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Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 1© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Customizing RadFrac Convergence
Advanced Distillation with Aspen Plus
© 2002 AspenTech. All Rights Reserved.
Lesson Objectives
• Understand how to customize RadFrac convergence algorithms
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 2© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
RadFrac Convergence Form
© 2002 AspenTech. All Rights Reserved.
Basic Convergence Input
From the Basic sheet, you can alter the following convergence parameters:
• Algorithm (only for Custom Convergence).
• Maximum iterations.
• Error tolerance.
• Initialization method (only for Custom Convergence).
• Damping level.
• Liquid-liquid phase splitting method.
• Solids handling.
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 3© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Convergence Methods
• To customize convergence methods, first choose Custom on the RadFrac Specifications form.
• This will allow you full freedom to select both the convergence method and the initialization strategy.
• For details on convergence methods, refer to lesson 6.
© 2002 AspenTech. All Rights Reserved.
Custom ConvergenceSpecifying Custom Convergence on the Setup Configuration sheet allows the user to select convergence strategy:
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 4© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Algorithms
• Four algorithms can be selected on the Algorithm field:– Standard– Sum-Rates– Nonideal– Newton
• Parameters for the specific algorithms are on the Convergence Algorithm sheet.
© 2002 AspenTech. All Rights Reserved.
Maximum Iterations
• Most problems converge within the standard default 25 iterations.
• Complex columns may need more.– Large numbers of components– Reactions– Chemistry– Liquid-liquid equilibrium
• In general, increase Maximum iterations if Err/Tol is decreasing but convergence is not achieved in 25 iterations.
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 5© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Error Tolerance
• The tolerance specified here is the outside loop convergence tolerance. Expect errors of this magnitude in:– Dew and bubble points– Mass balance (component and total)– Enthalpy balance
• The default value is 10-4. Do not increase this value.
• Decrease (tighten) when necessary.
© 2002 AspenTech. All Rights Reserved.
Tightening Error Tolerance
Tighten tolerance in these situations:
• Following trace components.
• LLE where one liquid phase is small.
• Any time greater accuracy is desired.
• RadFrac blocks inside recycle loops.– Default tear stream tolerance is 10-5
– Errors in RadFrac can impede tear convergence– Make RadFrac tolerance 1 or 2 orders of magnitude tighter than tear
stream tolerance (otherwise noise from RadFrac results may adversely affect higher level iterations)
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 6© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Flowsheet with Recycle
RadFrac convergence tolerance should be tighter than tear stream tolerance.
Recycle
© 2002 AspenTech. All Rights Reserved.
Initialization Option
Use this strategy For this situation……………………………………………………………………Crude For wide boiling systems with
multidraw columns……………………………………………………………………Chemical For narrow boiling chemical systems……………………………………………………………………Azeotropic For azeotropic distillation columns……………………………………………………………………Cryogenic For cryogenic applications (for
example, air separations)……………………………………………………………………
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 7© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Damping Level
• RadFrac offers four damping levels:– None (default): No damping– Mild: Mild level of damping– Medium: Medium level of damping– Severe: Maximum damping
• Use damping to stabilize convergence if excessive oscillation occurs during convergence (when Err/Toloscillates when near solution).
• Damping slows convergence – with Severe damping extra iterations are almost always needed.
© 2002 AspenTech. All Rights Reserved.
Liquid-Liquid Phase Split Algorithm
Gibbs (default)– Uses Gibbs Energy minimization– Stable, reliable– Requires that LLE model be thermodynamically consistent
Eq-Solve– Equates component fugacities– May find false roots– Fast
Hybrid (new for Aspen Plus 10)– Uses Eq-solve to find all roots and Gibbs to select one– Should be faster than Gibbs; requires consistency
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 8© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Solids Handling
Overall (default)– All solids are removed from feed streams and put into bottom
streams– Makes column heat balance easier to solve– Small errors in duties and bottoms temperature– Flow specifications (Bottoms flow, e.g.) do not include solids
Stage– Solids move down the column, are present on all stages below
feeds– Gives correct heat balances and temperatures– Flow specifications include solids
© 2002 AspenTech. All Rights Reserved.
Fixing Convergence Problems
Err/Tol StepsDecreasesbut slowly Increase the number of iterations
Oscillates Increase Damping Level
Diverges 1) Provide Initial Estimates
2) Check specifications for problems
3) Simplify the specifications
4) Try different algorithms
5) Tune algorithm parameters
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 9© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Convergence Algorithm Sheet
• This sheet allows input for details of three-phase calculations and for algorithm-specific parameters.
• It has four Subsections:1. Sum-rates parameters2. Newton parameters3. Non-ideal parameters4. Three-phase parameters
© 2002 AspenTech. All Rights Reserved.
Sum-Rates Parameters
Jacobian Method controls when and how the Jacobian(matrix of partial derivatives) is calculated.– Initial - Calculates the Jacobian initially, updates it with the
Broyden method once inside loop has converged– Rmsol - keeps recalculating Jacobian until outside loop error
falls below the Update threshold. Below the Update thresholdBroyden is used.
– The Update threshold default is 0.01– The Rmsol method is slower (more Jacobian calculations are
done) but more stable. Lowering the threshold increases this effect.
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 10© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Newton Parameters
Newton’s method requires large amounts of intermediate storage space. Two parameters control the amount of space allocated:– Maximum size– Storage factor
These may need to be increased for large problems. If so, you will get an explicit error message when you first run the column.
© 2002 AspenTech. All Rights Reserved.
Non-Ideal Method Parameters
Gamma model controls what activity coefficient model is used to extrapolate activity coefficients in the inside loop– Margules– Combined (Wilson and Margules)
User can also specify limits on extrapolated activity coefficients at the midpoint and at infinite dilution– Increasing max gamma at infinite dilution can reduce iterations
needed for highly non-ideal (high activity coefficient) systems– Increasing max gamma at midpoint may reduce outside loop
iterations but increase inside loops
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 11© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Three-Phase Parameters
Mole-fraction threshold for 2nd liquid key component
• Allows user to control whether a single liquid phase is called liquid1 or liquid2. Does not affect phase equilibrium calculations.
• Default is 0.5.
Water-Butanol Liquid-Liquid Phase Diagram
Tem
pera
ture
C25
5075
100
125
150
Mole Fraction of Water0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
© 2002 AspenTech. All Rights Reserved.
Additonal Three-Phase Parameters
Gamma-model option for 2nd liquid phase:– Constant (computed from the outside loop)– Margules (default -- acts like non-ideal algorithm)
Maximum liquid-liquid phase split iterations:– Controls LL iterations in inside loop– LLE done rigorously in outside loop– Default is 20
Maximum initialization passes:– Controls number of initialization iterations in which two liquid phases
are assumed to exist on all stages– Default is 0. Using 1 or 2 may aid convergence when initialization
does not predict the region of LLE well.
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 12© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Convergence Advanced Sheet
• A large number of unrelated options and parameters in alphabetical order.
• Duplicates parameters on some other forms (Maxol).
• Keeps all the old input language functionality.
• Often uses input language terminology.
» This section covers the most important parameters in a (somewhat) more logical order.
© 2002 AspenTech. All Rights Reserved.
Advanced Tab Parameters
• Absorber (Yes or No)– Controls inside-loop convergence for Standard algorithm– Use ‘Yes’ for absorbers and strippers without condenser or
reboiler
• Fminfac– Lower limit of tray flow rate (vapor or liquid) as a fraction of
sum of all feed flows– Default is 10-5. Lower this only if there are expected to be
some very small flows
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 13© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Design Spec Parameters
• Dsmeth– Controls the design spec method used with Newton algorithm– Simult (default) solves them with all the other equations
(usually quicker)– Nested solves them in a separate loop
(gives results even if spec is not satisfied)
• Rmsol0– Specifies the RMS outside loop error below which design spec
iterations are performed– Default is 0.1. Decreasing this may help if column appears to
diverge once middle loop iterations begin
© 2002 AspenTech. All Rights Reserved.
Feed Flash Parameters
• Flash-Maxit– Controls how many iterations are allowed for the feed flash– Default is 50 – Often 75 or more are needed for difficult flashes, especially
• Liquid-liquid Equilibrium• Electrolyte Chemistry
• Flash-tol– Controls feed flash tolerance (to override global flash-tol)– Rarely changed
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 14© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Local Enthalpy Model Parameters
• Hmodel1– Mole, Mass, or Pseudomass weighting
• Hmodel2– No-Temp: local enthalpy dependence only from ideal gas
(usually adequate)– Temp: local T dependence calculated initially (needed for
absorbers because sensible heat effects are important) – Update: recomputes local Temp dependence every pass
(rarely used)
Both are automatically chosen, but can be overridden
© 2002 AspenTech. All Rights Reserved.
Local K-value Parameter
• Kmodel choices:– X weights based on liquid mole fractions– Y weights based on vapor mole fractions– K based on (vapor mole fractions)/(k+1)
– This is chosen automatically based on type of distillation calculation
– Change to X or Y for systems with electrolytes
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 15© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Inside Loop Parameters (1)
• Maxil– Controls how many Inside Loop iterations are allowed per
outside loop– Increasing Maxil can help middle loop (design spec)
calculations converge– Do not increase unless there are design specs– Do not increase above about 20 (if 20 is not enough, the
problem lies elsewhere)
© 2002 AspenTech. All Rights Reserved.
Inside Loop Parameters (2)
Ilmeth– Controls how the inside loop is converged– Broyden (default)
• Usually the fastest
– Wegstein• May be necessary for very large columns (200+ stages)• Uses less storage than Broyden, but is slower• May require increasing Maxil
– Newton (bigger and faster than Broyden; good with tray efficiencies)
– Schubert (default when using Sum-Rates)
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 16© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Acceleration and Damping Controls
• Acceleration (and damping) for the Bounded Wegsteinmethod:– Qminbwol– Qmaxbwol– Qminbwil– Qmaxbwil
» In Aspen Plus 10, use of the Damping control on the Convergence Basic tab is preferred.
© 2002 AspenTech. All Rights Reserved.
Wegstein Parameters (1)
Qminbwol:The lower bound on qOL (Wegstein acceleration parameter). The default is 0.
(direct substitution)
(simple midpoint damping)
Set Qminbwol to 0.5 if outside loop convergence behavior is very erratic or is divergence occurs.
)ˆ(21
ˆ
kOL
kOL
1kOL
kOL
kOL
1kOL
kOL
xx x0.5q
x x 0q
+==
==
+
+
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 17© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Wegstein Parameters (2)
• QmaxbwolThe upper bound on qOL (default is 0.5).
NOTE: If you increase Qminbwol beyond 0.5, for consistency, you must increase Qmaxbwol.
• QminbwilThe lower bound onq IL (default is 0).
Set Qminbwil to 0.5 if outside loop convergence behavior is very erratic or if divergence occurs.
© 2002 AspenTech. All Rights Reserved.
Wegstein Parameters (3)
• QmaxbwilThe upper bound on q IL (default is 0.5).
NOTE: If you increase Qminbwil beyond 0.5, for consistency, you must also increase Qmaxbwil.
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 18© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Acceleration and Damping Controls
Rmsol1– Controls when Broyden method is used to accelerate the
convergence of selected variables– Below this Outside Loop RMS error, Broyden is used– Decrease this number if RadFrac converges to a point and
then stalls or becomes erratic– If Rmsol1 is less than the overall convergence tolerance,
Broyden is never used
» In Aspen 10, use of the Damping control on the Convergence Basic tab is preferred.
© 2002 AspenTech. All Rights Reserved.
Rmsol1
• RadFrac converges the outside loop by a combination of bounded Wegstein and Broyden methods:– Wegstein
– Broyden
( )( ) k
OL1-k
OLOL
kOL
kOLOL
kOL
kOL
kOL
kOL
1kOL
iii
iii
iiii
qx,x
qx,x
x̂)q-(1xqx
→
→
+•=+
kOL
kOL
kOL
1kOL
kOL xx̂)xx(J −=−+
variableOLith theof valuecalculated x̂
variableOLith theof valueassumed xindexiteration k
i
i
OL
OL
=
==
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 19© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
RadFrac Convergence Recap
• Recheck that the problem is well-posed (physically reasonable) and as simple as realistically possible.
• If Err/Tol is decreasing, give it more iterations.
• Change Convergence algorithm
• Improve Initialization by:
– Choosing a better initialization method, or – Giving estimates
• Increase Damping level
• Tune convergence parameters
• Reinitialize!
© 2002 AspenTech. All Rights Reserved.
Workshop 8A: Absorber Column
COLUMN
WATER
GAS-IN
GAS-OUT
BOTMS
START WITH: WS8A-ABSORBER.BKP
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 20© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Workshop 8B: Acetone/Water Column
COLUMN
HEATER
FEED COL-FEED
DIST
BOTMS
START WITH: WS8B-ACETONEWATER.BKP
© 2002 AspenTech. All Rights Reserved.
Workshop 8C: Sour Water Stripper
COLUMN
BOTTOMS
OVHD
FEED
STEAM
START WITH: WS8C-SOURWAT.BKP
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 21© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Workshop 8D: VCM Column
COLUMN
OVHD
BOTMS
FEED1
FEED2
START WITH: WS8D-VCMCOL.BKP
© 2002 AspenTech. All Rights Reserved.
Workshop 8E: Ethanol Dehydration
AZEO-COL
ETOH-PRO
FEED
CH-PHS
AZ-OVHD
START WITH: WS8E-ETOHDEHY.BKP
Advanced Distillation with Aspen Plus Customizing RadFrac Convergence
Aspen Technology, Inc.9 – 22© 2002 AspenTech. All Rights Reserved.
© 2002 AspenTech. All Rights Reserved.
Workshop 8F: Propanol/Butanol/Water
COLUMN
FEED
OVHD
BOTMS
SIDE
START WITH: WS8F-PBW.BKP
© 2002 AspenTech. All Rights Reserved.
Workshop 8G: Electrolyte Column
COLUMN
WATER
SOURGAS
GAS-PROD
BOTTOMS
START WITH: WS8G-ELEC.BKP