ion to catalysis kinetics and chemical processes

ISBN: 978-1-4665-5883-0

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While chemical products are useful in their own rightthey address the demands and needs of the massesthey also drain our natural resources and generate unwanted pollution. Green Chemical Engineering: An Introduction to Catalysis, Kinetics, and Chemical Processes encourages minimized use of non-renewable natural resources and fosters maximized pollution prevention. This text stresses the importance of developing processes that are environmentally friendly and incorporate the role of green chemistry and reaction engineering in designing these processes.

Focused on practical application rather than theory, the book integrates chemical reaction engineering and green chemical engineering, and is divided into two sections. The first half of the book covers the basic principles of chemical reaction engineering and reactor design, while the second half of the book explores topics on green reactors, green catalysis, and green processes. The authors mix in elaborate illustrations along with important developments, practical applications, and recent case studies. They also include numerous exercises, examples, and problems covering the various concepts of reaction engineering addressed in this book, and provide MATLAB software used for developing computer codes and solving a number of reaction engineering problems.

Consisting of six chapters organized into two sections, this text:

Covers the basic principles of chemical kinetics and catalysis Gives a brief introduction to classification and the various types of chemical reactors

Discusses in detail the differential and integral methods of analysis of rate equations for different types of reactions Presents the development of rate equations for solid catalyzed reactions and enzyme catalyzed biochemical reactions Explains methods for estimation of kinetic parameters from batch reactor data Details topics on homogeneous reactors Includes graphical procedures for the design of multiple reactors Contains topics on heterogeneous reactors including catalytic and non-catalytic reactors Reviews various models for non-catalytic gassolid and gasliquid reactions Introduces global rate equations and explicit design equations for a variety of non-catalytic reactors Gives an overview of novel green reactors and the application of CFD technique in the modeling of green reactors Offers detailed discussions of a number of novel reactors Provides a brief introduction to CFD and the application of CFD Highlights the development of a green catalytic process and the application of a green catalyst in the treatment of industrial effluent

Comprehensive and thorough in its coverage, Green Chemical Engineering: An Introduction to Catalysis, Kinetics, and Chemical Processes explains the basic concepts of green engineering and reactor design fundamentals, and provides key knowledge for students at technical universities and professionals already working in the industry.

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CHEMICAL ENGINEERING

Green ChemicalEngineering

An Introduction to Catalysis, Kinetics,and Chemical Processes

S. Suresh and S. Sundaramoorthy

SureshSundaram

oorthyG

reen C

hem

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S. Suresh and S. Sundaramoorthy

MATLAB is a trademark of The MathWorks, Inc. and is used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This books use or discussion of MATLAB software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLAB software.

CRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742

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To my beloved parents and my wife S. Arisutha.

S. Suresh

To all my teachers and inquisitive students.

S. Sundaramoorthy

vii

Contents

Foreword ...................................................................................................................................... xiiiPreface .............................................................................................................................................xvAcknowledgements .................................................................................................................... xixAuthors ......................................................................................................................................... xxiNomenclature ............................................................................................................................ xxiii

1 Introduction .............................................................................................................................11.1 Principles of Green Chemistry and Green Chemical Engineering .......................21.2 Chemical Reaction Engineering: The Heart of Green Chemical Engineering ..................................................................................................4

Section I Kinetics, Catalysis and Chemical Reactors

2 Introduction to Kinetics and Chemical Reactors .............................................................92.1 Kinetics of Chemical Reactions ...................................................................................9

2.1.1 Reaction Rate ....................................................................................................92.1.2 Extent of Conversion ..................................................................................... 102.1.3 Rate Equation ................................................................................................. 11

2.1.3.1 Activation Energy and Heat of Reaction .................................... 112.1.3.2 Limiting Reactant ........................................................................... 14

2.1.4 Elementary and Non-Elementary Reactions ............................................. 152.1.5 Reversible Reactions ...................................................................................... 162.1.6 Determination of Rate Equations for Single Reactions from

BatchReactor Data ......................................................................................... 172.1.6.1 A Graphical Method for the Estimation of k and n ................... 212.1.6.2 Estimation of Kinetic Parameters for the

Reactionbetween Reactants A and B ..........................................232.1.7 Integrated Forms of Kinetic Rate Equations for Some

SimpleReactions ............................................................................................ 242.1.7.1 First-Order Reaction ...................................................................... 242.1.7.2 Second-Order Reaction ..................................................................252.1.7.3 Third-Order Reaction .................................................................... 272.1.7.4 Second-Order Irreversible Reaction between A and B .............282.1.7.5 Reversible First-Order Reaction ................................................... 292.1.7.6 Zero-Order Reaction ......................................................................30

2.1.8 Multiple Reactions ......................................................................................... 392.1.8.1 Series Reaction ................................................................................ 392.1.8.2 Parallel Reaction .............................................................................43

2.1.9 Autocatalytic Reactions .................................................................................452.1.10 Non-Elementary Reactions and Stationary State Approximations ........ 47

2.1.10.1 Estimation of Kinetic Parameters for Non-Elementary Reactions by Linear Regression ...................................................48

viii Contents

2.1.11 Catalysis: Mechanism of Catalytic ReactionsA Brief Introduction .... 522.1.11.1 Kinetics of Solid Catalysed Chemical Reactions:

LangmuirHinshelwood Model ..................................................532.1.12 Kinetics of Enzyme-Catalysed Biochemical Reactions ............................ 62

2.2 Chemical Reactors: An Introduction ........................................................................ 672.2.1 Homogeneous Reactors: Holding Vessels .................................................. 67

2.2.1.1 Ideal Continuous Stirred Tank Reactor (CSTR) ......................... 692.2.1.2 Ideal Tubular Reactor ..................................................................... 70

2.2.2 Heterogeneous ReactorsMass Transfer Equipment .............................. 732.2.2.1 Heterogeneous Catalytic Reactors ............................................... 76

Appendix 2A: Catalysis and Chemisorption ..................................................................... 792A.1 Catalysis: An Introduction ........................................................................... 79

2A.1.1 Types of Catalysis ........................................................................... 792A.1.2 An Overview of the Basic Concepts of Catalysis....................... 82

2A.2 Heterogeneous Catalysis and Chemisorption ........................................... 822A.2.1 Adsorption Isotherms ...................................................................83

2A.3 Catalyst Deactivation and Regeneration ....................................................862A.4 Case Studies: Removal of Pollutants by Adsorption ................................88

2A.4.1 Adsorptive Removal of Phenol by Activated Palash Leaves ..................................................................................882A.4.2 Adsorptive Removal of Various Dyes by

SynthesisedZeolite ..................................................................... 982A.5 Conclusions ................................................................................................... 106

Appendix 2B: Fitting Experimental Data to Linear Equations by Regression ............ 1062B.1 Fitting Experimental Data to Linear Equations by Regression ............ 1062B.2 Fitting Data to a Linear Equation of the Type y = a1x1 + a2x2 + x0 .......... 108

Excercise Problems .............................................................................................................. 111MATLAB Programs ........................................................................................................... 114

3 Homogeneous Reactors ..................................................................................................... 1353.1 Homogeneous Ideal Reactors .................................................................................. 135

3.1.1 Design Equations for Ideal Reactors ......................................................... 1353.1.1.1 Design Equation for First-Order Irreversible Reaction ........... 1373.1.1.2 Design Equation for Second-Order Irreversible Reaction .................................................................... 1373.1.1.3 Design Equation for First-Order Reversible Reaction ............. 138

3.1.2 Graphical Procedure for Design of Homogeneous Reactors ................. 1433.1.3 Multiple Reactors: Reactors Connected in Series .................................... 147

3.1.3.1 System of N Numbers of Ideal CSTRs in Series ....................... 1473.1.3.2 Optimal Sizing of Two CSTRs Connected in Series ................ 1543.1.3.3 CSTR and PFR in Series ............................................................... 157

3.1.4 Design of Reactors for Multiple Reactions ............................................... 1633.1.4.1 Design of CSTR for Chain Polymerisation Reaction ............... 169

3.1.5 Non-Isothermal Reactors ............................................................................ 1743.1.5.1 Design Equations for Non-Isothermal Reactors ...................... 1753.1.5.2 Optimal Progression of Temperature for Reversible

Exothermic Reactions .................................................................. 1773.1.5.3 Design of Non-Isothermal Reactors with and without

Heat Exchange Q .......................................................................... 183

ixContents

3.1.5.4 Non-Isothermal CSTR Operation: Multiple Steady Statesand Stability ....................................................................... 193

3.2 Homogeneous Non-Ideal Reactors ......................................................................... 1973.2.1 Non-Ideal Reactors versus Ideal Reactors ................................................ 1973.2.2 Non-Ideal Mixing Patterns ......................................................................... 1983.2.3 Residence Time Distribution: A Tool for Analysis of

FluidMixingPattern ...................................................................................2003.2.3.1 Tracer Experiment ........................................................................ 2023.2.3.2 Mean and Variance 2 of Residence Time Distribution ......................................................................... 2063.2.3.3 Residence Time Distribution for Ideal Reactors ...................... 2063.2.3.4 RTD as a Diagnostic Tool ............................................................ 210

3.2.4 Tanks in Series Model ................................................................................. 2103.2.4.1 Estimation of Parameter N .......................................................... 2153.2.4.2 Conversion according to Tanks in Series Model ..................... 216

3.2.5 Axial Dispersion Model .............................................................................. 2193.2.5.1 Conversion according to Axial Dispersion Model ..................223

3.2.6 Laminar Flow Reactor ................................................................................. 2313.2.6.1 Conversion in Laminar Flow Reactor .......................................233

3.2.7 Non-Ideal CSTR with Dead Zone and Bypass ........................................ 2373.2.7.1 Conversion according to Non-Ideal CSTR with Dead

Zone and Bypass .......................................................................... 2393.2.8 Micro-Mixing and Segregated Flow ......................................................... 244

3.2.8.1 Micro-Mixing and the Order of Reaction ................................. 2483.2.8.2 Conversion of a First-Order Reaction in Ideal

Reactorswith Completely Segregated Flow .............................2503.2.8.3 Micro-Mixing and Ideal PFR ...................................................... 252

Appendix 3A: Estimation of Peclet NumberDerivation of Equation Using Method of Moments .................................................................................................254

Exercise Problems ................................................................................................................258MATLAB Programs ........................................................................................................... 262

4 Heterogeneous Reactors .................................................................................................... 2894.1 Heterogeneous Non-Catalytic Reactors ..................................................................... 289

4.1.1 Heterogeneous GasSolid Reactions ........................................................ 2894.1.1.1 Shrinking Core Model ................................................................. 2914.1.1.2 Reactors for GasSolid Reactions ............................................... 299

4.1.2 Heterogeneous GasLiquid Reactions ...................................................... 3174.1.2.1 Derivation of Global Rate Equations ......................................... 3204.1.2.2 Design of Packed Bed Reactors for GasLiquid Reactions .... 327

4.2 Heterogeneous Catalytic Reactions and Reactors ................................................3344.2.1 Reaction in a Single Catalyst Pellet ...........................................................334

4.2.1.1 Internal Pore Diffusion and Reaction in a Slab-Shaped Catalyst Pellet ................................................................................ 337

4.2.1.2 Internal Pore Diffusion and Reaction in a Spherical Catalyst Pellet ................................................................................ 341

4.2.1.3 Modified Thiele Modulus ......................................................3464.2.1.4 Modification of the Thiele Modulus for a Reversible

Reaction .........................................................................................348

x Contents

4.2.1.5 Diffusion and Reaction in a Single Cylindrical Pore within the Catalyst Pellet ............................................................350

4.2.1.6 Global Rate Equation ...................................................................3534.2.2 Catalytic Reactors ........................................................................................354

4.2.2.1 Two-Phase Catalytic Reactors ..................................................... 3554.2.2.2 Three-Phase Catalytic Reactors ..................................................365

Exercise Problems ................................................................................................................ 370MATLAB Programs ........................................................................................................... 372

Section II Green Chemical Processes and Applications

5 Green Reactor Modelling .................................................................................................. 3955.1 Novel Reactor Technology ....................................................................................... 395

5.1.1 Micro-Reactor ............................................................................................... 3955.1.1.1 Characteristics of Micro-Reactors .............................................. 396

5.1.2 Microwave Reactor ...................................................................................... 3995.1.3 High-Pressure Reactor ................................................................................4005.1.4 Spinning Disk Reactor ................................................................................400

5.2 Some Reactor Design Software and Their Applications .....................................4025.2.1 gPROMS: For Simulation and Modelling of Reactors ............................4025.2.2 ANSYSReactor Design ............................................................................403

5.2.2.1 Computational Fluid Dynamics.................................................4035.2.2.2 CFD Modelling of Multiphase Systems .................................... 407

5.3 ASPEN Plus Simulation of RCSTR Model ............................................................. 4185.3.1 Simulation of CSTR Model ......................................................................... 4195.3.2 Conclusions ...................................................................................................427

6 Application of Green Catalysis and Processes .............................................................4296.1 Introduction to Application of Green Catalysis and Processes ..........................4306.2 Case Study 1: Treatment of Industrial Effluents Using Various Green

Catalyses ..................................................................................................................... 4316.2.1 Introduction .................................................................................................. 432

6.2.1.1 Properties of Zeolites ...................................................................4346.2.1.2 Zeolite Na-Y ...................................................................................4366.2.1.3 Applications of Zeolites ...............................................................440

6.2.2 Adsorption of Dyes onto Zeolite ...............................................................4426.2.2.1 Acid Orange 7 Dye .......................................................................4436.2.2.2 Methyl Orange Dye ......................................................................4436.2.2.3 Methylene Blue .............................................................................4436.2.2.4 Safranine Dyes ..............................................................................444

6.2.3 Catalytic WPO ..............................................................................................4456.2.3.1 Experimental Design ...................................................................4456.2.3.2 Results and Discussions .............................................................. 4516.2.3.3 Conclusions and Recommendations ......................................... 459

6.3 Case Study 2: Thermolysis of Petrochemical Industrial Effluent ......................4666.3.1 Source of Wastewater .................................................................................. 4676.3.2 Experimental Procedure ............................................................................. 4676.3.3 Kinetic Studies .............................................................................................468

xiContents

6.3.4 Results and Discussion ............................................................................... 4706.3.5 Conclusions ................................................................................................... 472

6.4 Case Study 3: Catalytic Wet-Air Oxidation Processes ......................................... 4746.4.1 Introduction ................................................................................................. 475

6.4.1.1 Alcohol Production in India ....................................................... 4766.4.1.2 Wastewater Generation and Characteristics ............................ 4796.4.1.3 Wastewater Treatment Methods ................................................ 4816.4.1.4 Drawbacks of Different Technologies ....................................... 4816.4.1.5 Wet Air Oxidation ........................................................................ 482

6.4.2 Literature Survey .........................................................................................4836.4.3 Experimental Setup and Design ................................................................4866.4.4 Results and Discussions .............................................................................4866.4.5 Conclusions ................................................................................................... 491

References ................................................................................................................................... 493

Further Reading ......................................................................................................................... 499

Index ............................................................................................................................................. 501

xiii

Foreword

I am delighted to write this foreword for a timely and well-organised book that integrates chemical reaction engineering and green chemical engineering. Recently, clear signs have emerged informing us that we may have hit the carrying capacity of our planet as a result of our incessant endeavour towards economic growth. In this regard, there is an urgent need to educate everyone, particularly young chemical engineers, about environmentally benign processing and sustainability.

Having known the authors for a long time, I am not surprised that the book is com-prehensive and thorough in its coverage. With precise writing and elaborate illustrations, the book should be very accessible to both graduate and undergraduate students. The authors have also struck a neat balance between analytical and numerical approachesstudents will be enriched by the elegance of the analytical derivations and the numerical/CFD approaches that help them to solve problems not amenable to analytical approaches. Faculty teaching courses on numerical methods and students who are in the process of mastering computational methods for problem solving will also find this book a useful resource. Case studies discussed in Chapter 6 can form the basis for capstone design proj-ects or to introduce mini-projects within courses such as reaction engineering, sustain-ability and so forth.

I wish to congratulate the authors for putting together this informative and educational textbook. This book reflects the long-term student-centric approaches that the authors have adopted during their many years of university teaching as well as the concern they have for our planet. With this pedagogical base and care for the environment, the book is sure to resonate with readers for a long time to come.

Lakshminarayanan SamavedhamDepartment of Chemical and Biomolecular Engineering

National University of SingaporeSingapore

xv

Preface

The chemical industry, an important prime mover of economic growth and development of any nation, focuses on producing useful chemical products that are essential for meet-ing the high quality and standard of living demanded by humankind. Although the con-tribution of the chemical industry to wealth generation is well recognised, it is generally perceived as a major source of pollution and environmental degradation. Chemical indus-tries transform renewable and non-renewable chemical resources available in nature into useful chemical products and in the process generate unwanted side products that pollute the environment. Waste generated by these activities has slowly but surely impacted the environment and, if left unchecked, has the potential to threaten the very existence of humankind. Economic growth and development will become unsustainable unless the impact of these activities on the environment is reduced or minimised. There is a growing need to devise new technologies and methods of chemical processing that generate little or no pollution and are benign to the environment. Green Chemical Engineering is a novel approach that accounts for environmental impact in the design, development and opera-tion of various chemical engineering processes and helps in making those processes less environmentally impactive. The main goal of green chemical engineering is to achieve sustainability through pollution prevention and minimum utilisation of non-renewable natural resources. Sustainability is essentially about meeting the needs of the present gen-eration without compromising the ability of future generations to meet their own needs.

A chemical reactor is the most important component of any chemical processing indus-try in which key chemical transformations take place. It is the efficiency with which these transformations occur in a chemical reactor that determines the amount of waste gener-ated in the chemical process. Hence, designing a chemical reactor to achieve maximum performance is the key for waste minimisation. Chemical reaction engineering (CRE) pro-vides a scientific basis and methodology for quantifying the performance of a reactor as a function of design and operational variables. Thus, CRE plays a central role in green chemical engineering. Understanding various factors that influence the performance of a chemical reactor would provide a sound basis for designing the reactor to achieve maxi-mum performance. Some key factors which influence reactor performance are feed rate and reactor size, operating temperature and pressure, kinetic rate, transport rate, fluid flow and mixing pattern, adsorption characteristics, pore structure and surface topology of the catalyst. Reactor performance is a result of a complex interplay of all these factors. Incorporating the influence of these factors in the reactor design requires highly sophisti-cated computational tools such as CFD (computational fluid dynamics), advanced experi-mental measurement techniques and computer-based molecular simulation tools.

Organisation of the Book

In this book, we have made an attempt to integrate the concepts of chemical reaction engineering with green chemical engineering, highlighting the role of chemical reaction engineering in the design and development of green processes and green technologies

xvi Preface

that are benign to the environment. The book is organised into two main sections. Section I, which includes Chapters 2 through 4, covers the basic principles of chemical reaction engineering and reactor design. Section II, which includes Chapters 5 and 6, covers topics on green reactors, green catalysis and green processes.

Chapter 1 presents a brief introduction on green chemical engineering, highlighting the need for developing processes that are environment friendly and the role of green chemis-try and reaction engineering in designing such processes.

Chapter 2 covers the basic principles of chemical kinetics and catalysis and gives a brief introduction on classification and types of chemical reactors. Differential and integral methods of analysis of rate equations for different types of reactionsirreversible and reversible reactions, autocatalytic reactions, elementary and non-elementary reactions, and series and parallel reactions are discussed in detail. Development of rate equations for solid catalysed reactions and enzyme catalysed biochemical reactions are presented. Methods for estimation of kinetic parameters from batch reactor data are explained with a number of illustrative examples and solved problems.

Chapter 3 covers topics on homogeneous reactors including ideal, non-ideal and non-isothermal reactors. Explicit design equations are derived for ideal homogeneous reactors. Graphical procedures for design of multiple reactors are presented. Design of homoge-neous reactors for series parallel reactions and polymerisation reactions are discussed. Procedures are developed for optimal design of non-isothermal reactors and adiabatic reactors. Topics on non-ideal reactors highlighting various models for non-ideal mixing patterns are covered in detail. Concepts are explained through a number of illustrative examples and solved problems.

Chapter 4 covers topics on heterogeneous reactors including catalytic and non-catalytic reactors. Various models for non-catalytic gassolid and gasliquid reactions are presented and global rate equations are derived. Explicit design equations are derived for a variety of non-catalytic reactorsfluidised-bed, moving-bed and packed-bed reactors. Global rate equations are derived for reactions occurring in a catalyst pellet accounting for external and internal mass transfer and surface reaction. Design equations are derived for a num-ber of catalytic reactorspacked-bed, fluidised-bed and slurry reactors. The designs of catalytic and non-catalytic reactors are illustrated through solved examples.

Chapter 5 gives an overview of novel green reactors and the application of the CFD technique in modelling of green reactors. This chapter presents detailed discussions on a number of novel reactors, namely, the microreactor, microwave reactor and spinning disc reactor. A brief introduction on CFD and the application of CFD in modelling laminar mixing in a stirred tank reactor is presented.

Chapter 6 covers applications and case studies on the development of green catalysts and green processes. Three case studies are presented in this chapter highlighting the development of a green catalytic process and the application of a green catalyst in the treatment of industrial effluent.

A number of solved and exercise problems are included in Chapters 2 through 4 for better understanding of various concepts of reaction engineering covered in this book. An impor-tant feature of this book is the use of MATLAB software to develop computer codes for solving a number of reaction engineering problems. The listing of the codes are included at the end of Chapters 2 through 4. The MATLAB codes are also made available in the CD accompanying the book. The reader can make use of these codes for solving a variety of problems illustrated in the book and also for solving more advanced level problems.

In our opinion, the concepts of reaction engineering presented in this book will be use-ful reference material for teaching chemical reaction engineering at the undergraduate

xviiPreface

and graduate level. Graduate and research students will find the material on green chem-ical engineering presented in this book useful for understanding the basic concepts of green engineering and for pursuing further research in this area. Thus, this book intends to address both pedagogical and research interests of the readers in the area of green pro-cess engineering.

Any comments or suggestions on this book may kindly be mailed to [email protected] or [email protected].

S. Suresh Department of Chemical Engineering

Maulana Azad National Institute of TechnologyBhopal, Madhya Pradesh, India

S. Sundaramoorthy Department of Chemical Engineering

Pondicherry Engineering CollegePuducherry, India

MATLAB and Simulink are registered trademarks of The MathWorks, Inc. For product information, please contact:

The MathWorks, Inc.3 Apple Hill DriveNatick, MA 01760-2098 USATel: 508 647 7000Fax: 508-647-7001E-mail: [email protected]: www.mathworks.com

xix

Acknowledgements

A number of people have contributed directly or indirectly in our endeavour to write this book. Each of us would first wish to acknowledge separately those individuals who helped us achieve this goal.

S. Suresh

It is a pleasant privilege to acknowledge Professor C.N.R. Rao, director, International Centre for Material Science (India), Professor P.K. Bhattacharya of the Department of Chemical Engineering, Indian Institute of Technology Kanpur (India), Professor I.M. Mishra of the Department of Chemical Engineering, Indian Institute of Technology Roorkee (India), Professor K.K. Appukuttan, director, MANIT Bhopal (India) and Dr. V.C. Srivastava of the Department of Chemical Engineering, Indian Institute of Technology Roorkee (India), for their constant advice and encouragement.

A special thanks to Dr. Sachin Kumar Sharma, director of research, Viresco Energy LLC, USA and Dr. Amit Keshav, Department of Chemical Engineering, NIT Raipur (India) for helping me at different times during the preparation of chapters, problems and case studies.

Last but not the least, I wish to appreciate the role of my family members, my parents and my wife, S. Arisutha and my daughter Krithi for constant support and endurance dur-ing the period of writing this book

S. Sundaramoorthy

A special thanks to my teachers Professor M.G. Subba Rau and Professor P.N. Singh at Karnataka Regional Engineering College, Surathkal, India (now known as NIT Surathkal), who inspired me through their love and passion for teaching. I am very grateful to my beloved teacher and research supervisor Professor Ch. Durgaprasad Rao at IIT Madras, India for being a great source of inspiration and encouragement all through my research and academic career. My respectful thanks to Professor K. Ethirajulu, former princi-pal of Pondicherry Engineering College (PEC) whose valuable thoughts, words and deeds have profoundly influenced my academic career. My special thanks to Professor D.Govindarajulu, principal, PEC for his support and encouragement in writing this book.

Last but not the least, I wish to place on record my gratitude to my wife Hemalatha, and my son Arunsenthil for their constant encouragement, support, love and affection.

Both of us would jointly acknowledge the support extended by the following people in writing this book:

We profoundly thank Dr. Lakshminarayanan Samavedham, associate professor, National University of Singapore for going through the manuscript and for penning his valuable thoughts in the Foreword.

A number of faculty and staff members of MANIT Bhopal (India) and PEC Puducherry (India) have encouraged us in various ways, personal as well as professional and we are grateful to all of them.

xx Acknowledgements

We thank Mr Ravi Kusma Teja, Mr Shakti Nath Das, Mr Shashank Tiwari and all other students of MANIT who have cheerfully typeset pages and pages of handwritten manu-script of this book.

It has been a great pleasure working with CRC Press (Taylor & Francis Group) and we look forward to yet another opportunity of working with them again in future.

xxi

Authors

S. Suresh is an assistant professor of chemical engineer-ing at Maulana Azad National Institute of Technology, Bhopal, India. He earned a PhD from the Indian Institute of Technology, Roorkee, India, in the area of environmen-tal pollution control. He has held various research positions at a number of universities in India including Pondicherry University, Indian Institute of Technology Kanpur and the International Centre for Materials Science, JNCASR, Bangalore. His research interests are in the areas of sepa-ration processes, reactor design, adsorption, catalysis, waste utilisation and nanomaterials. He has written a number of research articles and books in his area of research. In rec-

ognition of his research contributions, he has received a number of awards and honours including the Young Scientist Award instituted by the Government of Uttarakhand, India and the Best Environmental Engineer award conferred by the Institution of Engineers (India).

S. Sundaramoorthy is a professor of chemical engineering at Pondicherry Engineering College, Puducherry, India. He earned his PhD from the Indian Institute of Technology Madras, India, in the area of process control. He has over 28 years of teaching and research experience. Dr. Sundaramoorthy has held teaching and visiting research positions, respectively, at the National Institute of Technology Karnataka, Surathkal and the National University of Singapore. His research inter-ests are in the areas of model-based predictive control, mem-brane separations, process integration and optimisation. He has published many research articles and has delivered a number of keynote and invited lectures at international and national conferences.

xxiii

Nomenclature

Notationsa activity of a catalysta,b,,r,s stoichiometric coefficients for reacting substances A, BR, Sa interfacial area per unit volume of tower (m2/m3)A cross-sectional area of a reactor (m2)A,B reactantsC concentration (mg/L)CM Monod constant (mol/m3); or Michaelis constant (mol/m3)Cp heat capacity (J/mol K)C CpA pA , mean specific heat of feed, and of completely converted product stream, per mole of key

entering reactant (J/mol A+ all else with it)d diameter (m)d order of deactivationD molecular diffusion coefficient (m2/s)De effective diffusion coefficient in porous structures (m3/m solid.s)ei(x) an exponential integralC degree CelsiusC0 initial concentration of adsorbate in solutionCe equilibrium liquid-phase concentrationCL heat capacity of a liquidCp heat capacity of a gas at constant pressureCS adsorbent concentration in the solutionCt equilibrium liquid phase concentration after time tC0 initial concentration of adsorbate in solutionC0,i initial concentration of each component in solutionCe unadsorbed concentration of the single-component at equilibriumD dipole moment; diffusivity; distillate flow rate; amount of distillate; desorbentDe , Deff effective diffusivity (m2/s)Di impeller diameterDp effective packing diameter; particle diameterDs surface diffusivityE() E-curve w. r. t. timeF feed rate (mol/s or kg/s)F() F-curve w. r. t. timeEa activation energy[ES] concentration of complex ES[S] concentration of substrate S[E] concentration of free enzyme EH phase distribution coefficient or Henrys law constant; for gas-phase systems H = p/C (Pa.m3/mol)H HA A , Enthalpy of unreacted feed stream, and of completely converted product stream, per mole of A

(J/mol A + all else)Hr, Hf, Hc heat or enthalpy change of reaction, of formation, and of combustion (J or J/mol)k reaction rate constant (mol/m3)1n s1

kd rate constant for the deactivation of catalystkeff effective thermal conductivity (W/m K)kg mass transfer coefficient of the gas film (mol/m2 Pa s)

xxiv Nomenclature

kl mass transfer coefficient of the liquid film (m3 liquid/m2 surface s)K equilibrium constant of a reaction for the stoichiometrym mass flow rate (kg/s)M mass (kg)n order of reactionN number of equal-size mixed flow reactors in seriesNA moles of component APA partial pressure of component A (Pa)pA* partial pressure of A in gas which would be in equilibrium with CA in the liquid; hence, p H CA A A

*= (Pa)

Q heat duty (J/s = W)qt adsorbed quantity of dye (mg/g)rc radius of unreacted core (m)R radius of particle (m)R ideal gas law constant, = 8.314 J/mol K = 1.987 cal/mol K = 0.08206 lit atm/mol KR recycle ratios space velocity (s1)S surface (m2)t time (s)T temperature (K or C)u* dimensionless velocityv volumetric flow rate (m3/s)V volume (m3)W mass of solids in the reactor (kg)XA fraction of A converted, the conversionXA moles A/moles inert in the liquidXA fractional conversion of AYA moles A/mole inert in the gasF(t) fractional uptake of adsorbate on adsorbent, 0 < F(t) < 1G Gibbs free energy; mass velocity; volumetric holdup on a tray; rate of growth of crystal sizeG Gibbs free energy for active complex formationG0 Gibbs free energy of adsorptionH enthalpy for active complex formationH0 enthalpy of adsorptionHw heat of adsorption of water (normally assumed to be zero)Hsol heat of solutionHst,0 isosteric heat of adsorption with zero coverageHst,a apparent isosteric heat of adsorptionHst,net net isosteric heat of adsorptionk mass-transfer coefficient that takes into account the bulk flow effect; rate constantK equilibrium ratio for vapour-liquid equilibria; equilibrium partition coefficient in and for a component

distributed between a fluid and a membrane; overall mass transfer coefficient; adsorption equilibrium constant

KG overall mass-transfer coefficient based on the gas phase with a partial pressure driving forceKL overall mass-transfer coefficient based on the liquid phase with a partial pressure driving forcek0B constant in Bangham equationk0 pre-exponential factor in Arrhenius equationkA adsorption rate constant for the adsorption equilibriumkB Boltzmann constantkD desorption rate constant for the adsorption equilibriumkd distribution coefficientkf rate constant of pseudo-first-order adsorption modelkid intra-particle diffusion rate constantkS rate constant of pseudo-second-order adsorption modelKF constant of Freundlich isothermKEL,i constant of extended-Langmuir isotherm for each component

xxvNomenclature

KL constant of Langmuir isothermKR constant of RedlichPeterson isothermKF mono-component (non-competitive) constant of Freundlich isotherm of the single componentKF,i individual Freundlich isotherm constant of each componentKL constant of Langmuir isothermKL,i individual Langmuir isotherm constant of each componentKT equilibrium binding constant corresponding to the maximum binding energyL liquid; length; height; liquid flow rate; under flow rate; crystal sizem slope of equilibrium curve; mean flow rate; mass; mass of adsorbent per litre of solutionM molecular weight; molar liquid holdupn molar flowrate; moles; constant in Freundlich equation; number of pores per cross-sectional area of

membrane; number of unitsN number of phases; number of moles; molar flux; number of equilibrium (theoretical stages); rate of

rotation; number of transfer units; cumulative number of crystals size; number of stable nodes; number of data points

NNu Nusselt number = temperature gradient at wall or interface/temperature gradient across fluidNRe Reynolds number = inertial force/viscous forceNPr Prandtl number = momentum diffusivity/thermal diffusivityNPe Peclet number = convective transport/molecular transferNSc Schmidt number momentum diffusivity/mass diffusivityNSh Sherwood number = concentration gradient at wall or interface/concentration gradient across

fluidNSt Stanton number for mass transfer, heat transferNOG number of overall gas-phase transfer units1/n mono-component (non-competitive) Freundlich heterogeneity factor of the single component,

dimensionless1/ni individual Freundlich heterogeneity factor of each component, dimensionlessP pressure; powerp partial pressureP percentage contributionpc critical pressurePM permeability; permeancePr reduced pressure; vapour pressurepH0 initial pH of the solutionqe equilibrium single-component solid-phase concentrationqe,i equilibrium solid-phase concentration of each component in binary mixtureqe,cal calculated value of solid-phase concentration of adsorbate at equilibriumqe,exp experimental value of solid-phase concentration of adsorbate at equilibrium, mg/gqm maximum adsorption capacity of adsorbentqmax constant in extended Langmuir isothermqt amount of adsorbate adsorbed by adsorbent at time tQ rate of heat transfer; volume of liquid; volumetric flow rate; adsorption energyQL volumetric liquid flow rateR universal gas constant, 8.314 J/K molR2 correlation coefficientsRa radius of the adsorbent particle assumed to be sphericalRmin minimum reflux ratior radius; reaction raterH hydraulic radius = flow cross section/wetted perimeterrp pore radiusRL liquid-phase withdrawal factor; separation factor, dimensionlessS solid; rate of entropy; total entropy; solubility; cross-sectional area for flow; solvent flow rate; mass of

adsorbent; stripping factor; surface area; inert solid flow rate; flow rate of crystals; supersaturationSg surface area per unit volume of a porous particleSST total sum of squaresS active site on the adsorbentS change in the entropy for active complex formationS0 change in the entropy for adsorption

xxvi Nomenclature

T overall mean of responset time; residence time; average residence timetb time to breakthrough in adsorptiontE elution time in chromatographyT absolute temperatureTc critical temperatureU superficial velocity; overall heat-transfer coefficient; liquid stream molar flow rateUa superficial vapour velocityu velocity; interstitial velocity; bulk average velocity; flow average velocityuc superficial liquid velocityUr usage rateus superficial velocityuv gas velocityV vapour; volume; vapour flow rate; overflow rate; volume of the solutionVp pore volume per unit mass of particlew mass fraction; mass of the adsorbentWi initial weight of the sampleW actual weight of the sampleWf final weight of the samplex mole fraction in liquid-phase; mole fraction in any phase; distance; mass fraction in raffinate; mass

fraction in underflow; mass fraction of particlesX equilibrium moistureXB bound moisture contentXc critical free moisture contentXT total moisture contentXAe fraction of the adsorbate adsorbed on the adsorbent under equilibriumXi mass of solute per volume of solidy mole fraction in vapour phase; distance; mass fraction in extract; mass fraction in overflowY mole or mass ratio; mass ratio of soluble material to solvent in overflow; concentration of solute in

solventz mole fraction in any phase; overall mole fraction in combined phases

Greek Letters dirac delta function, an ideal pulse occurring at time t = 0 (s1) viscosity of fluid (kg/m s) density or molar density (kg/m3 or mol/m3)2 variance of a tracer curve or distribution function (s2) time for complete conversion of a reactant particle to product (s); time Thiele modulus HATTA number constant of RedlichPeterson isotherm wavelength effectiveness factor mean residence timeB batch reaction times density of solid particlesB molal density of reactants fractional solids hold up of the bedd volume fraction of the dense phase bed porosity summation symbol Angstrom

xxviiNomenclature

AbbreviationsAAS atomic absorption spectrometryAC activated carbonAFS atomic fluorescence spectrometryASTM American Society of Testing MaterialAvg averagebar 0.9869 atmosphere or 100 kPabbl barrelBET BrunauerEmmettTellerBFA bagasse fly ashBFB bubbling fluidised bedBJH BarrettJoynerHalendaBOD Bio-chemical oxygen demandBR batch reactorBtu British thermal unitC degrees Celsius, K-273.3CAD computer aided designCAE computer aided engineeringcal calorieCEC cation exchange capacityCFB circulating fluidised bedCFD computational fluid dynamicscfs cubic feet per secondCI color indexcm centimetercmHg pressure in centimetres head of mercuryCOD chemical oxygen demandcP centipoiseCSTR continuous stirred tank reactorCT computed tomographyCWPO catalytic wet peroxide oxidationD-R DubininRadushkevichDOE design of experimentsDOF degree of freedomDTA differential thermal analysisDWW distillery wastewatere exponential functionE-factor environmental factorECCP European Climate Change ProgrammeECD electron capture detectorEDAX energy-dispersive atomic x-rayEOF electro osmotic floweq equivalentexp exponential functionF degree Fahrenheit, R 459.7FA fly ashFF fast fluidised bedFID flame ionisation detectorft feetFTIR fourier transform infra red spectroscopyg gramg-mol gram-moleGAC granular activated carbonGC gas chromatographyGCMS gas chromatographymass spectrometryGDP gross domestic product

xxviii Nomenclature

gpd gallons per dayGUI graphical user interfaceh hourHPLC high-performance liquid chromatographyHYBRID hybrid fractional error functionJCPDS Joint Committee on Powder Diffraction StandardsJ jouleK degrees Kelvinkg kilogramkmol kilogram-moleL litrelb poundlbf pound-forceLFR laminar flow reactorLHHW LangmuirHinshelwoodHouganWatsonLHS left-hand side of an equationln logarithm to the base elog logarithm to the base 10M-M MichaelisMentenmax maximummeq milliequivalentsMFR mixed flow reactormg milligrammin minute; minimumm metermm milimetermmHg pressure in mm head of mercuryM molarmmol millimole (0.001 mole)mol gram-moleMPSD Marquardts percent standard deviationMTBE methyl tert-butyl etherMW molecular weightmw molecular weight (kg/mol)N Newton; normalNm nanometerP phenolPCM progressive conversion modelPC pneumatic conveyingPe Peclet numberPFR plug flow reactorPLA poly lactic acidppm parts per millionpsi pounds force per square inchpsia pounds force per square inch absolutePTFE polytetrafluroethyleneR-P RedlichPetersonRHA rice husk ashRHS right-hand side of an equationRpm rotation per minRTD residence time distributionSBU secondary building unitSCM shrinking-core modelSDR spinning disk reactorSEM scanning electron microscopes secondSSE sum of square of error

xxixNomenclature

SSE sum of squares of errorstm steamSTP standard conditions of temperature and pressure (usually 1 atm and either 0C or 60F)SZ synthesised zirconiaTB turbulent fluidised bedTCD thermal conductivity detectorTGA thermogravimetric analysisTG thermal gravimetryUSEPA United States Environmental Agency ProtectionVOC volatile organic compoundvs versusWAO wet air oxidationwt WeightXRD x-ray diffractionyr yeary yearZr zirconiaZX zeolitem micrometer

Dimensions and Units

Conversion Factors for American Engineeringand CGS Units to SI Units

To Convert from To Multiply by

Areaft2

in.2m2

m20.09296.452 104

Accelerationft/h2 m/s2 2.325 108

DensityLbm/ft2

Lbm/gal (US)g/cm3

kg/m3

kg/m3

kg/m3 = g/L

16.02119.81000

Diffusivity, kinematic viscosityft2/hcm2/s

m2/sm2/s

2.581 1051 104

Energy, work, heatft. lbfBtu (IT)cal (IT)ergkW h

J = N mJ = N mJ = N mJ = N mJ = N m

1.35610554.1871 1073.6 106

continued

xxx Nomenclature


EnthalpyBtu (IT)/lbmCal (IT)/g

J/kg = N m/kgJ/kg = N m/kg

23264187

Forcelbfdyne

NN

4.4481 105

Heat-transfer coefficientBtu (IT)/h ft2 Fcal (IT)/s cm2 C

W/m2 KW/m2 K

5.6794.187 104

Interfacial tensionlbf/ftdyne/cm

N/m = kg/s2N/m = kg/s2

14.591 104

Lengthftin.

mm

0.30480.0254

Masslbmtontonne (metric ton)

kgkgkg

0.4536907.21000

Mass flow ratelbm/hlbm/s

kg/skg/s

1.26 1040.4536

Mass flux, mass velocitylbm/h ft2 Kg/s m2 1.356 103

Powerft lbf/hft lbf/shpbtu(IT)/h

W = J/s =N m/sW = J/s =N m/sW = J/s =N m/sW = J/s =N m/s

3.766 1041.356745.70.2931

Pressurelbf/ft2

lbf/in2

atmBartorr = mmHgIn. HgIn. H2O

Pa = N/m2Pa = N/m2Pa = N/m2Pa = N/m2Pa = N/m2Pa = N/m2Pa = N/m2

47.8868951.013 1051 105133.33386249.1

Specific heatBtu(IT)/lbm Fcal/g C

J/kg K = N m/kg KJ/kg K = N m/kg K

41874187

Surface tensionlbf/ftdyne/cmerg/cm2

N/mN/mN/m

14.590.0010.001

continued

xxxiNomenclature

Physical Constants

Universal (Ideal) Gas Law Constant, R

1987 cal/mol K or Btu/lbmol F8315 J/kmol K or Pa m3/kmol K8.315 kPa m3/kmol K0.08325 bar L/mol K82.06 atm cm3/mol K0.7302 atm ft3/lbmol R10.73 psia ft3/lbmol R1544 ft lbf/lbmol R62.36 mmHg L/mol K21.9 in. Hg ft3/lbmol R

Atmospheric Pressure (Sea Level)

101.3 kPa = 101300 Pa = 1.013 bar760 torr = 29.92 in. Hg1 atm = 14.696 psia

Avogadros Number

6.022 1023 molecules/mol

Boltzmann Constant

1.381 1023 J/K molecules


Thermal ConductivityBtu (IT) Ft/h ft2 Fcal (IT) cm/s cm2 C

W/m K = J/s m KW/m K = J/s m K

1.731418.7

Velocityft/hft/s

m/sm/s

8.467 1040.3048

Viscositylbm/ft slbm/ft hcP

kg/m skg/m skg/m s

1.4884.134 1040.001

Volumeft3

lgal (US)

m3

m3

m3

0.028321 1033.785 103

xxxii Nomenclature

Faradays Constant

96,490 charge/g-equivalent

Gravitational Acceleration (Sea Level)

9.807 m/s2 = 32.174 ft/s2

Joules Constant (Mechanical Equivalent of Heat)

4.184 J/cal778.2 ft lbf/Btu

Plancks Constant

6.626 1034 J s/molecule

Speed of Light in Vacuum

2.998 108 m/s

StefanBoltzmann Constant

5.671 108 W/m2 K4

0.1712 108 Btu/h ft2 R4

11Introduction

Chemical engineering deals with the transformation of chemicals from one form (raw materials which are renewable and non-renewable resources) to another form (products), which is useful to man for meeting the requirements of comfortable living. This process of accomplishing the transformation from raw material form to product form invariably leads to the generation of some unwanted side products (pollutants) and depletion of renewable and non-renewable resources. The demand for useful chemical products (such as cement, sugar, pulp and paper, pharmaceuticals, petroleum and petrochemicals, etc.) is ever growing with a rising population and affluent living standards driven by economic growth. By 2025, the production capacity of chemicals in Asia alone, which is the most populous region on the earth, is expected to increase by five to six times its capacity in 2000. This will result in faster consumption of natural resources such as fossil fuel and fresh water and steeper rise in the levels of pollution in the environment. The long-term and short-term impact of this on the environment, if left unchecked, has the potential to threaten the very existence of life on earth.

Environment factor or E-factor is defined as the mass of waste generated per unit mass of a product created and, hence, is a measure of impact on the environment caused by a chemical industry. The E-factor values for different categories of chemical industries are listed in Table 1.1. Contrary to common belief, chemical industries producing high value-added chemical products such as pharmaceuticals and fine chemicals contribute more to environmental degradation than refining and bulk chemical industries.

Economic growth and development at the expense of environmental degradation is unsustainable in the long run. The need and the necessity for striking a suitable balance between economic development and environmental degradation have given birth to the concept of sustainable development. In 1987, the United Nations World Commission on Environment and Development defined sustainable development as meeting the needs of the present without compromising the ability of future generations to meet their own needs. Bakshi and Fiksel have defined a sustainable process or product as one in which resource consumption and waste generation are kept at acceptable levels while making a positive contribution to societal needs, and generating long-term profit to the business enterprise. Thus, sustainable development involves complex interaction between industry, society and the ecosystem.

Although chemical industries contribute to 1215% of the gross domestic product (GDP) of any developing nation, they are generally perceived by society as a major source of pollution and a major cause for environmental degradation. So, there is a greater expec-tation on the chemical engineering community to play a responsible role in protect-ing the environment by way of developing and adapting newer technologies and newer methods of production that are friendlier to the environment. Any technology that is environment friendly is known as Green Technology. Green technologies are sustain-able technologies of higher material and energy efficiencies that use renewable resources and prevent or minimise pollution at the source rather than treating it at the end of the pipe. One may consider the total pollution generated in producing a chemical as a

2 Green Chemical Engineering

product of total global population, consumption per capita and the inefficiency of the process. It is not possible to regulate or reduce the population growth or consumption per capita as any attempt to curb these factors through governmental regulations would lead to social and political unrest and economic instability. So, the only viable option for reduction or minimisation of pollution is to develop newer technologies and processes that are more efficient in terms of material and energy utilisation and minimisation of waste generated.

1.1 Principles of Green Chemistry and Green Chemical Engineering

Paul Anastas and John Warner in their book published in 1998 introduced the concept of green chemistry as a philosophy for the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. The founders of green chemistry formulated 12 basic principles, the application of which in the practice of chemical engineering is expected to lead to the development of ecofriendly products and processes. The 12 basic principles of green chemistry are discussed below:

1. Prevent waste: Design the process such that waste is prevented at the outset rather than treating or cleaning up waste after it has been generated.

2. Maximise atom economy: Synthesise or design methods that maximise incorpora-tion of all material used in the process into the final product so that the generation of waste is minimised. Select an alternative synthesis route or raw material such that the amount of side reactions generating undesired by-products is minimal. Atom economy, which is defined as

Atom economy

Molecular weight of desired productSum of mol

=

eecular weight of all the products produced

is taken as a measure of how efficiently the raw material is used. Raw material yielding maximum atom efficiency is selected. For example, maleic anhydride can be produced using either benzene or n-butane as raw material and the cor-responding reactions are

2 9 2 46 6 2 4 2 3 2 2C H O C H O H O CO from benzene+ + + ( )

TABLE 1.1

E-Factor Values for Different Categories of Chemical Industries

Categories of Chemical IndustriesProduction Capacity

(Tonnes/Year)E-Factor (Waste/Product

Ratio by Weight)

Oil refinery 106108 0.1Bulk chemicals 104106 15Fine chemicals 102104 550Pharmaceuticals 101103 25100

3Introduction

C H O C H O H O from -butane4 10 2 4 2 2 23 5 4+ +. ( )n

Atom economy for the n-butane route is 57.6% and 44.4% for the benzene route. Thus, n-butane is preferred over benzene as a raw material for production of maleic anhydride. Choosing an atom efficient raw material is the first step in designing an environment-friendly chemical process.

3. Design less hazardous chemical synthesis: Synthesise or design methods that use and generate substances that minimise toxicity to the environment.

4. Design safer chemicals and products: Design chemical products with least toxic con-tamination to the environment.

5. Use safer solvents and auxiliaries: Reduce the use of solvents that cause pollution. If necessary, use solvents that are benign to the environment. In this context, super-critical CO2 (scCO2) and ionic liquids show remarkable potential as ecofriendly substitutes for solvents that are toxic to the environment.

6. Design for energy efficiency: Design processes for higher energy efficiency so that net consumption of energy (fuel) is minimised and the impact of energy usage on the environment is reduced. The processes that operate at ambient tempera-ture and pressure consume less energy and are preferred over high-temperature and high-pressure systems. The possibility of reducing energy and material con-sumption through appropriate processes and heat integration should be explored. Novel combo systems that integrate reactors with mass exchangers offer plenty of opportunities for the design of energy-efficient chemical processes.

7. Use renewable feed stocks: Choose a raw material or a feed stock that is renewable rather than depleting, wherever possible. The alternate route for synthesis of chemicals using biomass as feed stock is an option to be explored. For example, biochemical synthesis of adipic acid using d-glucose as a feed stock is an ecofriendly alternative to the traditional chemical synthesis of adipic acid using benzene as feed stock.

8. Reduce or avoid the use of chemical derivatives: The use of chemical derivatives should be avoided or minimised as this can lead to the need for additional reagents and further generation of waste.

9. Use catalysis in place of stoichiometric reagents: Catalyst-based synthesis of chemicals results in lower pollution generation compared to synthesis routes that make use of stoichiometric reagents. Thus, a catalyst plays a crucial role in the design of environmentally benign chemical processes. A significant improvement in waste reduction can be achieved through proper selection and design of solid catalysts.

10. Design chemicals to degrade after use: Chemical products should be so designed that at the end of their function, they break down into innocuous products and do not persist in the environment. For example, biopolymers such as poly lactic acid (PLA) and polyhydroxyalkanoates (PHA), used as substitutes for chemical plas-tics, exhibit excellent biodegradability, unlike plastics.

11. Analyse in real time for pollution prevention: Develop analytical methods that allow for real-time monitoring and control prior to the formation of hazardous substance.

12. Minimise potential for accident through safer chemistry: Substances used in chemi-cal processes should be chosen to minimise the potential for chemical accidents including releases, explosions and fire.


1.2 Chemical Reaction Engineering: The Heart of Green Chemical Engineering

Any chemical plant can be perceived as a system of units arranged in a particular sequence of material processing steps required to transform raw material to a final product. All pro-cessing units in a chemical industry can be broadly grouped into three sections: raw mate-rial pretreatment section, reactor section and separation or purification section. Of these three sections, the reactor section in which key chemical transformations take place is the heart of chemical processing and any improvement in the performance of the reactor sec-tion is likely to have a major impact on pollution prevention. Thus, chemical reaction engi-neering plays a central role in green chemical processing. Although the principles of green chemistry provide a road map to the development of green processes, it is the selection, design and operation of the reactor that determines whether a process would be successful or not. In most chemical processes, the choice of the reactor and its operation has a very strong influence on the number and type of separation units required on the upstream and downstream sides and hence has a profound impact on the environment.

Chemical reaction engineering provides the methodology for quantifying the reac-tor performance as a function of design and operational variables. Reactor performance, which is measured in terms of fractional conversion of reactants and product selectivity, is influenced by a number of factors such as feed rate, reactor size, temperature, kinetic rate, transport rate, mixing and flow patterns. Appropriate quantification (or modelling) of reactor performance requires a multi-scale approach involving system characterisation in a wide range of scales, from molecular to macro (reactor length in metres). So, pollu-tion prevention through proper choice and operation of the reactor requires the related issues to be addressed in all these scales. At the molecular level, understanding process chemistry helps to achieve maximum atom efficiency, understanding complex reaction mechanisms leads to the development of appropriate kinetic rate expressions used in reac-tor design and understanding the mechanism of catalytic reactions leads to the design of a catalyst for achieving maximum selectivity. At a meso scale, understanding fluid mix-ing and transport in eddies, transport in multiphase systems, transport within pores of a catalyst pellet and the effect of local transport on reaction rate is crucial for the design of reactors that achieve optimum performance. At a macro scale, understanding the effect of hydrodynamics on reactor performance is crucial for scale up and operation of reactors. Thus, a multiscale reactor modelling approach, which integrates the system description at all these three scales (molecular, meso and macro), is needed for the design of reactors to achieve optimum performance.

Multiscale reactor modelling and design approach requires highly sophisticated and advanced computational tools and experimental techniques. A proper quantification of reactor performance requires an appropriate description of how the reacting species are brought into contact by fluid mixing. In a multiphase reactor, one should be able to describe the flow and mixing patterns in each one of the phases. In the traditional approach to reac-tor design, either plug flow or completely mixed flow is usually assumed in each one of the phases. If this assumption does not match with the experimental observation, an axial dispersion model is assumed and the model parameter (Peclet number) tuned to match the experimental measurements. However, this traditional approach lacks the predict-ability required for design, scale-up and operation of novel and non-traditional reactors having complex fluid mixing patterns. A better description of the flow, mixing and phase

5Introduction

contacting pattern is required to develop more realistic reactor models. In this context, sophisticated computational fluid dynamics (CFD)-based models can capture and describe the complexities of fluid mixing more realistically. Thus, CFD is an important computa-tional tool useful for the design of novel reactors. However, sophisticated experimental measurement techniques are necessary to get information on fluid velocity and turbulence to validate the CFD-based models.

The selection and design of a catalyst play a crucial role in achieving high reactor per-formance and making the process environmentally benign. On selecting a catalyst that is appropriate for a particular application, the catalyst can be tailor made for optimum yield and selectivity. Advanced experimental and computer simulation techniques are used to study the surface topology, adsorption characteristics, pore structure and transport prop-erties that are useful for the design of a catalyst having requisite characteristics. A novel catalyst design combined with advanced reactor design technology can make any process economically viable and environmentally beneficial.

This book is written with a view of highlighting the importance and significance of chemical reaction engineering in green chemical processing. The book is organized into two sections. Section I includes Chapters 2 through 4 covering the concepts of reaction engineering, chemical kinetics, catalysis and chemical reactors. Section II, which includes Chapters 5 and 6, highlights the modelling of green reactors and the applications of green catalysis and processes.

Section I

Kinetics, Catalysis and Chemical Reactors

92Introduction to Kinetics and Chemical Reactors

Design and operation of chemical reactors in a chemical industry profoundly influence the impact that the industry may have on the surrounding environment. Understanding dif-ferent types of reactions and characterising their kinetic behaviour are important for opti-mal design and operation of chemical reactors. This chapter outlines the basic principles of chemical kinetics, methods of obtaining rate equations for different types of reactions, principles of catalysis and kinetics of catalytic reactions. A brief introduction on the types and classification of reactors is presented in this chapter.

2.1 Kinetics of Chemical Reactions

A chemical reaction is a process in which chemical compounds in one form (reactants) are transformed into another form (products). This transformation occurs at a speed that is influenced by a number of factors such as temperature, pressure and so on. Chemical kinetics deals with developing mathematical expressions for the speed or rate of chemical reactions. Studies on chemical kinetics provide data and information about the speed of reactions that are required for the engineering design of reactors. A review of some essen-tial principles of chemical kinetics is presented in this chapter.

2.1.1 Reaction Rate

Reaction rate is a measure of the speed of a chemical reaction. Consider an irreversible homogeneous reaction

aA + bB cC + dD

taking place in a vessel (batch reactor) of fixed volume V. Assume that a uniform condition is maintained in the reaction vessel by proper stirring of the fluid. Let nA, nB, nC and nD be the number of moles of A, B, C and D, respectively, in the reaction vessel at some particular time t. Let dnA, dnB, dnC and dnD be the change due to reaction in the number of moles of A, B, C and D, respectively, after a time lapse dt. The rate of reaction or specific reaction rate is defined as a change in the number of moles of a reactant or product per unit time per unit volume. Thus,

r AV

dndt

r

AA

B

= =

=

rate of conversion of per unit volume

rate

1

of conversion of per unit volumeBV

dndt

B=

1


r CV

dndt

r

CC

D

= =

=

rate of conversion of per unit volume

rate

1

of conversion of per unit volumeDV

dndt

D=

1

Rates of reactions of chemical species participating in the reaction are related to each other through the stoichiometric coefficients of the reaction as

r

ra

rb

rc

rd

A B C D=

=

= =( ) ( ) ( ) ( )

(2.1)

where r is called the specific reaction rate of the reaction in general (and not any particular chemical species) and its dimension is (kmol/m3) (s). For a constant-volume liquid-phase reaction, the rate can be expressed in terms of concentration of A, CA = nA/V as

r

dCdt

AA

= (2.2)

Note that rA is negative for reactants and (rA) is positive.

2.1.2 Extent of Conversion

The extent of conversion is a measure of fractional conversion of reactants achieved in a specified time. Let nA0 and nB0 be the initial number of moles of A and B, respectively, pres-ent in the reaction vessel of volume V. Let nA and nB be the number of moles of A and B, respectively, present in the vessel after some time t. The extent of conversion or fractional conversion of A is defined as the number moles of A converted in a specified time per moles of A present at the time of start-up. Thus,

x

n nn

nn

AAA A

A

A

A= =

=

Fractionalconversion of 0 0 01

Similarly,

x

n nn

nn

BBB B

B

B

B=

=

Fractionalconversion of = 0 0 01

In terms of concentrations of A and B, we can write

x

CC

xCC

AA

AB

B

B= = 1 1

0 0and

(2.3)

where CA0 and CB0 are initial concentrations of A and B. Substituting CA = CA0(1 XA) in Equation 2.2, we can write the rate equation rA in terms of conversion XA as

( ) = r C

dXdt

A AA

0

(2.4)


2.1.3 Rate Equation

According to the law of mass action, which is based on experimental observations, it is found that the reaction rate, rA, can be expressed as

=r kC CA An

Bm

(2.5)

wherek: specific reaction rate constant (kmol)/(m3) (s) (kmol/m3)m+nn: order of reaction with respect to Am: order of reaction with respect to B

Equation 2.5 is called the rate equation. n and m are called partial orders of the reaction with respect to A and B, and (n+m) is called the global order of the reaction. Studies on chemical kinetics of the reaction are aimed at developing an appropriate rate equation for the reaction. This is done by conducting the given reaction in a controlled manner in a batch or flow reactor, and observing the progress of the reaction over time through appropriate measurements of concentrations of chemical compounds participating in the reaction. The validity of law of mass action has been explained using collision theory. According to colli-sion theory, reaction between compounds A and B is caused primarily by a collision between molecules of A and B. The higher the concentrations of A and B, the higher the chances of collision between molecules of A and B. Thus, the rate of reaction increases with increase in concentrations of A and B (CA and CB). The higher the temperature of the reaction medium, the higher the internal energy of the molecules and the greater the intensity of collisions between the molecules. Thus, the rate of reaction increases with an increase in temperature. The influence of temperature on the rate of reaction is explained by the Arrhenius law, which defines the relationship between the reaction rate constant k and the temperature as

k k eE RT

=

0 /

(2.6)

wherek0: frequency factorE: activation energy of the reactionR: gas law constantT: temperature in K

Thus, the rate equation is a product of a temperature-dependent term and a concentra-tion-dependent term.

2.1.3.1 Activation Energy and Heat of Reaction

Every reaction is associated with some change in the energy levels of chemical species par-ticipating in the reaction. This energy transformation is shown in the energy level diagram (Figure 2.1).

Reactants at some energy state ER kJ/kmol (Point R in the diagram) pass through a high-energy activated state E* kJ/kmol (Point A) before finally getting transformed into prod-ucts that are at an energy state Ep kJ/kmol (Point P). The energy required to raise the reactants to an activated state is called activation energy E:

E E ER= ( )* kJ/kmol (2.7)


Activation energy E is also referred to as an energy barrier, as reactants have to neces-sarily cross this barrier before getting converted into products. The larger the value of E (energy barrier), the slower the rate of reaction. It is the intrinsic mechanism of a chemical reaction that determines the magnitude of activation energy. For some reactions, the acti-vation energy may be so high that the reaction may be rendered infeasible. The net change in energy (or enthalpy) associated with the chemical reaction is called the heat of reaction HR , defined as

H E ER P R= ( ) kJ/kmol (2.8)

Endothermic reactions are reactions in which the specific enthalpy of the products Ep is greater than the specific enthalpy of the reactants ER (Ep > ER), and the heat of reaction HR is positive. Endothermic reactions take away heat from the reaction medium and hence require a continuous supply of heat. On the contrary, exothermic reactions are reactions in which Ep < ER and the heat of reaction HR is negative. Exothermic reactions liberate heat to the reaction medium. The energy level diagram for the exothermic reaction is shown in Figure 2.2.

PROBLEM 2.1

The kinetic rate constant k for a first-order reaction was estimated at four different tem-peratures in a batch experiment. The estimated values of k are as follows:

Calculate the activation energy E.

T (K) 303 323 343 363k(s 1) 0.071 0.189 0.510 0.991

E* (activatedstate)

EP (product)

ER (reactant)

Heat of reaction

Reaction coordinates

Energy

Activation energy

HR

E

P

A

R

FIGURE 2.1Energy diagram for endothermic reaction.


According to the Arrhenius law,

k k e E RT= 0 /

which can also be written in the linear equation form as

ln lnk kE

RT= 0

Thus, a plot of ln k versus 1/T is a straight line with slope m = E/RT and intercept I = ln k0.

From the plot of ln k versus 1/T shown in Figure P2.1:Slope m = 4904.7 and intercept I = 13.55.

The activation energy = R m = 8.314 4904.7 = 40,778 kJ/kmol K

and k0 = eI = 7,64,756 kJ/kmol.Note: Refer MATLAB program: cal_active_energy.m

1T

(K1) 3.3 103 3.1 103 2.92 103 2.75 103

ln k 2.65 1.67 0.673 0.009

E* (activatedstate)

EP (product)

ER (reactant)

Heat of reaction

Reaction coordinates

Energy

Activation energy

HR

E

P

A

R

FIGURE 2.2Energy diagram for exothermic reaction.


2.1.3.2 Limiting Reactant

Consider the chemical reaction between compounds A and B, aA + bB cC, and let the rate equation be

( ) =r kC CA An

Bm

(2.9)

Both reactants A and B will get completely converted into products if the reactants A and B are present in the exact stoichiometric mole ratio.

CB0 is the minimum initial concentration of B required for the complete conversion of A.

C

ba

CB A0 0 =

(2.10)

where CA0 is the initial concentration of A.Assume that reactant B is made available very much in excess of the minimum quantity

required, that is, C CB B0 0 . Then reactant A, but not reactant B, will get completely con-verted into product. Even after complete conversion of A, a large amount of B will remain unconverted in the reaction vessel. As reactant B is present in excess, change in concentra-tion of B is negligible compared to the initial concentration of B, CB0 and hence CB can be treated as a constant for all practical purposes, that is, CB = CB0. The rate Equation 2.9 gets reduced to

= r k CA An

(2.11)

where k kCBm = .Reactant A is called the limiting reactant and it is the concentration of A, CA, that controls

the rate of reaction. Consider a second-order reaction between A and B with the rate equa-tion expressed as

=r kC CA A B (2.12)

Slope = 4904.7Intercept = 13.55

3

2.5

2

1.5

1

0.5

0

0.5

2.60 2.80 3.00 3.20 3.40

ln k

(1/T 103), K1

FIGURE P2.1Plot of ln k versus 1/T for estimation of activation energy.


If reactant B is used in excess quantity and A is the limiting reactant, then the rate Equation 2.12 gets reduced to

= r k CA A

which is a first-order rate equation. This reaction is called a pseudo-first-order reaction.

2.1.4 Elementary and Non-Elementary Reactions

Elementary reactions are single-step reactions in which a reaction leading to the forma-tion of product occurs on direct collision between reactant molecules and no intermediate compounds are formed. Consider the reaction between reactants A and B leading to the formation of product C, and let aA + bB cC be the stoichiometric equation for this reac-tion. Let the observed reaction rate of A be

=r kC CA An

Bm

(2.13)

For an elementary reaction, there is one-to-one correspondence between the order of the reaction observed and the stoichiometric coefficients. If this reaction is an elementary reaction, then n = a and m = b, and the rate equation is

=r kC CA Aa

Bb

(2.14)

The reaction is non-elementary if it occurs not in a single step but in multiple steps with the formation of intermediate transition compounds in each of these steps. Non-elementary reactions are split into a number of elementary reaction steps and this defines the mechanism of the non-elementary reaction. For non-elementary reactions, there is no one-to-one correspondence between the observed rate equation and the stoichiometric equation. For example, the rate equation for the reaction

H Br HBr2 2 2+

obtained from the experimental measurements (data) is reported as

rk C C

k C C

( )

/

HBrH Br

HBrBr

=

+ ( )11 2

2

2 2

2

(2.15)

which has no correspondence to the stoichiometric equation. Thus, the reaction between H2 and Br2 is a non-elementary reaction. The derivation of a rate equation for a non-elementary reaction is a trial-and-error procedure involving the following steps:

i. Propose a mechanism for the non-elementary reaction by splitting the non-elementary reaction into a number of elementary reaction steps.

ii. Write down the rate equations for each of the elementary steps using the law of mass action. Derive an overall rate equation by combining the rate expressions written for each of the elementary steps.


iii. Compare the rate equation derived for the proposed mechanism with the observed rate equation obtained from the experimental data. If it compares well, the pro-posed mechanism holds good. Otherwise, propose a new mechanism and repeat the above steps.

The deviation of rate equation for non-elementary reactions is discussed later (Section 2.1.10).

2.1.5 Reversible Reactions

Reversible reactions are reactions that proceed in both forward and reverse directions. Consider a reversible reaction between the reactants A and B leading to the formation of products C and D:

aA bB cC dDk

k+ +

1

2

The reaction is split into a forward step 1 and a reverse step 2, both of which are assumed to be elementary reaction steps. In the forward step 1, A reacts with B resulting in the for-mation of product C and D and the rate of forward reaction is

( ) =r k C CA Aa

Bb

1 (2.16)

where k1 is the kinetic rate constant of the forward reaction. In the reverse step 2, C reacts with D leading to the formation of A and B and the rate of reverse reaction is

r k C CA Cc

Dd

= 2 (2.17)

where k2 is the kinetic rate constant of the reverse reaction. The net rate of reaction of A is

( ) = r k C C k C CA Aa

Bb

Cc

Dd

1 2 (2.18)

An example of a reversible reaction is the reaction between H2 and N2 leading to the production of NH3.

N NH NH2 3 33 3+

However, this reaction is not an elementary reaction. A reversible reaction is said to be at an equilibrium state when the forward reaction rate is equal to the reverse reaction rate. Thus, at equilibrium, the net rate of reaction (rA) is zero. If CAe, CBe, CCe and CDe are the equilibrium concentrations of A, B, C and D, respectively, then k C C k C CAea Beb Cec Ded1 2= . The equilibrium constant K is defined as

K

kk

C CC C

Cec

Ded

Aea

Beb=

=

1

2 (2.19)

Equilibrium constant K is the ratio of the forward reaction rate constant k1 to the reverse reaction rate constant k2. The effect of temperature T on the equilibrium constant K is described by the Vant Hoff relation