Separation Trains
Azeotropes
S,S&L Chapt. 8
What is an Azeotrope?
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
Prevents the removal of certain species in
high concentrations
Binary
Distillation
IPA/IPE
x
IPA-IPE
x
Mininum-boiling or
Maximum-boiling
Azeotropes
x
Can multi-component Distillations
have Azeotropes?
• Yes!
Azeotrope Conditions
• Conditions on the Activity Coefficient
PT  x1 1L P1s  (1  x1 ) 2L P2s
 Lj  1, j  1,2...C , Positive Deviations from Raoult' s Law
 Lj  1, j  1,2...C , Negative Deviations from Raoult' s Law
• Minimum Boiling, γjL> 1
• Maximum Boiling, γjL< 1
• xj=yj, j=,1,2,…C
Raoult’s Law
Pi  xi Psat
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
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
Importance of Physical Property
Data Set
• In all cases
– Need sophisticated liquid phase model to
accurately predict the activity coefficient for the
liquid.
• For High Pressure Cases Only
– Also need sophisticated (non-ideal) gas phase
fugacity model
Two Types of Min. Boiling Azeotropes
• Homogeneous Azeotrope
• Heterogeneous Azeotrope
Overlay with
Liquid/Liquid Separation which is sometimes best separation method
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
Concepts Needed
• Phase Diagram for 3 phases
• Lever Rule on Phase Diagram
• Residue Curves
Basics: 3-Phase Diagrams
0.2 TBA
0.65
DTBP
0.2 DTBP
0.15 H2O
TBA = Tertiary-butyl alcohol
DTBP = Di-tertiary-butyl peroxide
Basics: 3-Phase Diagrams (Cont’d)
0.2 TBA
0.2 DTBP
0.6 H2O
Basics: The Lever Rule
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
j
,C  1
 x  y  x (1  K {T, P, x, y })
j
dx
j

dt
j
j
 x y
j
j
j


Multi-component Azeotropes
• Residue Curve Map
– dxj /dť = dxj /d ln(L) = xj – yj
• Integrate from various starting points
Arrows from low to High Temp
Path of the residue composition
Sketching Residue Curves (Exercise)
dx
j

dt
 x y
j
j
Distillation
• XB, XF and YD form a
line for a Distillation
Column
• Line can not cross
Feasible Region line
For Partial
Condenser
For Total
Condenser
Distillation Boundaries
• Equilibrium Trays in Total Reflux
Vn
dx
D
 xn 
yn 
y D  xn  y n , for D  0, L n -1  Vn
dh n 1
Ln 1
Ln 1
• Distillation Lines
xn  yn1 , for all n  0,1,...
– xn and yn lie on equilibrium tie lines
– Tangent to Residue Curve
To Create Residue Maps
• AspenPlus
– After putting in the components and selecting
the physical property method
– Choose
• Tools/Analysis/Property/Residue
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
Stripping section of
distillation column
n 1
At total reflux, D = 0 and Vn = Ln-1
dx
 x y
dh
n
n
n
Nodes
Residue Curves (Cont’d)
dx
j

dt
 x y
j
j
Residue curves for
zeotropic system
Residue curves for
Azeotropic system
Defining Conditions for Multicomponent Azeotrope
t goes from 0 to 1, ideal to non-ideal to find Azeotrope
Product Composition Regions
for Zeotropic Systems
Product Composition Regions
for Azeotropic Systems
Heterogeneous Azeotropic Distillation
Example: Dehydration of Ethanol
Try toluene
as an
entrainer
What are the zones of exclusion?
Ethanol/Water
Distillation with
Toluene to Break
Azeotrope
M2
S2
D1
S1
M1
Distillation Line
Tie Line
Ethanol/Water
Distillation with
Benzene
To Break
Azeotrope
How To Break Azeotropes with
Entrainer
• Separation Train Synthesis
– Identify Azeotropes
• Some distillations are not Azeotropic and can be
accomplished relatively easily
– Identify alternative separators
– Select Mass Separating Agent or Entrainer
– Identify feasible distillate and bottoms product
compositions
• Residue Curve Analysis
Pressure Swing to
Break Azeotrope
Temp. of
Azeotrope
vs. Pressure
Mole
Fraction
of
Azeotrope
Pressure-swing Distillation (Cont’d)
Example: Dehydration of Tetrahydrofuran (THF)
T-x-y diagrams for THF and water
Other Multi-component Distillation
Problems
• Multiple Steady States
– Run same distillation column with same set
points but different computational starting point
• Get Two or More Different Results
– Top or bottom compositions
– This is real in that the column will have two
different operating conditions!
– Happens most often with multi component
distillation