(116d) Hybrid Separations Including Organic Solvent Nanofiltration: Process Intensification for the Purification of Iso-Butylbenzene

The coupling of different traditional and/or new and innovative separation techniques called hybrid separations leads to process intensification at the process level within the development of new processes or within retrofitting of existing processes. Hybrid separations integrate the advantages of the individual separations techniques resulting in decreased energy consumption, increased process efficiency and/or product quality. Especially, the integration of distillation for high capacity processing and membrane for energy-efficient and selective processing offers large benefits.

Organic solvent nanofiltration (OSN) is a pressure-driven separation method in which components are separated by differences in molecular sizes or weights or by different ability to dissolve into the membrane. Pervaporation (PV) separates components by their differences in relative volatility but is not restricted by the Vapor-Liquid-Equilibrium. The main advantages of both processes are the low energy consumption compared to traditional separation methods like distillation; possibilities to recycle the separated compounds or solvents; and flexibility due to their modular design. In the case of OSN no phase transition occurs and mild processing conditions are attained by low operating temperatures [1].

In this work the catalytic alkylation of toluene by propene was investigated regarding the purification of the products: iso-butylbenzene and n-butylbenzene. The main product of the reaction is iso-butylbenzene, an intermediate to produce ibuprofen [2], while the unreacted aromatic component is recycled using distillation. Since the yield of the investigated reaction is low (<20%) recycling of toluene by distillation has a high energy demand and can also lead to polymerisation of the products [3].The conventional product purification is achieved by two distillation columns. In the first one, toluene is evaporated and recycled to the reactor. The bottom product of the first column is introduced to the second column where the separation of the two isomers is performed. Based on PV, OSN and distillation different process options for hybrid processing are generated. One of those consists of a membrane cascade for the pre-enrichment of the isomers, a distillation column for toluene recovery and recycling, and a second distillation column for the separation of butylbenzene isomers. Laboratory experiments were conducted for the membrane operations, e.g. with Puramem^TM280 nanofiltration membrane, at different temperatures and solute concentrations. Based on the experimental data membrane models were implemented in the simulation environment Aspen Custom Modeler^TMand used for the optimisation of the process options by an evolutionary optimisation method. The objective of the optimisation process was to minimise the production cost of iso-butylbenzene including investment and operational costs and to identify the best process option.

Results show that the pre-concentration of isomers by OSN significantly improves process economics by lowering the amount of toluene that has to be evaporated in the first distillation column.

References

[1] Vandezande, P., Gevers, L. E. M., Vankelecom, I.F. J., Solvent resistant nanofiltration: separating on a molecular level, Chemical Society Reviews, 2008, 37(2), 365-405.

[2] Sheldon, R. A., Organic synthesis past, present and future, Chemistry and Industry (London), (1992) 903-906.

[3] Stevens, M. G., Anderson, M. R., Foley, H. C., Side-chain alkylation of toluene with propene on caesium/nanoporous carbon catalysts, Chemical Communications, 1999, 413–414.