|Abstract||The separation of carbon dioxide from different sources (e.g. natural gas, flue gas, etc.) has become an important area of research. Some conventional methods of CO2 separation were used over the years including adsorption (with porous solids), absorption (with amines), cryogenic separation and membranes. Amongst these technologies, Supported Ionic Liquid Membranes (SILMs) technology has been developed in the past few years and became one of the promising techniques in CO2 separation from gas streams. SILMs technology combines the advantages of both membranes and ionic liquids (ILs) hence it has become an interest of many recent studies. Most of the synthesized SILMs in literature uses porous membranes to support the ionic liquids. Although these SILMs achieve high permeability of CO2, the separation selectivity to the other gas is very low due to the high permeance of the other gas. Another drawback of porous SILMs is the membrane failure with high pressures due to ionic liquid loss through the pores of the support membrane.
In this work, we look alternative solutions to overcome these disadvantages by synthesizing SILMs using dense (non-porous) polymeric support by which limiting or eliminating ILs loss through the membrane and increase the selectivity of CO2 separation. Four types of ionic liquids (ILs) were blended with polysulfone (PSF) to produce functional dense polymeric-supported ionic liquid membranes (DPSILMs). These ionic liquids are 1-alkyl-3-methylimidazolium bistriflamide [C4mim][NTf2] and Di-iso-propyl 1-alkyl-3-methylimidazolium bistriflamide [DIP-C4mim][NTf2], Tributylmethylphosphonium formate [P4441][formate], and Tributylmethylammonium formate [N4441][formate].
The main aim of this study is to investigate the potential use of the synthesized DPSILMs in the industrial gas processing applications for high-pressure CO2 separation from N2 and CH4 streams with less or no loss of ILs. The synthesized DPSILMs were analysed using FTIR and SEM and showed a clear chemical and physical change in the structure PSF and well distribution of ILs in PSF. Binary mixtures of CO2/N2 and CO2/CH4 (5 mol% CO2) were used in the study. Selectivity values for the prepared DPSILMs were obtained using a high-pressure membrane unit obtained from Rubotherm Präzisionsmesstechnik GmbH apparatus (System 2). The highest CO2/N2 selectivity values were 36 for both PSF-0.5 wt% [DIP-C4mim][NTf2], PSF-25 wt% [N4441][formate], 29 and 21 for PSF-0.5 wt% [C4mim][NTf2] and PSF-50 wt% [P4441][formate] respectively. Whereas the highest CO2/CH4 selectivity results were 70, 63, 47, and 32 for PSF-2.5 wt% [C4mim][NTf2], PSF-2.5 wt% [DIP-C4mim][NTf2], PSF-0.5 wt% [N4441][formate], and PSF-5 wt% [P4441][formate] respectively. Another system was used to measure the permeability of each gas (System 1) to be plotted then on Robeson's upper bound (2008) with other PSF blends in the literature for better comparison. The plot showed that the synthesized DPSILMs gave satisfying results and behave as well or better than different types of reported PSF blends. The highest CO2 permeabilities (with CO2/N2 separation measurements) obtained with each IL were 19, 13.6, 10.8, and 8.9 barrer with PSF-25 wt% [N4441][formate], PSF-5 wt% [p4441][formate], PSF-0.5 wt% [DIP- C4mim][NTf2], and PSF-5 wt% [C4mim][NTf2] respectively. However with CO2/CH4 separation measurements, the highest CO2 permeabilities were 17.3, 13.8, 12.5, and 11.5 barrer with PSF-12.5 wt% [P4441][formate], PSF-2.5 wt% [DIP-C4mim][NTf2], PSF-0.5 wt% [N4441][formate], and PSF-2.5 wt% [C4mim][NTf2] respectively.
Stability measurements of the synthesized DPSILMs were conducted regarding ILs loss and CO2/CH4 separation efficiency. Stability results showed that DPSILMs with 5 wt% [P4441][formate] and [N4441][formate] showed about 30% and 20% ILs loss respectively at 10 bar after 12 hours with small reduction in CO2/CH4 selectivity; while no loss of [DIP-C4mim][NTf2] and [C4mim][NTf2] was observed.