Separating Azeotropic Mixtures
CHEN 4460 – Process Synthesis,
Simulation and Optimization
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lecture No. 6 – Review of Non-ideal Thermodynamics
September 25, 2012
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Process Design/Retrofit Steps
Assess Primitive
Problem
Detailed Process
Synthesis Algorithmic
Methods
Development
of Base-case
PART II
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
Plant-wide
Controllability
Assessment
Algorithmic Methods
Lecture 6 – 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.
•
This prevents the removal of certain species in high
concentrations.
Lecture 6 – Objectives

Be able to sketch the residue curves on a ternary phase
diagram

Be able to define the range of possible product
compositions using distillation, given the feed composition
and the ternary phase diagram
Basics: The Lever Rule
Basics: Ternary Phase Diagrams
0.2 TBA
0.65
DTBP
0.2 DTBP
0.15 H2O
Basics: Ternary Phase Diagrams
0.2 TBA
0.2 DTBP
0.6 H2O
Homogeneous Azeotropes 1:4
At equilibrium:
_ V
_ L
j
j
f  f
y P  x f
V
j
j
L
j
j j
yP  xP
yP  xP
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
Homogeneous Azeotropes 2:4
Example – Phase diagrams for benzenetoluene mixture at 90 oC
Homogeneous Azeotropes 3:4
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
Homogeneous Azeotropes 4:4
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
Heterogeneous Azeotropes
For a minimum-boiling azeotrope with large deviation from
Raoult’s 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
Heterogeneous Azeotrope
Residue Curves 1:3
Simple Distillation
Mass balance on species j:
Lx  ( L) y  (L  L)(x  x ), j  1,
j
j
j
,C  1
j
As L  0:
Lx  y dL  Lx  Ldx  x dL  dLdx , j  1,
j
j
Rearranging:
j
j
dx
j
dL / L
j
j
,C  1
 x  y  x (1  K {T, P, x, y })
j
dx
j

dt
j
j
 x y
j
j
j


Residue Curves 2:3
•
Residue Curves  Liquid Compositions at Total Reflux
dx
j

dt
 x y
j
j
Residue curves for
zeotropic system
Residue curves for
Azeotropic system
Residue Curves 3:3
•
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
Rectifying section of
distillation column
n
n 1
D
n 1
At total reflux, D = 0 and Vn = Ln-1
dx
 x y
dh
n
n
n
Sketching Residue Curves

Plot pure components on
vertices along with Tb

Plot all azeotropes on
diagram along with their Tb

Plot
residue
curves
connecting all azeotropes,
azeotropes & vertices, and
finally vertices & vertices
with arrow heads pointing
towards increasing boiling
point temperatures

Plot
additional
residue
curves that “arch” towards
intermediate temperatures
on the way to the end
point
dx
j

dt
 x y
j
j
Product Compositions Regions
•
For zeotropic systems
–
L: Lowest boiling component, I: Intermediate boiling
component, H: Highest boiling component, F: Feed composition
Pure L distillate
Pure H bottoms
Product Compositions Regions
•
For azeotropic systems
–
Shaded regions: Feasible distillate and bottoms product
compositions
Two binary azeotropes
Three binary azeotropes
and one ternary azeotrope
Summary – Non-ideal Thermo
On completion of this part, you should:

Be able to sketch the residue curves on a ternary phase
diagram

Be able to define the range of possible product
compositions using distillation, given the feed composition
and the ternary phase diagram
Other Business
•
Homework
–
–
•
SSLW: 8.14b-d, 8.15
Due Tuesday October 2
Next Lecture (October 2)
–
–
•
Part 1: Sequencing Azeotropic Distillation Columns (SSLW 230-251)
Part 2: Review for Midterm Exam
Midterm Exam
–
–
October 9 during lecture
Open book or closed book?