Improving the mass transfer rate in G–L membrane contactors with ionic liquids as absorption medium. Recovery of propylene

Abstract

The use of membrane contactors for the G–L separation of olefins by means of reactive absorption with ionic liquid silver complexes presents a great potential to intensify the conventional separation process based on cryogenic distillation. Nevertheless, the key issue to be addressed is the high mass transfer resistance offered by the ionic liquid that flows through the shell side of the membrane contactor. It is therefore crucial to improve the fluid dynamics of the system in order to enhance the mass transfer rate in the liquid film. This work reports the analysis of the performance of a gas–liquid transverse flow membrane contactor applied to the reactive absorption of propylene in BMImBF 4–Ag + as reaction media. First the mass transfer phenomena developed in the system in the absence of chemical reactions were assessed; several experiments were carried out at 293 K flowing pure BMImBF 4 at flow rates between 6.42 × 10 −6 m 3 s −1 and 1.19 × 10 −5 m 3 s −1 with propylene as fed gas. Under the operational conditions used in this work, the resistances across the gas film and the microporous membrane are negligible so the overall mass transfer resistance is controlled by the diffusional resistance in the liquid boundary layer. The experimental data were fitted to an empirical correlation, Sh = 1.1 · Re 0.32 Sc 0.33, that allows to calculate the mass transfer rate in the ionic liquid flowing through the shell side of the module at low Reynolds numbers. Moreover it was observed that for the BMImBF 4 flowing at 6.42 × 10 −6 m 3 s −1 and 293 K the improved fluid-dynamics of the transverse flow membrane contactor leads to an overall mass transfer coefficient 17.6 times higher than that reported previously using a parallel-flow contactor. Furthermore, a detailed mathematical model was developed in order to predict and analyze the performance of the system under different operational conditions. This model was applied to simulate the performance of the system under the presence of silver ions in the reaction medium. The reactive absorption of propylene using ionic liquids–silver complexes in the transverse flow membrane contactor has provided promising results, showing a process intensification by a factor of 17.4 in terms of mass transfer per specific area compared to a conventional stirred tank reactor system ( K overall × a(MC)/ K overall × a(CSTR)).