5 - azeotropic distillation1 sequencing of azeotropic distillation columns ref: seider, seader and...
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5 - Azeotropic Distillation1
SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS
Ref: Seider, Seader and Lewin (2004), Section 7.5
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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 boundariesdistillation boundaries limit the product compositions of a column to lie within a bounded regionPrevent the removal of certain species in
high concentrations
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Instructional Objectives
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
When you have finished studying this unit, you should:
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Homogeneous Azeotropes
At equilibrium:
V L
j jff_ _
V L
j j j j jy P x f
s
1 1 1yP xP
s
2 2 2yP x P
s s s s
1 1 2 2 1 1 1 2P xP x P xP 1 x P( )
s s s
2 1 2 1P P P x( )
At fixed temperature
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Homogeneous Azeotropes (Cont’d)
Example – Phase diagrams for benzene-
toluene mixture at 90 oC
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Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the activity coefficients are different from unity:
S
1 1 1 1yP x P
S
2 2 2 2yP x P
s s
1 1 1 1 2 2P x P 1 x P( )
If the mixture has a minimumminimum-boiling azeotrope
i1
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol
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Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the activity coefficients are different from unity:
S
1 1 1 1yP x P
S
2 2 2 2yP x P
s s
1 1 1 1 2 2P x P 1 x P( )
If the mixture has a maximummaximum-boiling azeotrope
i1
Example – Phase diagrams for Acetone-Chloroform
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Heterogeneous Azeotropes
For a minimum-boiling azeotrope with large deviation from Raoult’s law ( ), phase splitting may occur and a minimum-boiling heterogeneous azeotrope forms, having a vapor phase in equilibrium with two liquid phases.
Homogeneous Azeotrope Heterogeneous Azeotrope
7usually ,1 i
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j j j j j jLx ydL Lx Ldx xdL dLdx j 1 C 1, , ,
Residue Curves
Mass balance on species j:
j j j jLx L y L L x x j 1 C 1( ) ( )( ), , ,
As L 0:
Distillation still
j
j j j j
dxx y x 1 K T P x y
dL L( { , , , })
/
Rearranging:
j
j j
dxx y
dt
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Residue curves for Azeotropic
system
Residue Curves (Cont’d)
j
j j
dxx y
dt
Residue curves for zeotropic
system
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Liquid Compositions at Total Reflux
Species balance on top n-1 trays:
n
n n 1
dxx x
dh
n 1 n 1 n nDL x Dx Vy
Approximation for liquid phase:
Substituting:
n n
n n D
n 1 n 1
dx V Dx y x
dh L L
At total reflux, D = 0 and Vn = Ln-1
n
n n
dxx y
dh Stripping section of
distillation column
The residue curves approximates the distillation curves at total The residue curves approximates the distillation curves at total refluxreflux
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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:
2- On each stage:
,1,01
nyxnn
lines tiemequilibriu theof ends at the lie and nn yx
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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)
j
j j
dxx y
dt
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Product Composition Regions for Zeotropic Systems
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Product Composition Regions for Azeotropic Systems
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Example: Dehydration of Ethanol
Heterogeneous Azeotropic Distillation
Try toluene as an entrainer
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S1
S2
M1
D2
M2
D1
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Pressure-swing Distillation
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T-x-y diagrams for THF and water
Example: Dehydration of Tetrahydrofuran (THF)
Pressure-swing Distillation (Cont’d)
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Azeotropic Distillation - Summary
On completing this unit, you should:
Next week:Next week: HEN Synthesis HEN Synthesis
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