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