azeotropic destillation
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054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion1
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin1
054402 Design and Analysis
LECTURE 6: SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS
Daniel R. LewinDepartment of Chemical Engineering
Technion, Haifa, Israel Ref: Seider, Seader and Lewin (2004), Section 7.5
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (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
054402 DESIGN AND ANALYSIS LECTURE SIX
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6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin3
Section B: Algorithmic Methods
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin4
IntroductionSeparation sequences are complicated by the presence of azeotropes, often involving mixtures of oxygenated organic compounds:
AlcoholsKetonesEthersAcidsWater
In these cases, distillation boundariesdistillation boundaries limit the product compositions of a column to lie within a bounded region
Prevent the removal of certain species in high concentrations
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion3
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin5
Instructional Objectives
Be able to sketch the residue curves on a tertiary phase diagramBe able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagramBe able to define the PFD for homogeneous and heterogeneous azeotropic distillation systemsBe able to define the PFD for a pressure swing distillation system
When you have finished studying this unit, you should:
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin6
Basics: The Lever Rule
054402 DESIGN AND ANALYSIS LECTURE SIX
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6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin7
0.15 H2O
0.2 DTBP
Basics: 3-Phase Diagrams
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin8
Basics: 3-Phase Diagrams (Cont’d)
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion5
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin9
Homogeneous Azeotropes
At equilibrium:=
V L
j jf f_ _
φ = γV L
j j j j jy P x f
= s
1 1 1y P xP
= s
2 2 2y P x P
= + = + −s s s s
1 1 2 2 1 1 1 2P x P x P x P 1 x P( )
= + −s s s
2 1 2 1P P P x( )
At fixed temperature
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin10
Homogeneous Azeotropes (Cont’d)
Example – Phase diagrams for benzene-toluene
mixture at 90 oC
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion6
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin11
Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity:
S
1 1 1 1y P x P= γ
S
2 2 2 2y P x P= γ
s s
1 1 1 1 2 2P x P 1 x P( )= γ + − γ
If the mixture has a minimumminimum-boiling azeotropei1γ >
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin12
Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity:
S
1 1 1 1y P x P= γ
S
2 2 2 2y P x P= γ
s s
1 1 1 1 2 2P x P 1 x P( )= γ + − γ
If the mixture has a maximummaximum-boiling azeotropei1γ <
Example – Phase diagrams for Acetone-Chloroform
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion7
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (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.
i1γ
Homogeneous Azeotrope Heterogeneous Azeotrope
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin14
j j j j j jLx y dL Lx Ldx x dL 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∧ = −
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion8
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (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
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin16
Residue Curves ≡≡≡≡ Liquid Compositions at Total Reflux
Species balance on top n-1 trays:
n
n n 1
dx x xdh −
≈ −
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 xdh L L
− −
≈ − +
At total reflux, D = 0 and Vn = Ln-1
n
n n
dx x ydh
≈ −
Rectifying section of distillation column
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion9
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin17
Sketching Residue Curves (Exercise)
j
j j
dxx y
d t∧ = −
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Product Composition Regions for Zeotropic Systems
054402 DESIGN AND ANALYSIS LECTURE SIX
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Product Composition Regions for Azeotropic Systems
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Example: Dehydration of Ethanol
Heterogeneous Azeotropic Distillation
Try toluene as an entrainer
054402 DESIGN AND ANALYSIS LECTURE SIX
Daniel R. Lewin, Technion11
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin21
S1
S2
M1
D2
M2D1B2B1
D2D1F M1 S1
S2
M2
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Pressure-swing Distillation
054402 DESIGN AND ANALYSIS LECTURE SIX
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T-x-y diagrams for THF and water
Example: Dehydration of Tetrahydrofuran (THF)
Pressure-swing Distillation (Cont’d)
6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin24
Azeotropic Distillation - Summary On completing this unit, you should:
Next week:Next week: HEN SynthesisHEN Synthesis
Be able to sketch the residue curves on a tertiary phase diagramBe able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagramBe able to define the PFD for homogeneous and heterogeneous azeotropic distillation systemsBe able to define the PFD for a pressure swing distillation system