1. Stathis Skouras and Sigurd Skogestad
AIChE Annual Meeting 2002
November 3-8, 2002,
Indianapolis, Indiana, USA
Separation of Ternary Heteroazeotropic Mixtures in a Multivessel Batch Distillation-Decanter Hybrid
Stathis Skouras and Sigurd Skogestad
N T N U

2. Multivessel and cyclic batch distillation columns Simulation results
Presentation Outline
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Project objective
The model
Multivessel and cyclic batch distillation columns
Simulation results
Zeotropic mixtures: Effect of vapor bypass
Azeotropic mixtures: Feasibility
Serafimov’s topological class 1.0-2
Serafimov’s topological class 1.0-1a
Conclusions

3. Project Objective Original: Azeotropic Mixtures
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Original: Azeotropic Mixtures
Is it feasible to separate heteroazeotropic mixtures in the multivessel column?
What kind of heteroazeotropes can be separated in the novel column?
Batch time (energy) requirements for the multivessel column
Additional topic: Effect of vapor bypass
Zeotropic mixture

4. The Model Staged column model No vapor holdup - constant liquid holdup
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Staged column model
No vapor holdup - constant liquid holdup
Constant vapor flows
Perfect mixing and equilibrium on all stages
Ideal gas
VLE activity coefficients from UNIQUAC
LLE from experimental data
Atmospheric pressure P = bar
Simulations performed in MATLAB

5. The Cyclic Batch Column (Treybal, 1970) Used as basis for comparison of batch times
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Characteristics:
Conventional batch column, but with top vessel where top product is accumulated (instead of continuous removal)
Closed operation in each cycle
N-component mixture may be separated in a sequence of N-1 cycles
Indirect level control in top vessel with temperature control (TC) (Skogestad, 1997)
Why cyclic operation?
Simple to operate and control.
Most cases: Batch time (energy) savings compared to conventional policy with continuous removal of top product (Sørensen, 1994)

6. Generalization: The Multivessel Batch Column (Hasebe, 1995)
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Characteristics:
Batch column with N vessels
Closed (total reflux) operation
N-component mixture: single closed operation with pure products accumulated in the N vessels
Here: Ternary mixture (N=3). Need 2 sections and 3 vessels
Indirect level control in the vessels using temperature control (Skogestad, 1997)
Why multivessel column?
Very simple to operate. No off-cut fractions. Column ‘runs itself’
Energy (time) savings due to the multieffect nature of the operation.

7. Multivessel column: Previous work on zeotropic mixtures
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Time savings of 50% compared to conventional (open) batch distillation (Hasebe, 1992, 1995)
Wittgens (1997): Proved both experimentally and theoretically the feasibility for a quaternary mixture (N=4)
Hilmen (1999): Time savings up to 50% compared to a cyclic column for ternary mixtures
Used vapor bypass configuration
This work
Ternary mixture: Methanol/Ethanol/1-Propanol
Effect of vapor bypass

8. Methanol/Ethanol/1-Propanol
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Cyclic column:
Multivessel column
with vapor bypass:
35% time savings compared to cyclic column

9. Multivessel with vapor bypass
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Dynamic response of light
component in vessels
Found: 35% time savings
BUT: Dynamics of middle vessel are slow
Propose: No vapor bypass

10. Multivessel without vapor bypass
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Response in middle vessel
Improved Time savings:
26% additional savings by eliminating vapor bypass
50% overall savings compared to the cyclic

11. AZEOTROPIC MIXTURES:
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Serafimov (1971): 26 feasible ternary VLE diagrams
Hilmen (2001): 4 existing elementary cells
Class : one separatrix
(Methanol/Water/1-Butanol)
Class 1.0-1a: no separatrix
(Ethyl Acetate/Water/Acetic Acid)

12. Azeotropic mixture: Serafimov’s topological class 1.0-2
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
DISTILLATION LINES MAP
One binary heteroazeotrope
One distillation boundary (unstable separatrix)
Two distillation regions
Final products in the vessels depend on the feed
The problem
Not all 3 original components can be recovered by simple distillation
The idea
The boundary is crossed by decantation and all 3 original components are recovered in a distillation-decanter hybrid

13. Separation in the multivessel
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Step 1: Build up the composition profile
Feed F in the left feed region
Methanol in the top (unstable node)
Heteroazeotrope in the middle (saddle)
1-Butanol in the bottom vessel (stable node)

14. Separation in the multivessel
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Step 2: Decant-Reflux the organic phase
Decanter at the middle of the column (internal)
Split the heteroazeotrope in the decanter
Reflux the organic phase in the column
Direct level control in the decanter

15. Separation in the cyclic column
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AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Cycle 1
Methanol in top vessel
Almost binary in the still
Cycle 2
Build up heteroazeotrope in top
Aqueous phase in the top vessel
1-Butanol in the still

16. Batch time (energy) requirements
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
System
Methanol/Water/1-Butanol
3 column configurations
Conventional multivessel (with vapor bypass)
Modified multivessel (w/o vapor bypass)
Cyclic column
2 product spec. sets
[0.99,0.97,0.99]
[0.99,0.98,0.99]
Multivessel w/o bypass
Spec 1: -29%
Spec 2: -37%
Cyclic
Spec 1: +28%
Spec 2: +24%
Base case
Multivessel with bypass
Multivessel column feasible for separation of heteroazeotrope
Multivessel less time consuming than cyclic column (24-28%)
Multivessel even better without vapor bypass (50% of cyclic)
However, vapor into middle vessel (decanter) not very practical

17. DISTILLATION LINES MAP
Azeotropic mixture: Serafimov’s topological class 1.0-1a
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
DISTILLATION LINES MAP
One binary heteroazeotrope
No distillation boundary
Final products in the vessels depend on the feed
The problem
Separation stops because of the heteroazeotrope accumulation in the top
The idea
The liquid-liquid split is used to overcome the azeotropic composition, thus enhancing the separation of the original mixture

18. Separation in the multivessel
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Step 1: Build up the composition profile
Feed F in the upper feed region
Heteroazeotrope in the top (unstable node)
Ethyl Acetate (EtAc) in the middle (saddle)
Acetic Acid (AcAc) in the bottom (stable node)

19. Separation in the multivessel
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Step 2: Decant-Reflux the organic phase
Decanter at the top of the column
Split the heteroazeotrope in the decanter
Reflux the organic phase in the column
Direct level control in the decanter

20. Separation in the cyclic
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Step 0: Build up heteroazeotrope in top vessel
Cycle 2
Ethyl Acetate in the top vessel
Acetic Acid in the still
Cycle 1
Aqueous phase in the top vessel
Almost binary in the still

21. Batch time (energy) requirements
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
System
Ethyl Acetate/Water/Acetic Acid
3 column configurations
Conventional multivessel (with vapor bypass)
Modified multivessel (w/o vapor bypass)
Cyclic column
2 specification sets
[0.97,0.97,0.99]
[0.99,0.98,0.99]
Multivessel w/o bypass
Spec 1: +7%
Spec 2: -6%
Cyclic
Spec 1: +54%
Spec 2: +44%
Base case
Multivessel with bypass
Multivessel column feasible
Multivessel much less time consuming than cyclic
No improvement without vapor bypass

22. (Water/Acetic Acid/Butyl Acetate)
Future work
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Class 1.0-1b is an example of Cell III
(Water/Acetic Acid/Butyl Acetate)
Our simulations show that also this mixture can be separated in the multivessel
Cell IV is very rare
References
Serafimov (1996), “Thermodynamic and Topological Analysis of Liquid-Vapor Phase Equilibrium Diagram and Problems of Rectification of Multicomponent Mixtures”,
Hilmen et al (2002). “Topology of Ternary VLE Diagrams: Elementary Cells”

23. Conclusions
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
Multivessel: Time savings often about 50% compared to conventional batch distillation
For separation of zeotropic mixtures: Propose modified multivessel column without vapor bypass. Additional time savings of 26% for case study.
A multivessel-decanter hybrid (with vapor bypass) is proposed for the separation of heteroazeotropic mixtures (Serafimov’s classes and 1.0-1a)

24. Discussion
N T N U
AIChE 2002 Annual Meeting, 3-8 November, Indianapolis
The multivessel performs always better than the cyclic column in the separation of both zeotropic and azeotropic mixtures
A modified multivessel (w/o vapor bypass) exhibits faster composition dynamics in the middle vessel
The modified multivessel is 26% faster than the conventional and 50% faster than the cyclic for zeotropic mixtures
A multivessel-decanter hybrid is proposed for the separation of heteroazeotropic mixtures of Serafimov’s classes and 1.0-1a
For practical reasons the conventional multivessel (with vapor bypass) is proposed for the separation of heteroazeotropes
The proposed multivessel-decanter hybrid exhibits time savings 25-50% compared to the cyclic column