09highly coupled dist syst
TRANSCRIPT
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 1© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Highly Coupled Distillation Systems
Advanced Distillation with Aspen Plus
© 2002 AspenTech. All Rights Reserved.
Lesson Objectives
• Understand alternative strategies for modeling Highly Coupled Distillation Systems in Aspen Plus.
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 2© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Modeling Distillation Systems (1)
Examples of highly coupled distillation systems include:
• Azeotropic distillation– Ethanol dehydration using benzene or cylcohexane– IPA dehydration using benzene– Acetic acid dehydration using butyl-acetate
• Extractive distillation– C4 extraction using DMF or acetonitrile– Separation of toluene and MCH using phenol
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Modeling Distillation Systems (2)
• Ethylene plant quench section– Quench tower + primary fractionator + fuel oil stripper
• Absorber/stripper systems (for acid gas removal)– Rectisol process (methanol)– Amine processes (MEA, DEA)
• Liquid-liquid extraction/stripper– BTX extraction using Sulfolane or ethylene glycol
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 3© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Approaches
Two approaches for modeling these systems:
1. Model them as several interconnected RadFrac blocks (The Flowsheet approach)
2. Use MultiFrac block (The Simultaneous Approachapplicable only to selected problems)
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Considerations for the Flowsheet Approach
• Flowsheet convergence is greatly affected by:– Solvent makeup using Fortran blocks or Balance blocks– Choice of tear streams– Choice of recycling convergence methods– Choice of column specifications– Tolerance balancing– Sequencing
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 4© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Extractive Distillation Case Study
Separation of toluene and methyl-cyclohexane (MCH)
• Uses phenol as the solvent
• Uses a flowsheet that consists of an extractive distillation column and a regeneration column
• Modeled with the UNIFAC Property Method to represent the Vapor-Liquid Equilibrium
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Extractive Distillation – Base Case (1)
Filename: EX9A-EXTDIST1.BKP
HEATER
FEED
HOT-FEED
EXT-COL
SOLVENT
MCH-PROD
COL-BTMS
REGEN
TOL-PROD
Temp 200Pres 20Mole Flow
MCH 200 lbmol/hrToluene 200 lbmol/hr
Temp 200Pres 20Mole Flow
Toluene 12 lbmol/hrPhenol 1188 lbmol/hr
Temp 220Pres 20
Pressure Drop Per Stage 0.2 psiTotal CondenserDistillate Rate 200 lbmol/hrReflux Ratio 8
20
22
7
14
10
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 5© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Extractive Distillation – Base Case (2)
Converging tear streams: SOLVENT* WARNING
CONVERGENCE BLOCK $OLVER01 NOT CONVERGED IN 30 ITERATIONS
3 vars not converged, Max Err/Tol 0.52165E+02** ERROR
Convergence block $OLVER01 did not convergenormally in the final pass
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Extractive Distillation – Base Case (3)
COL-BTMS FEED HOT-FEED MCH-PROD SOLVENT TOL-PRODTemperature F 311.8 77.0 220.0 219.3 355.9 232.9Pressure psi 20.20 20.00 20.00 16.00 19.80 16.00Vapor Frac 0 0 0 0 0 0Mole Flow lbmol/hr 1399.999 400.000 400.000 200.000 1199.999 200.000Mass Flow lb/hr 131351.120 38065.736 38065.736 19530.077 112815.462 18536.285Volume Flow cuft/hr 2342.544 751.920 828.128 445.729 1980.486 386.440Enthalpy MMBtu/hr -59.834 -15.234 -12.690 -13.427 -58.950 0.799Mole Flow lbmol/hr MCH 17.886 200.000 200.000 182.114 0.000 17.885 TOLUENE 243.287 200.000 200.000 17.576 60.863 182.106 PHENOL 1138.825 0.000 0.000 0.309 1139.135 0.008
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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Extractive Distillation – Add Makeup (1)
HEATER
FEED HOT-FEED
EXT-COL
SOLVENT MCH-PROD
COL-BTMS
REGEN
RECYCLE
TOL-PROD
MIXER
MAKEUP
Pres 20Temp 220Pres 20Mole Flow
Phenol 1 lbmol/hr
Filename: EX9A-EXTDIST2.BKP
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Extractive Distillation – Add Makeup (2)
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 7© 2002 AspenTech. All Rights Reserved.
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Extractive Distillation – Add Makeup (3)
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Extractive Distillation – Add Makeup (4)
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 8© 2002 AspenTech. All Rights Reserved.
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Extractive Distillation – Add Makeup (5)> Loop $OLVER01 Method: WEGSTEIN Iteration 30Converging tear streams: SOLVENT* WARNING
CONVERGENCE BLOCK $OLVER01 NOT CONVERGED IN 30 ITERATIONS
3 vars not converged, Max Err/Tol -0.59837E+02** ERROR
BLOCK MIXER IS NOT IN MASS BALANCE:MASS INLET FLOW = 0.14802432E+02, MASS OUTLET FLOW =
0.14798550E+02RELATIVE DIFFERENCE = 0.26230909E-03MAY BE DUE TO A TEAR STREAM OR A STREAM FLOW MAY
HAVEBEEN CHANGED BY A FORTRAN, TRANSFER, OR BALANCE
BLOCKAFTER THE BLOCK HAD BEEN EXECUTED.
** ERRORConvergence block $OLVER01 did not convergenormally in the final pass
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Extractive Distillation – Add Makeup (6)
COL-BTMS FEED HOT-FEED MAKEUP MCH-PROD RECYCLE SOLVENT TOL-PRODTemperature F 313.1 77.0 220.0 220.0 219.3 356.6 356.5 232.9Pressure psi 20.20 20.00 20.00 20.00 16.00 19.80 20.00 16.00Vapor Frac 0 0 0 0 0.00043561 0 0 0Mole Flow lbmol/hr 1449.260 400.000 400.000 0.334 200.000 1249.260 1249.260 200.000Mass Flow lb/hr 135987.288 38065.736 38065.736 31.468 19529.235 117450.128 117450.788 18537.160Volume Flow cuft/hr 2422.828 751.920 828.128 0.503 483.753 2061.649 2061.606 386.487Enthalpy MMBtu/hr -62.425 -15.234 -12.690 -0.020 -13.414 -61.377 -61.015 0.787Mole Flow lbmol/hr MCH 18.031 200.000 200.000 0.000 181.970 0.001 0.000 18.030 TOLUENE 243.555 200.000 200.000 0.000 17.704 61.593 61.259 181.962 PHENOL 1187.674 0.000 0.000 0.334 0.326 1187.666 1188.000 0.008
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 9© 2002 AspenTech. All Rights Reserved.
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Extractive Distillation –Redundant Specs
• Distillate rate is specified for both columns.
• Instead specify a bottoms rate of 1200 lbmol/hr for the REGEN column.
> Loop $OLVER01 Method: WEGSTEIN Iteration 4Converging tear streams: SOLVENT
# Converged Max Err/Tol 0.25089E-01
*** MASS AND ENERGY BALANCE ***IN OUT RELATIVE DIFF.
CONVENTIONAL COMPONENTS (LBMOL/HR)MCH 200.000 200.000 -0.305866E-10TOLUENE 200.000 200.000 0.155813E-06PHENOL 0.420749 0.420749 0.000000E+00
TOTAL BALANCEMOLE(LBMOL/HR) 400.421 400.421 0.778092E-07MASS(LB/HR ) 38105.3 38105.3 0.753368E-07
ENTHALPY(BTU/HR ) -0.152594E+08 -0.126200E+08 -0.172965
Filename: EX9A-EXTDIST3.BKP
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IPA Dehydration – Base Case (1)
AZEOCOL
FEED
REFLUX
IPAPROD
AZ-OVHD
STRIPPERH2O-PHS
STRPBTMS
STRPOVHD
MIXER
MAKEUP
BENZ-PHS
DECANTER
Filename: EX9B-IPADEHY1.BKP
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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IPA Dehydration – Base Case (2)
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IPA Dehydration – Base Case (3)
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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IPA Dehydration – Base Case (4)
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IPA Dehydration – Base Case (5)
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 12© 2002 AspenTech. All Rights Reserved.
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IPA Dehydration – Base Case (6)
Loosen Tear Stream Convergence Tolerance:
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IPA Dehydration – Base Case (7)
Increase Tear stream iterations:
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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IPA Dehydration – Base Case (8)
> Loop CVBLK Method: WEGSTEIN Iteration 39Converging tear streams: AZ-OVHD STRPOVHD
# Converged Max Err/Tol -0.87154E+00* WARNING
BLOCK STRIPPER IS NOT IN MASS BALANCE:MASS INLET FLOW = 0.92748289E+00, MASS OUTLET FLOW =
0.92783251E+00RELATIVE DIFFERENCE = 0.37695355E-03IMBALANCE IS DUE TO A LOOSE TEAR TOLERANCESTREAM STRPOVHD TOLERANCE = 0.10000000E-02
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IPA Dehydration – Improve Convergence
• Use Broyden Convergence Method
• Tighten the tolerance of the tear streams to 0.0001
> Loop CVBLK Method: BROYDEN Iteration 8Converging tear streams: AZ-OVHD STRPOVHD
# Converged Max Err/Tol -0.11036E+00
Filename: EX9B-IPADEHY2.BKP
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 14© 2002 AspenTech. All Rights Reserved.
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IPA Dehydration – Tear Stream Selection
Filename: EX9B-IPADEHY3.BKP
AZEOCOL
FEED
REFLUX
IPAPROD
AZ-OVHD
STRIPPER
H2O-PHS
STRPBTMS
STRPOVHD
MIXER
BENZ-PHS
MAKEUP
DECANTER
MIXOVHDS
MIXOVHD
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IPA Dehydration - Tear Stream Selection
• Select MIXOVHDS as the Tear stream to converge.
> Loop CVBLK Method: BROYDEN Iteration 8Converging tear streams: MIXOVHDS
# Converged Max Err/Tol 0.81613E-01
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 15© 2002 AspenTech. All Rights Reserved.
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Ethylene Plant Quench Section
The following table summarizes a typical configuration for an ethylene plant quench section:
………………………………………………………………...Number of components 28 real components
8 pseudocomponents………………………………………………………………...Quench tower 4 theoretical stages………………………………………………………………...Primary fractionator 7 theoretical stages
1 pumparound………………………………………………………………...Fuel oil stripper 3 theoretical stages………………………………………………………………...
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Ethylene Plant Quench Section Approach
• Flowsheet section approach– Model the quench tower with RadFrac– Model the primary fractionator and fuel oil stripper with
MultiFrac– Solve a recycle stream via iteration– Provide recycle estimates
• Simultaneous approach– Use the MultiFrac block to solve all units and recycle
simultaneously– Recycle estimates are not required
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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Flowsheet Approach
PrimaryFractionator
FEED
FOSTM
PFBTMS
WATER
GASOLINE
Quench Tower
QW1
QW2
QTOVHD
QTBTMS
1
2
3
4
1234
Stripper
FO
1
2
3
Decanter
HC
PFOVHD
HPHC
REFLUX
5
67
Filename: EX9C-QUENCH.BKP
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Simultaneous Approach with MultiFrac (1)
QT-PFR
FEED
QW1
QW2
FOSTM
QTOVHD
PFBTMS
WATER
GASOLINE
Filename: EX9C-QUENCHMULTI.BKP
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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Simultaneous Approach with MultiFrac (2)
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Simultaneous Approach with MultiFrac (3)
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
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Simultaneous Approach with MultiFrac (4)
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Simultaneous Approach with MultiFrac (5)
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Advanced Distillation with Aspen Plus Highly Coupled Distillation Systems
Aspen Technology, Inc.10 – 19© 2002 AspenTech. All Rights Reserved.
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Ethylene Plant Quench Section Results
Solution MethodPerformance Measure Flowsheet MultiFrac…………………………………………………………………………………Recycle estimates Good None…………………………………………………………………………………Number of recycle iterations 21 --…………………………………………………………………………………Recycle convergence 1E-4 --tolerance…………………………………………………………………………………CPU seconds (PII-266) 60 20…………………………………………………………………………………Maximum relative 3E-4 1E-16mass balance error…………………………………………………………………………………
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