Heteroazeotropic Batch Distillation
Feasibility and Operation
Stathis Skouras
7. May 2004
Department of Chemical Engineering, NTNU
1
NTNU
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
2
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)
3
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)
4
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)
5
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)
6
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)
7
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,)
8
Batch Distillation Arrangements
Rectifier
(two-vessel column)
9
Conventional multivessel
(with vapour bypass)
Modified multivessel
(without vapour bypass)
Time Requirements in Batch Columns
Zeotropic mixture: Methanol/Ethanol/1-Propanol
Base case-Equimolar
xF=[1/3,1/3,1/3]
Rich in light
xF=[0.7,0.15,0.15]
Rich in intermediate
xF=[0.15,0.7,0.15]
Rich in heavy
xF=[0.15,0.15,0.7]
10
Specification
Conventional multivessel
(with vapour bypass)
[h]
Modified multivessel
(no vapour bypass)
[%]
Two-vessel column
[%]
[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
[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
[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
[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
The modified multivessel (without vapour bypass) is the best
WHY?
Time Requirements in Various Batch Columns
Zeotropic mixture: Methanol/Ethanol/1-Propanol
1
1
ethanol
composition in the middle vessel
composition of main component
bottom vessel
middle vessel
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0
0.6
______ conventional multivessel
............. modified multivessel
0.4
0.2
1-propanol
0.5
1
1.5
2
2.5
Time (h)
3
3.5
4
0
0
0.5
1
1.5
methanol
2
2.5
Time (h)
3
3.5
4
(+) The vapour stream entering
the middle vessel improves
the composition dynamics of
the light component
top vessel
0.8
0.6
0.4
0.2
middle vessel
bottom vessel
11
0.8
1
composition of light component
Conventional multivessel
top vessel
0
0
0.5
1
1.5
2
2.5
Time (h)
3
3.5
4
(-) Practical difficulties with a
vapour stream entering the
middle vessel
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
12
Separation of Ternary Heteroazeotropic
Mixtures in the Multivessel Column
The mixture
The column
EtAc [s]
77.1 oC
Serafimov,s class 1.0-1a
-.-.-.- binodal curve at 30 oC
het.az [un]
71.6 oC
- - - - distillation lines
Acetic Acid [sn]
118.2 oC
13
Water [s]
100.0 oC
Separation of Ternary Heteroazeotropic
Mixtures in the Multivessel Column
Operation
Build-up step
Decantation step
EtAc [s]
77.1 oC
xM
-.-.-.- binodal curve at 30 oC
F
........ composition evolution
-.-.-.- binodal curve at 30 oC
middle
vessel
-o-o- column liquid profile
EtAc [s]
77.1 oC
xM
top
vessel
het.az [un]
xT 71.6 oC
-o-o- column liquid profile
F
xT0
het.az [un]
71.6 oC
+++ composition evolution
top
vessel
xB
Acetic Acid [sn]
118.2 oC
14
xB
Water [s] Acetic Acid [sn]
100.0 oC
118.2 oC
xT
Water [s]
100.0 oC
Time Requirements in Various Batch Columns
Ternary heteroazeotropic mixtures
Class 1.0-2
xF=[1/3,1/3,1/3]
Class 1.0-1a
xF=[0.6,0.2,0.2]
Class 2.0-2b
xF=[0.45,0.05,0.5]
Specification
Conventional
multivessel-decanter
hybrid
[h]
Modified
multivessel-decanter
hybrid
[%]
Rectifierdecanter
hybrid
[%]
[0.99,0.97,0.99]
3.4
-35
+29
[0.99,0.98,0.99]
4.9
-33
+41
[0.97,0.97,0.99]
2.8
-7
+39
[0.98,0.99,0.99]
3.7
-11
+32
[0.97,0.97,0.99]
3.3
0
+61
[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
15
(-) Practical difficulties with vapour streams entering a decanter
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
–
–
3.
Time requirements for heteroazeotropic mixtures
–
–
–
16
It is feasible
Showed how to separate the mixtures (operation, control, etc)
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
Heteroazeotropic Batch Distillation
A 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
–
–
–
–
17
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)
Process Description
Example
Close-boiling (AB) + Entrainer (E)
What has to be done
E [s]
77.1 oC
Step 1: Product recovery (LA)
LE, x
LE
binodal
curve
Strategy A
2nd step
xS,1
xS,2
B [sn]
118.2 oC
Pure B in the still at steady state
1st step
How to do
xF
SB2 Strategy B
Step 2: Entrainer recovery (E or LE)
het.az [un]
71.6 oC
Strategy A: Do the steps sequentially
F
xS,0
(+): Recovery of pure E
SB1
F'
(-): Time consuming
LA, xLA
A [s]
100.0 oC
Strategy B: Do the steps simultaneously
(+): Less time consuming
(-): Cannot recover pure E
18
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
19
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
Benzene [s]
80.1 oC
Water (A) / Dioxane (B)
I
+
binodal
curve
het.az [un]
69.0 oC
Benzene (E)
II
batch distillation
boundary
1,4-Dioxane [sn]
101.3 oC
III
distillation
boundary
hom.az [s]
86.6 oC
Water [sn]
100.0 oC
• Water (A) / Dioxane (B) is azeotropic
• Benzene (E) forms binary heteroazeotrope with water
• Two distillation boundaries and limit the products under distillation
21
• Three distillation regions complicate the synthesis of the process
Simulations for the Rectifier Column
Benzene [s]
80.1 oC
xLE
-.-.- binodal curve (25 oC)
- - - distillation boundaries
-o-o- column liquid profile
I
xD,0 het.az [un]
69.0 oC
II
total reflux
(t=1h)
t=2h
F,xF III
steady state
still path
xD,1
xD,f
xS,0
xS,f
1,4-Dioxane [sn]
101.3 oC
xS,1
hom.az [s]
86.6 oC
xLA
Water [sn]
100.0 oC
• Column profile restored during the process
• Still path crosses distillation boundaries
• These results cannot be obtained by homoazeotropic distillation
22
• Pure and anhydrous ethanol recovered in the still at steady state and
water recovered with the aqueous phase in the decanter
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
23
Feasibility Conditions
Feasibility
• Same for rectifier and multivessel
Operation
• Place (A+B+E) in the still
• Start the process
• Collect some of the
heteroazeotrope in the decanter
Feasibility condition 1:
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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 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:
25
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
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
26
composition LE with the still product composition B in the direction of
increasing temperature from LE to B
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:
27
•
Two entrainer selection rules
•
Two guidelines for avoiding infeasible entrainers
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)
28
2) The entrainer (E) should preferably not form a ternary saddle
homoazeotrope
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
29
Thank you for your attention…
30