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

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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

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0.15 H2O

0.65 DTBP

0.2 TBA

0.2 DTBP

Basics: 3-Phase Diagrams

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0.6 H2O

0.2 DTBP

0.2 TBA

Basics: 3-Phase Diagrams (Cont’d)

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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

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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:

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

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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.

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

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Residue curves for Azeotropic system

Residue Curves (Cont’d)

j

j j

dxx y

d t

Residue curves for zeotropic system

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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

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Sketching Residue Curves (Exercise)

j

j j

dxx y

d t

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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 Ethanol

Heterogeneous Azeotropic Distillation

Try toluene as an entrainer

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S1

S2

M1

D2

M2

D1

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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: 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