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Advanced Distillation

Technologies

Advanced Distillation

Technologies

Design, Control and Applications

Anton Alexandru Kiss

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Dedicated to the loving memory of mygrandparents, and to all who contributed so

much to my work over the years.

Contents

Preface xiii

Acknowledgements xv

1 Basic Concepts in Distillation 1

1.1 Introduction 11.2 Physical Property Methods 21.3 Vapor Pressure 61.4 Vapor–Liquid Equilibrium and VLE Non-ideality 8

1.4.1 Vapor–Liquid Equilibrium 81.4.2 VLE Non-ideality 11

1.5 Relative Volatility 131.6 Bubble Point Calculations 141.7 Ternary Diagrams and Residue Curve Maps 16

1.7.1 Ternary Diagrams 161.7.2 Residue Curve Maps 18

1.8 Analysis of Distillation Columns 241.8.1 Degrees of Freedom Analysis 261.8.2 McCabe–Thiele Method 271.8.3 Approximate Multicomponent Methods 33

1.9 Concluding Remarks 34References 35

2 Design, Control and Economics of Distillation 37

2.1 Introduction 372.2 Design Principles 38

2.2.1 Operating Pressure 392.2.2 Heuristic Optimization 40

2.2.3 Rigorous Optimization 412.2.4 Feed Preheating 422.2.5 Intermediate Reboilers and Condensers 422.2.6 Heat Integration 43

2.3 Basics of Distillation Control 442.3.1 Single-End Control 462.3.2 Dual-End Control 492.3.3 Alternative Control Structures 522.3.4 Constraint Control 532.3.5 Multivariable Control 54

2.4 Economic Evaluation 552.4.1 Equipment Sizing 562.4.2 Equipment Cost 592.4.3 Utilities and Energy Cost 622.4.4 Cost of Chemicals 63


3 Dividing-Wall Column 67

3.1 Introduction 673.2 DWC Configurations 703.3 Design of DWCs 75

3.3.1 Heuristic Rules for DWC Design 773.3.2 Approximate Design Methods 783.3.3 Vmin Diagram Method 793.3.4 Optimal Design of a DWC 82

3.4 Modeling of a DWC 833.4.1 Pump-Around Model 843.4.2 Two Columns Sequence Model 843.4.3 Four Columns Sequence Model 853.4.4 Simultaneous Models 863.4.5 Simulation of a Four-Product DWC 863.4.6 Optimization Methods 86

3.5 DWC Equipment 873.5.1 Liquid/Reflux Splitter 893.5.2 Column Internals 913.5.3 Equipment Sizing 913.5.4 Constructional Aspects 94

3.6 Case Study: Separation of Aromatics 973.7 Concluding Remarks 103

References 107

viii CONTENTS

4 Optimal Operation and Control of DWC 111

4.1 Introduction 1114.2 Degrees of Freedom Analysis 1124.3 Optimal Operation and Vmin Diagram 1144.4 Overview of DWC Control Structures 117

4.4.1 Three-Point Control Structure 1184.4.2 Three-Point Control Structure with

Alternative Pairing 1204.4.3 Four-Point Control Structure 1214.4.4 Three-Point Control Structure with

Nested Loops 1214.4.5 Performance Control of Prefractionator

Sub-system using the Liquid Split 1224.4.6 Control Structures Based on Inferential

Temperature Measurements 1234.4.7 Feedforward Control to Reject Frequent

Measurable Disturbances 1264.4.8 Advanced Control Techniques 127

4.5 Control Guidelines and Rules 1284.6 Case Study: Pentane–Hexane–Heptane Separation 1294.7 Case Study: Energy Efficient Control of a BTX DWC 132

4.7.1 Energy Efficient Control Strategies 1354.7.2 Dynamic Simulations 139


5 Advanced Control Strategies for DWC 153

5.1 Introduction 1535.2 Overview of Previous Work 1545.3 Dynamic Model of a DWC 1565.4 Conventional versus Advanced Control Strategies 163

5.4.1 PID Loops within a Multi-loop Framework 1635.4.2 Linear Quadratic Gaussian Control 1655.4.3 Generic Model Control 1675.4.4 Multivariable Controller Synthesis 167

5.5 Energy Efficient Control Strategies 1715.5.1 Background of Model Predictive Control 1735.5.2 Controller Tuning Parameters 1755.5.3 Dynamic Simulations 176

5.6 Concluding Remarks 180

CONTENTS ix

Notation 181References 183

6 Applications of Dividing-Wall Columns 187

6.1 Introduction 1876.2 Separation of Ternary and Multicomponent Mixtures 1886.3 Reactive Dividing-Wall Column 1956.4 Azeotropic Dividing-Wall Column 1986.5 Extractive Dividing-Wall Column 1996.6 Revamping of Conventional Columns to DWC 2036.7 Case Study: Dimethyl Ether Synthesis by R-DWC 2056.8 Case Study: Bioethanol Dehydration by A-DWC and

E-DWC 2126.9 Concluding Remarks 223

References 223

7 Heat Pump Assisted Distillation 229

7.1 Introduction 2297.2 Working Principle 2317.3 Vapor (Re)compression 232

7.3.1 Vapor Compression 2337.3.2 Mechanical Vapor Recompression 2337.3.3 Thermal Vapor Recompression 234

7.4 Absorption–Resorption Heat Pumps 2347.4.1 Absorption Heat Pump 2347.4.2 Compression–Resorption Heat Pump 235

7.5 Thermo-acoustic Heat Pump 2367.6 Other Heat Pumps 240

7.6.1 Stirling Cycle 2407.6.2 Vuilleumier Cycle 2417.6.3 Brayton Cycle 2417.6.4 Malone Cycle 2427.6.5 Solid–Sorption Cycle 242

7.7 Heat-Integrated Distillation Column 2447.8 Technology Selection Scheme 245

7.8.1 Energy Efficient DistillationTechnologies 246

7.8.2 Multicomponent Separations 2497.8.3 Binary Distillation 2547.8.4 Selected Scheme Applications 263

x CONTENTS


8 Heat-Integrated Distillation Column 271

8.1 Introduction 2718.2 Working Principle 2738.3 Thermodynamic Analysis 2778.4 Potential Energy Savings 280

8.4.1 Partial Heat Integrated Distillation Column(p-HIDiC) 280

8.4.2 Ideal Heat Integrated Distillation Column(i-HIDiC) 281

8.5 Design and Construction Options 2828.5.1 Inter-coupled Distillation Columns 2848.5.2 Distillation Column with Partition Wall 2858.5.3 Concentric Distillation Column 2878.5.4 Concentric Column with Heat Panels 2888.5.5 Shell & Tube Heat-Exchanger Column 2898.5.6 Plate-Fin Heat-Exchanger Column 2908.5.7 Heat Transfer Means 292

8.6 Modeling and Simulation 2958.7 Process Dynamics, Control, and Operation 2978.8 Applications of HIDiC 3008.9 Concluding Remarks 304

References 305

9 Cyclic Distillation 311

9.1 Introduction 3119.2 Overview of Cyclic Distillation Processes 3139.3 Process Description 3169.4 Mathematical and Hydrodynamic Model 319

9.4.1 Mathematical Model 3199.4.2 Hydrodynamic Model 3219.4.3 Sensitivity Analysis 323

9.5 Modeling and Design of Cyclic Distillation 3279.5.1 Modeling Approach 3299.5.2 Comparison with Classic Distillation 3319.5.3 Design Methodology 3319.5.4 Demonstration of the Design Procedure 333

9.6 Control of Cyclic Distillation 335

CONTENTS xi

9.7 Cyclic Distillation Case Studies 3389.7.1 Ethanol–Water Stripping and Concentration 3389.7.2 Methanol–Water Separation 341


10 Reactive Distillation 353

10.1 Introduction 35310.2 Principles of Reactive Distillation 35410.3 Design, Control and Applications 35710.4 Modeling Reactive Distillation 36210.5 Feasibility and Technical Evaluation 364

10.5.1 Feasibility Evaluation 36410.5.2 Technical Evaluation 367

10.6 Case Study: Advanced Control of a ReactiveDistillation Column 37110.6.1 Mathematical Model 37110.6.2 Open-Loop Dynamic Analysis 37410.6.3 Closed-Loop Performance 374

10.7 Case Study: Biodiesel Production byHeat-Integrated RD 378

10.8 Case Study: Fatty Esters Synthesis by Dual RD 38310.9 Concluding Remarks 387

References 388

Index 393

xii CONTENTS

Preface

Our modern society is currently facing an energy revolution, and it needsto identify properly the potential threats and use all the opportunities tomeet the needs of the growing population. Accordingly, chemical engi-neers have embarked on a quest for shaping a much needed sustainablefuture—especially considering that chemical industry is among the mostenergy demanding sectors. Distillation is a thermal separation methodwidely applied in the chemical process industry as the separation tech-nology of choice, despite its very low thermodynamic efficiency. Remark-ably, almost every single product on the market includes components thatwent through a distillation column. Even now, when changing from fossilfuels to a bio-based economy, it is clear that in the next two decadesdistillation will retain its significance as the main method for separatingmixtures—although this old workhorse of the chemical industry is facingsome new big and bold challenges.

Owing to the limitation of fossil fuels, the need for energy indepen-dence, and the environmental problem of the greenhouse gas effect, thereis a considerable increasing interest in the research and development ofintegrated chemical processes that require less capital investment,reduced operating costs, and have high eco-efficiency. Energy efficientdistillation is a hot topic in separation technology due to the keyadvantages of the integrated technologies, such as reduced investmentcosts and low energy requirements, as well as an increasing number ofindustrial applications. Although the research and development carriedout at universities and industrial companies in this exciting field isexpanding quickly, there is still no book currently available focusingon this important area in distillation technology—the largest consumer ofenergy in the chemical process industry.

Therefore, we feel that there is a significant gap that can be addressedwith this book and it will be of immense interest to a readership across theworld. The book provides engineers with a wide and relatively deepinsight into integrated distillations using non-conventional arrange-ments. Readers can learn from this material the background, recentdevelopments, fundamental principles, design and simulation methods,detailed case studies of distillation processes, as well as expected futuretrends. We believe that the abundant valuable resources included here—relevant equations, diagrams, figures, and references that reflect thecurrent and upcoming integrated distillation technologies—will be ofgreat help to all readers from the (petro-)chemical industry, bio-refineries,and other related areas.

This book is the first comprehensive work about advanced distillationtechnologies, covering many important topics such as key concepts indistillation technology, principles of design, control, equipment sizingand economics of distillation, DWC design and configurations, optimaloperation, controllability and advanced control strategies, industrial andpilot-scale DWC applications (in ternary separations, azeotropic distil-lation, extractive distillation, and reactive distillation), HIDiC design andconfigurations, heat pump assisted applications, cyclic distillation, andreactive distillation. Each chapter is independently written and consiststypically of an introduction, working principle, process design, modelingand simulation, process control and operation, specific equipment,industrial and applied research examples, concluding remarks, as wellas a comprehensive list of useful references for further reading.

Note that the author is aware about the unavoidable presence of someminor mistakes. That is why I would like to express my gratitude forevery observation and suggestion towards further improving thismaterial.

xiv PREFACE

Acknowledgements

This book is the result of several years of dedicated work on variousdistillation topics, and I truly hope that the reader will find it useful andreadable. Clearly, I am very grateful to everyone who has contributed inone way or another to make it a possible success. Therefore, I wish toexpress my deep gratitude and thanks to many of my collaborators andco-authors of scientific articles covering almost all topics in this book:Sorin Bildea, Radu Ignat and Ionela Lita (University “Politehnica”of Bucharest, RO), Eugeny Kenig and €Omer Yildirim (University ofPaderborn,DE), David Suszwalak (EcoleNationale Sup�erieure deChimiede Mulhouse, FR), Carlos Infante Ferreira and Ruben van Diggelen(Delft University of Technology, NL), Rohit Rewagad (University ofTwente, NL), Alexandre Dimian and Gadi Rothenberg (University ofAmsterdam, NL), Andr�e de Haan, Servando Flores-Landaeta, MayankShah, and Edwin Zonervan (Eindhoven University of Technology, NL),Florin Omota (Fluor, NL), Zoltan Nagy (LoughboroughUniversity, UK),Juan Gabriel Segovia-Hern�andez (Universidad de Guanajuato, MX),Vladimir Maleta (MaletaCD, UA), as well as Hans Pragt and Cornaldvan Strien (AkzoNobel, NL).

Furthermore, I had the privilege during this time to benefit from smartand appropriate observations, during personal discussions or indirectcontacts with some remarkable persons from academia and industry, towhom I am also truly indebted: 9Zarco Oluji�c, Andrzej Stankiewicz, andJohan Grievink (Delft University of Technology, NL), Igor Dejanovi�c(University of Zagreb, HR), Sigurd Skogestad (NTNU Trondheim, NO),Ivar Halvorsen (SINTEF ICT, NO), Norbert Asprion (BASF, DE),Levente Simon (BASF, CH), Thomas Gruetzner (Lonza, CH), Bj€ornKaibel (Julius Montz GmbH, DE), Jeffrey Felix (Sulzer Chemtech, CH),Henry Kister (Fluor/FRI, US), William Luyben (Lehigh University, US),

Larry Biegler (CarnegieMellonUniversity, US), Dolf Bruinsma and SimonSpoelstra (ECN,NL), Yuji Tanaka (CosmoResearch Institute, JP), Aris deRijke (DSM Research, NL), Jan Harmsen (Shell Global Solutions, NL),Andrzej G�orak (Technical University of Dortmund, DE), Panos Seferlis(Aristotle University of Thessaloniki, GR), Donato Aranda (Federal Uni-versity of Rio de Janeiro, BR) Sascha Kersten (University of Twente, NL),as well as a number of other colleagues.

In addition, the excellent support and valuable help from the editorsRebecca Stubbs, Sarah Tilley, Jasmine Kao and cover designer Dan Jubb(John Wiley & Sons, UK) is greatly acknowledged. My thankfulness isextended also to my friends around the world for their moral supportand for keeping in touch with me during these busy and turbulent times.And last but not least, my special thanks go to my loving family—especially my better half—for their tender affection, understanding,relentless support, and continuous encouragement.

xvi ACKNOWLEDGEMENTS

1

Basic Concepts in Distillation

1.1 INTRODUCTION

Distillation is a thermal separation method for separating mixtures of twoor more substances into its component fractions of desired purity, based ondifferences in volatilities of components—which are in fact related to theboiling points of these components—by the application and removal ofheat. Note that the term distillation refers to a physical separation processora unit operation.Remarkably, distillationcanbecombinedwithanotherdistillation operation, leading to a dividing-wall column (Harmsen, 2010),or with a chemical reaction, leading to reactive distillation (Sundmacherand Kienle, 2003; Sundmacher, Kienle, and Seidel-Morgenstern, 2005;Luyben and Yu, 2008; Sharma and Singh, 2010), and/or other chemicalprocess operations (Schmidt-Traub and Gorak, 2006).

At the commercial scale, distillation has many applications, such as theseparation of crude oil into fractions (e.g., gasoline, diesel, kerosene, etc.),water purification and desalination, the splitting of air into its components(e.g., oxygen, nitrogen, and argon), and the distillation of fermentedsolutions or the production of distilled beverages with high alcohol content(Forbes, 1970). Distillation underwent enormous development due to thepetrochemical industry, and as such it is one of the most importanttechnologies in the global energy supply system (Harmsen, 2010). Essen-tially, all transportation fuel goes through at least one distillation columnon its way from crude oil to readily usable fuel, with tens of thousands ofdistillation columns in operation worldwide. In view of the foreseendepletion of fossil fuels and the switch to renewable sources of energysuch as biomass, the most likely transportation fuel will be ethanol,

Advanced Distillation Technologies: Design, Control and Applications, First Edition.Anton Alexandru Kiss.� 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

methanol, or derivatives. The synthesis of alternative fuels leads typicallyto aqueous mixtures that require distillation to separate ethanol or metha-nol from water. Consequently, distillation remains the separation methodof choice in the chemical process industry. The importance of distillation isunquestionable in providing most of the products required by our modernsociety (e.g., transportation fuel, heat, food, shelter, clothing, etc.).

The analysis, design, operation, control, and optimization of distilla-tion columns were studied extensively in the last century but, until theintroduction of computers, only hand calculations and graphical meth-ods were developed and applied in distillation studies. As distillationanalysis involves many iterative vapor–liquid phase equilibrium calcula-tions, and tray-to-tray component balances that are ideal for digitalcomputation, the use of computers has had a beneficial effect in recentdecades (Luyben, 2011). Many companies still have their own in-houseprocess simulators, although commercial steady-state and dynamic pro-cess simulators (e.g., Aspen Plus1, Aspen Dynamics1, ChemCAD, AspenHYSYS1, PRO/II, etc.) are now available and dominate the field—withdistillation playing a key role in these simulators.

The topic of distillation is very broad and it would require manyvolumes to cover it in a comprehensive manner. Consequently, for moredetails the reader is directed to several good books, which cover thissubject in great detail: Kister (1992a), Kister (1992b), Taylor and Krishna(1993), Stichlmair and Fair (1998), Seader and Henley (1998), Dohertyand Malone (2001), Mujtaba (2004), Petlyuk (2004), Lei, Chen, andDing (2005), and more recently Luyben (2006, 2011).

It is important to note that distillation can separate chemical componentsonly if the compositions of the vapor and liquid phases that are in equili-brium with each other are different. Therefore, a practical understanding ofvapor–liquid equilibrium (VLE) is essential for the analysis, design, andcontrol of distillation columns. This introductory chapter presents in astructured and convenient way the basic concepts of distillation: propertymethods, vapor pressure, bubble point, relative volatility, VLE, vapor–liquid–liquid equilibrium (VLLE), ternary diagrams, residue curve maps(RCM), and theoretical stage and short-cut design methods for distillation.

1.2 PHYSICAL PROPERTY METHODS

An extremely important issue in distillation calculations is the selection ofan appropriate physical property method that will accurately describe thephase equilibrium of the chemical system. Missing or inadequate physical

2 ADVANCED DISTILLATION TECHNOLOGIES

properties can undermine the accuracy of a model or even prevent one fromperforming the simulation. For this reason, finding good values forinadequate or missing physical property parameters is crucial to a successfulsimulation. Nevertheless, this depends strongly upon choosing the rightestimation methods—an issue already recognized in the world of chemicalprocesses modeling by the axiom “garbage in, garbage out” which meansthat the simulation results have the same quality as the input data/parameters(Carlson, 1996). In most design situations there is some type of data—forexample, VLE reported in the literature, experimental measurements, anddata books (Gmehling et al., 1993; Perry and Green, 1997)—that can be usedto select the most appropriate physical property method. The processsimulators used nowadays (e.g., Aspen Plus, ChemCAD, HYSYS, PRO/II)have libraries with numerous alternative methods—the most commonly usedbeing NRTL, UNIQUAC, UNIFAC, Chao–Seader, van Laar, Wilson, Gray-son, Peng–Robinson, Soave–Redlich–Kwong (SRK), andderivatives of them.

Figure 1.1 provides a very convenient scheme that can be used for thequick and easy selection of an appropriate property model for virtuallyany chemical system (Aspen Technology, 2010a, 2010b). The propertymodel names used here are given as in the Aspen Plus process simulator.Table 1.1 summarizes the commonly used property methods available inAspen Plus (Aspen Technology, 2010b).

Figure 1.1 Property methods selection scheme

BASIC CONCEPTS IN DISTILLATION 3

1.3 VAPOR PRESSURE

Distillation is based on the fact that the vapor of a boiling mixture willbe richer in the components with lower boiling points. Consequently,when this vapor is sufficiently cooled the condensate will contain morevolatile (e.g., light, low-boiling) components, while at the same timethe original mixture will contain more of the less volatile (e.g., heavy,high-boiling) components.

Vapor pressure—a physical property of a pure chemical component—is the pressure that a pure component exerts at a given temperature whenboth liquid and vapor phases are present. In other words, the vaporpressure of a liquid at a particular temperature is the equilibrium pressureexerted by the molecules leaving and entering the liquid surface. Here aresome key issues to consider:

� Vapor pressure is related to boiling, and it increases with the energyinput.

� A liquid boils when its vapor pressure equals the ambient pressure.� The ease with which a liquid boils depends on its volatility.

Distillation occurs because of the differences in volatility of thecomponents in the liquid mixture.

� Liquids with a high vapor pressure (volatile liquids) boil at lowtemperatures, and vice versa.

� The vapor pressure (and also the boiling point) of a liquid mixturedepends on the relative amounts of the components in the mixture.

Table 1.2 provides the vapor pressure of some common substances atambient temperature. Note that chemicals with a non-zero vapor

Table 1.2 Vapor pressure of some common substances at 20 �C

Chemical componentVapor pressure(bar)

Vapor pressure(mmHg)

Ethylene glycol 0.005 3.75Water 0.023 17.5Propanol 0.024 18.0Ethanol 0.058 43.7Methyl isobutyl ketone (MIBK) 0.265 198.6Freon 113 (1,1,2-trichlorotrifluoroethane) 0.379 284Acetaldehyde 0.987 740Butane 2.2 1650Formaldehyde 4.357 3268Carbon dioxide 57 42753


pressure lower than atmospheric are liquids, while those with a vaporpressure higher than atmospheric are gases, under normal conditions.

Raoult’s law states that the vapor pressure of an ideal solution isdependent on the vapor pressure of each chemical component and onthe mole fraction of the component present in the solution. Once thecomponents in the solution have reached equilibrium, the total vaporpressure (p) of the solution is:

p ¼XNC

j¼1

pj ¼XNC

j¼1

p�j xj (1.1)

with the individual vapor pressure for each component defined as:pj ¼ p�j xj, where pj is the partial pressure of component j in the mixture(in the solution), p�j is the vapor pressure of the pure component j, and xjis the mole fraction of component j in the mixture.

The vapor pressure depends only on temperature and not on composition,since it is a pure component property. The dependence on temperature isusually a strong one, with an exponential increase of the vapor pressure athigher temperatures. Figure 1.2a shows some typical vapor pressure curves,for benzene, toluene, and xylene—with the exponential increase clearlyobservable at high temperatures. Figure 1.2b plots the natural log of thevapor pressure versus the reciprocal of the absolute temperature. It can beseen that as temperature increases (to the left of the plot) thevaporpressure ishigher. In both plots of Figure 1.2, a vertical (constant temperature) lineshows that, at a given temperature, benzene has a higher vapor pressurethan toluene and xylene. Therefore, benzene is the lightest component,while xylene is the heaviest component—from the volatility (notdensity) standpoint. Correspondingly, a horizontal (constant-pressure)

Figure 1.2 Vapor pressure of pure components: benzene, toluene, and p-xylene


line shows that benzene boils at a lower temperature than does tolueneor xylene. Therefore, benzene is the lowest boiling component, whilexylene is the highest boiling component. Note also that in Figure 1.2athe vapor pressure lines for benzene, toluene, and xylene are fairlyparallel, meaning that the ratio of the vapor pressures does not changemuch with the temperature or pressure. Consequently, the ease ordifficulty of benzene/toluene/xylene separation—directly translatedinto the energy requirements for the specified separation—does notchange much with the operating pressure. However, other chemicalcomponents can have temperature dependences of the vapor pressurethat are quite different to this example (Luyben, 2006).

In the case of distilling the binary mixture benzene–toluene, theconcentration of the lighter (low-boiling) benzene in the vapor phasewill be higher than that in the liquid phase—while the reverse is true forthe heavier (high-boiling) toluene. As a result, benzene and toluene can beseparated in a distillation column into a top distillate stream that isalmost pure benzene and a bottoms stream that is fairly pure toluene.Using experimental vapor pressure data for each component, equationscan be fitted by means of two, three, or more parameters. The Antoineequation—derived from the Clausius–Clapeyron relation—relates thevapor pressure and temperature for pure components:

log p ¼ A� B=ðCþ TÞ (1.2)

where p is the vapor pressure, T is temperature, and A, B, and C areconstants specific for each pure chemical component—their numericalvalues depend on the units used for vapor pressure (e.g., bar, mmHg, kPa)and on the units used for temperature (�C or K). The simplified form withthe constant C set to zero (log p¼A�B/T) is known as the Augustequation, and describes a linear relation between the logarithm of thepressure and the reciprocal temperature—assuming that the heat ofvaporization is independent of temperature.

1.4 VAPOR–LIQUID EQUILIBRIUM AND VLENON-IDEALITY

1.4.1 Vapor–Liquid Equilibrium

Vapor–liquid equilibrium data for two-component (binary) systems iscommonly represented by means of T–xy and xy diagrams—where Tis the temperature, and x, y are the liquid and vapor composition,


respectively, expressed in mole fraction. Basically, the T–xy diagram plotsthe temperature versus the liquid and vapor composition, while the xydiagram plots only y versus x. Although these types of diagrams aregenerated at a constant pressure, the T–xy an xy diagrams are extremelyconvenient for the analysis of binary distillation systems—especially sincethe operating pressure is relatively constant in most distillation columns.

Figure 1.3 shows the T–xy diagram (also known as the boiling pointdiagram) for the benzene–toluene system at atmospheric pressure—thatis, how the equilibrium compositions of the components in a liquidmixture vary with temperature at a fixed pressure. The boiling point ofbenzene is that at which the mole fraction of benzene is 1, while theboiling point of toluene is that at which the mole fraction of benzene is 0.As illustrated by the T–xy diagram, benzene is the more volatile compo-nent and therefore has a lower boiling point than toluene. The lowercurve in the T–xy diagram is called the bubble-point curve (saturatedliquid curve), while the upper one is known as the dew-point curve(saturated vapor curve). The saturated liquid/lower curve gives the molefraction of benzene in the liquid phase (x) while the saturated vapor/upper curve gives the mole fraction of benzene in the vapor phase (y).Drawing a horizontal line at a certain temperature and reading off theintersection of this line with the two curves give the compositions of thetwo liquid and vapor phases. Note that the bubble-point is defined asthe temperature at which the liquid starts to boil, while the dew-pointis the temperature at which the saturated vapor starts to condense. The

Figure 1.3 T–xy diagram for the mixture benzene–toluene at atmospheric pressure


region below the bubble-point curve shows the equilibrium composition ofthe subcooled liquid, while the region above the dew-point curve shows theequilibrium composition of the superheated vapor. Note that in the regionbetween the lower and upper curves, there are two phases present—bothliquid and vapor. For example, when a subcooled liquid is heated (point A,at 0.4 mole fraction of benzene) its concentration remains constant until itreaches the bubble-point (point B) when it starts to boil. The vaporsproduced during the boiling have the equilibrium composition of pointC (ca. 0.65 mole fraction of benzene), and are thus over 60% richer inbenzene than the original liquidmixture. This difference between the liquidand vapor compositions is in fact the basis for distillation operations.

The T–xy diagram can be easily generated in process simulatorssuch as Aspen Plus, and the results at several pressures can be plotted(Figure 1.4). It is important to note that the higher the pressure, thehigher the temperatures.

The xy diagram is also an effective tool in the analysis of distillationsystems. Figure 1.5 illustrates the xy diagrams for the binary mixturebenzene–toluene (Figure 1.5a) and propane–propylene (Figure 1.5b). Asbenzene and toluene have a relatively large difference in boiling points, thecurve is noticeably shifted from the diagonal (x¼ y). However, propyleneand propane have quite close boiling points, which leads to a very difficultseparation—as illustrated in the xy diagram by the fact that the curve isvery close to the diagonal (x¼ y). Remarkably, bothT–xy and xy diagrams

Figure 1.4 T–xy diagram for the mixture benzene–toluene at various pressures


provide valuable insight into the phase equilibrium of binary systems, asthey can be used for quantitative analysis of distillation (Luyben, 2006).

1.4.2 VLE Non-ideality

Liquid-phase ideality—equivalent to activity coefficients gj¼ 1 (unity)—occurs only when the components are very similar. The benzene–toluenesystem described earlier is a common example, where the activitycoefficients of both components are very close to unity. However, ifthe components are quite different then non-ideal behavior occurs. Forexample, let us consider a methanol–water mixture; here water is verypolar but methanol is polar only at the -OH end of the molecule while the-CH3 end is non-polar. This difference results in some non-ideality(Figure 1.6). Figure 1.6a gives the T–xy curve at atmospheric pressure,

Figure 1.5 The xy diagram for the mixture benzene–toluene (a) and propane–propylene (b)

Figure 1.6 T–xy diagram (a) and activity coefficient plot (b) for methanol–water


while Figure 1.6b shows the variation of activity coefficients for bothwater and methanol over the composition space. The NRTL physicalproperty method was used in this example to generate these plots. Theactivity coefficient values range up to g¼ 2.3 for methanol at the x¼ 0limit and g¼ 1.66 for water at x¼ 1 (Luyben, 2006).

Let us consider now an ethanol–water mixture, in which case the-CH2CH3 (ethyl) end of the ethanol molecule is more non-polar thanthe -CH3 end of methanol. As expected, the non-ideality is morepronounced—as clearly illustrated by the T–xy and xy diagrams shownin Figure 1.7. Note that the xy curve shown in Figure 1.7b crosses thediagonal (45� line, where x¼ y) at about 90 mol.% ethanol—this clearlyindicates the presence of an azeotrope. Note also that the temperature atthe azeotropic composition (351.0 K) is slightly lower than the boilingpoint of ethanol (351.5 K).

In fact, the most intriguing VLE curves are generated by azeotropicsystems that give rise to VLE plots where the equilibrium curves crossesthe diagonal (on the xy diagram). Note that an azeotrope is defined as thecomposition at which the liquid and vapor compositions are equal. Whenthis occurs in a distillation column, there is no further change in the liquidand vapor compositions from tray to tray—hence the azeotrope repre-sents a distillation boundary. Azeotropes can be classified according tothe phase as homogeneous (single liquid phase) or heterogeneous (twoliquid phases), according to the boiling temperature as minimum-boilingor maximum-boiling, and they can occur in binary, ternary, and multi-component systems. The ethanol–water mixture described in the previ-ous example has a minimum-boiling homogeneous azeotrope (singleliquid phase boiling at 78 �C, with the composition of 89.3 mol.%ethanol). The VLE non-ideality and the types of azeotropic systemsare tackled in more detail by Stichlmair and Fair (1998).

Figure 1.7 T–xy diagram, activity coefficient plot (a) and xy diagram (b) forethanol–water


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