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SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS. Ref: Seider, Seader and Lewin (2004), Section 7.5. Introduction. Separation sequences are complicated by the presence of azeotropes, often involving mixtures of oxygenated organic compounds: Alcohols Ketones Ethers Acids Water - PowerPoint PPT Presentation
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5 - Azeotropic Distillation1
SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS
Ref: Seider, Seader and Lewin (2004), Section 7.5
5 - Azeotropic Distillation2
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 boundariesdistillation boundaries limit the product compositions of a column to lie within a bounded regionPrevent the removal of certain species in
high concentrations
5 - Azeotropic Distillation3
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 Distillation4
Homogeneous Azeotropes At equilibrium:
V L
j jff_ _
V Lj j j j jy P x f
s1 1 1yP xP
s2 2 2yP x P
s s s s1 1 2 2 1 1 1 2P xP x P xP 1 x P( )
s s s2 1 2 1P P P x( )
At fixed temperature
5 - Azeotropic Distillation5
Homogeneous Azeotropes (Cont’d)
Example – Phase diagrams for benzene-
toluene mixture at 90 oC
5 - Azeotropic Distillation6
Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity:
S1 1 1 1yP x P
S2 2 2 2yP x P
s s1 1 1 1 2 2P x P 1 x P( )
If the mixture has a minimumminimum-boiling azeotrope
i 1
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol
5 - Azeotropic Distillation7
Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity:
S1 1 1 1yP x P
S2 2 2 2yP x P
s s1 1 1 1 2 2P x P 1 x P( )
If the mixture has a maximummaximum-boiling azeotrope
i 1
Example – Phase diagrams for Acetone-Chloroform
5 - Azeotropic Distillation8
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.
Homogeneous Azeotrope Heterogeneous Azeotrope
7usually ,1 i
5 - Azeotropic Distillation9
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
jj j j j
dx x y x 1 K T P x ydL L ( { , , , })/
Rearranging:
jj j
dx x ydt
5 - Azeotropic Distillation10
Residue curves for Azeotropic
system
Residue Curves (Cont’d)j
j j
dx x ydt
Residue curves for zeotropic
system
5 - Azeotropic Distillation11
Liquid Compositions at Total Reflux Species balance on top n-1 trays:
nn 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
nn n
dx x ydh Stripping section of distillation column
The residue curves approximates the distillation curves at total The residue curves approximates the distillation curves at total refluxreflux
5 - Azeotropic Distillation12
Distillation Curves An exact representation of the distillation curve (operating line) at total reflux can be calculated based on: 1- Between two adjacent stages: 2- On each stage:
,1,01
nyxnn
lines tiemequilibriu theof ends at the lie and nn yx
5 - Azeotropic Distillation13
Comparison of Residue and Distillation Curves
When the residue curve is linear (as for binary mixtures), the distillation and residue curves are coincide. Their differences are pronounced in the regions with extensive curvature.
5 - Azeotropic Distillation14
Sketching Residue Curves (Exercise)j
j j
dx x ydt
5 - Azeotropic Distillation15
Product Composition Regions for Zeotropic Systems
5 - Azeotropic Distillation16
Product Composition Regions for Azeotropic Systems
5 - Azeotropic Distillation17
Example: Dehydration of EthanolHeterogeneous Azeotropic Distillation
Try toluene as an entrainer
5 - Azeotropic Distillation18
S1S2
M1
D2
M2D1
5 - Azeotropic Distillation19
Pressure-swing Distillation
5 - Azeotropic Distillation20
T-x-y diagrams for THF and water
Example: Dehydration of Tetrahydrofuran (THF)
Pressure-swing Distillation (Cont’d)
5 - Azeotropic Distillation21
Azeotropic Distillation - Summary On completing this unit, you should:
Next week:Next week: HEN Synthesis 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