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.
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
