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Distillation Design and Control Using Aspen Simulation, 2nd Edition

ISBN: 978-1-118-41143-8
510 pages
April 2013
Distillation Design and Control Using Aspen Simulation, 2nd Edition (1118411439) cover image

Learn how to develop optimal steady-state designs for distillation systems

As the search for new energy sources grows ever more urgent, distillation remains at the forefront among separation methods in the chemical, petroleum, and energy industries. Most importantly, as renewable sources of energy and chemical feedstocks continue to be developed, distillation design and control will become ever more important in our ability to ensure global sustainability.

Using the commercial simulators Aspen Plus® and Aspen Dynamics®, this text enables readers to develop optimal steady-state designs for distillation systems. Moreover, readers will discover how to develop effective control structures. While traditional distillation texts focus on the steady-state economic aspects of distillation design, this text also addresses such issues as dynamic performance in the face of disturbances.

Distillation Design and Control Using Aspen Simulation introduces the current status and future implications of this vital technology from the perspectives of steady-state design and dynamics. The book begins with a discussion of vapor-liquid phase equilibrium and then explains the core methods and approaches for analyzing distillation columns. Next, the author covers such topics as:

  • Setting up a steady-state simulation
  • Distillation economic optimization
  • Steady-state calculations for control structure selection
  • Control of petroleum fractionators
  • Design and control of divided-wall columns
  • Pressure-compensated temperature control in distillation columns

Synthesizing four decades of research breakthroughs and practical applications in this dynamic field, Distillation Design and Control Using Aspen Simulation is a trusted reference that enables both students and experienced engineers to solve a broad range of challenging distillation problems.

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PREFACE TO THE SECOND EDITION xv

PREFACE TO THE FIRST EDITION xvii

1 FUNDAMENTALS OF VAPOR–LIQUID–EQUILIBRIUM (VLE) 1

1.1 Vapor Pressure / 1

1.2 Binary VLE Phase Diagrams / 3

1.3 Physical Property Methods / 7

1.4 Relative Volatility / 7

1.5 Bubble Point Calculations / 8

1.6 Ternary Diagrams / 9

1.7 VLE Nonideality / 11

1.8 Residue Curves for Ternary Systems / 15

1.9 Distillation Boundaries / 22

1.10 Conclusions / 25

Reference / 27

2 ANALYSIS OF DISTILLATION COLUMNS 29

2.1 Design Degrees of Freedom / 29

2.2 Binary McCabe–Thiele Method / 30

2.2.1 Operating Lines / 32

2.2.2 q-Line / 33

2.2.3 Stepping Off Trays / 35

2.2.4 Effect of Parameters / 35

2.2.5 Limiting Conditions / 36

2.3 Approximate Multicomponent Methods / 36

2.3.1 Fenske Equation for Minimum Number of Trays / 37

2.3.2 Underwood Equations for Minimum Reflux Ratio / 37

2.4 Conclusions / 38

3 SETTING UP A STEADY-STATE SIMULATION 39

3.1 Configuring a New Simulation / 39

3.2 Specifying Chemical Components and Physical Properties / 46

3.3 Specifying Stream Properties / 51

3.4 Specifying Parameters of Equipment / 52

3.4.1 Column C1 / 52

3.4.2 Valves and Pumps / 55

3.5 Running the Simulation / 57

3.6 Using Design Spec/Vary Function / 58

3.7 Finding the Optimum Feed Tray and Minimum Conditions / 70

3.7.1 Optimum Feed Tray / 70

3.7.2 Minimum Reflux Ratio / 71

3.7.3 Minimum Number of Trays / 71

3.8 Column Sizing / 72

3.8.1 Length / 72

3.8.2 Diameter / 72

3.9 Conceptual Design / 74

3.10 Conclusions / 80

4 DISTILLATION ECONOMIC OPTIMIZATION 81

4.1 Heuristic Optimization / 81

4.1.1 Set Total Trays to Twice Minimum Number of Trays / 81

4.1.2 Set Reflux Ratio to 1.2 Times Minimum Reflux Ratio / 83

4.2 Economic Basis / 83

4.3 Results / 85

4.4 Operating Optimization / 87

4.5 Optimum Pressure for Vacuum Columns / 92

4.6 Conclusions / 94

5 MORE COMPLEX DISTILLATION SYSTEMS 95

5.1 Extractive Distillation / 95

5.1.1 Design / 99

5.1.2 Simulation Issues / 101

5.2 Ethanol Dehydration / 105

5.2.1 VLLE Behavior / 106

5.2.2 Process Flowsheet Simulation / 109

5.2.3 Converging the Flowsheet / 112

5.3 Pressure-Swing Azeotropic Distillation / 115

5.4 Heat-Integrated Columns / 121

5.4.1 Flowsheet / 121

5.4.2 Converging for Neat Operation / 122

5.5 Conclusions / 126

6 STEADY-STATE CALCULATIONS FOR CONTROL STRUCTURE SELECTION 127

6.1 Control Structure Alternatives / 127

6.1.1 Dual-Composition Control / 127

6.1.2 Single-End Control / 128

6.2 Feed Composition Sensitivity Analysis (ZSA) / 128

6.3 Temperature Control Tray Selection / 129

6.3.1 Summary of Methods / 130

6.3.2 Binary Propane/Isobutane System / 131

6.3.3 Ternary BTX System / 135

6.3.4 Ternary Azeotropic System / 139

6.4 Conclusions / 144

Reference / 144

7 CONVERTING FROM STEADY-STATE TO DYNAMIC SIMULATION 145

7.1 Equipment Sizing / 146

7.2 Exporting to Aspen Dynamics / 148

7.3 Opening the Dynamic Simulation in Aspen Dynamics / 150

7.4 Installing Basic Controllers / 152

7.4.1 Reflux / 156

7.4.2 Issues / 157

7.5 Installing Temperature and Composition Controllers / 161

7.5.1 Tray Temperature Control / 162

7.5.2 Composition Control / 170

7.5.3 Composition/Temperature Cascade Control / 170

7.6 Performance Evaluation / 172

7.6.1 Installing a Plot / 172

7.6.2 Importing Dynamic Results into Matlab / 174

7.6.3 Reboiler Heat Input to Feed Ratio / 176

7.6.4 Comparison of Temperature Control with Cascade CC/TC / 181

7.7 Conclusions / 184

8 CONTROL OF MORE COMPLEX COLUMNS 185

8.1 Extractive Distillation Process / 185

8.1.1 Design / 185

8.1.2 Control Structure / 188

8.1.3 Dynamic Performance / 191

8.2 Columns with Partial Condensers / 191

8.2.1 Total Vapor Distillate / 192

8.2.2 Both Vapor and Liquid Distillate Streams / 209

8.3 Control of Heat-Integrated Distillation Columns / 217

8.3.1 Process Studied / 217

8.3.2 Heat Integration Relationships / 218

8.3.3 Control Structure / 222

8.3.4 Dynamic Performance / 223

8.4 Control of Azeotropic Columns/Decanter System / 226

8.4.1 Converting to Dynamics and Closing Recycle Loop / 227

8.4.2 Installing the Control Structure / 228

8.4.3 Performance / 233

8.4.4 Numerical Integration Issues / 237

8.5 Unusual Control Structure / 238

8.5.1 Process Studied / 239

8.5.2 Economic Optimum Steady-State Design / 242

8.5.3 Control Structure Selection / 243

8.5.4 Dynamic Simulation Results / 248

8.5.5 Alternative Control Structures / 248

8.5.6 Conclusions / 254

8.6 Conclusions / 255

References / 255

9 REACTIVE DISTILLATION 257

9.1 Introduction / 257

9.2 Types of Reactive Distillation Systems / 258

9.2.1 Single-Feed Reactions / 259

9.2.2 Irreversible Reaction with Heavy Product / 259

9.2.3 Neat Operation Versus Use of Excess Reactant / 260

9.3 TAME Process Basics / 263

9.3.1 Prereactor / 263

9.3.2 Reactive Column C1 / 263

9.4 TAME Reaction Kinetics and VLE / 266

9.5 Plantwide Control Structure / 270

9.6 Conclusions / 274

References / 274

10 CONTROL OF SIDESTREAM COLUMNS 275

10.1 Liquid Sidestream Column / 276

10.1.1 Steady-State Design / 276

10.1.2 Dynamic Control / 277

10.2 Vapor Sidestream Column / 281

10.2.1 Steady-State Design / 282

10.2.2 Dynamic Control / 282

10.3 Liquid Sidestream Column with Stripper / 286

10.3.1 Steady-State Design / 286

10.3.2 Dynamic Control / 288

10.4 Vapor Sidestream Column with Rectifier / 292

10.4.1 Steady-State Design / 292

10.4.2 Dynamic Control / 293

10.5 Sidestream Purge Column / 300

10.5.1 Steady-State Design / 300

10.5.2 Dynamic Control / 302

10.6 Conclusions / 307

11 CONTROL OF PETROLEUM FRACTIONATORS 309

11.1 Petroleum Fractions / 310

11.2 Characterization Crude Oil / 314

11.3 Steady-State Design of Preflash Column / 321

11.4 Control of Preflash Column / 328

11.5 Steady-State Design of Pipestill / 332

11.5.1 Overview of Steady-State Design / 333

11.5.2 Configuring the Pipestill in Aspen Plus / 335

11.5.3 Effects of Design Parameters / 344

11.6 Control of Pipestill / 346

11.7 Conclusions / 354

References / 354

12 DIVIDED-WALL (PETLYUK) COLUMNS 355

12.1 Introduction / 355

12.2 Steady-State Design / 357

12.2.1 MultiFrac Model / 357

12.2.2 RadFrac Model / 366

12.3 Control of the Divided-Wall Column / 369

12.3.1 Control Structure / 369

12.3.2 Implementation in Aspen Dynamics / 373

12.3.3 Dynamic Results / 375

12.4 Control of the Conventional Column Process / 380

12.4.1 Control Structure / 380

12.4.2 Dynamic Results and Comparisons / 381

12.5 Conclusions and Discussion / 383

References / 384

13 DYNAMIC SAFETY ANALYSIS 385

13.1 Introduction / 385

13.2 Safety Scenarios / 385

13.3 Process Studied / 387

13.4 Basic RadFrac Models / 387

13.4.1 Constant Duty Model / 387

13.4.2 Constant Temperature Model / 388

13.4.3 LMTD Model / 388

13.4.4 Condensing or Evaporating Medium Models / 388

13.4.5 Dynamic Model for Reboiler / 388

13.5 RadFrac Model with Explicit Heat-Exchanger Dynamics / 389

13.5.1 Column / 389

13.5.2 Condenser / 390

13.5.3 Reflux Drum / 391

13.5.4 Liquid Split / 391

13.5.5 Reboiler / 391

13.6 Dynamic Simulations / 392

13.6.1 Base Case Control Structure / 392

13.6.2 Rigorous Case Control Structure / 393

13.7 Comparison of Dynamic Responses / 394

13.7.1 Condenser Cooling Failure / 394

13.7.2 Heat-Input Surge / 395

13.8 Other Issues / 397

13.9 Conclusions / 398

Reference / 398

14 CARBON DIOXIDE CAPTURE 399

14.1 Carbon Dioxide Removal in Low-Pressure Air Combustion Power Plants / 400

14.1.1 Process Design / 400

14.1.2 Simulation Issues / 401

14.1.3 Plantwide Control Structure / 404

14.1.4 Dynamic Performance / 408

14.2 Carbon Dioxide Removal in High-Pressure IGCC Power Plants / 412

14.2.1 Design / 414

14.2.2 Plantwide Control Structure / 414

14.2.3 Dynamic Performance / 418

14.3 Conclusions / 420

References / 421

15 DISTILLATION TURNDOWN 423

15.1 Introduction / 423

15.2 Control Problem / 424

15.2.1 Two-Temperature Control / 425

15.2.2 Valve-Position Control / 426

15.2.3 Recycle Control / 427

15.3 Process Studied / 428

15.4 Dynamic Performance for Ramp Disturbances / 431

15.4.1 Two-Temperature Control / 431

15.4.2 VPC Control / 432

15.4.3 Recycle Control / 433

15.4.4 Comparison / 434

15.5 Dynamic Performance for Step Disturbances / 435

15.5.1 Two-Temperature Control / 435

15.5.2 VPC Control / 436

15.5.3 Recycle Control / 436

15.6 Other Control Structures / 439

15.6.1 No Temperature Control / 439

15.6.2 Dual Temperature Control / 440

15.7 Conclusions / 442

References / 442

16 PRESSURE-COMPENSATED TEMPERATURE CONTROL IN DISTILLATION COLUMNS 443

16.1 Introduction / 443

16.2 Numerical Example Studied / 445

16.3 Conventional Control Structure Selection / 446

16.4 Temperature/Pressure/Composition Relationships / 450

16.5 Implementation in Aspen Dynamics / 451

16.6 Comparison of Dynamic Results / 452

16.6.1 Feed Flow Rate Disturbances / 452

16.6.2 Pressure Disturbances / 453

16.7 Conclusions / 455

References / 456

17 ETHANOL DEHYDRATION 457

17.1 Introduction / 457

17.2 Optimization of the Beer Still (Preconcentrator) / 459

17.3 Optimization of the Azeotropic and Recovery Columns / 460

17.3.1 Optimum Feed Locations / 461

17.3.2 Optimum Number of Stages / 462

17.4 Optimization of the Entire Process / 462

17.5 Cyclohexane Entrainer / 466

17.6 Flowsheet Recycle Convergence / 466

17.7 Conclusions / 467

References / 467

18 EXTERNAL RESET FEEDBACK TO PREVENT RESET WINDUP 469

18.1 Introduction / 469

18.2 External Reset Feedback Circuit Implementation / 471

18.2.1 Generate the Error Signal / 472

18.2.2 Multiply by Controller Gain / 472

18.2.3 Add the Output of Lag / 472

18.2.4 Select Lower Signal / 472

18.2.5 Setting up the Lag Block / 472

18.3 Flash Tank Example / 473

18.3.1 Process and Normal Control Structure / 473

18.3.2 Override Control Structure Without External Reset Feedback / 474

18.3.3 Override Control Structure with External Reset Feedback / 476

18.4 Distillation Column Example / 479

18.4.1 Normal Control Structure / 479

18.4.2 Normal and Override Controllers Without External Reset / 481

18.4.3 Normal and Override Controllers with External Reset Feedback / 483

18.5 Conclusions / 486

References / 486

INDEX 487

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WILLIAM L. LUYBEN, PhD, is Professor of Chemical Engineering at Lehigh University where he has taught for over forty-five years. Dr. Luyben spent nine years as an engineer with Exxon and DuPont. He has published fourteen books and more than 250 original research papers. Dr. Luyben is a 2003 recipient of the Computing Practice Award from the CAST Division of the AIChE. He was elected to the Process Control Hall of Fame in 2005. In 2011, the Separations Division of the AIChE recognized his contributions to distillation technology by a special honors session.

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