Permeability of dense (homogeneous) cellulose acetate membranes to methane, carbon dioxide, and their mixtures at elevated pressures

Mean permeability coefficients for CH 4 and CO 2 (P CH4 and P CO2 ) in cellulose acetate (CA, DS = 2.45) were determined at 35°C (95°F) and at pressures up to about 54 atm (800 psia). The measurements were made with pure CH 4 and CO 2 as well as with CH 4 /CO 2 mixtures containing 9.7, 24.0, and 46.1 mol % CO 2 . In the measurements with the pure gases, P CH4 was found to decrease with increasing pressure, as expected from the dual-mode sorption model. By contrast, P CO2 passes through a minimum and then increases with increasing pressure, probably due to the plasticization (swelling) of CA by CO 2 . The values of P CH4 and P CO2 determined with the mixtures containing 9.7 and 24.0 mol % CO 2 decrease with increasing total pressure ; this behavior is adequately described by the extended dual-mode sorption model for mixtures. By contrast, the values of P CH4 and P CO2 obtained with the mixture containing 46.1 mol % CO 2 pass through a minimum and then increase as the total pressure is raised, probably also due to the plasticization of CA by CO 2 . The CO 2 /CH 4 selectivity (=P CO2 /P CH4 ) of the CA membranes decreases with increasing total pressure and, at constant pressure, decreases with increasing CO 2 concentration in the feed mixture. The effects of exposing the CA membranes to high-pressure CO 2 prior to the permeability measurements (conditioning effects) on P CH4 and P CO2 have also been studied.