Heteroazeotropic Batch Distillation Feasibility and Operation Stathis Skouras

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Heteroazeotropic Batch DistillationFeasibility
and OperationStathis 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., Perrys Chemical Engineers
  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., Perrys Chemical Engineers
  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
Modified multivessel (without vapour bypass)

• Rectifier
• (two-vessel column)

Conventional multivessel (with vapour bypass)
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Time Requirements in Batch ColumnsZeotropic
mixture Methanol/Ethanol/1-Propanol
Specification Conventional multivessel (with vapour bypass) h Modified multivessel (no vapour bypass) Two-vessel column
Base case-Equimolar xF1/3,1/3,1/3 0.99,0.97,0.99 3.8 -26 32
Base case-Equimolar xF1/3,1/3,1/3 0.99,0.99,0.99 4.9 -31 16
Base case-Equimolar xF1/3,1/3,1/3 0.995,0.995,0.995 5.8 -33 16
Rich in light xF0.7,0.15,0.15 0.99,0.97,0.99 3.6 -19 8
Rich in light xF0.7,0.15,0.15 0.99,0.99,0.99 4.1 -22 2
Rich in light xF0.7,0.15,0.15 0.995,0.995,0.995 4.5 -22 2
Rich in intermediate xF0.15,0.7,0.15 0.99,0.97,0.99 4.0 -33 28
Rich in intermediate xF0.15,0.7,0.15 0.99,0.99,0.99 6.6 -36 -2
Rich in intermediate xF0.15,0.7,0.15 0.995,0.995,0.995 7.9 -34 -8
Rich in heavy xF0.15,0.15,0.7 0.99,0.97,0.99 2.4 0 71
Rich in heavy xF0.15,0.15,0.7 0.99,0.99,0.99 2.4 0 104
Rich in heavy xF0.15,0.15,0.7 0.995,0.995,0.995 2.8 0 104
The modified multivessel (without vapour bypass)
is the best WHY?
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Time Requirements in Various Batch
ColumnsZeotropic mixture Methanol/Ethanol/1-Prop
anol
Conventional multivessel

• () 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

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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
The mixture
The column
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Separation of Ternary Heteroazeotropic Mixtures
in the Multivessel Column
Operation
Build-up step
Decantation step
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Time Requirements in Various Batch
ColumnsTernary heteroazeotropic mixtures
Specification Conventional multivessel-decanter hybrid h Modified multivessel-decanter hybrid Rectifier-decanter hybrid
Class 1.0-2 xF1/3,1/3,1/3 0.99,0.97,0.99 3.4 -35 29
Class 1.0-2 xF1/3,1/3,1/3 0.99,0.98,0.99 4.9 -33 41
Class 1.0-1a xF0.6,0.2,0.2 0.97,0.97,0.99 2.8 -7 39
Class 1.0-1a xF0.6,0.2,0.2 0.98,0.99,0.99 3.7 -11 32
Class 2.0-2b xF0.45,0.05,0.5 0.97,0.97,0.99 3.3 0 61
Class 2.0-2b xF0.45,0.05,0.5 0.999,0.999,0.999 4.3 0 88
() Multivessel configurations perform better
than the rectifier column (-) Modified
multivessel less attractive for heteroazeotropic
mixtures (-) Practical difficulties with vapour
streams entering a decanter
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Heteroazeotropic Batch Distillation The story so
far

• Time requirements for zeotropic mixtures
• Multivessel configurations perform better
• Modified multivessel better than conventional
  multivessel
• Practical considerations regarding the modified
  multivessel
• Separation of heteroazeotropic mixtures in the
  multivessel column
• It is feasible
• Showed how to separate the mixtures (operation,
  control, etc)
• 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
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
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
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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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An Example
Water (A) / Dioxane (B) Benzene (E)

• 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

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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

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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

• Feasibility
• Same for rectifier and multivessel

• Operation
• Place (ABE) 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
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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 (LE?A or LE?B)

• 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