1. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 1 NEW COLUMN CONFIGURATIONS FOR PRESSURE SWING BATCH DISTILLATION I. FEASIBILITY STUDIES Modla G. and P. Láng Budapest University of Technology and Economics, Department of Process Engineering, lang@mail.bme.hu Goal: to study the pressure swing batch distillation separation of binary homoazeotropes by feasibility studies. Besides studying the well known configurations (rectifier, stripper, middle vessel column) we also suggest three novel configurations such as -the combination of the batch rectifier and stripper, -the double column batch rectifier and -double column batch stripper.

2. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 2 I. Introduction II. Theoretical bases III. Column configurations IV. Results V. Conclusion Outline

3. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 3 I. Introduction Distillation is the separation method most frequently applied in the chemical industry, which is based on the difference of volatility of the components of a liquid mixture. For the separation of the two components (A and B) forming an azeotrope a special distillation method must be applied such as the pressure swing distillation, extractive or heteroazeotropic distillation. Batch distillation has always been an important part of seasonal, uncertain or low capacity and high-purity chemicals’ production. It is a process of key importance in the pharmaceutical and several other industries and in the regeneration of waste solvent mixtures. The main advantage of batch distillation over the continuous one is that a single apparatus can process many different liquid mixtures. Even multicomponent mixtures can be separated by batch distillation in a single column.

4. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 4 II.1 Theoretical bases Azeotropic mixture Mixture that forms one or several azeotrope(s). The azeotrope can be a -homoazeotrope, when one liquid and one vapour phases are in equilibrium or -heteroazeotrope, when two liquid and one vapour phases are in equilibrium. x-y equilibrium curve of a minimum boiling point homoazeotropic mixture

5. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 5 II.2 Theoretical bases Pressure sensitive azeotropic mixtures Many mixtures contain azeotrope whose position can be shifted substantially by changing system pressure. Pressure swing distillation Distillation process for the separation of the azeotropes, where the pressure sensitivity helps to separate the azeotropes. Condition of application For pressure-swing distillation to be practical, the azeotropic composition must vary at least 5%, over a moderate pressure range (not more than ten atmospheres between the two pressures). x-y equilibrium curve of a minimum boiling point homoazeotropic mixture at the different pressures

6. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 6 II.3 Theoretical bases Feasibility method When making feasibility studies we suppose that maximal (perfect) separation can be produced. This involves the following simplifying assumptions: - very high reflux/reboil ratio, -negligible liquid plate hold-up, - negligible vapour hold-up, -instantaneous pressure change. The method is based on the determination of the feasible compositions of products (continuously withdrawn) and those of residues (remaining in the vessel). - Feasibility interval of a component (FI) all feed compositions, from where the given component can be produced as pure (top or bottom) product by maximal separation. - Feasibility interval of the binary separation (FS) all feed compositions, from where both components can be purely produced by maximal separation at the given pressure or by applying pressure swing, that is, two different pressures.

7. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 7 III. Column configurations III.1 Continuous system Minimum boiling point azeotropic mixture Maximum boiling point azeotropic mixture

8. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 8 III.2 Batch column configurations The pressure swing can be performed in time or in different place Pressure swing batch distillation Pressure swing in time -Batch rectifier -Batch stripper -Combination of the batch rectifier and stripper Pressure swing in place -Middle vessel column -Double column batch rectifier -Double column batch stripper

9. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 9 III.2 Batch column configurations Pressure swing in time – batch rectifier and batch stripper Batch rectifier (BR)Batch stripper (BS) Evolution of the product and vessel compositions of the BR (BS) for max. (min.) azeotropes

10. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 10 III.2 Batch column configurations Pressure swing in time – combination of the batch rectifier and stripper Combination of the batch rectifier and stripper Evolution of top vapour and bottom liquid compositions of the BR-BS for max. (min.) azeotropes

11. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 11 III.2 Batch column configurations Pressure swing in different places – Middle vessel column Middle vessel column Possible pressures: 1. P RS >>P MV =P SS 2. P RS =P MV >>P SS 3. P RS =P MV <<P SS 4. P RS <<P MV =P RS Stripping section (SS) Rectifying section (RS) Middle vessel (MV) There are four different cases, but neither of them is feasible, because one of the two products is always of azeotropic composition. UNFEASIBLE PROCESS

12. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 12 III.2 Batch column configurations Pressure swing in different places – Double column batch rectifier Double column batch rectifier (DCBR) Evolution of product and vessel compositions of the DCBR for max. azeotropes

13. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 13 III.2 Batch column configurations Pressure swing in different places – Double column batch stripper Double column batch stripper (DCBS) Evolution of product and vessel compositions of the DCBS for min. azeotropes

14. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 14 III.2 Batch column configurations Advantages of the double column configurations - the separation A/B is possible in only one production step, - both column sections can be operated in steady-state, - there is no pressure change during the process, - the two columns are operating simultaneously, so they can be thermally integrated, - the dimension (number of stages, diameter) of the two columns can be different (higher flexibility).

15. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 15 IV. Results of the investigations F: feasible NF: unfeasible +:acceptable ++:good Column configurationMax. azeotrope Min. azeotrope Batch rectifierF,+F Batch stripperFF,+ Batch rectifier-stripperFF Middle vessel columnNF Double column batch rectifierF,++NF Double column batch stripperNFF,++

16. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 16 V. Conclusion The pressure swing batch distillation separation of binary homoazeotropes was investigated by assuming maximal separation. Three new column configurations were studied. We stated that: - The separation of both minimum and maximum azeotropes is feasible by the alternate application of a batch rectifier and stripper. - The double column configurations have several advantages: - there is only one production step, the two components can be produced simultaneously, - both column sections can be operated in steady-state, - there is no pressure change during the process (shorter process time), - the two columns can be thermally integrated. These results were verified by rigorous simulation calculations, as well. (Poster section, P2.19)

17. 8th International Symposium on Dynamics and Control of Process Systems (2007) Mexico/Cancun 17 THANK YOU FOR YOUR ATTENTION