Solubility and diffusivity of light gases in ethyl cellulose at elevated pressures Effects of ethoxy content

Abstract

The solubility of He, O 2, N 2, CH 4, and CO 2 in ethyl cellulose (EC) membranes with ethoxy contents of 47.2, 47.9, and 49.6% was measured at 35.0°C and at pressures up to ca. 30 atm (3,040 kPa). Mean diffusion coefficients, D ̄ , for these gases in EC were obtained from the solubility measurements in conjunction with mean permeability coefficients determined in a previous study. The solubility coefficients, S, for the above-mentioned gases in EC increase in the gas order: S(He)⪡ S(N 2)< S(O 2)< S(CH 4)< S(CO 2); this is also the order of increasing critical temperatures of these gases. The values of D ̄ for He, O 2, N 2, and CH 4 at low concentrations in EC increase in the order of decreasing ‘kinetic’ molecular diameters: D ̄ ( CH 4 )< D ̄ ( N 2 )< D ̄ ( O 2 )< D ̄ ( He) . The values of D ̄ ( CO 2 ) are lower than those of D ̄ ( O 2 ) , even thrugh CO 2 has a smaller ‘kinetic’ diameter than O 2. The lower diffusivity of CO 2 in EC could be due to specific interactions between polar groups in the polymer and the polar CO 2 molecules. In general, the values of S and D ̄ for the five gases in EC tend to increase with increasing ethoxy content in the polymer, probably due to an increase in the polymer free volume. EC and cellulose acetate (CA) are compared as membrane materials for gas separations, with emphasis on the CO 2 CH 4 separation. EC has a higher gas permeability and a lower selectivity than CA because of its larger free volume and higher chain mobility.