Stathis Skouras and Sigurd Skogestad

1. N T N U 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

2. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Presentation Outline • 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Project Objective • 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis The Model • 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 = 1.013 bar • Simulations performed in MATLAB

5. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis The Cyclic Batch Column (Treybal, 1970)Used as basis for comparison of batch times 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Generalization: The Multivessel Batch Column (Hasebe, 1995) 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Multivessel column: Previous work on zeotropic mixtures • 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Cyclic column: Methanol/Ethanol/1-Propanol Multivessel column with vapor bypass: • 35% time savings compared to cyclic column

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

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

11. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis AZEOTROPIC MIXTURES: • Serafimov (1971): 26 feasible ternary VLE diagrams • Hilmen (2001): 4 existing elementary cells Class 1.0-2 : one separatrix (Methanol/Water/1-Butanol) Class 1.0-1a: no separatrix (Ethyl Acetate/Water/Acetic Acid)

12. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Azeotropic mixture:Serafimov’s topological class 1.0-2 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Separation in the multivessel Step 1: Build up the composition profile • FeedF 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Separation in the multivessel 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Separation in the cyclic column 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Batch time (energy) requirements 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] Cyclic Spec 1: +28% Spec 2: +24% Multivessel w/o bypass Spec 1: -29% Spec 2: -37% 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Azeotropic mixture: Serafimov’s topological class 1.0-1a 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Separation in the multivessel 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Separation in the multivessel 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Separation in the cyclic 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Batch time (energy) requirements System • Ethyl Acetate/Water/Acetic Acid 2 specification sets • [0.97,0.97,0.99] • [0.99,0.98,0.99] 3 column configurations • Conventional multivessel (with vapor bypass) • Modified multivessel (w/o vapor bypass) • Cyclic column 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Future work 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. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Conclusions • 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 1.0-2 and 1.0-1a)

24. N T N U AIChE 2002 Annual Meeting, 3-8 November, Indianapolis Discussion • 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 1.0-2 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