Distillation Design and Control Using Aspen Simulation 2e

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.

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