SEQUENCING OF AZEOTROPIC
DISTILLATION COLUMNS
Ref: Seider, Seader and Lewin (2004), Section 7.5
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Azeotropic Distillation - 5
Introduction
 Separation sequences are complicated by the
presence of azeotropes, often involving mixtures of
oxygenated organic compounds:
 Alcohols
 Ketones
 Ethers
 Acids
 Water
 In these cases, distillation boundaries limit the
product compositions of a column to lie within a
bounded region
Prevent the removal of certain species in high
concentrations
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Azeotropic Distillation - 5
Instructional Objectives
When you have finished studying this unit, you should:
 Be able to sketch the residue curves on a tertiary
phase diagram
 Be able to define the range of possible product
compositions using distillation, given the feed
composition and the tertiary phase diagram
 Be able to define the PFD for a heterogeneous
azeotropic distillation system
 Be able to define the PFD for a pressure swing
distillation system
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Azeotropic Distillation - 5
Homogeneous Azeotropes
At equilibrium:
_ V
_ L
j
j
f  f
y P  x f
V
j
j
j
j
yP  xP
yP  xP
L
j
s
1
2
1 1
s
2 2
P  x P  x P  x P  (1  x )P
s
s
1 1
s
2 2
1 1
1
s
2
 P  (P  P )x
s
2
s
1
s
2
1
At fixed temperature
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Azeotropic Distillation - 5
Homogeneous Azeotropes (Cont’d)
Example – Phase diagrams
for benzene-toluene
mixture at 90 oC
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Azeotropic Distillation - 5
Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the
activity coefficients are
different from unity:
yP  x P
yP  x P
S
1
1
1 1
2
2
2 2
P  x  P  (1  x )  P
s
1
1 1
1
S
s
2 2
If   1 the mixture has a minimum-boiling azeotrope
i
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol
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Azeotropic Distillation - 5
Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the
activity coefficients are
different from unity:
yP  x P
yP  x P
S
1
1
1 1
2
2
2 2
P  x  P  (1  x )  P
s
1
1 1
1
S
s
2 2
If   1 the mixture has a maximum-boiling azeotrope
i
Example – Phase diagrams for Acetone-Chloroform
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Heterogeneous Azeotropes
For a minimum-boiling azeotrope with large deviation from
Raoult’s law (  i  1, usually  7 ), phase splitting may occur
and a minimum-boiling heterogeneous azeotrope forms,
having a vapor phase in equilibrium with two liquid phases.
Homogeneous Azeotrope
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Heterogeneous Azeotrope
Azeotropic Distillation - 5
Residue Curves
Distillation still
Mass balance on species j:
Lx  ( L ) y  (L  L )( x  x ), j  1,
j
j
j
j
,C  1
As L  0:
Lx  y dL  Lx  Ldx  x dL  dLdx , j  1,
j
j
Rearranging:
j
j
dx
j
dL / L
j
,C  1
 x  y  x (1  K {T, P, x, y })
j
dx
j

dt
9
j
j
j
j


 x y
j
j
Azeotropic Distillation - 5
Residue Curves (Cont’d)
dx
j

dt
 x y
j
j
Residue curves for
zeotropic system
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Residue curves for
Azeotropic system
Azeotropic Distillation - 5
Liquid Compositions at Total Reflux
Species balance on top n-1 trays:
L x  Dx  Vy
n 1
n 1
n
D
n
Approximation for liquid phase:
dx
 x x
dh
n
n
n 1
Substituting:
dx
V
D
 x 
y 
x
dh
L
L
n
n
n
n
D
n 1
n 1
At total reflux, D = 0 and Vn = Ln-1
Stripping section of
distillation column
dx
 x y
dh
n
n
n
The residue curves approximates the distillation curves at total reflux
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Distillation Curves
An exact representation of the distillation curve
(operating line) at total reflux can be calculated based on:
1- Between two adjacent stages: x n  y n1 n  0, 1, 
2- On each stage: x n and y n lie at the ends of the equilibriu m tie lines
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Comparison of Residue and Distillation Curves
When the residue curve is linear (as for binary mixtures), the
distillation and residue curves are coincide. Their differences
are pronounced in the regions with extensive curvature.
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Sketching Residue Curves (Exercise)
dx
j

dt
14
 x y
j
j
Azeotropic Distillation - 5
Product Composition Regions
for Zeotropic Systems
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Product Composition Regions
for Azeotropic Systems
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Heterogeneous Azeotropic Distillation
Example: Dehydration of Ethanol
Try toluene
as an
entrainer
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D1
M2
M1
S1
S2
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D2
Azeotropic Distillation - 5
Pressure-swing Distillation
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Azeotropic Distillation - 5
Pressure-swing Distillation (Cont’d)
Example: Dehydration of Tetrahydrofuran (THF)
T-x-y diagrams for THF and water
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Azeotropic Distillation - Summary
On completing this unit, you should:
 Be able to sketch the residue curves on a tertiary
phase diagram
 Be able to define the range of possible product
compositions using distillation, given the feed
composition and the tertiary phase diagram
 Be able to define the PFD for a heterogeneous
azeotropic distillation system
 Be able to define the PFD for a pressure swing
distillation system
Next week: HEN Synthesis
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Azeotropic Distillation - 5