Advances in Distillation Retrofit

Nguyen Van Duc Long · Moonyong Lee

Advances in DistillationRetrofit

Nguyen Van Duc LongSchool of Chemical EngineeringYeungnam UniversityGyeongsanSouth Korea

Moonyong LeeSchool of Chemical EngineeringYeungnam UniversityGyeongsanSouth Korea

ISBN 978-981-10-5899-8 ISBN 978-981-10-5901-8 (eBook)DOI 10.1007/978-981-10-5901-8

Library of Congress Control Number: 2017954336

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Preface

As a thermal separation method, distillation is considered the most important andmature separation technology in the chemical process industry. While this unitoperation has many advantages, one major drawback is its large energy require-ment, which significantly influence the overall plant profitability. Therefore, theincreasing cost of energy has forced industry to reduce its energy consumption. Inaddition, many processes are required to increase capacity due to an increase ofdemand. To accomplish these tasks, retrofit of distillation processes become animportant issue. To carry out effectively retrofit, an innovation solution is the useof advanced process integration and process intensification techniques, which hasbeen a rapid growth in technological and commercial opportunities. In particular,multi-effect distillation, heat pump assisted distillation are the best examples ofproven process integration technology, whereas thermally coupled distillationsequence (TCDS), dividing wall column (DWC), and reactive distillation are thebest examples of proven process intensification technology in distillation becausethey have significantly lower investment and operating costs while also reducingthe equipment and carbon footprint.

How can one decide quickly what techniques and whether they are good pro-cess concepts for distillation retrofit? This field requires a book that will capturean accurate snapshot regarding the field and provide an insightful review to all thekey techniques in the revolution in distillation retrofit. Considering that there arealready books dealing with known and basic issue in distillation, this book aims toprovide the readers with some issues related to retrofit of distillation usingadvanced distillation techniques, emphasizing the use of multi-effect distillation,heat pump assisted distillation, TCDS, DWC, reactive distillation, and innovativehybrid systems, which have not been comprehensively discussed in the publishedbooks. Rather than prepare a textbook on advanced distillation in the usual andtraditional format, the authors considered writing a book that highlighted conceptsand practical applications rather than theory.

This book presents a comprehensive review of contemporary advanced techni-ques employed for enhancing the distillation process with the purpose is bridgethe gap between developers of advanced distillation design procedures and those

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ultimately use them. Thus this book is a source of information for undergraduateand postgraduate students of chemical engineering, practicing process designersand chemical engineers.

Several companies supported our work. In particular we would like to thankSamsung BP Chemicals, Samsung Cheil Industries, SKC, LG Chemical, andKOGAS.

Nguyen Van Duc LongMoonyong Lee

vi Preface

List of Abbreviations and Greek Letters

Abbreviations

A Area [ft2]ABDWC Azeotropic bottom dividing wall columnBDWC Bottom dividing wall columnBC Bare cost [$]BMC Updated bare module cost [$]C2 EthaneC3 PropaneC5+ GasolineCD Coordinate decent methodologyCE Cellulosic ethanolCO2 Carbon dioxideCOP Coefficient of performanceCsteam Cost of the steam [$]CCW Cost of cooling water [$]Celectricity Cost of electricity [$]Crefrigeration Cost of refrigeration [$]CGCC Column grand composite curveD Diameter [ft]DWC Dividing wall columnDWPC Dividing wall prefractionator configurationEo Overall efficiency [%]EMV Murphree stage efficiency [%]ECMD Enhanced Capacity Multiple DowncomerETDWC Extractive top dividing wall columnEPC Engineering, procurement and constructionF Feed flow [kg/hr]FEED Front-end engineering design

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FL Liquid flow [kg/hr]FV Vapor flow [kg/hr]FTCDC Fully thermally coupled distillation columnh Enthalpy [kJ/kg]HP High pressureiC4 IsobutaneK Distribution coefficientL Length [ft]LLE Liquid-liquid equilibriumLP Low pressureMD Multiple DowncomerMF Module factorMPF Material and pressure factorMVR Mechanical vapor recompressionN Number of traysnBuOH Normal butyl alcoholNGL Natural gas liquidnC4 Normal butaneNRTL Nonrandom two-liquidOp Operating cost [$]P Total vapor pressure of the solutionpi Partial pressure of component i in the vaporp′i Vapor pressure of pure component iPSA Pressure swing adsorptionPFMD Parallel Flow Multiple Downcomerq Feed thermal conditionR Reflux ratioRDWC Reactive dividing wall columnRSM Response surface methodologyR2 Coefficient of determinationS Area of the heat exchanger [ft2]SSC Side stream columnT Temperature [oC]TSA Temperature swing adsorptionTAC Total annual cost [$]TCDS Thermally coupled distillation sequenceTCEDS-SR Thermally coupled extractive distillation sequencesTCRDS Thermally coupled reactive distillation sequenceTDWC Top dividing wall columnVC Vapor compressionVLE Vapor-liquid equilibriumxi Composition of component i in the liquid phaseyi Composition of component i in the vapor phase

viii List of Abbreviations and Greek Letters

UF Update factorUNIQUAC Universal quasi-chemicalΔT Temperature difference

Greek letters

γ Liquid activity coefficientµi Chemical potential of component iα Relative volatilityλ Latent heat [kJ/kg]

ixList of Abbreviations and Greek Letters

Contents

1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Distillation Fundamentals and Principles . . . . . . . . . . . . . . . . . . . 1

1.2.1 Vapor Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.2 Vapor-Liquid Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . 41.2.3 Dew Point and Bubble Point . . . . . . . . . . . . . . . . . . . . . . 51.2.4 Equilibrium Flash Calculations . . . . . . . . . . . . . . . . . . . . 6

1.3 Distillation Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.3.1 Shortcut Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.3.2 Rigorous Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Distillation Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4.1 Azeotrope Search. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4.2 Physical Property Methods. . . . . . . . . . . . . . . . . . . . . . . . 91.4.3 Binary Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5 Distillation Process Retrofit/Revamp and Debottlenecking. . . . . . 101.5.1 Process Integration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.5.2 Process Intensification . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.5.3 Combination of Heat Integration and Process

Intensification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Retrofit and Debottlenecking by Modifying Column Internal . . . . . 172.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Types of Column Internal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.1 Trays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.2.2 Packings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.3 Column Internal Performances . . . . . . . . . . . . . . . . . . . . . 202.2.4 Criteria for Selection of Vapor-Liquid Contactors. . . . . . . 23

2.3 Retrofit and Debottlenecking by Modifying Column Internal. . . . 242.3.1 Using High Capacity/Efficiency Trays . . . . . . . . . . . . . . . 24

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2.3.2 Using High Capacity/Efficiency Packings. . . . . . . . . . . . . 312.3.3 Column Internal Structure Modifications . . . . . . . . . . . . . 322.3.4 Industrial Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.4 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.4.1 Existing Conventional Distillation . . . . . . . . . . . . . . . . . . 362.4.2 Retrofitted Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3 Promising Retrofit Technologies for Single Column. . . . . . . . . . . . . 433.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.2 Heat Pump Assisted Distillation . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.2.2 Working Principle of MVR . . . . . . . . . . . . . . . . . . . . . . . 453.2.3 MVR Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.2.4 Pros and Cons of MVRs . . . . . . . . . . . . . . . . . . . . . . . . . 473.2.5 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.3 Self-Heat Recuperation Technology . . . . . . . . . . . . . . . . . . . . . . 503.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.3.2 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.4 Feed Thermal Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.4.2 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.5 Side Reboiler and Side Condenser . . . . . . . . . . . . . . . . . . . . . . . 593.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.6 Operating Pressure Changing . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.7 Multi-Effect Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633.7.2 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.8 Cyclic Distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.8.2 Retrofit to Cyclic Distillation . . . . . . . . . . . . . . . . . . . . . . 67

3.9 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4 Promising Retrofit Technologies for Multi-Column System . . . . . . . 714.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.2 Re-Arranging the Distillation Sequence. . . . . . . . . . . . . . . . . . . . 71

4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.2.2 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.3 Prefractionator Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.3.2 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.4 Thermally Coupled Distillation Sequence . . . . . . . . . . . . . . . . . . 784.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

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4.4.2 Retrofit to Thermally Coupled Distillation Sequence . . . . 794.4.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.5 Alternative Coupled Schemes of the Petlyuk Column . . . . . . . . . 894.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

4.6 Dividing Wall Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.6.2 Design and Optimization of DWC . . . . . . . . . . . . . . . . . . 954.6.3 Retrofit Using DWC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964.6.4 Column Modification and Hardware. . . . . . . . . . . . . . . . . 1014.6.5 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

4.7 Dividing Wall Prefractionator Configuration . . . . . . . . . . . . . . . . 1134.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.7.2 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

4.8 Multi-Effect Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1144.8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

4.9 Combination of Internal and External Heat Integration . . . . . . . . 1194.9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1194.9.2 TCDS or DWC with Heat Pump or Self-Heat

Recuperation Technology . . . . . . . . . . . . . . . . . . . . . . . . 1194.9.3 TCDS or DWC with Multi-Effect . . . . . . . . . . . . . . . . . . 1224.9.4 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

4.10 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5 Promising Retrofit Technologies for NonconventionalDistillation Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.2 Batch Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

5.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.2.2 Retrofit of Batch Distillation . . . . . . . . . . . . . . . . . . . . . . 134

5.3 Side Stream Column. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355.3.2 Retrofit of a SSC to a DWC. . . . . . . . . . . . . . . . . . . . . . . 1375.3.3 Retrofit of a SSC to a Heat Pump Assisted Dividing Wall

Column (HPDWC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385.3.4 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

5.4 Pressure Swing Azeotropic Distillation . . . . . . . . . . . . . . . . . . . . 1445.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.4.2 Pressure Swing Azeotropic Distillation. . . . . . . . . . . . . . . 1455.4.3 Retrofit of Pressure Swing Sequence . . . . . . . . . . . . . . . . 146

5.5 Extractive Distillation Sequence . . . . . . . . . . . . . . . . . . . . . . . . . 1475.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.5.2 Retrofit Design and Optimization . . . . . . . . . . . . . . . . . . . 1495.5.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

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5.6 Azeotropic Distillation Sequence . . . . . . . . . . . . . . . . . . . . . . . . 1555.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1555.6.2 Retrofit of Azeotropic Distillation . . . . . . . . . . . . . . . . . . 158

5.7 Reactive Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595.7.2 Reactive Distillation Retrofit . . . . . . . . . . . . . . . . . . . . . . 1655.7.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

5.8 Hybrid System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1755.8.1 Hybrid Distillation-Membrane System . . . . . . . . . . . . . . . 1765.8.2 Hybrid Distillation-Extraction System . . . . . . . . . . . . . . . 1835.8.3 Hybrid Distillation-Adsorption System. . . . . . . . . . . . . . . 1865.8.4 Outlook on Hybrid Systems . . . . . . . . . . . . . . . . . . . . . . . 189

5.9 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

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About the Authors

Nguyen Van Duc Long is currently a Foreign Assistant Professor at YeungnamUniversity. He received his PhD in the Department of Chemical Engineering atYeungnam University in August, 2012. Subsequently, he joined Samsung CheilIndustries as a Senior Process Engineer. He has worked on many long-term indus-trial projects, which were collaborative efforts between his laboratory and industrialmanufacturers or engineering companies. These industrial projects mainly focus onprocess systems engineering, process retrofit and debottlenecking technologies, pro-cess integration and intensification technologies, dividing wall column, thermallycoupled distillation sequence, reactive distillation, extractive distillation, azeotropicdistillation, pressure swing distillation, batch distillation, heat pump assisted distilla-tion, self-heat recuperation technology, separation processes, reactive separationprocesses, sustainable chemical processes, exergy and energy efficiency, naturalgas processing, petroleum refinery, carbon capture, storage and utilization, carbondioxide emission reduction, gasification, biofuels and green energy, biochemicals,biomass conversion and biorefineries. He published 38 scientific articles in peer-reviewed journals, 3 review papers and 1 book chapter.

Moonyong Lee has been a professor at Yeungnam University, School ofChemical Engineering, since 1994. Before joining the university, he had workedas Senior Process Engineer in SK refinery and petrochemical company for 10years. He has an MSc and a PhD in Chemical Engineering from KAIST. Sinceentering the university, he has worked on many industrial projects related toprocess modeling, design, and control. His current research areas are design,modeling, optimization, and control of various energy and chemical processes(especially distillation based separation processes). He is the author of over 170publications in international journals and the recipient of the GoMyeungBokScientific Award from ICASE (2006), Conventional Chemical EngineeringSpecial Award from KIChE (2013), and Korean Government Minister Award forEngineering Industry Contribution (2016).

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Chapter 1Introduction

1.1 Introduction

Without distillation, there would be no modern living (Rosenzeig, 1997) because itis the most well-defined separation process that is used widely to produce the world’spetroleum fuels, to treat most natural gas, and is a critical element in a host of processesmaking the chemicals and other products that world needs (Sorensen and Darton,2006). The performance of these industries is strongly dependent on the engineeringand engineers. One typical drawback of distillation processes is that they require the lar-gest portion of energy in industrial separation technologies with an estimated 3% of theworld’s energy consumption (Hewitt et al., 1999). This can significantly influence theoverall plant profitability. Therefore, development of a new type of column and/or per-formance enhancement of existing distillation processes for improved energy efficiencyhave been imperative issues associated with distillation (Olujic et al., 2009). In addition,saving energy in this process is also an important issue from an environmental stand-point or carbon dioxide (CO2) mitigation because the huge amount of energy consumedin distillation has a large impact on greenhouse gas emissions (Matsuda et al., 2012).

A broad range from improving capacity, energy and separation efficiency usingprocess modification and/or process integration, such as multi-effect distillation orheat pump assisted distillation and/or process intensification, such as reactive distilla-tion, and/or innovative hybrid systems, such as configuration combining distillationand pervaporation, have created enormous concern. Basic knowledge of distillationis required to develop a new type of column and/or enhance the performance ofexisting distillation processes for improved separation and energy efficiency.

1.2 Distillation Fundamentals and Principles

The separation operation in distillation is based on intimate contact between thevapor and liquid phases using various kinds of devices, such as random or struc-tured packings and plates or trays. Because of the difference in gravity between

1© Springer Nature Singapore Pte Ltd. 2017N.V.D. Long, M. Lee, Advances in Distillation Retrofit, DOI 10.1007/978-981-10-5901-8_1

the vapor and liquid phases, liquid runs down the column, cascading from tray totray, while vapor flows up the column, contacting liquid at each tray (Seaderet al., 2008). Liquid reaching the bottom of the column is vaporized partially in aheated reboiler to provide boil-up, which is sent back up the column, while theremainder of the bottom liquid is withdrawn as the bottom product. Vapor reach-ing the top of the column is cooled and condensed to a liquid in the overhead con-denser. A part of this liquid is returned to the column as reflux to provide liquidoverflow, while the remainder of the overhead stream is withdrawn as distillate, oroverhead product. In some cases, only a part of the vapor is condensed so that avapor distillate can be withdrawn. Note that the vapor and liquid leaving an equili-brium stage are assumed to be in complete equilibrium with each other; thus, ther-modynamic relations can be used to determine the temperature and relate theconcentrations in the equilibrium stream at a given pressure (Seader et al., 2008).

The lighter (lower-boiling) components tend to concentrate in the vapor phase,whereas the heavier (higher-boiling) components tend toward the liquid phase(Seader et al., 2008). The result is a vapor phase that becomes richer in the lightercomponents as it passes up the column and a liquid phase that becomes richer inheavy components as it cascades downward. The overall separation achievedbetween the distillate and the bottom depends primarily on the relative volatilitiesof the components, the number of contacting trays, and the ratio of the liquidphase flow rate to the vapor-phase flow rate.

1.2.1 Vapor Pressure

The vapor pressure, which is an important property of liquids, is the pressure atwhich both liquid and vapor exist at a given temperature. At this point, dynamicequilibrium exists, in which vaporization and condensation take place at equalrates and the pressure in the vapor space remains constant (Theodore and Ricci,2010). The pressure at equilibrium is equal to the vapor pressure of the liquid. Thevapor pressure is related to the boiling temperature. In particular, liquids with ahigh vapor pressure boil at low temperatures while liquids with a low vapor pres-sure boil at high temperatures. The vapor pressure depends only on temperature(Fig. 1.1). In particular, the vapor pressure increases with increasing temperature.

Raoult’s law states that the partial pressure of each component (pi) in a solutionis proportional to the mole fraction (xi) of that component in the liquid mixturebeing studied with the “proportionality constant” being its vapor pressure ðp′iÞ(Theodore and Ricci, 2010). Therefore, for component i in a mixture, Raoult’s lawcan be expressed as

pi = p′ixi (1.1)

where pi is the partial pressure of component i in the vapor, p′i is the vapor pres-sure of pure component i at the same temperature, and xi is the mole fraction of

2 1 Introduction

component i in the liquid. This expression may be applied to all components sothat the total pressure, P, is given by the sum of all the partial pressures.

If the gas phase is ideal, Dalton’s law applies

pi = yiP (1.2)

where yi is the mole fraction of component i in the vapor, and P is the total vaporpressure of the solution, which can be calculated as follows:

P=XNC

i=1

pi =XNC

i=1

pi′xi (1.3)

Therefore, Eq. (1.1) can then be written as

yi = xipi′P

� �(1.4)

Raoult’s law applications require vapor pressure information, which can befound in the literature (Green and Perry, 2008). However, there are twoequations that can be used in lieu of the vapor pressure information, theClapeyron equation and the Antoine equation (Theodore et al., 2009). The twoconstant (A, B) Clapeyron equation is given by

ln p′=A−B

T(1.5)

where p′ and T are the vapor pressure and temperature, respectively. The threeconstant (A, B, C) Antoine equation is given by

ln p′=A−B

T +C(1.6)

Fig. 1.1 Vapor pressure of a pure component

31.2 Distillation Fundamentals and Principles

1.2.2 Vapor-Liquid Equilibrium

The equilibrium principles between the vapor and liquid require understandingbecause the separation of chemical mixtures in distillation is based on the phaseequilibrium behavior. They play an important role in designing and predictingthe performance of distillation processes. Consider the vapor–liquid equilibriumof mixtures, where each component appears in both phases. At thermodynamicequilibrium,

PV =PL (1.7)

TV = TL (1.8)

μVi = μLi (1.9)

where P, T and µi are the pressure, temperature, and chemical potential of compo-nent i, respectively.

Because the pressure and temperature fix the equilibrium vapor and liquid com-positions, the experimental data is frequently presented in the form of tables of thevapor mole fraction y and liquid mole fraction, x, for one constituent over a rangeof temperatures, T, for a fixed pressure, P, or over a range of pressures for a fixedtemperature (Seader et al., 2008).

The distribution of a component in two phases is represented by the distributioncoefficient, K, as follows:

K =yixi

(1.10)

where yi and xi are the composition of component i in the vapor phase and liquidphase, respectively. A component with a high K-value tends to concentrate in thevapor, while it tends to concentrate in the liquid with a low K-value. The K-valueis a function of temperature and pressure. The K values are used widely in distilla-tion calculations, and the ratio of the K values of two components is defined as therelative volatility,

αij =Ki

Kj=

yi=xiyj=xj

(1.11)

The relative volatility is a measure of the ease of separation, in particular, thehigher it is, the easier the separation. For binary mixtures:

α12 =K1

K2=

y1=x1y2=x2

=y1ð1− x1Þx1ð1− y1Þ (1.12)

4 1 Introduction

Rearranging this equation, results in

y1 =α12x1

1+ ðα12 − 1Þx1 (1.13)

Equation (1.13) expresses the more volatile component mole fraction in thevapor as a function of the mole fraction of that component in the liquid and the rela-tive volatility (Kister, 1992), which can be plotted in Fig. 1.2 as an x–y diagram.

1.2.3 Dew Point and Bubble Point

To estimate the stage, condenser, and reboiler temperatures, set procedures arerequired for calculating the dew and bubble points (Towler and Sinnott, 2013). At thebubble-point temperature, the total vapor pressure exerted by the mixture becomesequal to the confining drum pressure, and it follows that

Pyi =

PKxi = 1:0 in the

bubble formed (Seader et al., 2008). At the dew-point temperature, the relationship,Pxi =

P yiKi= 1:0, must be satisfied. A calculation of these temperatures must be

found by iteration at a given system pressure.

x, mole fraction propane in liquid

0.0 0.2 0.4 0.6 0.8 1.0

y,

mo

le f

ractio

n p

rop

an

e in

va

po

r

0.0

0.2

0.4

0.6

0.8

1.0

α = 1.1

α = 2

α = 5

α = 1

Fig. 1.2 x–y diagram with various relative volatilities

51.2 Distillation Fundamentals and Principles

1.2.4 Equilibrium Flash Calculations

Suppose the temperature, pressure, and overall composition of the inlet streamshown in Fig. 1.3 is known. The mole fractions of component i in the inlet, vapor,and liquid streams are zi, xi and yi, respectively, with

Pyi = 1:0 and

Pxi = 1:0.

Subtracting these two equations results in:

Xyi −

Xxi = 0 (1.14)

When the vapor and liquid are in equilibrium, the relationship is yi =Kixi.Equation (1.14) can be rearranged to:

XKixi −

Xxi = 0 or

XðKi − 1Þxi = 0: (1.15)

The mole balance for each component over the phase separator (Finlayson,2006) is

F =V + L and Fzi =Vyi + Lxi: (1.16)

Dividing by F and defining v as the fraction of the feed that is vapor gives thefollowing:

zi = yiv; + xið1− vÞ with v=

V

F: (1.17)

Using the equilibrium expression again, Eq. (1.17) can be written as

zi =Kixiv+ xið1− vÞ= ðKi − 1Þxiv+ xi: (1.18)

Solving for the mole fractions in the liquid gives

xi =zi

1+ ðKi − 1Þv : (1.19)

Fig. 1.3 Flash distillation

6 1 Introduction

Placing this expression into Eq. (1.15) results in the final equation.

X ðKi − 1Þzi1+ ðKi − 1Þv = 0 (1.20)

This is called the Rachford–Rice equation (Finlayson, 2006). If the K valuesand inlet composition zi are known, this is a nonlinear equation to solve for v.Once the value of v is known, the value of the liquid composition xi, vapor com-position yi, vapor flow rate, and liquid flow rate can be calculated.

1.3 Distillation Design

1.3.1 Shortcut Methods

Shortcut methods can be used to estimate the stage and energy requirements with-out the aid of computers. They generally depend on the assumption of constantrelative volatility, and should not be used for severely non-ideal systems (Towlerand Sinnott, 2013).

The minimum number of theoretical stages at the total reflux (Nm) was esti-mated using the Fenske equation (Fenske, 1932).

Nmin =ln

xD;LKxD;HK

� �=

xB;LKxB;HK

� �� �ln αLK−HK

(1.21)

where αLK–HK is the average relative volatility of the light key with respect to theheavy key; and xLK and xHK are the light and heavy key concentration, respectively.

The Underwood equations calculating the minimum reflux ratio in a multicom-ponent system is used widely as follows:

X αixD;iαi − θ

=Rmin + 1 (1.22)

where θ is the root of the equation,

X αixF;iαi − θ

= 1− q (1.23)

where q is the feed thermal condition with quantity is the liquid fraction of the feed.Many reflux-stages relationships have been proposed with the most popular

method being the Gilliland correlation. This plot correlating the reflux and stagescan be used to estimate the number of stages, N, after Nmin, minimum reflux ratio(Rmin) and reflux ratio (R) are determined using the Fenske and Underwoodequations. The procedure uses a diagram plot (R − Rmin)/(R + 1) on the x-axis and

71.3 Distillation Design

(N − Nmin)/(N + 1) on the y-axis (Kister, 1992). One enters the diagram with theabscissa value, which is known, and then it reads the ordinate of the correspondingpoint on the Gilliland curve (Kiss, 2013). The only unknown of the ordinate is thetotal number of stages (N).

To estimate the feed stage location, the most popular shortcut relationships arethe Fenske equation and the Kirkbridge equation. The Kirkbridge equation isexpressed as

NR

NS=

zHKzLK

xB;LKxD;HK

� �2 B

D

" #0:206

: (1.24)

1.3.2 Rigorous Methods

Although shortcut methods can be performed simply and fast by hand, they haveinferior accuracy. With the rapid development of computer technology, rigorous cal-culations describing a column as a group of equations and solving these equations tocalculate the operating conditions of the column are the primary tools for the designand optimization in modern distillation practice. In particular, rigorous methods areused to solve the MESH equations, which stand for (Kister, 1992):

• Material or flow rate balance equations, both component and total,• Equilibrium equations, including the bubble-point and dew-point equations,• Summation or stoichiometric equations or composition constraints, and• Heat or enthalpy or energy balance equations.

Normally the inside-out method is used to calculate most columns because ofits robustness and its ability to solve a wide variety of columns. On the otherhand, it is incapable of handling reactions in the column, which can be changed torate-based methods.

1.4 Distillation Simulation

The analysis, conceptual design, optimization, operation, and control of distillationprocesses can be done using process simulators, such as Aspen Plus, AspenHysys, ProII, which are now available and accessed.

1.4.1 Azeotrope Search

Many mixtures of chemical components form azeotropes, which have vapor andliquid phases with identical compositions. This phenomenon affects the distillationdesign and simulation in terms of the feasibility, effort and time. These mixtures

8 1 Introduction