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Distillation columns in CHEMCAD
Short Cut Tower Tower Plus SCDS - Fenske-Underwood-Gilliland - Limited design, Rating - Rigorous - Inside-Out Algorithm - Usual specifications - Simple columns - Rigorous - Inside-Out Algorithm - Extended specifications - Complex columns - Rigorous - Simultaneous corrections method - Usual specifications
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
SHOR column
Short Cut Method (SHOR)
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Fenske-Underwood-Gilliland method
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One feed and two products
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Design mode – Calculates reboiler and condenser duties, determines number of theoretical stages, finds optimum feed tray location (Fenske, Kirkbride)
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Rating mode – Calculates reboiler and condenser duties, computes fractional recovery of key components
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Generally not useful beyond initial column estimates: constant molar underflow requirement is too unrealistic
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
SCDS Column
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Simultaneous Correction Method: Large number of stages Stable with non-linear (non-ideal) thermodynamics Run time = B * (NCOMP) Dynamics 2 * NSTAGES Mass transfer and tray efficiencies
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
TOWR column
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Inside-Out Method: Fast with EOS thermodynamics Large number of components, small number of stages Run time = A * NCOMP * (NSTAGES) 2 No mass transfer or stage efficiencies Complex columns Dynamics
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
General Recommended Selection
EOS thermodynamics Non-ideal thermodynamics Three phase distillation Reactive distillation Mass transfer Complex column Trays > 100 Components > 20 Tower SCDS SCDS SCDS SCDS Tower Plus SCDS Tower
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Distillations Models…
Degrees of Freedom..
Simple absorber Reboiler Condenser Side exchanger Pumparound Side stripper 0 1 1 1 2 1
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Distillations Models…
Typical Specifications Relatively “Safe”: One material balance specification (D, B) One heat balance specification (R/D, V/B) May require estimates: Compositions Temperatures Recoveries Heat loads Poor Choices: Two material balance specifications Pure stream temperature
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Distillations Models…
Column Estimate Guidelines
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No estimate better than poor estimate
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Avoid pure component temperature
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Avoid azeotrope temperature
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Tray 2, N-1 temperatures for unusual problems
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Use thermodynamic (phase) plots for guidance
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Use profile options for difficult problems
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Use preliminary material balances
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Distillations Models…
Column Convergence – Problem Recognition SCDS: Profile oscillations Increasing errors Many NR steps, decreasing size Inside-Out: Increasing errors Many Jacobian Inversions
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Distillations Models…
Column Convergence – Problem Management
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Review thermo
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Poor specifications
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Specification conflicts?
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Poor estimates?
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High pressure?
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Close boiling? Dry trays? (adjust T profiles)
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Create restart profiles
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Use damping factors
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Converged with poor mass balance? (tighten tolerances)
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All else fails? (reduce tray efficiencies)
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Dehydration of ethanol using normal pentane as the entrainer
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Note: To find multicomponent azeotropes you can use CC-Batch
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Benzene/Ethanol/Water Residue Curve Map
1 0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 0 0.1
0.2
0.3
0.4
0.5
Benzene mole fraction
0.6
0.7
0.8
0.9
1 Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
material balance line excluding condenser tie line 1. Fix bottoms composition at 99.99% EtOH.
2. Pick tie line that crosses relevant boundaries. Fixes composition of decanter liquid phases.
3. Determine distillate flowrate by pegging water flowrate.
4. Pick y(n) on tie line so that it is in equilibrium with homogeneous liquid.
5. Determine organic phase flowrate using lever rule.
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Simulation results Liquid compositions on trays Composition of combined feed y(n) Composition of top tray liquid
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Design considerations in Azeotropic Distillation
• Entrainer requirements • Column sequence • Reflux ratio • Number of stages • Effect of trace impurities
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
distillation boundary Distillation boundaries can be crossed but it is almost always very difficult to take advantage of it.
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
VOC Stripping…
T = 85 F P = 14.7 psia H2O = 696,000 lb/hr TCE = 69 lb/hr Air
Distillations Models…
H(TCE,25C) = 650 atm.
H(TCE in H2O) = 650 atm.
TCE = 1 ppb
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
Distillations Models…
Debutanizer – Split Iso and N butanes Feed: C3 iC4 nC4 iC5 nC7 83.06 lbmoles/hr 4329.755
725.09
62.12
1286.98
Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net