Guidelines for Process Safety in Batch Reaction Systems

Guidelines for

Process Safety in Batch Reaction Systems

@

INTERSCIENCE AMERICAN INSTITUTE OF CHEMICAL ENGINEERS

CENTER FOR CHEMICAL PROCESS SAFETY

of the AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 3 Park Avenue, New York, New York 10016-5991

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Guidelines for

Process Safety in Batch Reaction Systems

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This book is one of a series of publications available from the Center for Chemical Process Safety. A complete list of titles appears at the end of this book.

Guidelines for

Process Safety in Batch Reaction Systems

@

INTERSCIENCE AMERICAN INSTITUTE OF CHEMICAL ENGINEERS

CENTER FOR CHEMICAL PROCESS SAFETY

of the AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 3 Park Avenue, New York, New York 10016-5991

Copyright 8 1999 American Institute of Chemical Engineers 3 Park Avenue New York, New York 10016-5991 All rights reserved. No part of this publication may be reproduced,

stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or othenvise without the prior permission of the copyright owner. ISBN 0-8169-0780-3

This book is available at a special discount when ordered in bulk quantities. For information, contact the Center for Chemical Process Safety at the address shown above.

It is sincerely hoped that the information presented in this document will lead to an even more impressive record for the entire industry; however, the American Institute of Chemical Engineers, its consultants, CCPS Subcommittee members, their employers, their employers’officers and directors, and Arthur D. Little, Inc., disclaim making or giving any warranties or representations, express or implied, including with respect to fitness, intended purpose, use or merchantability and/or correctness or accuracy of the content of the information presented in this document. As between (1) American Institute of Chemical Engineers, its consultants, CCPS Subcommittee members, their employers, their employers’officers and directors, and Arthur D. Little, Inc., and (2) the user of this document, the user accepts any legal liability or responsibilitywhatsoever for the consequence of its use or misuse.

CCPS and members of the Batch Reaction Subcommittee dedicate this book to the memory of Felix Freiheiter and A1 Noren

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Contents Preface

Acknowledgments Acronyms and Abbreviations

1 Process Safety in Batch Reaction Systems 1.1. Scope 1.2. Special Concerns of Batch Reaction Systems 1.3. Approach Used in Guidelines

1 2 3

2 Chemistry 2.1. Introduction 2.2. Case Study 2.3. Key Issues 2.4. Process Safety Practices Table 2: Chemistry Appendix 2A. Chemical Reactivity Hazards Screening A. 1. A.2. A.3. A.4.

Understand the Problem Conduct Theoretical Screening Conduct ExperimentalScreening Conduct ExperimentalAnalysis

7 8 9 9 11 21 21 21 23 25 vii

...

CONTENTS

Vlll

3

.IyIILIAIIRIy

Equipment Configuration and Layout 3.1. Introduction 3.2. Case Studies Pump Leak Incidents Tank Farm Fire

3.3. Key Issues 3.4. Process Safety Practices Table 3: Equipment Configuration and Layout

4

Equipment ~~

4.1. Introduction Vessels Including Reactorsand Storage Vessels Centrifuges Dryers Batch Distillation Columns and Evaporators Process Vents and Drains Charging and Transferring Equipment Drumming Equipment Milling Equipment Filters

4.2. Case Studies

Batch Pharmaceutical Reactor Accident Seveso Runaway Reaction Pharmaceutical Powder Dryer Fire and Explosion

4.3. Key Issues 4.4. Process Safety Practices Table 4.0: General Table 4.1: Reactors and Vessels Table 4.2: Centrifuges Table 4.3: Dryers Table 4.4: Batch Distillation and Evaporation Table 4.5: Process Vents and Drains

27 28 28 28 29 29 30

35 35 36 38 39 40 40 41 41 42 42 43

43 44 44

45 45 48 54 64 70 73 75

ix

Contents

Table 4.6: Transferring and Charging Equipment Table 4.7: Drumming Equipment Table 4.8: Milling Equipment Table 4.9: Filters Appendix 4A. Storage and Warehousing

76 90 96 100 105

5

InstrumentationlControl Systems 5.1. Introtluction 5.2. Case Study 5.3. Key Issues 5.4. Process Safety Practices Table 5: InstrumentatiodControl Systems

Operations and Procedures 6.1. Introduction 6.2. Case Studies Initiator OverchargingIncident Reactant StratificationIncident

6.3. Key Issues 6.4. Process Safety Practices Table 6: Operations and Procedures

References Glossary

109 112 113 114 115

125 129 129 130 13 1 131 132

143 159

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Preface

The Center for Chemical Process Safety (CCPS) was established in 1985 by the American Institute of Chemical Engineers (AIChE) for the express purpose of assisting the Chemical and Hydrocarbon Process Industries in avoiding or mitigating catastrophic chemical accidents. To achieve this goal, CCPS has focused its work in four areas: Establishing and publishing the latest scientific and engineering practices (not standards) for the prevention and mitigation of incidents involving hazardous materials, Encouraging the use of such information by dissemination through publications, seminars, symposia and continuing education programs for engineers, Advancing the state-of-the-art in engineering practices and technical manageiment through research in prevention and mitigation of catastrophic: events, and Developing and encouraging the use of undergraduate education curricula which will improve the safety knowledge and consciousness of engineers. The Center for Chemical Process Safety (CCPS) identified the need for a publication dealing with process safety issues unique to batch reaction systems. Guidelines for Process Safety in Batch Reaction Systems, is the result of a project begun in 1997 in which a group of volunteer professionals representing major chemical, pharmaceutical and hydrocarbon processing companies, worked with Arthur D. Little Inc., to produce a book that attempts to describe the safe design and operation of batch reaction systems. The objectives of the book are to Identify safety concerns unique to batch reaction systems; Provide a how-to guide for the practicing engineer to identify, define and address unique safety issues in batch reaction systems; Xi

xii

PREFACE

Provide a range of criteria and techniques to be considered in the development, design, operation, and maintenance of batch reaction systems to reduce risk and ensure safety of people, environment, and property; Provide an aid to identify potential sources of unsafe conditions encountered in batch reaction systems; Provide guidance in applying appropriate practices to prevent accidents; and Identify sources for specific expertise and reference them accordingly. The book does not focus on occupational safety and health issues, although improved process safety can benefit each area. Detailed engineering designs are outside the scope of the book. This book intends to identify issues and concerns in batch reaction systems and provides potential solutions to address these concerns. This should be of value to process design engineers, operators, maintenance personnel, as well as members of process hazards analysis teams. While the book offers potential solutions to specific issueskoncerns, ultimately the user needs to make the case for the solutions that best satisfy their company’s requirements for a balance between risk reduction and cost. In many instances the book provides one or more sources of additional information on the subject which could be of value to the reader.

Acknowledgments

The American Institute of Chemical Engineers (AIChE) wishes to thank the Center for Chemical Process Safety (CCPS) and those involved in its operation, including its many sponsors whose funding and technical support made this project possible. Particular thanks are due to the members of the Batch Reaction Subcommittee for their enthusiasm, tireless effort and technical contributions. Members of the subcommittee played a major role in the writing of this book by suggesting examples, by offering failure scenarios for the major equipment covered in the book and by suggesting possible solutions to the various Concerns/Issues mentioned in the tables. AIChE and CCPS would also like to express their appreciation to Arthur D. Little, Inc. for their contribution in preparing this book for publication. It is the collective industrial experience and know-how of the subcommittee members plus the experience and expertise of Arthur D. Little, Inc. that makes this book especially valuable to the process and design engineer. Dr. Georges A. Melhem was the Director-in-Charge of this project, for Arthur D. Little, Inc.; Dr. Sanjeev Mohindra of Arthur D. Little, Inc. was the author; and Christina Hourican handled the somewhat complex word processing for this project. The Batch Reaction Subcommittee was chaired by Walter L. Frank of EQE International. Current members of the subcommittee, listed alphabetically are: David J. Christensen, Union Carbide Corporation; Warren Greenfield, International Specialty Products; Philip P. Malkewicz, Nalco Chemical Company; Peter F. McGrath, Olin Corporation; Louisa A. Nara, Bayer Corporation; Leslie A. Scher, CCPS Staff Consultant; Robert Schisla, Eastman Chemical Company; Anthony Torres, Eastman Kodak Company; Dr. Jan C. Windhorst, Nova Chemicals; and Paul Wood, Eli Lilly & Company. Former subcommittee members who contributed much in getting this project started were Felix Freiheiter, CCPS Staff Consultant (deceased); Al Noren, Monsanto Company-Searle (deceased); John Noronha, Eastman Kodak Company (retired) and Robert Stankovich, Eli Lilly & Company. xiii

xiv

ACKNOWLEXMENTS

The Batch Reaction Subcommittee would also like to acknowledge the following peer reviewers for their meaningful suggestions and contributions: Robert E. Hollenbeck, Fred Maves, Gary Paulu, Arlyn H. Poppen, and Monica R. Stiglich of 3M; Pete Lodal of Eastman Chemical Company; Michael Hofler, Lisa Morrison and Peter Monk of Nova Chemicals; Stanley S. Grossel of Process Safety 8z Design, Inc.; Linda Hicks of Reilly Industries, Inc.; Gary York of Rhodia, Inc.; Steve Getz of Union Carbide Corporation; and John Davenport, CCPS staff consultant. Lastly, the members of the Batch Reaction Subcommittee would like to thank their employers for providing the time and support to participate in this project.

Acronyms and Abbreviations

ACGIH ACS AGA AIChE AIHA AIT ANSI APFA API APTAC ARC ASM ASME ASNT ASSE ASTM AWS BLEVE BPCS CAA CAAA CART CCPS CEM CFR CGA

American Conference of Government Industrial Hygienists American Chemical Society American Gas Association American Institute of Chemical Engineers American Industrial Hygiene Association Autoignition Temperature American National Standards Institute American Pipe Fittings Association American Petroleum Institute Automated Pressure Tracking Adiabatic Accelerating Rate Calorimeter American Society for Metals American Society of Mechanical Engineers American Society for Nondestructive Testing American Society of Safety Engineers American Society for Testing and Materials American Welding Society Boiling Liquid Expanding Vapor Explosion Basic Process Control System Clean Air Act Clean Air Act Amendments Computed Adiabatic Reaction Temperature Center for Chemical Process Safety Continuous Emissions Monitor Code of Federal Regulations Compressed Gas Association xv

xvi CIA CMA CSTR DCS DIERS DIPPR DOT DPC DSC DTA EPA ERD ERPG ERRF ERS ESD FC F&EI FIBC FL FMEA FMEC FO FRP HAZOP HSE HVAC IChemE IDLH IEC IEEE IRI ISA

IS0

LEL LFL LNG LOC LPG MAWP MEC MIE

ACRONYMS AND ABBREVIATIONS

Chemical Industries Association Chemical Manufacturers Association Continuous-Flow Stirred-Tank Reactor Distributed Control System Design Institute for Emergency Relief Systems Design Institute for Physical Property Data Department of Transportation Deflagration Pressure Containment Differential Scanning Calorimetry Differential Thermal Analysis Environmental Protection Agency Emergency Relief Design Emergency Response Planning Guideline External Risk Reduction Facilities Emergency Relief System Emergency Shutdown Device Fail Closed Fire and Explosion Index Flexible Intermediate Bulk Containers Fail Last Position Failure Mode and Effects Analysis Factory Mutual Engineering Corporation Fail Open Fiber Reinforced Plastic Hazard and Operability Study Health and Safety Executive Heating, Ventilation, and Air Conditioning The Institution of Chemical Engineers Immediately Dangerous to Life or Health International Electrotechnical Commission Institute of Electrical and Electronics Engineers Industrial Risk Insurers Instrument Society of America International Standards Organization Lower Explosive Limit Lower Flammable Limit Liquefied Natural Gas Limiting Oxidant Concentration Liquefied Petroleum Gas Maximum Allowable Working Pressure Minimum Explosible Concentration Minimum Ignition Energy

Acronyms and Abbreviations

MOC MSDS NACE NBIC NDE NEC NEMA NESC NFPA NIOSH NPSH NTIAC OSHA P&ID PEL PES PFD PFR PHA PID PLC PPE PRV PSD PSI PSS PSV PVRV RP RSST RT RTD SADT SAE SCBA SCC SIL SIS SFPE

SRS

TGA TEMA

Management of Change Material Safety Data Sheet National Association of Corrosion Engineers National Board Inspection Code Nondestructive Examination National Electrical Code National Electrical Manufacturers Association National Electrical Safety Code National Fire Protection Association National Institute of Occupational Safety and Health Net Positive Suction Head Nondestructive Testing Information Analysis Center Occupational Safety and Health Administration Piping and Instrumentation Diagram Permissible Exposure Limit Programmable Electronic System Process Flow Diagram Plug Flow Reactor Process Hazard Analysis Proportional Integral Derivative Programmable Logic Controller Personal Protection Equipment Pressure Relief Valve Process Safety Device Process Safety Information Process Safety System Pressure Safety Valve Pressure-Vacuum Relief Valve Recommended Practice Reactive Systems Screening Tool Radiographic Testing Resistance Temperature Detector Self Accelerating Decomposition Temperature Society of Automotive Engineers Self-contained Breathing Apparatus Stress Corrosion Cracking Safety Integrity Level Safety Instrumented System Society of Fire Protection Engineers Safety Related Systems Thermogravimetric Analysis Tubular Exchanger Manufacturer Association

xvii

ACRONYMS AND ABBREVIATIONS

xviii THA TLV UBC UEL UFL UL UPS

UT

VCE VDI VDE

voc VSP

Thermal Hazardous Analysis Threshold Limit Value Uniform Building Code Upper Explosive Limit Upper Flammable Limit Underwriters Laboratory Inc. Uninterruptible power supply Ultrasonic testing Vapor Cloud Explosion Verein Deutsche Ingenieure Verein Deutsche Elektrotechnike Volatile Organic Compound Vent Sizing Package

1 Process Safety in Batch Reaction Systems 2.1. Scope

The Center for Chemical Process Safety (CCPS) has identified the need for a publication dealing with process safety issues unique to batch reaction systems. This book, Guidelines for Process Safety in Batch Reaction Systems, attempts to aid in the safe design, operation and maintenance of batch and semi-batch reaction systems. I[n this book the terms “batch” and “semi-batch” are used interchangeably for simplicity. The objectives of the book are to: Provide a how-to guide for the practicing engineer to identify, define, and address unique safety issues typically encountered in batch reaction systems,. Provide a range of criteria and techniques to be considered in the development, design, operation, and maintenance of batch reaction systems to reduce risk and ensure safety of people, environment, and property. Provide an aid to identify potential sources of unsafe conditions encountered in batch reaction systems. Provide guidance in applying appropriate process safety practices to prevent accidents. Identify sources for specific expertise and reference them accordingly. The book does not focus on occupational safety and health issues, although improved process safety can benefit these areas. Detailed engineering designs are outside the scope of this work. This book intends to identify issues and concerns in batch reaction systems and provide potential solutions to address these concerns. This should be of value to process design engineers, operators, maintenance personnel, as well as members of process hazards analysis teams. While this book offers potential solutions to specific issuedconcerns, ultimately the user needs to make the case for the solutions that provide a balance between risk

2

1. PROCESS SAFETY IN BATCH REACTION SYSTEMS

reduction and cost. The solutions presented in the book, are possible approaches for dealing with a particular issue. The authors of this book could not anticipate all the possible issues, or all applicable solutions for a specific issue. Therefore, it is intended that the use of the suggested solutions be combined with sound engineering judgment and consideration of all relevant factors. Furthermore, all the solutions presented may not be applicable to a given situation. It should also be recognized that the solutions presented might themselves introduce potential hazards that were not originally present. Therefore, it is necessary to use the information in the context of the total design concept to ensure that all hazards have been considered, and that all applicable laws and regulations have been complied with.

1.2. Special Concerns of Batch Reaction Systems Batch reaction systems present unique challenges for process safety (Hendershot 1987). Batch operations consist of a series of processing steps, which must be carried out in the proper order, and at the proper time. By their very nature, batch-type processes do not operate in a steady state. As the process is being carried out, the holdup of materials in the vessel varies with time as materials are charged, reacted and perhaps withdrawn, thus changing mixing characteristics and effective heat transfer area. There is a continuous variation in the physical properties, chemical compositions, and physical state of the reaction mixture with time. This makes it more difficult, both for the operators and control systems, to monitor and diagnose the process. The sequence of processing steps, and frequent start-ups and shutdowns increase the probability of human errors and equipment failures. Moreover, batch reaction systems often handle multiple processes and products in the same equipment. This can also lead to increased probability of human error. Batch plants are often designed for general use, rather than dedicated to a specific process. The piping and layout of the equipment is often modified to meet the needs of the current process, or the process is modified to use the existing equipment. Use of the same equipment in different campaigns, complex process piping, and the use of shared auxiliary equipment, such as columns and condensers, presents greater challenges in preventing cross contamination; in selecting materials of construction; and in selecting instrumentation and control systems. Additionally, the complexity of equipment and the frequency of changes complicate the process documentation task. These frequent changes often result in complex management of change (MOC) issues. ’ The nature of batch operations (unsteady-state), frequently involving manual intervention, creates significant issues pertaining to the design of control systems, design of operating procedures, and the interaction between the

1.3. Approach Use:din Guidelines

3

control system and the operators. The operator is a more integral part of the process control, supervision loop, and the safe operation of the process. The operator managing a batch process has a greater number of duties and responsibilities than his counterpart in a continuous system. Several of these duties are either specific to batch operations or are done more often in batch operations than in continuous processes. The number and variety of functions that the operator has to perform during a batch process requires effective management systems, including more rigorous training, to minimize human error. Furthermore, the batch operator is more involved and is often in closer proximity to the process. This close proximity puts the operator at increased risk to direct exposure to the hazards associated with larger inventory of raw materials and semi finished products than continuous systems with comparable throughput. All of these issues make batch reaction systems unique, in terms of the challenges they pose for managing process safety. Figure 1 shows a typical batch reaction system.

1.3. Approach Used in Guidelines The book presents information pertaining to the safety issues in batch reaction systems in five chapters. Each chapter starts with a description of the topic covered in the chapter. This is followed by a short example highlighting a reported incident involving a batch reaction system. The case study is followed by a listing of key issues and process safety practices unique to the topic. The issues and concerns presented in this book, as well as potential design solutions and sources of additional information are presented in the tables. This format concisely conveys the necessary and relevant information in a familiar and convenient format. The organization of the tables is described below. Concei:dIssue: Identifies a specific safety issue or concern and its safety implications. Potential Solutions and Control Mechanisms: Lists the potential solutions and control mechanisms that may be employed to reduce the risk of a specific issue or concern. Additional Resources: Provides additional sources of information on the concerns/issues identified in the tables. Please note that the “Additional Resources” column does not attempt to include all sources of additional information. It should be recognized that the solutions and control mechanisms presented in the table are possible approaches for dealing with a particular issue.

0

0

Figure 1. Typical batch reaction system.

To waste solvent recovery

J

Product

1.3. Approach Uscd in Guidelines

5

They are solely meant to create awareness and assist designers and operators, and are not meant to imply that these solutions and control mechanisms are Recognized and Generally Acceptable Good Engineering Practices (RAGAGEP). The authors of this book could not anticipate all the possible issues, or all applicable solutions for a specific issue. The potential solution and controls are intended to stimulate thought and initiate discussions about the appropriateness of the potential solutions presented, and potentially stimulate the generation of other solutions not included in the table. It is intended that the use of the tables should be combined with sound engineering judgment and consideration of all relevant factors. Furthermore, all the solutions presented may not be applicable to a given situation. It should also be recognized that the solutions presented could introduce potential hazards that were not originally present. Therefore, it is necessary to use the table in the context of the total design concept to insure that all hazards have been considered. The information pertaining to the safety issues in batch reaction systems is presented in the following chapters:

Chapter 2. Chemistry Understanding the behavior of all the chemicals involved in the process-raw materials, intermediates, products and by-products, is a key aspect to identifying and understanding the process safety issues relevant to a given process. The nature of the batch processes makes it more likely for the system to enter a state (pressure, temperature, and composition) where undesired reactions can take place. The opportunities for undesired chemical reactions also are far greater in batch reaction systems due to greater potential for contamination or errors in sequence of addition. This chapter presents issues, concerns, and provides potential solutions related to chemistry in batch reaction systems.

Chapter 3. Equipment Configuration and Layout Proper equipment configuration and layout can make a significant contribution to the safety of a processing facility. In batch processes, where the material handled by the process can change frequently, providing safe separation distances presents an even greater challenge than continuous processes. Other important considerations for facility layout are the electrical classification and fire protection requirements. It also is quite common for batch processes to be located inside buildings. This leads to the need to provide adequate building ventilation to avoid buildup of hazardous vapordgases due to leaks. This chapter presents issues/concerns, and provides potential solutions related to equipment configuration and la.yout in batch reaction systems.

6

1. PROCESS SAFETY IN BATCH REACTION SYSTEMS

Chapter 4. Equipment Frequently a piece of equipment is used in different processes during its lifecycle. This could result in process conditions that exceed the safe operating limits of the equipment. Equipment inspection may provide a poor prediction of the equipment’s useful life and reliability, due to the change of material handled or change in process chemistry over the life of equipment. Batch operations are also characterized by frequent start-up and shut-down of equipment. This can lead to accelerated equipment aging and may lead to equipment failure. This chapter presents issues and concerns related to the safe design, operation, and maintenance of various pieces of equipment in batch reaction systems, and provides potential solutions.

Chapter 5. InstrumentationlControl Systems The fact that batch processes are not carried out at steady state conditions imposes broad demands on the control system. The instrumentation and control system have to be selected to provide adequate control for a wide variety of operating conditions and a wide variety of processes. In addition, basic process control and shutdown systems have to deal with sequencing issues. This chapter presents issues and concerns related to safety of instrumentation and control in batch reaction systems, and provides potential solutions.

Chapter 6. Operations and Procedures The operator is an integral part of the process in a batch reaction process. Some of the functions a typical operator working in a batch processing facility may have to perform are scheduling, equipment setup, cleaning, charging, executing and controlling procedure, monitoring, fault diagnosis and corrective action, sampling, handling of finished and off-spedpartially finished products, maintenance, emergency response, process logging and communication. Several of these are either specific to batch operations, or are done more often in batch operations than in continuous processes. The greater number and variety of functions that the operator has to perform during a batch process presents more opportunities for errors than for continuous operations. This chapter presents issues related to operations and procedures in batch reaction systems, and provides potential solutions.

2 Chemistry 2.1. Introduction Understanding the behavior of all the chemicals involved in the process-raw materials, intermediates, products and by-products is a key aspect of understanding the process safety issues relevant to a given process. A knowledge of how these chemicals behave individually and how they interact with other chemicals, utilities, materials of construction, potential contaminants or other materials that they can come in contact with during shipment, storage, and processing is essential for understanding and managing process safety. Understanding the chemistry of the process also provides the greatest opportunity in applying the principles of inherent safety at the chemical synthesis stage. Process chemistry greatly determines the potential impact of the processing facility on people and the environment. It also determines such important safety variables as inventory, ancillary unit operations, by-product disposal, etc. Creative design and selection of process chemistry can result in the use of inherently safer chemicals, a reduction in the inventories of hazardous chemicals and/or a minimization of waste treatment requirements. Reactors often represent a large portion of the risk posed by a batch chemical operation. A better understanding of the reaction behavior and kinetics allows for an optimization of reactor control and safety systems. Knowledge of the reaction behavior includes desired reactions as well as undesired sidereactions that can take place in the reactor itself and other parts of the process. Knowledge of the physical properties of the materials involved in the process and the effects of physical phenomena such as mass transfer, heat transfer, mixing, phase of reaction on the overall reaction rate may be used to identify designs that maximize the economical benefits while reducing risk. As outlined in Chapter 1,batch chemical reactors present unique challenges to the designers in terms of process safety. The transient nature of the batch processes makes it more likely for the system to reach a condition (pressure, temperature, and composition) where undesired reactions could take place. The opportunities for 7

8

2. CHEMISTRY

undesired chemical reactions also are far greater in batch reaction systems due to greater potential for contamination. The importance of understanding the chemistry of the process is not limited to the research and development phase of the process lifecycle. It manifests itself again and again in all the lifecycle stages ranging from research and development to plant decommissioning. Process development in the conceptual design stage and the pilot plant stage can identify opportunities for inherent process safety in selecting the process chemistry and simplifying, i.e., improving control and operation schemes to keep the chemical reactants within safe operating limits. Knowledge of chemistry also can be used to design mitigation measures for releases of hazardous chemicals to the environment (CCPS-G12). In the detailed engineering stage, engineers and chemists can identify safety issues and provide design solutions to reduce the risk posed by the process. During the construction and start-up phase, knowledge of process chemistry can be used to identify operations that need to be strictly followed to reduce potential hazards. Special attention needs to be focused on cleaning to prevent cross contamination. It also can be used to provide safer equipment layout and ergonomics. The operator of the process needs to have an understanding of the process chemistry and the hazards associated with the various chemicals in order to perform routine operations and to identify, diagnose and respond to incipient hazardous situations. Plant modifications, plant expansions, proposed catalysts modifications and changes to the feedstock composition, or other raw materials need to undergo a management of change review. It is also important to analyze the effect of such modifications on the by-product and waste streams. During shutdown and decommissioning, attention needs to be focused on hazards associated with the residues left in the unit after shutdown. There are several issues that are unique to batch reaction system design. The process development time is often shorter due to the need to respond quickly to market demands. The small-scale batch process development may not receive the same effort and rigor as larger batch or continuous processes. Moreover, batch processes are often made to fit the existing facilities. This could lead to operating key equipment and emergency relief systems at the edge of their original design limits.

2.2. Case Study A weigh tank containing chlorosulfonic acid needed to be cleaned to remove salt

deposits. The salt deposits precipitated from the material and occasionally plugged the downstream control valve. Since the material was water reactive, heptane was chosen to clean the vessel. Chemists had not anticipated the material would be reactive with heptane. While cleaning the vessel the pressure

2.4. Process Safety Practices

9

started to rise from the reaction, causing the vessel bottom head to fail at the weld seam. The force from the escaping gases propelled the tank into the ceiling and overhead structural steel. A small fire erupted which was quickly brought under control by the automatic sprinkler system. Even though the chemists had reviewed the chemistry and did not anticipate any problems, use testing could have identified this problem in the laboratory rather than the plant.

2.3. Key Issues Process chemistry issues and their effects on batch reaction systems safety are presented in Table 2, beginning on page 11.This table is meant to be illustrative but not comprehensive. Some key issues are listed below. The hazardous materials used in the process may be raw materials, intermediates, products, by-products, cleaning materials, decomposition, or uninten'ded products. Inadvertent contact between two or more chemicals may lead to a hazardous condition. Process materials may be pyrophoric, water reactive, strong oxidizers or strong reducers. Process chemistry should be selected to fit existing batch equipment. Disposal issues pertaining to unreacted batches, incomplete batches or off-spec products.

2.4. Process Safety Practices Listed below are safety practices aimed at minimizing the incidents caused by process chemistry issues. Select ii process chemistry or synthesis route that is inherently safer. Perform chemical reactivity testing, including the analytical verification of reactants, catalyst, quenches, initiators, and inhibitors. More details are provided in Appendix 2A of this chapter. Use reactor calorimetry testing to determine thermodynamics and kinetics of process. See Appendix 2A (Chemical reactivity hazards screening). Pilot process before putting into operation. Provide system to maintain process safety information (PSI) - Systems for the identification, compilation, and update of information - Assign responsibility for developing new PSI, updating existing PSI and approval of changes to PSI.

10

2. CHEMISTRY

- Controls to verify and/or cross check completion and accuracy of development and update of PSI.

Maintain Process Safety Information (PSI) related to chemistry such as: Information pertaining to the hazards of the chemicals used in the process. This should contain at least the following information: toxicity, flammability, permissible exposure limits, physical data, reactivity data, corrosivity data, thermal and chemical stability data, and hazardous effects of inadvertent mixing of different materials that could occur. - Document safety issues pertaining to process chemistry - Share knowledge between chemists, engineers and operators. Use chemical interaction matrices to identify potential incompatibilities between combinations of materials (not just binary reactions) and interactions with cleaning solvents, heat transfer fluids and other utilities, equipment lubricants, scrubbing media, materials of construction, etc. Implement management of change procedures for changes in design, operation, equipment and chemistry. Provide emergency relief where needed. Provide for addition of diluent, poison, or inhibitor directly to reactor. Provide for automatic or manual actuation of bottom discharge valve to drop batch into a dump tank with diluent, poison or inhibitor, or to an emergency containment area. Design equipment to accommodate maximum operating envelope. Appropriate use of Safety-Related Systems (SRS) such as Safety-Instrumented Systems (SIS). Inert equipment where appropriate. Specialized training or technical literature by raw material suppliers to address special use or handling requirements. Careful analysis of cleaning practices, especially nonperiodic or special purpose cleaning.

-

11

Table 2: Chemistry

Table 2: Chemistry

Selection of ChemistryProcess Chemistry I.

Raw materials, intermediates, praiducts, by-products, decomposition or unintended products are hazardous. Use of the hazardous materials poses a potential risk to the people and the environment.

Select a process chemistry or synthesis route that is inherently safer (e.g., nontoxic, nonflammable materials, less severe operating condition) Use Process Safety Management techniques to minimize the risk to people, and the environment Select equipment design for high integrity containment .

CCPS G-1 CCPS G-6 CCPS G-10 CCPS G-21 CCPS G-24 CCPS G-25 CCPS G-31 CCPS (2-41

!.

Use of maaerials sensi-

Select a process chemistry that is inherently safer (e.g., replace shock sensitive, high temperature sensitive and high pressure sensitive materials with more benign materials, less severe operating conditions) Prevent exposure to shock, high temperature or pressure Design for pressure containment Provide adequately designed relief device Use less severe operating conditions

ASME VIlI Div I and II CCPS G-11 CCPS G-13 CCPS G-23 CCPS G-30 CCPS G-41 DIEM NFPA 68 NFPA 69

ChemicaVmaterials that may potentially come in contact are incompatible. Inadvertent contact between two or more chemicals may lead to a hazardous condition.

Use inherently safer chemistry (e.g., when

API RP750 CCPS G-1 CCPS G13 CCPS G-22 CCPS G 3 CCPS E 3 0 CCPS G-32 CCPS G-41 Hendershot

).

tive to shock, high temperature or high pressure. If the material is inadvertently exposed to an unsuitable condition, or if the process moves out of the safe operating limits, it could result in a loss of containment.

phosgene is cooled in a heat exchanger, consider use of an inert oil as the coolant rather than water as heat exchanger tubes may fail) Use chemical interaction matrices to identify potential incompatibilities between chemicals including utilities, solvents and materials of construction Use chemical reactivity testing to identify and evaluate the hazards Use written operating procedures and provide training Use consistent labeling system Select equipment to minimize inadvertent contact as a result of equipment failure Isolate process from sources of incompatible material

1987

Kletz 1991 Lees 1996

12

2. CHEMISTRY

Selection of Chemistry/Process Chemistry ~

l".ll__ll

I.

Ingress of air into reactor containing pyrophoric material. Of fire / deflagration.

Use management systems (tagging) or interlocks to prevent opening of reactor during reaction progress Provide emergency purge and/or isolation activated on detection of oxygen Provide isolation valves to isolate equipment Design system to accommodate deflagration pressure Provide fire and/or deflagration suppression system Provide closed feed system Provide explosion venting

IGA-XK0775 SCPS G-29 qFPA 2001 \JFPA 68 VFPA 69

;.

Water reactivity of chemicals involved in reaction* Of runaway.

Avoid use of water as cooling/heating medium Avoid use of watedsteam for cleaning of reactor Avoid direct water connection to reactor Prevent backflow from scrubber into reactoi Eliminate water as a mechanical seal barrier fluid Clean and chemically dry vessel prior to charging water reactive material Provide dry compressed gas feeds Ensure that alternate sources of inert gas feeds are dry

SCPS G-13 X P S G-41

Confirm that raw materials feeds are dry Preplan fire protection requirements and procedures Use inherently safer equipment (e.g., jacketed vessels instead of tube heat exchangers] When water or steam is used as a utility, insure appropriate mechanical integrity program for equipment _ " I

._

I

13

Table 2: Chemistry

Selection of ChemistryProcess Chemistry i.

Raw materials, intermediates, products, by-products and/or undesired byproducts are subject to runaway reactions that produce extreme heat and/or extreme amounts of gaseoudvapor products.

Test suspect materials for undesired properties, (e.g., endothermic compounds, compounds containing oxidizing and reducing group such as ammonium nitrate) Substitute or attenuate hazardous materials (inherently safer alternative) Construct equipment to handle extreme temperatures and or pressures Provide emergency relief systems Provide process monitoring and control systems Provide External Risk Reduction Facility (ERRF)

:cps (3-11 NERS EC 61508

7.

Unknown intermediate / side reaction. Unknown

Test suspect materials to characterize undesired properties Conduct thermal hazards analysis Evaluate methods for controlling runaway reactions (e.g., short stop, inhibitors) Determine consequences of runaway reactions and ensure mitigation techniques are in place

;CPS G-13 MERS

bility for runaway reaction.

5.

Chemicalis for use in the process are selected because they are convenient and not necessarily because they are the most suited. Nonoptirnal system design in terms of safety and economics.

XPS (3-23 Carefully and deliberately select process chemicals and synthesis route :CPS G-41 Emphasize consequences of chemical choices to Research and Design Engineering

14

__-9.

10.

2. CHEMISTRY

Selection of Chemistry/F'rocess Chemistry Process chemistry

Use inherently safer chemistry

to fit in existing batch equipment. Nonoptimal process design in terms of safety and economics. possibility of operating close to, or outside of, the safe

Consider the consequences of using existing equipment for new processes (e.g., batch size, corrosion, cost) Anticipate future product needs before purchase of equipment Match batch sizes to equipment capabilities

Operating Of the equipment and the relief capability.

Provide equipment with comparable pressure rating for the entire system Implement management of change orocedures

Short development time may in a less than complete knowledge of the hazards.

Allocate enough time for development Use more time-efficient PHA techniques Use administrative controls to decide when to go to full scale production Establish minimum requirements "transfer package" for process knowledge Require development chemist to be present during initial product runs

X P S G-1 X P S G-8 X P S (2-23 ZCPS G-41

API RP 750 CCPS G-1 CCPS (2-10 CCPS G-25

Chemical Identification

11.

Establish procedures for testing and verifica Trade name of process tion of raw materials, including use testing chemical changes or trade name prevents Implement management of change immediate recognition procedures of harmful effects/ inter* Implement operating procedures and action of chemical. training Sometimes different Use consistent internal labeling system vendors use different names for chemicals. Use of incorrect chemicals leads to hazardous conditions.

CCPS G-13 CCPS G-22 NFPA 325M NFPA 401 NFPA 704

15

Table 2: Chemistry

Composition 2.

L3.

Addition of incorrect reactant Or unanticipated to the reactor. Possibility for runaway reaction.

Change in feed composition. Thiis may happen due to change in suppliers or due to introduction of reworked material. 'Unwanted effect on reaction produ c t ~ by-products. , Varying inhibitor concentrations in monomers from different vendors. Potential for runaway reaction.

Use dedicated feed tank and reactor Implement procedure for double checking reactant identification and quality Implement procedure for double checking addition of correct reactant in correct order Develop operating instructions on the correct or permitted connections between tank! and vessels Color code and label lines Provide dedicated storage areadunloading facilities for reactants Use dedicated connections and/or unique couplings Physically separate points of connection of incompatible materials Use interlocks which prevent addition of certain combination of chemicals Use batch sequencing in control systems when possible Require certificate of analysis for raw material

V1 RP 750 XPS G-13 XPS G-22 X P S G-29 XPS G 3 0

Design for feed variations Obtain certificate of analysis Establish purity limits for each feed Bench scale use testing Sample and analyze feed stocks before addition Design system to accommodate maximum expected pressure Provide adequately designed emergency relief device Implement procedure for double checking reactant identification and quality Implement management of change

CCPS G-1 CCPS Ell CCPS E l 3 CCPS G-23 CCPS Y-28 DIERS

2

.... .... ....... .. .....

16

2. CHEMISTRY

Runaway Reaction 14.

Runaway reaction (caused by, e.g., generation of excessive heat during "fast" charging).

Provide automatic or manual addition of diluent, poison, or inhibitor directly to reactor Provide automatic or manual actuation of bottom discharge valve to drop batch into a dump tank with diluent, poison or inhibitor, or to an emergency containment area Provide automatidmanual isolation based on detection of undesired reaction rate System design accommodating maximum expected pressure and temperature Provide adequately designed relief device Provide emergency cooling Design equipment to limit excessively fast feedrate

XPS G-11 :CPS G-13 :CPS G-23

>IERS (letz 1991

_ I

15.

Overchargdoverfeed of reactants Possibility of overfilling vessel, or initiating runaway reaction.

Use of dedicated reactant charge tank sized only to hold amount of reactant needed Interlock reactant feed charge ed via feed totalizer or weight comparison in charge tank Provide automatidmanual response to level or other indication of abnormal quantity of vessel contents Monitor reaction initiation and progress during charging Provide batch sequencing interlocks that demand operator action Provide adequately designed relief device

JCPS G-13 X P S G-23

16.

Undercharge/ underfeed of reactants. Possibility of unreacted mixed reactants left at end of batch, leading to a subsequent runaway reaction.

Use dedicated reactant charge tank sized to hold correct amount of reactant needed Interlock reactant feed charge via feed totalizer or weight comparison in charge tank

ZCPS G-11 X P S G-13 X P S G-23

DIERS Provide automatidmanual response to level or other indication of abnormal quantity of Kletz 1991 vessel contents Provide means for detecting reaction completion before proceeding Design reactor and/or downstream system to accommodate maximum expected pressure Install adequately designed emergency relief device Establish procedure for disposal of material!

17

Table 2: Chemistry

Runaway Reaction 7.

Overcharge of catalyst or initiator, too much or too fast. Possibility of runaway reaction.

Use dedicated catalyst or initiator charge

XPS G-15 XPS G-23 ZCPS G-29

18.

Undercharge of catalyst. Potential fsor accumulation of reactants and subsequent runaway reaction. Possibility of no reaction resulting in a waste disposal issue.

Use dedicated catalyst charge tank sized to hold only the amount of catalyst needed Implement administrative (procedural) controls for catalyst on the amount or concentration to be added Use staging area for preweighed single catalyst charges Provide means of detecting reaction progress and completion before proceeding further Design reactor and downstream system to accommodate maximum expected pressure Provide adequately designed relief device Establish procedure for disposal of unreacted materials

XPS G-11 XPS G-13 X P S G-23

m

__ -_

tank sized to hold only the amount of catalyst needed Limit quantity of catalyst or initiator added by flow totalizer Implement procedural controls on the amount or concentration of catalyst or initiator to be added Use staging area for preweighed single catalyst charges Design equipment to prevent excessively fast feed. Do not oversize pumps or control valves Install flow restriction orifice Provide means of detecting reaction progress and completion before proceeding further Design reactor and downstream system to accommodate maximum expected pressure

m

-

18

2. CHEMISTRY

Runaway Reaction 1.

20.

Overactive and/or wrong catalyst. Possibility for runaway reaction.

Use dedicated catalyst charge tank sized to

:cps :cps :cps :cps

Establish procedures for testing and verification of catalyst activity and identification including use testing Provide means of detecting reaction completion before proceeding Design reactor and/or downstream system to accommodate maximum expected pressure

X P S GCCPS GCCPS GDlERS Kletz 19'

hold only the amount of catalyst needed Passivate fresh catalyst prior to use or use prediluted catalyst Establish procedures for testing and verification of catalyst activity and identification including use testing. Include procedure to monitor shelf life of catalyst to maintain activity Develop and install emergency system and procedures to shortstop runaway reaction. Establish administrative (procedural) controls on the amount or concentration of catalyst to be added Use staging area for preweighed single catalyst charges Provide means of detecting reaction progress and completion before proceeding Design reactor and downstream system to accommodate maximum expected pressure Provide adequately designed relief device Establish procedure for disposal of unreacted materials Require certificate of analysis for catalyst

Inactive and/or wrong catalyst. Possibility for accumulation of reactant and subsequent runaway reaction in reactor or downstream vessel. Possibility of no reaction resulting in a waste disposal issue.

- .

Provide adequately designed relief device Establish procedure for disposal of unreacted reactant mixture

G-. GGG:CPS GX P S GIIERS

19

Table 2: Chemistry

Runaway Reaction :cps G-11 :CPS G-13 :CPS G-23 )IERS 3etz 1991

21.

Incorrect inhibitor / initiator composition or concentration or amount* Reaction proceeds too rapidly.

Provide automatic control of inhibitodinitiator addition Provide analytical verification of inhibitor / initiator effectivenessincluding use testing (including shelf life issues) Avoid conditions for precipitating, or otherwise separating inhibitor from reacting species Design system to accommodate maximum expected pressure and temperature Provide emergency cooling Provide adequately designed relief device

22.

lnsufficientdiluent due to under feled or excessive evaporation ing in insufficient heat sink. possit,ility of runawaY reaction due to high temperature excursion Or high concentfation of reac+ng species

:cPS G-11 Provide automatic control of diluent addition XPS G-23 Select diluent less susceptible to evaporation Install automatidmanual isolation based on detection of unexpected reaction rate Provide emergency cooling Provide adequately designed relief device Monitor liquid level

23.

Incomplete reaction due to insufficient residence time* low Overactive inhibitor etc* Possibility of no tion. Possibility of unexpected reaction in processing steps. Problem of disposing Of unreacted mixture.

Auto/Manual response to low reaction progress Decision not to proceed to next step based on detection of low reactor temperature and/or reactor composition sampling Design reactor or downstream vessel to accommodate maximum expected pressure Provide adequately designed relief device Implement procedure for disposing of unreacted mixture

SCPS G-11 X P S G-13 ZCPS G-15 X P S G-22 CCPS G-23 CCPS G-31 DIEM

Develop written procedures to clean and verify reactor readiness Implement checklist verification Analyze used cleaning solvent

CCPS G-15 CCPS (3-22 CCPS G-29

Contamination 24.

Chemical reaction due to equipment not being proper'y drained from previous run. Possibibty of unwantedreaction or insufficient desired reaction.

20

2. CHEMISTRY

Contamination 25.

Contamination from leakage of heating/cooling media or introduction of other foreign substances (e.g., corrosion) or possibility of unwanted reaction between the heatingkooling medium and the reactor contents, leading to runaway reaction. Possibility of no reaction or inhibited reaction resulting in accumulation of reactants and delayed runaway.

Use heating/cooling medium which does na

react with or inhibit reactor contents Use external heaterkooler (panel coil) Use electrical heating with proper consider ation of maximum possible heating elemeni temperature Use lower pressure heating or cooling medium to avoid flow into reactor in the event of a leak Consider impact of reactor contents leakin] into utility implement procedures for IeaUpressure testing of jacket, coil or heat exchanger prior to operation Provide emergency cooling Transfer reactor contents to dump tank with diluent quench.

X P S G-23

Off-spec producthntermediate raw material i 26.

The raw materials are Off-Wec in generation of excessive waste products.

Implement effective quality ~. control .prograi Employ good operating procedures Provide procedures to safely handle the unplanned waste generation and/or neutral ize off-spec materials.

X P S G-29

Implement strict quality control program Employ good operating procedures Develop effluent handling procedures

X P S G-29 X P S G-32

I

Waste Minimization

27. i

Variation in waste by batch. Feed to downstream waste processing equipment. Possibility of reaction in waste streams, flammable/ toxic hazard.

Appendix 2A. Chemical Reactivity Hazards Screening

21

Appendix 2A. Chemical Reactivity Hazards Screening Characterizing chemical reactivity hazards involves a review of the inherent thermal hazards of the pure process materials as well as the thermal hazards of the materials under processing conditions. Gaining this understanding and characterizing thermally hazardous systems is a multistep process.

A. 1. Understand the Problem The first step is to understand the context in which the thermal hazard information is needed. This might include information on materials, reactions, processing conditions, previous incidents, if any, and any other available information that can help with the characterization.

A.2. Conduct Theoretical Screening After understanding the problem, the second step is to conduct a theoretical screening to determine the expected thermal hazards of a system. Table A.l identifies properties of materials to be considered, and some potential sources of information, in formulating an opinion about the thermal hazards of particular materials and reactions. The first place to look for information describing the physical properties and known reaction hazards of an individual chemical or process is the literature. Once literature sources have been exhausted, theoretical information should be developed. This determination of theoretical values involves the development of worst-case theoretical estimates based on chemical compatibility information and thermophysical properties such as formation energies, heats

22

2. CHEMISTRY

Table A. 1: Potential Sources of Theoretical Screening Data 1. Basic chemical data

MSDSs, manufacturer’s data, The Merck Index

2. Reactivity data

Bretherick‘s Handbook, NFPA 49,325 and 432 hazard ratings, Sax, Handbook of Hazardous Chemical Properties, Kirk-Othmer Encyclopedia of Chemical Technology or as determined

3. Incident data

Open literature

4. Chemical compatibility

Literature or as determined

5. Chemical structure

Supplied by research scientist, CRC Handbook of Chemistry and Physics

6. Formation energies

Literature (e.g., Pedley’s Handbook) or as determined, Perry’s Chemical Engineers’ Handbook, DIPPR, CRC Handbook of Chemistry and Physics

7. Heats of reaction, decomposition, solution

Literature or as determined CRC Handbook of Chemistry and Physics

8. Chetah hazard criteria

ASTM Chetah Software (see discussion)

9. Computed Adiabatic Reaction Temperature (CART) at constant pressure and/or volume

As calculated

matrix

~

-*-

w-

~

of reaction, decomposition and solution, hazard criteria, and computed adiabatic reaction temperature (CART). Chemical Compatibility

Chemical incompatibility charts can help in organizing available data on the incompatibilities existing between expected mixtures. Frurip (Frurip et al., 1997) gives one procedure for developing a chemical compatibility chart while describing some of the tools available. CCPS G-13 also provides a table of known incompatibility hazards. Data can also be gathered experimentally on the compatibility of materials. Incompatibility charts have been published by the U.S. Coast Guard (1994), ASTM (1980) as well as others. See Frurip (Frurip et al., 1997) for a description of experimental tests and published compatibility charts.

23

Appendix 2A. Chemical Reactivity Hazards Screening

Themopbysical Properties

Much information can be understood by a review of certain thermophysical properties of materials and mixtures. In comparing the values of heats of reaction, heats of decomposition and CART to values for known hazardous compounds, an estimation of thermal hazard potential can be made. Table A 2 outlines thermal hazard ranking values that could be used in classifying materials and processes based on heats of reaction and CART determinations (Melhem and Shanley 1997). Two standard estimation methods for heat of reaction and CART are Chetah 7.2 and NASA CET 89. Chetah" Version 7.2 is a computer program capable of predicting both thermochemical properties and certain reactive chemical hazards of pure chemicals, mixtures or reactions. Available from ASTM, Chetah 7.2 uses Benson's method of group additivity to estimate ideal gas heat of formation and heat of decomposition. NASA CET 89 is a computer program that calculates the adiabatic decomposition temperature (maximum attainable temperature in a chemical system) and the equilibrium decomposition products formed at that temperature. It is capable of calculating CART values for any combination of materials, including reactants, products, solvents, etc. Melhem and Shanley (1997)describe the use of CART values in thermal hazard analysis.

A.3. Conduct I5xperimental Screening Experimental screening involves conducting experimental tests to gauge the thermal hazard of materials and processes. The goal of these tests is to provide information by which the materials and processes may be characterized. Experimental screening can be performed for the following: Self-reactivity Mechanical sensitivity Thermal sensitivity Deflagration and explosion, including dust explosibility and ignitability

Table A.2: Theoretical Hazard Rankings Negligible to Low

Less exothermic than -1.2 kJ/g (-0.28 kcaVg)

< 700K

Intermediate

-1.2 kJ/g < AHr < -3.0 kJlg (-0.28 kcaVg -z AHr < -0.7kcaVg)

1600 K

24

2. CHEMISTRY

Self-Reactivity Hazards

Self-reactivity can be defined as the potential for a material to decompose or undergo energetic changes. Some of the methods for characterizing selfreactivity hazards are listed in Table A.3. Mechanical Sensitivity

Mechanical sensitivity can be divided into two categories-mechanical friction and mechanical shock. Mechanical friction can be defined as mechanical energy imposed by materials being wedged between surfaces and mechanical shock can be defined as mechanical energy imposed by materials undergoing an impact. Several tests for measuring the sensitivity to friction and the impact of materials are detailed in CCPS G-13. Thermal Sensitivity

Thermal sensitivity is the potential for a material to explode under a thermal stimulus. Test methods are outlined in CCPS G-13. Explosion Testing, Including Dust Explosibility and Ignitability

Explosion testing should be performed to establish safe operating limits. Dust explosibility and ignitability are measurements of the potential for a combustible material, in dust form, to explode or ignite. Any combustible material has the potential to cause a dust explosion if dispersed in air as a dust and ignited. Further details on explosibility testing can be found in Palmer (1973), Bartknecht (1989) and Eckhoff (1997).

Table A.3: Methods for Conducting Self-Reactivity Experimental Screening

i3

Differential scanning calorimetry (DSC)

Onset temperature of exotherms, heat of reaction

Thermogravimetric analysis (TGA)

Onset temperature of weight loss

Differential thermal analysis (DTA)

Onset temperature of exotherms, heat of reaction, C,, approximate kinetics

Reactive Systems Screening Tool (RSST'")

Temperature history of runaway reaction, rates of temperature and pressure rise (for gas producing reactions)

ARC

Temperature history of runaway reaction, rates of temperature and pressure rise (for gas producing reactions)

L

t,

I

25

Appendix 2A. Chemical Reactivity Hazards Screening

A.4. Conduct Experimental Analysis Experimental analysis involves the use of thermal hazard analysis tests to verify the results of screening as well as to identify reaction rates and kinetics. The goal of this level of testing is to provide additional information by which the materials and processes may be characterized. The decision on the type of experimental analysis that should be undertaken is dependent on a number of factors, including perceived hazard, planned pilot plant scale, sample availability, regulations, equipment availability, etc. Table A.4, taken from the CCPS Guidelines for Chemical Reactivity Evaluation and Application to Process Design, shows the questions which need to be asked regarding the safety of the proposed reaction, the data required to answer those questions and some selected methods of investigation. The experimental analysis is extremely specialized, and companies should consider outsourcing the tests if they do not have specialists in this area.

Table A.4: Essential Questions on Safety Aspects of Reactions

1. What is the potential temperature rise by the desired reaction? What is the rate of the temperature rise? What are the consequences? What is the maximum pressure?

Enthalpy of desired reaction Specific heat Vapor pressure of solvent as a function of temperature Gas evolution

Table of data Thermodynamic data Calculations; estimations Differential Thermal Analysis (DTA) / Differential Scanning Calorimetry (DSC) Dewar flask experiments Reaction calorimetry with pressure vessel Thermometryhnanometry APTAC" /ARC" /RSST/VSP

2. What is the potential temperature rise by undesired reactions or thermal decomposi- tion, such as from contaminants, impurities, etc.? What are the: consequences? What is the maximum pressure?

Enthalpy of undesired reaction Specific heat Rate of undesired reaction as a function of temperature

DTA/DSC Dewar flask experiments APTAC" /ARC" /RSST/VSP

3. Is reactant accumulation possible? What are thc consequences?

Steady state concentrations Kinetic data Data from 1 and 2

Reaction calorimetry combined with analysis Potential energy by DSC/DTA VSP / APTAC"

4. What is the safe storage tempera-

Kinetic data

Isothermal Storage Test

ture for shelf life?

**

I ye

,

",,-

This page intentionally left blank

3 Equipment Configuration and Layout 3.1. Introduction Proper equipment configuration and layout can make a significant contribution to the safety of a processing facility. Safe separation distances are usually based on hazard considerations, but often the demands for safe access during construction, operation, and maintenance are governing factors. In batch processes, where the material utilized in the process can change frequently, providing safe separation distances presents an even greater challenge. In general, larger spacing between equipment leads to a safer layout. However, this may lead to an increase in pipe work, which in itself may increase the probability of accidental releases. The l.arger spacing between equipment may also increase operator effort and workload in operating the process. Often batch process equipment needs to be located inside buildings. This is usually the case when the process needs to be shielded from extreme headcold conditions, the elements, and/or needs to be kept sterile. This leads to the need to provide adequate building ventilation to avoid buildup of hazardous material due to leaks and other process emissions. When the operation of a process involves opening, cleaning, charging etc., point source ventilation may also need to be provided. Layout also has a significant role in minimizing the probability of ignition of a flammable release. Area electrical classification provides the basis for the control of electrical ignition sources. This classificationis also used to determine the areas that require protection from vehicular access, etc. Frequently, highly hazardous processes that can result in overpressure (e.g., hydrogenation) are placed behind blast resistant struaures/walls. Another important issue in layout is the provision of safe access to equipment for emergency response needs such as fire-fighting etc. The layout also needs to provide for safe escape and rescue routes. As far as off-site population is concerned, the most important siting factor is the distance between the process 27

3. EQUIPMENT CONFIGURATION AND LAYOUT

28

and the off-site receptors. Physical effects of accidental releases, fires and explosions decay rapidly with distance. Low population density in the immediate vicinity of the plant reduces the number of people potentially affected by the accidental releases.

3.2. Case Studies Pump Leak Incidents A high-pressure reciprocating pump, originally used for pumping heavy hydrocarbons, was put into service to pump propylene in an unventilated building. A leak occurred from the gland due to failure by fatigue of the studs holding the gland in position. The escaping liquid vaporized and was ignited by a furnace 76 meters away. Four men were badly burned and the glass windows on the buildings were broken. The failure was attributed to the fact that plant management had not implemented effective management of change procedures. As a result of the deflagration, gas detectors and remote isolation capability were provided. Also, the pump was moved to an open building where small leaks would be dispersed by natural ventilation (CCPSG-39).

Tank Farm Fire In November 1990 a fire occurred at a flammable liquid tank farm supporting Denver’s Stapleton international airport. Eight of the farm’s twelve storage tanks contained jet fuel, totaling almost 4.2 million gallons. The fire burned for 55 hours, destroying seven tanks. Investigators concluded that a damaged pump in a valve pit near the storage tanks may have caused the initial leak and also may have ignited the fuel. In addition, the investigators concluded that a pipe simultaneously cracked, thus releasing fuel into the fire area. The subsequent fire fed on the fuel collecting in the pit and spewing from the two leaks, and impinged on piping and related equipment in the valve pit. As this fire continued to burn, flange gaskets deteriorated, causing more leaks and allowing more fuel to flow out of the storage tanks. The growing fire encroached on two storage tanks adjacent to the valve pit. Approximately 12 hours into the incident, a friction coupling parted, allowing fuel from one storage tank to suddenly increase the fire size. The fire spread to an impounding area and involved two more fuel tanks. The following changes to the tank farm site would have mitigated the outcome of this incident: Increased distance between the tanks and the pumpinghalve area Increased tank-to-tank separation

3.4. Process Safety ]Practices

29

Installation of internal excess flow or fail-safe remotely operated valves for tanks at locations where piping connects Provisions for the removal of fuel in the event the storage tanks' primary discharge means becomes inoperable Simple and recognizable means for fire fighters to shut off fuel flow into the facility Increased structural support for piping (CCPS G-39)

3.3. Key Issues Safety issues in batch reaction systems relating to equipment configuration and layout are pres'ented in Table 3. This table is meant to be illustrative but not comprehensive. A few key issues are presented below. Shared vent systems, utility systems, or equipment may result in incompatible materials coming together. Potential for fire traveling through the shared vent system. Possibility of combining incompatible materials in drainage and dikes. There is a greater need to provide ready access to equipment in batch plants because these require more manual operations. If the access is difficult, it may lead to operator injury and/or inability of operator to carry out responsibilities. Close proximity of hazardous processes may result in releases or other hazardous conditions in one process affecting the neighboring process areas, thereby resulting in escalation of the hazard.

3.4. Process Safety Practices Listed below are safety practices aimed at minimizing hazards due to equipment configuration and layout. Provide safe separation distances for normal operation, maintenance, emergency egress, ergonomics Design systems to prevent incompatible materials coming together Provide appropriate area electrical classification Provide appropriate building, and point source ventilation Provide ignition source control Monitor utility systems for contamination Proper control room design Use daimage limiting construction Provide spill control Install adequate sprinkler protection

30

3. EQUIPMENT CONFIGURATION AND LAYOUT

Table 3: Equipment Configuration and Layout

Shared Systems 1.

2.

Shared vent systems. Possibility of incompatible materials coming together.

Design to avoid incompatible materials present in the same vent system Install deflagration suppression systems Design vent to prevent backflow/accumulation Prescrub vent discharge before transfer to vent header Monitor circulating utility systems for contamination

Shared utility supply systems. Possibility of incompatible materials coming together via contamination of the shared utility system

Design to avoid common utility supply headers and/or systems to processes with incompatible materials Install backflow protection on supply lines Implement mechanical integrity program to prevent contamination of utility systems Monitor circulating utility systems for contamination

ICGIH 1986

X P S G-11 VFPA-69 YFPA-91

API RP 750 CCPS G-7 CCPS G-22 CCPS G-29 CCPS G-57 NFPA-91

_ I

API RP 750 CCPS G-11 CCPS (2-22 Kletz 1991 Lees 1996 NFPA-91

3.

Shared equipment (e.g. auxiliary processing "scrubbers"). POSsibility of incompatible materials coming together.

Design to avoid or minimize use of common equipment for incompatible materials Implement proper cleaning procedure between incompatible uses to prevent cross contamination Prescrub or treat process streams before transfer to common equipment

4.

Shared transfer

Avoid the use of incompatible materials in API RP 750 shared transfer systems Klen 1991 Ensure cleaning procedures are followed Lees 1996 NFPA-91

.

31

Table 3: Equipment Configuration and Layout

\

Ignition Sources

0

5.

Ignition of flammable re!iultingin fire or explosion.

Provide safe separation distances Develop appropriate area electrical classification Provide ignition source control Place ignition sources in positive pressure enclosure and buildings Provide adequate ventilation

LPI RP 500 IS 5345 6 5958 4FPA-70 *PA-77

osion 6.

Shared vent systems. Potential for fire travthe shared vent system.

Design vent system to prevent backflow/accumulation Prescrub or treat vent discharge before transfer to the vent header Install detonation and / or deflagration arresters Install deflagration suppression system Provide explosion venting and isolation mechanism Provide vent system inecting or purging Install dedicated vent systems

I3 CFR 154 WPA-69 WPA-91

7.

Liquid spills. Possibility of accumulation of liquids resulting in fire or explosion hazard.

Provide spill control through adequate drainage and curbs or dikes Provide adequate ventilation Wash down systems Minimize possibility of ignition Minimize possibility of spills

API RP 750 CCPS G-22 CCPS G-24 CCPS G-30 Lees 1996 NFPA 69 NFPA-15

Provide segregation storage of incompatible materials Don't put incompatible materials in the same dike Use segregated drainage & sewer systems Wash down systems Minimize possibility of ignition

API RP 750

spills. Possibilin drainage and dikes.

--%r-&nmr

CCPS G-22 CCPS G-22 CCPS (2-24 CCPS G-30 NFPA-328 NFPA-329 "

\

"

I

*

3. EQUIPMENT CONFIGURATIONAND LAYOUT

32

FireExplosion 9.

10.

Control room sited closer to the batch process due to need for more operator interaction with batch processes. Infiltration of flammablehoxic release from outside. Possible overpressure from external explosion.

\PI RP 752. Proper location of air intake Provide adequate control room ventilation X P S G-26 system gFPA-101 Provide positive control room pressure to prevent inflow of hazardous material Provide flammable/toxic detection systems in buildings Provide control room or facility alarm to warn occupants Provide personal protective equipment Provide sufficient battled air / SCBA Provide doors on the side of the control room opposite to expected hazard sources Provide wind direction indication visible from inside the building I control room Employ damage limiting construction Develop emergency response procedures Develop evacuation plans Provide exterior (and interior) fire extinguishing equipment Design control room to withstand blast overpressure

Batch equipment located indoors. A release of flammabldtoxic material tends to disperse slower than if the release is outdoors. May lead to large concentration buildup and result in operator exposure. Confined flammable releases are also more likely to result in explosion with larger overpressures.

Provide adequate building ventilation Install flammablehoxic detection systems in buildings with alarms to warn building occupants of hazardous accumulations Provide personal protective equipment Provide sufficient bottled air/SCBA Develop emergency response procedures Develop evacuation plans Install explosion venting for room and/or building Damage limiting construction of processing building

ACGIH 1986 CCPS G-3 CCPS (3-13 CCPS G-26 NFPA-68

33

Table 3: Equipment Configuration and Layout

Operator Exposure 1.

Operating equipment is opened, cleaned, emptied, or charged frequently. Operator exposure to toxic or flammablc materials during normal process operation.

Install point source ventilation Install building ventilation Install flammabldtoxic detection systems in buildings with alarms to warn building occupants of hazardous accumulations Use personal protective equipment Provide sufficient bottled air/SCBA Develop emergency response procedures Develop appropriate evacuation plans

,PI 2007

:cPs G-22 XPS (3-32

General

.

.2.

Close proximity of Provide segregated storage feed chemicals for difseparate the processes ferent processes result* Provide unique loading devices ing- in possibility of . using wrong material. See also Chapter 6

XPS G-29 :cps G-3

.3.

Close proximity of process equipment and process areas impedes response and evacuation. Possibility of exposure and/or reduction in efficiency of emergency response.

XPS G-29 :lea 1991 @PA-101 decklenburgh 985

Design equipment layout to accommodate emergency needs-response, ingress, egress Maintain good house keeping Schedule materials used Investigate alternate methods of delivery to occupy less workspace (pipeline instead of drums) Perform prestartup walk-through Perform auditdinspection Clearly mark and maintain the integrity of routes and pathways Schedule processes to reduce amount of material Redesign and modification Use dedicated staging and storage areas

3. EQUIP=

34

CONFIGURATIONAND LAYOUT

General

t

14.

Operator access to equipment. There is a greater need to provide to equipment in batch plants because these require more manual operations. If the access is difficult, it may lead to operator injury and/or inability of operator to carry out responsibilities.

X P S G-23 Provide shortest, most direct and safest route to items requiring most frequent NFPA-101 attention Mecklenburgh Consider ergonomics during layout design 1985

___^__

_ I _ -

15.

API RP 752 Close proximity of Maintain safe separation distances hazardous processes* Consider the need for fire walls, solid Of re'eases floors, etc. in building design and or other hazardous construction conditions in one pro* Provide emergency relief design to vent to ceSSaffecting the safe location neighboring process areas resulting in escalation of the hazard. _ I _

16.

Close proximity of hazardous process. High pressure vessels which may fail explosively.

Maintain safe separation distances

API RP 750 CCPS G-26 Dow F&EI

i

4 Eauhment 1

.

4.1. Introduction This chapter discusses safety issues related to the design and operation of key equipment used in the batch reaction systems. Some of the equipment covered includes: Vessels, including reactors and storage vessels Centrifuges Dryers Batch distillation columns and evaporators Process vents and drains Charging and transferring equipment Drumming equipment Milling equipment Filters Batch process systems impose an additional dimension to the design of equipment. A piece of equipment in batch operations is frequently used in different processes during its life cycle. Surplus equipment or existing equipment is often reused for a different purpose. These practices introduce the possibility of equipment being inadvertently used outside its intended operating envelope. In addition, using existing equipment for new process may overtax existing ancillary units such as utilities, disposal facilities, fire protection etc. Inspection alone may be an inadequate predictor of the equipment reliability due to change of material handled or change in process chemistry over the life of the equipment. Batch operations are characterized by frequent start-up and shutdown of equipment. This can lead to accelerated equipment aging, and may lead to unexpected equipment failure. Some of the types of equipment used in batch reaction systems are discussed in more detail below. 35

36

4. EQUIPMENT

Vessels Including Reactors and Storage Vessels (Table 4.1) Vessels are key components of a batch reaction process facility. While reactors may be the first type of vessel to come to mind, vessels also include storage tanks for feedstocks, intermediates, products, waste streams, etc. Vessels can vary widely in design with respect to factors such as size, pressure and temperature ratings, and materials of construction. However, some common concerns result from the inventories of hazardous materials present in the vessels, the potentially severe operating conditions (e.g., high temperature and pressure) that might pose hazards, and the fact that, in the case of reactors, we are intentionally releasing the chemical potential energy of the process, with the attendant risks of doing so. Reactors are generally, but not always, of robust construction in keeping with the elevated temperatures and pressures commonly associated with the process chemistry. Significant emphasis is placed on integrity of containment, with key considerations including proper temperature and pressure ratings for design, and proper consideration of materials of construction. Adequate mixing and heat exchange capabilities are important with respect to both the intended process function of the vessel, and the safe operation of the vessel; inadequate cooling and/or mixing are common causal factors for runaway reactions that can lead to vessel rupture. Reactors also often share the safety significant performance issues described below for storage vessels. As previously discussed, the flexibility of processing, typical in batch facilities, can complicate the provision of design features that address all of these above concerns for all potential uses of the reactor. Storage tanks are generally designed based upon the vapor pressure of their contents, and can range from low pressure (API-type) tanks to high pressure tanks for compressed gases or pressurized liquids. Nonrefrigerated, pressure-liquefied gases such as liquefied petroleum gases (LPGs) will flash upon release and cool equipment to the extent that the equipment may fail due to cold embrittlement. Boiling liquid expanding vapor explosions (BLEVEs) can result when vessels containing these materials are exposed to external fires. Releases of flammable liquefied gases can also give rise to fires, vapor cloud explosions, and fireballs (e.g., during BLEVEs). Refrigerated liquefied gases are stored at much lower pressures and, accordingly, generally pose much less of a hazard. However, BLEVE hazards still exist for fire exposure situations. Both pressurized and refrigerated liquefied gases pose concerns of exposure to personnel to extremely cold liquids and vapors upon release, along with any toxicity or asphyxiation hazards inherent to the particular liquid. Pressurized and refrigerated storage is covered in detail by industry standards, codes and guidelines, specifically by the NFPA for smaller quantities and API for larger quantities. Atmospheric storage tanks are normally used for liquid materials that are below their boiling point at ambient conditions. Hazards associated with

4.1. Introduction

37

atmospheric tanks (ambient pressure to 15 psig) include overpressure and underpressure, vapor generation, spills, tank rupture, fire, and product contamination. In addition, settling of foundations, and seismic and wind loadings are important concerns. (See MI RP 620 and RP 650.) Although atmospheric storage tanks are not subject to BLEVEs, releases of flammable or combustible liquids can lead to pool fires. Since the potential consequences of fires increase as inventories increase, it is advisable to apply principles of inherent safety through reduction of iiiventories and elimination, where possible, of known ignition sources. The contamination of material in tanks by the introduction of incompatible materials or material of the wrong temperature can cause runaway reactions, polymerization, high temperature excursions, or underpressurization of the tank. To avoid potential contamination of products or routing wrong materials to tanks, safeguards should be implemented, such as clearly labeling piping, valves and manifolds to the tank; use of clear and well-defined operating procedures; and provision of periodic operator training. For vessels containing flammable liquids, where the vessel design pressure is insufficient to contain a deflagration or open loading is performed, consideration should be given to providing an inert gas blanket (e.g., nitrogen) to reduce the oxygen concentration and prevent fires or explosions. Storage vessels also include bins and silos used for the storage of solid materials such as pellets, granules, or dusts. The primary hazard in the storage of such materials comes from the dust that is generated during the mechanical handling of these materials. Suspensions of combustible dusts in the vessel vapor space above the material can be ignited leading to fires and explosions. Since dust production typically cannot be prevented, other means of explosion prevention must be applied. Ignition sources should be minimized, and explosion venting of vessels (including bin vent filters or baghouses) should be considered. Care should be taken during the design of a bin to reduce horizontal surfaces inside the bin where material can remain and create a hazard when the bin is opened for maintenance; the air above such areas has been known to explode while work inside the bins was being performed during normal repairs. Additionally, the vessels can be inerted in a manner similar to that used for atmospheric storage tanks (NFPA 68 and 69). The pneumatic transfer of solids can also be performed using an inert or a reduced oxygen concentration gas with a closed loop return to the sending tank. Among the principal reasons for providing inerting on reactors and vessels is the desirability of eliminating flammable vapor-air mixtures that can be caused by: Addition of solids through the manhole. Materials having low minimum spark ignition energies, or autoignition temperatures

38

4. EQUIPMENT

Potential ignition sources that cannot be controlled adequately, such as: - spontaneous combustion - reactive chemicals: pyrophoric materials, acetylides, peroxides, and water-reactive materials - static electricity: material transfer where lines and vessels are not grounded properly, agitation of liquids of high dielectric strength (low conductivity), addition of liquids of high dielectric strength to vessels, addition to or agitation of liquids in vessels having nonconductive liners Another purpose of inerting is to control oxygen concentrations where process materials are subject to peroxide formation or oxidation to form unstable compounds (acetylides, etc.) or where materials in the process are degraded by atmospheric oxygen. An inert gas supply of sufficient capacity must be ensured. The supply pressure must be monitored continuously. The designer should consider the need for additional measures to supply inert gas. Particular attention must be given to the following situation: In the case of locally high nitrogen consumption (i.e., when a large kettle is inerted), the pressure in the main line may drop so far that the mains could be contaminated by gases or vapors from other apparatus connected at the same time. Depending upon the application, the quality of inert gas (e.g., water content, contaminants) can be important to process safety. The required level of inerting must be ensured by technical and administrative measures, for example: control and monitoring of inert gas flow and inert gas pressure continuous or intermittent measurement of oxygen concentration explicit information in the standard operating procedures or in the process computer program for the correct procedure to achieve a sufficient level of inerting A rigorous mechanical integrity program to ensure the proper design, construction, and maintenance of reactors and storage vessels is essential in order to prevent leaks or more serious vessel failures arising from corrosion or other mechanical failure. The leaking of flammable and toxic liquids can have serious safety and environmental consequences, which are compounded by the large inventories that can be held in these vessels.

Centrifuges (Table 4.2) Since centrifuges are subject to the hazards inherent in all rotating equipment, the designer should first consider whether other, safer methods of separation (such as decanters or static filters) can be used. If it is determined that a

4.1. Introduction

39

centrifuge must be used, the design should be reviewed to ensure that it is as safe and reliable as possible. A good discussion of centrifuge safety design features and operating practices is found in an IChemE publication (1987). Potential problems associated with centrifuges include mechanical friction from bearings; vibration; leaking seals; static electricity; and overspeed. Vibration is both a cause of problems and an effect of equipment problems. The potential destructive force of an out-of-balance load has led to setting lower shutdown limits on the magnitude of vibration than other rotating equipment. Flexible connections for process and utility lines become a must so these vibration problems are not transmitted to connected equipment. Flexible hoses with liners having concentric convolutions (bellows type) avoid the sharp points inherent with spiral metallic liners. By avoiding the sharp point the liner is less likely to cut the exterior covering. Grounding of all equipment components, including internal rotating parts, must be ensured initially and periodically thereafter. Grounding via some type of brush or other direct contact is preferred to grounding via the bearing system through the lubricating medium (unless conductive greases are used). Use of nonconductive solvents complicates the elimination of static electricity concerns; use of conductive solvents or antistatic additives should be considered where feasible. For flammable and/or toxic materials all of the precautions for a pressurized system should be considered. For example, when a centrifuge is pressurized, overpressure protection is required, even if the pressurization is an inert gas. Relieving of the pressure to a closed system or safe location must be considered.

Dryers (Table 4.3) The choice between different types of dryers is often guided by the chemicals involved and their physical properties, particularly heat sensitivity. As when selecting other equipment, the designer should first ask if the step is necessary; if so, whether this is the correct or safest process step. Does the material being processed have to have all of the liquid removed? Can the downstream step or customer use the material in a liquid, slurry or paste form? Some of the hazards in drying operations are: vaporization of flammable liquids; presence of combustible dusts; overheating leading to decomposition; and inerting leading to an asphyxiation hazard. For heat sensitive material, limiting the temperature of the heating medium and residence time of the material are used to prevent decomposition. Inventories of hazardous materials should be minimized. Preventive measures include adequate ventilation and explosion venting, explosion containment, explosion suppression, inerting, elimination of ignition sources, and vapor recovery. Instrumentation may include oxygen

40

4. EQUIPMENT

analyzers and sensors for temperature, humidity, etc. Effluent gases should be monitored for flammability limits. The IChemE book (1990) should be consulted for a thorough review of fires and explosions in dryers. Several general principles may be applied to equipment handling combustible dusts: design equipment to withstand a dust explosion; minimize volume filled by dust suspension; minimize (monitor) mechanical failure and overheating (bearing, rollers, mills); eliminate static electricity and other sources of ignition; minimize passage of burning dust by isolating equipment; provide explosion prevention (e.g., by inerting) and protection (e.g., suppression, venting, or isolation); provide fire protection; maintain design operating conditions via management of change.

Batch Distillation Columns and Evaporators (Table 4.4) Batch distillation equipment can range from a free-standing column with a reboiler, condenser, receiver, and vacuum system, to the use of a jacketed reactor with a condenser. Distillation often involves the generation of combustible vapors in the process equipment. This necessitates the containment of the vapor within the equipment, and the exclusion of air from the equipment, to prevent the formation of combustible mixtures that could lead to fire or explosion. Since distillation is temperature, pressure, and composition dependent, special care must be taken to fully understand the potential thermal decomposition hazards of the chemicals involved. Other potential hazards can result from the freezing or plugging in condensers, or blocked vapor outlets, which may lead to vessel overpressurization if the heat input to the system is not stopped. Emphasis should be placed upon the use of inherently safer design alternatives using concepts such as: limiting the maximum heating medium temperature to safe levels; selecting solvents which do not require removal prior to the next process step; using tempered heat transfer medium to prevent freezing in the condenser; and locating the vessel temperature probe on the bottom head to ensure accurate measurement of temperatures, even a low liquid levels.

Process Vents and Drains (Table 4.5) Process vents and drains, including emission control devices, are often overlooked but are important elements in the safety of batch systems. Inadequate attention to these items can result in incompatible chemical mixtures within the

4.1. Introduction

41

system; formation of combustible atmospheres, or overloading of emission control equipment,, Some items requiring special attention are: elimination of pockets or traps in pipelines; identification and consideration of all process fluids or equipment that could simultaneously drain or vent into common pipelines or equipment; the potential need to prescrub the stream being vented prior to mixing with other streams; proper selection of materials of construction. In addition to the information presented in this chapter, refer to Chapter 3, Equipment Configuration and Layout, for further discussions on shared vent and drain systems.

Charging and TransferringEquipment (Table 4.6) Due to the nature of batch operations, transferring and charging of process materials is a common activity. This can entail gas, liquids, and/or solids handling via open equipment. This may include pumping of liquids from drums or dumping of solids from other containers into an open vessel, shoveling material into a dryer, or making temporary connections such as at hose stations. Primary concerns include the of loss of containment and the potential for exposure of operating personnel to hazardous materials; the potential for other hazards such as fires or explosions; and the ergonomic issues inherent in manipulating large, heavy containers. The first two concerns are of particular significance in batch operations, since operating personnel are often more frequently and more intimately exposed to the batch processes than is typically the case with continucius processes. Some commonly applied controls include providing enclosed charging systems, where feasible; use of localized ventilation; proper selection and use of personal protective equipment; use of ]mechanicalassists for handling drums and other containers; procedures and training; and interlocking vessel openings to prevent opening while the vessel is pressurized.

Drumming Equipment (Table 4.7) Many of the material hazards present in batch processing are also present during the drumming of materials out of the process. However, there are additional considerations unique to this operation, including the mechanical handling of massive objects, potential for puncture of containers, and loss of liner integrity. Some of the hazards present in the drumming stage have the potential for overpressurization leading to release of chemicals and operator exposure,

42

4. EQUIPMENT

underpressurization of drums, o r uncontrolled reactions occurring after drumming, leading to potential fires or explosions. Special consideration needs to be given to drummed materials that are shocwheat sensitive as well as drummed materials that degrade over time.

Milling Equipment (Table 4.8) Milling equipment may be used in batch systems where it is necessary to reduce particle size or product agglomeration. A primary hazard associated with milling equipment is the temperature increase that can be imparted to the material during the milling operation, particularly when product flow through the mill is significantly reduced or interrupted (similar concerns exist for other solids handling operations such as blending and, to a lesser degree, particle size separations such as screening or sieving). This can lead to ignition or decomposition of combustible or unstable materials that could lead to fires or explosions in the milling equipment. Additionally, fires or explosions can result from the presence of combustible dusts typically present in the milling equipment, should other ignition sources be present. Other concerns include the potential for exposure of operating personnel to chemical hazards. A number of design alternatives should be considered when milling materials that are combustible or are temperature sensitive, such as monitoring of milling temperature; shaft speed sensors to detect pluggage in the mill; and instrumentation or inspections to ensure product flow, thus limiting material temperature rise to a safe level. Other ignition sources should be identified and excluded through consideration of static electricity concerns, including proper bonding and grounding; proper area electrical classification; proper selection, location, and maintenance of bearings; and removal of tramp materials from the feed to the milling equipment. Milling of impact-sensitive materials should generally be avoided.

Filters (Table 4.9) One of the primary concerns for filters is the loss of containment of flammable and toxic materials and operator safety during the frequent opening and closing of the equipment (e.g., for changing filter elements or unloading filters). Inherently safer process alternatives should be considered to eliminate or lessen the need for filtration. Self-cleaning, automatic backwashing, or sluicing filters should be considered for pyrophoric or toxic materials as they do not have to be

4.2. Case Studies

43

opened or disassembled to remove the filter cake. Filters for liquid service should be provided with fire-relief valves and safe operating procedures for out-of-service conditions. Bag house filters are normally low-pressure units. They can vary in operating conditions from hot and chemically aggressive to cool and inert. Hot feed may lead to exceeding the temperature rating of the filters and could even result in a bag house fire. As with all filters, not exceeding the design differential pressure is important to both the process stability and safety. As the solid is removed from the gas stream and is subsequently handled for recovery or disposal, all of the conventions and concerns for handling dust, powders and other solids apply. The system should be protected from the potential of dust deflagration by the use of pressure relief or suppression devices. A discussion of safety considerations for these types of systems is found in Dust Explosion Prevention and Protection Part 1-3. (IChemE 1992). In summary, it must be remembered that both design and operations are important in maintaining the integrity of the process and equipment. 4.2. Case Studies

Batch Pharmaceutical Reactor Accident While two operators were charging penicillin powder from fiber drums into a reactor containing a mixture of acetone and methanol, an explosion occurred at the reactor manhole. The two operators were blown back by the force of the explosion, and were covered with solvent-wet powder. The incident was initiated by the ignition of solvent vapors, which resulted in a dust explosion of the dry powder. The solvent mixture in the reactor did not ignite. Tests on the polyethylene liners inside the fiber drums showed that they were nonconclucting; while an attempt had been made to ground the liners, this would not have been effective for the nonconductive polyethylene. The most probable cause of the ignition was an electrostatic discharge from the polyethylene liner during reactor charging. which had been grounded at the time of the incident After this accident, the company instituted the following procedures (Drogaris 1993): Requiring nitrogen inerting when pouring dry solids into flammable solvents Adding dry powder to the reactor by means of grounded metal scoops, where possible, rather than by pouring in directly from drums with polyethylene liners Using only conductive polyethylene liners

44

4. EQUIPMENT

Using a closed charging system rather than pouring dry powders into flammable solvents directly via an open manhole Performing an electrostatic hazard review of the whole plant and all the processes whenever powders and flammable solvents are used

Ed. Note: Even though this incident involved a reactor, it applies as well to any vessel, open-manhole, charging operation. Most likely the liners were loose and the operators not grounded. I f fixed liners were in place and the operators grounded, the accident might not have occurred. Another problem that can be avoided by using closed charging systems is the volumetric displacement of fluids from the vessel during addition of solids. Seveso Runaway Reaction On July 10, 1976 an incident occurred at a chemical plant in Seveso, Italy, which had far-reaching effects on the process safety regulations of many countries, especially in Europe. An atmospheric reactor containing an uncompleted batch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its temperature was 158"C, well below the temperature at which a runaway reaction could start (believed at the time to be 230"C, but possibly as low as IUOC). The reaction was carried out under vacuum, and the reactor was heated by steam in an external jacket, supplied by exhaust steam from a turbine at 190°C and a pressure of 12 bar gauge. The turbine was on reduced load, as various other plants were also shutting down for the weekend (as required by Italian law), and the temperature of the steam rose to about 300°C. There was a temperature gradient through the walls of the reactor (300°C on the outside and 160°C on the inside) below the liquid level because the temperature of the liquid in the reactor could not exceed its boiling point. Above the liquid level, the walls were at a temperature of 300°C throughout. When the steam was shut off and, 15 minutes later, the agitator was switched off, heat transferred from the hot wall above the liquid level to the top part of the liquid, which became hot enough for a runaway reaction to start. This resulted in a release of TCDD (dioxin), which killed a number of nearby animals, caused dermatitis (chloracne) in about 250 people, damaged vegetation near the site, and required the evacuation of about 600 people (Kletz 1994).

Pharmaceutical Powder Dryer Fire and Explosion An operator had tested dryer samples on a number of occasions. After the last sampling, he closed the manhole cover, put the dryer under vacuum, and started rotation of the dryer. A few minutes later an explosion and flash fire occurred, which self-extinguished. No one was injured. Investigations revealed that after

4.4. Process Safety Practices

45

the last sampling, the dryer manhole cover had not been securely fastened. This allowed the vacuum within the dryer to draw air into the rotating dryer and create a flammable mixture. The ignition source was probably an electrostatic discharge (the Teflon" coating on the internal lining of the dryer could have built up a charge). No nitrogen inerting had been used (Drogaris 1993). After this incident, the following precautions were instituted to prevent similar incidents from occurring in the future: Nitrogen purging is carried out before charging or sampling of the dryer If the absolute pressure rises to about 4 psia, the rotation stops, an alarm sounds, and a nitrogen purge starts automatically

4.3. Key Issues Safety issues in batch reaction systems relating to equipment are presented in Tables 4.0 through Table 4.9. The various tables are organized as follows: Table 4.0 Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 4.5 Table 4.6 Table 4.7 Table 4.8 Table 4.9

General Reactors and Vessels Centrifuges Dryers Batch Distillation columns and evaporators Process Vents and Drains Charging and Transferring Equipment Drumming Equipment Milling Equipment Filters

Tables 4.0,4.1, and 4.6 contain information that may be applicable to the whole range of equipment and operations. These tables are meant to be illustrative but not comprehensive.

4.4. Process Safety Practices Listed below are practices that should be considered in the design and safe operation of equipment in batch reaction systems. When using inert gas, provide protection against personnel asphyxiation hazards Protect against the accumulation of electrostatic charges which can cause ignition. This may include the bonding and grounding of the tank, piping,

46

4. EQUIPMENT

and other ancillary equipment and the use of bottom or diptube addition of liquids to minimize material splashing in the tank. Provide adequate fixed fire protection for tanks and vessels containing flammable, unstable or reactive materials. This can include fire loops with hydrants and monitors in the storage area, foam systems for individual tanks, and deluge spray systems to keep the exposed surfaces of tanks cool in case of fire in an adjacent tank. Install flame arresters on atmospheric vents to prevent fire on the outside of the tank from propagating back into the vapor space inside the tank. Provide fire resistant insulation for critical vessels, piping, outlet valves on tanks, valve actuators, instruments lines, and key electrical facilities. Provide remote controlled, automatic, and fire-actuated valves to stop loss of tank contents during an emergency; provide fire protection to these valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids. Vessels should be provided with overpressure relief protection. Provide the capability to add a considerable amount of coolant or diluent to reduce the reaction rate if required. This measure requires: - choice of an appropriate fluid which does not react with the reaction mixture - sufficient free volume in the reactor - piping, instrumentation, etc. to add the fluid in the time required Provide the capability to rapidly depressurize the reactor to a safe location, if needed. Add an inhibitor to stop the reaction. This measure requires intimate knowledge of how the reaction rate can be influenced and whether effective mixindinhibition is possible. Dump the reactor contents into a vessel which contains cold diluent. This option also requires particular care that the dumping line is not blocked or does not become blocked during the dumping procedure. For reactors containing flammable liquids, where the reactor design pressure is insufficient to contain a deflagration, consideration should be given to providing an inert gas blanket (usually nitrogen). Match batch size to container size of critical components, using an integral number of whole containers, where possible Double check materials being added to reactor Complete batch loading sheets for each batch run Use of operator sign-off sheets Preweigh reactants before transferring to reactor Verify raw materials (certificate of analysis for critical materials) Use of a staging area Use of dedicated and proper storage and unloading areas that don't expose other operating and production facilities

4.4. Process Safety Practices

47

Maintain safe handling and storage practices Provide fire suppression deluge protection in areas having high concentrations of flammables or combustibles Test reactive and critical raw materials prior to use Sample to confirm concentrations Label all containers Use unique containers (e.g., colors, shapes) where appropriate Identify all process and utility lines (written material name and color coded) Indicate direction of flow, where applicable Use unique fimngdconneaiondcoupliigs(e.g., colors or sizes) where needed Match batch size to equipment capabilities Use appropriate materials of construction Consider Inherently Safer Design alternatives (e.g., to withstand maximum upset conditions-temperature, pressure, flow) Use hard piping where possible Minimize pipe lengths where possible Use heating media that will not exceed the safe temperature limits for the process Design for ease of cleaning Remove abandoned lines and equipment Install valves and local instrumentation where they will be accessible and visible Where used, include check valves in mechanical integrity program Provide adequately designed relief devices Provide separate vent systems for incompatible materials

48

4. EQUIPMENT

Table 4.0: General

Overpressure Blockage of piping, valves or flame arresters due to solid deposition. Potential for system overpressure.

Size piping system to maintain minimum required velocity to avoid deposition If appropriate, eliminate flame arrester or use parallel switchable flame arresters with flow monitoring Monitor flow in line Remove solids from process stream (use knockout pot, filter, etc.) Install insulatiodtracing of piping to minimize solid deposition (freezing/precipitation) Recirculate material in lines prone to solid deposition Use flush mounted valves where required Periodically clean via flushing, blowdown, internal line cleaning devices (e.g., "pigs") Design piping for maximum expected pressure Install adequately designed emergency relief device (ERD)

2.

Failure of vacuum

Design vessel to accommodate maximum vacuum (full vacuum rating) Provide vacuum relief devicehystem (can be a source of oxygen in vapor space resulting in flammable atmosphere) Provide a vacuum alarm Interlock to inject inert gas Select vacuum source to limit vacuum capability

4PI 2000 X P S G-11 DIERS FMEC 7-59 VFPA 69

Design vessel to accommodate maximum vacuum (full vacuum rating) Use blanketing gas pressure control system to minimize vacuum

4SMG VlII FMEC 7-59

1

resulting in POssibility of vessel

3. t g

iPI 2028 X P S G-1 1 \SME VIll .iptak 1982 Vilday 1991

1.

Uncontrolled condensatiodabsorpOf vapor phase component resulting in vacuum creation inside vessel.

Provide vacuum relief devicehystem Blanket the condenser Insulate equipment to mitigate effect of ambient temperature changes, e.g., thunderstorm Interlock cold liquid feeds with heat source (e.g. distillation column)

_

NFPA 99C

.

.

49

Table 4.0: General

FirelExplosion '.

Deflagration of vapor caused by air leakage into equipment "Perating under vacuum. Possibility of fire/explosion.

Design vessel to accommodate maximum expected deflagration pressure Provide deflagration pressure relief device/system

Ignition of condensed flammable vapor or solid deposits in ductwork! Possibility of fire,exo~osion.

Design system to prevent condensation in ductwork or buildup of deposits by providing smooth surfaces, elimination of potential points of soliddliquid accumulation. Periodicall; flush and/or steam clean piping/ducts Include cleaning procedure in process write-up Provide written cleaning procedure and responsibility Provide provision for drainage of ducts (e.g., sloped, low point drains) Eliminate ignition sources within the ductwork Bond and ground all pipe and duct work Eliminate flammables or combustibles Provide inert atmosphere Install dilution system to keep flammable concentration below lower flammable limit (LFL) Install on-line flammable gas detection and activation of inerting system Install automatic sprinkler system Install deflagration vents Provide automatic isolation of associated equipment via quick closing valves Provide design system to contain overpressure where practical Provide weak sections in piping and duct work Operate above dew point or sublimation point Avoid use of static generating materials (plastic or rubber) for piping and ductwork systems in hazardous service

JFPA 68 JFPA 69

Provide oxygen analyzer with activation of inert gas addition on detection of high oxygen concentration Provide continuous inert purge to check for leaks before Start-up Operate below the Lower Flammable Limit (LFL) ZCPS G-41 'MEC 7-59 4FPA 69 JFPA 77 JFPA 68

50

4. EQUIPMENT

FireExplosion

7.

I.

hadequate ventilation in ducts due to partial obstructions or closed dampers leading to creation of flammable atmosphere. Possibility of firdexplosion.

Design dampers so that system will handle the minimum safe ventilation rate at maximum damper throttling Provide damper mechanical position stop to prevent complete closure of damper Eliminate ignition sources within the ductwork Use bonding and grounding Eliminate flammables or combustible by material substitution Use inert atmosphere Design ventilation system to keep flammable concentration below lower flammable limit Provide on-line flammable gas detection and activation of inerting system Install automatic sprinkler system Install deflagration vents Provide automatic isolation of associated equipment via quick closing valves Provide weak sections (for pressure relief) in piping and duct work Design system to accommodate maximum expected deflagration pressure Provide prescrubberdcondensers to reduce load in duct

:cps G-4 ?FPA 13 JFPA 15 JFPA 1 6 JFPA 68 JFPA 69

Inadequate circulation in equipment causing ~ w d a t i o of n flammable pockets. Possibility of fire/explosion.

Design where natural circulation is sufficient to prevent accumulation of flammables Eliminate flammable solvent (e.g., substitute water-based solvent) Design system for deflagration pressure containment where practical

X P S G-41 rlFPA 69

Premature shutdown of fandventilation system immediately following shutdown of heat input (prior to sufficientcoolin& resulting in hot spots and flammable pockets (dryers, carbon beds, and thermal oxidizers). Possibility of subsequent ignition resulting in fire or explosion.

Design where natural circulation is sufficient to \IFPA 69 prevent accumulation of flammables and/or creation of hot spots Design to contain overpressure where practical Provide postventilation interlocks and/or operating procedures to keep fans running for a sufficient time after shutdown of heating system

51

Table 4.0: General

Fire/Explosion

11.

Production of fine powder during auxiliary processing. Possibility of a dust or dudhybrid explosion.

Operate below minimum oxygen concentration Maintain good housekeeping Crindhlend under inert atmosphere Provide damage limiting construction Provide design to contain overpressure where practical Maintain inlet temperature of heating medium sufficiently below the minimum ignition temperature

?FPA 68 JFPA 69 JFPA 650 JFPA 654

Manifolding of ventilation exhaust ducts of several pieces of equipment from several processes. Possibility of spread of fire or deflagration from one location to the next.

Use dedicated exhaust ducts Vent individual pieces of equipment through conservation vents to prevent back flow Install flame arresters at vessel vents, where applicable Design to contain overpressure where practical Maintain ignition source control Maintain use of inert atmosphere Provide automatic isolation via quick closing valves of manifold duct system on detection of firelflammable atmosphere or overpressure in duct system Provide automatic sprinkler systedinerting gas Provide deflagration vents Provide deflagration suppression system Monitor flammable atmospherdfire Provide nitrogen blocks (nitrogen injection to stop flame propagation) or other explosion isolation measures

JFPA 13 JFPA 15 JFPA 16 JFPA 68 rlFPA 69

Pyrophoric material exposed to air when equipment is opened for cleaninghaintenance. Possibility of fire and operator exposure.

^

-

Maintain good operating and cleaning procedures X P S G-32 X P S G-41 Provide fixed water spray, if appropriate Use inherently safe material, where possible VFPA 15 Provide inert purge Deactivate pyrophoric material prior to exposing to air Purchase/design equipment that does not require opening Ensure operating procedures are in place to purge with inert gas prior to opening

--

+ ,.-

"-* >

&

La">

I -

52

4. EQUIPMENT _ . “

.

Operator Exposure

2

Emission of toxic, flammable or corrosive vapors

Provide local exhaust ventilation connected to a disposal system (vent condenser, adsorber, scrubber or incinerator)

when equipment is Opened for deanindmaintenance or during charging of hazardous material. Possibility for operator exposure.

Operator shuts down operation in response to vapor detection alarm Develop and implement appropriate operating procedures Provide operation to remove operator from zone of danger Purge vessel prior to opening Use inherently safer materials, where possible

Management of Change

13.

14.

15.

-

~

-

-

-

”

”

~

-

-

Equipment used in different processes during its lifecycle. equipment Or existing equipment reused for different use. passibility of equipment being used outside its safe operating envelope.

Procure equipment that can be used in other processes (current or future) without operating close to its design envelope Design equipment for the entire system to accommodate the maximum expected pressure

Using existing equipment for new process may Overtax existing ancillary units e.g., utilitieddisposav fire protection etc. Possibility of hazardous event.

Ensure that the equipment is able to handle the new process chemistry, and that the demands of the new process on ancillary units are also met Perform process hazards analysis

Use of temporary equipment for processing

:CPS G-41

XX‘S Y-28

Select a material of construction that has a wide application range Verify suitability of equipment for new service (material of construction, pressure and temperature rating, etc.) Verify suitability of relief device for new service Develop and implement appropriate cleaning and decontamination procedures

Perform management of change review

”

.”

I

Implement management of change procedure

CCPS G-1

53

Table 4.0: General

Management of Change

16.

Equipment inspection may provide a poor prediction of equipment due to change of material handled or change in process chemistry over the life of equipment.

Reevaluate and possibly reset inspection intervals when equipment is used for handling different chemistry Perform management of change review

Not "in-kind" replacements (e.g., gaskets, ruI'mre disks, packing, mechanical seals) resulting in failure. Possibility of hazardous release.

Ensure that the replacement satisfies the requirements of duty Implement management of change review process

j

I

18. Cyclic nature of batch process (e.g., stadstop, thermal cycling). Possibility of mechanical wear and tear. Possible loss of containment.

Implement mechanical integrity program Design equipment for easy replacement Consider demand of cycling while designing equipment and controls

19, Available equipment determines the process chemistry selected. Operating 'lose to the safe operating envelope of the equipment and the relief capability.

Procure equipment that can be used in other processes (current or future) without operating close to its operating envelope. Provide equipment with comparable pressure rating for the entire system Match batch sizes to equipment capabilities

20.

Frequent stardstop of equipment may lead to equipment failure.

CPS Y-28

,SHA

910.119

Loss of Containment

1

cps G-22 CPS G-27

Minimize frequent stadstop by proper sizing of equipment (e.g. pump capacity) Implement mechanical integrity program Develop procedure to investigate causes for frequent reset of control Minimize frequent stadstop of equipment