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5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin1
054402 Design and Analysis II
LECTURE 5: SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS
Daniel R. Lewin
Department of Chemical Engineering
Technion, Haifa, Israel
Ref: Seider, Seader and Lewin (1999), Section 5.6
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin2
Assess Primitive Problem
Steps in Process Design and Retrofit
Development of Base-case
Plant-wide Controllability Assessment
Detailed Design, Equipment sizing, Cap.
Cost Estimation, Profitability Analysis,
Optimization
Detailed Process Synthesis -Algorithmic
Methods
SECTION B
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin3
Section B: Algorithmic Methods
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin4
Introduction
Separation sequences are complicated by the presence of azeotropes, often involving mixtures of oxygenated organic compounds: Alcohols Ketones Ethers AcidsWater
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
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin5
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:
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin6
Basics: The Lever Rule
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin7
0.15 H2O
0.65 DTBP
0.2 TBA
0.2 DTBP
Basics: 3-Phase Diagrams
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin8
0.6 H2O
0.2 DTBP
0.2 TBA
Basics: 3-Phase Diagrams (Cont’d)
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin9
Homogeneous Azeotropes
At equilibrium:
V L
j jf f_ _
V L
j j j j jy P x f
s
1 1 1yP xP
s
2 2 2y P 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
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin10
Homogeneous Azeotropes (Cont’d)
Example – Phase diagrams for benzene-toluene
mixture at 90 oC
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin11
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 2y P x P
s s
1 1 1 1 2 2P x P 1 x P( )
If the mixture has a minimum-boiling azeotropei
1
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin12
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 2y P x P
s s
1 1 1 1 2 2P x P 1 x P( )
If the mixture has a maximum-boiling azeotropei
1
Example – Phase diagrams for Acetone-Chloroform
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin13
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.
i1
Homogeneous Azeotrope Heterogeneous Azeotrope
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin14
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
d t
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin15
Residue curves for Azeotropic system
Residue Curves (Cont’d)
j
j j
dxx y
d t
Residue curves for zeotropic system
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin16
Residue Curves 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
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin17
Sketching Residue Curves (Exercise)
j
j j
dxx y
d t
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin18
Product Composition Regions for Zeotropic Systems
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin19
Product Composition Regions for Azeotropic Systems
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin20
Example: Dehydration of Ethanol
Heterogeneous Azeotropic Distillation
Try toluene as an entrainer
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin21
S1
S2
M1
D2
M2
D1
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin22
Pressure-swing Distillation
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin23
T-x-y diagrams for THF and water
Example: Dehydration of Tetrahydrofuran (THF)
Pressure-swing Distillation (Cont’d)
5-Azeotropic DistillationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin24
Azeotropic Distillation - Summary
On completing this unit, you should:
Next week: 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