09highly coupled dist syst

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


Lesson Objectives

• Understand alternative strategies for modeling Highly Coupled Distillation Systems in Aspen Plus.




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


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




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)


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




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


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





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



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




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







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



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




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



IPA Dehydration – Base Case (1)

AZEOCOL

FEED

REFLUX

IPAPROD

AZ-OVHD

STRIPPERH2O-PHS

STRPBTMS

STRPOVHD

MIXER

MAKEUP

BENZ-PHS

DECANTER

Filename: EX9B-IPADEHY1.BKP





Loosen Tear Stream Convergence Tolerance:



Increase Tear stream iterations:





> 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


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





IPA Dehydration – Tear Stream Selection


AZEOCOL

FEED

REFLUX

IPAPROD

AZ-OVHD

STRIPPER

H2O-PHS

STRPBTMS

STRPOVHD

MIXER

BENZ-PHS

MAKEUP

DECANTER

MIXOVHDS

MIXOVHD


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




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………………………………………………………………...


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




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


Simultaneous Approach with MultiFrac (1)

QT-PFR

FEED

QW1

QW2

FOSTM

QTOVHD

PFBTMS

WATER

GASOLINE

Filename: EX9C-QUENCHMULTI.BKP




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…………………………………………………………………………………


09highly coupled dist syst

Documents