Distillation columns in CHEMCAD

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Distillation columns in CHEMCAD
Short Cut - Fenske-Underwood-Gilliland
- Limited design, Rating Tower
- Rigorous -
Inside-Out Algorithm -
Usual specifications -
Simple columns Tower Plus - Rigorous
- Inside-Out Algorithm
- Extended specifications
- Complex columns SCDS - Rigorous
- Simultaneous corrections
method - Usual specifications
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SHOR column

• Short Cut Method (SHOR)
• Fenske-Underwood-Gilliland method
• One feed and two products
• Design mode Calculates reboiler and condenser
  duties, determines number of theoretical stages,
  finds optimum feed tray location (Fenske,
  Kirkbride)
• Rating mode Calculates reboiler and condenser
  duties, computes fractional recovery of key
  components
• Generally not useful beyond initial column
  estimates constant molar underflow requirement
  is too unrealistic

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SCDS Column

• Simultaneous Correction Method
• Large number of stages
• Stable with non-linear (non-ideal) thermodynamics
• Run time B (NCOMP)2 NSTAGES
• Mass transfer and tray efficiencies
• Dynamics

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TOWR column

• 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

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General Recommended Selection
EOS thermodynamics Tower Non-ideal
thermodynamics SCDS Three phase
distillation SCDS Reactive distillation SCDS M
ass transfer SCDS Complex column Tower
Plus Trays gt 100 SCDS Components gt
20 Tower
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Distillations Models
Degrees of Freedom.. Simple absorber 0 Reboiler
1 Condenser 1 Side exchanger 1 Pumparound 2
Side stripper 1
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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
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Distillations Models
Column Estimate Guidelines

• No estimate better than poor estimate
• Avoid pure component temperature
• Avoid azeotrope temperature
• Tray 2, N-1 temperatures for unusual problems
• Use thermodynamic (phase) plots for guidance
• Use profile options for difficult problems
• Use preliminary material balances

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Distillations Models
Column Convergence Problem Recognition
SCDS Profile oscillations Increasing
errors Many NR steps, decreasing
size Inside-Out Increasing errors Many
Jacobian Inversions
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Distillations Models
Column Convergence Problem Management

• Review thermo
• Poor specifications
• Specification conflicts?
• Poor estimates?
• High pressure?
• Close boiling? Dry trays? (adjust T profiles)
• Create restart profiles
• Use damping factors
• Converged with poor mass balance? (tighten
  tolerances)
• All else fails? (reduce tray efficiencies)

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Dehydration of ethanol using normal pentane as
the entrainer
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Note To find multicomponent azeotropes you can
use CC-Batch
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binodal plot (fixed temperature)
liquid boiling envelope (fixed P)
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tie line
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Simulation results
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Design considerations in Azeotropic Distillation
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distillation boundary
Distillation boundaries can be crossed but it is
almost always very difficult to take advantage of
it.
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Distillations Models
VOC Stripping
H(TCE,25C) 650 atm.
T 85 F P 14.7 psia H2O 696,000
lb/hr TCE 69 lb/hr
H(TCE in H2O) 650 atm.
Air
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Distillations Models
Debutanizer Split Iso and N butanes
Feed C3 83.06 lbmoles/hr iC4 4329.755 nC4
725.09 iC5 62.12 nC7 1286.98