sequencing of azeotropic distillation columns

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

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Homogeneous Azeotropes (Cont’d)

Example – Phase diagrams for benzene-

toluene mixture at 90 oC

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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

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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

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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

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Residue curves for Azeotropic

system

Residue Curves (Cont’d)j

j j

dx x ydt

Residue curves for zeotropic

system

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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

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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

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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.

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Sketching Residue Curves (Exercise)j

j j

dx x ydt

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Product Composition Regions for Zeotropic Systems

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Product Composition Regions for Azeotropic Systems

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Example: Dehydration of EthanolHeterogeneous Azeotropic Distillation

Try toluene as an entrainer

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S1S2

M1

D2

M2D1

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Pressure-swing Distillation

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T-x-y diagrams for THF and water

Example: Dehydration of Tetrahydrofuran (THF)

Pressure-swing Distillation (Cont’d)

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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