.8) the ratio with a value of almost 1 required for an oxo-alcohol synthesis can be adjusted with a membrane process. 5.2.3.6

Cryogenic Separation Processes

Separation and purification processes at low temperatures make use of the big difference of the boiling points of hydrogen and the other components contained in the raw synthesis gas, like CO, CH4 (in the case of residual gases higher hydrocarbons as well) or N2. All processes have in common the necessity to remove upstream water and carbon dioxide traces completely from the feedstock to prevent these components from freezing at low temperatures, thus avoiding the blockage of equipment and piping. Nowadays, in modern processes this drying is done almost always with adsorption at molsieves, where the adsorber beds are regenerated thermally with heated gas. All equipment and piping of the cryogenic part are compactly cased in the so-called coldbox. In order to reduce the heat penetration into the coldbox, it is filled with insulation material, e.g. perlite, cf. also Section 2.2.5.7. Depending on the composition of the synthesis gas, two different kinds of processes are technically applied for the separation of raw synthesis gas into the components H2 and CO: x Condensation processes x Methane scrubbing With both processes almost all desired CO-purities are achievable; according to the requirements, the purity of the raw hydrogen can be improved with a downstream pressure swing adsorption.

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Fig. 5.10 Cryogenic condensation process. (1, 2) Plate fin heat exchanger; (3) Separator; (4) Hydrogen stripper; (5) CO/CH4-separation column; (6) Compressor.

Fig. 5.10 shows a condensation process with two separation columns. The dried raw synthesis gas is cooled in compact aluminium plate fin heat exchangers (1, 2) to such an extent that the gas phase, the raw hydrogen, can be separated from the CO/CH4-rich liquid phase at feedstock pressure (3). The “refrigeration” required for cooling is provided by raw hydrogen to be heated and the evaporating CO. In the hydrogen stripper (4), hydrogen still dissolved in the liquid phase is separated. The CO/CH4-mixture of the stripper bottom is separated in the second column (5), thus reaching the CO-purity at the top through the reflux of liquefied and sub-cooled pure CO from the multistage CO-compressor (6). The mixture from the top of the hydrogen stripper and the bottom of the CO/CH4-column results in an waste gas at low pressure and is suitable to be used as fuel gas. Condensation processes are preferably used for the separation of CO-rich high pressure synthesis gases with low methane content from partial oxidations. The raw hydrogen can be purified further in a PSA-plant. For raw synthesis gas from steam reformers with a higher H2/CO-ratio and a higher residual methane content, the methane scrubbing for the extraction of pure CO and crude hydrogen is more suitable than the condensation process. The process flow diagram is shown in Fig. 5.11. In the first, so-called scrubbing

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Fig. 5.11 Cryogenic methane scrubbing. (1, 2) Plate fin heat exchanger; (3) Scrubbing column; (4) Hydrogen stripper; (5) CO/CH4-separation column; (6) Compressor; (7) Expansion turbine.

column (3), the major part of the hydrogen is separated from the raw synthesis gas, which is cooled in the plate fin heat exchangers (1, 2), by scrubbing out the other components. Sub-cooled and highly pure, liquid methane is used as solvent, which is cooled with evaporating CO for the removal of the heat of solution. In the hydrogen stripper (4), the bottom product of the first column is purified from the still dissolved hydrogen. The CO/CH4-mixture of the stripper bottom is separated in the third column (5). The CO-purity at the top of this column is adjusted via the reflux of sub-cooled pure CO. A refrigeration balance of the process is achieved by the CO-loop through compressor (6) and expansion turbine (7). Excessive methane is released at low pressure together with the top product of the hydrogen stripper and usually used as fuel gas. Hydrogen can be delivered for numerous applications without further purification. The energy requirement of cryogenic separation methods is mainly determined by the energy required by the CO-compressor. There are a lot of variations for both processes, for example, too much nitrogen in the raw synthesis gas reduces the CO-purity. In such cases, an additional separation column for N2-separation is required, although this increases the energy requirement considerably. If besides methane higher hydrocarbons are found in the raw synthesis gas as well they can be separated by means of a suitable process arrangement.

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Fig. 5.12 Nitrogen scrubbing.

A technically important process for the production of ammonia synthesis gas is the nitrogen wash process. The synthesis gas, usually from a rectisol scrubbing process and mainly consisting of hydrogen, is purified from CO, Ar and CH4. High-pressure nitrogen, e.g. from the air separator for the PO-unit, is cooled and expands into the synthesis gas, due to which the “refrigeration” required for the process is generated [5.30]. The nitrogen serves as solvent for the removal of CO, simultaneously the H2/N2 ratio of almost 3, required for the ammonia synthesis, is adjusted (see Fig. 5.12). 5.2.4 Processes for the Production of Synthesis Gas from Hydrocarbons

This section exemplarily describes selected complete processes for the generation of synthesis gases of different composition.

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5.2.4.1

157

Reformer Plant for the Production of Hydrogen

Figure 5.13 shows the process flow diagram of a modern hydrogen plant with steam reforming, HT-shift and pressure swing adsorption (PSA) [5.31]. Feedstock is natural gas that has to be pretreated because of the contained sulphur traces. Before feeding to the reformer, the natural gas is heated to about 380 °C with a small quantity of recompressed hydrogen against hot synthesis gas. After the addition of steam and further heating against flue gas, the hydrocarbon/steam mixture is fed to the reformer. In the radiation zone, the tubes filled with the catalyst are heated by burners from outside. Hot gas at about 850 °C leaving the reformer contains with S/C = 2.7 about 50% H2, 10% CO, 5% CO2, 30% H2O and 5% CH4. After cooling down to about 330 °C, during which high-pressure steam is generated, the gas passes the adiabatic catalyst bed of the HT-shift reactor. The outlet flow still contains about 3% residual CO. Under further energy recovery (preheating of feedstock, preheating of boiler feed water) and the discharge of waste heat to air and cooling water, the synthesis gas is cooled down to the PSAentrance temperature of about 30–40 °C. In the PSA-plant, high-purity hydrogen is produced, the PSA-tail gas together with additional natural gas is used for the firing of the steam reformer. In the radiation zone, about 50% of this heat is absorbed by the catalyst tubes. The flue gas is routed into the convection zone at about 1000 °C, where it is cooled down to a stack temperature of about 150° (feedstock superheating, steam generation and superheating, air preheating). The steam generated in the plant is partially required as process steam, the rest is available for export.

Fig. 5.13 Steam-reformer plant for the generation of hydrogen from natural gas.

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Fig. 5.14 Photography of a steam-reformer plant.

Over the last 10–20 years, this type of plant has gained global acceptance, although there are slight differences in the various processes with regard to energy integration. Before PSA-plants with efficient H2-yield were available, process configurations with low-temperature shift, CO2-scrubbing and methanation were common. For modern hydrogen plants for the application in refineries, however, hydrogen purities are required that are only achievable by means of pressure swing adsorption [5.32]. The picture in Fig. 5.14 shows a modern steam reformer plant for hydrogen generation. Steam reformer plants are also suitable for the processing of light, liquid hydrocarbons like naphtha or LPG. The liquid feedstock is evaporated against cooling process gas and then, together with hydrogen, it is further heated for desulphurization. For the reforming in the catalyst tubes a special catalyst with alkaline components, usually potash, has to be chosen. Thereby, soot formation on the catalyst can be avoided. An alternative to this is the so-called pre-reformer that generates a pre-reformed mixture of CH4, CO and H2 at temperatures around 450–550 °C in an adiabatic fixed bed reactor from a hydrocarbon/steam mixture. Since the methane content is still high at these temperatures due to the chemical equilibrium, the gas is downstream fed to a conventional steam reformer. 5.2.4.2

Reformer Units for the Generation of CO and H2

The block diagram of a H2/CO unit (HyCO unit) is shown in Fig. 5.15. For the synthesis gas generation unit, the aspects described in the previous section apply. In contrast to the hydrogen plant, however, carbon dioxide is removed by means of chemical scrubbing process, then it is compressed and, completely part of it, fed to the steam reformer. In case additional CO2 is available, a further reduction of the H2/CO ratio in the raw synthesis gas is possible by means of CO2 import. This process does not allow the exclusive generation of CO, but H2

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Fig. 5.15 Steam-reformer plant for the generation of CO- and H2 from natural gas.

is always generated as a by-product. Thus, the most economical process is always the production of both gases. After the scrubbing, the synthesis gas has to be dried in an adsorber station and purified from CO2-traces. The separation to pure CO and raw hydrogen takes place in a methane scrubbing, the fuel gas produced there is used for firing of the reformer. In a PSA-unit, the raw hydrogen is processed to the required purity, with the tail gas being also used as fuel gas in the reformer. Contrary to the more simple hydrogen plant shown in Fig. 5.13, the H2/CO plant requires additional energy for the CO and CO2 compressors. Furthermore, thermal energy from the synthesis gas to be cooled is required for the regeneration of the solvent, which results in reduced steam export. 5.2.4.3

PO-plant for the Production of CO and H2

The generation of synthesis gas from heavy oil by means of steam reforming is not possible. Usually, a PO-process for the gasification of heavy oil is chosen instead. Figure 5.16 shows the required process steps of a PO plant for the production of hydrogen and carbon monoxide. As by-product, the rectisol-wash supplies a H2S-rich gas suitable for being processed to elementary sulphur. Due to the inevitable soot formation in the PO-reactor, the soot has to be scrubbed out of the synthesis gas and then be separated from the soot water. In the Texaco-process, usually naphtha is used for this, which is mixed with a part of the heavy oil and then evaporated. The soot does not evaporate, together with the heavy oil, it is fed back to the PO-reactor. In a condensation process, the CO-rich synthesis gas is separated into pure CO and raw hydrogen, the purity of the product hydrogen is adjusted in a PSA-unit. In the process described, the tail gas of the PSA is recompressed and recycled to the scrubbing unit, thus a CO-yield of almost 100% is achieved. The photo in Fig. 5.17 shows a modern PO-plant for the production of CO and H2.

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Fig. 5.16 PO-plant for the generation of CO- and H2-generation from heavy oil.

Fig. 5.17 Photography of a PO-plant.

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5.3 Process Analytics

A number of process appliances is used for the production monitoring in synthesis gas plants. Depending on concentration and kind of component to be examined, they are based on different measuring principles. Analytics support the control of process steps such as partial oxidation, CO2-scrubbing, adsorption on molecular sieve and product purities. Apart from the process analyzers, a number of gas analyzers with electrochemical sensors are applied to ensure that operating staff is not exposed to explosive methane/H2/air mixtures or increased O2-concentrations, and that the threshold limit value (TLV) of 30 vppm for CO is not permanently exceeded. Such gas analyzers are installed at potential leakage points (e.g. natural gas compressor, reformer, hydrogen compressor). Special safety measures are to be taken for the analysis room. Via methane and H2-monitoring, the non-explosion-proof analyzers are switched dead e.g. at a concentration of 50% of the lower explosion limit (LEL) in the room. Further measures are: Limiting the feeding of combustible or toxic media into the analysis room by a measuring orifices; ventilation has to ensure that in case of a line break 50% LEL are not exceeded. An alarm system outside the analysis room indicates: Failure ventilation, combustible gas > 20% LEL, CO-alarm, oxygen deficiency, collected faults for gas alarm system, O2-monitoring and smoke alarm. The safety regulation for analysis rooms according to DIN EN 61 285 has to be obeyed. In the following, a typical example of a synthesis gas plant on the basis of partial oxidation is described. The process analytics of which is specified in Table 5.4. The feedstock consists of natural gas with methane (87.7% mole fraction), ethane (6.8% mole fraction), propane (1.5% mole fraction), nitrogen (1.3% mole fraction), carbon dioxide and hydrogen (1% mole fraction each) as the main components. The process steps are as follows: After a catalytic sulphur removal the non-catalytic partial oxidation takes place at 1400 °C generating CO, CO2, H2, H2O. After a soot scrubbing, a partial flow of the product is directed from the partial oxidation reactor to the shift reactor, in which the adjustment of the desired CO/H2-ratio in the synthesis gas takes place. This includes the conversion of CO with water vapour to CO2 and H2 on a Cu-catalyst. After an ensuing CO2-scrubbing on methyldiethanolamine basis, the combined synthesis gas flows reach a membrane separation unit, consisting of a bundle of organic high-performance capillaries, where the hydrogen is separated as permeate. The separated hydrogen is fine scrubbed in a pressure swing unit (molecular sieve filling). The retentate (residual H2, CH4, N2, CO) of the membrane is passed to the compression as synthesis gas product.

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28.4% 2.2% 51% 0.45% 17.5% 70.5% 24.4% 3% 0.4% 0.4%

CO: CO2: H2: CH4: H2O: H 2: CO2: CO: CH4: N2: H2: CO: CH4: N2: CO2: H2: N2: CH4: CO:

Synthesis gas after partial oxidation and soot separator

Synthesis gas after shift reactor

Synthesis gas after CO2-scrubbing

Hydrogen 18 bar Outlet pressure swing adsorption 99.98% 200 ppm 1 ppm 1 ppm

66.4% 32.2% 0.6% 0.5% 0.07%

Composition of gas mixture1)

Measuring point

Process gas chromatograph: Flame ionization after CO-methanization redundant: infrared

Infrared spectrometry

Process gas chromatograph: Thermal conductivity

Process gas chromatograph: Thermal conductivity

Measuring principle

Table 5.4 Process analysis in a synthesis plant based on partial oxidation.

0–50% 0–5% 0–10% 0–70% 0–3%

CO: 0–3 ppm (GC) CO: 0–10 (IR)

CO2: 0–0.2%

CO: 0–5%

CO: CO: CO2: H2: CH4:

Measuring range1)

Hydrogen product control

Control CO2-scrubbing

Adjustment H2/CO-ratio in the synthesis gas to membrane separation: 1.5–2.5 (mol/mol)

Monitoring of partial oxidation

Cause

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1)

Conductivity: < 0.2 µS cm–1

Boiler feed water

All concentrations indicated in % molar fraction or ppm molar fraction.

Electrodes

pH-electrodes

pH:

Boiler feed water

9–10

Amperometry

dissolved O2 < 0.1 mg L–1

Boiler feed water

0–0.5%

0–8%

5–10 Cond.: 0–5 µS cm–1

pH:

diss. O2: 0–0.2 mg L–1

O2:

ZrO2-probe

O2: 0% Composition: variable

Off-gas for combustion

O2:

Paramagnetism

71.5% 16.3% 8.9% 2.5%

N2: H2O: CO2: O2:

Flue gas

0–70% 0–70% 0–3000 ppm 0–0.5 ppm 0–0.5 ppm 0–3%

Measuring range1) H 2: CO: CO2: COS: H2S: CH4:

48.7% 49.4% 0.11% 0.87% 0.7% 0.15%

H2: CO: CO2: CH4: N2: Ar:

Synthesis gas 65 bar, Retentate from membrane separation

Measuring principle Process gas chromatograph S-components: Flame photometry Other components: Thermal conductivity

Composition of gas mixture1)

Measuring point

Table 5.4 (continued)

Suppression of corrosion



Exclusion of reverse flow of air

Verification of the completeness of combustion

Product control Synthesis gas

Cause

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5.4 Applications of Hydrogen, Carbon Monoxide and Syngas

Typical applications of hydrogen, carbon monoxide and synthesis gases are listed in the following chapter. In the case of hydrogen they are divided by industry segments (Section 5.4.1). The use of hydrogen in the chemical industry/refineries is described in more detail (Section 5.4.1.1). Encouraged by the rapidly increasing importance of hydrogen as a sustaining and emission-free energy carrier, Section 5.4.1.2 additionally deals with some aspects of the speedily developing hydrogen technology. The ensuing Section 5.4.1.3 discusses the application of hydrogen in fuel cells. 5.4.1 Applications of Hydrogen

The applications of gaseous hydrogen (GH2) take advantage of its low density, its high thermal conductivity and/or its reducing/hydrogenating effect. The application of liquid hydrogen (LH2) is especially determined by its energy density and the environmentally friendly overall balance regarding recovery, handling and combustion. In the processing industry/metallurgy hydrogen is used x as a welding gas component for TIG-welding (Ar/H2-mixtures) x as a root shielding gas component for electric arc welding (together with nitrogen or argon) x as a shielding gas component for the generation of float glass (in mixture with N2) x to provide a shielding atmosphere in the stainless steel manufacturing x to boost bell-type annealing furnaces for sheet metal coils in cold rolling mills (reduced heating and cooling phase with H2) x for high-pressure gas quenching after annealing furnaces for tempering and hardening (e.g. substitute of oil baths) In chemistry/refining hydrogen is used x to gain a synthesis gas for the generation of ammonia (with N2) or methanol (with CO) x to hydrogenate higher-boiling oil distillates for the recovery of fuels (see Section 5.4.1.1) x to hydrogenate inedible oils to produce soaps, ointments and other special chemicals x to hydrogenate and remove sulphur compounds (e.g. desulphurization of fuels) x to hydrogenate unsaturated compounds and functional groups such as aldehydes, ketones or nitriles

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In environmental protection/energy engineering hydrogen is used x to substitute existing combustion engines by H2-operated drive systems to reduce exhaust gas pollution x to develop new LH2/GH2-storage systems for motor cars and busses to supply the necessary infrastructure (e.g. tanks and filling stations) x to develop carbon-free power stations and energy supply chains (avoidance of CO2-emissions and “greenhouse gases”) (see Section 5.4.1.2) x to operate fuel cells for mobile (“hydrogen-vehicles”) and stationary applications (e.g. energy emergency supply) (see Section 5.4.1.3) x to serve as a fuel for missiles/spacecrafts (LH2/LOX-engines) x to refrigerate high-speed turbines in power stations In others industries hydrogen is used to x to hydrogenate unsaturated fatty acids (e.g. production of margarine from vegetable oils by fat hardening in autoclaves) x to remove nitrogen compounds from drinking water (e.g. denitration) x to remove oxygen from cooling water in power stations (to avoid stress cracking corrosion in piping, boilers and heat exchangers) x to serve as a buoyancy medium for balloons x to serve in highest purities as shielding or carrier gas for the manufacturing of semiconductor components (e.g. ion implantation, epitaxy) 5.4.1.1

Hydrogen Use in the Chemical Industry

Oil refineries are both producers and consumers of hydrogen, the volumes consumed usually exceeding those internally produced. Hydrogen is needed as a reactant in hydrogenating processes, as hydro cracking and hydro treating. Since refineries are shifting their output to larger quantities of low-sulphur, lowaromatics and brighter products, demand for hydrogen will continue to grow. The main processes for production of hydrogen are: x Platforming as the main hydrogen producing process in the refining process x Recovery from refinery gases, e.g. by pressure swing adsorption after platforming step x Steam reforming of methane as an external hydrogen source x Gasification of oil refining residues and recovery from synthesis gas The main processes for the consumption of hydrogen are: x Hydro-treating 0– 90 Nm3 H2/m3 feed x Hydro-desulphurization 15–275 Nm3 H2/m3 feed 20–100 Nm3 H2/m3 feed x Hydro-refining 0–100 Nm3 H2/m3 feed x Hydro-dealkylation 300–450 Nm3 H2/m3 feed x Hydro-cracking Basic Chemicals and Catalyst Production: Hydrogen is a reactant in the manufacture of basic chemicals and intermediates as well as specialty chemicals and

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Fig. 5.18 Hydrogenation of phenol to cyclohexanone.

pharmaceuticals. Synthesis gas, a mixture of hydrogen and carbon monoxide, is employed above all for synthesis of alkanes, methanol and in the hydroformylation of olefins to aldehydes and alcohols. Ammonia is manufactured from a different synthesis gas mixture of hydrogen and nitrogen. Hydrogenation is generally of high importance and involves the homogeneous or heterogeneous catalytic addition of hydrogen to organic compounds. Examples are: x Hydrogenation of adipic acid dinitrile to hexamethylene diamine x Hydrogenation of benzene to cyclohexane x Hydrogenation of phenol to cyclohexanone (see Fig. 5.18) Industrial hydrogen is also used in the manufacture of catalysts for reducing metal oxides to the active metallic form and to regulate chain length in the polymerization of propylene to polypropylene and in the manufacture of polyethylene. 5.4.1.2

Hydrogen as an Energy Carrier

Introduction: The crude oil era cannot last forever, although exactly when it will end is something no-one can reliably predict at present. However, experts [5.33] estimate that global oil production is almost certain to peak within the decade (between 2000 and 2010) and push the fossil fuel supply and demand curve out of balance as a result (Fig. 5.19). For every barrel of crude oil discovered in new reserves, four barrels are now consumed. Peak oil production will mark a decisive turning point in market perceptions surrounding the availability of crude oil [5.34]. For the first time, depletion of fossil fuel reserves will start to have a direct impact on national economies, with increasing global energy demands inevitably also inflating oil prices, as is already the case. Particularly given that petrol and diesel engines alone consume half of today’s entire crude oil production, researchers

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Fig. 5.19 Global oil production will probably exceed its vertex in this decade. This is one of the significant reasons, to look for alternative energy sources. (Source: Jolin J. Campbell and Jean H. Laherrère, “The End of Cheap Oil”, Scientific American, March 1998).

and developers worldwide are searching for suitable energy carriers and sources to ensure a sustainable supply of energy for automotive transport. And since global markets will continue to require enhanced mobility for people, goods and services, tomorrow’s energy sources must also be expandable. Increased mobility has also exacerbated the CO2 problem (see Section 6.1.1). Experts meanwhile agree that our use of fossil fuels is a significant factor in climate change, if not the main cause [5.35–5.37]. In view of these facts, the spotlight is increasingly turning to research and technology players to find new, sustainable energy carriers and sources (wind, water, sun, biomass and biogas). The most promising energy carrier of the future is currently hydrogen. Hydrogen and Mobility: Hydrogen is not a source of energy, but an energy carrier that has to be produced – similar to electricity. However, it has a significant advantage over the latter: hydrogen stores well. This means it is an ideal way to store electricity. However, its real potential lies in mobile applications. It is the only sustainable, zero-emission fuel source capable of powering an unlimited number of vehicles and other applications. Hydrogen is also able to absorb a local energy surplus and release it in a different place on demand. This enables a reliable, constant flow of energy – even when drawing from energy sources that are not continuously available. As an energy carrier, H2 could therefore play an important role both medium and long term in securing our mobility. Because it is a zero-emission fuel, it can power cars, aeroplanes, buses and ships, for instance, with water as the only waste product. Since transport accounts for a major part of our energy consumption and draws almost exclusively on fossil fuels, this is the most significant potential application of this new energy carrier.

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Fig. 5.20 The carbon share of energy carriers has decreased continuously over the course of history. The ultimate goal is to completely do without carbon and abandon CO2 emissions as a consequence.

Feasibility: Any new, cutting-edge energy carrier can only be successful if it fulfils certain criteria. For instance, it must reduce CO2 emissions throughout the entire energy chain. With hydrogen, this can be achieved by generating it either from sustainable energy sources or from fossil fuels (coal, oil, natural gas) with CO2 capture and storage [5.38]. Figure 5.20 shows the carbon-hydrogen ratio of various typical hydrocarbons. Energy carriers of the future must also guarantee unrestricted availability. Only traces of free hydrogen are present in the lower part of earth’s atmosphere, whereas chemically bound hydrogen as a component of water and other compounds is widely distributed. In fact, on the average, every sixth atom in the earth’s crust (including the hydrosphere and atmosphere) is a hydrogen atom [5.39] Releasing it from these compounds requires energy. For decades hydrogen has been a key element in chemical industry. The technologies required to produce hydrogen are already available and widely used in industry (see Section 5.2). When implementing hydrogen applications for use by the general public, the challenge particularly lays in the future energy source, rather than in the development of the energy transport means (read hydrogen or electricity). Diverse Supply Models: Although there are many possible hydrogen supply routes from today’s perspective there will not be one ideal route for all purposes. Rather than following a one-size-fits-all production path for all applications, a far more varied strategy is called for here. The main issue is diversification of supply. Initially, fossil fuels will continue to be the main source of energy production for electricity or hydrogen. However, in the medium and long term, the use of sustainable energy sources such as wind and hydropower, biomass and solar energy will account for a growing share of global energy and subsequent hydrogen production until they finally supplant fossil sources (Fig. 5.21).

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Fig. 5.21 Overview of hydrogen infrastructure for automotive application: Several production and supply paths are conceivable; particularly a broad diversification of production is the basis of a meaningful hydrogen infrastructure [5.55].

The best hydrogen production process must be selected on an individual basis and respectively take the application, location and region into account. Typical selection criteria include production capacity, availability of raw materials, demand patterns, product purity and pressure, energy and operating costs, investment costs and the value of by-products. Factors such as plant reliability or expandability as demand rises may also play a decisive role. Analysis of emissions over the entire energy chain (well-to-wheel analysis) [5.40, 5.41] shows that certain methods can already reduce emissions by using natural gas in hydrogen production. Ultimately, however, it is only methods that use sustainable energy sources or fossil fuels with CO2 capture and storage that are able to meet long-term CO2 reduction targets. Some supply paths may show higher or equivalent greenhouse gas (GHG) emissions than the current petrol or diesel-driven options. These paths would therefore only be acceptable for a relatively short transitional period. The idea is to leverage “low hanging fruit” approaches to push technological advances and drive down costs. The main conventional methods of hydrogen production are steam reforming, partial oxidation (POX) and electrolysis (see Section 5.2). The most economical method of hydrogen production is currently steam reforming of natural gas [5.42, 5.43]. CO2 generation is unavoidable in both first mentioned processes. The raw material used (fossil or regenerative) determines whether the CO2 produced results in a net increase in the atmosphere. Electrolysis has been in commercial use for over eighty years in its conventional form. It provides an obvious way to capitalize on regenerative energy sources (i.e. electricity generated

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with regenerative sources). Electrolysis will therefore gain in importance in the long term. Sustainability: Hydrogen production from sustainable energy sources (sun, wind, water, and biomass) is an almost perfect cycle, both sustainable and environmentally friendly. The current obstacle is that installed capacities of energy collection from sustainable sources are not sufficient to meet total global energy demand cost effectively. However, hydrogen production from sustainable energy is already cost-effective at locations that meet certain preconditions. A well-known example of this is the plan to build a hydrogen economy in Iceland by 2030. On one hand, geothermal springs and hydropower mean sustainable energy is available in abundance, and, on the other, importing crude oil is particularly expensive for such a remote island. Construction, maintenance and operation of the grid networks currently accounts for a large proportion of electricity costs, for instance [5.44]. These costs could be reduced with distributed, sustainable facilities with hydrogen as the storage medium. Supply Chain and Transport: To successfully build a hydrogen infrastructure, existing resources such as roads, natural gas pipelines and electricity grids must be integrated in the new infrastructure concept [5.45]. There are two basic options for distributing hydrogen to customers via a fuelling station network: the hydrogen can either be directly produced at fuelling stations or concentrated in large-scale plants and then transported to the fuelling stations. Taking these two general distribution paths as the departure point, there are currently five hydrogen supply paths under consideration: x Hydrogen is produced from natural gas in concentrated large scale, reforming plants (the same principle would also work biogas-, or biomass-based) and then transported as a liquid or as a compressed gas via the road to the H2 fuelling stations. x Hydrogen is produced in concentrated large scale, reforming plants attached to a gas pipeline network. It is then transported by pipeline to the H2 fuelling stations. x Hydrogen is produced directly at the fuelling station using small scale gas reforming facilities. It is then compressed before distribution. x Hydrogen is produced directly at the fuelling station using small scale electrolysers and compressed before distribution. x Hydrogen for early, short-term applications at fuelling stations can be supplied from an existing nearby H2 pipeline system serving refineries and chemical parks (after having been purified). All these supply paths must be weighed up in terms of overall energy consumption, emissions and cost. Producing hydrogen via electrolysis has the advantage that the electricity can be generated from almost all other energy sources, so it allows for a high degree of diversification across energy sources. Large scale concentrated hydrogen production is well suited to refinery locations, where steam reformers

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usually already produce industrial quantities of hydrogen for desulphurising fuels and hydrogenating crude oil fractions. Reformers could also be installed at locations with high demand such as urban centres, keeping transport distances short. Liquid hydrogen is transported to the fuelling stations in trailers or superinsulated tankers such as have been in daily use for many years. This is the most cost-effective option for distributing high quantities of hydrogen, although it requires significant investment. The technology for hydrogen liquefaction is already established (see Section 5.2.3.6). Looking beyond liquefaction, industrialscale compressed gaseous hydrogen can also be distributed via a pipeline network similar to that currently used for natural gas. The disadvantage of this path is a loss of flexibility. Liquid hydrogen can be easily turned into gaseous hydrogen at the station, but the reverse operation is currently only economically feasible on an industrial scale. Storage plays a key role in the hydrogen supply chain. Hydrogen is an extremely light gas and its low density poses technical challenges to its adoption as a fuel. At normal pressure, three thousand litres of gaseous hydrogen contain the same amount of energy as one litre of petrol, for example. To store and transport the gas effectively, it therefore has to be highly compressed. So it is either compressed (CGH2, compressed gaseous hydrogen) or cooled down to –253 °C so it liquefies (LH2, liquid hydrogen). In terms of mass (gravimetric energy density), LH2 has an outstanding storage value of 120 MJ/kg (petrol: 44 MJ/kg), but in terms of volume (volumetric energy density) it performs less well at 9 MJ/L (petrol: 33 MJ/L)

Fig. 5.22 Important elements of hydrogen infrastructure: Transport vehicles for compressed gaseous and liquid hydrogen. To transport an equal amount of hydrogen gaseous transport vehicles have to travel up to seven times more frequent [5.55].

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(see Section 5.1.3.2). This means that a car would either have to be fitted with a relatively large fuel tank to cover the same distance or it would have to refuel at much shorter intervals. Whether future vehicles will consume more or less energy than today’s vehicles depends very strongly on the type of drive train. With fuel cell electric drive trains the best fuel economy is possible [5.41]. The mobile storage challenge is best exemplified by a transport comparison: around six times more hydrogen – and therefore energy – can be transported by lorry trailer when the hydrogen is in liquid form. Figure 5.22 shows the different hydrogen transport capacities of two 40-tonne trailers with a net payload of approximately 500 kg (gas) and 3500 kg (liquid). In practice the differences are even larger, since the pressure differential with the customers’ stationary hydrogen tanks mean gas tankers cannot be completely emptied, but have to return for refilling with a certain residual pressure. The higher investments, personnel expenses and traffic volume involved in storing gaseous hydrogen must also be factored into economic viability assessments. Fuelling: The cost-effectiveness of different delivery systems must be taken into account when assessing fuelling, since for example the costs of distributing compressed hydrogen are higher than those of liquid hydrogen. Compressed hydrogen tanks make sense wherever volume and weight are not key factors, for example storage facilities on industrial premises. When considering storage systems for liquid hydrogen evaporation rates have to be taken into account. The latter is inevitable – despite super-insulation – a small portion of liquid hydrogen always gasifies inside the storage vessel due to the impact of external temperature. When, after some time, the maximum pressure is reached, hydrogen must be released to reduce pressure inside the storage vessel (boil-off gas). It is therefore important to develop vessels whose boil-off rates are as low as possible. The boiloff effect is heavily dependent on the volume to surface ratio of the vessel [5.46]. For example a 10 000 litres storage vessel used at a refuelling station can have a value of 0.1 vol.% per day after two weeks; an 80 litres passenger car tank can have a value of 3 vol.% per day after 4 days [5.47]. Significant progress has been made in the development of liquid hydrogen storage systems for vehicles. Linde AG has developed an innovative tank system, for example, which significantly extends the window before the maximum pressure is reached and hydrogen has to be blown off because of the tank design [5.47]. However, pressure has only to be released if the vehicle is not driven for an extended period. As soon as the vehicle is started, hydrogen is consumed and the tank pressure drops, so the boil-off clock is reset. Hence for fleet vehicles in constant use, for example, boil-off is rarely an issue. Alternatively, fuel cells could be used to easily convert evaporated hydrogen into electricity, thus avoiding pressure build-up if a vehicle is not driven for a long time. Alternative hydrogen storage methods such as chemical bonding in alanates and metal hydrides or graphite storage based on nanotechnology are still in development and currently nowhere near the efficiency of liquid and gaseous hydrogen storage.

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Safety: Particularly when it comes to personal mobility, the safety of a new energy carrier has to match that of established fuels, mainly petrol. The light hydrogen molecules volatilize very rapidly in the atmosphere which is a major advantage. Petrol evaporates more slowly and is heavier than air, so stays on the ground longer, where it is most likely to ignite. They also burn differently. As a liquid, petrol can leak and spread on the ground, burning with a wide flame that radiates very strong heat. Hydrogen, on the other hand, originating from the safety valve, burns with a narrow, almost upright flame that emits little heat [5.48]. However, in contrast to bright petrol flames, hydrogen flames are barely visible in daylight. The absence of carbon ensures soot-free combustion when burning hydrogen only producing water vapour. Hydrogen thus fares well when its combustion properties are compared with those of the energy carriers commonly used today (see Section 5.1.3). We can already draw on around thirty years’ experience surrounding the safety of gases in cars and the supporting infrastructure. This shows that liquid gas vehicles do not present any increased risk as long as the regulations in force are adhered to. Recent testing, for instance by TÜV SÜD [5.49], confirm that this also applies to hydrogen. In all hydrogen systems, safety concerns focus on parts that gas flows around or through, such as containers, pipelines, fuelling nozzles and valves. Hydrogen storage is therefore already subject to an almost seamless series of quality assurance measures, covering construction, manufacture and expert inspections. The properties of metallic equipment are influenced by hydrogen that entered the metallic matrix. Some metallic materials are sensitive to degradation mechanisms caused by the latter. In order to prevent damages on steel pressure equipment induced by gaseous hydrogen, material selection, design and fabrication of such equipment should be thoroughly assessed [5.50, 5.51]. Strict safety regulations apply to both the on-board components and to the supply infrastructure. Extensive experience is also available in this area, since natural gas, liquefied petroleum gas (LPG) and hydrogen have already seen decades of industrial use for power generation and as fuel. Safety measures and guidelines for novel mobile applications of hydrogen technology are being developed and to some extent already in place [5.52], and experience shows hydrogen is at least as safe to use as a vehicle fuel as conventional options such as petrol and diesel [5.48, 5.49]. Costs: Alongside availability and greenhouse gas (GHG) related issues, cost is a decisive factor governing the viability of a new energy carrier. Every well-towheel-analysis must include the given requirements concerning local, regional or long-distance conversion steps and transport options. Given that some hydrogen production paths include the CO2 emissions at some location along the path it is evident that the evaluation should extend beyond the fuelling station to include the wider picture. As a rule, low-emission and, especially GHG-free-energy carriers will entail higher costs. However, it should also be noted that fossil fuels in general are also becoming more expensive in the medium term and the cost

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gap between regenerative and fossil fuels is closing. All evaluations must also take account of the usage efficiency profile. In the case of vehicles, for example, this includes efficient use of hydrogen thanks to hydrogen-optimised internal combustion engines or fuel-cell systems (see Section 5.4.1.3). General cost analyses [5.53–5.56] show that: x Viewed across the entire supply chain, hydrogen liquefaction expenses are partially compensated by lower transport costs compared with the compressed hydrogen option. x Hydrogen management at the fuelling station determines the costs of fuelling with compressed or liquid hydrogen. x Local supply is influenced by yet unclear unit costs, and current electricity and natural gas prices. x In the long-term, pipeline delivery will only enable low hydrogen supply costs if there is a high substitution rate. A detailed examination of hydrogen supply costs at fuelling stations [5.55] shows that a hydrogen infrastructure can only be established successfully if it is integrated in the current filling station infrastructure. The report concludes that central (large scale and concentrated) hydrogen production and liquefaction followed by liquid hydrogen (LH2) delivery to the fuelling stations in cryogenic containers is the most cost-efficient option, both for liquid and compressed hydrogen (CGH2) fuelling (Fig. 5.23).

Fig. 5.23 Logistics of hydrogen can be arranged according to the supply concept of conventional fuel; additional to conventional fuels, refuelling stations are supplied with liquid hydrogen and are able to supply both liquid-, and compressed hydrogen vehicles.

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Fig. 5.24 In order to allow for long distance travel, up to the year 2020 approximately 2800 hydrogen dispensing refuelling stations would be needed in various European conurbations. Until 2030 the network would need to be continuously extended [5.55].

The costs of a comprehensive European infrastructure of hydrogen fuelling stations, including production and distribution, were determined for the first time in a study by E4tech [5.57]. This clearly demonstrates that the costs in real terms depend on the strategy and technical parameters adopted in developing the network. The study concludes that expansion of the fuelling network should concentrate on a few major European urban areas, ideally in countries such as France, Germany and the United Kingdom. Figure 5.24 shows how the supply network in these countries would have to be expanded to support long-distance travel in the future. Applications: Engineers in all the major automobile companies are working on drive-train concepts for hydrogen-powered vehicles. They are currently pursuing two main strategies to advance hydrogen technology – fuel cells and internal combustion engines. Developers now have around six hundred vehicles on the road worldwide. Some vehicle manufacturers are primarily focusing on the conventional spark ignited internal combustion engine, which is relatively simple to modify so that it can also operate on hydrogen. This strategy benefits from the technically evolved status of internal combustion engines, which hydrogenfuelled engines can of course draw on, as well as the possibility of transitional options until the hydrogen fuelling station network is dense enough to ensure long-distance travel. A car with a hydrogen combustion engine can switch to petrol when the hydrogen tank is empty. However, viewed long-term, everything

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points to intensive use of fuel cell technologies (see Section 5.4.1.3). Fuel cell development is therefore also regarded as a key success factor in the widespread use of hydrogen in transport. In stationary usage, fuel cells could also operate heating systems and mini power plants, for instance. Looking beyond cars, there are numerous other potential uses of hydrogen as an alternative vehicle fuel, with hydrogen-powered buses, commercial vehicles and forklifts already operational. The first H2-propelled submarines are also in active use. Hydrogen in submarines is stored in metal hydrides, since the relatively high weight of hydride storage units is not a disadvantage. Hydrogen-powered fuel cells are also silent, and low temperature operated, helping submarines to remain undetected. The aviation industry is also refining concepts to implement hydrogen as an energy carrier [5.58]. Since weight reduction is essential in aviation, liquid hydrogen is the only viable option for hydrogen-fuelled aviation. A European project designed an airport infrastructure concept in which large quantities of liquid hydrogen are produced directly on-site. A decade ago a wide-ranging industry consortium, including Linde, was already involved in the German development of the Sänger horizontal take-off space transportation system, fuelled by a mixture of liquid and solid hydrogen (slush hydrogen), since this has an even higher density than liquid hydrogen [5.59]. There are also designs for the use of hydrogen to fuel the on-board electrical system of conventional aircrafts, operated by an APU (auxiliary power unit). Essentially, hydrogen is potentially suitable for all applications that currently run on batteries and require minimal recharging times whilst offering a maximum of power output and operating hours. Hydrogen is expected to grow in popularity for portable applications: as a replacement for batteries and accumulators in handheld electrical appliances and as portable power generators. Micro fuel cells are viewed as alternative energy suppliers for electronic devices such as notebooks, torches, cameras or pocket computers, for instance. These devices currently require between 5 and 15-watt electrical power. Two economic aspects make fuels highly attractive in this area – the high volume sales and high proceeds that energy carriers can generate here. Annual European demand is estimated at over half a million fuel cells, and due to high battery costs, strong manufacturer loyalty and the significant consumer benefit, this market segment is also prepared to pay a price premium per kilowatt hour. Portable power sets appear to open up another promising niche application for hydrogen technologies and fuel cells. They are used as portable generators, emergency power backup’s, uninterruptible power supplies, drive units for special vehicles and fixed generators for aeroplanes, boats and mobile homes. These applications are much closer technically to the requirements posed by road transport, for example, but standards here with regard to performance, lifetime and costs are far less strict. These niche markets could therefore help to drive the move towards mass production of fuel cells and with that the widespread implementation of hydrogen technologies.

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177

Fuel Cells

The Function of Fuel Cells: In future energy supply scenarios fuel cells will play an important role. Fuel cell systems are more environmentally friendly than conventional systems and their size can be adapted to the market requirements, ranging from fuel cells used to drive cars to those for electricity generation in power plants. Fuel cells directly convert chemical into electrical energy – without noise emission or open combustion [5.60]. Fuel cells use hydrogen or hydrogen-containing gases as fuel and pure oxygen or air as oxidant. In contrast to the detonating oxy-hydrogen reaction, which releases thermal energy by way of explosion, the electrochemical reactions in a fuel cell release energy in the form of electrical current and only in minor amounts as heat. In an ideal case only pure water is formed as the reaction product. In order to control the generation of energy, the gases are separately converted on catalyst layers, which function as anodes and cathodes. Hydrogen (H2) is chemically oxidized at the anode. The electrons released in this process flow through the external circuit to perform electrical work before they reduce oxygen molecules to oxonium anions (O2–) at the cathode. Hydronium ions (protons, H+), which are produced by the oxidation of hydrogen, move through the gas-impermeable electrolyte and recombine with oxonium anions to form water. Figure 5.25 shows the structure of a polymer electrolyte membrane fuel cell (PEMFC), the fuel cell with the highest perspective for use in car engines. Oxidation reaction at the anode: 2 H2 o 4 H+ + 4 e– Reduction reaction at the cathode: O2 + 4 H+ + 4 e– o 2 H2O

Fig. 5.25 Schematic of the structure of a PEMFC.

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Fig. 5.26 Schematic of Grove’s original gas battery apparatus [5.61]. Table 5.5 Development steps of fuel-cell technology. Year

Development

1800

Johannes v. Ritter produces hydrogen and oxygen through the electrolysis of a sulfuric acid solution

1839

Sir W. Grove recognizes the reversibility of Ritter’s experiments and constructs the first prototype of a fuel cell (see Fig. 5.26 and [5.61])

1889

Ludwig Mond and Charles Langer construct a fuel cell using air and coal gas, on the basis of Grove’s experiments

1894

William Ostwald points out the relevance of Grove’s discovery in a discourse

1939

Francis T. Bacon constructs the first fuel cell with an alkaline electrolyte (200 °C hot caustic potash solution and pressurized hydrogen).

1950s Siemens and Varta investigate AFC technology 1960s The PEMFC is tested as an energy source in NASAs Gemini space travel program 1960s Use of alkaline fuel cells in the Apollo space travel program

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1982

Submarines with fuel cells

1994

The first cogeneration power plant on phosphoric acid fuel cell (PAFC)-basis comes into service in Hamburg

1997

Daimler-Benz introduces a public-service bus (new electric bus, NEBUS) with fuel cells

1997

Start of the NECAR (new electric car) model ranges as an experimental platform for automobile applications

2000

Introduction of a go-cart operated by direct methanol fuel cell (DMFC)

2002

Crossing the USA with a NECAR 5 from Daimler-Chrysler

2003

Daimler-Chrysler, Ford Honda, Nissan and Toyota announce first demonstration fleets

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History: The working principal of the fuel cell is known since the 19th century: in 1839 the physicist Sir William Robert Grove discovered the reversibility of the electrolysis of water at a platinum wire surface (Fig. 5.26). In the following years the development of fuel cells was mainly performed by scientific and research institutions. Due to the high costs the applications were limited to niche markets as military applications in submarines. For the Apollo space program the fuel cell was also used. An overview of the most important steps of the development of fuel-cell technology is presented in Table 5.5 [5.63]. Growing environmental awareness in the late 20th century and the large increase in environmental pollution due to emission of waste gases of all kinds resulted in strongly growing interest in the low-emission fuel-cell technology. Efforts to improve air quality led to the introduction of the Clean Air Act in California in 1988. This law demanded a set fraction of emission-free automobiles (zero emission vehicles) to be in operation from 2004 onward. This put the automobile industry under strong pressure to develop clean driving techniques. Different prototypes of fuel-cell vehicles have been developed in the past 10 years [5.62]. Types of Fuel Cells: All fuel cells consist of a gas-impermeable ion conductor as the electrolyte, which is catalytically coated on both sides or which has catalytic characteristics itself. This ion conductor separates the anodic and cathodic spaces of the electrochemical cell from each other. Fuel-cell systems and their components must meet different requirements, depending on the desired electrical output. Fuel cells can cover a range from 0.1 W to 30 kW power plants. Depending on the working temperature, which can range from room temperature to 1000 °C and depending on the fuel cell type, different fuels can be used like hydrogen, alcohols or hydrocarbons (e.g. natural gas, gasoline, diesel fuel) used as fuel and air or oxygen as oxidant. Table 5.6 summarizes different operating conditions, the catalysts used, and application areas [5.64]. Applications for Hydrogen: The role of hydrogen as an energy carrier has been described in Section 5.4.1.2. In the following the main applications for fuel cells and their demand in the hydrogen quality are shortly described. The main focus in recent fuel cell development is the use for propulsion. All major car manufacturers are investing in this technique. For fuel cell driven cars high power densities are required as 100 kW have to be installed within a passenger car. One other requirement is the fast start up time and the short reaction time for load changes. This is only fulfilled by a PEM fuel cell. As the PEMFC operates at temperatures around 100 °C the start up time is rather short. On the other hand the low operating temperature causes very high demand for the hydrogen quality. As the catalysts for hydrogen oxidation in PEMFC are Platinum based all substances adsorbing strongly on Platinum are a potential poison for the fuel cell. Especially Carbon Monoxide (CO) has to be removed from the feed gas. Therefore the hydrogen has to be of high purity with a maximum CO content of 1 ppmv. Furthermore NH3 is a strong poison for the electrocatalysts. Therefore a car application demands the highest grade of hydrogen which is stored on board.

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5 Hydrogen and Carbon Monoxide: Synthesis Gases Table 5.6 Different operating conditions of fuel cell types. AFC

PEMFC

PAFC

MCFC

SOFC

Operation temperature [°C]

60–90

80–110

160–200

600–800

800–1000 new development: 500–600

Fuel

H2 (high purity)

H2 (high purity)

H2 natural gas

H2 natural gas

H2 natural gas, LPG

Oxidation

O2 (high purity)

O2, Air

Air

Air

Air

Anode catalyst (example)

Raney-Ni; Pt/C

Pt/C; Pt–Ru/C

Pt/C

Ni/Al; Ni/Cr

Ni

Electrolyte

KOH

sulfonated PTFE

phosphoric acid

Li/Na carbonate

Y–ZrO2

Cathode catalyst

Raney Ni

Pt/C

Pt/C; Pt–Co–Cr/C

NiO

La–Sr–MnO3

Application

Space craft

mobile, portable; automotive

stationary

stationary

stationary

AFC: PEMFC: PAFC: MCFC: SOFC:

Alkaline Fuel Cell Polymer Electrolyte Fuel Cell Phosphoric Acid Fuel Cell Molten Carbonate Fuel Cell Solid Oxide Fuel Cell

Another application of great interest is the use of fuel cells in electronic devices. For laptops or mobile phones the energy density of hydrogen is too small. Therefore the development concentrates on a special form of the PEMFC the Direct Methanol fuel cell (DMFC) in which the fuel is converted within the electrocatalyst. Further applications for fuel cells are auxiliary power units. These devices generate additionally electric power for instance in trucks or campers. For these applications the fuels already used should be applied. This means the feed gas has to be reformed on site. As mentioned above the quality demands for PEMFC are very high and therefore the fuel preparation is complex. For this application the SOFC is more appropriate. Due to the high temperature of the SOFC not only hydrogen can be converted but also light hydrocarbons and carbon monoxide. Therefore only one catalytic step prior to the fuel cell is needed which converts higher hydrocarbons to hydrogen, CO and short chain hydrocarbons. As the anode catalyst is Ni based and the metal is in the reduced state an oxygen contact at the

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operating temperatures leads to formation of NiO in an exothermic reaction. This destroys the anode catalyst. Therefore even traces of oxygen have to avoided in the feed. Other poisons for the SOFC are higher hydrocarbons leading to coking as well as sulphur compounds. Another application for fuel cells is combined heat and power (CHP) generation devices. The choice of the fuel cell type depends on the desired power generation. For small decentralized units in the range up to 3 kWel PEMFC and SOFC are in the focus of development. The higher temperature of the SOFC makes a SOFC more efficient. On the other hand the applied temperatures cause material problems. The PEMFC requires the highest gas quality what is reflected in a more complex reforming system. The common feed for the FC system is natural gas. One residue in the reforming process is Methane which is inert to the PEMFC electrode and can be used as heating gas in the off gas burner. In case of the SOFC the not converted methane can be converted itself in the SOFC. For larger power generation units (up to 100 kWel) PAFC and MOFC are an attractive alternative. The gas quality requirements for the PAFC are not as strict as for the PEMFC. The electrocatalyst of the PAFC is Pt based. Due to the higher operating temperature the PAFC can tolerate higher CO concentration compared to the PEMFC. CO concentrations up to a few 100 ppmv CO do not affect the performance of the FC. One major disadvantage of the PAFC is the use of a highly corrosive electrolyte (Phosphoric Acid) which can damage the materials. Also the liquid electrolyte leaks out of the matrix in which it is embedded with operating time. Even higher temperatures are used for the MCFC. This type of fuel cell can convert also CO and light hydrocarbons directly in the FC. Often a pre-reforming is used to produce a mixture of hydrogen, CO and methane out of natural gas or even liquid fuels. As the catalyst is Ni based like in the case of SOFC small amounts of Oxygen can lead to major damage of the catalysts. Also sulphur in form of H2S has to be removed prior to the FC as the catalyst is poisoned strongly by it. A major problem of the MCFC technique is the aggressive conditions of the operation. The electrolyte is a molten carbonate and it has to be prevented that this corrosive material leaks out the FC unit. Resume: The required quality of hydrogen for fuel cells strongly depends on the type of fuel cell used. In terms of CO content the general rule is: the lower the working temperature the lower has to be the content of CO in the hydrogen. For fuel cells for propulsion with the highest potential to become a mass market the hydrogen should contain less than 1 ppmv CO. A second catalyst poison for all fuel cell types is sulphur. Sulphur in form of H2S strongly bonds to all anode catalysts and blocks active sites for oxidation reactions. Sulphur requires even lower maximum concentrations (< 0.5 ppmv) than CO. For all FC types there are other contaminants which differ from type to type.

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5.4.2 Applications of Carbon Monoxide

It is used as: x Carbon donor in the heat treatment of metals (e.g. carbonisation of iron and steel) x Basic compound for the organic chemistry (e.g. production of higher alcohols, aldehydes, carbon acids) x Reactant in the inorganic chemistry (e.g. production of phosgene, metal carbonyls) 5.4.3 Applications of Synthesis Gas (Mixtures of CO and H2)

It is used as: x Raw material for the methanol synthesis x Raw material for the hydrocarbon/fuel-production (Fischer–Tropsch synthesis) x Raw material for the formation of aldehydes and alcohols from olefins (oxosynthesis) x Reduction gas for the production of metals from oxides or ores (in special furnaces) x Heat treatment gas for neutral annealing or carbonisation of iron and steel (e.g. on site production in gas generators starting from hydrocarbons or by cracking of CH3OH) x Fuel for the generation of electricity in power stations

References [5.1] M. Geitel: Das Wassergas und seine Verwendung in der Technik, 3rd edition, Siemens, Berlin, 1900, p. 4. [5.2] F. Ullmann: Enzyklopädie der technischen Chemie, Vol. 11, Urban & Schwarzenberg, Berlin, 1922, p. 608 ff. [5.3] Georges Patart, French Patent 540, p. 343 (19/08/1921, published 12/07/1922). [5.4] F. Fischer, H. Tropsch: Ber. Dtsch. Chem. Ges. 1926, 59, 830–832. [5.5] C. D. Frohning, H. Kölbel, M. Ralek, W. Rottig, F. Schnur, H. Schulz in J. Falbe (Ed.): Chemierohstoffe aus Kohle, Georg Thieme Verlag, Stuttgart, 1977. [5.6] Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 101st edition, W. de Gruyter, Berlin, 1995, p. 249 ff. [5.7] Römpp, 10th edition, Keyword: Wasserstoff, Georg Thieme Verlag, Stuttgart, 1996. [5.8] R. D. McCarty: Hydrogen Technology Survey: Thermophysical Properties, National Bureau of Standards, NASA Technical Reports, NASA SP. 3089, January 1975. [5.9] Ullmann’s, 5th edition, Vol. A 13, p. 299 ff., VCH, Weinheim, 1989. [5.10] F. Ullmann: Enzyklopädie der technischen Chemie, Vol. 7, Urban & Schwarzenberg, Berlin, 1919, p. 33 ff. [5.11] Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 101st edition, W. de Gruyter, Berlin, 1995, p. 863.

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[5.12] Ullmann’s, 4th edition, Band 6, p. 226 ff., Verlag Chemie, Weinheim, 1977. [5.13] G. Sorbe: Sicherheitstechnische Kenndaten chem. Stoffe, 41. Erg. Lfg., 9/94; Handbook of Compressed Gases, 3th ed, Compr. Gas. Assoc., Van Nostrand Reinhold, New York, 1989. [5.14] Kirk-Othmer, 4th edition, 5, Volume 5, p. 97–101. [5.15] L’Air Liquide: Encyclopedie des Gaz, Elsevier, Amsterdam, 1976. [5.16] Ullmann’s, 5th edition, A13, p. 297 ff., VCH, Weinheim, 1989. [5.17] Ullmann’s, 5th edition, A12, p. 169 ff., VCH, Weinheim, 1989. [5.18] M. Appl: Ammonia, Methanol, Hydrogen, Carbon Monoxide – Modern Production Technologies, Nitrogen, British Sulphur Publishing, 1997, ISBN 1-873387-26-1. [5.19] T. Dreier, U. Wagner: Perspektiven einer Wasserstoff-Energiewirtschaft. Brennstoff Wärme Kraft 2000, 52(12), 41–46. [5.20] C. E. G. Padro: Back to the Future. American Chemical Society 1999, 44(2), 235–239. [5.21] J. R. Rostrup-Nielsen, T. Rostrup-Nielsen: Large-Scale Hydrogen Production. Cattech. 2002, 6(4), 150–159. [5.22] K. Andreassen: Hydrogen Production by Electrolysis. Hydrogen Power 1998, 91–102. [5.23] S. Michel: Low Cost Production of Hydrogen, Carbon Monoxide and Synthesis Gas. DGMK Conference Proceedings 2000, 2, 31–24. [5.24] H. H. Gunardson, J. M. Abrado: Produce CO-rich Synthesis Gas. Hydrocarbon Process. 1999, 78(4), 87–93. [5.25] G. Bourbonneux: Hydrogen Production, in Petroleum Refining, 2001, Vol. 3. [5.26] I. Dybkjaer, S. Winter Madsen: Advanced Reforming Technologies for Hydrogen Production. Hydrocarbon Eng. 1998, 3(1), 56–65. [5.27] M. V. Twigg: ICI Catalyst Handbook, 2nd edition, Manson Publishing, London, 1996. [5.28] M. Lembeck: LAC – The Linde Ammonia Concept. Linde-Report on Science and Technology 1995, 55. [5.29] W. Baade, K. Cambell, U. Parekh: 50 Years of Continuous Innovation in Hydrogen. Hydrocarbon Eng. 2002, 7(8), 37–39. [5.30] R. Fabian, W. Förg: Modern Liquid Nitrogen Wash Process for the Purification of NH3 Synthesis Gas at High Pressure. Linde-Report on Science and Technology 1975, 22. [5.31] B. Kandziora: Hydrogen Technology – Advanced Technologies. Hydrocarbon Eng. 2002, 7(8), 47–50. [5.32] D. Steen, A. Zagoria: Hydrogen Technology – A Uniform Approach. Hydrocarbon Eng. 2002, 7(8), 40–45. [5.33] C. J. Campbell: Peak Oil – A Turning Point for Mankind, Presentation at the Technical University of Clausthal, Germany, December 2000. [5.34] J. Schindler, W. Zittel (Ludwig Bölkow Systemtechnik): Written statement on the public hearing of experts by the Enquete Commission of the German Bundestag “Nachhaltige Energieversorgung unter den Bedingungen der Globalisierung und der Liberalisierung” on the topic “Weltweite Entwicklung der Energienachfrage und der Ressourcenverfügbarkeit”, October 2000. [5.35] Norbert Strohschen (Gerling Global Reinsurance Company): Climate Change and Environmental Protection – a Global Interest for Insurers and Reinsurers, Paper presented at HYFORUM 2000. [5.36] Thomas Loster: Getting to the Root of Natural Catastrophes, Allianz Global Risk Report 3/99. [5.37] Prof. Dr. H. Graßl: Strategies for Slowing Climate Change, revised version of presentation made at Allianz Environmental Protection Foundation’s annual symposium in 1997, Allianz Global Risk Report 3/99. [5.38] G. Ondrey: Carbon Dioxide Gets Grounded, Chemical Engineering, March 2000. [5.39] Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 102nd edition, Walter de Gruyter, Berlin 2007, pp. 259–260. [5.40] Well-to-Wheels Report, Eucar Concawe JRC, Well-to-wheels analysis of future automotive fuels and powertrains in the European context, May 2006, European commission, http://ies.jrc.cec.eu.int/wtw.html.

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5 Hydrogen and Carbon Monoxide: Synthesis Gases [5.41] Well-to-Wheel Analysis of Energy Use and Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems – A European Study. L-B-Systemtechnik GmbH, Ottobrunn, September 2002; www.lbst.de/gm-wtw. [5.42] A. König, F. Seyfried, I. Drescher: Fuel Reforming as Part of a Hydrogen Economy, HYFORUM 2000, The International Hydrogen Energy Forum 2000 Policy – Business – Technology, 11–15 September 2000, Munich, Germany. [5.43] B. Gaudermack, S. Lynum: Hydrogen from natural Gas without release of CO2 to the Atmosphere, International Journal of Hydrogen Energy, Vol. 23, No. 12, pp. 1087–1093. [5.44] Iceland, Shell, DaimlerChrysler, Norsk Hydro form Company to develop hydrogen economy, The Hydrogen & Fuel Cell Letter, March 1999. [5.45] Hermann Scheer (Alternativer Nobelpreis 1999): Solare Weltwirtschaft, Verlag Kunstmann, 1999. [5.46] Andreas Züttel: Hydrogen Storage, University of Fribourg Switzerland, presentation at The International German Hydrogen Energy Congress, Essen, Feb. 2004. [5.47] C. J. J. Reijerkerk: Potential of cryogenic hydrogen storage in vehicles, paper published at NHA Conference, Apr. 2004, Los Angeles. [5.48] Dr. Michael R. Swain University of Miami, Fuel Leak Simulation, proceedings of the 2001 DOE Hydrogen Program Review. [5.49] A. Stepken: TÜV-Süd, Presentation Wasserstoff – so sicher wie Benzin, Medienforum Deutscher Wasserstofftag, München, Oct. 2003. [5.50] R. A. Oriani et al.: Hydrogen Degradation of Ferrous Alloys, Noyes Publications USA (1985). [5.51] P. F. Timmins: Solutions to Hydrogen Attack in Steels, ASM International 1997, ISBN 0-87170-597-4. [5.52] Preliminary draft proposal for a regulation of the European parliament and of the Council relating to the type-approval of hydrogen powered motor vehicles, Version 2, July 13th 2006, http://ec.europa.eu/enterprise/automotive/pagesbackground/hydrogen/ consultation/hydrogen_draft_proposal.pdf. [5.53] B. Hoehlein, T. Grube, C. J. J. Reijerkerk: Beitrag zur FVS-Jahrestagung 2004: Wasserstoff und Brennstoffzellen – Energieforschung im Verbund, Wasserstofflogistik – Produktion, Konditionierung, Verteilung, Speicherung und Betankung. [5.54] C. J. J. Reijerkerk: Hydrogen Filling Stations Commercialisation, Final Project for University of Hertfordshire in conjunction with University of Applied Sciences Hamburg, Linde AG, Munich Sep. 2001. [5.55] B. Valentin: Wirtschaftlichkeitsbetrachtung einer Wasserstoffinfrastruktur für Kraftfahrzeuge, University of Applied Sciences München, Linde AG, Munich, Nov. 2001. [5.56] Jaco Reijerkerk: “Wasserstofflogistik”, Page 4–5, BWK 1/2, 2006. [5.57] David Hart: “The Economics of a European Hydrogen Automotive Infrastructure”, presented at International Hydrogen Day, Berlin 2005. [5.58] Andreas Westenberger: “Ausblick auf zukünftige H2-Anwendungen in der zivilen Luftfahrt”, hydrogen.tech 2006, Munich. [5.59] M. Bracha: Sänger – An Advanced Space Transport System, Study by Linde AG on behalf of MBB, 1989. [5.60] Ullmann fuel cells. [5.61] W. R. Grove: On a New Voltaic Combination of Gases by Platinum, The London and Edinburgh Philosophical Magazine and Journal of Science, Philos. Mag. 14 (1839) 127. [5.62] A. E. Hammerschmidt, M. F. Waidhas: Spektrum der Wissenschaft (1999) No. 2, A44. [5.63] a) Internet page from Smithonian Institutes http://americanhistory.si.edu/csr/fuelcells/ index.htm; b) http://dc2.uni-bielefeld.de/dc2/fc/folien/f-geschi.htm; c) http://www.fuelcelltoday.com. [5.64] R. Dittmeyer, W. Keim, G. Kreysa, A. Oberholz, Winnacker-Küchler: Chemische Technik, Bd. 4, 2005.

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6 Carbon Dioxide 6.1 History, Occurrence, Properties and Safety 6.1.1 History

Carbon dioxide (CO2, carbonic acid anhydride), previously often called carbonic acid, is a natural component of man’s environment, i.e. of its respiratory gas. In the first half of the 16th century Paracelsus was the first to distinguish between CO2 and air, while Black identified CO2 as an element of the respiratory air around 1760 [6.1]. More or less at the same time, Lavoisier established proof of the composition of CO2 through synthesis [6.2]. In the 19th century, man learned to deal with CO2 and extended its utilization increasingly. In 1835, Thilorier carried out first experiments for liquefying CO2 and producing dry ice [6.3]. The first plant for the industrial generation of CO2 in Germany was erected in 1875 by the “Maschinenfabrik Sürth” [6.4]. Since the drawing up of a property table for CO2 by Mollier (1895) [6.5] engineering has made great progress regarding both the CO2 liquefaction plants and the application of the industrial gas CO2 [6.6]. 6.1.2 Occurrence

The occurrence of CO2 on the earth should be regarded as an essential part of the carbon cycle. Carbon is to a large extent organically bound and stored in fossil fuels such as petroleum, natural gas and coal (cf. Chapter 7). Large quantities of CO2 are found in the form of different carbonates in minerals of the earth’s crust, such as limestone (about 5.5 · 1016 t) as well as in the oceans (about 1.4 · 1014 t). And the atmosphere contains CO2 to a volume fraction of meanwhile 0.038%, which in fact adds up to 2.7 · 1012 t. Large quantities of CO2 are also involved in the biological carbon cycle in the narrower sense, i.e. in the photosynthesis of CO2 to sugar (glucose) and the respective reverse combustion reaction:


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6 Carbon Dioxide light + chlorophyll

Photosynthesis: 6 CO2 + 6 H2O → C6H12O6 + 6 O2 Combustion:

C6H12O6 + 6 O2 o 6 CO2 + 6 H2O + energy

It is estimated that about 110 to 120 · 109 t of carbon are currently bound through photosynthesis each year. By determining the CO2 content of the atmosphere and deep ice, it has been established that in the last 100 years more CO2 has been generated by the combustion of fossil fuel than bound through photosynthesis [6.7, 6.8]. In recent years, carbon dioxide has fallen into disrepute as the major greenhouse gas and thus possibly as one of the causes of global warming. The reason for this is that energy generation through the combustion of fossil fuels producing carbon dioxide has increased dramatically in the 20th century. In fact, since about 1900, an increase in the CO2 content of the atmosphere from an initial volume fraction of 0.03% to the above mentioned volume fraction of 0.037% has been observed [6.9]. The CO2 which is being traded industrially at the present time is only produced separately through combustion in exceptional cases. In general it is recovered from industrial off-gases that would otherwise escape directly into the atmosphere. Only a small percentage of the traded CO2 is recovered from earth sources, e.g. mineral waters. On the other hand, the CO2 obtained and liquefied this way has not disappeared, but turns up sooner or later in a dispersed form. With worldwide about 20 · 106 t a–1 in the year 2002, the industrial gas CO2 to be discussed here makes up only a tiny share of the global CO2 cycle. Therefore the CO2 traded as industrial gas is not subject to the Emissions Trading Directive 2003/87/EG, the requirements established in view of complying with the Kyoto Protocol regarding the framework agreement of the United Nations on climatic changes of December 11, 1997 for an EU-wide emission trading system. 6.1.3 Physical and Chemical Properties

Under normal conditions, carbon dioxide is an odourless and colourless gas with the following properties (gaseous carbon dioxide): Molar mass Standard density Rel. density to air Spec. gas constant Molar heat capacity (25 °C) Thermal conductivity (25 °C, 1 bar) Viscosity (gaseous, 20 °C) Dielectric constant (0 °C, 1 bar)

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44.011 1.977 1.5291 0.1927 37.13 1.64 · 10–4 20.3 · 106 1.000989

kg kmol–1 kg mN–3 kJ kg–1 K–1 J mol–1 K–1 W cm–1 K–1 Pa s

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187

Fig. 6.1 Pressure/temperature diagram for CO2.

Carbon dioxide is easily liquefiable and has a triple point (see Fig. 6.1). Some of the essential properties of the liquid carbon dioxide are listed below [6.10, 6.11]: Critical temperature Critical pressure Temperature at triple point Pressure at triple point Boiling pressure

at 220 K 250 K 280 K 300 K

Heat of vaporization (triple point) Melting heat (triple point) Density (–50 °C) Thermal conductivity (300 K, 1 bar) Viscosity (gaseous, 300 K) Dielectric constant (liquid, 0 °C)

304.13 73.75 216.58 5.18 5.99 17.85 41.61 67.13 347.86 9.02 1152.6 16.8 · 10–3 15.0 · 106 1.58

K bar K bar bar bar bar bar kJ kg–1 kJ mol–1 kg m–3 W m–1 K–1 Pa s

Solid carbon dioxide occurs as pressed CO2 snow, so-called dry ice, with a density of 1300 to 1500 kg m–3. CO2 is a very stable component that degrades only at very high temperatures (under atmospheric pressure at 1205 °C to 0.032%, at 2367 °C to 21%, at 2843 °C to 76.1%) into CO and O2. At even higher temperatures, carbon and O2 are formed.

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CO2 o CO + 0.5 O2

'H = + 238.17 kJ · mol–1

CO2 o C + O2

'H = + 393.77 kJ · mol–1

For this reason, CO2 is a very weak oxidizing agent that reacts to carbon monoxide and carbon only at high temperatures with strong reducing agents such as H2, C, P, Mg, Na, K. Thus, magnesium oxide and carbon (soot) are produced under a strong flash of light when carbon-dioxide snow is ignited with magnesium powder. The equilibrium arising from the reaction with hydrogen and coal plays an important role, for instance, in the production of synthesis gas and steel (see Chapter 5). Water–gas equilibrium: CO2 + H2 o CO + H2O(g)

'H = + 41.19 kJ · mol–1

Boudouard-equilibrium: CO2 + C o 2 CO

'H = + 172.58 kJ · mol–1

An aqueous solution of CO2 shows only weak acidity (solubility at 20 °C, 1 bar: 0.9 L CO2 per litre of water). The reason for this is that only 0.2% of the CO2 reacts with water to carbonic acid H2CO3. CO2 + H2O o H2CO3 o H+ + HCO3– The remainder occurs as hydrated CO2. With ammonia, CO2 forms ammonium carbamates as the result of a CO2 insertion reaction: CO2 + 2 NH3 o H2N–C(=O)–O– + NH4+ The dehydration of the carbamate provides urea (H2N–C(=O)–NH2) which is an important compound for the fertilizer and plastics industry [6.12]. 6.1.4 Safety Issues

Gaseous CO2 is part of the human respiratory system. The CO2 concentration in our exhaled air is about 4 to 4.5 vol.%. But inhalation of air with CO2 concentrations higher than atmospheric will have the following physiological effects [6.13]: x x x x x x

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up to 1 to 1.5%, slight effect after several hours (TLV-TWA: 0.5%) up to ca. 3% slightly narcotic, higher breathing and pulse rate 4 to 5% headache, higher blood pressure, signs of intoxication 5 to 10% breathing more laborious, loss of judgement over 10% unconsciousness after 1 min further exposure to levels between 10 and 100% will result in death

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189

It should be observed that CO2 is heavier than air, so it spreads along the ground and is collected in ditches, cellars, cavities etc. Liquid CO2 is stored under pressure and at low temperatures. The relevant rules and recommendations must be observed: x national and local regulations, industrial code of practice x European Union’s Pressure Equipment Directive 97/23/EC, and x relevant rules and recommendations of the EIGA und CGA [6.14, 6.15] Dry ice exists at a temperature of –78.5 °C. When handling liquid CO2 and dry ice the wearing of personal protection equipment (protective goggles and gloves) is mandatory.


Today, most of the carbon dioxide coming on to the market as industrial gas is recovered from CO2 sources which already exist. A CO2 source is understood to be gases and off-gases with a significant CO2 content. About 70% of the CO2 on the European market is actually recovered from synthesis gas plants (see also Section 5.2.3). Only a small part of the CO2 is generated by the combustion of fossil fuels, preferably natural gas. This usually occurs in smaller units (capacity < 2 t h–1), which are so far away from a suitable CO2 source that the transport to the consumer is economically not feasible. Apart from brewery plants, today the predominant part of CO2 is produced in plants with a capacity of 1 to 25 t h–1. This usually requires the process steps shown in Fig. 6.2.

Fig. 6.2 Process steps for the CO2-production.

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6.2.1 Sources of Carbon Dioxide Recovery

The most important criterion for the economic efficiency of a CO2 source is the CO2 partial pressure. Gas mixtures which are suitable for the economical recovery of liquefied CO2 with high purity are understood to be carbon dioxide sources preferably with a CO2 partial pressure > 1 bar. Table 6.1 shows examples of CO2 sources listed by decreasing CO2 partial pressure. A criterion for not selecting a CO2 source is the presence of additional gas components the removal of which may be costly. Some examples of such sourcespecific secondary components are given in Table 6.2. Table 6.1 CO2 Sources. Source

CO2 partial pressure

CO2-fraction from acid-gas scrubbings in ammonia or other synthesis gas plants or H2-generation plants (cf. Section 5.2)

1.0 to 1.2 bar

CO2-containing off-gas from fermentation plants, e.g. in breweries

0.9 to 0.95 bar

CO2 from underground deposits (also in mixtures with hydrocarbons)

1 to 30 bar

Natural gas purification plants, so-called sweetening plants

1.0 to 1.2 bar

Ethylene oxide plants

0.8 to 0.95 bar

Acid neutralisation plants

around 1 bar

Lime and cement furnaces

0.2 to 0.5 bar

Flue gas

0.09 to 0.11 bar

Table 6.2 Specific impurities in different CO2 sources [6.16]. Often occurring impurities

H2 N2 CO O2 Ar CH4 Water

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Selection of additional components found in certain CO2 sources CO2 fraction of synthesis gas plant

Natural CO2 source

CO2 fraction from a fermentation plant

Residual gas of an ethylene oxide plant

Flue gas

Alcohols Org. acids Aldehydes Ketones Amines H2S COS Mercaptanes HCN

C2+ Oil Benzene Toluene H2S R–SH COS Radon Mercury Salts

Alcohols Org. acids Aldehydes Ketones Yeasts Germs S-compounds

C2H4 C2+ Ethyl chloride Vinyl chloride Aldehydes

NOx SO2 SO3 HCl HCN Amines Metal oxides Mercury Dust

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6.2.2 Pre-purification, Enrichment, Extraction, Capture

If CO2 sources with low partial pressure (< 1 bar) are utilised an additional process step for the extraction or enrichment of the CO2 has to be applied upstream of the liquefaction unit. Most commonly an absorption process (CO2 scrubbing) is used but a pressure swing adsorption (PSA) process (cf. Section 5.2.3.4) or a combination of partial condensation and wash unit can also be applied. The scrubbing unit consists of an absorption column, where the CO2 is absorbed in a wash liquid and a regeneration column, where the CO2 is stripped from the wash liquid. The feed gas enters the absorption column at the bottom and is absorbed in the lean wash liquid, which flows countercurrently from the top to the bottom of the column. On top of the column traces of wash liquid are removed from the residual gas by means of fresh water. The CO2-rich wash liquid is pumped to the upper section of the stripper. On its way it is heated against hot-regenerated wash liquid from the stripper bottom. The CO2 is absorbed by means of the stripping steam generated in the reboiler, which is heated by LP steam. The lean wash liquid is withdrawn from the bottom of the stripper, cooled against loaded wash liquid and finally against cooling water and fed to the top of the absorber column. The stripping steam is condensed against cooling water in the top of the regeneration column. The surplus water is withdrawn as waste water. This process is typical for a number of wash liquids but mainly amine solvents. Depending on the selected solvent some additional equipment is required, i.e. x x x x x

a filter system in a side stream to the lean wash solution a device for the addition of anti-foam agent a solvent storage of the complete liquid inventory a reclaimer for removal of heat-stable salts and a pump for the feeding of the amine solution from storage into the process

A large variety of wash liquids are available which can be divided into physical absorbents and chemical absorbents [6.17]. Among the first group are solvents like water or methanol, where the CO2 solubility is basically a function of the CO2 partial pressure. The chemical absorption is characterised by a temporary chemical bond at ambient temperature which is released at higher temperatures. An important group of these solvents are the alkanolamines. Among these is Monoethanolamine (MEA) commonly used for the extraction of CO2 from the flue gas, lime-kiln off-gas or blast furnace off-gas. An aqueous solution of 15 to 30 wt-% MEA reacts with CO2 as follows: C2H5O–NH2 + H2O + CO2 C2H5O–NH3+ + HCO3– about 120 °C m o about 50 °C

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Fig. 6.3 Sketch of CO2 scrubbing unit.

Advantage: high reaction velocity and high pick-up at low CO2 partial pressures. Disadvantage: high reaction enthalpy i.e. high energy demand for regeneration, sensitive against oxygen, i.e. higher MEA losses if O2 is present in the feed gas. Methyldiethanolamine (MDEA) is often used for the removal of CO2 from synthesis gases produced in steam reformers (cf. Section 5.2.2.1). The aqueous solution of 35 to 50 wt-% MDEA reacts with CO2: +

(C2H5O)2N–CH3 + H2O + CO2 (C2H5O)2NH–CH3 + HCO3– about 120 °C m o about 50 °C. Advantage: less sensitive to O2, lower reaction enthalpy, high pick-up at high CO2 partial pressure. Disadvantage: low reaction velocity, this can be almost compensated by adding an activator, e.g. piperazine, to the MDEA solution (e.g. BASF’s activated aMDEA®). The steam demand for the regeneration of some commercially available absorption solutions based on amines is shown in Fig. 6.4. To reduce the cost of the post-combustion capture of CO2 from power stations in the future large research efforts are being made to develop new absorption solvents or to improve the efficiency of the existing ones [6.18–6.21]. For raw gases with a CO2 partial pressure higher than 3 to 5 bar a two-stage MDEA wash can be used, which is characterised by two wash cycles, i.e. one with a completely regenerated wash solution and a second one with a partially regenerated wash solution. The two-stage wash process is characterised by a significantly lower steam demand but an increase in the required pump energy and investment cost.

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193

Fig. 6.4 Specific steam consumption of different CO2 scrubbing processes.

6.2.3 Standard Process for the Liquefaction of Carbon Dioxide

A frequently used process for the purification and liquefaction of CO2 is described (Fig. 6.5).

Fig. 6.5 Standard process for the liquefaction of CO2.

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6.2.3.1

Compression and Water Separation

The raw CO2 has approximately 1.1 to 1.4 bar at ambient temperature. It is compressed to about 18 bar in an oil-flooded two-stage screw compressor followed by a highly efficient oil separation, consisting of coalescers and a carbon bed. The lubrication oil has to be suitable for food applications. Non-lubricated twostage piston compressors are occasionally used for small-capacity units. After compression, the CO2 is cooled against cooling water to just above ambient temperature and further cooled against evaporating refrigerant to about 10 °C. Possibly condensed water is separated in separators which are installed before the compressor and after the coolers. The condensate (water and probably traces of wash solution and/or oil) is fed to the waste water system. 6.2.3.2

Adsorber Station

The residual traces of water and other components, such as odorants, are removed in regenerative adsorbers filled with molecular sieve or silica gel and activated carbon. Typically the unit operates according to the following automatic sequence: Adsorption, expansion, regeneration, cooling, pressure build-up and adsorption again in possibly parallel operation, (see also Sections 2.2.4 and 2.2.5.6). For the regeneration of the adsorbent either residual gas of the rectification or dried CO2 is used. But air can also be used for regeneration of adsorbers. In this case the cooling is normally done with dry CO2 or residual gas to sweep the air from the adsorbent. The regeneration gas is heated to about 200 °C either electrically or by means of steam. By these means, the components adsorbed on the molecular sieve as well as on the activated carbon are desorbed. The regeneration gas is then emitted to the atmosphere. If only water is desorbed by means of dry CO2, it can be recycled to the compressor to improve the recovery rate. 6.2.3.3

Liquefaction and Stripping of Lighter Components

The purified CO2 is first cooled against cold residual gas and in the reboiler of the stripping column. Here it serves as heating medium for the generation of the required stripping steam. The major part of the CO2 is liquefied against an evaporating refrigerant. The liquefied portion is fed to the top of the stripping column. Part of the stripping steam is also reliquefied in the liquefier to increase the CO2 recovery rate. This depends both on the portion of the lighter components, such as N2, CH4 etc., and on pressure and temperature in the liquefier (see Fig. 6.6). The remaining residual gas serves to pre-cool the CO2 and, if required, to regenerate the adsorbers. 6.2.3.4

Refrigerating Unit

The liquefaction of the CO2 requires a considerable refrigeration unit; the theoretical minimal value amounts to 85.8 kWh per ton of CO2 at –33 °C. Generally, the total amount of refrigeration required (liquefaction, insulation losses, losses due to temperature differences through heat exchangers) is covered by a closed compression refrigeration unit. All modern refrigerants can be used, but often

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195

Fig. 6.6 CO2 recovery rate depending on the CO2 content and final liquefaction temperature.

water-free ammonia is selected for its favourable properties, such as high specific evaporation enthalpy and volumetric refrigeration effect as well as low price and environment-friendly behaviour. The gaseous refrigerant is compressed either in a two-stage piston compressor or more frequently in two screw compressors or a so-called compound compressor from slightly over ambient pressure to about 16 to 17 bar. It is liquefied against cooling water (often in a so-called evaporative condenser, where the main part of the cooling duty is directly provided by evaporating water) and stored in an accumulator. From here, it can be expanded either directly into the CO2 liquefier or, as is common for thermodynamically efficient plants, first into a flash drum at the compressor interstage pressure. Together with the gaseous refrigerant from the CO2 precooler, the expansion gas is fed to the second compression stage. The remaining liquid can first be supercooled against pure CO2 from the sump of the stripping column and then expanded into the CO2 liquefier and evaporated there. The flash is significantly less compared to direct expansion from liquefaction condition to evaporation pressure. Apart from the investment costs for the plant, a decisive cost factor of the CO2 recovery is the total energy consumption, i.e. mainly the energy demand for the CO2 compressor and the refrigerating unit. For medium to large liquefaction capacities (3 to 20 t h–1 liquid CO2), the specific energy demand can be estimated with the help of the curve shown in Fig. 6.7. 6.2.4 Process Steps to Obtain High Product Purity and Recovery Rate

Today the CO2 market generally requires high product purities (total impurities < 100 ppmv, for individual components and applications < 1 ppmv and occasionally in the lower ppbv-range). With the process shown in Fig. 6.5 not all of the

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Fig. 6.7 Specific power demand of the CO2 liquefaction.

trace components found in the different CO2 sources can be removed to achieve the required purity. Quality standards for CO2 used in food and beverages for example are to be found in papers published by the following organizations: x International Society of Beverage Technologists (isbt) [6.22]: “Quality Guidelines and Analytical Method Bibliography for Bottlers” x European Industrial Gases Association (EIGA) [6.14]: Publication: Doc. 76/01/E “CO2 Specification Guide for Analytical Steps and Frequencies” Publication: Doc. 70/99/E “Carbon Dioxide Source Certification, Quality Standards and Verification” x Compressed Gas Association, Inc. [6.15]: Publication: CGA G-6-2003 “Carbon Dioxide, 6th Edition” Publication: CGA G-6.2 “Commodity Specification for Carbon Dioxide” The trace components given in Table 6.2 can be removed with the following process steps (see Fig. 6.8). 6.2.4.1

Scrubbing

Water scrubbing: All easily water-soluble components (e.g. alcohols, organic chlorides, amines, etc.) can be separated by means of water scrubbing. Potassium permanganate: An aqueous solution of KMnO4 is used to oxidise organic components such as germs, yeasts and other easily oxidable matters. Soda: An aqueous solution of Na2CO3 can be used for the removal of SO2/SO3 as well as the corresponding acids. Caustic soda can also be added to the soda wash cycle. All these wash processes comprise of similar equipment.

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197

Fig. 6.8 Process steps for the removal of trace components.

A wash unit consists of a scrubbing column in which the raw CO2 is fed in countercurrent to a cooled water/wash solution circulation. The liquid enriched with impurities is drawn from the sump of the column and partly replaced by fresh water/solution at the top of the column. Water scrubbing is only applied when the raw CO2 is not already the CO2 fraction of a gas scrubbing. It is sometimes installed upstream of the compressor to be used as a direct gas cooler. 6.2.4.2

Adsorption and Chemisorption

Apart from the usual adsorptive dryer station, additional adsorption steps can serve to remove trace components (e.g. sulphur compounds, alcohols, aldehydes, ketones, ester, other odorants, aromatics, etc.). All molsieves and all types of activated carbon are used as adsorbents, but also zinc and ferric oxides which, in contrast to the dryers, are usually not regenerated in situ but have to be replaced by fresh adsorbents after saturation. According to the quantity of the component(s) to be removed, adsorption occurs in single adsorbers or in series-connected twin-adsorbers. This series-connection (also called “Lead/Lag” configuration) is designed in such a way that the container with the unloaded adsorbent follows the “active” one during normal operation of the plant. When the adsorption capacity of the “active” adsorber is exhausted, it is taken out of operation and the vessel with the fresh adsorbent is turned into the “active” one. The loaded adsorbent can be replaced by a fresh one while the plant is kept in operation.

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6.2.4.3

Catalytic Combustion

Today the removal of hydrocarbons, with boiling points higher than CO2, is carried out almost exclusively by catalytic combustion. The potassium permanganate wash once used for oxidising easily oxidable hydrocarbons is nowadays only used for the purification of CO2 obtained in connection with fermentation processes (e.g. in breweries). The catalytic oxidation takes place with precious metal catalysts (platinum and/or palladium on aluminium oxide carriers) at temperatures between 200 and 600 °C. When the reactor exit temperature reaches only about 450 °C the remaining methane has to be removed in the CO2 stripper. If the required oxygen is added to the CO2 in the form of air, then the nitrogen mixed with the CO2 has also to be separated again in the CO2 stripper. The CO2 partial pressure is lower and therefore the recovery rate is lower (see Fig. 6.6). The CO2 is first heated against the combustion product and then, in an electrical heater, up to the required starting temperature. If only small amounts of combustible components are contained in the CO2, the temperature rise in the combustion reactor is small and either the electrical heater has to provide more energy or the countercurrent gas/gas heat exchanger requires significantly more exchanger surface. In other words, the capital to be invested for the countercurrent heat exchanger has to be optimized in view of the operating costs of the electric heater. 6.2.4.4

Improvement of the Carbon Dioxide Recovery Rate

With low concentration of CO2 in the raw gas, the CO2 recovery rate at common liquefaction temperatures is small, as is apparent from Fig. 6.6. An improvement in the recovery rate can, for example, be achieved by means of lower liquefaction temperatures. In order to avoid the required installation of a cascade refrigeration unit with different refrigerants and additional compressors and equipment, liquefied CO2 can be used as refrigerant for the cold generation in a so-called “open” refrigeration cycle at a sufficiently low temperature level. A large part of the CO2 in the residual gases of the stripping column and the liquefier can be liquefied against evaporating CO2, at –50 °C, for example. The liquefied CO2 is separated and evaporated at about 5.5 bara. The evaporation enthalpy is exactly adequate to compensate the required liquefaction enthalpy. The evaporated CO2 is mixed with the inlet flow of the second stage of the CO2 compressor. Thus the open CO2 cycle with only small amounts of additional equipment (see process flows depicted in bold print in Fig. 6.9) represents an economical alternative to a cascade refrigeration unit. 6.2.5 Carbon Dioxide Recovery from Flue Gas

For the recovery of CO2 from flue gas or calcinations kiln exhaust, further process steps are required as shown in Fig. 6.10, in addition to the CO2 purification and liquefaction.

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Fig. 6.9 Enhancement of CO2 recovery.

Fig. 6.10 CO2 from flue gas.

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Initially, solid particles (dust) have to be removed from the flue gas in order not to endanger the subsequent devices, e.g. blowers or compressors. The flue gas is then cooled down in the heat recovery (countercurrent heat exchanger or recuperator). The blower compensates the pressure drop of the additional downstream process steps. Before CO2 is removed from the flue gas, SO2 and NOx have to be removed to a remaining level of about 10 ppmv SO2 and about 20 ppmv NOx [6.23]. The NOx must be removed with a Selective Catalytic Reduction (SCR) using aqueous ammonia. This process step takes place at about 200 to 300 °C and must be carried-out upstream the heat recovery section. The bulk removal of SO2 is normally done by a wet flue gas desulphurisation system using limestone and producing gypsum [6.24] or other wash processes producing SO2, e.g. SOLINOX® [6.25]. The remaining sulphur oxides can be removed with a soda scrubbing unit [6.26]. The resulting sodium sulfite (Na2SO3) is discharged with the waste water. For the following CO2 scrubbing, an oxygen resistant wash solution has to be applied, e.g. Econamine FG (cf. Section 6.2.2). Compared to the recovery of CO2 from other sources, the cost of this process is so high that flue gas or similar CO2 sources cannot be regarded as economically attractive for CO2 recovery. However, with the increasing political pressure regarding CO2 emissions, e.g. from power stations burning fossil fuel and chemical plants, great efforts are being made to develop new processes for the capture of CO2 [6.27, 6.28]. This captured CO2 may be stored in the ground (e.g. in exploited oil or gas fields (EOR) or aquifiers) or in the deep sea. For this sequestration the CO2 must be compressed to pressures up to 100 bar and more. The flow rates of such CO2 streams are huge compared to the CO2 flow rates in the normal gas market, e.g. a 600 MWthermal coal-fired power station produces about 240 t/h CO2. After the necessary purification steps those CO2 streams will be excellent sources for the CO2 liquefaction and will change the entire situation on the CO2 market. 6.2.6 Production of Dry Ice

If liquid CO2 is expanded from storage conditions (e.g. boiling fluid at 15 bara) to a pressure slightly above ambient pressure (e.g. 1.1 bara, –77.6 °C), about 52% of the CO2 occur as dry ice snow and the rest in gaseous form [6.29, 6.30]. This dry ice snow is pressed into blocks or into pellets. The remaining expansion gas is compressed to a little over the storage pressure by means of compressors and liquefied in a liquefaction unit so that it can be fed back into the storage tank. The required energy for the production of dry ice is about 170 to 210 kWh t–1, including compression and re-liquefaction of the arising residual gas. Dry ice block weights can vary from 5 to 20 kg per block, with a density of about 1.4–1.6 kg dm–3. Pellets vary from 5 to 15 g per pellet, smaller pellets, 0.3 to 0.7 g are used for dry ice blasting, bulk density is about 0.8–1.0 kg dm–3 Dry ice is shipped in insulated boxes.

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6.3 Applications

Carbon dioxide has certain special properties (see Section 6.1.3) that must be taken into account in its application: x Liquid CO2 can only be used at pressures over 5.2 bar (e.g. in pressurised heat exchangers or spray nozzles) x When pressurised liquid CO2 expands, gaseous and solid CO2 form as a result. Solid CO2 is known as “dry ice” and available on the market as snow, slices, pellets, nuggets and blocks x Solid CO2 or dry ice sublimates at atmospheric pressure, allowing a number of special applications (e.g. cooling and blasting) x Over approximately 32 °C and 74 bar, CO2 enters a supercritical condition, facilitating another range of special applications (e.g. dissolving and cleaning) x Inerting and purging processes with gaseous CO2 follow the same basic pattern as those using nitrogen (see Section 2.5.1) In welding and the processing industry, carbon dioxide is used to: x Inert and purge pipes and vessels for safety and maintenance purposes x Replace harmful propellants in plastic foam (e.g. polystyrene) (see Example B) x Pressurise spray cans as a substitute propellant for harmful chlorofluorocarbons x Shrink and join construction components, e.g. shrink fitting and positive grouting of shafts, gears, valve seats and other machinery components. Dry ice is often used for this purpose x Operate CO2 lasers, e.g. in combination with nitrogen and helium (see Example A) x Weld construction steel and fine-grained steel using the MAG process (preferably in combination with Ar or Ar/O2) (see Section 2.5.3, Example B) In the chemistry, petrochemistry, pharmaceutical and medical industries, carbon dioxide is used to: x Provide an inert solvent or co-reactant for chemical synthesis (e.g. of carbonates) x Extract ingredients and separate them by chromatography (e.g. in analytical chemistry) x Produce inorganic carbonates (e.g. sodium carbonate/bicarbonate) and catalysts x Provide a protective atmosphere in chemical production x Regenerate ion exchangers (e.g. for the partial desalination of drinking water) x Accelerate enhanced oil recovery (EOR) and natural gas recovery x Ensure inert atmospheres during manufacturing and packaging of pharmaceuticals (e.g. pills, powders, pastes, capsules, ampoules)

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x Produce organic and pharmaceutical compounds (e.g. acetyl salicylate, or aspirin) x Enable various medical applications (see Chapter 10) In food technology, biology and environmental protection, carbon dioxide is used to: x Shock-freeze food, e.g. meat or fish, in tunnel, spiral or rotary freezers x Cool food during mixing, chopping and transport (e.g. by mixing dry-ice pellets or CO2 snow into the product) x Protect food during packaging and storage (using pure CO2 or mixtures with N2 and/or O2) x Produce refreshments such as fizzy or sparkling drinks (carbonating with dissolved CO2) x Harden foodstuffs by cooling before cutting (e.g. raw meat or ham) x Stun animals before slaughter (e.g. pigs and fowl) x Extract substances using supercritical CO2 (e.g. caffeine from coffee beans, hop extracts) x Accelerate the growth of plants in greenhouses (see assimilation, chlorophyll) x Neutralise alkaline wastewaters (see Example C) x adjust the calcite saturation in drinking-water recovery and supply (see avoidance of pipe corrosion and Example D) In other industries, carbon dioxide is used to: x Clean surfaces by blasting with CO2 pellets (e.g. heat exchangers, building facades) x Remove raw oil from rocks and beaches after oil-tanker disasters (see Example E) x Remove paint from automotive parts or other coated equipment (e.g. in underground stations) x Clean garments or metal parts in washing machines (substitution of chlorinated carbons) x Protect magnesium melts during handling x Prevent oxidation, fire and explosions (e.g. fire-prevention in warehouses and fire-extinguishing in waste-incineration bunkers) (see Example F) x Clean semiconductor components, e.g. wafers, by dry-ice blasting using highest purity CO2 x Improve pulp and paper production processes (see Example G) Example A: Laser Processes

All laser processes basically involve the conversion of electrical energy into a light beam of a single wavelength. This laser beam is generated in the laser resonator. It is mostly parallel, allowing easy transfer over long distances to wherever it is needed. At the processing area, the laser beam is focused by lenses, providing

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Fig. 6.11 Absorption rate [%] for laser radiation of cold metal.

the energy density needed for immediate heating, melting and even evaporation of metals. Many different laser types have been developed for the treatment of different materials. The higher the absorption rate of the laser radiation, the faster the material can be heated. While some highly reflective materials (e.g. aluminium and copper) absorb short wavelengths best (e.g. Nd:YAG/Neodynium:Yttrium Aluminium Garnet or diode lasers), less reflective materials (e.g. iron and steel) can best be treated with lasers operating at longer wavelengths (e.g. CO2 lasers) (see Fig. 6.11). The most common application area for high-power lasers is laser cutting of metals, since high cutting speed can be combined with high cutting precision (see Fig. 6.12). The advantages of laser welding include very narrow seam widths with considerably fewer weld distortions compared with traditional welding methods (see Fig. 6.13). Welding with CO2 and Nd:YAG lasers is becoming increasingly widespread in industrial production. High-power CO2 lasers (2–12 kW) are used to weld car bodies, automotive transmission components, heat exchangers and tailored blanks. Other laser applications include marking (e.g. product codes), drilling (e.g. where extremely small holes are required) and surface treatment (e.g. annealing, hardening, spraying, coating and cleaning). Technical gases are widely used in laser applications. On one hand, laser gases are used to generate radiation (e.g. LASERMIX® gases), and on the other process gases (e.g. O2, N2 and He) support applications such as cutting, welding and surface treatment.

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Fig. 6.12 Laser cutting.

Fig. 6.13 Laser welding.

As CO2 laser implies, carbon dioxide is the active component of this laser type. The laser gas also contains helium and nitrogen. Besides these main components, some special CO2 lasers require admixing of oxygen, hydrogen, carbon monoxide, and/or xenon, which additionally supports the physical and chemical laser-beam formation. The cutting gas used (see process gases) is crucial to the result. Oxygen generally yields good cutting performance in carbon steels and low-alloyed steels. However, it can react with the base metal and cover the cut edge with an oxide layer. This is why nitrogen is the gas of preference for cutting high-alloyed steels, especially where high laser-power is available. The welding gas (see process gases) has various functions. It protects the focusing optics (lenses) against fumes and spatters and inhibits the formation of a plasma cloud along the laser beam. Helium is most often used for this purpose with CO2 lasers. The welding gas often also plays an active role in the welding process. It increases welding speed and improves the mechanical properties of the joint. Mixtures of helium, argon, carbon dioxide, and/or oxygen are frequently used here. In summary, lasers provide a precise and easily adjustable tool, removing the need for mechanical contact with the work piece. The evolution of this equipment is fascinating, with new applications emerging almost daily.

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Example B: Plastic Foaming

Foamed polymers are characterised by a cellular structure and reduced density compared to solid material. They may be divided into open and closed-cell foams as well as foams with homogeneous cell structures and those with a porous core and compact (unfoamed) outer skin. Foam densities range from as low as approx. 10 kg/m3 in some polymers to close to that of compact materials. The main advantages of foamed plastics are low consumption of raw materials, reduced weight, excellent heat and sound-insulation and mechanical damping. Important applications of polymer foams include packaging materials, insulation, sound absorption and upholstery. Economically important production processes especially include extrusion foaming, polyurethane foaming, production of expanded polystyrene (EPS) and polyolefines (EPP, EPE) and injection-moulded foaming. The cellular structure of synthetic foams is created by so-called blowing agents. Additives are also often required, particularly nucleation agents and stabilisers. Depending on the process and desired foam density, either chemical or physical blowing agents are applied. Chemical blowing agents are mixed into the plastic in powder or pellet-form. Above a certain temperature, the blowing agent disaggregates and releases gaseous reaction products, usually nitrogen or CO2, inflating the plastic to form a high-density foam. One of the most common chemical blowing agents is azodicarbamide (ADC). Physical blowing agents are metered into the molten plastic during foam extrusion or injection-moulded foaming. They may also be applied to one of the initial components in polyurethane (PUR) foaming. Physical blowing agents are used to create low-density foams with a more homogenous foam structure. Hydrocarbons (particularly butane and pentane) and inert gases (e.g. CO2 and N2) are widely used here. Inert gases have many advantages, such as being environmentally friendly, non-flammable, non-toxic, chemically inert and inexpensive. The blowing agent is homogeneously distributed and solved under high pressure (usually 100 to 400 bar) in the melt or in a single reaction component (PUR foaming). At the die exit, the pressure drops abruptly and the blowing agent becomes highly super-saturated in the polymer. The foaming process then starts, i.e. the existing nuclei grow and form bubbles. The physical blowing agent selected has a strong influence on foam quality and costs of the foamed product. Environmental safety also plays an increasingly important role. The blowing agents currently used in industrial countries have no ozone depletion potential (ODP) and aim for very low global warming potential (GWP). The successful application of CO2, whose use is growing in the industry, strongly depends on a powerful pressurising and precise metering system. Starting with a cryogenic tank, the liquid CO2 passes through an initial pressure-boosting step such as a compressor station, followed by a mass flow-controlled high-pressure dosing pump (usually of a diaphragm design) (see Fig. 6.14). Other specially customised systems have also been implemented successfully.

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Fig. 6.14 High-pressure CO2 supply and metering concept for extrusion foaming.

Example C: Wastewater Neutralisation

Many industrial processes generate alkaline wastewater that has to be neutralised prior to further internal usage or external discharge. The release of untreated waste water into public sewers can lead to drastic consequences and serious measures being imposed by the local authorities. The mandatory pH range varies between pH 9 and 6.5. One method of neutralisation is treatment with mineral acids such as hydrochloric or sulphuric acid. However, the potential drawbacks of this include complex storage and metering units, insufficient or excessive acidification, corrosion problems and accumulation of salts such as chlorides or sulphates. When carbon dioxide is mixed into water, it quickly forms carbonic acid and neutralises alkaline compounds. This process is very efficient and requires only very simple dissolving and metering devices. One common application is the SOLVOCARB®-B process (see Fig. 6.15). The carbon dioxide process may be used in wastewater treatment basins or buffer tanks, for example. The gas diffuser hoses release CO2 uniformly into the water, ensuring optimum utilisation. Fixed at the bottom of the neutralisation tank, these perforated hoses are made of resistant elastomer. When the carbon dioxide is switched on, the pores open and small bubbles of gas are emitted. No additional energy source is required for CO2 introduction, which is controlled by a pH-measurement device.

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Fig. 6.15 Introduction of CO2 into wastewater via the SOLVOCARB®-B process (Linde).

The benefits of carbon dioxide for wastewater neutralisation include: x x x x x

Flat neutralisation curve, quick and safe adjustment to mandatory pH range Easy and secure storage and dosage of CO2 Low risk of excessive acidification, corrosion or salt precipitation Economic, safe and eco-friendly operation Also suitable for water treatment in swimming pools and on construction sites

Example D: Conditioning of Drinking Water

High-quality drinking water is one of life’s everyday essentials. However, as demand rises and suitable resources shrink, low-quality raw water is increasingly being used in drinking-water production. Drinking water is subject to stringent statutory requirements. In the developed world, numerous directives define the required minimum quality and associated parameters. To ensure high-quality drinking water supplies, the raw water usually has to be treated to comply with these regulations. Industrial gases provide a variety of conditioning methods to achieve this. Examples of CO2 applications in water treatment are: x Water hardening: In case of insufficient hardness, CO2 and suitable calcium compounds can be added to adjust the desired mineral content (cf. remineralisation) x Utilisation following rapid decarbonisation: Excessive CO2 and/or calciumhydrogen-carbonate can be eliminated by adding calcium hydroxide. After calcite precipitation, the pH can be adjusted as appropriate by adding small amounts of CO2 (cf. calcite equilibrium)

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Fig. 6.16 Enrichment of drinking water with CO2 via the SOLVOCARB®-R process (Linde).

x pH regulation prior to process stages: Many processes can only be performed within a defined pH range. CO2 is used to adjust the pH prior to flocculation stages, for example There are a variety of ways to add CO2 in water works, such as via reactors. One common application is the SOLVOCARB®-R process (see Fig. 6.16). It is a closed system in bypass operation, guaranteeing high standards of hygiene. The benefits of carbon dioxide for the conditioning of drinking water include: x Accurate adjustment of the desired pH and water hardness x Improved taste x Prevention of precipitation and limestone formation x Prevention of corrosion in pipeline mains Example E: Blast-Cleaning with Dry Ice and Liquid CO2

Dry-ice blasting is a non-abrasive method using dry-ice pellets. Liquid blasting starts with CO2 liquid, which is then transformed into CO2 snow, producing an even softer cleaning agent. In both cases, the CO2 particles are accelerated by compressed air through a nozzle focused on the object for cleaning (see Fig. 6.17). The process is similar to traditional gun-shot blasting using solid blasting media such as steel or glass particles. Today, nearly all industries use the CO2 blast-cleaning method. Example applications include cleaning of tire moulds, electronic elements, ship inner walls, bakery machinery, printing equipment, gear drives, foundry moulds, chemical plants, as well as paint removal from all types of surface.

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Fig. 6.17 Ship restoration – removing the antifouling layer via dry-ice blasting.

CO2 pellets are available in various sizes, but the optimum diameter for this purpose is 3 mm. They are produced in pelletisers, which press CO2 snow through a matrix to form the pellets. These are then transported in insulated boxes, or on-site production is also possible. In the liquid blasting method, CO2 liquid is drawn from an insulated tank and supplied via pipeline to a so-called gun. This consists of a dedicated chamber producing the snow and a special nozzle directing it at the target. The main advantage of these CO2 cleaning systems is that they operate without any blast-media residue. The CO2 sublimates completely. For tougher jobs it is possible to add more abrasive matter such as lava sand, leaving only minor deposits. There is a wide variety of cleaning systems on the market catering to different abrasion requirements. As the expanding gas creates considerable noise at the tip of the nozzle, ear protection and sound insulation are mandatory. Customised systems are also available for various purposes, e.g. automatic tire-mould cleaning. Example F: Extinguishing Smouldering Fires in Waste-Incineration Bunkers

Smouldering fires can occur in many sites, such as coal-dust storage, corn silos or wood-chip silos in chipboard factories. Conventional methods of fire-extinguishing often fail in these types of situation, mainly because neither water (with or without additives), foam nor powder can reach the fire if it is deep within a pile of stored material. Gas-extinguishing is then the method of choice. Fire-extinguishing methods for a smouldering fire may be differentiated as follows: Water: x Only soaks upper layers, cannot reach fires deep in waste piles x Makes waste sticky, so clearing by crane becomes difficult x May overload the bunker walls due to relatively high weight x Impedes subsequent waste-incineration in furnaces

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Fig. 6.18 CO2 injection in a waste-incineration bunker.

Nitrogen: x Weighs less than air (molecular weight 28 kg/kmol; air 28,8) x Rises and rapidly leaves seat of fire, diminishing its extinguishing effect x Draws air in as it leaves the fire Carbon dioxide: x Weighs much more than air (molecular weight 44 kg/kmol) and accumulates in bunker x Remains near seat of fire for longer and does not draw in air x Cools due to its high heat capacity x Allows loss-free storage This comparison shows that CO2 provides clear advantages for the fighting of smouldering fires in container like waste-incineration bunkers (see Fig. 6.18). Example G: Process Improvement in Pulp and Paper Production

Soap Acidulation Using CO2 Crude tall oil (CTO) is produced from soap by means of a reaction using sulphuric acid. The consumption of sulphuric acid is normally about 200 kg per tonne of CTO, depending on the soap quality. This represents a major portion of total sulphur intake for a modern kraft pulp mill. Using carbon dioxide as a pre-treatment for soap acidulation in CTO production can cut sulphuric acid consumption by 30 to 50%. Dissolving carbon dioxide in water forms carbonic acid, which reacts with the CTO soap, reducing the solution’s pH from around 12 to below 8. At this pH level, two phases separate, producing both a creamy soap oil and a bicarbonate brine in which black liquor components are dissolved. The creamy soap oil phase is acidulated into CTO. This pre-treatment process may be used in batch or continuous mode and implemented using the regular control system (see Fig. 6.19).

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Fig. 6.19 Example of soap acidulation with CO2 (pretreatment).

Benefits of carbon dioxide in soap acidulation include: x x x x x

Better control of sulphur/sodium balance in the pulp mill Simple process design Use of existing soap tanks and equipment Low investment cost (no pressurised equipment) Improved run ability in existing CTO plant

Pulp Washing Pulp washing is a key process stage in all types of pulp mill. Poor washing conditions may lead to higher water and chemical consumption, reducing the effectiveness of effluent treatment and chemical recovery operations. All these factors tend to increase production costs and effluent problems. CO2 pulp washing technology (see Fig. 6.21) can significantly improve pulp washing results, producing substantial economic benefits. Key effects of CO2 pulp washing include improved ion exchange and reduced fibre swelling, which has an impact on dewatering capability. The carbonates formed react with waterinsoluble sticky calcium soaps, which can then act as detergents. Depending on the specific mill situation, benefits of CO2 pulp washing may include: x x x x x x x x

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Increased production rate Reduced COD from wash plant Decreased chemical consumption Reduced wash-water volumes Decreased steam consumption Lower demand for defoamer agents (see Fig. 6.20) Reduced effluent loads Lower maintenance costs

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Fig. 6.20 Defoamer consumption based on the pulp washing process.

Fig. 6.21 Example of a CO2 pulp washing installation.

The pH Control Concept (ACTICO®) Adjusting pH levels with carbon dioxide instead of a strong acid has several advantages, allowing stable and reliable control and eliminating the risk of pH shocks in the system (see Fig. 6.22). For systems containing calcium carbonate as filler, pH is an important parameter. Below pH 8, calcium carbonate starts to dissolve and the calcium concentrations in the process waters increase. Using carbon dioxide it is possible to adjust safely the right pH that is low enough for further process requirements but high enough to secure reduced dissolution of calcium carbonate.

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Fig. 6.22 pH variations in the head box without ACTICO® (Linde).

Fig. 6.23 Stable pH in the head box with ACTICO® (Linde).

The ACTICO® concept is a sophisticated combination of an automation and CO2 injection system which can be tailored to individual paper machines. The pH control system provides total pH control in the wet end and minimises pH variations (see Fig. 6.23). Benefits of ACTICO® pH control concept include: x Automated adjustment of optimum pH with general improvements in the whole papermaking process x Carbon dioxide may be added in different positions from the stock preparation to the headbox in the paper machine (see Fig. 6.24). Process Stabiliser (ADALKA®) Calcium carbonate is often used as a filler when high brightness levels are desired in the final paper, as its own high brightness increases that of the final product. In systems containing calcium carbonate, pH is an important parameter because calcium carbonate starts to dissolve below pH 8 (see ACTICO®).

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Fig. 6.24 Example of an ACTICO® installation for controlling the pH level in the head box.

Fig. 6.25 Example of ADALKA® installation (Linde) at the incoming pulp lines and the coated broke tower in the papermaking process.

During the ADALKA® process, a sodium bicarbonate buffering solution is added to stock preparation to regulate and stabilise pH, alkalinity and calcium levels in the papermaking process (see Fig. 6.25). This buffering solution is formed on-site by combining carbon dioxide and sodium hydroxide, preferably in an alkalinity control unit (ACU™). The solution enhances the system’s buffering capacity, allowing it to handle more acids and bases without substantial changes in pH. The ADALKA® process stabiliser reduces the dissolution of calcium carbonate due to the common ion effect, resulting in lower concentrations of calcium ions in the process (see Fig. 6.26). Benefits of ADALKA® process stabilizer include: x Addition of bicarbonate buffering solution to reduce calcium hardness which is a process obstacle that negatively impacts several additives and the process x The solution may also be used in many different positions in the papermaking process for pH stabilisation and alkalinity control

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Fig. 6.26 Reduction of calcium hardness with ADALKA® process stabiliser.

References [6.1] J. Black: Experiments upon Magnesia Alba, Quicklime and Other Alcaline Substances, Alembic Club, Edinburgh, 1777. [6.2] A. L. Lavoisier: Opuscules Physiques et Chimiques, Paris, 1774. [6.3] M. Thilorier: Ann. Chim. Phys. 1835, 60 (2), 427. [6.4] H. Dünkelmann: Extraction and Use of Carbon Dioxide (CO2) and Dry Ice, LINDE Reports on Science and Technology, 1968, 12. [6.5] www.coolpage4u.de. [6.6] E. Almquist: History of Industrial Gases, p. 93 ff., New York, Boston, Dortrecht, London, Moscow, 2003. [6.7] W. Stoll, F. Berit: Carbondioxide – where does it come from, where does it go?, special print, edition 9040/II, Messer Griesheim GmbH, Düsseldorf, 1990. [6.8] T. Banuri et al.: IPCC Third Assessment Report, Climate Change 2001, Mitigation, Technical Summary, p. 27. www.ipcc.ch. [6.9] D. L. Albritton et al.: IPCC Third Assessment Report, Climate Change 2001, The Scientific Basis, Technical Summary, p. 36. www.ipcc.ch. [6.10] Ullmann’s, 6th edition, 6, p. 394 ff., Wiley-VCH, Weinheim, 2003. [6.11] Handbook of Chemistry and Physics, 86th edition, CRC Press, London, 2005. [6.12] Ullmann’s, 6th edition, 6, p. 397, Wiley-VCH, Weinheim, 2003. [6.13] EIGA Publication: IGA Doc 66/99/E, Appendix B. [6.14] European Industrial Gas Association (EIGA). www.eiga.be/catalogue.asp. [6.15] Compressed Gas Association (CGA). www.cganet.com/Publications.asp. [6.16] EIGA Publication: IGC Doc 70/99/E, Appendix B. [6.17] A. L. Kohl, R. B. Nielsen, R. B.: Gas Purification, Gulf Publishing Company, Houston, 1997. [6.18] Publication of the German Patent Application No.: DE 10 2004 011 428 A1. [6.19] Publication of the German Patent Application No.: DE 10 2004 011 429 A1. [6.20] T. Mimura, K. Matsumoto, M. Iijima, S. Mitsuoka: Development and Application of Flue Gas Carbon Dioxide Recovery technology. www.CO2cr.com.au/PUBFILES/CAP0304/ PUBFT-0209.pdf. [6.21] International Patent Application, No.: 2004/110595 A2. [6.22] International Society of Beverage Technologists (isbt). www.bevtech.org/order_publications.html.

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6 Carbon Dioxide [6.23] M. Simmonds, P. Hurst, M. B. Wilkinson, C. Watt, C. A. Roberts: A Study of Very Large Scale Post Combustion CO2 Capture at Refining & Petrochemical Complex, 6th International Conference on Greenhouse Gas Control Technologies, pp. 39–44, Kyoto. [6.24] K. Muramatsu, T. Shimizu, N. Shinoda, A. Taani: Development of Mitsubishi Wet Flue Gas Desulfurization System, Cemical Economy & Engineering Review, No. 11, Vol. 16, Nov. 1984. [6.25] J. Sporer, The SOLINOX® Process, Technology and Operating Experience of Regenerative Desulfurization Method, LINDE Reports on Science and Technology, 1992, 50. [6.26] J. D. Brady, Flue Gas Scrubbing Process for Sulfur Dioxide and Particulate Emissions Preceding CO2 Absorption, Enviromental Progress, Vol. 6, No. 1, Feb. 1987. [6.27] B. Mertz, O. Davidson, H. de Coninck, M. Loos, L. Meyer: Carbon Dioxide Capture and Storage, IPCC Special Report (Summery for Policymakers & Technical Summery) WMO, UNEP 2003. [6.28] G. Marsh: Carbon Dioxide Capture and Storage – A Win-Win Option? Special report by AEA Technology plc, May 2003. www.dti.gov.uk/files/file18798.pdf. [6.29] Ullmann’s, 6th edition, 6, p. 405, Wiley-VCH, Weinheim, 2003. [6.30] CGA Publication: G-6.9 – 2004, Dry Ice.

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Natural gas just like petroleum developed during the mesozoic (250 to 65 million years ago) and the tertiary (65 to 1.6 million years ago) during the conversion from organic substances of predominantly maritime origin that deposited at the bottom of the sea. With more and more material being taken up, the pressure on the lower layers increased sharply and the temperature rose to 100 to 200 °C. Thus crude oil and natural gas developed from the residues of the dead organisms (Figs. 7.1 and 7.2). In China, natural gas was used as a fuel a number of centuries earlier than in Western cultures, maybe even in the 5th century B.C. Methane-rich natural gas

Fig. 7.1 Natural gas development. Industrial Gases Processing. Edited by Heinz-Wolfgang Häring Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31685-4

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Fig. 7.2 Natural gas bubble.

found in brine deposits was gathered together with the brine. Here, the combustible gas was often used to evaporate the brine for the extraction of salt. Later on in China gas pipelines of bamboo canes were built with which the distance of a day’s journey could be bridged. Old texts already report on heating value adjustment of the natural gas by the controlled admixture of air before being fed into the pipelines. The transport of natural gas in big leather sacks was also very common. The ancient “perpetual fires” in the area of today’s Iraq, as they were already mentioned in the paper of Plutarch of the period from about 100 to 125 B.C., presumably came from natural gas which escaped out of crevices and which was ignited by flashes of lightning. In the 19th century, the commercial utilization of natural gas began in Europe and North America. An essential contribution to this was the invention of the Bunsen burner by Robert Bunsen in the year 1885. With this device, natural gas could be mixed with air in the proper ratio to enable safe combustion.

7.2 Occurrence

Natural gas is found in many different layers in the underground. This comprises formations of slate, sandstone, coal and underground salt water accumulations. On the ground of deep oceans, solid methane hydrate is found that develops from water and methane under pressure and at low temperatures. Rich methane hydrate deposits are situated in the North Siberian Sea. The natural gas deposits detected today are shown in Fig. 7.3. The development of the detected deposits over the last twenty years is to be found in Fig. 7.4. It was above all the discoveries in Russia and around the Persian Golf that considerably increased the known reserves.

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Fig. 7.3 Proved natural gas deposits. (Source: BP statistical review of world energy 2006).

Fig. 7.4 Development of proved natural gas reserves. (Source: BP statistical review of world energy 2006).

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Natural gas can be subdivided into two categories: dry and wet natural gas. Here, dry and wet do not refer to the water content, but to the content of higher hydrocarbons. Dry natural gas is produced from relatively low depths in pure gas deposits and disposes of a calorific value of about 35 000 kJ m–3. In contrast to wet natural gas, the dry one is immediately available for use and needs no special cleaning. Wet natural gas, also called associated gas, usually occurs when crude oil is pumped from greater depths. Owing to the higher reservoir pressure, the hydrocarbons gaseous at atmospheric pressure are dissolved in the crude oil. With the crude oil being pumped, the light and medium-weight hydrocarbons evaporate owing to decompression and are absorbed again by the natural gas. The carbon dioxide possibly contained in the natural gas is not only capable of contaminating the product. In different research project, experts try to develop techniques that help to avoid the emission of large amounts of this greenhouse gas. Promising seem to be recent projects in which the carbon dioxide occurring during the drillings is pumped through the drilling tunnel back under the seabed. In these experiments, the carbon dioxide accumulates in a porous layer of sandstone enclosed by slate rocks. Natural gas is one of the most important energy sources, the availability of which is of great importance for the global economy. After its pumping together with crude oil, natural gas is often stored in former already depleted gas fields. In some cases, these natural reservoirs dispose of a capacity of up to one billion cubic meters.

7.3 Consumption

The consumption of natural gas differs widely from country to country (Fig. 7.5). Countries with large own reserves tend to handle the raw material natural gas more generously, while countries with scarce or lacking resources are of course more economical. Despite the considerable findings, the predicted availability of the natural gas reserves has hardly changed. If consumption and resources develop similarly, as it was the case in the last years, a serious natural gas shortage is to be expected at the beginning of the second half of the 21st century (Fig. 7.6).

7.4 Natural Gas Trade

As a rule, over shorter and medium distances up to about 3000 km, natural gas is transported as gas in pipelines. In case the laying of pipelines is not possible for geographical or political reasons, or the distances between source and consumer exceed 3000 km by far, then today natural gas is usually liquefied near the source, transported by ship as LNG (liquefied natural gas) and again converted into its gaseous state in the vicinity of the consumer (Fig. 7.7).

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Fig. 7.5 Natural gas consumption per capita. (Source: BP statistical review of world energy 2006).

Fig. 7.6 Ratio natural gas reserves/extraction. (Source: BP statistical review of world energy 2006).

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Fig. 7.7 Cross border trade in natural gas. (Source: BP statistical review of world energy 2006).

At the beginning of the 21st century, the total amount of natural gas consumed amounted to about 2500 · 109 m3 a–1 or 2500 BCM (see Table 7.1). With 150 · 109 m3 or 150 BCM (corresponds to about 120 · 106 t or 120 Mtpa) per year, the share of LNG in the consumption seems to be quite small. However, in relation to the cross-border natural gas market, this results in a trade share of more than 20–25% with a tendency to rise (Fig. 7.8). Table 7.1 Regional classification of natural gas consumption in 109 m3. Marketed production

Imports

Consumption

120.0

121.8

739.3

North America

737.5

Latin America

133.4

12.0

12.0

133.4

Europe

283.0

117.3

263.4

428.9

22.9

0.0

45.5

68.4

FSU

719.7

131.1

–

588.7

Africa

127.2

66.3

1.2

62.1

Middle East

229.4

36.6

9.6

202.5

Asia/Oceania

272.3

76.6

106.5

302.1

2525.4

559.7

559.7

2525.4

Central Europe

Total World

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7.5 Composition

223

Fig. 7.8 Development of the global LNG-quantities in trade.

7.5 Composition

Depending on the deposit, there are large differences in the composition of natural gas. It is often the case that heavier natural gas, i.e. rich in higher hydrocarbons, can be found in deep reservoirs and vice versa. Increased helium contents are often accompanied by high nitrogen content. Table 7.2 Composition of natural gas. Components

Typical

Extreme

Methane CH4

80–95

50–95

% Mol. frac.

Ethane C2H6

2–5

2–20

% Mol. frac.

Propane C3H8

1–3

1–12

% Mol. frac.

Butane C4H10

0–1

0–4

% Mol. frac.

C5 Alkanes and higher hydrocarbons

0–1

0–1

% Mol. frac.

Carbon dioxide CO2

1–5

0–99

% Mol. frac.

Nitrogen N2

1–5

0–70

% Mol. frac.

Hydrogen sulfide H2S

0–2

0–6

% Mol. frac.

Oxygen O2

0

0–0.2

% Mol. frac.

Helium

0–0.1

0–1

% Mol. frac.

Other inert gases

traces

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7 Natural Gas

7.6 Process of Natural Gas Treatment

Usually, natural gas treatment on the basis of thermal process engineering takes place in three steps (see Fig. 7.9). The first step that may consist of partial steps just like all other subsequent steps, serves the preparation of the crude gas for its processing. Here, for example, acid-forming gas components, such CO2, H2S and other sulphuric compounds are removed. Usually, chemical scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent is being regenerated. Then the natural gas is dried. In case of moderate water dew point requirements, glycol is used as wash liquor. The lowest water contents (< 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally, mercury is removed in case aluminium will be used as material of construction for equipment. Mercury in contact with aluminium may lead to catastrophic corrosion. In the central process step, the pre-treated natural gas is separated into a light and a heavy fraction. As a rule, this separation takes place by means of partial condensation below ambient temperature. The light fraction always contains methane and nitrogen, sometimes even lighter hydrocarbons. For further use it is either compressed to pipeline pressure or liquefied and used as LNG. Beginning with ethane, the heavy fraction can contain all higher hydrocarbons that may be isolated, if required, by means of fractioning and then be marketed in technically pure quality.

Fig. 7.9 Diagram of natural gas treatment.

7.6.1 Dew-point Adjustment

Dew-point adjustment (see Fig. 7.10) serves the reduction of the concentration of water and heavy hydrocarbons in natural gas to such an extent that no condensation occurs during the ensuing transport in the pipeline. This would lead to a multiphase flow that places higher demands on the design and laying of the pipelines.

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Fig. 7.10 Dew point adjustment. (1) Condenser; (2) Separator; (3) Column; (4) Reboiler; (5) Compressor.

Water can be removed down to a dew point of –30 °C by means of glycol scrubbing. Then the water-containing glycol is regenerated through rectification and is reused. Owing to diverse impurities, the arising waste water has to be reconditioned before it is discharged into the environment. Usually, the ensuing separation of higher hydrocarbons occurs through cooling and partial condensation in a heat exchanger (1). The formed liquid phase is separated (2) and, if required, stabilized in a column (3) through the stripping of light components by means of a reboiler (4). Now, after being heated up again, the remaining gas has the required margin from the dew point of the water and the organic compounds. Usually, the top product of column (3) is fed back to the sales gas again by means of compression (5). 7.6.2 Separation of Liquefied Petroleum Gas

By liquefied petroleum gas, also called LPG, a mixture of propane and butane is understood that is often sold in small containers as cylinder gas. Above all, it is important for a sufficient fuel gas supply of infrastructurally weak areas without connection to a gas pipeline. In some countries, LPG is very popular as fuel for motor vehicles. Moreover, LPG is a valuable raw material in the petrochemical and chemical industry. The commercial recovery of liquefied petroleum gas from natural gas is carried out in plants with a capacity range of 10 000 mN3 h–1 up to over 1 000 000 mN3 h–1 of crude gas. The larger the plant, the higher the propane yield may be for an economic optimum, since the increased investment costs are amortized faster.

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Simple process versions such as in the case of dew point adjustment are not suitable here, since at a typical crude gas pressure of 50 to 100 bar and a singlestage, partial condensation with ensuing separation of the developed fluid, too much propane remains in the gas phase, thus limiting the propane separation from the sales gas. All modern plants dispose of a cryogenic separation in the core of the process. Earlier concepts, based on the scrubbing of light hydrocarbons from the crude gas by means of oil, are no longer market-relevant. A process version with high propane yield (US-patent 4,157,904 of The Ortloff Corporation) is shown in Fig. 7.11. The crude gas is cooled and partially condensed under high pressure in a heat exchanger (1) that may also consist of a heat exchanger network. If required, even an external refrigerating plant can be used for cooling, which, as a rule, uses propane as refrigerant. The formed liquid is separated in a separator (2), expanded, heated up again in the heat exchanger (1) and fed to a column (4). The column (4) is operated by a reboiler in a way that the sump product consists of propane and higher hydrocarbons (C3plus in short). Only small amounts of ethane and lighter hydrocarbons are admitted in the bottom product. The gas phase from the separator (2) is now divided into two split flows. The usually larger part is expanded in a turbine (3) and works on the pressure of the column (4), at the same time it is partly condensed and then fed to the column. Due to this procedure, a cold flow is fed to the column that reduces the overhead propane losses. Now the decisive step for the propane yield is to completely condense the remaining gas flow from the separator (2) against

Fig. 7.11 C3plus separation GSP. (1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster; (6) Compressor; (7) Heat exchanger; (8) Reboiler.

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the cold overhead product of column (4) in a further heat exchanger (7) under pressure, to subcool it and feed it to column (4) as reflux. This step is the basis for the customary expression GSP (Gas Subcooled Process). The overhead product, heated again to ambient temperature in heat exchanger (1) is then recompressed to a pressure under which it can be discharged to a pipeline. Sales-gas compression is driven by the mechanical power of the expansion turbine (3) in a booster (5) and the external energy in a compressor (6). Now the sales gas consists of all feed gas components with a vapor pressure higher than that of propane, thus basically of nitrogen, methane and ethane. With this process version, a propane yield of about 90% can be achieved efficiently. The propane yield of the GSP-process is limited by the fact that the reflux on the top of the column (4) is still relatively rich in propane, thus owing to the gas/liquidequilibrium propane gets lost overhead. A possibility for the enhancement of the propane yield is shown in Fig. 7.12 (US patent 4,854,955 of the Elcor Corporation). In contrast to the GSP-process, the crude gas flow condensed in the heat exchanger (7) is not directly fed to column (4) after the expansion to column pressure, but used as refrigerant in an overhead condenser (11). Propane still getting lost in the GSP-process can now be recovered in a reflux separator (9) after partial condensation and again be fed to column (4) by means of the reflux pump (10). The name SFR (Split Flow Reflux) derives from the additional reflux. Due to this economical development of the GSP-process, the propane yield can be boosted to the range between 97 to 99% at constant power consumption.

Fig. 7.12 C3plus separation SFR. (1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster; (6) Compressor; (7) Heat exchanger; (8) Reboiler; (9) Reflux separator; (10) Reflux pump; (11) Overhead condenser.

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The two processes presented so far share the fact that the crude gas is completely led through column (4). On the one hand, this makes a larger column diameter necessary in the upper part of column (4). Moreover, the large amount of methane in this area dilutes the hydrocarbons to be recovered, thus rendering their separation more difficult. In a new approach (see Fig. 7.13) differing from the GSP and SFR-processes, the upper section of column (4) is separated and designed as a recontactor (9) (US Patent 4,617,039 of the Pro-Quip Corporation, today incorporated in the Linde AG). The partially condensed crude gas flow of the expansion turbine (3) is fed to the sump of the recontactor. The overhead product of the reduced column (4) is cooled against the top product of the recontactor (9) and at the same time partially condensed. Here, a larger amount of ethane is condensed in a methane-lean gas. In the recontactor (9), this liquid ethane now meets a methane-rich gas phase with significantly lower ethane content. As a result, part of the ethane-rich reflux of the recontactor (9) evaporates and, owing to the heat of evaporation required for this, it leads to a significant cooling of gas and liquid in the recontactor (9). Ultimately, in the OHR-process the lowest process temperature is no longer caused by the outlet condition at the expansion turbine (9), but by the cooling effect of this integrated open absorption-heat pump, through which ethane in the column (4) is stripped under high pressure, liquefied in the condenser (7) and re-evaporated in the recontactor (9) under low partial pressure. The permanent ethane losses overhead of the recontactor (9) are continuously replaced by ethane from the crude gas.

Fig. 7.13 C3plus separation OHR. (1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster; (6) Compressor; (7) Heat exchanger; (8) Reboiler; (9) Recontactor, (10) Reflux pump.

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229

Apart from the usual process steps of partial condensation and rectification, this aspect of the OHR-process brings up the variant of the self-cooling physical scrubbing, thus opening up new variation possibilities. One of these variants is increasing the pressure of the recontactor (9) over the pressure of the column (4). Now, in the partially condensed top product of column (4) a separator is required from which the reflux can be transported to the recontactor (9) by means of a pump. As a counter move, pump (10) is no longer required and can be replaced by a valve. Owing to this modification, the pressure in recontactor (9) can now be chosen independently from the critical pressure of the C3plus fraction in the sump of column (4). Instead of the previous limitation to about 30 bar, the pressure of the methane fraction in recontactor (9) can now be maintained at 40 to 50 bar, owing to which the expenditure for recompression in the sales gas compressor (6) can be reduced. Although the OHR-process does allow the highest propane yields, it is still very popular due to its attractive investment costs and the high overall efficiency resulting from it. 7.6.3 Ethane Separation

Ethane recovered from natural gas is mainly used in ethylene plants in which ethylene and other light alkenes are produced from ethane and even higher hydrocarbons. Since today ethylene plants are only efficient with annual capacities starting at 500 000 t of ethylene, even the demand of natural gas in the preceding ethane recovery is high. With a typical ethane content of a mole fraction of 5% in the natural gas, about 1 200 000 mN3 h–1 of natural gas are required for the ethylene production mentioned before. Large petrochemical complexes process about two or three times this amount. Ethane separation from natural gas (see Fig. 7.14) is carried out in similar processes as the propane (or liquefied petroleum gas) separation previously described in detail. Since the patent protection for the GSP-process has expired for some time (original patent grant in the year 1979), this process belongs to common knowledge in the separation of natural gas. Since in the case of ethane separation, column (4) shows a significantly colder temperature profile compared to propane separation owing to the lighter components, the heat integration between crude gas cooling and column heating has to be carried out differently. The liquid from separator (2) is not heated again in heat exchanger (1), but fed directly to column (4). For the crude gas cooling, cold liquid flows are rather routed from column (4) and heated against crude gas. Thus, an advantageous coupling of column heating and crude gas cooling is to be achieved, rendering superfluous the separate sump heating of column (4) as well as a crude gas cooling by external sources. Similar to the propane separation, with this simple process the economic ethane yield can only be brought to slightly over 90%. For higher yields, the further development of the GSP will be useful, which is known under the term RSV (Recycle Split Vapour) (US patent 5,568,737 of the Elcor Corporation, see Fig. 7.15).

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7 Natural Gas

Fig. 7.14 C2plus separation GSP. (1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster; (6) Compressor; (7) Heat exchanger.

Fig. 7.15 C2plus separation RSV. (1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster; (6) Compressor; (7) Heat exchanger.

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Here, with the otherwise same basic process, a split flow of the compressed sales gas is recycled, completely condensed in the heat exchanger (7), subcooled and finally fed to column (4) as reflux. This reflux is significantly leaner in ethane compared to the reflux used in the GSP-process. Thus, better retention of the ethane at the top of column (4) is possible owing to which ethane yields between 95 and 99% are economically achievable. 7.6.4 Liquefaction

With the liquefaction of natural gas, an energy density is obtained that corresponds to about three times the pressure storage at 200 bar. This fact is favourably used for storage and transport purposes. Natural-gas liquefaction was originally used to balance the seasonally different demand for natural gas. In these so-called peak-shaving plants, natural gas is typically liquefied in the in-between seasons and in summer, and is stored as LNG in large tanks. In times of peak demand, often occurring in January, natural gas is re-evaporated and fed into the pipeline system. The first plant of this kind was built in 1939 in West Virginia, USA. Even today, a lot of these plants are operated mainly by local energy suppliers. Today, however, there are hardly any new projects for peak-shaving LNG-plants, since caverns are used for the storage of highpressure natural gas. This renders the expensive liquefaction and cold storage of the natural gas superfluous. For a large part of the existing peak-shaving plants, refrigeration is based on expander cycles as they are common, for example, for air separation and liquefaction. This uncomplicated technique is particularly suited for the intermittent operation of a typical peak-shaving plant. Apart from serving as permanent energy store, LNG is also an alternative to the natural gas transport via pipeline. In case the laying of pipelines is not possible for geographical or political reasons, or if distances between source and consumer significantly exceed 3000 km, today natural gas is usually liquefied near the source, transported by ship as LNG and converted into its gaseous state again near the consumer. Liquefaction plants of this kind serve the permanent basic supply of gas customers and are therefore called Base-Load-Plants. The first Base-Load LNG-Plant was put into operation in Algeria in 1964. From today’s point of view, the early plants disposed of a low liquefaction capacity of below one million tons of LNG per year (< 1.0 mtpa LNG). The cold required for the liquefaction was usually generated in a series arrangement of pure component refrigeration cycles (pure component cascade) or in one single mixed refrigerant cycle. Since in the case of pure refrigerants, the temperature does not change during evaporation at constant pressure, an energetically advantageous small temperature difference between warm and cold flows can be realized through the application of a number of evaporation stages (see Fig. 7.16). Despite the high equipment cost of a pure-component cascade resulting from the multitude and complexity of the recycle compressors, the specific energy

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7 Natural Gas

Fig. 7.16 Cooling of a typical natural gas against a pure component refrigerant cascade.

consumption is still quite high. Various companies developed improved concepts for the reduction of energy consumption and investment costs of the refrigeration cycles – criteria that are increasingly important for plants with a capacity of more than 1.0 mtpa of LNG. Over the last decades, Air Products & Chemicals, Inc. held an outstanding market position mainly based on the C3MR-process (LNG Air Products C3MR, see Fig. 7.17). In this process, for the first time ever applied in Brunei in 1973, the pretreated natural gas is first cooled (1) to about –30 °C by a multi-stage propane refrigerating unit installed around a large recycle compressor (2) and a condenser

Fig. 7.17 LNG Air Products C3MR. (1) Heat exchanger; (2) Recycle compressor; (3) Condenser; (4) Heat exchanger; (5) Separator; (6) Compressor; (7) Condenser.

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Fig. 7.18 Cooling of a typical natural gas against pure component precooling with ensuing mixed refrigerant cycle.

(3). Liquefaction and subcooling of the natural gas to –160 °C occur against a mixed refrigerant in a heat exchanger (4) of special design described in more detail at the end of this chapter. This mixture, normally consisting of light hydrocarbons from methane to pentane and nitrogen, is chosen in a way that the evaporation process over a wide temperature range approaches the cooling curve of the warm process flows (see Fig. 7.18). In detail, the mixed refrigerant cycle is structured as follows: the gaseous refrigerant is fed to the recycle compressor at about –30 °C and at low pressure of about 2 to 3 bar and compressed to a high discharge pressure of more than 50 bar. As a rule, two separate compressor casings are required for this compression, as there is not enough space in a common casing for the multitude of compressor impellers necessary owing to the high pressure ratio. In the aftercooler (7), the compression heat is then discharged to the ambient. Now the C3-precooling cycle is used to partly condense the mixed refrigerant. In the cold separator (5) a high-boiling liquid and a low-boiling gas develop which are further cooled down separately in the heat exchanger (4). At the same time, the gas flow of (5) is being completely condensed and fed as refrigerant to the top of the heat exchanger (4) after expansion. At the same pressure, the liquid phase separated in separator (5) shows a boiling range shifted towards higher temperatures owing to the heavier components, and is therefore suited to the cooling of the warm flows entering the heat exchanger (4). Thus the mixed refrigerant is first separated into two fractions in the separator (5) and then mixed again in heat exchanger (4). The C3MR-process, extremely successful on the market is predominantly applied in the performance range of 1–5 mtpa of LNG. In the case of higher plant capacities per train, limiting factors are above all the main dimensions of the mixed refrigerant cycle compressor (6) and the heat

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7 Natural Gas

Fig. 7.19 LNG Linde/Statoil MFC.

exchanger (4), and here mainly in the lower, warm range. With the aim of shifting the capacity barrier of base-load LNG-plants upwards and reducing specific investment and operating costs, the Linde AG company, in cooperation with the Norwegian Statoil ASA, has developed the Mixed Fluid Cascade (MFC process) (see Fig. 7.19). In contrast to the C3MR-process, in the MFC-process all refrigerant flows of different composition are kept separate at any time. The cold separator (5) is redundant. Instead of this, mixed refrigerants are used in separate closed cycles the composition of which corresponds more or less to the gas respectively liquid phase of the separator (5). The additional cold mixed refrigerant cycle has now a separate refrigerant evaporator (8), serving the subcooling of the LNG, as well as a recycle compressor (9) with aftercooler (10). Since the refrigerant completely evaporated in the heat exchanger (8) is now led directly to the corresponding compressor (9) and is no longer first superheated in the warm lower part of the heat exchanger (4) and than led to compressor (6) together with the heavy fraction of separator (5), as it is the case in the C3MR-process, both compressor (6) and the warm lower part of heat exchanger (4) are significantly relieved. Owing to this measure, the upper capacity barrier of a single-line LNG-plant is raised to about 12 mtpa of LNG. Moreover, the specific energy consumption is reduced due to precooling (1) based on a further third mixed refrigerant cycle. A first LNG-plant according to the MFC-principle is currently being built and will be put into commercial operation in the year 2007, with a liquefaction capacity of 4.3 mtpa of LNG. Almost all base-load LNG-plants built so far are using a special construction for the heat exchangers (4) respectively (8), which is termed as coil-wound heat exchanger (see Figs. 7.20 and 7.21). Here, a number of heat-exchanger tubes, sometimes several thousand, are wound on a central core tube (mandrel). This process is comparable to the winding up of yarn on a bobbin. Thus, huge heating surfaces of several ten thousand square meters can be accommodated in one apparatus. In the tubes, the flows to be cooled down are arranged upwards. On

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Fig. 7.20 Coil-wound heat exchanger on the winding bench.

the shell side, the cold flow, in this case the evaporating refrigerant, is falling, owing to which all tubes are equally cooled down. Owing to their mechanical robustness, coil-wound heat exchangers are highly esteemed. For design reasons, one shell flow only can be led against several tube flows. Brazed aluminium heat exchangers, a frequently used alternative in cryogenic processes, are much more flexible with regard to the flow arrangement. However, owing to the limited core dimensions many parallel cores are required. At the time of printing, however, it was still not possible to apply brazed aluminium heat exchangers successfully in large base-load LNG-plants with mixed refrigerant cooling. Besides Air Products & Chemicals, Inc., today the Linde AG is the only manufacturer worldwide of coil-wound heat exchangers suitable for base-load LNG-plants. Figure 7.21 shows two of these items manufactured by the Linde AG for an LNG-plant in Australia that works according to the C3MR-process.

Fig. 7.21 Coil-wound heat exchanger ready for shipment.

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7.6.5 Nitrogen Separation

Natural gas is a widespread energy carrier used by a multitude of consumers. Since pipeline systems often connect a lot of providers with a lot of users, the adherence to certain specifications for the required compatibility of the different kinds of natural gas is necessary. Very important quality features in this connection are the calorific value and the Wobbe number. In case natural gases with high nitrogen content cannot be adapted to pipeline standards by mixing them with other natural gases of suitable composition, nitrogen separation is often required. As a rule, the separated nitrogen is emitted to the atmosphere, consequently it has to comply with the stringent conditions regarding the residual content of hydrocarbons. The admissible residual content of nitrogen in natural gas usually amounts to some percent. For separation units with these demands, i.e. high purity and at the same time high yield in relation to one gas component, even nowadays cryogenic plants (see Fig. 7.22, N2-separation without pre-separation) are economically superior to newer processes on the basis of adsorption or permeation. After the pretreatment usual for cryogenic plants, the crude gas is cooled down in a first heat exchanger (1) and partially condensed. The developing liquid, which contains only little dissolved nitrogen, is evaporated again under a pressure as high as possible and fed to the second stage of the two-stage sales gas compressor (8/9). The remaining gas phase of (2) is cooled down further in the heat exchanger (3) and fed to the lower part of a double column. The sump heating of the lower column part (5) is also integrated into the heat exchanger (3). The task of the lower

Fig. 7.22 N2-separation without pre-separation. (1) Heat exchanger; (2) Separator; (3) Heat exchanger; (4) Heat exchanger; (5) Column; (6) Heat exchanger; (7) Methane pump; (8, 9) Compressor.

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column part is the separation of the crude gas into a methane-lean top product and a nitrogen-lean sump product. Then, both primary products are fed to the upper part of the column, with the sump product being subcooled in heat exchanger (3) and the top product being condensed, also subcooled, in heat exchanger (4), and finally used as reflux. Quantity and composition of this reflux are responsible for the quality of the top product – the nitrogen to be separated – of the upper column. The highly concentrated nitrogen is heated up in the heat exchanger chain 4–3–1 and discharged to the ambient. Now, the sump of the upper column is a nitrogen-lean methane fraction that evaporates in heat exchanger (3) after internal compression by means of pump (7) and is heated to ambient temperature in heat exchanger (1). Afterwards, this flow is brought to discharge pressure to the pipeline with the help of the sales gas compressor (8/9). The heat integration of these two sections of column (5) occurs similarly to an air separation unit to the effect that the operating pressure of the lower (pressure) column is chosen high enough to enable the top product of the pressure column to be condensed in a heat exchanger (6) against the evaporating sump product of the upper (low-pressure) column. During the operation of a plant, quality and yield of the separated nitrogen are only to be influenced by the discharge pressure of the methane pump (7). High pressure of the pump results in poor separation and vice versa. In case the nitrogen content of the natural gas falls below 25% by volume, the process described above is no longer suitable for the separation of nitrogen with the required volume and purity. In such cases, a process with pre-separation column (10) (see Fig. 7.23) is applied. Here, in the first heat exchanger the feed gas is

Fig. 7.23 N2-separation with pre-separation. (1) Heat exchanger; (2) Separator; (3) Heat exchanger; (4) Heat exchanger; (5) Column; (6) Heat exchanger; (7) Methane pump; (8, 9) Compressor; (10) Column.

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deeper precondensed, and from the liquid separated in the separator a nitrogenrich overhead fraction is stripped. The nitrogen-lean methane quantity removed via the sump of the column (10) is now large enough for the gases discharging from separator (2) and column (10) to be further separated in the double-column operation described above, owing to their now increased nitrogen content. The initially surprising fact that the reduction of a low nitrogen concentration in the crude gas involves higher expenditures than a high nitrogen concentration, is to be explained by the fact that in the first case the concentration factor of the nitrogen to be separated is higher and therefore represents the more demanding processing task. For this reason, natural gas fields with a nitrogen content increasing over the time represent a special procedural challenge. This situation is more and more caused by tertiary oil production in which gases, among them also nitrogen, are re-injected in order to maintain the natural reservoir pressure. After some time, nitrogen finds its way into the neighbouring natural gas fields and makes a nitrogen separation from the natural gas necessary to keep the natural gas still salable.

7.7 Applications

Natural gas is used as: x x x x x

a fuel for industrial heating and desiccation processes a fuel for the operation of public and industrial power stations a household fuel for cooking, heating and providing hot water a fuel for environmentally friendly liquid natural gas vehicles a raw material for chemical synthesis (see also Methane and other Fuel Gases, Section 8.5) x a raw material for large-scale fuel production using gas-to-liquid (GTL) processes (e.g. to produce sulphur- and aromatics-free diesel with low-emission combustion)

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8 Fuel Gases 8.1 Introduction

Combustible gases have an ignition range and an ignition temperature when mixed with air or oxidizing substances. Combustible gases are self-igniting if the ignition temperature is below 100 °C. Even at room temperature these gases can react so strongly that the exothermal energy leads to flame occurrence. Instable gases are prone to exothermal spontaneous decomposition without requiring air or an oxidizing substance. In general combustible gases with a high calorific value are referred to as fuel gases (Table 8.1). Table 8.1 Overview of important fuel gases [8.1, 8.2]. Name of the gas

Chemical formula

Ignition range in air (% vol. fraction)

Ignition temperature (°C)

Specific calorific value (kJ kg–1)

Acetylene

C2H2

2.4–83.0

325

49 912

Butane

C4H10

1.5–8.5

365

49 500

1-Butene

C4H8

1.6–10.0

440

48 426

Natural gas

Mixtures esp. of CH4, C3H8, CO2,N2

depending on composition


Ethane

C2H6

3.0–15.5

515

51 877

Ethene

C2H4

2.7–34.0

425

50 283

Methane

CH4

5.0–15.0

595

55 498

Propane

C3H8

2.1–9.5

470

50 345

Propene

C3H6

2.0–11.1

455

48 918

Hydrogen

H2

4.0–75.6

560

141 800



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8 Fuel Gases

They are used for specific purposes on an industrial scale, such as acetylene for welding and cutting steel. The fuel gases acetylene and ethene are discussed below.

8.2 Acetylene C2H2 8.2.1 Acetylene and the Beginnings of Welding Engineering

The historical development of acetylene runs parallel to the history of carbide from which it was initially formed after the addition of water. The first reference to carbide and acetylene came from the Irish chemist Edmund Davy in England in 1836. When trying to produce metal potassium by heating a mixture of calcined potassium tartrate with charcoal, he obtained a black mass as by-product – i.e. calcium carbide – which reacted with water and formed a combustible gas. In a scientific report he described the characteristic properties of acetylene in detail. At that time, he called the gas “bicarburet of hydrogen”, which means more or less “double carbon bond of hydrogen”. In 1860, the Frenchman Berthelot again came across the gas discovered by Davy and investigated its properties more thoroughly. He named it “acetylene“ and determined its chemical formula as C2H2. He also recognized that this gas formed the first element in a range of hydrogens whose formula he determined as CnH2n–2. Berthelot also succeeded in producing acetylene from organic raw materials, i.e. from hydrocarbons and was thus to all intents and purposes one of the founders of organic chemistry. In 1862 F. Wöhler found the (at that time) more natural way to produce acetylene when, searching for metal aluminum, he succeeded for the first time in producing calcium carbide that releases acetylene when treated with water. However, Wöhler’s discovery remained on an academic scale since the large amounts of electric energy, which are today applied as a matter of course, were not available at that time. The basis of the carbide industry – inexpensive energy in large amounts – was not formed until the invention of the dynamo machine by Werner von Siemens in the year 1866. The first electric furnaces to be powered with electrical energy gained from water power were put into operation after 1892, which enabled the production of calcium carbide on an industrial scale and also with suitable purity. The German-Canadian chemist T. L. Wilson and the Frenchman and later noble laureate H. Moissan are closely associated with these events. In 1892 Wilson succeeded in producing calcium carbide for the first time in an electric furnace he himself had built and is therefore regarded as the founder of the acetylene industry. In the USA Wilson was granted a patent for his process in 1893. At that time, acetylene won from carbide was very much in demand as it burned with a bright shining flame. Its luminance easily surpassed that of candles,

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petroleum lamps and coke gas. Euphorically people referred “to the sun having been brought to earth”. Acetylene lighting spread quickly, but the success story hoped for by the fast expanding carbide industry was only fulfilled to a small extent. The American Edison invented the filament lamp in 1879 with which electric current was used directly for lighting purposes. Even the brightest acetylene lamp did not have a chance in the long run against this competition. At the turn of the century, however, another property of acetylene gained growing importance in industry – its extremely high combustion temperature with oxygen, i.e., 3160 °C. Carl von Linde’s invention of liquefying air and isolating oxygen from it greatly accelerated the development of autogenous technology. Autogenous technology is the process in which ferrous metals can be separated or bonded by means of the acetylene-oxygen flame. The biggest step forward for the autogenous technology was the ability to safely store and transport gases under pressure in special steel cylinders. This is especially valid for acetylene for which special measures had to be taken as a result of its thermal instability (proneness to decomposition). Acetylene can only be stored and transported safely in steel cylinders if it is dissolved in a suitable liquid, such as acetone, at a maximum pressure of 25 bar and if the solvent with the acetylene dissolved in it is absorbed inside the steel cylinder by a porous mass which does not conduct heat well. The most urgent task of the national and international associations, e.g. “Association for Autogenous Metal Processing e.V.”, “DVS – German Welding Society”, “IIW – International Institute of Welding” and “EWF – European Federation for Welding, Joining and Cutting” was the safety aspect. The necessity of establishing rules and regulations which have to be observed by gas producers and users was and still is obvious. The accidents, some of which were serious, occurring initially during the production and handling of acetylene (and oxygen) had to be prevented at all costs. In the period before the development of petrochemistry, acetylene was the basis of organic chemistry, as is still the case in some industries today. Calcium carbide, the ideal energy store, can be easily won from coal, lime and electric energy and can be stored in large quantities without difficulty. Similarly acetylene can easily be produced from carbide. A variety of products can then be produced from the acetylene after bringing it together with other elements and compounds. In Germany today about 60% of the total acetylene production is supplied by the petrochemical industry and 40% is still produced in carbide-acetylene plants. 8.2.2 Physical Properties

The main physical properties of acetylene (ethyne) are shown in Table 8.2 [8.1, 8.2].

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8 Fuel Gases Table 8.2 Physical properties of acetylene. Property

Symbol

Unit

Value

Molar weight

M

g mol–1

26.038

°C bar

–80.55 1.2819

Triple point temperature Pressure Critical point temperature Pressure Density

TC PC UC

°C bar g m–3

35.18 61.9 231

Standard density at 15 °C/1bar

UN

kg m–3

1.095

Density relative to air at 15 °C/1bar Specific heat (at 0 °C/1.013bar)

J g–1

0.1018

Ignition temperature (1.013 bar) x with air x with oxygen

°C °C

355 300

Max. flame temperature with oxygen

°C

3160

Ignition limits at room temperature x with air x with oxygen

% volume fraction % volume fraction

2.3 … 82 2.5 … 93

Combustion velocity

m s–1

1.35

Formation enthalpy

kJ mol–1

226.9

kJ kg–1 kJ kg–1

50 400 48 700

Thermal conductivity (at 0 °C, 1.013 bar)

W m–1

0.0184

Molar volume (at 0 °C, 1.013 bar)

m3 kmol–1

22.223

Conversion figures Data: 1 kg = 0.909 m3

kg m3

1 0.909

Upper calorific value Lower calorific value

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0.905

cP

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8.2.3 Acetylene Decomposition – Deflagration

Acetylene (C2H2, ethyne) has a high reactivity as a result of the triple bond. The triple bond can be broken down through external influences, such as heat supply. This process is called decay or decomposition, in special cases even deflagration. When this happens the acetylene molecule decomposes into its components carbon and hydrogen and the large amount of energy stored in the triple bond, which is required to keep it together, is released. The energy released in the form of heat can stimulate the neighboring molecules to decompose too, which may lead to a chain reaction. This property is the source of the risk potential of acetylene. The safety risk can be controlled but it requires an exact knowledge of possible reactions to certain external influences. Applied to modern acetylene technology, this results in the following safety principle: Acetylene under pressure is only permitted to be enclosed in cavities of limited volume that can be sealed off from each other or in pipes of limited nominal diameters and limited maximum allowable pressure. The mechanical stability of the casing of these cavities and pipes has to be great enough to safely withstand the possible consequences of decomposition under operating conditions. 8.2.4 Ignitable Mixtures

Table 8.2 shows that acetylene is ignitable in air within the wide range of 2.3–82.0. It has a combustion velocity of 1.35 m s–1 and an enthalpy of 226 kJ mol–1. Acetylene can to all intents and purposes be regarded as a gaseous explosive, comparable to other better known explosive agents. As reference explosive one quantity unit of TNT (trinitrotoluene) is internationally valid and here the quantity of 1 kg of TNT is used for the comparison: The following is applicable: 1 kg 1 kg 1 kg 1 kg

Nitroglycerine C2H2 in decomposition C2H2 with O2 Oxyhydrogen

to to to to

1.25 kg TNT 1.72 kg TNT 2.41 kg TNT 3.14 kg TNT

8.2.5 Liquefaction of Acetylene – Acetylene Hydrate

In the aggregate state “liquid” acetylene has particularly high energy content and therefore it has explosive character. In the vapor-pressure curve (Fig. 8.1) basic data such as the critical point and the dew point are shown. The formation of liquid acetylene should be absolutely avoided when handling this gas, as this state is not manageable without elaborate measures.

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8 Fuel Gases

Fig. 8.1 Critical point and dew point of acetylene.

Liquefaction of acetylene in acetone-treated cylinders is impossible as the pressure in the cylinder cannot reach the liquefaction pressure at low temperatures [8.3]. 8.2.6 Acetylene Hydrate

Acetylene hydrate originates through the addition of water to compressed acetylene in a temperature range of –5 to +10 °C. Its formula is C2H2 · 5.75 H2O and it has a waxy appearance. Its risk potential is not as high as that of liquid acetylene. It can however ignite when hit by shock waves. Similar to liquid acetylene, it can originate during compression at winter temperatures. This has to be avoided by taking appropriate safety measures. When acetylene hydrate occurs in the equipment of an acetylene plant, especially in decomposition barriers and other safety accesories, it is less the danger of explosion than the blockage of the gas paths through the waxy mass that has to be taken into consideration. 8.2.7 Acetylides

Under certain circumstances acetylene combines with the metals copper, silver and mercury to form acetylides. As a dry substance these acetylene compounds are explosive and ignite through impact or friction. Compared to the other acetylides, silver acetylide releases the largest amount of energy in an explosion. Acetylides can originate and precipitate from watery saline solutions of the metals mentioned above under certain conditions regarding temperature, pH value and concentration. These precipitation reactions were previously used in the wet chemical acetylene analytics.

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In acetylene engineering – and especially in the generation of acetylene from carbide as well as in supply engineering – the possibility of generating acetylide is an important factor when selecting the material for the equipment. The formation of acetylides is not only possible in saline solutions in laboratory experiments but it also occurs when moist crude acetylene comes into contact with metallic silver or copper surfaces. Corrosion products encourage acetylide formation on copper surfaces. Tests have shown that there is considerable acetylide formation when pure copper or copper alloys (brass) have a Cu-content > 70%. The layer thicknesses can reach a strength that allows the separation of pure acetylide as particles, similar to scale. With Cu contents of 70% and below, very thin acetylide layers are still possible but there is no danger of ignition.

Fig. 8.2 Plant sketch of an acetylene generator (Type Sirius-Linde 400, designed for a gasometer overpressure of 750 mm water column { 7.35 kPa).

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Regulations concerning the use of materials containing silver or copper can be found in the appropriate guidelines, such as the TRAC 204 “Technical regulations for acetylene plants and calcium carbide storage” or EIGA “Code of practice acetylene”. If these rules are observed, there is no risk in using these metals in acetylene plants [8.3]. 8.2.8 Extraction Processes 8.2.8.1

Acetylene Generated via Carbide

Acetylene is traditionally generated by the reaction of calcium carbide with water. The low-pressure development system Sirius-Linde is described in systems engineering terms as a highly efficient system with trouble-free operation. Acetylene is purified and dried after generation [8.3]. 8.2.8.2

Petrochemically Generated Acetylene

Acetylene which is won petrochemically is usually offered in very large quantities at the respective plant as acetylene is no longer of prime importance in the chemical industry as a raw material for chemical products. One advantage over carbide acetylene is that there are no by-products and therefore no facilities required for their disposal. The petrochemical manufacture of acetylene is based on reprocessing cracked gases with the ensuing extraction of C2 hydrocarbons [8.4]. 8.2.9 Gas Supply 8.2.9.1

Storage of Dissolved Acetylene in Cylinders

Acetylene can only be stored in a gaseous state. The supply as liquefied gas, as with air gases (argon, nitrogen, oxygen and the like), is ruled out due to the very high risk of explosion. In order to prevent acetylene decomposition, initiated by a flashback or external heating, from spreading and to bring it to a standstill, the steel cylinders have to be specially prepared with a porous or high-porous mass. In the past, Mikropor A, consisting of pumice, kieselguhr, charcoal and magnesium carbonate, was used as a filling mass. A high-porous monolithic mass of 90% porosity consisting of calcium oxide, silica flour and water has, since the 1960s, taken the place of the previous filling mass. It is filled into the steel cylinder in viscous form and is then steam cured in the furnace. For the improvement of the mechanical stability a special kind of glass fiber is now used instead of asbestos fibers. The solvent plays the main role in the safety system “acetylene cylinder“, in which acetylene can be dissolved under pressure. Two solvents, which are comparable regarding their protective properties in acetylene cylinders, are used world wide – acetone and Dimethyl Formamide (DMF).

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Due to the differences in vapor pressure and dissolving power for acetylene, acetone can be favorably applied in climate zones with summer and winter. DMF should be used in warmer climate zones, where temperatures remain considerably above freezing all the year round. The reason for this lies in the higher dissolving power of DMF for acetylene at lower temperatures. DMF is detrimental to health and as a liquid can be absorbed through the skin. When handling these two solvents, the respective rules of conduct contained in the safety specifications, laws and rules and regulations must be observed. 8.2.9.2

Design of a Gas Supply System

The design of a gas supply system depends on the one hand on the maximum discharge quantity of an acetylene cylinder, type 40, 48, 50 (500 l per hour in normal single-shift operation), on the other hand on the weekly average consumption and also on cost-effective logistics. Gas supply systems can comprise of single-cylinder units, gas cylinder manifold units, cylinder bundle units (6 or 16 cylinders) and mobile acetylene supply plants, e.g. containers, acetylene trailers with 8 or 16 bundles (16 cylinders per bundle) that can provide up to 2304 kg of acetylene with one vehicle. An acetylene supply plant consists of a high-pressure, medium-pressure and possibly a low-pressure section. In addition to the pressure regulators, safety devices such as manual safety cylinder or bundle connectors, non-return valves, automatic quick acting shut-off devices, flame arrestors and multifunctional safety devices have to be installed. As basic rules and regulations for the planning of an acetylene supply unit EN or ISO standards, e.g. ISO 14 114, ISO 5175, ISO 7291, ISO 2503, ISO 14 113, ISO 15 615 have to be complied with. The initial approval of acetylene plants has to be carried out by qualified persons and/or a notified body. Recurrent inspections are also required. Before planning or modifying such plants it is advisable to contact a gas supplier who employs specialized application engineers and qualified staff. 8.2.10 Autogenous Engineering Applications

The molecule acetylene has an enormous amount of combustible energy with high flame efficiency and ignition velocity in the acetylene flame. As soon as molecular decomposing begins energy is released – in contrast to other fuel gases. This energy is called heat of formation or enthalpy. Acetylene releases 8714 kJ kg–1 for use at this stage. The energy of the first combustion phase with oxygen, the primary flame, has to be added. Only this energy is of importance in autogenous engineering, a considerable advantage compared to other fuel gases.

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Main application fields of autogenous engineering (cf. Section 8.5): x x x x x x x x x x x

Flame cutting Flame cleaning Flame grooving Flame straightening Flame heating Flame hardening Gas welding Hard-face welding Flame spraying Flame brazing Carbon boating

8.2.11 Regulations

When operating, planning, constructing, at the initial and recurrent inspections, during maintenance and service of acetylene gas cylinders, bundles and their supply units national, European or international rules, orders, regulations have to be complied with, e.g.: 1. Directive 97/23/EC of the European Parliament and of the Council of 29 May 1997 2. EC Directive 1999/92/EC of the European Parliament and of the Council of 14 Dec 1999 explosion protection 3. ISO 5175/EN 730-1 and -2 Equipment used in gas welding, cutting and allied processes – safety devices for fuel gases and oxygen or compresses air – general specifications, requirements and tests 4. ISO 7291/EN 961 Welding, cutting and allied processes – manifold regulators 5. ISO 2503/EN ISO 2503 Gas welding equipment – pressure regulators for gas cylinders used in welding, cutting and allied processes up to 300 bar 6. ISO 14 113/EN ISO 14 113 Gas welding equipment – rubber and plastic hoses assembled for compressed or liquefied gases up to a maximum design pressure of 450 bar 7. ISO 14 114/EN ISO 14 114 Gas welding equipment – acetylene manifold systems for welding, cutting and allied processes, general requirements 8. EIGA – European Industrial Gases Association Code of practice acetylene, IGC Doc 123/04/E

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249

8.3 Ethene C2H4 8.3.1 Physical Properties

In Table 8.3 the fundamental physical properties of ethylene (ethene), C2H4 are shown [8.1, 8.2]. 8.3.2 Production Processes

The petrochemical production of ethene is based on reprocessing cracked gases and their isolation and the ensuing extraction of C2 hydrocarbons [8.5]. 8.3.3 Application and Use

Ethene is also used for industrial production in autogenous engineering due to its more favorable chemical-physical properties with regard to flame temperature, ignition velocity, flame efficiency and heat of formation. These advantages distinguish ethane from the other more slowly burning fuel gases. The values are however below those of acetylene. Main application fields in autogenous engineering (cf. Section 8.5): x x x x x x x

Flame cutting (cutting efficiency lies between propane and acetylene) Flame grooving Flame straightening Flame heating Gas welding Flame spraying Flame brazing

8.3.4 Gas Supply and Safety

The design of the gas supply depends on the one hand on the required maximum gas amount in m3/h together with the supply pressure, on the other hand on the weekly average consumption and in addition on cost-effective logistics. Gas supply systems can comprise of single-cylinder units, gas cylinder manifold units as well as cylinder bundle units (e.g. 12 cylinders per bundle). At constant volumes of more than approx. 500 m3 per month, there is the possibility of a supply via heat-insulated stationary tanks with downstream evaporators. The low-temperature ethene is delivered to the customer in a liquid state by means of special tank trucks and then transferred loss-free into the

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8 Fuel Gases Table 8.3 Physical properties of ethylene. Property

Symbol

Unit

Value

Molar weight

M

g mol–1

28.054

Triple point resp. melting point at 1.013 bar Vapor pressure Melting heat

°C bar kJ kg–1

–169.43 0.0012 119.45

Boiling point at 1.013 bar Heat of evaporation

°C kJ kg–1

–103.72 482.86

°C bar g/L

9.5 50.76 218

In liquid state: x Density at the boiling point at 1.013 bar x Specific heat at the boiling point

g L–1 kJ kg–1 K–1

567.92 2.42

In gaseous state: x Density at 0 °C and 1.013 bar x Specific heat at 25 °C and 1.013 bar x Thermal conductivity at 15 °C and 1 bar

kg m–3 kJ kg–1 K–1 µW cm–1 K–1

1.261 1.54 188

Critical point temperature Pressure Density

TC PC UC

Density relative to air Ignition temperature (1.013 bar) x with air

°C

425

Max. flame temperature with oxygen

°C

2924

Ignition limits at room temperature x with air x with oxygen

% volume fraction % volume fraction

2.7 … 34.0 2.9 … 80.0

Formation enthalpy

kJ kg–1

1865

kJ kg–1

47 600

m3 kmol–1

22.245

L kg m3

1 0.568 0.482

Lower calorific value Molar volume (at 0 °C, 1.013 bar) Conversion figures for different states Date: 1 L liquid = 0.568 kg = 0.482 m3

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0.974

HL

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customer tank via a rotary pump. If necessary, the liquefied ethene is evaporated in the evaporator and conducted via pipes to the points of consumption, each of which is equipped with a multifunctional safety device. The capacity of the ethene storage depots ranges from 1500 kg up to 30 000 kg and higher. The admissible operating pressure of the tank is 18 bar. The required supply pressure is adjustable over the whole pressure range. The fundamental rules and regulations for the design of ethene supply plants laid down in international and national rules, orders or directives have to be followed. The initial approval of ethene plants has to be carried out by qualified persons and/or a notified body. Recurrent inspections are also required. Before planning or modifying such plants it is advisable to contact a gas supplier who employs specialized application engineers and qualified staff. 8.3.5 Regulations

When operating, planning, constructing, at the initial and recurrent inspections, during maintenance and service of acetylene gas cylinders, bundles and their supply units the following has to be observed (among others): 1. Directive 97/23/EC of the European Parliament and of the Council of 29 May 1997 2. EC Directive 1999/92/EC of the European Parliament and of the Council of 14 Dec 1999 explosion protection 3. ISO 5175/EN 730-1 and -2 Equipment used in gas welding, cutting and allied processes – safety devices for fuel gases and oxygen or compresses air – general specifications, requirements and tests 4. ISO 7291/EN 961 Welding, cutting and allied processes – manifold regulators 5. ISO 2503/EN ISO 2503 Gas welding equipment – pressure regulators for gas cylinders used in welding, cutting and allied processes up to 300 bar 6. ISO 14 113/EN ISO 14 113 Gas welding equipment – rubber and plastic hoses assembled for compressed or liquefied gases up to a maximum design pressure of 450 bar

8.4 Other Fuel Gases

The preceding chapters dealt exclusively with the fuel gases acetylene and ethene. Other fuel gases, such as e.g. ammonia, chlorine ethane, chlorine methane,

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dimethyl ether, isobutane, isobutene, phosphine etc. are described in [8.1, 8.2, 8.6, 8.7].

8.5 Applications

Acetylene (ethyne) is used as: x high-performance fuel gas for autogenous processes (see Section 2.5.2) – in combination with O2, e.g. for welding, cutting, straightening, brazing, soldering, hotforming – in combination with compressed air, e.g. for preheating before – and past – heating after welding x separating agent that deposits pure carbon (e.g. for the glass and aluminium industries) x combustible carrier gas for flame-photometry in analytical chemistry x basic substance for the production of organic compounds (e.g. acetaldehyde, monomer vinyl chloride, acetanhydride) Ethylene (ethene) is used as: x pure fuel gas and main component in fuel gas mixtures for autogenous processes (allowing transport and storage in the liquid phase) x ripening gas for the controlled ripening of stored fruit (e.g. with N2/ethylene mixtures for bananas) x basic substance for the production of organic compounds (e.g. polyethylene, polyvinyl chloride, polyether, polyvinyl ether, anthracene, ethylene oxide, isoprene) Methane is used as (see also Chapter 7): x universal and environmentally friendly fuel gas for heating purposes x fuel gas component for explosion deburring x reactant to create specific atmospheres in metallurgical furnaces x operating gas for radiation-counter tubes x standard for calorimetric measuring x basic substance for the production of organic compounds (e.g. via formation of synthesis gas) Propane is used as: x common liquid gas for many types of general heating x technical fuel gas for applications such as heating, warming, flame cutting and annealing x propellant for aerosol cans (substitute for chlorofluorocarbons or CFCs) x refrigerant, e.g. for household or industry refrigerators x fuel gas for hot air balloons (buoyancy through hot combustion gas products) x basic substance for the production of propene and polypropylene

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References

253

References [8.1] Linde AG, 82049 Höllriegelskreuth: Spezialgase-Katalog (8550/2). [8.2] Linde AG, Division Linde Gas: catalogues, data sheets and special prints on specialty gases and gases for metalworking (fuel gases), 82049 Höllriegelskreuth. [8.3] H.-J. Sontag: Linde AG, 82049 Höllriegelskreuth: Handbuch des Carbidacetylens. [8.4] Ullmann’s, 6th edition, 1, p. 215 ff., Wiley-VCH, Weinheim, 2003. [8.5] Ullmann’s, 6th edition, 1, p. 531 ff., Wiley-VCH, Weinheim, 2003. [8.6] F. Schuster: Handbuch der Brenngase und ihre Eigenschaften, Vieweg, Braunschweig. [8.7] The John Zink Combustion Handbook, Printed in USA, ISBN 0-8493-2337-1.

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9 Specialty Gases 9.1 Introduction

Under the generic term “specialty gases”, far more than 100 different gaseous and liquid substances and many thousands of different gas mixtures are subsumed. The boundaries of the product group of specialty gases are fluid and therefore only difficult to define. Nevertheless, in the following an attempt is made to compile criteria defining a gas’ or a gas mixture’s characteristics classifying them as specialty gases. x Higher technical specification of the gas, e.g. regarding its purity or its specific components x Higher expenditure on the availability, e.g. for the pre-treatment of the tanks, purification, filling x Rare occurrence of a gas (even with regard to the application volume), e.g. stable isotopes such as helium-3, deuterium x Origin beyond the classical field of industrial gases (in general, from the chemical and petrochemical industry), e.g. chlorine, hydrogen chloride, silane and related to these x Certain chemical (corrosivity, self ignition) and toxicological properties x Supply in special packing such as compressed gas packing (non-returnable containers) In the following, specialty gases are subdivided into pure gases, gas mixtures and the product group of electronic gases derived from the application (may be pure gases and gas mixtures). The pure gases/gas mixtures dealt with in this chapter are compressed gases often filled and transported in compressed gas containers. Accordingly, the applicable regulations (Technical Rules on Compressed Gases = TRG and Traffic Laws, ADR) are being referred to.


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9.2 Pure Gases 9.2.1 Definitions

In general, the definitions of “gases” are almost congruent in different literature sources. According to ADR 2003: “Gases are substances with a vapour pressure of more than 3 bar at +50° or which are completely gaseous at +20° and a standard pressure of 1.013 bar.” From this, the definition of pure gases can be derived [9.1]. According to pr EN 13 096: “A pure gas is a substance as described above, which occurs technically pure in the compressed gas container [9.2]. A pure gas can contain other components stemming from the production process or added to maintain the stability of the product, provided that the concentrations of these components do not change the classification or the transport regulations such as the reason for filling, filling pressure or test pressure”. According to TRG 100: “Pure gases are compressed gases consisting of only one molecule type and occurring technically pure in the compressed gas container” [9.3]. These definitions take already into account that 100% pure gases do not exist. The different stages of purity are characterized by more or less large portions of impurities stemming from production and filling processes. 9.2.2 Quality Criteria

The purity of a gas corresponds to the content of the main molecule and is given in percent, usually with reference to the mole fraction. Another indication of purity, the dot notation [9.4] has also gained acceptance, as already described in Section 2.2.5.5. The dot notation serves the clear indication of the minimum content of a gas by means of two digits separated by a dot. x The digit in front of the dot indicates the number of “nines” in the percentage for the content of the pure gas x The digit behind the dot indicates the first decimal place deviating from the “nine”, e.g. argon 5.6 means: minimum content of argon 99.9996% mole fraction nitrogen 6.0 means: minimum content of nitrogen 99.9999% mole fraction Apart from the dot notation, there are also terms that point to particularly low impurities or to an application. x Nitrogen “CO-free” (Zero gas for CO-gauges) x Helium ECD (Helium for the operation of Electron Capture Detectors)

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Essential additional criteria are defined by listing components (specifications) with limiting concentrations that must not be exceeded. Owing to the low proportions, the following smaller units have gained acceptance. ppm (part per million, 1 · 10–6) ppb (part per billion, 1 · 10–9) ppt (part per trillion, 1 · 10–12) 9.2.3 Sources/Production

Today, a number of large-scale units for the generation of gases can achieve the purities (6.0 and higher) required for specialty gases. These plants are described, inter alia, in the Sections 2.2.5.5, 3.3.2, 3.4.2, 4.2, 5.2.4 and 6.2.4. The generation of all other gases occurs in complex chemical processes in which split streams are branched off for further utilization. 9.2.4 Purification/Processing

The product quality achieved in large-scale plants not always is sufficient for the different applications of specialty gases. This means the products have to be subsequently purified. The multitude of pure gases and the descriptions of the related individual purification-processes would go beyond the scope of this chapter. Therefore, Table 9.1 shows only examples of processes, pure gases and impurities to be removed [9.5]. Irrespective of the purification carried out, it is essential for maintaining the quality of the raw material to ensure suitable pretreatment of the containers and adequate gas transfer in the filling station. These topics are being referred to in Sections 9.3.2.2 and 9.6. The supply of special purities is also possible on the basis of analytical control (“selection procedure”).

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9 Specialty Gases Table 9.1 Processes for the purification of specialty gases. Process

Pure gas

Impurities to be removed

Notes

Distillation cylinder to cylinder

Gases liquefied under pressure

lower-boiling components like N2, O2

Heating of the output cylinder, cooling of the target cylinder over condenser

Distillation in the tank

i. a. CO2

lower-boiling components like N2, O2

Distill off top gas, isobar if possible

Adsorption on molecular sieve

inert gases

CO2, moisture

Adsorption on activated carbon

inert gases

SO2, NO2, HCl

Heterogeneous catalysis

Air H2, N2, Ar, He CO2

CO, H2, CH4 O2

Catalyst Pd b. 300–450 °C Catalyst Pd b. 20–120 °C

H2, HC1)

Catalyst Pd/Pt 230–350 °C

inert gas O2 H2 N2

O2, H2O, CO, CO2, CH4, H2, N2 H2O, CO, CO2, CH4, H2 O2, H2O, CO, CO2 O2, H2O, CO, CO2, CH4, H2

Getter acc. SAES-patent Catalyst + Absorber Getter/Catalyst + Absorber Getter/Catalyst + Absorber 500–600 °C

Xe

SF6

Getter/gas scrubber

1)

HC = hydrocarbons

9.2.5 Application Examples

Here again the multitude of products renders the representation difficult. This means that only a small part of the possible applications, as given in Table 9.2, can be described. In view of its importance, one field of application shall be described more closely, namely instrumentation gases for analytical measuring methods. Instrumentation gases are used in sample processing as extraction medium, stripping medium or refrigerant to extract samples, to strip out highly volatile substances or to enable the enrichment in a cold trap. As zero gas, they are only permitted to contain the component to be measured in a concentration not detectable for the applied measuring devices. Zero gases serve to the adjustment of the zero point in gas-analysis devices.

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259

Table 9.2 Application examples for pure gases. Gas kind

Application

Use as/for

Ar

x Insulation glass panels x Atomic absorption spectrometer (AAS) for metal traces

Filling gas Shielding gas and purge gas

CO2

x Biological growth control

Anaerobic organisms

He

x Glass fiber production

Inert atmosphere and thermal discharge

Kr

x Light and gas discharge lamps x Insulation glass panels

Filling gas Filling gas

Ne

x Light and gas discharge lamps x Flatpanels in plasma technology (Plasma-Display-Panel, PDP)

Filling gas Filling gas

N2

x Biological growth control x Semiconductor manufacturing

anaerobic organisms Inert gas

N2O

x Food technology x Flame atom absorption spectrometer

Propellant for spray cream Operating gas

SF6

x Medium and high voltage switch x Nonferrous metal melts (aluminium, magnesium) x Glass fiber production x Insulation glass panels

Insulation and cooling gas Shielding gas Adjustment of the refraction gradient Filling gas

Xe

x Light and gas discharge lamps x Flat panels in plasma technology x Particle accelerator for nuclear-physical examinations x Ion engines for e.g. satellites

Filling gas Filling gas Filling gas Propellant

Usually, for this purpose are used: x x x x

Nitrogen in purity levels of up to 6.0, CO-free and ECD Hydrogen in purity levels of up to 6.0 Helium in purity levels of up to 6.0 and ECD Argon in purity levels of up to 6.0

Taking gas chromatography as an example, Table 9.3 shows the various applications in analytics as carrier and instrumentation gas.

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9 Specialty Gases Table 9.3 Application of gases in the gas chromatography. Detector

Carrier gas

Instrumentation gas

Gas purity for measuring range

Remarks

ppt – 100 ppb – > 10 ppm 100 ppb 10 ppm Thermal conductivity detector (TCD)

Hydrogen

5.3

5.0

Helium Argon Nitrogen

5.3 5.3 5.3

5.0 5.0 5.0

Flame ionization detector (FID)

Hydrogen Helium Nitrogen

6.0

5.6

5.3

5.0

6.0 6.0

5.6 5.6

5.3 5.3

5.0 5.0

HC-free1)

Synthetic air Electron capture detector (ECD)

Flame photometric detector (FPD)

Helium

Nitrogen

ECD

Nitrogen Helium Hydrogen

ECD

P10/P5 – gas (%-methane in Argon)

Hydrogen Helium Nitrogen

6.0

5.6

5.3

5.0

6.0 6.0

5.6 5.6

5.3 5.3

5.0 5.0

HC-free

Helium

6.0

5.6

5.3

5.0

6.0

5.6

5.3

5.0

Helium

7.0–6.0

Hydrogen

6.0

5.6

5.3

5.0

6.0 6.0 6.0

5.6 5.6 5.6

5.3 5.3 5.3

5.0 5.0 5.0

Helium Argon Nitrogen

Mass selective detector (GC-)MS 1)

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Helium Nitrogen Hydrogen Oxygen Methane Helium

6.0

C

D

A

HC-free1)

Synthetic air Atomic emission detector (AED)

A

1)

Nitrogen

Helium ionization detector (HID) Thermoionic detector (TID)

B

ECD ECD

Synthetic air Photo ionization detector (PID)

A

6.0

6.0

6.0 5.0 5.0 4.5

5.3 5.0 5.0 4.5

7.0–6.0

6.0

E

Hydrocarbons

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9.3 Gas Mixtures/Calibration Gas Mixtures 9.3.1 Definitions

Gas Mixtures (Generic Term) Just as in the case of pure gases, there are again definitions from different sources. According to pr EN 13 099: “A gas mixture is a mixture of two or more components, liquid or gaseous, filled in a cylinder to be withdrawn as mixture, and fulfilling the criteria of a gas” (see Section 9.2.1) [9.6]. According to TRG 100 : “Gas mixtures are compressed gases consisting of several kinds of molecules (components). Apart from gases fluids may also be components of a gas mixture” [9.3]. Calibration Gas Mixtures Generally, gas mixtures intended for the application as calibration gas mixtures have to fulfil higher demands on manufacturing and analysis as well as on the purity of the raw materials. There are two official German definitions for the term “calibration gas mixtures”: According to TRG 102 : “A calibration gas mixture is a gas mixture intended to be used in analytical technology.” [9.7].

Notes for Table 9.3 A Hydrocarbon impurities (HC) in the instrumentation gases cause strong reference line noise and consequently to a deterioration in the detection limit. Therefore, the HC-concentration in the instrumentation gases should be as low as possible. For the FID/FPD, a gas mixture of 40% of hydrogen, balance helium is used as fuel gas. B The ECD reacts very sensitively to impurities in the gases, pipes, fittings and seals from substances with a high electron affinity like oxygen and chlorofluorocarbons (CFC). Oxygen, moisture and CFCs deteriorate the detection limit. C Easily ionizable HC impurities in instrumentation gases increase the reference line noise. Therefore, the HC-share in the instrumentation gases should be as low as possible. D Due to the interference liability of the HID, the detector should be operated under protective atmosphere. E Besides high purity helium as carrier and plasma gas, the spectrometer needs high purity nitrogen as purge gas and various reagent gases, depending on which elements are to be measured.

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According to VDI-Guidelines 3490 P. 1: “A mostly compressed gas mixture, normally consisting of a balance gas and one or more components” [9.8]. This VDI-definition contains further essential terms. Balance gas: “Pure gas or gas mixture, which as the major component supplements the components used for calibration”. Component: “Gaseous or vaporized component of a calibration gas mixture, known in quantity and quality, and directly used for examination of the calibration”. In Table 9.4, some customary synonyms for these “official” terms are given. Table 9.4 Terms and synonyms. Calibration gas mixture

=

Balance gas

+

Component

Calibration gas

Main component

Measuring component

Standard

Carrier gas

Component

Reference standard

Residual gas

Portion

Sample gas

Component Element

Mole Fractions and Concentration Units For a comprehensive description of a gas mixture/calibration gas mixture, apart from the indication of the kind of balance gas and components, even the indication of the mole fraction resp. of the concentration is required. x The mole fraction of a component is the ratio of the number of moles of the component to the sum of the number of moles of all components of the calibration gas mixture. x The concentration of a component represents the ratio of the quantity of this component to the volume of the mixing phase. For a clear characterization, inter alia, the following details are possible: x x x x

Mole fraction, e.g. mol mol–1, mmol mol–1, µmol mol–1 Mass concentration, e.g. kg m–3, g m–3, mg m–3 Volume concentration, e.g. m3 m–3, L m–3, mL m–3 Mole concentration, e.g. mol m–3, mol L–1, mmol L–1

The volume indications always refer to the standard state (1.013 bar, 273.15 K); parts by volume are based on ideal gas volumes (mole fractions). For smaller units, see Section 9.2.2.

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9.3.2 Production [9.9] 9.3.2.1

Technical Feasibility

Physical Constraints If gas mixtures contain condensable components (acc. to TRG 102, each gas with a critical temperature t –10 °C, as well as each liquid) the filling pressure has to be limited in a way that these components do not condense under the filling pressure. As practice-oriented condensation temperatures +10 °C are determined for summer (manufacturer-specific exception, according to TRG +5 °C) resp. –10 °C in winter that should not be exceeded or fallen below. Determination of the maximum filling pressure according to TRG 102 [9.7]: Pfill + 1 ≤

100 ⋅ Pi Ki

Pfill = Filling pressure (gauge) at +15 °C in bar Pi = absolute vapour pressure of the condensable component i in bar Ki = Concentration of the condensable component in the mixture in % vol X Example: Butane Pi = 1.3 bar at +10 °C Ki = 10% volume concentration Result: Pfill = 12 bar i.e. a mixture with a butane volume fraction of 10% may only be filled up to 12 bar (based on the temperature +10 °C). Chemical Constraints Gases that can react amongst themselves must not be mixed (e.g. CO2 + NH3, SO2 + NH3). See also TRG 102, Appendix 2 “Gas mixture diagram” [9.7]. Safety-related Constraints Mixtures of fuel gases and oxygen resp. synthetic air are only allowed to be filled, if after all, their concentration lies in a concentration range, which has to be sufficiently below or above the respective explosion limit. For the filling of such mixtures (in Germany) a documented approval of the Federal Institute for Materials Research and Testing (BAM) has to be obtained. 9.3.2.2

Pretreatment of Containers

Only consequent pretreatment of the containers [9.9] enables the production of stable calibration gas mixtures. Since calibration gas cylinders can be used for the most different compositions, both residues of the previous mixture and a possibly existing moisture film on the inner surface have to be removed during the pretreatment, if possible quantitatively. For this purpose, the containers are being

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heated, evacuated at regular intervals and purged with the gas used as balance gas later on. In case of special requirements (e.g. ppm-mixtures with corrosive components), the additional moisture measurement of the purge gas is advisable. Only when the discharging purge gas shows the same moisture content as the inflowing dry purge gas, the quality of the pretreatment is guaranteed. 9.3.2.3

Preparation Methods

Basically, calibration gas mixtures are produced by combining defined quantities of different components. The preparation methods are characterized by special features: Method of mixing procedure: x Static, i.e. certain gas amounts are filled one after the other into a container (unique mixing procedure) x Dynamic, i.e. gas flows are continuously being mixed Method of determining quantity: x Volumetric, i.e. by determination of volumes x Manometric, i.e. by measurement of pressures x Gravimetric, i.e. by determination of masses (weighing) The combination of these procedures results in a multitude of possible methods Here, however, only such methods are being dealt with that are suitable for a production in compressed gas cylinders. Dynamic-Volumetric Method The basic principle is the blending of different volume or mass flows. The simplest mixing arrangement consists of one pressure regulator and one flow meter per type of gas. The low uncertainties of the content of components desired for the calibration gas mixtures cannot be achieved with this simple arrangement. If manufacturing tolerances of 2 to 3% rel. shall be obtained, the volume ratio control has to be taken over by the measured variable of the respective analytical device. Moreover, the filling into compressed gas cylinders requires a suitable compressor. Advantage of the method: Constant composition when filling larger numbers of cylinders. Disadvantage of the method: The number of components is limited in this method (mainly binary mixtures) and the investment costs are considerable. Manometric Method (Manometric-Static Partial Pressure Method) This principle is applied for the production of technical gas mixtures (e.g. shielding gases for welding). It is based on the measuring of the pressure changes after the addition of individual components resp. the balance gas. For example, in order to obtain a composition of 10% of A, 10% of B, balance C at 150 bar, the component A

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is filled into the compressed gas cylinder up to 15 bar, than component B at further 15 bar up to 30 bar and the balance gas C up to a final pressure of 150 bar. However, problems may arise in practice, since this method requires isothermal filling and neglects the different compressibility of the gases. Consequently, with this method, production tolerances of only r5 … 10% rel. are achievable. Due to the application of equations of state, deviations from the “ideal behaviour” as well as temperature influences can be taken into account. With the help of these corrections, the production tolerance can be increased to r2 … 5% rel. Advantage of this method: Economical large-scale production of calibration gas mixtures with constant composition is possible. Disadvantage of this method: The mixture tolerance is relatively low. The use of different equations of state leads to varying results. Gravimetric Method At this method, the components are filled into a compressed gas cylinder one after the other and, after each dosing the mass increase is determined by weighing. Thus, the direct relation of the weighed gases to the basic unit “kg” or “mol” is given, and corrections with the help of equations of state are obsolete. For the reasonable application of this method, some preconditions have to be fulfilled. Requirement on the weighing equipment: On the one hand, the balances have to dispose of a high capacity owing to the high weight of the cylinders, and on the other hand as high a resolution as possible owing to the small “gas weights”. Precision balance (Beam balance): With a resolution of 3 mg, for instance, these balances dispose of a capacity of 30 kg. They are also applied for the production of calibration gas mixtures according to the method of re-weighing with an inaccuracy of up to 0.01% rel. (Fig. 9.1). In the production of these extremely precise calibration gas mixtures [9.11], apart from the inaccuracies of the balances even other influencing factors, such as gas impurities, filling errors or lift forces have to be considered. The balance itself only appears at the third level in the uncertainty hierarchy [9.12]. Electronic balances: In routine production, electronic balances are predominantly used (see Fig. 9.2). At a capacity of 150 kg, for example, electronic balances show a weighing inaccuracy of up to r0.1 g. Meanwhile, there are also solutions for the re-weighing method with fully-automatic connection and disconnection of the filling pipe under the application of electronic balances [9.13]. Mass of the component to be dosed: The mass of the smallest component has to be significantly higher than the absolute uncertainty of the balance. With electronic balances of a capacity of 150 kg and a resolution of 0.1 g (s.a.), a production tolerance of r 1% rel. can only be achieved with the weighed gas sample being at least 10 g. In case this mass is not achieved owing to the desired content of the calibration gas mixture, premixtures that are also gravimetrically produced have to

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Fig. 9.1 Precision balance for the production of calibration gas mixtures.

Fig. 9.2 Filling station with electronic balances.

be used. These are gas mixtures, in which the required component occurs with a higher percentage of mole fraction. The balance gas of the premixture corresponds to that of the calibration gas to be produced. Advantage of this method: Very low preparation tolerance. Independent from pressure, temperature and compressibility. This method is supported by standards (e.g. ISO 6142) [9.14]. Disadvantage of this method: Increased manufacturing expenditure (time-, staffand plant-consuming). Meanwhile, these disadvantages have been reduced by automation to the extent that today, the gravimetric method is the preferred method for calibration gas mixture production.

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Homogenization After filling, the gas fractions have to be mixed completely. According to the Brownian molecular movement, the gas molecules would mix automatically after a certain time. To accelerate the mixing process, the gas cylinders, for instance, are being rotated around their longitudinal axis in a mechanical device (Fig. 9.3). A provable mixture separation of a homogeneous mixture occurs only when the condensation temperature of a component is fallen below.

Fig. 9.3 Homogenization of gas mixtures.

9.3.2.4

Analytical Quality Assurance

The importance of the analytical control depends on the preparation tolerance of the mixture method applied. In case of preparation methods with higher inaccuracies it is common practice to determine the mole fraction by means of analysis. Even if the mixture is gravimetrically produced, analytical control cannot be renounced. On the one hand, it can never be excluded, for instance, that systematic weighing errors or individual errors of the operator occur. On the other hand, above all in case of corrosive components, adsorption effects and reactions with the inner surface of the gas cylinder cannot be excluded. Such effects can only be detected analytically. The common analytical methods shall only be listed concisely: x x x x x x x x x x

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Wet-chemical absolute methods Electrochemical methods Optical methods (FTIR, IR, UV-VIS) Special oxygen and moisture measuring systems Gas chromatography (GC) with a multitude of detector systems Chemiluminescence method (CLD) Mass spectrometry (MS) Atomic absorption spectrometry (AAS) Ion chromatography (IC) Inductively coupled plasma spectrophotometry (ICP)

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9 Specialty Gases Table 9.5 Examples for the application of gas mixtures. Application

Mixture Components

Balance gas

Notes

Emission control at furnaces

O2 CO SO2 NO

N2 N2 N2 N2

according to the legal regulations of BImSchG and TA-Air

Exhaust-emission check (AU)

CO, CO2, C3H8

N2

i.a. calibration gas mixtures with official test certificate

Car industry x Exhaust control x Optimization of engines x Development of catalytic converters

O2 CO NO C3H8

N2 N2 N2 Synth. air

Indoor air monitoring x Explosive H2 atmospheres CH4 C3H8 other flammable gases x TLV CO (personal security, PH3 workplace SO2 monitoring) NH3 other toxic gases

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Synth. air Synth. air Synth. air Synth. air Synth. air N2 N2 N2 N2

Immission measuring

Formaldehyde NO NO2 SO2 Benzene, toluene, xylene (BTX)

N2 N2 Synth. air Synth. air Synth. air/N2

Laser

CO2, N2

He

H2, CO, CO2, N2

He

F2 HCl

He He

Filling gas for light bulbs

N2

Ar

Leak detection

He

N2

Process monitoring and control

Hydrocarbons C1–C6

N2, CH4 and other hydrocarbons

Calibration gas mixtures for ex-alarm devices Calibration gas mixtures for TLV alarm devices

Operating gases for CO2-lasers Operating gases for marker laser Operating gases for excimer-laser

Calibration gas mixtures for process chromatographs

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269

Only wet-chemical analysis methods are absolute methods which refer to basic values, and thus do not require calibration. All other measuring methods are working comparatively (ISO 6143) [9.15]. Here, apart from internal standards gravimetrically produced on precision balances (see Section 9.2.2.3, precision balances), even worldwide recognized standards are used (Federal Institute for Materials Research and Testing, Bundesanstalt für Materialforschung und -prüfung, BAM; National Institute of Standards and Technology, NIST; Nederlands Meetinstituut, NMi). 9.3.3 Application Examples

Gas mixtures/calibration gas mixtures are used in a number of applications in environmental protection, process optimization, protection of people and plants and in research. Table 9.5 shows an excerpt of the most important applications.

9.4 Electronic Gases 9.4.1 Definition/Special Demands

In accordance with their name, electronic gases are used for the manufacturing of semiconductor components, but also in related fields of high-technology (e.g. manufacturing of glass fibres, solar cells, microscopical components). Due to the high complexity of the manufacturing processes with several hundred process steps resulting in structures in the range of less than 0.1 µm, high demands regarding the content of gaseous impurities (particularly O2 and H2O) and particles are placed on the gases applied. Moreover, some of the gases have chemical properties (corrosive, highly toxic, self-igniting) that require the use of special equipment on the compressed gas containers (i.e. remotely controlled, pneumatically operated cylinder valves, flow restrictors, metal-to-metal seal between cylinder valve and process lines). Containers for electronic gases are subject to special cleaning procedures to remove particles, organic impurities, deposits and corrosion products from their inner surface. Depending on the chemical properties of the respective product and the specific demands in the respective field of application, apart from the usual steel containers also inside polished containers of steel, stainless steel or aluminium are used. Filling stations for electronic gases are equipped with particle filters, products liquefied under pressure are filled by application of a distillation step. In addition, adsorptive cleaning methods are often applied (see also Section 9.2.4). Regarding the gas analysis methods used in quality control, apart from the already described methods for specialty gases, the field of metal-trace analysis has to be emphasized.

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9 Specialty Gases Table 9.6 Examples for applications of electronic gases. Application

Gas/Gas mixture

Notes

Manufacturing of semiconductors Deposition of silicon Deposition of monocrystalline layers (Epitaxy) x Si-containing gases x Doping gases x Etching gases Dry etching x of Si, SiO2 x of Al, Al2O3 x of compound semiconductors

SiH4 SiH2Cl2

SiH4, SiH2Cl2, ‚Si2H6 AsH3, PH3, B2H6 HCl CF4, SF6, CHF3, Cl2, C2F6, C3F8, C4F8, NF3, HBr, CH2F2, CH3F CClF3, BCl3, Cl2 HCl, Cl2

Thermal oxidation

HCl, Cl2

Deposition from the gas phase (CVD, Chemical Vapour Deposition)

SiH4, NH3, N2O, SiF4, NO

Doping x Diffusion

For the removal of impurities

x Ion implantation

PH3 Donor o n-conductive AsH3 Donor o n-conductive B2H6 Acceptor o p-conductive BCl3, BF3, AsH3 in H2, PH3 in H2

Metallization (CVD)

WF6

Cleaning of reaction chambers C2F6, C3F8, C4F8, C4F8O, NF3, SF6 Manufacturing of solar cells Deposition of silicon

SiH4

Doping

PH3 in H2, (CH3)3B

Chamber cleaning

C2F6, NF3, SF6

Protective layer

SiH4, NH3

Si3N4-layer

Glass fiber manufacturing Etching of surface and removal of moisture

Cl2

Other applications

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Coating of flat glass

SiH4

Reflective layer

Tempering of surfaces

SiH4, NH3

Si3N4-layer

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9.5 Disposal

271

9.4.2 Application Examples

In Table 9.6, selected applications are listed.

9.5 Disposal

As defined, specialty gases can be combustible, self-igniting, toxic or corrosive. The handling of such gases requires special safety measures. This includes the disposal of residues that has to be carried out without polluting the environment. The selection of the disposal method depends on the gas properties, the quantities and the local and plant-specific conditions. In Table 9.7, examples of the most important methods for the disposal of certain gases are given. Further details, mainly according to safety-related aspects, can be found in [9.16].

Table 9.7 Examples for gas disposal methods. Method

Suitable for

Notes

Recovery

All gases

Preferred method for the benefit of environmental protection. Appropriate for high-quality gases or gases the disposal of which is costly due to properties or quantities.

Emission into the atmosphere

Inert gases that do not pollute the environment, like N2, O2, noble gases

Neutralization with acids

Alkaline gases like NH3

Neutralization with lyes

Acid gases like chlorosilanes, HCl, HBr, SO2, NO2

Fixed-bed adsorption

AsH3, COS, amines

Combustion

Hydrocarbons, CO

Combustion combined with adsorption

Halogenated hydrocarbons, PH3

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Combustion product must not be ecologically harmful

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9 Specialty Gases

9.6 Transfer of Gases

Unsuitable fittings and pipes can reduce the quality of the pure gases and calibration gas mixtures considerably. The selection of the adequate gas supply system depends on the chemical, physical and physiological properties of the gases applied and the application-specific demands. 9.6.1 Selection of the Materials

Special attention has to be paid to the selection of the materials when reactive gases are applied. For example, considerable corrosion problems are to be expected when halogens, sulphuric compounds and nitrogen oxides are used in presence of moisture. Some gases, such as H2S, SO2, C2H2 and C2H4 are prone to thermal decomposition on hot metal surfaces. CO can form carbonyl compounds with nickel, iron and chrome. Hydrogen reduces metal oxides under the formation of water vapour. 9.6.2 Physical Interaction Forces Adsorption and Desorption on Surfaces

Apart from the chemical influence of the fluid-contacting inner surfaces of fittings and piping, the microscopic surface (roughness) has to be paid attention to. It decisively influences the physical interaction forces between gas and surface, i.e. the adsorption and desorption of gas molecules. In case of the metal materials copper and stainless steel, mainly applied in ultra-pure gas supply systems, the surface reactive in the adsorption/desorption of gases and vapours can be reduced significantly by means of polishing (e.g. electro chemical polishing). Relatively rough surfaces are found in plastics used for seals, membranes, valves and piping. Here, such gas/surface reactions occur frequently. Each surface, thus also the fluid-contacting inner surface of a fitting or pipe relevant here, is covered with a layer of adsorbed gases and vapours in equilibrium with their environment. The equilibrium state and therefore the transport of the adsorbed gases from the wall into the gas and vice versa is a function of the temperature and the concentration or partial pressure drop between wall and gas flow. Moreover, the surface structure has considerable influence. The microscopic surface active for adsorption can have many times the surface of the geometrical surface. If, for example, a fitting was exposed to ambient air during its storage, oxygen and moisture have accumulated on the surface which diffuse into the ultra-pure gas during later operation.

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273

Adsorption and Desorption Out of Materials

Adsorption and desorption on and off materials are negligible in the case of metals, in the case of plastic materials, however, as they are used for seals, membranes, valve seats, they develop values that have to be noted. Plastics are able to adsorb considerable amounts of gases and vapours and to desorb them again. Even the plastic material itself can desorb volatile components such as softening agents. Therefore, care has to be taken that independent from the application the amount of plastic components in the ultra-pure gas systems is as low as possible. Diffusion Through Materials

The diffusion through materials is also relevant for plastic material. Air components, for example, diffuse through plastic tubes. Therefore, they should generally not be used in ultra-pure gas system. At first sight, the assertion that air is able to diffuse into fittings and pipes even against higher pressure seems to be paradox. However, an exchange of gas molecules in both directions actually takes place. Decisive is the partial pressure difference of the involved gases inside and outside a pipe, for example. This phenomenon called permeation occurs practically only in plastic materials. In the case of metals, permeation (within the scope of gas engineering) is absolutely negligible. 9.6.3 Tightness of the Gas Supply System

Utmost attention has to be paid to the tightness of the supply system. Potential leakages in the pipe system are soldered or welded joints and screwings, e.g. flanges, screwings and valve seats. While soldered and welded joints, provided they are manufactured according to the rules, can be regarded as tight, in the case of detachable connections this is only possible to a certain extent. Metal seals in general show lower leakage rates than soft seals, and should therefore be used exclusively. The number of screwings should be minimized and pipe connections should generally be welded. 9.6.4 Purging of the Gas Supply System

In general, the contamination of pure gases or calibration gas mixtures owing to desorption processes can be avoided by means of intensive purging of the gas supply system before operation. Evacuating is only advisable if the fittings used are vacuum-tight and the ports on the valves are not too small. When purging fittings and piping some factors regarding the maintenance of the gas quality have to be taken into account. For example, it is wrong to completely open the cylinder valve at once, if a pressure regulator is connected. In this case, pressure compensation with the cylinder pressure occurs in the high-pressure section of the pressure regulator. Owing to back diffusion, the air in the pressure regulator would mix with the process gas of the cylinder and contaminate it. Even continuous

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9 Specialty Gases Table 9.8 Design of the components in specialty gas supply systems.

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Gas purity

Class 5.0

Class 6.0

Class 6.0 and higher, and semiconductor process gases

Pressure regulator

Material brass or stainless steel, specially cleaned, He-leak rate d 10–8 kPa L s–1, elastomere or a metal diaphragm

Material predominantly stainless steel, specially cleaned, He-leak rate d 10–10 kPa L s–1, stainless steel diaphragm

Material stainless steel, electropolished, high surface quality, He-leak rate d 10–10 kPa L s–1, stainless steel diaphragm, minimized dead volume, free of non-ferrous metals, cleanroom assembled, minimum particulate emission

Valves

Diaphragm-type seal, rarely packing seal or O-ring seal

Diaphragm-type seal, Bellows-type seal

Diaphragm-type seal, bellows-type seal electropolished, minimized dead volume

Piping

Material copper or stainless steel, specially cleaned

Material stainless steel specially cleaned or electropolished

Material stainless steel, electropolished

Pipe connections

Flux-free brazed, orbital welded

Orbital welded

Orbital welded

Detachable connections

Metal-to-metal sealed tube fittings

Metal-to-metal sealed tube fittings, metal-to-metal sealed VCR unions

Metal-to-metal sealed VCR unions

Application examples

Gas supply for the general laboratory needs, for gas analyzers, for production plants using high-quality working gases e.g. CO2-Lasers, for the manufacturing of lamps, production of special ceramics and metals

Gas supply for laboratory needs involving high purity gases, for gas analyzers using calibration gas mixtures in the ppm-range and/or corrosive components, for production plants using highest purity gases and gas mixtures, e.g. excimer-lasers, for the manufacturing of optical fibers, discrete components and less highly integrated circuits

Gas supply for R&D applications involving ultra-high purities, e.g. in microelectronics, for production plants using ultra-high purity gases and gas mixtures as well as corrosive and toxic process gases, e.g. for VLSI circuits, sensors and solar cells

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References

275

Fig. 9.4 Pressure swing purging.

purging is not very effective because of the numerous dead volumes. Therefore, the pressure swing method is recommended (Fig. 9.4). In Table 9.8, the most important components for specialty gas supply systems depending on gas purity and application are shown.

References [9.1] European Agreement on the International Transport of hazardous goods on roads (ADR 2003) Deutscher Bundesverlag, Bonn, 2003. [9.2] per EN 13 096, Conditions for filling gases into cylinders, draft, 1999. [9.3] TRG 100, Technical Rules for Compressed Gases, Register of Compressed Gases, Carl Heymanns Verlag, Cologne, 1998. [9.4] Dot notation, in the internet at www.industriegaseverband.de. [9.5] H. Schön: Handbuch der reinsten Gase, Springer-Verlag, Berlin, Heidelberg, New York, 2005. [9.6] per EN 13 099, Conditions for filling gas mixtures in cylinders, draft 1999. [9.7] TRG 102, Technical Rules for Compressed Gases, Gas Mixtures, Carl Heymanns Verlag, Cologne, 1985. [9.8] VDI-Guidelines 3490 Sheet 1, Measuring of gases – calibration gas mixtures – terms and definition VDI-Verlag, Düsseldorf, 1980. [9.9] K. Wilde, K. Studtrucker: Linde Reports 69, 1993. [9.10] H. Schön: Before the Gas Cylinder is Filled. Linde Reports 1998, 60, 28. [9.11] D. Heller: Production of precision gas mixtures on a high-resolution weigh beambalance. Linde Reports 77, 1998, 21. [9.12] H. Gaier, H. Heller: Computer simulation of the error calculation for precision gas mixtures. Linde Reports 77, 1998, 21. [9.13] H. Schön: Methods and devices for the gravimetric preparation of calibration gas mixtures by means of reweighing, EP Pat. W 097/42447 (1996). [9.14] ISO 6142, Gas analysis – Preparation of calibration gas mixtures – gravimetric method, 2001. [9.15] ISO 6143, Gas analysis, Comparison methods for determining and checking the composition of calibration gas mixtures, 2001. [9.16] EIGA 30/03, Disposal of Gases, European Industrial Gases Association, Brussels, 2003.

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10 Gases in Medicine 10.1 Introduction

For decades medical gases1) have been regarded by the healthcare community as low value commodities, delivered to the hospital and reordered when necessary. However, things are evolving rapidly. The most significant change is the classification from former medical gases to pharmaceutical quality products and medical devices. This process is largely driven in Europe by the regulatory authorities. This means that for medical gases the same requirements are applicable as for the traditional medical pharmaceutical industry – all for the safety of the patient. The regulatory authorities are conducting an evaluation and assessment based on submitted comprehensive quality, non-clinical and clinical documentation aiming to demonstrate the benefit and safety for the patient. The authority evaluation includes the entire finished product, i.e. active substance, excipients and container closure system (cylinder and valves). In US most medical gases are sold as US Pharmacopoeia products. These products have no indications connected with them. Manufacturing of these gases needs to be approved by the FDA (Food and Drug Administration/US regulatory agency for medicinal products). Medical gases and the services around them are needed in various clinical situations and can be found in ambulances, in intensive care units and may also be delivered right to the patient’s home. Medical Gases can be produced in bulk and delivered in special customized packages (compressed gas cylinders of different sizes, compressed gas bundles, as refrigerated liquid) in mobile cryo-containers or in stationary cryo-tanks including evaporator. In addition, a large number of gas mixtures are individually produced by medical institutes according to specific orders and, due to lacking authorizations, delivered as specific pharmaceutical. If a medical gas is classified as a medical device in accordance with the European classification criteria for medical devices, fulfilment of so-called “Essential 1) The term “medical gas” is generally used for gases in medicine, although they are designated “medicinal gas” if being approved as pharmaceutical. Industrial Gases Processing. Edited by Heinz-Wolfgang Häring Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31685-4

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10 Gases in Medicine

requirements” has to be proved with the help of a conformity assessment procedure according to the Medical Device Directive/MDD (MPG in Germany), and the product has to be provided with the mandatory CE-marking. The demands on the quality of medical gases are laid down bindingly in pharmacopoeia (e.g. European Pharmacopoeia/Ph. Eur. – US Pharmacopoeia/ USP) in the form of monographs. A pharmacopoeia monograph contains, inter alia, requirements for the quality of a medical gas as well as for the measuring methods to be applied in testing, e.g. its identity, content, purity. (A monograph can be compared with a technical standard). Production sites for the manufacturing of pharmaceutical preparations are subject to the control of regulatory authorities. Therefore, all locations where medical gases are produced or transferred need a manufacturing license for the grant of which the fulfilment of the GMP-guidelines (Good Manufacturing Practise) is a prerequisite. A complete documentation of production, quality control and distribution channels (possibility to traceability) is a basic requirement for the handling of medical products. Medical products have to be released by a special qualified and authorized person (“QP”).

10.2 Medicinal Oxygen

Recovery/Processing: During the process of air separation, atmospheric air is purified, liquefied and separated into its components, cf. Section 2.2. The recovered oxygen (O2) is either filled into large storage tanks in liquid form (LOX med) or after its evaporation in gaseous form (GOX med) into compressed gas cylinders. Medical oxygen fulfils the requirements defined in the “Oxygen” monograph of the European Pharmacopoeia. This is subject to continuous control by the mandatory analytical processes. Medical oxygen is one of the pharmaceutical preparations used most world wide. Traditionally, oxygen is administered in case of prevailing or imminent oxygen deficiency. 10.2.1 Home-therapy

Currently oxygen is the only medical gas finding wide application at the patient’s home. Patients benefiting from short-term oxygen therapy include cluster headache patients, where inhalation of 100% oxygen has proven to bring relief of symptoms in up to 82% of patients [10.1]. Long-term oxygen-therapy is indicated for all patients with a stable chronic lung disease (such as chronic obstructive pulmonary disease, cystic fibrosis etc.), who have an arterial PO2 (oxygen partial pressure) consistently less than or equal to 55 mm Hg when breathing air, at rest and awake [10.2]. These patients need a

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continuous oxygen supply for their treatment for more than 15 hours a day. The dose is indicated in L/min (e.g. 2 L/min) in the medical prescription. There are three modes to supply oxygen to the patient at home, offering different levels of mobility. Gaseous oxygen: Compressed gas cylinders with oxygen are filled at a typical pressure of 200 bar (= 20 MPa). Therefore, 1 L (litre) of this compressed oxygen corresponds to 200 L oxygen at ambient pressure. Typical cylinder sizes for home supply contain 2 or 10 L. The administration of oxygen is done via pressure regulators that are connected to or permanently integrated in the cylinder valves (IVR, combivalves). Oxygen concentrators are electric devices and thus require a power connection, limiting the patient’s mobility. The ambient air sucked-in by a compressor is filtered and pressed through sieve beds that adsorb nitrogen from the air. The oxygen-enriched air is collected in a product tank and delivered to the patient via a pressure regulator at the outlet of the device. Depending on the adjusted flow rate, oxygen concentrations of up to 98% are achieved. Recently developed concentrating devices operating with fuel cell technology deliver 100% oxygen. Liquid oxygen: Oxygen is a refrigerated liquid (–183 °C). One litre of liquid oxygen corresponds to 853 L of gaseous oxygen at ambient conditions of 15 °C and 1 bar. The liquid oxygen is directly supplied to the home patient and stored there in a special cryogenic vessel. The administration of the oxygen to the patient prior to application (e.g. by means of nasal cannula) occurs either directly from the storage container in gaseous state after evaporation via a downstream heat exchanger or from a smaller portable cryogenic tank (with integrated evaporator), into which the patient himself fills a defined quantity of liquid oxygen. Feeding of the evaporated oxygen occurs by means of a plastic tube (Fig. 10.1).

Fig. 10.1 Mobile O2-therapy with portable vessels.

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10.2.2 Hospitals and Other Fields of Application

Hospitals, rehab-clinics, emergency services and registered doctors with own practices receive medical oxygen either in large quantities in liquid state by tank trucks or in compressed gas cylinders (single cylinders or cylinder bundles), if smaller quantities are required. Oxygen is either centrally provided via pipe systems and withdrawn in the hospital wards via quick couplings (wall outlets) or delivered directly to the patient by means of mobile compressed gas cylinders. The cylinder pressure (e.g. 200 bar) has to be reduced to a pressure suitable for the patients’ needs by means of pressure regulators or an innovative cylinder valve with permanently integrated pressure regulator and flow meter (medical device with CE-marking). Current applications are the HBO-Chambers (Hyperbaric Oxygenation), in which oxygen is administered under increased pressure.

10.3 Gases for Anaesthesia 10.3.1 Medical Nitrous Oxide (Laughing Gas)

Nitrous oxide (N2O) existing in the atmosphere emanates from different sources. The main pollution of the atmosphere with nitrous oxide arises from fertilizers in agriculture. In relation to the total emission of nitrous oxide in the atmosphere, the share of medical nitrous oxide is less than 0.05%. Here it has to be considered that nitrous oxide enhances the greenhouse effect, however, without any influence on the ozone layer. Recovery/Processing

Medical nitrous oxide is obtained by heating of ammonium nitrate: NH4NO3 o N2O + 2 H2O (240 °C) It is mainly supplied liquefied under pressure in compressed gas cylinders. At high demands, nitrous oxide is also supplied by special tank trucks and filled into storage tanks installed on-site from where the centralized supply in the hospital is possible. Application

For more than 150 years, nitrous oxide has been clinically used for anaesthesia and pain treatment all over the world. Owing to the multitude of patients to be treated, nitrous oxide can be regarded as one of the pharmaceutical products broadest examined with regard to its spectrum of side effects and resulting

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281

Fig. 10.2 Application of medical gases in surgery.

contra-indications. Even today, nitrous oxide is an essential part of general anaesthesia. The good controllability and the possibility of continuous control of the concentration render nitrous oxide a safe and easy-to-handle anaesthetic (Fig. 10.2). 10.3.2 Xenon

Xenon (Xe) represents an innovation in the field of anaesthesia. Its narcotic effect was discovered only in 1951. Clinical studies proved the principle applicability even as mono-anaesthetic. The marketing of xenon as finished pharmaceutical requires authorization according to the current drug law regulations. The high price for xenon can be reduced by its recycling from exhaled gas. In order to enable the substitution of all anaesthetics by xenon, considerable increases in capacity of air separation plants would be required.

10.4 Medical Carbonic Acid (Carbon Dioxide)

For recovery/processing see Section 6.2. Application

Recently, medical carbonic acid (CO2) has gained more and more importance in a number of medical fields of application. Especially in the promising field of minimal invasive surgery, the stabilization of visceral cavities (aeroperitoneum, pneumothorax) in laparoscopic and thoracoscopical interventions can not be imagined without the application of this gas – which in case of being correctly used reabsorbs easily.

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However, even the external application of medical carbonic acid is of growing importance, as for example in sanatoria, where carbonic acid baths have become firmly established for the successful treatment of certain dysfunctions of the cardiac and circulatory system as well as of the vascular system.

10.5 Medical Air

Recovery/Processing This product can either be obtained according to the European Pharmacopoeia monograph “Air medicinal (Aer medicinalis)” by the filling of purified and compressed ambient air, or even synthetically by mixing of oxygen and nitrogen. Application

Medical air is indispensible for a number of therapeutical measures, in particular in neonatology, anaesthesia and intensive medicine. It supports respiration in case of: x x x x

anaesthesia mechanical ventilation artificial respiration in case of transports supply of respiratory air in the incubator

Medical air must not be confused with the product “Compressed air for breathing apparatuses” which is intended for non-medical purposes, and the demands on production and quality of which are not defined in the pharmaceutical laws and regulations (GMP, Ph. Eur.), but in a standard norm (EN 12 021). Therefore, they differ significantly from the demands on the quality of a pharmaceutical preparation. For these reasons, compressed air for breathing apparatuses must not be used for medical purposes. Since August 1, 2001, the quality requirements on air for artificial respiration purposes according Ph. Eur. have also become valid for air used for surgical tools, see ISO 739-1 (EN 737-3).

References [10.1] L. Kudrow: Response of cluster headache attacks to oxygen inhalation. Headache 1981, 21: 1–4. [10.2] C. McDonald, A. Crockett, I. Young: Adult domiciliary oxygen therapy. Position statement of the Thoracic Society of Australia and New Zealand Med J Aust 2005, 182 (12): 621–626 [Position Statement].

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11 Logistics of Industrial Gas Supply 11.1 Introduction

In principle, logistics of customers’ supply with industrial gases is determined by the required quantity and the transport distance. In order to adapt to the specific requirements of the consumers, nowadays suppliers avail of several possibilities that are schematically depicted in Fig. 11.1. The customer receives the industrial gases either in gaseous state in compressed gas cylinders or in liquid state, mostly cryogenic, in insulated special containers. Bulk consumers, such as refineries or steel mills with constantly high demand are supplied via pipelines. Table 11.1 gives an overview of the currently relevant possibilities of industrial gas supply.

Table 11.1 Possibilities of supply with some industrial gases as examples. Form of supply

N2

O2

Ar

H2

CO CO2 He

Ne

Kr

Xe

Gaseous

u

u

u

u

u

u1)

u

u

u

u

u

1)

u

u

u

u

1)

u

Compressed gas cans Compressed gas cylinders Cylinder bundles

u u

u u

u u

Compressed gas trailers

Liquid

1) 2)

2)

2)

u u

u

u

u

2)

Pipeline system

u

u

Cryogenic jugs

u

u

u

u

Tank truck

u

u

u

u

Railroad tank car

u

u

u

u

u u

u 2)

u

u u

u

u

Liquefied under pressure. Locally limited networks.


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11 Logistics of Industrial Gas Supply

Fig. 11.1 Logistics of industrial gases supply.

11.2 Storage and Transport of Compressed Gases 11.2.1 Fundamentals

Owing to its low specific density, gases and gas mixtures are usually stored in compressed gas cylinders. As a rule, compressed gas cylinders consist of steel or aluminium materials, even fibrous composite materials are used. The European Standard EN 1089 part 3 provides for a corresponding colour label on the collar of the cylinder, according to the content: Compressed gas cylinders for argon are labelled dark green, cylinders for helium brown, those for oxygen white and red for hydrogen.

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285

The TRG (Technical Rules for Compressed Gases) subdivide gases and gas mixtures according to their chemical and physical behaviour and determine the compressed gas cylinders to be used, including their equipment, inspection periods, filling factors and filling pressures. Cylinders with inflammable content are generally equipped with a left-hand thread at the outlet valve. In general, the transport of compressed gases is carried out by lorries, and both national and international regulations have to be followed. To mention on national level, e.g. the x German directive for road transport of dangerous materials (“Gefahrgutverordnung Straße”/GGVS) x US “Hazardous Materials Regulations” (Title 49 CFR, Parts 100–199) x European agrement concerning the international carriage of dangerous goods by road (“Accord européen relativ au transport international des marchandises dangereuses par route”/ADR) The United Nation’s “Recommodations on the Transport of Dangerous Goods – Model Regulations” provides a uniform regulatory framework which can be applied in all countries for national or international transport by any mode of transport. 11.2.2 Kinds of Transport and Storage for Compressed Gases

Storage and transport of compressed gases occurs in different kinds of containers, depending on the quantities: Small quantities are delivered to the user in handy compressed gas cans of aluminium or in small steel cylinders with volumes of 1 resp. 0.38 litres and a maximum filling pressure of 12 resp. 200 bar. These non-returnable compressed gas containers mainly used for laboratory purposes are the smallest possible transport unit [11.1]. As a rule, larger quantities are transported in compressed gas cylinders of steel or aluminium, usually with a volume of 10, 20, 40 and 50 L and a maximum filling pressure of 200 bar. Partly, even compressed gas cylinders with a maximum filling pressure of 300 bar are used for the transport of helium, nitrogen, oxygen and welding protective gases [11.1]. Pallets, in which up to 12 cylinders can be transported, depending on the type, are used for the safe and efficient transport of compressed gas cylinders. In case the demand exceeds the efficient and reasonable supply by single cylinders, compressed gas cylinder bundles are used. Those are 12 single compressed gas cylinders with a content of 40 or 50 L each, connected with each other via pipe and forming one unit. The cylinders are arranged vertically or horizontally; each bundle frame disposes of a filling and discharge valve. Compressed gas cylinder bundles can be connected to larger units at the consumer. The four mentioned types of compressed gas cylinders are customary for the storage and transport of nearly all gas kinds. In the case of helium and hydrogen, a special form of vehicles with mounted compressed

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gas cylinders, so-called compressed gas trailers, are applied for the supply to bulk consumers. As a rule, the gas is transferred into a large stationary pressure vessel, in rare cases it is even withdrawn directly from the trailer. However, the transport and storage means described above, are inadequate for industrial bulk consumers of the chemical and steel industry. They are supplied by On-site-plants (gas production plants on site, also called “over-the-fence-supply”) respectively by a pipeline system. Such pipeline system for nitrogen, oxygen and hydrogen, for instance, is operated by Linde in the German chemistry triangle Buna/Leuna/Bitterfeld. This form of transport of natural gas has been popular in Europe for a long time and enables the transport of large quantities of compressed air over large distances (cf. Chapter 7). 11.2.3 Efficiency of Compressed Air Gas Transport

The quantity of gas to be transported is determined by container pressure and container size. However, in the case of withdrawal of overflowing, the pressure of the transport container in dependence on the consumer pressure can only be insufficiently used. The efficiency of the transport of compressed gases is still characterized by the fact that the respective pressure containers are several times heavier than the transported quantity of gas itself. For this reason, constructions of fiber reinforced composite materials in combination with steel or aluminium are increasingly applied. By using such combinations of materials, the transport capacity of a compressed gas trailer for hydrogen, for instance, can almost be doubled at a constant gross vehicle weight rating of 40 t. A highly efficient form of transport is the pipeline; however, the high investment costs should be commensurate with the transported gas quantity. Therefore, pipeline systems for industrial gases are only regionally available and to a limited extent to bulk consumers such as to refineries (H2) or steel mills (O2), for instance.

11.3 Storage and Transport of Liquefied Compressed Gases 11.3.1 Fundamentals

Liquefied cryogenic gases are transported and stored in thermally insulated compressed gas vessels. The insulation of the vessels is achieved by their design with cylinder jacket insulation. Here, the inner vessel is concentrically arranged in the outer vessel and the insulation is located in the clearance. This helps to minimize heat penetration into the cryogenic gas of inner vessel which results from thermal conductivity, thermal radiation and convection:

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x For gases with boiling temperatures above 70 K (e.g. N2, O2, Ar) usually a perlitepowder-vacuum insulation is used x For gases with boiling points below 70 K (e.g. H2, He, Ne) superinsulation has become established. This consists of a number of double layers of aluminium foil and insulation foil in the high vacuum The quality of the insulation depends among other factors significantly on the quality of the vacuum. Despite good insulation, stored liquefied compressed gases are subject to heat penetration which makes the cryogenic content evaporate slowly. The insulation quality and thus the thermal flow are described by the evaporation rate. It is indicated in percent per day, based on the maximal filling capacity of the vessel. In closed storage vessels, i.e. without extraction, this evaporation rate causes a slow pressure rise. Thus, in case of very long periods of standstill, the admissible operating pressure can be achieved and trigger off a safety valve. For safety reasons, only special materials, such as austenitic, low temperature steels or aluminium alloys are used for cryogenic liquefied gases, that still dispose of sufficient fracture toughness at low design temperatures of down to 4 K (liquid He). 11.3.2 Forms of Transport and Storage of Liquefied Gases

The liquefaction of gases takes place in a process step following the production, partly involving high expenditure of energy. Nevertheless, especially the air gases nitrogen, oxygen and argon are mostly stored and transported in liquid state as a consequence of their considerably reduced volume compared to the gaseous state. Cylindrical vertical tanks for liquefied compressed gases are available in a volume range between 1500 and 80 000 L. These tanks are suitable for both the direct extraction of liquid gas and for the extraction of compressed gas with a subsequently installed evaporator. For the storage of huge quantities sometimes even spherical tanks are used, for example as buffer tanks in production plants. For the transport of liquefied cryogenic gases such as N2, O2, H2, Ar and He, cryogenic jugs with volumes of 38 to 1000 L [11.2] are used. For lager quantities special double-wall tank trucks are used. Optimized tank trucks at a gross vehicle weight rating of 40 t (depending on national laws and regulations) and a volume of 13 000 to 28 000 L are able to transport a payload of up to 22 000 kg. Road tank trucks are mostly designed perlite-insulated; in case higher demands regarding the evaporation rate are to be fulfilled, even superinsulated tank trucks are deployed, see Fig. 11.2. For the transport over very large distances, e.g. intercontinental shipping, in addition to the superinsulation of the vessel, a so-called nitrogen shield that cools the insulation by means of liquid nitrogen reduces the heat penetration into the cryogenic gas of tank trucks for He and H2. As a rule, trucks for road

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Fig. 11.2 Special tank truck for the transport of liquid hydrogen filling vertical tanks.

transport are designed for a maximum operational overpressure of 1.7 bar [11.1]. Transport vehicles for rail transport are available for liquid O2, N2, Ar and CO2. For railroad tank cars, the gross vehicle weight rating amounts to 80 t and the payload depending on the gas to about 60 000 kg (CO2). 11.3.3 Efficiency of the Transport of Liquefied Gases

The decisive advantage of liquefied gases is their significantly higher density compared to the compressed gas. Owing to this fact, the transport capacity in road transport is considerably higher. At identical vehicle weight, a special tank truck for liquid hydrogen, for example, is able to transport more than six times the quantity of hydrogen a compressed gas trailer is able to transport. The justification for the liquid transport can be found in the favourable ratio of payload to gross weight of the tank truck compared to the transport in compressed gas cylinders. After liquefying, for instance, the gas volume of oxygen is reduced by the factor of 850 at 15 °C and 1 bar [11.2]. Thus, the higher costs for the liquefaction of gases are faced with lower transport costs. Depending on the transport distance, the energetic extra expenditure is compensated for.

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References

289

11.4 Special Forms of Supply

Carbon dioxide is stored in pressure-liquefied state in small containers like compressed gas cylinders and cylinder bundles. Large quantities of CO2 are transported in special tank trucks respectively railroad tank cars in cryogenic liquefied state, analogous to the gases described in Table 11.1. For the transport of solid CO2 (dry ice), coolers e.g. of polystyrene are used (cf. Chapter 6). Natural gas is usually transported via pipelines. In order to be able to exploit natural gas deposits even in more remote areas without pipelines being available for transportation, natural gas, liquefied to cryogenic LNG is transported by tank ships (cf. Chapter 7). This enables the efficient transport of large quantities of LNG over far distances even in intercontinental transport.

References [11.1] http://www.linde-spezialgase.de. [11.2] Ullmann’s, 6th edition, 23, p. 189 ff., Wiley-VCH, Weinheim, 2003.

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Subject Index a accompanying gas 220 acetylene 240 – application in autogeneous engineering 247 – critical point 244 – decomposition 243 – dew point 244 – hydrate 244 – ignitable mixtures with air 243 – liquefaction 243 – petrochemical 246 – recovery from calcium carbide 246 – storage 246 – supply system 247 acetylene generator 245 acetylene hydrate 244 acetylide 244 air, medical 282 air booster 21 air separation by cryogenic rectification 20 air separation unit – process analysis 64 – safety 59 – safety, ignition source 60 air separator – cold section 24 – cryogenic 23 – safety, ignition in reboilers 63 – two-column 113 – warm section 23 alkaline electrolyser, electrolysis of water 143 aluminium plate-fin type, air separation 49 ambient air, helium content 125 aMDEA 150 ammonia synthesis gas, processing, nitrogen wash process 156

ammonium nitrate for processing of nitrous oxide 280 Andrews 3 AOD 4 argon – applications 104 – history 12 – occurrence 13 – properties 13 argon bulge, air separation 29 Asia Industrial Gases Association (AIGA) 7 ATR 148, see also autothermal reforming autogeneous engineering 247 autogenous technology 241 autothermal reactor 149 autothermal reformer 148 autothermal reforming 148 – production of synthesis gas 144

b Barrer 16 Bartlett, N. 112 Base-Load-Plants, natural gas 231 Benfield-process 150 Berthelot 240 Birkeland 9 Black 185 Bosch 4, 9 Boudouard-equilibrium 188 box reformers 146 Bunsen, Robert 218

c C2H2 243 C2H4 249 C2plus 230 C3MR 232 C3MR-process C3plus 226

232


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292

Subject Index Cailletet 9, 141 calibration gas mixtures 261 – fittings and pipes 272 – production 264 – production, dynamic-volumetric method 264 – production, gravimetric method 265 – production, manometric-static method 264 – supply systems 273 can-type reformers 146 carbon dioxide – as greenhouse gas 186 – high purity 195 – liquefaction 193 – liquid, properties 187 – occurrence 185 – pre-purification 191 – properties 186 – recovery 189 – recovery from flue gas 198 – sources 190 carbonic acid 185, 188 – medical 281 carbonic acid anhydride 185 carbon molecular sieves, O2 production 18 carbon monoxide – applications 182 – occurrence 141 – properties 141 carbonyl complexes 141 Caro 9 Cavendish 3, 9, 12, 136 CGA 6 Charles, C. 136 Charles, J. 3 chemical scrubbings 150 China Industrial Gases Industry Association (CIGIA) 7 CIGIA 7 Claude, G. 2, 3 Clement 141 CMS 18, see also carbon molecular sieves CO 141 CO2 185 CO2 reforming, synthesis gas production 144 CO2 scrubbing 191, 192 CO2 source 189 cold box 21, 23 columns, air separation 54 Commission Permanente Internationale (CPI) 6

1345vch12.indd 292

compressed air for breathing apparatuses 282 compressed gas 287 – liquefied, storage 286 – liquefied, storage in cryogenic jugs 287 – liquefied, storage in vertical tanks 287 – pipeline 286 – pipeline systems 286 Compressed Gas Association 6 compressed gas cans 285 compressed gas cylinder 283, 284 – colour labelling 284 compressed gas cylinder bundles 285 compressed gas trailers 286 compressor, air separation 45 concentration units 262 convective reformer 149 crude argon column – air separation 26 – McCabe–Thiele diagram 29

d D 137 Davy, E. 1, 240 Desormes 141 deuterium 137 Dewar 3, 136 Dirty Shift 150 dot notation, pure gases dry ice 187 – production 200 DVS 241

256

e EIGA 6 electronic gases 269 ethane, recovery from natural gas 229 ethene 249, see also ethylene – application in autogeneous engineering 249 – recovery 249 ethylene, properties 249 ethyne 243, see also acetylene European Dry Ice Association (EDIA) 6 European Industrial Gases Association 6 exergy – definition 35 – loss 35 expansion turbines, air separation 48 external compression, oxygen recovery 33 Eyde 9

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Subject Index

f Fenske, formula by 30 Fick’s Law 16 Fischer 4, 136 Fischer–Tropsch synthesis 4, 136 flooding in the downcomer 57 Fontana, Felice 136 Frank 9 FT-synthesis 136 fuel gases 239

– – – – – – – – –

bridge bonds 140 frequency 137 heavy 137 normal 138 occurrence 136 ortho 137 para 137 production by electrolysis of water properties 137

293

143

i g GAN 2 gas-to-liquid 238 gas companies, shares in the world market 5 Gases and Welding Distributors Association (GAWDA) 7 gases in medicine 277 gasification reactor 147 gas mixtures 261 – production 263 Gas Subcooled Process 227 GAWDA 7 generator gas 135 glycol scrubbing, natural gas 225 GOX 2 GOX med 278 GSP 226, 227 GTL 2, 238

h Haber 4, 9 heat exchanger – air separation 49 – coil-wound 234, 235 He I 127 He II 127 helium – high purity, recovery 128 – liquefaction 130 – occurrence 125 – properties 127 – recovery 127 helium-method, age determination of minerals 125 helium content, ambient air 125 Henry’s Law 16 high-temperature shift 150 Hoppe 112 HTS 150 HyCO-unit 158 hydrogen – applications 164

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industrial gas companies 5 industrial gases, supply, logistics 283 Industriegasverband e.V. (IGV) 7 instrumentation gases for analytical measuring methods 258 internal compression, oxygen recovery 33 International Oxygen Manufacturers Association (IOMA) 7

j Japanese Industrial Gas Association (JIGA) 7 jet flooding 57 Joule, J. P. 3 Joule–Thomson effect 1, 3

k Kamerlingh-Onnes, H. 3, 125 Kr/Xe – fine purification 115 – fine purification, combustion of hydrocarbons at the catalyst 115 – fine scrubbing 117 – pre-enrichtment in the air separator krypton 111 – occurrence 111 – recovery 112

113

l Lasonne 141 laughing gas, see nitrous oxide laughing gas for anaesthesia 280 Lavoisier 11, 185 Linde, C. v. 1, 3, 9 Linde air liquefier 1 Linde air separation 2 Linz-Donawitz (LD) process 4 liquefied natural gas 220 liquefied petroleum gas 225 – recovery from natural gas 225 liquefiers 37 liquid turbines, air separation 48 LNG 220

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294

Subject Index – applications 238 low-pressure column, air separation low-temperature shift 150 LOX 59 LOX med 278 LPG 225 LTS 150

28

m MAG 4 McCabe–Thiele diagram – binary O2/Ar-mixture 29 – crude argon column 29 MDEA 192 MEA 150, 191 Medical Device Directive, medical gases 278 medical gases 277 medium-temperature shift 150 membrane, nitrogen recovery 16 membrane module, parameters 17 metal hydrides 140 methanation 151 methane hydrate 218 methane scrubbing, cryogenic 155 methyldiethanolamine 192 – as chemical solvent for synthesis gas 150 MFC process 234 Mikropor A 246 Mixed Fluid Cascade process 234 Moissan, H. 1, 3, 240 molecular sieves – for pressure swing adsorption 18 – zeolitic, O2 recovery 18, 20 mole fraction 262 monoethanolamine 191 – as chemical solvent for synthesis gas 150 mtpa 231 MTS 150

n N2 10 nasal cannula 279 natural gas 217 – basic feed for synthesis gas 136 – calorific value 220 – composition 223 – dew-point adjustment 224 – dry 220 – ethane separation 229 – glycol scrubbing 224 – liquefaction 231

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– – – –

nitrogen separation 236 occurrence 218 occurrence, detected 218 separation from liquefied petroleum gas 225 – treatment 224 – wet 220 natural gas bubble 218 natural gas deposits 219 natural gas development 217 natural gas reserves 219 neon 111 – occurrence 111 – properties 112 – recovery 118 – recovery, fine purification 119 – recovery, pre-enrichtment 118 nitrogen – applications 67 – chemical properties 10 – fixation 10 – history 9 – inversion temperature 10 – occurrence 9 – properties 10 nitrogenase 10 nitrogen generators 36 nitrogen recovery – by means of PSA 19 – membranes 16 nitrogen scrubbing 156 nitrous oxide, medical, processing 280 noble gas hydrates 112 normal-hydrogen 138

o OHR 228 OHR-process 228 OMA 7 ortho-hydrogen 137 oxygen – applications 83 – concentrators 279 – for medical application 278 – history 11 – inversion temperature 11 – occurrence 11 – properties 11 – refining 12

p para-hydrogen 137 partial oxidation, production of synthesis gas 144, 146

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Subject Index Patart 4, 136 peak-shaving plants, natural gas 231 permeabilities of membranes 16 photosynthesis 186 PO 146 PO-plant 160 – gasification of heavy oil 159 PO-reactor 147 – Texaco 148 polymeric membranes, gas separation 16 potash as a chemical solvent for synthesis gas 150 pre-reformer 158 pressure column, air separation 25 Pressure Swing Adsorber Unit – helium recovery 128 pressure swing adsorption 18 – nitrogen recovery 18 – oxygen recovery 18 – production of high-purity hydrogen from synthesis gas 151 Priestley 9, 11, 141 primary reformer 148 process analytics, synthesis gas plants 161 production of hydrogen, reformer plant 157 production of pure argon 29 promoted combustion-test 61 PSA 18, 151 Puls Discharge Detector 120 pure argon column, air separation 26 pure gases 256 – dot notation 256 – fittings and pipes 272 – supply systems 273

q Q-T-Diagram 33 quantum fluid 127 quantum solid 127

r radon, occurrence 111 Ramsay 111 Rayleigh, J. W. 12 rectisol process 151 rectisol scrubbing 151 Recycle Split Vapor 229 RSV 229, 230

s Scheele 9 secondary reformer 148 separation factor 40, 114

1345vch12.indd 295

295

SFR 227 Shell-method, partial oxidation 147 sieve tray columns, air separation 55 Sirius-Linde, development system for acetylene 246 specialty gases, disposal 271 Split Flow Reflux 227 steam reformer 145, 157, 159 steam reforming, see steam reformer superfluid 127 synthesis gas – applications 182 – definition 135 – generation by autothermal reforming 148 – generation by partial oxidation 146 – generation by steam reforming 145 – processing 150 – processing, condensation process 154 – processing, cryogenic processes 153 – processing, membrane process 152 – processing, methanation 151 – processing, methane scrubbing 154 – processing, removal of carbon dioxide 150 – production 143 – production from hydrocarbons 144 – removal of acid gases 150 synthol-process 136

t T 137 tandem reformer 149 Technical Rules for Compressed Gases 285 Texaco-method, partial oxidation 147 Texaco-process, partial oxidation 159 thermosiphon and downflow evaporator 53 Thilorier 185 Thomson, W. 3 TIG 4 TRG 285 tritium 137 Tropsch 4, 136 turbines, air separation 47 turbo compressor, radial, air separation 45 two-column nitrogen generator 36

v vacuum pressure swing adsorption 20 Verband der Chemischen Industrie e.V. (VCI) 7 VPSA 20

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296

1345vch12.indd 296

Subject Index

w

x

water–gas shift equilibrium 146, 148 water–gas shift reaction 145, 147 water–gas shift reactor 150 water gas equilibrium 188 weeping 57 welding engineering, history 240 Wilson, T. L. 1, 3, 240 Wöhler, F. 1, 240

xenon 111 – in anaesthesia 281 – occurrence 111 – properties 112 – recovery 112 XePtF6 112

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