Experimental mixed-gas permeability, sorption and diffusion of CO 2 -CH 4 mixtures in 6FDA-mPDA polyimide membrane: Unveiling the effect of competitive sorption on permeability selectivity

Abstract

The nonideal behavior of polymeric membranes during separation of gas mixtures can be quantified via the solution-diffusion theory from experimental mixed-gas solubility and permeability coefficients. In this study, CO 2 -CH 4 mixtures were sorbed at 35°C in 4,40-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA)-m-phenylenediamine (mPDA)-a polyimide of remarkable performance. The existence of a linear trend for all data of mixed-gas CO 2 versus CH 4 solubility coefficients-regardless of mixture concentration-was observed for 6FDA-mPDA and other polymeric films; the slope of this trend was identified as the ratio of gas solubilities at infinite dilution. The CO 2 /CH 4 mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value. The analysis of CO 2 -CH 4 mixed-gas concentration-averaged effective diffusion coefficients of equimolar feeds showed that CO 2 diffusivity was not affected by CH 4 . Our data indicate that the decrease of CO 2 /CH 4 mixed-gas diffusion, and permeability selectivity from the pure-gas values, resulted from an increase in the methane diffusion coefficient in mixtures. This effect was the result of an alteration of the size sieving properties of 6FDA-mPDA as a consequence of CO 2 presence in the 6FDA-mPDA film matrix.