Towards a systematic determination of multicomponent gas separation with membranes: the case of CO2/CH4 in cellulose acetates

This work aims at obtaining a systematic description and prediction of the multicomponent gas separation behavior of membranes, using the Non-Equilibrium Lattice Fluid (NELF) model for gas sorption and the Standard Transport (ST) model for gas permeation. The scheme is applied to a comprehensive analysis of CO2/CH4 separation with cellulose acetate membranes. A dedicated experimental campaign (pressure-volume-temperature (PVT), differential scanning calorimetry (DSC) and pure gas sorption tests) was performed to obtain reliable model parameters, accounting also for crystallinity. The approach was validated against a complete set of literature data, including mixed gas sorption and permeation. The parameters obtained were used to perform predictive simulations of mixed CO2/CH4 sorption and permeation in a wide mixture composition range. The NELF model accurately predicts the effect of temperature on sorption, as well as the strong competitive exclusion of CH4 when CO2 is present, that enhances the solubilityselectivity. The ST model correctly estimates the experimentally observed lower-than-ideal CO2/CH4 permselectivity. The model shows that low perm-selectivity is due to mixed gas diffusivity-selectivity (which is not available experimentally): even though solubility-selectivity increases in the mixed gas case, diffusivityselectivity suffers a larger departure from pure gas conditions, ultimately leading to a lower perm-selectivity under multicomponent conditions.

Automated summary

This study utilized the NELF model and ST model to describe and predict gas separation behavior of membranes, particularly focusing on CO2/CH4 separation. Experimental results showed that the presence of CO2 caused strong competitive exclusion of CH4, enhancing solubility selectivity of the membrane. Additionally, the study found that under multicomponent conditions, low perm-selectivity was due to a decrease in diffusivity-selectivity.