Binary Distillation

1
Binary Distillation

• Zeotropic Azeotropic

• Azeotropic Cannot reach all compositions
  (unless change conditions)
• Products from a single ordinary distillation
  column depend on feed conditions

2
Residue Plot for Ternary Systems

• Ternary diagram representing liquid-residue
  composition for high/total reflux
• Each curve is the locus of possible equilibrium
  liquid-phase compositions during distillation
• Example Methanol/Ethanol/1-Propanol
• No azeotropes
• Only one distillation region
• Distillate can be nearly pure methanol or the
  bottoms can be nearly pure 1-propanol
• Nearly pure ethanol cannot be produced either as
  distillate or bottoms

3
Residue Plot for Ternary Systems

• Construction on ASPEN Properties
• Components Methanol/Ethanol/1-Propanol
• Property Method UNIQUAC
• Pressure 1 atm
• Now make residue plots for the following systems

4
Residue Plots for Ternary Systems

• Acetone/Methanol/Ethanol
• No distillation boundaries (even with azeotrope)
• Feed can be separated into two binary products
• Can avoid ternary distillate or bottoms products

5
Residue Plots for Ternary Systems

• Octane/2-Ethoxy-Ethanol/Ethylbenzene
• Two azeotropes!
• Distillation boundary connecting the two
  azeotropes
• Material balance cannot cross the distillation
  boundary

6
Product-Composition Regions for Distillation
Design

• Superimpose material balance line on residue plot
• Material balance line connects distillate and
  bottoms compositions and passes through feed
  composition

L Low-Boiling H High-Boiling I Intermediate
7
Product-Composition Regions for Distillation
Design

• Limiting distillate composition _at_ L apex
• Corresponding bottoms composition with least
  amount of component L _at_ B
• Limiting bottoms composition _at_ H apex
• Corresponding distillate composition with least
  amount of component H _at_ D
• Two material balance lines and residue curve
  define product-composition region (shaded)

Note Operating region on convex side of residue
curve (due to equilibrium requirement)
8
Product-Composition Regions for
Octane/2-Ethoxy-Ethanol/Ethylbenzene

• Recall two distillation regions caused by two
  minimum-boiling azeotropes, with boundary in
  between
• Region 1
• L N-Octane/2-E-E azeotrope
• H 2-E-E
• For given feed F1, draw MB lines starting from L
  and H components
• Feasible product composition regions are bounded
  by these lines and residue curve (shaded)
• D1 distillate composition region
• B1 bottoms composition region

D1 B1
9
Product-Composition Regions for
Octane/2-Ethoxy-Ethanol/Ethylbenzene

• Region 2
• L N-Octane/2-E-E azeotrope
• H Ethylbenzene
• For given feed F2, draw MB lines starting from L
  and H components
• Feasible product composition regions are bounded
  by these lines and residue curve (shaded)
• D2 distillate composition region
• B2 bottoms composition region

D2
Note Inflection point on residue curve where F2
is located gives larger D2 region