LECTURE SIX
054402 DESIGN AND ANALYSIS
054402 Design and Analysis LECTURE 6: SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS
Daniel R. Lewin Department of Chemical Engineering Technion, Haifa, Israel
Ref: Seider, Seader and Lewin (2004), Section 7.5
1 DESIGN AND ANALYSIS - (c) Daniel R. Lewin 6 - Azeotropic Distillation
Steps in Process Design and Retrofit
Assess Primitive Problem
Detailed Process Synthesis Algorithmic Methods
Development of Base-case
Plant-wide Controllability Assessment
SECTION B
Detailed Design, Equipment sizing, Cap. Cost Estimation, Profitability Analysis, Optimization
2 DESIGN AND ANALYSIS - (c) Daniel R. Lewin 6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Section B: Algorithmic Methods
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
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
DESIGN AND ANALYSIS - (c) Daniel R. Lewin 6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
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 homogeneous and heterogeneous azeotropic distillation systems Be able to define the PFD for a pressure swing distillation system
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Basics: The Lever Rule
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Basics: 3-Phase Diagrams
0.2 DTBP
0.15 H2O
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Basics: 3-Phase Diagrams (Contd)
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Homogeneous Azeotropes
At equilibrium:
f = f
j
V j j
_ V
_ L j
L
y P = x f
j j j
s
yP = xP yP = xP
1 1 1 2 s s s 1 1 s 2 2 1 1 1
2 2 s
P = x P + x P = x P + (1 x )P = P + ( P P )x
s s 2 1 2 1
At fixed temperature
9 DESIGN AND ANALYSIS - (c) Daniel R. Lewin 6 - Azeotropic Distillation
Homogeneous Azeotropes (Contd)
Example Phase diagrams for benzene-toluene mixture at 90 oC
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Homogeneous Azeotropes (Contd)
For non-ideal mixtures, the activity coefficients are different from unity:
i
yP = x P yP = x P
1 1 1 1 2 2 2
2 s
P = x P + (1 x ) P
s 1 1 1 1
2 2
If > 1 the mixture has a minimum-boiling azeotrope
Example Phase diagrams for Isopropyl ether-Isopropyl alcohol
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Homogeneous Azeotropes (Contd)
For non-ideal mixtures, the activity coefficients are different from unity:
i
yP = x P yP = x P
1 1 1 1 2 2 2
2 s
P = x P + (1 x ) P
s 1 1 1 1
2 2
If < 1 the mixture has a maximum-boiling azeotrope
Example Phase diagrams for Acetone-Chloroform
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Heterogeneous Azeotropes
For a minimum-boiling azeotrope with large deviation from Raoults law ( 1 ), phase splitting may occur and a minimum-boiling heterogeneous azeotrope forms, having a vapor phase in equilibrium with two liquid phases.
i
Homogeneous Azeotrope
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Heterogeneous Azeotrope
6 - Azeotropic Distillation
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
Residue Curves
Distillation still
Mass balance on species j:
Lx = ( L ) y + ( L L )( x + x ), j = 1, , C 1
j j j j
As L 0: Lx = y dL + Lx + Ldx x dL dLdx , j = 1, , C 1
j j j j j j
Rearranging:
dx
dL / L
= x y = x (1 K {T, P, x, y })
j j j j
dx
dt
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= x y
j
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Residue Curves (Contd)
dx
j
dt
= x y
j
Residue curves for zeotropic system
Residue curves for Azeotropic system
6 - Azeotropic Distillation
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
Residue Curves Liquid Compositions at Total Reflux
Species balance on top n-1 trays:
L x + Dx = Vy
n 1 n 1 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 n 1 n 1
Rectifying section of distillation column
At total reflux, D = 0 and Vn = Ln-1
dx x y dh
n n
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Sketching Residue Curves (Exercise)
dx
j
dt
= x y
j
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Product Composition Regions for Zeotropic Systems
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Product Composition Regions for Azeotropic Systems
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Heterogeneous Azeotropic Distillation Example: Dehydration of Ethanol
Try toluene as an entrainer
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
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Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
S2 F M1 D1 M2 B1 B2
M2 M1 S1 S2 D2 D1
D2
S1
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Pressure-swing Distillation
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
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Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Pressure-swing Distillation (Contd) Example: Dehydration of Tetrahydrofuran (THF)
T-x-y diagrams for THF and water
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DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
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 homogeneous and heterogeneous azeotropic distillation systems Be able to define the PFD for a pressure swing distillation system Next week: HEN Synthesis
24 DESIGN AND ANALYSIS - (c) Daniel R. Lewin 6 - Azeotropic Distillation
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Daniel R. Lewin, Technion
