5 - Azeotropic Distillation1 SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS Ref: Seider, Seader and...

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 L

j j j j jy P x f

s

1 1 1yP xP

s

2 2 2yP 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 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:

S

1 1 1 1yP x P

S

2 2 2 2yP x P

s s

1 1 1 1 2 2P x P 1 x P( )

If the mixture has a minimumminimum-boiling azeotrope

i1

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:

S

1 1 1 1yP x P

S

2 2 2 2yP x P

s s

1 1 1 1 2 2P x P 1 x P( )

If the mixture has a maximummaximum-boiling azeotrope

i1

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

j

j j j j

dxx y x 1 K T P x y

dL L( { , , , })

/

Rearranging:

j

j j

dxx y

dt

5 - Azeotropic Distillation10

Residue curves for Azeotropic

system

Residue Curves (Cont’d)

j

j j

dxx y

dt

Residue curves for zeotropic

system

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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

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

dxx y

dt

5 - Azeotropic Distillation15

Product Composition Regions for Zeotropic Systems

5 - Azeotropic Distillation16

Product Composition Regions for Azeotropic Systems

5 - Azeotropic Distillation17

Example: Dehydration of Ethanol

Heterogeneous Azeotropic Distillation

Try toluene as an entrainer

5 - Azeotropic Distillation18

S1

S2

M1

D2

M2

D1

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