efficiency and sustainability in the energy and chemical industries scientific principles and case studies

235 14 Effi ciency and Sustainability in the Chemical Process Industry ..... Concepts such as ideal work, entropy production, and lost work were clearly related to the effi cient use o

EFFICIENCY and SUSTAINABILITY

in the ENERGY and CHEMICAL INDUSTRIES

Scientific Principles and Case Studies

Missouri University of Science and Technology, Rolla, USA

Efficiency and Sustainability in the Energy and Chemical Industries: Scientific Principles

and Case Studies, Second Edition Krishnan Sankaranarayanan, Hedzer J van der Kooi, and Jakob de Swaan AronsProton Exchange Membrane Fuel Cells: Contamination and Mitigation StrategiesHui Li, Shanna Knights, Zheng Shi, John W Van Zee, and Jiujun Zhang

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EFFICIENCY and SUSTAINABILITY

in the ENERGY and CHEMICAL INDUSTRIES

Krishnan Sankaranarayanan

Hedzer J van der Kooi

Jakob de Swaan Arons

Scientific Principles and Case Studies

Boca Raton, FL 33487-2742

© 2010 by Taylor and Francis Group, LLC

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Preface xv

About This Book xix

Acknowledgments xxi

Authors xxiii

I Part Basics 1 Introduction 3

References 6

2 Thermodynamics Revisited 7

2.1 The System and Its Environment 7

2.2 States and State Properties 7

2.3 Processes and Their Conditions 8

2.4 The First Law 8

2.5 The Second Law and Boltzmann 11

2.6 The Second Law and Clausius 12

2.7 Change in Composition 13

2.8 The Structure of a Thermodynamic Application 18

References 21

3 Energy “Consumption” and Lost Work 23

3.1 Introduction 23

3.2 The Carnot Factor 24

3.3 Lessons from a Heat Exchanger 25

3.4 Lost Work and Entropy Generation 29

3.5 Conclusion 31

References 31

4 Entropy Generation: Cause and Effect 33

4.1 Equilibrium Thermodynamics 33

4.2 On Forces and Flows: Cause and Effect 34

4.3 Cause and Effect: The Relation between Forces and Flows 36

4.4 Coupling 38

4.5 Limited Validity of Linear Laws 40

4.6 Conclusion 46

References 46

5 Reduction of Lost Work 47

5.1 A Remarkable Triangle 47

5.2 Carnot Revisited: From Ideal to Real Processes 49

5.3 Finite-Time, Finite-Size Thermodynamics 54

5.4 The Principle of Equipartitioning 55

5.5 Conclusions 58

References 58

I Part I Thermodynamic Analysis of Processes 6 Exergy, a Convenient Concept 63

6.1 Exergy 63

6.2 The Convenience of the Exergy Concept 66

6.2.1 Out of Equilibrium with the Environment: What It Takes to Get There 67

6.2.2 Out of Equilibrium with the Environment: What It Takes to Stay There 68

6.2.3 Dissipative Structures 69

6.2.4 Physical and Chemical Exergy 70

6.3 Example of a Simple Analysis 71

6.4 The Quality of the Joule 74

6.5 Example of the Quality Concept 77

6.6 Conclusions 80

References 81

7 Chemical Exergy 83

7.1 Introduction 83

7.2 Exergy of Mixing 83

7.3 Chemical Exergy 84

7.3.1 Reference Components from Air 85

7.3.2 Exergy Values of the Elements 86

7.3.3 Chemical Exergy Values of Compounds 88

7.3.4 The Convenience of the Chemical Exergy Concept 89

7.4 Cumulative Exergy Consumption 90

7.5 Conclusions 91

References 92

8 Simple Applications 93

References 105

9 Energy Conversion 109

9.1 Introduction 109

9.2 Global Energy Consumption 110

9.3 Global Exergy Flows 112

9.4 Exergy or Lost Work Analysis 115

9.5 Electric Power Generation 115

9.5.1 Steam Plants 116

9.5.2 Gas Turbines 116

9.5.3 Combined Cycle 117

9.5.4 Nuclear Power 118

9.5.5 Hydropower 120

9.5.6 Wind Power 120

9.5.7 Solar Power 121

9.5.8 Geothermal Energy 121

9.6 Coal Conversion Processes 121

9.6.1 Fixed or Moving Beds 122

9.6.2 Suspended Beds 122

9.6.3 Fluidized Beds 122

9.6.4 Thermodynamic Analysis of Coal Combustion 123

9.6.5 Discussion 124

9.6.6 Coal Gasifi cation 125

9.7 Thermodynamic Analysis of Gas Combustion 128

9.7.1 Exergy In 128

9.7.2 Air Requirements 128

9.7.3 Exergy Out 130

9.7.4 Effi ciency 132

9.7.5 Discussion 133

9.8 Steam Power Plant 134

9.9 Gas Turbines, Combined Cycles, and Cogeneration 135

9.9.1 Gas Turbines 136

9.9.2 Thermodynamic Analysis of Gas Turbines 136

9.9.3 Combined Cycles, Cogeneration, and Cascading 137

9.9.4 Example 138

9.10 Concluding Remarks 139

References 140

10 Separations 141

10.1 Introduction 141

10.2 Propane, Propylene, and Their Separation 141

10.2.1 Single-Column Process 142

10.2.2 Double-Column Process 142

10.2.3 Heat Pump Process 143

10.3 Basics 144

10.3.1 Flash Distillation 144

10.3.2 Multistage Distillation and Refl ux 145

10.4 The Ideal Column: Thermodynamic Analysis 149

10.5 The Real Column 152

10.6 Exergy Analysis with a Flow Sheet Program 155

10.7 Remedies 157

10.7.1 Making Use of Waste Heat 157

10.7.2 Membranes 158

10.7.3 Other Methods 159

10.8 Concluding Remarks 160

References 161

11 Chemical Conversion 163

11.1 Introduction 163

11.2 Polyethylene Processes: A Brief Overview 164

11.2.1 Polyethylene High-Pressure Tubular Process 166

11.2.2 Polyethylene Gas-Phase Process 167

11.3 Exergy Analysis: Preliminaries 168

11.4 Results of the HP LDPE Process Exergy Analysis 169

11.5 Process Improvement Options 171

11.5.1 Lost Work Reduction by the Use of a Turbine 172

11.5.2 Alternative to the Extruder 172

11.5.3 Process Improvement Options: Estimated Savings 173

11.6 Results of the Gas-Phase Polymerization Process Exergy Analysis 174

11.7 Process Improvement Options 175

11.7.1 Coupling Reactions and Chemical Heat Pump System 176

11.7.2 Exergy Loss Reduction by Recovering Butylene and Ethylene from Purge Gas 177

11.7.3 Heat Pump and Preheating of Polymer 177

11.7.4 An Alternative to the Extruder 179

11.7.5 Process Improvement Options: Estimated Savings 180

11.8 Concluding Remarks 181

References 181

12 A Note on Life Cycle Analysis 183

12.1 Introduction 183

12.2 Life Cycle Analysis Methodology 184

12.2.1 Goal and Scope 184

12.2.2 Inventory Analysis 186

12.2.3 Impact Assessment 187

12.2.4 Interpretation and Action 188

12.3 Life Cycle Analysis and Exergy 188

12.4 Zero-Emission ELCA 189

12.5 Concluding Remarks 191

References 191

I Part V Sustainability 13 Sustainable Development 195

13.1 Sustainable Development 195

13.1.1 Three Views 197

13.1.2 Some Other Views 197

13.2 Nature as an Example of Sustainability 198

13.3 A Sustainable Economic System 200

13.3.1 Thermodynamics, Economics, and Ecology 200

13.3.2 Economics and Ecology 203

13.3.3 Nature’s Capital and Services 205

13.3.4 Adjustment of the Gross National Product 206

13.3.5 Intermezzo: Thermodynamics and Economics—A Daring Comparison and Analogy 206

13.4 Toward a Solar-Fueled Society: A Thermodynamic Perspective 211

13.4.1 Thermodynamic Analysis of a Power Station 211

13.4.2 Some Observations 213

13.4.3 From Fossil to Solar 213

13.5 Ecological Restrictions 214

13.5.1 Ecological Footprint 214

13.5.2 Waste 218

13.6 Thermodynamic Criteria for Sustainability Analysis 221

13.6.1 Introduction 221

13.6.2 Sustainable Resource Utilization Parameter α 222

13.6.3 Notes on Determining Depletion Times and Abundance Factors 227

13.6.4 Exergy Effi ciency η 228

13.6.5 The Environmental Compatibility ξ 229

13.6.6 Determining Overall Sustainability 232

13.6.7 Related Work 234

13.7 Conclusions 234

References 235

14 Effi ciency and Sustainability in the Chemical Process Industry 239

14.1 Introduction 239

14.2 Lost Work in the Process Industry 239

14.3 The Processes 242

14.4 Thermodynamic Effi ciency 243

14.5 Effi cient Use of High-Quality Resources 244

14.6 Toward Sustainability 245

14.7 Chemical Routes 246

14.8 Concluding Remarks 247

References 248

15 CO 2 Capture and Sequestration 251

15.1 Introduction 251

15.2 CO2 Emissions 251

15.3 The Carbon Cycle 254

15.4 Carbon Sequestration: Separation and Storage and Reuse of CO2 256

15.5 Carbon Capture Research 258

15.6 Geologic Sequestration Research 259

15.6.1 Oil and Gas Reservoirs 259

15.6.2 Coal Bed Methane 260

15.6.3 Saline Formations 260

15.6.4 CO2 Mineralization 261

15.6.5 Effi ciency of CO2 Capture and Sequestration 261

15.7 Carbon Tax and Cap-and-Trade 261

15.8 Concluding Remarks 262

References 262

16 Sense and Nonsense of Green Chemistry and Biofuels 265

16.1 Introduction 265

16.1.1 What Is Green 265

16.1.2 What Is Biomass 266

16.1.3 Biomass as a Resource 267

16.1.4 Structure of This Chapter 268

16.2 Principles of Green Chemistry 268

16.3 Raw Materials 269

16.3.1 Biomass 270

16.3.2 Recycling 272

16.4 Conversion Technologies 273

16.4.1 Combustion 274

16.4.2 Pyrolysis 275

16.4.3 Gasifi cation 275

16.4.4 Upgrading Biomass 277

16.5 How Green Are Green Plastics 278

16.5.1 Optimism in the United States 278

16.5.2 Initiatives in Europe 278

16.5.3 From a Hydrocarbon to a Carbohydrate Economy 280

16.5.4 Feelings of Discomfort 280

16.5.5 Short Memory: Ignorance or Not Welcome 283

16.6 Biofuels: Reality or Illusion 283

16.6.1 Multidisciplinarity 283

16.6.2 Second-Generation Biofuels 287

16.6.3 The Fossil Load Factor 288

16.6.4 Sustainability and Effi ciency 289

16.6.5 Algae 290

16.6.6 The Future 290

16.6.7 Sense or Nonsense? Discussion 291

16.7 Concluding Remarks 294

References 295

17 Solar Energy Conversion 299

17.1 Introduction: “Lighting the Way” 299

17.2 Characteristics 303

17.3 The Creation of Wind Energy 307

17.4 Photothermal Conversion 311

17.5 Photovoltaic Energy Conversion 313

17.6 Photosynthesis 315

17.7 Concluding Remarks 317

References 319

18 Hydrogen: Fuel of the Future 321

18.1 Introduction 321

18.2 The Hydrogen Economy 321

18.3 Current Hydrogen Economy 324

18.4 Conventional Hydrogen Production from Conventional Sources 324

18.5 Hydrogen from Renewables 325

18.6 Hydrogen as an Energy Carrier 325

18.7 Hydrogen as a Transportation Fuel 325

18.8 Effi ciency of Obtaining Transportation Fuels 326

18.9 Challenges of the Hydrogen Economy 328

18.10 Hydrogen Production: Centralized or Decentralized 329

18.11 Infrastructure 329

18.12 Hydrogen Storage 330

18.13 Fuel Cells as a Possible Alternative to Internal Combustion 332

18.14 Costs of the Hydrogen Economy 332

18.15 Concluding Remarks 334

References 334

19 Future Trends 337

19.1 Introduction 337

19.2 Energy Industries 338

19.3 Chemical Industries 340

19.4 Changing Opinions on Investment 341

19.5 Transition 343

19.6 Concluding Remarks 344

References 345

Epilogue 347

Problems 349

Index 359

For some of us, the energy crisis of the 1970s and 1980s may still be fresh

in our memory The crisis was of political origin, not one of real shortage The developed countries responded by focusing on increasing energy effi -ciency, at home and in industry, and by taking initiatives to make them less dependent on liquid fossils from the Middle East More than ever before, attention shifted to coal as an alternative energy resource—its exploration, production, transportation, and marketing Massive research and develop-ment programs were initiated to make available clean and effi cient coal uti-lization and more easily handled materials as gaseous and liquid conversion products Obviously, large multinational oil companies played an important role in these initiatives, as they considered energy, not oil, their ultimate business

At the same time, there was growing concern worldwide for the ment With the industrial society proceeding at full speed with mass produc-tion and consumption, the world became aware that this was accompanied

environ-by mass emission of waste Air pollution, water pollution, deterioration of the soil, and so forth became topics that started worrying us immensely The

“irreversibility” of most of our domestic and industrial activities seemed to ask a price for remediation that could become too high, if not for the present generations, then for later ones This insight developed a sense of respon-sibility that went beyond political, national, or other specifi c interests and seemed to be shared by all aware world citizens Earlier, and triggered by activities of the Club of Rome, computer simulations showed the possible limits to growth for a growing world population with limited resources The

1987 Brundtland Report [1] emphasized our responsibility for future tions and pointed to the need for “sustainable development.” This showed the emergence of a trilemma—with economic growth, need for resources, and care for the environment in a delicate balance The sun as a renewable source

genera-of energy became more and more prominent, as exemplifi ed in Japan’s sive New Sunshine Program and by the emergence of “green chemistry,” a development to fulfi ll our needs for chemicals in a sustainable way

mas-Our desire to write this book originated from the aforementioned need to increase the energy effi ciency of industrial processes Of all factors that are important to our future, energy may well be the single-most critical prob-lem that we have to face in the twenty-fi rst century Octave Levenspiel, the famous American chemical engineer and scientist, has emphasized that no sensible decision on energy and its transformation could be well founded without understanding the concepts of thermodynamics After all, thermo-dynamics is the ultimate science of the transformation of energy and matter, irrespective of whether we talk about industrial, ecological, or even economic

systems From its scientifi c concepts, thermodynamics has emerged as the ultimate accountant of energy Thermodynamics, in particular the second law, seemed indispensable to fi nd one’s way in a labyrinth—or so it seems—

of resource and process alternatives But with the emergence of the wide call for sustainability, our interest extended to include factors other than effi ciency to deal with this concept In doing so, we discovered that in nature and its cycle of life, energy and chemistry are more or less synony-mous, and that nature has its own ways to be sustainable However complex its ways and processes, nature is the prime example of sustainability and the source of inspiration for developing from an industrial society to what some like to call a metabolic society: a society that makes use of an immaterial energy source and recycles its products, including its waste This is not only

world-a fworld-ascinworld-ating chworld-allenge but, more importworld-antly, world-a necessity! Effi ciency will still be an important factor, as there are serious indications that the world’s ecological opportunity to exploit the sun as a resource is limited

Angela Merkel, physicist and former German Minister of Environment, defi ned sustainable development as “using resources no faster than they can regenerate themselves and releasing pollutants to no greater extent than natural resources can assimilate them” [2]

Living systems are out of equilibrium with the dead and inorganic ronment Thermodynamics provides us with some very useful concepts to tell us how far out of equilibrium a system is and what it takes to main-tain this state The late French scientist Bernard Spinner pointed out that these concepts allow us to integrate the environment into the analysis of any system we are interested in, a wonderful thermodynamic principle: always study the system in interaction with its environment Science, in this instance thermodynamics, can hardly offer society something better “to live in har-mony with the Environment.” In conclusion, the main objective of this book

envi-is to study the effi ciency and sustainability of industrial systems In doing so,

we will be looking at these systems through the glasses of thermodynamics and apply this impressive science wherever possible In this second edition, the book’s structure of “Basics,” “Thermodynamic Analysis of Processes,”

“Case Studies,” and “Sustainability” has been unaffected, but a few things have changed Wherever relevant, problems have been added to a chapter, testing the students on understanding, reproduction, and application of the discussed concepts In Part II, special attention has been given to the pos-sibility of integrating the environment into the thermodynamic analysis of the systems or processes considered The authors are no experts on “climate” but accept the fact that the emissions of CO2 are increasing at an increased rate.* And so a new chapter is dedicated to this subject with special atten-tion for the removal and storage of CO2 A CO2-free industry emerging from hydrocarbon resources could imply an industry based on H2, a topic to which another new chapter has been devoted Chapter 12 on life cycle analysis has

*Nature, 458, 1091–1094, April 29, 2009.

been extended to include the fate of the quality of energy during the “cycle”

of the process or product The separate chapters on biomass conversion and green chemistry have been integrated into one new chapter The chapter

“Economics, Ecology and Thermodynamics” (Chapter 18 in the fi rst edition) has now been absorbed into the other chapters, as economics, ecology, and thermodynamics should always be considered simultaneously Frequently, researchers consider economics as an afterthought, and those skilled in economics and business are not fully aware of the physical constraints that follow from the laws of thermodynamics

Finally, the senior authors (HJvdK and JdSA) have invited their former dent and junior coauthor (KS) to assume the responsibility of fi rst author His youth, exceptional skills, and critical scientifi c and engineering attitude will ensure that however distracted we may be by short-term events and

stu-“wisdom,” in the long term, scientifi c truth will prevail

Krishnan Sankaranarayanan Hedzer J van der Kooi Jakob de Swaan Arons

In the last three decades, important political events and authoritative reports have drawn our attention to the limits of economic growth, and caused grow-ing concern on our living environment, the latter even taking global dimen-sions with issues as climate change and reduction of biodiversity Most of us now seem to be aware that our technological and economic activities should

serve also the quality of our natural environment This is often called

sus-tainable development The question is now what, more precisely, is meant by

this The term becomes more relevant with talk of alternative energy sources, hydrogen, CO2, and terms such as “green.” This book aims to quantify these terms, determine the feasibility and possibility of claims, and allow for a rational evaluation and discussion based on sound scientifi c principles.This book answers this question for industrial processes, in particular those

in the energy and chemical industry Having a long experience in joint efforts with industry and with teaching, the authors use the fundamental laws of

thermodynamics as a point of departure They contrast the present industrial

society with the emerging metabolic society, in which mass production and

consumption are in harmony with the natural environment through closure

of material cycles These are ultimately driven by the primary new energy

source, the sun This book provides keys to a quantifi cation of process effi ciency

and sustainability This is illustrated in case studies, examples, and problems.

The book is meant for the practicing engineer and anybody else who is interested or engaged in the transition from a fossil-based, non-sustainable industry to a sustainable, low-waste industry based on renewable energy and resources Thus, it is hoped, the book itself will contribute to the devel-opment of a sustainable society

The authors wish to acknowledge the various useful contributions to this book by Dr Ir Sander Lems We also wish to thank the parents (K Sankaranarayanan and Kokila Sankaranarayanan) and wife (Ayshwarya Srinivasan) of the fi rst author (KS) for all the support during the fi nal phase

of the book and the wife (Kozue Takamura) of the third author (JdSA) for all the help with the typing and editing of the documents

Krishnan Sankaranarayanan received his MSc at Delft University of

Technology, the Netherlands and his PhD at Princeton University, New Jersey At Delft, he did an extensive study of the energy effi ciency of the polyolefi n industry, for which activity DSM acted as host He is currently group head reactor engineering and mixing at ExxonMobil Research and Engineering, Fairfax, Virginia

Hedzer J van der Kooi received his MSc and PhD degrees from Delft

University of Technology and specialized in phase equilibria In the last decade, he worked closely together with Sankaranarayanan on the sub-ject of this book, assisted by many students He is currently active in the Department of Architecture at Delft University of Technology

Jakob de Swaan Arons received his MSc and PhD degrees from the

Delft University of Technology, the Netherlands He spent some 20 years with Shell International, before he was appointed to the chair of Applied Thermodynamics and Phase Equilibria at Delft University of Technology

He is an elected member of the Royal Netherlands Academy of Arts and Sciences, and an honorary professor of the Beijing University of Chemical Technology, China From 2003 to 2009, he served as chair in the chemical engineering department of Tsinghua University, Beijing, China Much of his inspiration was drawn from his many visits to Japan and its research cen-ters He received the Hoogewerff Gold Medal for his lifetime contributions

to process technology in 2006

Learn the fundamentals of the game and stick to them

Jack Nicklaus, golf legend

In Chapter 2, we pay a renewed visit to thermodynamics We review its essentials and the common structure of its applications In Chapter 3, we focus on so-called energy consumption and identify the concepts of work available and work lost The last concept can be related to entropy produc-tion, which is the subject of Chapter 4 This chapter shows how some of the fi ndings of nonequilibrium thermodynamics are invaluable for process analysis Chapter 5 is devoted to fi nite-time fi nite-size thermodynamics, the application of which allows us to establish optimal conditions for operating

a process with minimum losses in available work

Introduction

Some years ago, we were teaching an advanced course in thermodynamics to process engineers of a multinational industry Subjects included phase equilib-ria, the thermodynamics of mixtures, and models from molecular thermody-namics applied to industrial situations Some participants raised the question whether some time could be spent on the subject of “the exergy analysis of processes.” At that time this was a subject with which we were less familiar because energy-related issues fell less within the scope of our activities We fell back on a small monograph by Seader [1] and the excellent textbook by Smith

et al [2], who dedicated the last chapter of their book not so much to exergy

but to the thermodynamic analysis of processes Concepts such as ideal work,

entropy production, and lost work were clearly related to the effi cient use of energy in industrial processes The two industrial examples given—one on the liquefaction of natural gas, the other on the generation of electricity in a natural gas-fi red power station—lent themselves very well not only for illustrative pur-poses but also for applying the exergy concept and exergy fl ow diagrams [3,4] The latter concepts appealed to us because of their instrumental and visual power in illustrating the fate of energy in the processes (Figure 1.1)

After this experience in industry, we started to include the subject in advanced courses to our own chemical engineering students at Delft University of Technology A colleague had pointed out to us that the design of a process is more valuable if the process has also been analyzed for its energy effi ciency For mechanical engineers, who were tradition-ally more engaged in energy conversion processes, this was obvious; for chemical engineers, until then more concerned with chemical conversion processes, this was relatively new The subject grew in popularity with our students because it became more and more obvious that the state of the environment and energy consumption are closely related and that excessive energy consumption appeared to be one of the most important factors in affecting the quality of our environment

In performing such an analysis, either for industry, or out of our own osity, we became more and more aware of the very important role that the second law is playing in our daily lives and how the thermodynamics of irreversible processes, until then for us a beautiful science but without sig-nifi cant applications, appeared to have a high “engineering” content Atkins’ statement that the second law is the driving force behind all change [5] had

curi-a lcuri-asting impcuri-act on us, curi-as much curi-as Goodstein’s suggestion [6] thcuri-at the second law may well turn out to be the central scientifi c truth of the twenty-fi rst

century We discovered the importance of the relation between results from classical, engineering, and irreversible thermodynamics as we have tried to make visible in what we like to call “the magic triangle” behind the second law (Figure 1.2).

Later, when we were struck by the observation that complex trial schemes and life processes or living systems have much in common,

indus-Exchanger

Sep./Val.

218

4 Liquid CH4

53

449 5

2 6

2 622

1000

1 0

378

118 722

220

Compression

3

FIGURE 1.1

Grassmann diagram for the Linde liquefaction process of methane One thousand exergy units

of compression energy result in 53 exergy units of liquid methane The thermodynamic effi ciency of this process is 5.3% The arrowed curves, bent to the right, show the losses in the various process steps.

-The magic triangle behind the second law

Engineering thermodynamics

Irreversible thermodynamics

our attention was again attracted to the meaning of the second law and the role of entropy production This led us to the topic of energy flow in biology and the invaluable monographs by Schrödinger [7] and Morowitz [8] This part of our education had come in a timely fashion,

as became apparent when, more and more often, the words ity” and “sustainable development” were brought in relation with the efficient use of energy We were forced to see our analysis in the light

“sustainabil-of these concepts and to make efforts to extend our analysis to indicate,

in quantitative terms, the extent to which processes or products are not only efficient but also contribute to sustainability Once again we were stimulated by ideas and questions from colleagues within multinational industries All these elements and influences can be found in this book and its structure

Part I of this book, “Basics” (Chapters 2 through 5), reviews the main results of classical thermodynamics and identifi es the important concepts

of ideal work, lost work, and entropy generation, from using and ing the fi rst and second laws for fl owing systems Having identifi ed these concepts, we further interpret them in everyday technical terms by using the main results of irreversible thermodynamics After reviewing possible ways to minimize the work lost, we conclude this part by giving atten-tion to the thermodynamic cost of performing a process in fi nite time and space

combin-Part II, “Thermodynamic Analysis of Processes” (Chapters 6 through 8), discusses the thermodynamic effi ciency of a process and how effi ciency can be established and interpreted A very useful thermodynamic prop-

erty, called exergy or available work, is identifi ed that makes it relatively

easy to perform and integrate the environment into such an analysis Some simple examples are given to illustrate the concept and its applica-tion in the thermodynamic or exergy analysis of chemical and nonchemi-cal processes

Part III, “Case Studies” (Chapters 9 through 12), takes these illustrations

a bit further, namely, by demonstrating the analysis for some of the most important processes in industry: energy conversion, separations, and chemi-cal conversion Chapter 12 briefl y discusses the concept of life cycle analysis, which aims to compare the consolidated inputs and outputs of a process

or a product “from the cradle to the grave” [9], and its extension to include the minimization of process irreversibilities in a so-called exergetic life cycle analysis [10]

Part IV, “Sustainability” (Chapters 13 through 18), deals with the topics of sustainable development, effi ciency, and sustainability in the chemical pro-cess industry and a very topical topic, carbon dioxide (CO2) The sense and nonsense of green chemistry and biofuels is expounded in this part as well, followed by solar energy conversion and musings on hydrogen in the fi nal chapter of this part

Chapter 19, contains the authors’ thoughts on what the future may hold

1 Seader, J.D Thermodynamic Effi ciency of Chemical Processes, Industrial Conservation Manual 1, Gyftopoulos, E.P (ed.), MIT Press: Cambridge, MA,

Energy-1982.

2 Smith, J.M.; Van Ness, H.C.; Abbott, M.M Introduction to Chemical Engineering

Thermodynamics, 4th edn., The McGraw-Hill Companies Inc.: New York, 1987.

3 Sussman, M.V Availability (Exergy) Analysis, A Self Instruction Manual, Mulliken

House: Lexington, MA, 1985.

4 de Swaan Arons, J.; van der Kooi, H.J Exergy analysis, adding insight and

preci-sion to experience and intuition In Precipreci-sion Process Technology Perspectives for

Pollution Prevention, Weijnen, M.P.C and Drinkenburg, A.A.H (eds.), Kluwer

Academic Publishers: Dordrecht, the Netherlands, 1993, pp 83–113.

5 Atkins, P.W Educating chemists for the next millennium ChemTech, 1992, July,

390–392.

6 Goodstein, D Chance and necessity Nature 1994, 368, 598.

7 Schrödinger, E What Is Life? Cambridge University Press: Cambridge, U.K.,

1980.

8 Morowitz, H.J Energy fl ow in biology In Biological Organisation as a Problem in

Thermal Physics, OxBow Press: Woodbridge, CT, 1979.

9 Ayres, R.U.; Ayres, L.W.; Martinas, K Exergy, waste accounting, and life-cycle

analysis Energy 1998, 23, 355–363.

10 Cornelissen, R.L Thermodynamics and sustainable development PhD thesis, Twente University, Enschede, the Netherlands, 1997.

Thermodynamics Revisited

In this chapter, we briefl y review the essentials of thermodynamics and its principal applications We cover the fi rst and second laws and discuss the most important thermodynamic properties and their dependence on pressure, temperature, and composition, being the main process variables Change in composition can be brought about with or without the transforma-tion of phases or chemical species The common structure of the solution of

a thermodynamic problem is discussed

2.1 The System and Its Environment

In thermodynamics, we distinguish between the system and its environment The system is that part of the whole that takes our special interest and that we wish to study This may be the contents of a reactor or a separation column or

a certain amount of mass in a closed vessel We defi ne what is included in the system The space outside the chosen system or, more often, a relevant selected part of it with defi ned properties, is defi ned as the environment

Next, we distinguish between closed, open, and isolated systems All are defi ned in relation to the fl ow of energy and mass between the system and its environment A closed system does not exchange matter with its environ-ment, but the exchange of energy (e.g., heat or work) is allowed Open systems may exchange both energy and matter, but an isolated system exchanges neither energy nor mass with its environment

2.2 States and State Properties

The system of our choice will usually prevail in a certain macroscopic state, which is not under the infl uence of external forces In equilibrium, the state

can be characterized by state properties such as pressure (P) and ture (T), which are called “intensive properties.” Equally, the state can be characterized by extensive properties such as volume (V), internal energy (U), enthalpy (H), entropy (S), Gibbs energy (G), and Helmholtz energy (A)

tempera-These properties are called “extensive” because they relate to the amount of mass considered; once related to a unit amount of mass, they also become intensive properties.

The equilibrium state does not change with time, but it may change with

loca-tion as in a fl owing system where P, T, and other state properties can gradually

change with position Then we speak of a steady state If the state temporarily changes with time, as in the startup of a plant, we call it a “transient” state

If an isolated system is in a nonequilibrium state, its properties will usually differ from its equilibrium properties and it will not be stable

If such a system can absorb local fl uctuations, it is in a metastable state; otherwise, the system and state are called unstable (Figure 4.2)

2.3 Processes and Their Conditions

Often our system of interest is engaged in a process If such a process takes place at a constant temperature, we speak of an isothermal process Equally, the process can be defi ned as isobaric, isochoric, isentropic, or isenthalpic

if pressure, volume, entropy, or enthalpy, respectively, remains unchanged during the process A process is called “adiabatic” if no heat exchange takes place between the system and its environment Finally, a process is called

“reversible” if the frictional forces, which have to be overcome, tend to zero The unrealistic feature of this process is that energy and material fl ows can take place in the limit of driving forces going to zero; for example, in an iso-thermal process, heat can be transferred without a temperature difference within the system or between the system and its environment In a real pro-cess, frictional forces have to be overcome, requiring fi nite “driving forces”

as ΔP, ΔT, ΔG, or when driving forces are already present in the system, this leads to processes where spontaneously is given in to such forces in a spon-taneous expansion, mixing process, or reaction Such processes are called

“irreversible” processes and are a fact of real process life As we will see later, the theory of irreversible thermodynamics identifi es the so-called thermo-dynamic forces, for example, Δ(1/T) instead of ΔT, and the associated fl ow

rate—in this instance, the heat fl ow rate Q˙

2.4 The First Law

Thermodynamics is solidly founded on its main laws The fi rst law is the law

of conservation of energy For a closed system that receives heat from the

envi-ronment, Q , and performs work on the environment, W , we can write

Δ =U Qin−Wout (2.1)

Heat and work are forms of energy in transfer between the system and the

environment If more heat is introduced into the system than the system

per-forms work on the environment, the difference is stored as an addition to the internal energy U of the system, a property of its state In a more abstract

way, the fi rst law is said to defi ne the fundamental thermodynamic state

property, U, the internal energy.

Equation 2.1, in differential form, can be written as

The δ-character is used to indicate small amounts of Q and W because heat

and work are not state properties and depend on how the process takes place between two different states

If the process is reversible and the sole form of work that the system can exert on its environment is that of volume expansion, then rev

been defi ned for our convenience; it has no fundamental meaning other than that, under certain conditions, its change is related to the heat absorbed by the system It can be shown that the specifi c heat at constant volume and

pressure, c v and c p, respectively, can be expressed as

For process engineering and design, it is important to know how enthalpy

is a function of pressure, temperature, and composition The last variable is discussed later It can be found in any standard textbook that the differential

of H can be expressed as a function of the differential of T and P as follows:

The fi rst term depends on what is sometimes called the caloric equation of

state, describing how intramolecular properties, the properties within the

molecules, are a function of the state variables The expression in brackets requires the mechanical equation of state, which expresses the dependency of

a property, for example, V on the intermolecular interactions, the interactions

between molecules Process simulation models usually contain information and models for both types of equation of state.

Most often, we are not interested in the “absolute” value of H, but rather in

its change, Δtr H The subscript “tr” refers to the nature of the change If the

change involves temperature and/or pressure for a one-phase system only,

no subscript is used for Δ But in case of a phase transition, of mixing, or of a chemical reaction, the subscript is used and may read “vap” for vaporization,

“mix” for mixing, or “r” for reaction, and so forth

When a superscript is used as in Δtr H°, it indicates that the change in H is

considered for a transition under standard pressure, which usually is chosen

as 1 bar In the case of chemical reactions, the superscript ° refers to dard pressure and to reactants and products in their pure state or otherwise defi ned standard states such as infi nitely dilute solutions

stan-Finally, we present the fi rst law for open systems as in the case of streams

fl owing through a fi xed control volume at rest [1]

For one ingoing and outgoing stream in the steady state (Figure 2.1), this equation simplifi es into

m . refers to the mass fl ow rate considered

u is the velocity of the fl owing system

z is its height in the gravitational fi eld

2.5 The Second Law and Boltzmann

The second law is associated with the direction of a process It defi nes the

fundamental property entropy, S, and states that in any real process the

direc-tion of the process corresponds to the direcdirec-tion in which the total entropy increases, that is, the entropy change of both the system and environment should in total result in a positive result or in equation form

thermody-fi lled with gas, at t = 0 half of the molecules are nitrogen, the other half are

oxygen, and all nitrogen molecules fi ll the left half of the vessel whereas all oxygen molecules fi ll the right half of the vessel, then this makes for a highly unlikely distribution, that is, one of a low thermodynamic probability Ω0 As time passes, the system will evolve gradually into one with an even distri-bution of all molecules over space This new state has comparatively a high thermodynamic probability Ω, and the generated entropy is given by

Standard textbooks give ample examples of how Ω can be calculated [1].Notice that the direction of the process and time have been determined:

This has been called the arrow of time [2] Time proceeds in the direction of

entropy generation, that is, toward a state of greater probability for the total

of the system and its environment, which, in the widest sense, makes up the universe Finally, we wish to point out that an interesting implication of

Equation 2.10 is that for substances in the perfect crystalline state at T = 0 K,

the thermodynamic probability Ω = 1 and thus S = 0

2.6 The Second Law and Clausius

As the fi rst law is sometimes referred to as the law that defi nes the

funda-mental thermodynamic property U, the internal energy of the system, the ond law is considered to defi ne the other fundamental property, the entropy S

sec-Classical thermodynamics, via Clausius’s thorough analysis [3] of namic cycles that extract work from available heat, has produced the relation

thermody-between S and the heat added reversibly to the system at a temperature T:

rev in

Q dS T

From this relation the differential dS can be expressed as the following

function of the differentials of pressure and temperature:

P, T, and also composition are the state variables most often used to

charac-terize the state of the system, as they can be easily measured and controlled

As we show in Part II, Equations 2.5 and 2.14 are important to perform the thermodynamic analysis of a process ΔT S0298, which expresses the change

in entropy of a reaction at 298 K and at standard pressure The reaction is defi ned to take place between compounds in their standard state, that is, in the

most stable aggregation state under standard conditions, like liquid water for water at 298 K and 1 bar Analogous to Equation 2.6, for the fi rst law for open systems, the second law reads

generated

i i j j cv

and simplifi es to Equation 2.8 for single fl ows in and out the control volume

in the steady state

Finally, equilibrium processes can be defi ned as processes between and passing states that all have the same thermodynamic probability On the one hand, these processes proceed without driving forces; on the other hand, and this is inconsistent and unrealistic, there is no incentive for the process

to proceed These imaginary processes function only to establish the mum amount of work required, or the maximum amount of work available,

mini-in proceedmini-ing from one state to the other

2.7 Change in Composition

So far our discussion of thermodynamic concepts has referred to systems

that did not seem to change with composition, only with pressure P, perature T, or the state of aggregation Thermodynamics, however, is much

tem-more general than being limited to these conditions, fortunately, for changes

in composition are the rule rather than the exception in engineering ations Pure homogeneous phases may mix, and a homogeneous mixture may split into two phases A homogeneous or heterogeneous mixture may spontaneously react to one or more products In all these cases changes in composition will take place This part of thermodynamics is usually referred

situ-to as “chemical” thermodynamics, and its spiritual father is Josiah Willard Gibbs [4] It has been the merit of Lewis [5] to “decipher” Gibbs’ achieve-ments and to translate these into readily applicable practical and compre-

hensible concepts such as the Gibbs energy of a substance i, or the individual

thermodynamic potential μi , fugacity f i , or activity a i, concepts now widely used in process design The thermodynamic or chemical potential can be considered to be the decisive property for an individual molecular species transport or chemical behavior It has been one of the main achievements of Prausnitz et al [6] to be instrumental in quantifying thermodynamic proper-ties for ready application by taking into account the molecular characteristics

and properties of those molecules making up the mixture in a particular thermodynamic state This branch of thermodynamics is often referred to

as “molecular thermodynamics,” and many consider Prausnitz as its most prominent founding father

If a mixing process or chemical transformation is brought about, neously or by applying work on the system, the process will take place with entropy generation:

P and T is much larger than that of the respective pure states (in molar

units):

mixed mix generated

with Ωmixed >>> Ωseparated

If the change is in composition only, at constant P and T, and confi ned to

the system we wish to consider, for instance, in a mixer, separation column,

or a reactor, then a system property G, the Gibbs energy, can be identifi ed

and has been defi ned as follows:

It has the property that in time (t) and at constant pressure and temperature it

tends to a minimum value that will be reached when the system has reached equilibrium (Figure 2.2);

If the process of mixing takes place with negligible change of the internal

energy U and volume V, we speak of ideal mixing and it can be shown then

that for 1 mol of mixture

in which

x i is the molar fraction of constituent i in the mixture

R is the universal gas constant

For an ideal mixture, ΔmixUideal and ΔmixV ideal are zero for mixing at constant

P and T, and so ΔmixGideal is given by

ideal

Notice that ΔmixH ideal is also zero and thus, with Equation 2.3 in mind, ideal

mixing at constant P and T will take place without heat effects.

For deviations from ideal mixing, the excess property M E is defi ned as

ideal

E

An important excess property is the excess Gibbs energy G E Many

mod-els have been developed to describe and predict G E from the properties of

the molecules in the mixture and their mutual interactions G E models often refer to the condensed state, the solid and liquid phases In case signifi cant changes in the volume take place upon mixing, or separation, the Helmholtz

energy A, defi ned as

and its excess property A E are the preferred choices for describing the cess This requires an equation of state that expresses the volumetric behav-ior of the mixture as a function of pressure, temperature, and composition