1. CHEMICAL ENGINEERINGDESIGN

2. This page intentionally left blank 3. CHEMICAL ENGINEERINGDESIGNPrinciples, Practice andEconomics of Plant andProcess DesignGAVIN TOWLERRAY SINNOTTAMSTERDAM BOSTON HEIDELBERG LONDONNEW YORK OXFORD PARIS SAN DIEGOSAN FRANCISCO SINGAPORE SYDNEY TOKYOButterworth-Heinemann is an imprint of Elsevier 4. Butterworth-Heinemann is an imprint of Elsevier30 Corporate Drive, Suite 400, Burlington, MA 01803, USA525 B Street, Suite 1900, San Diego, California 92101-4495, USA84 Theobalds Road, London WCIX 8RR, UKThis book is printed on acid-free paper.Copyright 2008, Elsevier Inc. All rights reserved.No part of this publication may be reproduced or transmitted in any form or by any means, electronic ormechanical, including photocopy, recording, or any information storage and retrieval system, withoutpermission in writing from the publisher.Permissions may be sought directly from Elseviers Science & Technology Rights Department in Oxford,UK: phone: (+44) 1865 843830, fax: (+44) 1865 853333, E-mail: permissions@elsevier.com. You mayalso complete your request on-line via the Elsevier homepage (http://elsevier.com), by selecting Support&Contact then Copyright and Permission and then Obtaining Permissions.No responsibility is assumed by the publisher or the authors for any injury and/or damage to persons orproperty as a matter of products liability, negligence or otherwise, of from any use or operation of anymethods, products, instructions, data or ideas contained in the material herein.Library of Congress Cataloging-in-Publication DataApplication SubmittedBritish Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.ISBN 13: 978-0-7506-8423-1For information on all Butterworth-Heinemann publicationsvisit our Web site at www.books.elsevier.comPrinted in the United States of America07 08 09 10 9 8 7 6 5 4 3 2 1Cover Design: Joe Tenerelli 5. Gavin Towler is the Senior Manager of Process Design, Modeling and Equipment at UOP LLC.He manages the areas of process design and optimization, equipment design, and physical and kineticmodeling for UOP Research and Development. As adjunct professor at Northwestern University,he teaches the chemical engineering senior design classes. He is a Chartered Engineer and Fellow ofthe Institute of Chemical Engineers.Ray Sinnott began his career in design and development with several major companies, includingDuPont and John Brown. He later joined the Chemical Engineering Department at the University ofWales, Swansea, UK, publishing the first edition of Chemical Engineering Design in 1983. He is aChartered Engineer, Eur. Ing. and Fellow of the Institute of Chemical Engineers. 6. This page intentionally left blank 7. CONTENTSPREFACE xvHOW TO USE THIS BOOK xviiACKNOWLEDGMENTS xix1 INTRODUCTION TO DESIGN 11.1. Introduction 21.2. Nature of Design 21.3. The Anatomy of a Chemical Manufacturing Process 81.4. The Organization of a Chemical Engineering Project 111.5. Project Documentation 131.6. Codes and Standards 161.7. Design Factors (Design Margins) 171.8. Systems of Units 181.9. Optimization 191.10. References 441.11. Nomenclature 441.12. Problems 452 FUNDAMENTALS OF MATERIAL BALANCES 492.1. Introduction 502.2. The Equivalence of Mass and Energy 502.3. Conservation of Mass 512.4. Units Used to Express Compositions 512.5. Stoichiometry 522.6. Choice of System Boundary 532.7. Choice of Basis for Calculations 562.8. Number of Independent Components 572.9. Constraints on Flows and Compositions 582.10. General Algebraic Method 59vii 8. viii CONTENTS2.11. Tie Components 602.12. Excess Reagent 622.13. Conversion, Selectivity, and Yield 632.14. Recycle Processes 682.15. Purge 702.16. Bypass 712.17. Unsteady-State Calculations 722.18. General Procedure for Material-Balance Problems 742.19. References 752.20. Nomenclature 752.21. Problems 763 FUNDAMENTALS OF ENERGY BALANCES AND ENERGYUTILIZATION 813.1. Introduction 823.2. Conservation of Energy 823.3. Forms of Energy (Per Unit Mass of Material) 833.4. The Energy Balance 843.5. Calculation of Specific Enthalpy 893.6. Mean Heat Capacities 903.7. The Effect of Pressure on Heat Capacity 923.8. Enthalpy of Mixtures 943.9. Enthalpy-Concentration Diagrams 953.10. Heats of Reaction 983.11. Standard Heats of Formation 1013.12. Heats of Combustion 1023.13. Compression and Expansion of Gases 1043.14. Energy Balance Calculations 1123.15. Unsteady State Energy Balances 1133.16. Energy Recovery 1143.17. Heat Exchanger Networks 1243.18. References 1453.19. Nomenclature 1463.20. Problems 1504 FLOWSHEETING 1534.1. Introduction 1544.2. Flowsheet Presentation 1554.3. Process Simulation Programs 1624.4. Specification of Components and Physical Property Models 165 9. CONTENTS ix4.5. Simulation of Unit Operations 1694.6. User Models 2044.7. Flowsheets with Recycle 2074.8. Flowsheet Optimization 2194.9. Dynamic Simulation 2244.10. References 2244.11. Nomenclature 2254.12. Problems 2265 PIPING AND INSTRUMENTATION 2355.1. Introduction 2365.2. The P and I Diagram 2365.3. Valve Selection 2415.4. Pumps and Compressors 2435.5. Mechanical Design of Piping Systems 2625.6. Pipe Size Selection 2655.7. Control and Instrumentation 2755.8. Typical Control Systems 2775.9. Alarms, Safety Trips, and Interlocks 2855.10. Computers in Process Control 2875.11. References 2895.12. Nomenclature 2915.13. Problems 2936 COSTING AND PROJECT EVALUATION 2976.1. Introduction 2986.2. Costs, Revenues, and Profits 2986.3. Estimating Capital Costs 3066.4. Estimating Production Costs and Revenues 3346.5. Taxes and Depreciation 3526.6. Project Financing 3586.7. Economic Evaluation of Projects 3636.8. Sensitivity Analysis 3806.9. Project Portfolio Selection 3846.10. References 3886.11. Nomenclature 3906.12. Problems 392 10. x CONTENTS7 MATERIALS OF CONSTRUCTION 3977.1. Introduction 3987.2. Material Properties 3987.3. Mechanical Properties 3997.4. Corrosion Resistance 4027.5. Selection for Corrosion Resistance 4077.6. Material Costs 4087.7. Contamination 4097.8. Commonly Used Materials of Construction 4107.9. Plastics as Materials of Construction for Chemical Plants 4177.10. Ceramic Materials (Silicate Materials) 4197.11. Carbon 4217.12. Protective Coatings 4217.13. Design for Corrosion Resistance 4217.14. References 4227.15. Nomenclature 4247.16. Problems 4248 DESIGN INFORMATION AND DATA 4278.1. Introduction 4288.2. Sources of Information on Manufacturing Processes 4288.3. General Sources of Physical Properties 4308.4. Accuracy Required of Engineering Data 4328.5. Prediction of Physical Properties 4338.6. Density 4348.7. Viscosity 4368.8. Thermal Conductivity 4408.9. Specific Heat Capacity 4428.10. Enthalpy of Vaporization (Latent Heat) 4498.11. Vapor Pressure 4518.12. Diffusion Coefficients (Diffusivities) 4528.13. Surface Tension 4558.14. Critical Constants 4578.15. Enthalpy of Reaction and Enthalpy of Formation 4608.16. Phase Equilibrium Data 4608.17. References 4728.18. Nomenclature 4778.19. Problems 479 11. CONTENTS xi9 SAFETY AND LOSS PREVENTION 4819.1. Introduction 4829.2. Materials Hazards 4869.3. Process Hazards 4939.4. Analysis of Product and Process Safety 5029.5. Failure-Mode Effect Analysis 5039.6. Safety Indices 5069.7. Hazard and Operability Studies 5179.8. Quantitative Hazard Analysis 5269.9. Safety Checklists 5319.10. References 5349.11. Nomenclature 5389.12. Problems 53810 EQUIPMENT SELECTION, SPECIFICATION, AND DESIGN 54110.1. Introduction 54210.2. Separation Processes 54310.3. Solid-Solid Separations 54310.4. Liquid-Solid (Solid-Liquid) Separators 55010.5. Separation of Dissolved Solids 57710.6. Liquid-Liquid Separation 58210.7. Separation of Dissolved Liquids 59010.8. Gas-Solid Separations (Gas Cleaning) 59110.9. Gas-Liquid Separators 60310.10. Crushing and Grinding (Comminution) Equipment 60910.11. Mixing Equipment 60910.12. Transport and Storage of Materials 62010.13. Reactors 62610.14. References 63010.15. Nomenclature 63510.16. Problems 63711 SEPARATION COLUMNS (DISTILLATION, ABSORPTION,AND EXTRACTION) 64111.1. Introduction 64211.2. Continuous Distillation: Process Description 64211.3. Continuous Distillation: Basic Principles 64511.4. Design Variables in Distillation 65011.5. Design Methods for Binary Systems 65211.6. Multicomponent Distillation: General Considerations 665 12. xii CONTENTS11.7. Multicomponent Distillation: Shortcut Methods for Stage andReflux Requirements 66711.8. Multicomponent Systems: Rigorous Solution Procedures(Computer Methods) 69311.9. Other Distillation Systems 69711.10. Plate Efficiency 69811.11. Approximate Column Sizing 70811.12. Plate Contactors 70911.13. Plate Hydraulic Design 71611.14. Packed Columns 74111.15. Column Auxiliaries 77111.16. Solvent Extraction (Liquid-Liquid Extraction) 77211.17. References 77911.18. Nomenclature 78411.19. Problems 78912 HEAT TRANSFER EQUIPMENT 79312.1. Introduction 79412.2. Basic Design Procedure and Theory 79512.3. Overall Heat Transfer Coefficient 79612.4. Fouling Factors (Dirt Factors) 79812.5. Shell and Tube Exchangers: Construction Details 80112.6. Mean Temperature Difference (Temperature Driving Force) 81512.7. Shell and Tube Exchangers: General Design Considerations 82012.8. Tube-Side Heat Transfer Coefficient and Pressure Drop (Single Phase) 82312.9. Shell-Side Heat Transfer and Pressure Drop (Single Phase) 82912.10. Condensers 87012.11. Reboilers and Vaporizers 89012.12. Plate Heat Exchangers 91812.13. Direct-Contact Heat Exchangers 92912.14. Finned Tubes 93012.15. Double-Pipe Heat Exchangers 93112.16. Air-Cooled Exchangers 93212.17. Fired Heaters (Furnaces and Boilers) 93212.18. Heat Transfer to Vessels 93812.19. References 94512.20. Nomenclature 95112.21. Problems 95713 MECHANICAL DESIGN OF PROCESS EQUIPMENT 96113.1. Introduction 96213.2. Pressure Vessel Codes and Standards 963 13. CONTENTS xiii13.3. Fundamental Principles and Equations 96613.4. General Design Considerations: Pressure Vessels 98013.5. The Design of Thin-Walled Vessels Under Internal Pressure 98613.6. Compensation for Openings and Branches 99313.7. Design of Vessels Subject to External Pressure 99513.8. Design of Vessels Subject to Combined Loading 99913.9. Vessel Supports 101313.10. Bolted Flanged Joints 102013.11. Heat Exchanger Tube Plates 102813.12. Welded-Joint Design 103113.13. Fatigue Assessment of Vessels 103313.14. Pressure Tests 103413.15. High-Pressure Vessels 103513.16. Liquid Storage Tanks 103813.17. Pressure-Relief Devices 103813.18. References 105313.19. Nomenclature 105613.18. Problems 106014 GENERAL SITE CONSIDERATIONS 106514.1. Introduction 106614.2. Plant Location and Site Selection 106614.3. Site Layout 106814.4. Plant Layout 106914.5. Utilities 107414.6. Environmental Considerations 107614.7. References 1086APPENDIX 1089A Graphical Symbols for Piping Systems and Plant 1089B Corrosion Chart 1099C Physical Property Data Bank 1119D Conversion Factors for Some Common SI Units 1141E Design Projects I 1145F Design Projects II 1165G Equipment Specification (Data) Sheets 1193H Typical Shell and Tube Heat Exchanger Tube-Sheet Layouts 1207I Material Safety Data Sheet 1213 14. This page intentionally left blank 15. PrefaceThis book was first published as Volume 6 of the Chemical Engineering series editedby Coulson and Richardson. It was originally intended to be a standalone designtextbook for undergraduate design projects that would supplement the other volumesin the Coulson and Richardson series. Emphasis was placed on the practice of processand equipment design, while the reader was referred to the other volumes in the seriesand other chemical engineering textbooks for details of the fundamental principlesunderlying the design methods.In adapting this book for the North American market, we have followed the samephilosophy, seeking to create a comprehensive guide to process plant design thatcould be used as part of the typical chemical engineering curriculum, while providingreferences to more detailed and specialized texts wherever necessary. The designprocedures can be used without the need for reference to the other books, researchpapers, or websites cited.We recognize that chemical engineers work in a very diverse set of industries,and many of these industries have their own design conventions and specializedequipment. We have attempted to include examples and problems from a broadrange of process industries, but where space or our lack of expertise in the subjecthas limited coverage of a particular topic, references to design methods available inthe general literature are provided.In writing this book, we have drawn on our experience of the industrial practice ofprocess design, as well as our experience teaching design at the University of WalesSwansea, University of Manchester, and Northwestern University. Since the book isintended to be used in practice and not just as a textbook, our aim has been to describethe tools and methods that are most widely used in industrial process design. We havedeliberately avoided describing idealized conceptual methods developed by researchersthat have not yet gained wide currency in industry. The reader can find good descrip-tionsof these methods in the research literature and in more academic textbooks.Standards and codes of practice are an essential part of engineering; therefore,the relevant North American standards are cited. The codes and practices coveredby these standards will be applicable to other countries. They will be covered byequivalent national standards in most developed countries, and in some cases therelevant British, European, or International standards have also been cited. Briefxv 16. xvi PREFACEsummaries of important U.S. and Canadian safety and environmental legislation havebeen given in the relevant chapters. The design engineer should always refer to theoriginal source references of laws, standards, and codes of practice, as they areupdated frequently.All of the costs and examples have been put on a U.S. basis, and examples havebeen provided in both metric and conventional units. Where possible, the termi-nologyused in the U.S. engineering and construction industry has been used.Most industrial process design is carried out using commercial design software.Extensive reference has been made to commercial process and equipment designsoftware throughout the book. Many of the commercial software vendors providelicenses of their software for educational purposes at nominal fees. We stronglyrecommend that students be introduced to commercial software at as early a stagein their education as possible. The use of academic design and costing software shouldbe discouraged. Academic programs usually lack the quality control and supportrequired by industry, and the student is unlikely to use such software after graduation.All computer-aided design tools must be used with some discretion and engineeringjudgment on the part of the designer. This judgment mainly comes from experience,but we have tried to provide helpful tips on how to best use computer tools.The art and practice of design cannot be learned from books. The intuitionand judgment necessary to apply theory to practice will come only from practicalexperience. We trust that this book will give its readers a modest start on that road.Ray SinnottGavin Towler 17. How to Use This BookThis book has been written primarily for students in undergraduate courses in chem-icalengineering and has particular relevance to their senior design projects. It shouldalso be of interest to new graduates working in industry who find they need tobroaden their knowledge of unit operations and design. Some of the earlier chaptersof the book can also be used in introductory chemical engineering classes and by otherdisciplines in the chemical and process industries.As a Senior Design Course TextbookChapters 1 to 9 and 14 cover the basic material for a course on process designand include an explanation of the design method, including considerations of safety,costing, and materials selection. Chapters 2, 3, and 8 contain a lot of backgroundmaterial that should have been covered in earlier courses and can be quickly skimmedas a reminder. If time is short, Chapters 4, 6, and 9 deserve the most emphasis.Chapters 10 to 13 cover equipment selection and design, including mechanicalaspects of equipment design. These important subjects are often neglected inthe chemical engineering curriculum. The equipment chapters can be used asthe basis for a second course in design or as supplementary material in a processdesign class.As an Introductory Chemical Engineering TextbookThe material in Chapters 1, 2, 3, and 6 does not require any prior knowledgeof chemical engineering and can be used as an introductory course in chemicalengineering. Much of the material in Chapters 7, 9, 10, and 14 could also be usedin an introductory class. There is much to be said for introducing design at an earlypoint in the chemical engineering curriculum, as it helps the students have a betterappreciation of the purpose of their other required classes, and sets the context for therest of the syllabus. Students starting chemical engineering typically find the practicalapplications of the subject far more fascinating than the dry mathematics they areusually fed. An appreciation of economics, optimization, and equipment design candramatically improve a students performance in other chemical engineering classes.xvii 18. xviii HOW TO USE THIS BOOKIf the book is used in an introductory class, then it can be referred to throughoutthe curriculum as a guide to design methods.Supplementary MaterialMany of the calculations described in the book can be performed using spreadsheets.Templates of spreadsheet calculations and equipment specification sheets are availablein Microsoft Excel format online and can be downloaded by all readers of this bookfrom http://books.elsevier.com/companions.Resources for InstructorsSupplementary material is available for registered instructors who adopt ChemicalEnginering Design as a course text. Please visit http://textbooks.elsevier.com forinformation and to register for access to the following resources.Lecture SlidesMicrosoft PowerPoint presentations to support most of the chapters are available freeof charge to instructors who adopt this book. To preview PDF samples of the slidesplease register with the site above. A complete set of slides on CD, in customizablePowerPoint format, will be sent to qualifying adopters on request.Image BankA downloadable image bank of artwork from the book to use in lecture presentationsis available.Instructors ManualA full solutions manual with worked answers to the exercises in the main text isavailable for download. 19. AcknowledgmentsAs in my prefaces to the earlier editions of this book, I would like to acknowledge mydebt to those colleagues and teachers who have assisted me in a varied career as aprofessional engineer. I would particularly like to thank Professor J. F. Richardson forhis help and encouragement with earlier editions of this book. Also, my wife, Muriel,for her help with the typescripts of the earlier editions.Eur. Ing. R. K. SinnottCoed-y-bryn, WalesI would like to thank the many colleagues at UOP and elsewhere who have workedwith me, shared their experience, and taught me all that I know about design.Particular thanks are due to Dr. Rajeev Gautam for allowing me to pursue this projectand to Dick Conser, Peg Stine, and Dr. Andy Zarchy for the time they spent reviewingmy additions to Rays book and approving the use of examples and figures drawnfrom UOP process technology. My contribution to this book would not have beenpossible without the love and support of my wife, Caroline, and our childrenMiranda, Jimmy, and Johnathan.Gavin P. TowlerInverness, IllinoisMaterial from the ASME Boiler and Pressure Vessel Code is reproducedwith permission of ASME International, Three Park Avenue, New York, NY 10016.Material from the API Recommended Practices is reproduced with permission ofthe American Petroleum Institute, 1220 L Street, NW, Washington, DC 20005.Material from British Standards is reproduced by permission of the British StandardsInstitution, 389 Chiswick High Road, London, W4 4AL, United Kingdom. Completecopies of the codes and standards can be obtained from these organizations.We are grateful to Aspen Technology Inc. and Honeywell Inc. for permission toinclude the screen shots that were generated using their software to illustrate theprocess simulation and costing examples. Laurie Wang of Honeywell also providedvaluable review comments. The material safety data sheet in Appendix I is reproducedwith permission of Fischer Scientific Inc. Aspen Plus1, Aspen Kbase, Aspen ICARUS,xix 20. xx ACKNOWLEDGMENTSand all other AspenTech product names or logos are trademarks or registered trade-marksof Aspen Technology Inc. or its subsidiaries in the United States and/or in othercountries. All rights reserved.The supplementary material contains images of processes and equipment frommany sources. We would like to thank the following companies for permission to usethese images: Alfa-Laval, ANSYS, Aspen Technology, Bete Nozzle, Bos-Hatten Inc.,Chemineer, Dresser, Dresser-Rand, Enardo Inc., Honeywell, Komax Inc., RigginsCompany, Tyco Flow Control Inc., United Value Inc., UOP LLC, and The ValveManufacturers Association.Jonathan Simpson of Elsevier was instrumental in launching and directing thisproject. He and Lyndsey Dixon provided guidance and editorial support throughoutthe development of this edition. We would also like to thank Heather Scherer,Viswanathan Sreejith and Jay Donahue for their excellent work in assembling thebook and managing the production process.The cover illustration shows the 100th CCR Platforminge unit licensed by UOPand is reproduced with permission of UOP LLC. 21. 1 INTRODUCTIONTO DESIGNChapter Contents1.1. Introduction1.2. Nature of Design1.3. The Anatomy of a Chemical Manufacturing Process1.4. The Organization of a Chemical Engineering Project1.5. Project Documentation1.6. Codes and Standards1.7. Design Factors (Design Margins)1.8. Systems of Units1.9. Optimization1.10. References1.11. Nomenclature1.12. ProblemsKey Learning Objectives& How design projects are carried out and documented in industry& Why engineers in industry use codes and standards and build margins into their designs& How to improve a design using optimization methods& Why experienced design engineers very rarely use rigorous optimization methods inindustrial practice1 22. 2 CHAPTER 1 INTRODUCTION TO DESIGN1.1. INTRODUCTIONThis chapter is an introduction to the nature and methodology of the design processand its application to the design of chemical manufacturing processes.1.2. NATURE OF DESIGNThis section is a general discussion of the design process. The subject of this book ischemical engineering design, but the methodology described in this section appliesequally to other branches of engineering.Chemical engineering has consistently been one of the highest paid engineeringprofessions. There is a demand for chemical engineers in many sectors of industry,including the traditional processing industries: chemicals, polymers, fuels, foods, phar-maceuticals,and paper, as well as other sectors such as electronic materials and devices,consumer products, mining and metals extraction, biomedical implants, and powergeneration.The reason that companies in such a diverse range of industries value chemicalengineers so highly is the following:Starting from a vaguely defined problem statement such as a customer need or a set ofexperimental results, chemical engineers can develop an understanding of the importantunderlying physical science relevant to the problem and use this understanding to create aplan of action and set of detailed specifications which, if implemented, will lead to apredicted financial outcome.The creation of plans and specifications and the prediction of the financial outcomeif the plans were implemented is the activity of chemical engineering design.Design is a creative activity, and as such can be one of the most rewarding andsatisfying activities undertaken by an engineer. The design does not exist at the start ofthe project. The designer begins with a specific objective or customer need in mindand, by developing and evaluating possible designs, arrives at the best way of achiev-ingthat objectivebe it a better chair, a new bridge, or for the chemical engineer,a new chemical product or production process.When considering possible ways of achieving the objective, the designer will beconstrained by many factors, which will narrow down the number of possible designs.There will rarely be just one possible solution to the problem, just one design. Severalalternative ways of meeting the objective will normally be possible, even several bestdesigns, depending on the nature of the constraints.These constraints on the possible solutions to a problem in design arise in manyways. Some constraints will be fixed and invariable, such as those that arise fromphysical laws, government regulations, and standards. Others will be less rigid andcan be relaxed by the designer as part of the general strategy for seeking the best design.The constraints that are outside the designers influence can be termed the externalconstraints. These set the outer boundary of possible designs, as shown in Figure 1.1.Within this boundary there will be a number of plausible designs bounded by the other 23. 1.2. NATURE OF DESIGN 3Region of all designsResourcesProcessconditionsChoiceofprocessMethodsPlausibledesignsSafety regulationsMaterialsPersonnelTimePossible designsGovernment controlsEconomic constraintsPhysical lawsStandards and codesExternal constraintsInternal constraintsFigure 1.1. Design constraints.constraints, the internal constraints, over which the designer has some control, such aschoice of process, choice of process conditions, materials, and equipment.Economic considerations are obviously a major constraint on any engineering design:plants must make a profit. Process costing and economics are discussed in Chapter 6.Time will also be a constraint. The time available for completion of a design willusually limit the number of alternative designs that can be considered.The stages in the development of a design, from the initial identification of theobjective to the final design, are shown diagrammatically in Figure 1.2. Each stage isdiscussed in the following sections.Figure 1.2 shows design as an iterative procedure; as the design develops, thedesigner will be aware of more possibilities and more constraints, and will be constantlyseeking new data and ideas, and evaluating possible design solutions.1.2.1. The Design Objective (The Need)All design starts with a perceived need. In the design of a chemical process, the need is thepublic need for the product, creating a commercial opportunity, as foreseen by the salesand marketing organization. Within this overall objective, the designer will recognizesubobjectives, the requirements of the various units that make up the overall process.Before starting work, the designer should obtain as complete, and as unambiguous,a statement of the requirements as possible. If the requirement (need) arises fromoutside the design group, from a customer or from another department, then thedesigner will have to elucidate the real requirements through discussion. It is impor-tantto distinguish between the needs that are must haves and those that are should 24. 4 CHAPTER 1 INTRODUCTION TO DESIGNDetermineCustomer NeedsSet DesignSpecificationsR&D if NeededGenerate DesignConceptsEvaluate Economics,Optimize & SelectDesignBuild PerformanceModelsPredict Fitnessfor ServiceProcurement& ConstructionBegin OperationCustomerApprovalDetailed Design &Equipment SelectionFigure 1.2. The design process.haves. The should haves are those parts of the initial specification that may bethought desirable, but that can be relaxed if required as the design develops. Forexample, a particular product specification may be considered desirable by the salesdepartment, but may be difficult and costly to obtain, and some relaxation of thespecification may be possible, producing a saleable but cheaper product. Wheneverpossible, the designer should always question the design requirements (the project andequipment specifications) and keep them under review as the design progresses. It isimportant for the design engineer to work closely with the sales or marketing depart-mentor with the customer directly, to have as clear as possible an understanding ofthe customers needs.When writing specifications for others, such as for the mechanical design or pur-chaseof a piece of equipment, the design engineer should be aware of the restrictions(constraints) that are being placed on other designers. A well-thought-out, compre-hensivespecification of the requirements for a piece of equipment defines the externalconstraints within which the other designers must work.1.2.2. Setting the Design BasisThemost important step in starting a process design is translating the customer need intoa design basis. The design basis is a more precise statement of the problem that is to besolved. It will normally include the production rate and purity specifications of the mainproduct, together with information on constraints that will influence the design, such as1. The system of units to be used.2. The national, local or company design codes that must be followed.3. Details of raw materials that are available. 25. 1.2. NATURE OF DESIGN 54. Information on potential sites where the plant might be located, includingclimate data, seismic conditions, and infrastructure availability. Site design isdiscussed in detail in Chapter 14.5. Information on the conditions, availability, and price of utility services such asfuel (gas), steam, cooling water, process air, process water, and electricity, thatwill be needed to run the process.The design basis must be clearly defined before design can be begun. If the design iscarried out for a client, then the design basis should be reviewed with the client at the startof the project. Most companies use standard forms or questionnaires to capture designbasis information. A sample template is given in Appendix G and can be downloadedin MS Excel format from the online material at http://books.elsevier.com/companions.1.2.3. Generation of Possible Design ConceptsThe creative part of the design process is the generation of possible solutions to theproblem for analysis, evaluation, and selection. In this activity, most designers largelyrely on previous experiencetheir own and that of others. It is doubtful if any designis entirely novel. The antecedents of most designs can usually be easily traced. Thefirst motor cars were clearly horse-drawn carriages without the horse, and thedevelopment of the design of the modern car can be traced step by step from theseearly prototypes. In the chemical industry, modern distillation processes have devel-opedfrom the ancient stills used for rectification of spirits, and the packed columnsused for gas absorption have developed from primitive, brushwood-packed towers.So, it is not often that a process designer is faced with the task of producing a designfor a completely novel process or piece of equipment.Experienced engineers usually prefer the tried-and-tested methods, rather thanpossibly more exciting but untried novel designs. The work that is required to developnew processes and the cost are usually underestimated. Commercialization of newtechnology is difficult and expensive, and few companies are willing to make multi-milliondollar investments in technology that is not well proven (known as me thirdsyndrome). Progress is made more surely in small steps; however, when innovation iswanted, previous experience, through prejudice, can inhibit the generation andacceptance of new ideas (known as not invented here syndrome).The amount of work, and the way it is tackled, will depend on the degree of noveltyin a design project. Development of new processes inevitably requires much moreinteraction with researchers and collection of data from laboratories and pilot plants.Chemical engineering projects can be divided into three types, depending on thenovelty involved:A. Modifications, and additions, to existing plant; usually carried out by the plantdesign group.B. New production capacity to meet growing sales demand and the sale of estab-lishedprocesses by contractors. Repetition of existing designs, with only minordesign changes, including designs of vendors or competitors processes carriedout to understand whether they have a compellingly better cost of production. 26. 6 CHAPTER 1 INTRODUCTION TO DESIGNC. New processes, developed from laboratory research, through pilot plant, toa commercial process. Even here, most of the unit operations and processequipment will use established designs.The majority of process designs are based on designs that previously existed. Thedesign engineer very seldom sits down with a blank sheet of paper to create a newdesign from scratch, an activity sometimes referred to as process synthesis. Even inindustries such as pharmaceuticals, where research and new product development arecritically important, the types of processes used are often based on previous designsfor similar products, so as to make use of well-understood equipment and smooth theprocess of obtaining regulatory approval for the new plant.The first step in devising a new process design will be to sketch out a rough blockdiagram showing the main stages in the process and to list the primary function(objective) and the major constraints for each stage. Experience should then indicatewhat types of unit operations and equipment should be considered. The steps in-volvedin determining the sequence of unit operations that constitute a processflowsheet are described in Chapter 4.The generation of ideas for possible solutions to a design problem cannot beseparated from the selection stage of the design process; some ideas will be rejectedas impractical as soon as they are conceived.1.2.4. Fitness TestingWhen design alternatives are suggested, they must be tested for fitness of purpose. Inotherwords, the design engineermust determine howwell each design conceptmeets theidentified need. In the field of chemical engineering, it is usually prohibitively expensiveto build several designs to find out which one works best (a practice known as proto-typing,which is common in other engineering disciplines). Instead, the design engineerbuilds a mathematical model of the process, usually in the form of computer simulationsof the process, reactors, and other key equipment. In some cases, the performancemodelmay include a pilot plant or other facility for predicting plant performance and collectingthe necessary design data. In other cases, the design data can be collected froman existingfull-scale facility or can be found in the chemical engineering literature.The design engineer must assemble all of the information needed to model theprocess so as to predict its performance against the identified objectives. For processdesign this will include information on possible processes, equipment performance,and physical property data. Sources of process information and physical propertiesare reviewed in Chapter 8.Many design organizations will prepare a basic data manual, containing all theprocess know-how on which the design is to be based. Most organizations will havedesignmanuals covering preferredmethods and data for themore frequently used designprocedures. The national standards are also sources of design methods and data. Theyare also design constraints, as new plants must be designed in accordance with thenational standards. If the necessary design data or models do not exist, then researchand development work is needed to collect the data and build new models. 27. 1.2. NATURE OF DESIGN 7Once the data has been collected and a working model of the process has beenestablished, then the design engineer can begin to determine equipment sizes andcosts. At this stage it will become obvious that some designs are uneconomical andthey can be rejected without further analysis. It is important to make sure that all ofthe designs that are considered are fit for the service, i.e., meet the customers musthave requirements. In most chemical engineering design problems, this comes downto producing products that meet the required specifications. A design that does notmeet the customers objective can usually be modified until it does so, but this alwaysadds extra costs.1.2.5. Economic Evaluation, Optimization, and SelectionOnce the designer has identified a few candidate designs that meet the customerobjective, then the process of design selection can begin. The primary criterion fordesign selection is usually economic performance, although factors such as safety andenvironmental impact may also play a strong role. The economic evaluation usuallyentails analyzing the capital and operating costs of the process to determine the returnon investment, as described in Chapter 6.The economic analysis of the product or process can also be used to optimize thedesign. Every design will have several possible variants that make economic senseunder certain conditions. For example, the extent of process heat recovery is a trade-offbetween the cost of energy and the cost of heat exchangers (usually expressed asa cost of heat exchange area). In regions where energy costs are high, designs that use alot of heat exchange surface to maximize recovery of waste heat for reuse in theprocess will be attractive. In regions where energy costs are low, it may be moreeconomical to burn more fuel and reduce the capital cost of the plant. The math-ematicaltechniques that have been developed to assist in the optimization of plantdesign and operation are discussed briefly in Section 1.9.When all of the candidate designs have been optimized, the best design can beselected. Very often, the design engineer will find that several designs have very closeeconomic performance, in which case the safest design or that which has the bestcommercial track record will be chosen. At the selection stage an experienced engin-eerwill also look carefully at the candidate designs to make sure that they are safe,operable, and reliable, and to ensure that no significant costs have been overlooked.1.2.6. Detailed Design and Equipment SelectionAfter the process or product concept has been selected, the project moves on to detaileddesign.Here the detailed specifications of equipment such as vessels, exchangers, pumps,and instruments are determined. The design engineer may work with other engineeringdisciplines, such as civil engineers for site preparation, mechanical engineers for designof vessels and structures, and electrical engineers for instrumentation and control.Many companies engage specialist Engineering, Procurement, and Construction(EPC) companies, commonly known as contractors, at the detailed design stage. 28. 8 CHAPTER 1 INTRODUCTION TO DESIGNThe EPC companies maintain large design staffs that can quickly and competentlyexecute projects at relatively low cost.During the detailed design stage there may still be some changes to the design, andtherewill certainly be ongoing optimization as a better idea of the project cost structureis developed. The detailed design decisions tend to focus mainly on equipment selec-tionthough, rather than on changes to the flowsheet. For example, the design engineermay need to decide whether to use a U-tube or a floating-head exchanger, as discussedin Chapter 12, or whether to use trays or packing for a distillation column, as describedin Chapter 11.1.2.7. Procurement, Construction, and OperationWhen the details of the design have been finalized, the equipment can be purchased andthe plant can be built. Procurement and construction are usually carried out by an EPCfirm unless the project is very small. Because they work on many different projects eachyear, the EPC firms are able to place bulk orders for items such as piping, wire, valves,etc., and can use their purchasing power to get discounts on most equipment. The EPCcompanies also have a great deal of experience in field construction, inspection, testing,and equipment installation. They can therefore normally contract to build a plant for aclient cheaper (and usually also quicker) than the client could build it on its own.Finally, once the plant is built and readied for startup, it can begin operation. Thedesign engineer will often then be called upon to help resolve any startup issues andteething problems with the new plant.1.3. THE ANATOMY OF A CHEMICAL MANUFACTURING PROCESSThe basic components of a typical chemical process are shown in Figure 1.3, in whicheach block represents a stage in the overall process for producing a product from theraw materials. Figure 1.3 represents a generalized process; not all the stages willbe needed for any particular process, and the complexity of each stage will dependon the nature of the process. Chemical engineering design is concerned with theselection and arrangement of the stages and the selection, specification, and designof the equipment required to perform the function of each stage.RawmaterialstorageFeedpreparationReactionProductseparationProductpurificationProductstorageSalesRecycle of unreactedmaterialBy-productsWastesStage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6Figure 1.3. Anatomy of a chemical process. 29. 1.3. THE ANATOMY OF A CHEMICAL MANUFACTURING PROCESS 9Stage 1. Raw material storage: Unless the raw materials (also called feed stocks orfeeds) are supplied as intermediate products (intermediates) from a neighboringplant, some provision will have to be made to hold several days or weeks worthof storage to smooth out fluctuations and interruptions in supply. Even when thematerials come from an adjacent plant, some provision is usually made to hold afew hours or even days worth of inventory to decouple the processes. Thestorage required depends on the nature of the raw materials, the method ofdelivery, and what assurance can be placed on the continuity of supply. Ifmaterials are delivered by ship (tanker or bulk carrier), several weeks stocksmay be necessary, whereas if they are received by road or rail, in smaller lots, lessstorage will be needed.Stage 2. Feed preparation: Some purification and preparation of the raw materialswill usually be necessary before they are sufficiently pure, or in the right form, tobe fed to the reaction stage. For example, acetylene generated by the carbideprocess contains arsenic and sulfur compounds, and other impurities, whichmust be removed by scrubbing with concentrated sulfuric acid (or other pro-cesses)before it is sufficiently pure for reaction with hydrochloric acid toproduce dichloroethane. Feed contaminants that can poison process catalysts,enzymes, or micro-organisms must be removed. Liquid feeds need to be vapor-izedbefore being fed to gas-phase reactors and solids may need crushing,grinding, and screening.Stage 3. Reaction: The reaction stage is the heart of a chemical manufacturingprocess. In the reactor the raw materials are brought together under conditionsthat promote the production of the desired product; almost invariably, somebyproducts will also be formed, either through the reaction stoichiometry, byside reactions, or from reactions of impurities present in the feed.Stage 4. Product separation: After the reactor(s) the products and byproducts areseparated from any unreacted material. If in sufficient quantity, the unreactedmaterial will be recycled to the reaction stage or to the feed purification andpreparation stage. The byproducts may also be separated from the products atthis stage. In fine chemical processes there are often multiple reaction steps, eachfollowed by one or more separation steps.Stage 5. Purification: Before sale, the main product will often need purification tomeet the product specifications. If produced in economic quantities, the bypro-ductsmay also be purified for sale.Stage 6. Product storage: Some inventory of finished product must be held tomatch production with sales. Provision for product packaging and transport isalso needed, depending on the nature of the product. Liquids are normallydispatched in drums and in bulk tankers (road, rail, and sea); solids in sacks,cartons, or bales. 30. 10 CHAPTER 1 INTRODUCTION TO DESIGNThe amount of stock that is held will depend on the nature of the product and themarket.Ancillary ProcessesIn addition to the main process stages shown in Figure 1.3, provision must be madefor the supply of the services (utilities) needed, such as process water, cooling water,compressed air, and steam. Facilities are also needed for maintenance, firefighting,offices and other accommodation, and laboratories; see Chapter 14.1.3.1. Continuous and Batch ProcessesContinuous processes are designed to operate 24 hours a day, 7 days a week, through-outthe year. Some downtime will be allowed for maintenance and, for some processes,catalyst regeneration. The plant attainment or operating rate is the percentage of theavailable hours in a year that the plant operates, and is usually between 90 and 95%.Attainment % hours operated8760 100Batch processes are designed to operate intermittently, with some, or all, of theprocess units being frequently shut down and started up. It is quite common for batchplants to use a combination of batch and continuous operations. For example, a batchreactor may be used to feed a continuous distillation column.Continuous processes will usually be more economical for large-scale production.Batch processes are used when some flexibility is wanted in production rate orproduct specifications.The advantages of batch processing areA. Batch processing allows production of multiple different products or differentproduct grades in the same equipment.B. In a batch plant, the integrity of a batch is preserved as it moves from operationto operation. This can be very useful for quality control purposes.C. The production rate of batch plants is very flexible, as there are no turn-downissues when operating at low output.D. Batch plants are easier to clean and maintain sterile operation.E. Batch processes are easier to scale up from chemists recipes.F. Batch plants have low capital for small production volumes. The same piece ofequipment can often be used for several unit operations.The drawbacks of batch processing areA. The scale of production is limited.B. It is difficult to achieve economies of scale by going to high production rates.C. Batch-to-batch quality can vary, leading to high production of waste productsor off-spec product.D. Recycle and heat recovery are harder, making batch plants less energy efficientand more likely to produce waste byproducts. 31. 1.4. THE ORGANIZATION OF A CHEMICAL ENGINEERING PROJECT 11E. Asset utilization is lower for batch plants, as the plant almost inevitably is idlepart of the time.F. The fixed costs of production are much higher for batch plants on a $/unit massof product basis.Choice of Continuous versus Batch ProductionGiven the higher fixed costs and lower plant utilization of batch processes, batchprocessing usually makes sense only for products that have high value and areproduced in small quantities. Batch plants are commonly used for Food products Pharmaceutical products such as drugs, vaccines, and hormones Personal care products Specialty chemicalsEven in these sectors, continuous production is favored if the process is wellunderstood, the production volume is large, and the market is competitive.1.4. THE ORGANIZATION OF A CHEMICAL ENGINEERING PROJECTThe design work required in the engineering of a chemical manufacturing process canbe divided into two broad phases.Phase 1: Process design, which covers the steps from the initial selection of theprocess to be used, through to the issuing of the process flowsheets and includesthe selection, specification, and chemical engineering design of equipment. In atypical organization, this phase is the responsibility of the Process Design Group,and the work is mainly done by chemical engineers. The process design groupmay also be responsible for the preparation of the piping and instrumentationdiagrams.Phase 2: Plant design, including the detailed mechanical design of equipment;the structural, civil, and electrical design; and the specification and design ofthe ancillary services. These activities will be the responsibility of specialistdesign groups, having expertise in the whole range of engineering disciplines.Other specialist groups will be responsible for cost estimation, and the purchaseand procurement of equipment and materials.The sequence of steps in the design, construction and startup of a typical chemicalprocess plant is shown diagrammatically in Figure 1.4, and the organization of atypical project group is shown in Figure 1.5. Each step in the design process will notbe as neatly separated from the others as is indicated in Figure 1.4, nor will thesequence of events be as clearly defined. There will be a constant interchange ofinformation between the various design sections as the design develops, but it is clearthat some steps in a design must be largely completed before others can be started.A project manager, often a chemical engineer by training, is usually responsible forthe coordination of the project, as shown in Figure 1.5. 32. 12 CHAPTER 1 INTRODUCTION TO DESIGNProject specificationInitial evaluation.Process selection.Preliminary flow diagrams.Material and energy balances.Preliminary equipment selectionand design.Process flow-sheeting.Preliminary cost estimation.Authorisation of funds.Detailed process design.Flow-sheets.Chemical engineering equipmentdesign and specifications.Reactors, Unit operations, Heat exchangers,Miscellaneous equipment.Materials selection.Process manualsPiping and instrument designInstrument selectionand specificationPumps and compressors.Selection and specificationVessel design Heat exchanger design Utilities and other services.Design and specificationElectrical,Motors, switch gear,substations, etc.Piping design Structural design Plant layout General civil work.Foundations, drains,roads, etc.Buildings.Offices, laboratories,control rooms, etc.Project cost estimation.Capital authorisationPurchasing/procurement Raw material specification.(contracts)ConstructionStart-up Operating manualsOperationSalesFigure 1.4. The structure of a chemical engineering project. 33. 1.5. PROJECT DOCUMENTATION 13Process sectionProcess evaluationFlow-sheetingEquipment specificationsProjectmanagerSpecialist design sectionsVesselsConstruction sectionConstructionStart-upLayout PipingvalvesElectricalControland instrumentsCompressorsand turbinespumpsProcurementsectionEstimatingInspectionSchedulingHeat exchangersfired heatersCivil workstructuresbuildingsUtilitiesFigure 1.5. Project organization.As was stated in Section 1.2.1, the project design should start with a clear specifi-cationdefining the product, capacity, raw materials, process, and site location. If theproject is based on an established process and product, a full specification can bedrawn up at the start of the project. For a new product, the specification will bedeveloped from an economic evaluation of possible processes, based on laboratoryresearch, pilot plant tests and product market research.Some of the larger chemical manufacturing companies have their own projectdesign organizations and carry out the whole project design and engineering, andpossibly construction, within their own organization. More usually, the design andconstruction, and possibly assistance with startup, are entrusted to one of the inter-nationalEngineering, Procurement, and Construction contracting firms.The technical know-how for the process could come from the operating com-panyor could be licensed from the contractor or a technology vendor. The operatingcompany, technology provider, and contractor will work closely together throughoutall stages of the project.On many modern projects, the operating company may well be a joint venturebetween several companies. The project may be carried out between companies basedin different parts of the world. Good teamwork, communications, and project man-agementare therefore critically important in ensuring that the project is executedsuccessfully.1.5. PROJECT DOCUMENTATIONAs shown in Figure 1.5 and described in Section 1.4, the design and engineering ofa chemical process requires the cooperation of many specialist groups. Effective 34. 14 CHAPTER 1 INTRODUCTION TO DESIGNcooperation depends on effective communications, and all design organizations haveformal procedures for handling project information and documentation. The projectdocumentation will include1. General correspondence within the design group and withGovernment departmentsEquipment vendorsSite personnelThe client2. Calculation sheetsDesign calculationsCost estimatesMaterial and energy balances3. DrawingsFlowsheetsPiping and instrumentation diagramsLayout diagramsPlot/site plansEquipment detailsPiping diagrams (isometrics)Architectural drawingsDesign sketches4. Specification sheetsThe design basisFeed and product specificationsAn equipment listSheets for equipment, such as heat exchangers, pumps, heaters, etc.5. Health, Safety and Environmental information:Materials safety data sheets (MSDS forms)HAZOP or HAZAN documentation (see Chapter 9)Emissions assessments and permits6. Purchase ordersQuotationsInvoicesAll documents are assigned a code number for easy cross-referencing, filing, andretrieval.Calculation SheetsThe design engineer should develop the habit of setting out calculations so that theycan be easily understood and checked by others. It is good practice to include oncalculation sheets the basis of the calculations, and any assumptions and approxima-tionsmade, in sufficient detail for the methods, aswell as the arithmetic, to be checked.Design calculations are normally set out on standard sheets. The heading at the top ofeach sheet should include the project title and identification number, the revisionnumber and date and, most importantly, the signature (or initials) of the person who 35. 1.5. PROJECT DOCUMENTATION 15checked the calculation. A template calculation sheet is given in Appendix G and canbe downloaded in MS Excel format from the online material at http://books.elsevier.com/companions.DrawingsAll project drawings are normally drawn on specially printed sheets, with the companyname, project title and number, drawing title and identification number, draftersname and person checking the drawing, clearly set out in a box in the bottom-rightcorner. Provision should also be made for noting on the drawing all modifications tothe initial issue.Drawings should conform to accepted drawing conventions, preferably those laiddown by the national standards. The symbols used for flowsheets and piping andinstrument diagrams are discussed in Chapters 4 and 5. In most design offices, com-puter-aided design (CAD) methods are now used to produce the drawings requiredfor all the aspects of a project: flowsheets, piping and instrumentation, mechanical andcivil work. While the released versions of drawings are usually drafted by a profes-sional,the design engineer will often need to mark up changes to drawings or makeminor modifications to flowsheets, so it is useful to have some proficiency with thedrafting software.Specification SheetsStandard specification sheets are normally used to transmit the information requiredfor the detailed design, or purchase, of equipment items, such as heat exchangers,pumps, columns, pressure vessels, etc.As well as ensuring that the information is clearly and unambiguously presented,standard specification sheets serve as check lists to ensure that all the informationrequired is included.Examples of equipment specification sheets are given in Appendix G. Thesespecification sheets are referenced and used in examples throughout the book.Blank templates of these specification sheets are available in MS Excel format in theonline material at http://books.elsevier.com/companions. Standard worksheets arealso often used for calculations that are commonly repeated in design.Process ManualsProcess manuals are usually prepared by the process design group to describe theprocess and the basis of the design. Together with the flowsheets, they provide acomplete technical description of the process.Operating ManualsOperating manuals give the detailed, step-by-step instructions for operation of theprocess and equipment. They would normally be prepared by the operating companypersonnel, but may also be issued by a contractor or technology licensor as part of thetechnology transfer package for a less-experienced client. The operating manuals areused for operator instruction and training and for the preparation of the formal plantoperating instructions. 36. 16 CHAPTER 1 INTRODUCTION TO DESIGN1.6. CODES AND STANDARDSThe need for standardization arose early in the evolution of the modern engineeringindustry; Whitworth introduced the first standard screw thread to give a measure ofinterchangeability between different manufacturers in 1841.Modern engineering stand-ardscover a much wider function than the interchange of parts. In engineering practicethey cover1. Materials, properties, and compositions.2. Testing procedures for performance, compositions, and quality.3. Preferred sizes; for example, tubes, plates, sections, etc.4. Methods for design, inspection, and fabrication.5. Codes of practice for plant operation and safety.The terms standard and code are used interchangeably, though code should reallybe reserved for a code of practice covering, say, a recommended design or operatingprocedure; and standard for preferred sizes, compositions, etc.All of the developed countries and many of the developing countries have nationalstandards organizations, which are responsible for the issue and maintenance of stand-ardsfor the manufacturing industries and for the protection of consumers. In theUnited States, the government organization responsible for coordinating informationon standards is the National Bureau of Standards; standards are issued by federal, state,and various commercial organizations. The principal ones of interest to chemicalengineers are those issued by the American National Standards Institute (ANSI), theAmerican Petroleum Institute (API), the American Society for Testing Materials(ASTM), the American Society of Mechanical Engineers (ASME) (pressure vessels andpipes), the National Fire Protection Association (NFPA; safety), and the Instrumenta-tion,Systems and Automation Society (ISA; process control). Most Canadian provincesapply the same standards used in the United States. The preparation of the standards islargely the responsibility of committees of persons from the appropriate industry, theprofessional engineering institutions, and other interested organizations.The International Organization for Standardization (ISO) coordinates the publica-tionof international standards. The European countries used to maintain theirown national standards, but these are now being superseded by common Europeanstandards.Lists of codes and standards and copies of the most current versions can beobtained from the national standards agencies or by subscription from commercialwebsites such as I.H.S. (www.ihs.com).As well as the various national standards and codes, the larger design organizationswill have their own (in-house) standards. Much of the detail in engineering designwork is routine and repetitious, and it saves time and money, and ensures conformitybetween projects, if standard designs are used whenever practicable.Equipment manufacturers also work to standards to produce standardized designsand size ranges for commonly used items, such as electric motors, pumps, heat exchan-gers,pipes, and pipe fittings. They will conform to national standards, where they exist, 37. 1.7. DESIGN FACTORS (DESIGN MARGINS) 17or to those issued by trade associations. It is clearly more economic to produce a limitedrange of standard sizes than to have to treat each order as a special job.For the designer, the use of a standardized component size allows for the easyintegration of a piece of equipment into the rest of the plant. For example, ifa standard range of centrifugal pumps is specified, the pump dimensions will beknown, and this facilitates the design of the foundation plates, pipe connections, andthe selection of the drive motors: standard electric motors would be used.For an operating company, the standardization of equipment designs and sizesincreases interchangeability and reduces the stock of spares that must be held inmaintenance stores.Though there are clearly considerable advantages to be gained from the use ofstandards in design, there are also some disadvantages. Standards impose constraintson the designer. The nearest standard size will normally be selected on completing adesign calculation (rounding up), but this will not necessarily be the optimum size;though as the standard size will be cheaper than a special size, it will usually be thebest choice from the point of view of initial capital cost. The design methods given inthe codes and standards are, by their nature, historical, and do not necessarilyincorporate the latest techniques.The use of standards in design is illustrated in the discussion of the pressure vesseldesign in Chapter 13. Relevant design codes and standards are cited throughout thebook.1.7. DESIGN FACTORS (DESIGN MARGINS)Design is an inexact art; errors and uncertainties arise from uncertainties in thedesign data available and in the approximations necessary in design calculations.Experienced designers include a degree of over-design known as a design factor,design margin, or safety factor, to ensure that the design that is built meetsproduct specifications and operates safely.In mechanical and structural design, the design factors used to allow for uncertain-tiesin material properties, design methods, fabrication, and operating loads are wellestablished. For example, a factor of around 4 on the tensile strength, or about 2.5 onthe 0.1% proof stress, is normally used in general structural design. The recom-mendeddesign factors are set out in the codes and standards. The selection of designfactors in mechanical engineering design is illustrated in the discussion of pressurevessel design in Chapter 13.Design factors are also applied in process design to give some tolerance in thedesign. For example, the process stream average flows calculated from materialbalances are usually increased by a factor, typically 10%, to give some flexibility inprocess operation. This factor will set the maximum flows for equipment, instrumen-tation,and piping design. Where design factors are introduced to give some contin-gencyin a process design, they should be agreed upon within the project organizationand clearly stated in the project documents (drawings, calculation sheets, and man-uals).If this is not done, there is a danger that each of the specialist design groups will 38. 18 CHAPTER 1 INTRODUCTION TO DESIGNadd its own factor of safety, resulting in gross and unnecessary over-design.Companies often specify design factors in their design manuals.When selecting the design factor, a balance has to be made between the desire tomake sure the design is adequate and the need to design to tight margins to remaincompetitive. Greater uncertainty in the design methods and data requires the use ofbigger design factors.1.8. SYSTEMS OF UNITSMost of the examples and equations in this book use SI units; however, in practicethe design methods, data, and standards that the designer will use are often onlyavailable in the traditional scientific and engineering units. Chemical engineeringhas always used a diversity of units, embracing the scientific CGS and MKS systemsand both the American and British engineering systems. Those engineers in theolder industries will also have had to deal with some bizarre traditional units, suchas degrees Twaddle or degrees API for density and barrels for quantity. Althoughalmost all of the engineering societies have stated support for the adoption of SIunits, this is unlikely to happen worldwide for many years. Furthermore, muchuseful historic data will always be in the traditional units, and the design engineermust know how to understand and convert this information. In a globalizedeconomy, engineers are expected to use different systems of units even within thesame company, particularly in the contracting sector where the choice of units is atthe clients discretion. Design engineers must therefore have a familiarity with SI,metric, and customary units, and a few of the examples and many of the exercisesare presented in customary units.It is usually the best practice to work through design calculations in the units inwhich the result is to be presented; but, if working in SI units is preferred, data can beconverted to SI units, the calculation made, and the result converted to whatever unitsare required. Conversion factors to the SI system from most of the scientific andengineering units used in chemical engineering design are given in Appendix D.Some license has been taken in the use of the SI system. Temperatures are given indegrees Celsius (8C); degrees Kelvin are used only when absolute temperature isrequired in the calculation. Pressures are often given in bar (or atmospheres) ratherthan in Pascals (N=m2), as this gives a better feel for the magnitude of the pressures. Intechnical calculations the bar can be taken as equivalent to an atmosphere, whateverdefinition is used for atmosphere. The abbreviations bara and barg are often used todenote bar absolute and bar gauge, analogous to psia and psig when the pressure isexpressed in pound force per square inch. When bar is used on its own, withoutqualification, it is normally taken as absolute.For stress, N=mm2 have been used, as these units are now generally accepted byengineers, and the use of a small unit of area helps to indicate that stress is theintensity of force at a point (as is also pressure). The corresponding traditional unitfor stress is the ksi or thousand pounds force per square inch. For quantity, kmol are 39. 1.9. OPTIMIZATION 19TABLE 1.1. Approximate Conversions Between Customary Units and SI UnitsQuantity Customary Unit SI Unit Approx. ExactEnergy 1 Btu 1 kJ 1.05506Specific enthalpy 1 Btu/lb 2 kJ/kg 2.326Specific heat capacity 1 Btu/lb8F 4 kJ/kg8C 4.1868Heat transfer coeff. 1 Btu/ft2h8F 6 W/m2 8C 5.678Viscosity 1 centipoise 1mNs=m2 1.0001 lbf/ft h 0:4mNs=m2 0.4134Surface tension 1 dyne/cm 1 mN/m 1.000Pressure 1 lbf=in2 (psi) 7kN=m2 6.8941 atm 1 bar 1.01325105 N=m2Density 1 lb=ft3 16 kg=m3 16.01851g=cm3 1kg=m3Volume 1 US gal 3:8103 m3 3:7854103Flow rate 1 US gal/min 0:23m3=h 0.227Note:1 U.S. gallon 0.84 imperial gallons (UK)1 barrel (oil) 42 U.S. gallons0:16m3 (exact 0.1590)1 kWh 3.6 MJgenerally used in preference to mol, and for flow, kmol/h instead of mol/s, as this givesmore sensibly sized figures, which are also closer to the more familiar lb/h.For volume and volumetric flow, m3 and m3=h are used in preference to m3=s, whichgives ridiculously small values in engineering calculations. Liters per second are usedfor small flow rates, as this is the preferred unit for pump specifications.Where, for convenience, other than SI units have been used on figures or diagrams,the scales are also given in SI units, or the appropriate conversion factors are given inthe text. Where equations are presented in customary units, a metric equivalent isgenerally given.Some approximate conversion factors to SI units are given in Table 1.1. These areworth committing to memory, to give some feel for the units for those more familiarwith the traditional engineering units. The exact conversion factors are also shownin the table. A more comprehensive table of conversion factors is given inAppendix D.1.9. OPTIMIZATIONOptimization is an intrinsic part of design: the designer seeks the best, or optimum,solution to a problem.Many design decisions can be made without formally setting up and solvinga mathematical optimization problem. The design engineer will often rely on acombination of experience and judgment, and in some cases the best design will beimmediately obvious. Other design decisions have such a trivial impact on process 40. 20 CHAPTER 1 INTRODUCTION TO DESIGNcosts that it makes more sense to make a close guess at the answer than to properly setup and solve the optimization problem. In every design though, there will be severalproblems that require rigorous optimization. This section introduces the techniquesfor formulating and solving optimization problems, as well as some of the pitfalls thatare commonly encountered in optimization.In this book, the discussion of optimization will, of necessity, be limited to a briefoverview of the main techniques used in process and equipment design. Chemicalengineers working in industry use optimization methods for process operations farmore than they do for design, as discussed in Section 1.9.11. Chemical engineeringstudents would benefit greatly from more classes in operations research methods,which are generally part of the Industrial Engineering curriculum. These methods areused in almost every industry for planning, scheduling, and supply-chain manage-ment:all critical operations for plant operation and management. There is an exten-siveliterature on operations research methods and several good books on theapplication of optimization methods in chemical engineering design and operations.A good overview of operations research methods is given in the classic introductorytext by Hillier and Lieberman (2002). Applications of optimization methods inchemical engineering are discussed by Rudd and Watson (1968), Stoecker (1989),Biegler et al. (1997), Edgar and Himmelblau (2001), and Diwekar (2003).1.9.1. The Design ObjectiveAn optimization problem is always stated as the maximization or minimization of aquantity called the objective. For chemical engineering design projects, the objectiveshould be a measure of how effectively the design meets the customers needs. Thiswill usually be a measure of economic performance. Some typical objectives are givenin Table 1.2.The overall corporate objective is usually to maximize profits, but the designengineer will often find it more convenient to use other objectives when working onsubcomponents of the design. The optimization of subsystems is discussed in moredetail in Section 1.9.4.The first step in formulating the optimization problem is to state the objective as afunction of a finite set of variables, sometimes referred to as the decision variables:z f (x1, x2, x3, . . . , xn) (1:1)wherez objectivex1, x2, x3, . . . , xn decision variablesThis function is called the objective function. The decision variables may be inde-pendent,but they will usually be related to each other by many constraint equations.The optimization problem can then be stated as maximization or minimization of theobjective function subject to the set of constraints. Constraint equations are discussedin the next section. 41. 1.9. OPTIMIZATION 21Design engineers often face difficulties in formulating the objective function. Someof the economic objectives that are widely used in making investment decisions lead tointrinsically difficult optimization problems. For example, discounted cash flow rateof return (DCFROR) is difficult to express as a simple function and is highly non-linear,while net present value (NPV) increases with project size and is unboundedunless a constraint is set on plant size or available capital. Optimization is thereforeoften carried out using simple objectives such as minimize cost of production.Health, safety, environmental, and societal impact costs and benefits are difficult toquantify and relate to economic benefit. These factors can be introduced as con-straints,but few engineers would advocate building a plant in which every piece ofequipment was designed for the minimum legally permissible safety and environmen-talperformance.An additional complication in formulating the objective function is the quanti-ficationof uncertainty. Economic objective functions are generally very sensitive tothe prices used for feeds, raw materials, and energy, and also to estimates of projectcapital cost. These costs and prices are forecasts or estimates and are usuallysubject to substantial error. Cost estimation and price forecasting are discussedin Sections 6.3 and 6.4. There may also be uncertainty in the decision variables,either from variation in the plant inputs, variations introduced by unsteady plantoperation, or imprecision in the design data and the constraint equations. Opti-mizationunder uncertainty is a specialized subject in its own right and is beyondthe scope of this book. See Chapter 5 of Diwekar (2003) for a good introduction tothe subject.1.9.2. Constraints and Degrees of FreedomThe constraints on the optimization are the set of equations that bound the decisionvariables and relate them to each other.If we write x as a vector of n decision variables, then we can state the optimizationproblem asOptimize (Max: or Min:) z f (x)subject to (s:t:): g(x) # 0h(x) 0(1:2)TABLE 1.2. Typical Design Optimization ObjectivesMaximize MinimizeProject net present value Project expenseReturn on investment Cost of productionReactor productivity per unit volume Total annualized costPlant availability (time on stream) Plant inventory (for safety reasons)Process yield of main product Formation of waste products 42. 22 CHAPTER 1 INTRODUCTION TO DESIGNwherez the scalar objectivef(x) the objective functiong(x) a mi vector of inequality constraintsh(x) a me vector of equality constraintsThe total number of constraints is m mi me.Equality constraints arise from conservation equations (mass, mole, energy, andmomentum balances) and constitutive relations (the laws of chemistry and physics,correlations of experimental data, design equations, etc.). Any equation that is intro-ducedinto theoptimizationmodel that contains an equal () sign will becomean equalityconstraint. Many examples of such equations can be found throughout this book.Inequality constraints generally arise from the external constraints discussed inSection 1.2: safety limits, legal limits, market and economic limits, technical limitsset by design codes and standards, feed and product specifications, availability ofresources, etc. Some examples of inequality constraints might includeMain product purity $ 99:99 wt%Feed water content # 20 ppmwNOx emissions # 50 kg=yrProduction rate # 400,000 metric tons per yearMaximum design temperature for ASME Boiler and Pressure Vessel Code SectionVIII Division 2 # 9008FInvestment capital # $50MM (50 million dollars)The effect of constraints is to limit the parameter space. This can be illustratedusing a simple two-parameter problem:Max: z x21 2x22s:t: x1 x2 5x2 # 3The two constraints can be plotted on a graph of x1 vs. x2, as in Figure 1.6.In the case of this example, it is clear by inspection that the set of constraints doesnot bound the problem. In the limit x1 !1, the solution to the equality constraint isx2 !1, and the objective function gives z!1, so no maximum can be found.Problems of this kind are referred to as unbounded. For this problem to have asolution, we need an additional constraint of the formx1 # a (where a2)x2 $ b (where b3)orh(x1, x2) 0to define a closed search space. 43. 1.9. OPTIMIZATION 23x25302 + 2x2Max z = x12s.t. x1 + x2 = 5x2 35 x1The inequality constraintlimits us to values on orbelow this lineThe equality constraintlimits us to valueson this lineFigure 1.6. Constraints on a simple optimization problem.It is also possible to overconstrain the problem. For example, if we set the problemMax: z x12 2x22s:t: x1 x2 5x2 # 3x1 # 1In this case, it can be seen from Figure 1.7 that the feasible region defined bythe inequality constraints does not contain any solution to the equality constraint.The problem is therefore infeasible as stated.Degrees of FreedomIf the problem has n variables and me equality constraints, then it has nme degreesof freedom. If n me then there are no degrees of freedom and the setof me equations can be solved for the n variables. If men, then the problem isx25302 + 2x2Max z = x15 x12s.t. x1 + x2 = 5x2 3x1 1The feasible regiondefined by the inequalitieshas no solution for theequality constraintFigure 1.7. An over-constrained problem. 44. 24 CHAPTER 1 INTRODUCTION TO DESIGNoverspecified. In most cases, however,men and nme is the number of parametersthat can be independently adjusted to find the optimum.When inequality constraints are introduced into the problem, they generally setbounds on the range over which parameters can be varied and hence reduce the spacein which the search for the optimum is carried out. Very often, the optimum solutionto a constrained problem is found to be at the edge of the search space, i.e., at one ofthe inequality constraint boundaries. In such cases, that inequality constraint becomesequal to zero and is said to be active. It is often possible to use engineering insightand understanding of chemistry and physics to simplify the optimization problem. Ifthe behavior of a system is well understood, then the design engineer can decide thatan inequality constraint is likely to be active. Converting the inequality constraint intoan equality constraint reduces the number of degrees of freedom by one and makes theproblem simpler.This can be illustrated by a simple reactor optimization example. The size and costof a reactor are proportional to residence time, which decreases as temperature isincreased. The optimal temperature is usually a trade-off between reactor cost and theformation of byproducts in side reactions; but if there were no side reactions, then thenext constraint would be the maximum temperature allowed by the pressure vesseldesign code. More expensive alloys might allow for operation at higher temperatures.The variation of reactor cost with temperature will look something like Figure 1.8,where TA, TB, and TC are the maximum temperatures allowed by the vessel designcode for alloys A, B, and C, respectively.The design engineer could formulate this problem in several ways. It could be solvedas three separate problems, one corresponding to each alloy, each with a constraint ontemperature TTalloy. The design engineer would then pick the solution that gave thebest value of the objective function. The problem could also be formulated as a mixedinteger nonlinear program with integer variables to determine the selection of alloyand set the appropriate constraint (see Section 1.9.10). The design engineer could alsorecognize that alloy A costs a lot less than alloy B, and the higher alloys give only arelatively small extension in the allowable temperature range. It is clear that costdecreases with temperature, so the optimum temperature will be TA for alloy A andReactorcostAlloy A B CTA TB TCTemperatureThere is a step changein cost when a higheralloy is neededFigure 1.8. Variation of reactor cost with temperature. 45. 1.9. OPTIMIZATION 25TB for alloy B. Unless the design engineer is aware of some other effect that has animpact on cost as temperature is increased, it is safe to write T TA as an equalityconstraint and solve the resulting problem. If the cost of alloy B is not excessive, then itwould be prudent to also solve the problem with T TB, using the cost of alloy B.The correct formulation of constraints is the most important step in setting up anoptimization problem. Inexperienced engineers are often unaware of many con-straintsand consequently find optimal designs that are dismissed as unfeasible bymore experienced designers.1.9.3. Trade-OffsIf the optimal value of the objective is not at a constraint limit, then it will usually bedetermined by a trade-off between two or more effects. Trade-offs are very commonin design, because better performance in terms of increased purity, increased recovery,or reduced energy or raw materials use usually comes at the expense of higher capitalexpense, operating expense, or both. The optimization problem must capture thetrade-off between cost and benefit.A well-known example of a trade-off is the optimization of process heatrecovery. A high degree of heat recovery requires close temperature approaches inthe heat exchangers (see Section 3.17), which leads to high capital cost as the exchang-ersrequire more surface area. If the minimum temperature approach is increased, thenthe capital cost is reduced but less energy is recovered.We can plot the capital cost andenergy cost against the minimum approach temperature, as shown schematically inFigure 1.9. If the capital cost is annualized (see Section 6.7), then the two costs can beadded to give a total cost. The optimum value of the approach temperature, DToptimum,is then given by the minimum point in the total cost curve.Some common trade-offs encountered in design of chemical plants include More separations equipment and operating cost vs. lower product purity; More recycle costs vs. increased feed use and waste formation; More heat recovery vs. cheaper heat exchange network; Higher reactivity at high pressure vs. more expensive reactors and highercompression costs;Cost ToptimumTotal CostEnergy CostCapital CostMinimum approachtemperatureFigure 1.9. The capital-energy trade-off in process heat recovery. 46. 26 CHAPTER 1 INTRODUCTION TO DESIGN Fast reactions at high temperature vs. product degradation; Marketable byproducts vs. more plant expense; Cheaper steam and electricity vs. more off-site capital cost.Stating an optimization problem as a trade-off between two effects is often usefulin conceptualizing the problem and interpreting the optimal solution. For example, inthe case of process heat recovery, it is usually found that the shape of the total costcurve in Figure 1.9 is relatively flat over the range 15CDToptimum40C. Know-ingthis, most experienced designers would not worry about finding the value ofDToptimum, but would instead select a value for the minimum temperature approachwithin the range 158C to 408C, based on knowledge of the customers preference forhigh energy efficiency or low capital expense.1.9.4. Problem DecompositionThe task of formally optimizing the design of a complex processing plant involvingseveral hundred variables, with complex interactions, is formidable, if not impossible.The task can be reduced by dividing the process into more manageable units, identi-fyingthe key variables and concentrating work where the effort involved will give thegreatest benefit. Subdivision and optimization of the subunits rather than the wholewill not necessarily give the optimum design for the whole process. The optimizationof one unit may be at the expense of another. For example, it will usually besatisfactory to optimize the reflux ratio for a fractionating column independentlyof the rest of the plant; but if the column is part of a separation stage following areactor, in which the product is separated from the unreacted materials, then thedesign of the column will interact with, and may well determine, the optimization ofthe reactor design. Care must always be taken to ensure that subcomponents are notoptimized at the expense of other parts of the plant.1.9.5. Optimization of a Single Decision VariableIf the objective is a function of a single variable, x, the objective function f(x) can bedifferentiated with respect to x to give f (x). Any stationary points in f(x) can then befound as the solutions of f (x) 0. If the second derivative of the objective function isgreater than zero at a stationary point, then the stationary point is a local minimum. Ifthe second derivative is less than zero, then the stationary point is a local maximum;and if it is equal to zero, then it is a saddle point. If x is bounded by constraints, thenwe must also check the values of the objective function at the upper and lower limitingconstraints. Similarly, if f(x) is discontinuous, then the value of f(x) on either side ofthe discontinuity should also be checked.This procedure can be summarized as the following algorithm:Min: z f(x)s:t: x $ xLx # xU(1:3) 47. 1.9. OPTIMIZATION 271. Solve f 0 df (x)dx 0 to find values of xS.2. Evaluate f 00 d2f (x)dx2 for each value of xS. If f 0, then xS corresponds to alocal minimum in f(x).3. Evaluate f (xS), f (xL), and f (xU).4. If the objective function is discontinuous, then evaluate f(x) on either side of thediscontinuity, xD1 and xD2.5. The overall optimum is the value from the set (xL, xS, xD1, xD2, xU) that givesthe lowest value of f(x).This is illustrated graphically in Figure 1.10a for a continuous objective function.In Figure 1.10a, xL is the optimum point, even though there is a local minimum at xS1.Figure 1.10b illustrates the case of a discontinuous objective function. Discontinuousfunctions are quite common in engineering design, arising, for example, whenchanges in temperature or pH cause a change in metallurgy. In Figure 1.10b theoptimum is at xD1, even though there is a local minimum at xS.If the objective function can be expressed as a differentiable equation, then it isusually also easy to plot a graph like those in Figure 1.10 and quickly determinewhether the optimum lies at a stationary point or a constraint.1.9.6. Search MethodsIn design problems, the objective function very often cannot be written as a simpleequation that is easily differentiated. This is particularly true when the objectivefunction requires solving large computer models, possibly using several differentprograms and requiring several minutes, hours, or days to converge a single solution.In such cases, the optimum is found using a search method. The concept of searchmethods is most easily explained for single variable problems, but search methods areat the core of the solution algorithms for multivariable optimization as well.Unrestricted SearchIf the decision variable is not bounded by constraints, then the first step is todetermine a range in which the optimum lies. In an unrestricted search we make anxL xUxzxD2xD1xSxL xUxzxS1xS2(a) (b)Figure 1.10. (a, b) Optimization of a single variable between bounds. 48. 28 CHAPTER 1 INTRODUCTION TO DESIGNinitial guess of x and assume a step size, h. We then calculate z1 f (x), z2 f (x h),and z3 f (xh). From the values of z1, z2, and z3, we determine the direction ofsearch that leads to improvement in the value of the objective, depending on whetherwe wish to minimize or maximize z.We then continue increasing (or decreasing) x bysuccessive steps of h until the optimum is passed.In some cases, it may be desirable to accelerate the search procedure, in which casethe step size can be doubled at each step. This gives the sequence f(x h), f(x 3h),f(x 7h), f(x 15h), etc.Unrestricted searching is a relatively simple method of bounding the optimum forproblems that are not constrained. In engineering design problems, it is almost alwayspossible to state upper and lower bounds for every parameter, so unrestricted searchmethods are not widely used in design.Once a restricted range that contains the optimum has been established, thenrestricted range search methods can be used. These can be broadly classified as directmethods that find the optimum by eliminating regions in which it does not lie, andindirect methods that find the optimum by making an approximate estimate of f (x).Regular Search (Three-Point Interval Search)The three-point interval search starts by evaluating f(x) at the upper and lowerbounds, xL and xU, and at the center point (xL xU)/2. Two new points are thenadded in the midpoints between the bounds and the center point, at (3xL xU)/4 and(xL 3xU)/4, as shown in Figure 1.11. The three adjacent points with the lowestvalues of f(x) (or the highest values for a maximization problem) are then used todefine the next search range.By eliminating two of the four quarters of the range at each step, this procedurereduces the range by half each cycle. To reduce the range to a fraction of the initialrange therefore takes n cycles, where 0:5n. Since each cycle requires calculating f(x)for two additional points, the total number of calculations is 2n 2 log = log 0:5.The procedure is terminated when the range has been reduced sufficiently to givethe desired precision in the optimum. For design problems, it is usually not necessaryto specify the optimal value of the decision variables to high precision, so is usuallynot a very small number.f (x)xxL xUx xFigure 1.11. Regular search. 49. 1.9. OPTIMIZATION 29Golden-Section SearchThe golden-section search, sometimes called the golden-mean search, is as simple toimplement as the regular search, but is more computationally efficient if 0:29.In the golden-section search, only one new point is added at each cycle.The golden-section method is illustrated in Figure 1.12. We start by evaluatingf (xL) and f (xU) corresponding to the upper and lower bounds of the range, labeled Aand B in the figure. We then add two new points, labeled C and D, each located adistance vAB from the bounds A and B, i.e., located at xL v(xUxL) andxUv(xUxL). For a minimization problem, the point that gives the highest valueof f(x) is eliminated. In Figure 1.12, this is point B. A single new point, E, is added,such that the new set of points AECD is symmetric with the old set of points ACDB.For the new set of points to be sym