1

Heteroazeotropic Batch DistillationFeasibility and Operation

Stathis Skouras

7. May 2004

Department of Chemical Engineering, NTNU

NTNU

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Introduction & Overview

Introduction:

• Distillation, azeotrope, heterogeneous azeotrope (heteroazeotropic), heteroazeotropic distillation - what are they actually?

• Motivation - industrial relevance• Batch distillation - Background

Overview of talk:

• Time requirements for zeotropic mixtures• Separation of heteroazeotropic mixtures in the multivessel column • Time requirements for heteroazeotropic mixtures• Heteroazeotropic batch distillation: A systematic approach

– Process description and column operation– Feasibility and entrainer selection

• Main contributions

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Introduction

Distillation:

A technique for separating mixtures into their constituent components by exploiting differences in vapour- and liquid phase compositions arising from partial vaporisation of the liquid phase and partial condensation of the vapour phase

Perry et al., Perry’s Chemical Engineer’s Handbook, (1997)

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Introduction

Distillation, azeotrope:

An azeotrope occurs for a boiling mixture of two or more species when the vapour and liquid phases in equilibrium have the same composition. As a consequence, we cannot separate such a mixture by boiling or condensing it and enhanced distillation techniques have to be applied

Biegler et al., Systematic Methods of Chemical Process Design, (1997)

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Introduction

Distillation, azeotrope, heterogeneous azeotrope (heteroazeotrope):

Heterogeneous behaviour means that the liquid phase partitions into two or more liquid phases at equilibrium. Two-liquid phase formation provides a means of breaking this azeotrope.

Biegler et al., Systematic Methods of Chemical Process Design, (1997)

Perry et al., Perry’s Chemical Engineer’s Handbook, (1997)

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Introduction

Distillation, azeotrope, heterogeneous azeotrope (heteroazeotrope), heteroazeotropic distillation:

An enhanced distillation technique which uses minimum-boiling azeotropes and liquid-liquid immiscibilities in combination to defeat the presence of other azeotropes or tangent pinches that would otherwise prevent the desired separation

Doherty and Malone, Conceptual Design of Distillation Systems, (2001)

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Introduction

• Distillation, azeotrope, heterogeneous azeotrope (heteroazeotropic), heteroazeotropic distillation - what are they actually?

• Motivation – industrial relevance

• Heteroazeotropic distillation is a very common enhanced distillation technique:– Ethanol/water separation by using benzene, cyclohexane, toluene, etc

– First successful application (patent) in 1902 in Germany by Young

• Heteroazeotropic distillation is a very powerful and flexible process:– Exploits several physical phenomena (enhanced vapour-liquid behaviour

and liquid-liquid immiscibilities)

– More possibilities for the separation of azeotropic mixtures than homoazeotropic distillation

– Simplified distillation sequences (decantation + distillation)

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Introduction

• Distillation, azeotrope, heterogeneous azeotrope (heteroazeotropic), heteroazeotropic distillation - what are they actually?

• Motivation – industrial relevance

• Batch distillation - Background

• Well suited for small-scale production (pharmaceutical, fine/specialty chemical industry)

• Separation of multicomponent mixtures in one single column. Various mixtures of different feeds can be processed

• More labour and energy intensive

• Heteroazeotropic distillation in batch columns not well understood. The presence of azeotropes complicates the design and synthesis of the process (what is feasible, how to operate the columns,)

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Batch Distillation Arrangements

Rectifier(two-vessel column)

Conventional multivessel(with vapour bypass)

Modified multivessel(without vapour bypass)

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Time Requirements in Batch ColumnsZeotropic mixture: Methanol/Ethanol/1-Propanol

The modified multivessel (without vapour bypass) is the best WHY?

SpecificationConventional multivessel

(with vapour bypass)[h]

Modified multivessel (no vapour bypass)

[%]

Two-vessel column[%]

Base case-EquimolarxF=[1/3,1/3,1/3]

[0.99,0.97,0.99] 3.8 -26 +32

[0.99,0.99,0.99] 4.9 -31 +16

[0.995,0.995,0.995] 5.8 -33 +16

Rich in lightxF=[0.7,0.15,0.15]

[0.99,0.97,0.99] 3.6 -19 +8

[0.99,0.99,0.99] 4.1 -22 +2

[0.995,0.995,0.995] 4.5 -22 +2

Rich in intermediatexF=[0.15,0.7,0.15]

[0.99,0.97,0.99] 4.0 -33 +28

[0.99,0.99,0.99] 6.6 -36 -2

[0.995,0.995,0.995] 7.9 -34 -8

Rich in heavyxF=[0.15,0.15,0.7]

[0.99,0.97,0.99] 2.4 0 +71

[0.99,0.99,0.99] 2.4 0 +104

[0.995,0.995,0.995] 2.8 0 +104

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Time Requirements in Various Batch ColumnsZeotropic mixture: Methanol/Ethanol/1-Propanol

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

Time (h)

com

posi

tion

of

ligh

t co

mpo

nent

top vessel

middle vessel

bottom vessel

0 0.5 1 1.5 2 2.5 3 3.5 40.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (h)

com

posi

tion

of

mai

n co

mpo

nent

top vessel bottom vesselmiddle vessel

Con

vent

iona

l mul

tive

ssel

(+) The vapour stream entering the middle vessel improves the composition dynamics of the light component

(-) Practical difficulties with a vapour stream entering the middle vessel

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

Time (h)

com

pos

itio

n i

n t

he

mid

dle

ves

sel ethanol

methanol1-propanol

______ conventional multivessel

............. modified multivessel

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Separation of Ternary Heteroazeotropic Mixtures in the Multivessel Column

• Is it feasible?– No study in the literature for a multivessel column

• How should we perform the separation?– Operation

– Control

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Separation of Ternary Heteroazeotropic Mixtures in the Multivessel Column

-.-.-.- binodal curve at 30oC

EtAc [s]77.1 oC

Water [s]100.0 oC

Acetic Acid [sn]118.2 oC

het.az [un]71.6 oC

Serafimov,s class 1.0-1a

- - - - distillation lines

The mixture The column

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Separation of Ternary Heteroazeotropic Mixtures in the Multivessel Column

EtAc [s]77.1 oC

Water [s]

100.0 oC

Acetic Acid [sn]

118.2 oC

het.az [un]71.6 oCF

xM

xB

xT

middlevessel

topvessel

-o-o- column liquid profile

........ composition evolution

-.-.-.- binodal curve at 30oC

Operation

Build-up step Decantation step

EtAc [s]77.1 oC

Water [s]

100.0 oC

Acetic Acid [sn]

118.2 oC

het.az [un]71.6 oC

F

xM

xB

xT0

xT

-o-o- column liquid profile

+++ composition evolution

topvessel

-.-.-.- binodal curve at 30oC

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(+) Multivessel configurations perform better than the rectifier column

(-) Modified multivessel less attractive for heteroazeotropic mixtures

(-) Practical difficulties with vapour streams entering a decanter

Time Requirements in Various Batch ColumnsTernary heteroazeotropic mixtures

Specification

Conventional multivessel-decanter

hybrid[h]

Modified multivessel-decanter

hybrid[%]

Rectifier-decanter hybrid

[%]

Class 1.0-2xF=[1/3,1/3,1/3]

[0.99,0.97,0.99] 3.4 -35 +29

[0.99,0.98,0.99] 4.9 -33 +41

Class 1.0-1axF=[0.6,0.2,0.2]

[0.97,0.97,0.99] 2.8 -7 +39

[0.98,0.99,0.99] 3.7 -11 +32

Class 2.0-2bxF=[0.45,0.05,0.5]

[0.97,0.97,0.99] 3.3 0 +61

[0.999,0.999,0.999] 4.3 0 +88

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Heteroazeotropic Batch Distillation The story so far

1. Time requirements for zeotropic mixtures– Multivessel configurations perform better

– Modified multivessel better than conventional multivessel

– Practical considerations regarding the modified multivessel

2. Separation of heteroazeotropic mixtures in the multivessel column– It is feasible

– Showed how to separate the mixtures (operation, control, etc)

3. Time requirements for heteroazeotropic mixtures– Multivessel configurations better than the rectifier column

– Practical considerations regarding the modified multivessel

– Use the conventional multivessel for such mixtures

UNTIL NOW THE MIXTURES WERE TERNARY AND ALREADY CONTAINED A HETEROAZEOTROPE

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Heteroazeotropic Batch DistillationA systematic approach

• Formulation of the problem

– The original mixture is binary (AB) azeotropic or close-boiling – The separation by simple distillation is impossible (AB is

azeotropic) or uneconomical (AB is close-boiling) – An entrainer (E) is added that forms heteroazeotrope with at

least one (preferably) of the original components

• The tasks

– What has to be done? (process description)– How to operate the columns in a simple way? (operation)– Which separations are feasible? (feasibility) – How to choose entrainers for the process? (entrainer selection)

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Process Description

E [s]

A [s]B [sn]

F

LE

,

LA

,

1st step2nd step

77.1 oC

100.0 oC118.2 oC

71.6 oChet.az [un]

binodal curve

Strategy B

Strategy A

xS,1

xS,2

xS,0

xLE

SB2

SB1

xF

xLA

F'

How to do

Strategy A: Do the steps sequentially

(+): Recovery of pure E

(-): Time consuming

Strategy B: Do the steps simultaneously

(+): Less time consuming

(-): Cannot recover pure E

Example

Close-boiling (AB) + Entrainer (E)

What has to be done

Step 1: Product recovery (LA)

Step 2: Entrainer recovery (E or LE)

Pure B in the still at steady state

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Operation

Rectifier column

Use a T-controller to indirectly adjust the holdup of the entrainer-lean phase (LE)

• No need to predetermine holdups of the immiscible phases in the decanter

• Simple realisation of the desired steady state results

• Both strategies A and B can be realised by adjusting the temperature setpoint

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Operation

Multivessel column

Use a L-controller to reflux all of the entrainer-lean phase (LE)

Use a T-controller to indirectly adjust the holdup in the middle vessel

• No need to predetermine holdups in the vessels

• Simple realisation of the desired steady state results

• Strategy A is implemented. Both process steps are performed simultaneously in the same column

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Benzene [s]80.1 oC

Water [sn]

100.0 oC101.3 oC

het.az [un]

69.0 oC

hom.az [s]

86.6 oC

1,4-Dioxane [sn]

II

I

batch distillation boundary

distillation boundary

binodal curve

III

An Example

• Water (A) / Dioxane (B) is azeotropic

• Benzene (E) forms binary heteroazeotrope with water

• Two distillation boundaries and limit the products under distillation

• Three distillation regions complicate the synthesis of the process

Water (A) / Dioxane (B)

+

Benzene (E)

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Simulations for the Rectifier Column

• Column profile restored during the process

• Still path crosses distillation boundaries

• These results cannot be obtained by homoazeotropic distillation

• Pure and anhydrous ethanol recovered in the still at steady state and water recovered with the aqueous phase in the decanter

Benzene [s]80.1 oC

Water [sn]100.0 oC

1,4-Dioxane [sn]

101.3 oC

xLE

xLA

het.az [un]

69.0 oC

hom.az [s]

86.6 oC

xS,0

xD,0

- - - distillation boundaries

-o-o- column liquid profile

F,xF

-.-.- binodal curve (25 oC)

I

xS,1x

S,f

total reflux

t=2h

steady state still path

III

II

xD,f

xD,1

(t=1h)

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Feasibility and Entrainer Selection

• Which separations are feasible with the proposed processes?

– Develop a method to check feasibility without doing simulations

– Use only the distillation lines map of the mixture and the binodal curve (VLLE)

• How to choose entrainers for the processes?

– propose simple rules for “screening” feasible entrainers

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Feasibility Conditions

Operation

• Place (A+B+E) in the still

• Start the process

• Collect some of the heteroazeotrope in the decanter

Feasibility condition 1:

It should exist a feed region where the heteroazeotrope is the unstable node so as it will boil overhead and start accumulated in the decanter

Feasibility

• Same for rectifier and multivessel

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Feasibility Conditions

Operation

• The heteroazeotrope splits in two phases

• Reflux the entrainer-rich phase (LE)

• Accumulate (remove) the entrainer-lean phase (LA)

• Pure B in the still

Feasibility condition 2:

It should, at steady state, exist a distillation line connecting the reflux composition LE with the still product composition B in the direction of increasing temperature from LE to B

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Checking Feasibility: An example

Example

Azeotropic (AB) +

Light entrainer (E)

Steady State Products

• LA and LE in the decanter

• B in the still

Feasibility conditions

1) It exists a feed region where the heteroazeotrope is the unstable node

2) It exists, at steady state, a distillation line connecting the reflux composition LE with the still product composition B in the direction of increasing temperature from LE to B

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Checking Feasibility

Three general cases for the original mixture (AB):

a) Close-boiling (low relative volatility) mixture (10 cases, 5 feasible)

b) Minimum-boiling (min) homoazeotropic mixtures (9 cases, 4 feasible)

c) Maximum-boiling (max) homoazeotropic mixtures (7 cases, 2 feasible)

The results for all cases helped us to formulate:

• Two entrainer selection rules

• Two guidelines for avoiding infeasible entrainers

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Entrainer Selection

Simple rules for entrainer selection:

1) The entrainer (E) should form a heteroazeotrope (AzEA or AzEB) with one of the original components (A or B) and/or a ternary heteroazeotrope (AzEAB)

2) The vertex of the original component to be obtained in the still at steady state (A or B) should be connected with the steady state reflux point of the entrainer-rich phase (LE) with a distillation line (residue curve) in the direction of increasing temperature from the top of the column to the bottom (LEA or LEB)

Guidelines for avoiding infeasible entrainers:

1) The entrainer (E) must not form a max. azeotrope with any of the original components (A or B)

2) The entrainer (E) should preferably not form a ternary saddle homoazeotrope

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Main Contributions

• Comparison of different batch column configurations, in terms of

time requirements, for zeotropic and heteroazeotropic mixtures – The vapour stream configuration in the middle vessel plays significant role

– Practical considerations for eliminating the vapour bypass

• Addressing separation of ternary heteroazeotropic mixtures in the

multivessel column– Showing how to perform the separation (control, operation)

• Systematic and comprehensive study of the heteroazeotropic batch

distillation process– Detailed analysis of the process

– Proposing control schemes for simple column operation

– Addressing feasibility issues

– Proposing rules for entrainer selection

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Thank you for your attention…