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
Plant-wide
Controllability
Assessment
Development
of Base-case
SECTION B
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
2
1
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Section B: Algorithmic Methods
3
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:
n Alcohols
o Ketones
p Ethers
q Acids
r Water
 In these cases, distillation boundaries limit the
product compositions of a column to lie within a
bounded region
ÖPrevent the removal of certain species in high
concentrations
4
2
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
5
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Basics: The Lever Rule
6
3
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
7
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Basics: 3-Phase Diagrams (Cont’d)
8
4
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Homogeneous Azeotropes
At equilibrium:
_ V
_ L
j
j
f = f
yφ P = xγ f
V
j
j
j
L
j j
yP = xP
yP = xP
s
1
1 1
2
2 2
s
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
9
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Homogeneous Azeotropes (Cont’d)
Example – Phase diagrams
for benzene-toluene
mixture at 90 oC
10
5
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the
activity coefficients are
different from unity:
yP = xγ P
yP = xγ P
S
1
1
1 1
2
2
2
P = x γ P + (1 − x ) γ P
s
1
1 1
1
S
2
s
2 2
If γ > 1 the mixture has a minimum-boiling azeotrope
i
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol
11
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the
activity coefficients are
different from unity:
yP = xγ P
yP = xγ P
S
1
1
1 1
2
2
2
P = x γ P + (1 − x ) γ P
s
1
1 1
1
S
2
s
2 2
If γ < 1 the mixture has a maximum-boiling azeotrope
i
Example – Phase diagrams for Acetone-Chloroform
12
6
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
Raoult’s law ( γ
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
13
Heterogeneous Azeotrope
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
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
Rearranging:
j
dx
j
dL / L
j
= x − y = x (1 − K {T, P, x, y })
j
dx
j
∧
dt
14
7
j
j
j
j
−
−
= x −y
j
j
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Residue Curves (Cont’d)
dx
j
∧
dt
= x −y
j
j
Residue curves for
zeotropic system
15
Residue curves for
Azeotropic system
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Residue Curves ≡ 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
Rectifying section of
distillation column
At total reflux, D = 0 and Vn = Ln-1
dx
≈ x −y
dh
n
n
16
8
n −1
n
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
17
= x −y
j
j
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Product Composition Regions
for Zeotropic Systems
18
9
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
19
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Heterogeneous Azeotropic Distillation
Example: Dehydration of Ethanol
Try toluene
as an
entrainer
20
10
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
S2
F
M1
D2
D1
S1
M2
B1
B2
D1
M2
M1
S1
S2
21
D2
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Pressure-swing Distillation
22
11
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion
LECTURE SIX
054402 DESIGN AND ANALYSIS
Pressure-swing Distillation (Cont’d)
Example: Dehydration of Tetrahydrofuran (THF)
T-x-y diagrams for THF and water
23
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
12
DESIGN AND ANALYSIS - (c) Daniel R. Lewin
6 - Azeotropic Distillation
Daniel R. Lewin, Technion