Distillation Design and Control Using Aspen Simulation

The new edition of this book greatly updates and expands the previous edition. It boasts new chapters on the divided wall column and carbon dioxide capture from stack gas, revises the design and control of distillation systems, and explains the use of dynamic simulation to study safety issues in the event of operating failures. Using Aspen Plus to develop rigorous simulations of single distillation columns and sequences of columns, the book considers the economics of capital investment and energy costs to create an optimal system for separation methods in the chemical and petroleum industries.

Table of Contents:

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

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.