Comparison of theoretical and experimental mass transfer coefficients of gases in supported ionic liquid membranes

A complex study of the operation of supported ionic liquid membranes from practical point of view was made. It was shown that supported ionic liquids are selective for the binary mixture of CH4/CO2. Unfortunately, the literature reports certain pure ionic liquid data, which are mainly focused on CO2 and there is lack of data for CH4. The main problem seems to be the lack of data of the diffusion coefficients of gases in ionic liquids and, consequently, there are no correlations avialable between the transport properties and the molecular properties of the gas and of the IL.. The data available in literature are measured and calculated for pure ionic liquids only but there is no reasonable way how to apply them to the supported ionic liquid membranes. All the literature data sets overestimate at least twice the membrane performance in terms of pemeability and selectivity. Such a discrepancy brings the supported ionic liquid membranes into an inconvenient position from the point of view of practical application because it is not possible to design a process reliably although the supported ionic liquid membranes are comparable in separation characteristics to the best polymeric membrane (see Robeson plot Fig. 7). The reasons why the theoretical models overpredict the real values of permeation fluxes may be summarized below: o The mass transfer resistance of the support is not negligible, as often assumed o Some pores of the support are not opened for permeation o The structure of the support obstructs the maximal saturation of the liquid with the gas or in other words the initial concentration of the gas in the liquid does not reach the expected equilibrium value. None of these reasons should be totally excluded. A simple model of transport of gases through the liquid membrane in permeation cell was suggested. It showed that in the used pressure range (150-350kPa), the mass transfer coefficient may be assumed constant with reasonable accuracy. The model could be applied in the scale-up of the process because it takes into account the decrease of the driving force that occurs along the membrane. This decrease is not really significant in the lab scale process, where the membrane area is small, but it may be important in larger scale processes, especially when the high enrichment of retentate is required. Generally, it can be concluded that the ionic liquid membranes are convenient for gas separation processes, but it is necessary to rely on the data connected to the supported liquid membranes themselves rather than on the data of pure (free) ionic liquids.