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

Abstract

Mean permeability coefficients for CH4 and CO2 (P̄ and P̄) 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 CH4 and CO2 as well as with CH4/CO2 mixtures containing 9.7, 24.0, and 46.1 mol % CO2. In the measurements with the pure gases, P̄ was found to decrease with increasing pressure, as expected from the “dual-mode” sorption model. By contrast, P̄ passes through a minimum and then increases with increasing pressure, probably due to the plasticization (swelling) of CA by CO2. The values of P̄ and P̄ determined with the mixtures containing 9.7 and 24.0 mol % CO2 decrease with increasing total pressure; this behavior is adequately described by the extended “dual-mode” sorption model for mixtures. By contrast, the values of P̄ and P̄ obtained with the mixture containing 46.1 mol % CO2 pass through a minimum and then increase as the total pressure is raised, probably also due to the plasticization of CA by CO2. The CO2/CH4 selectivity (≡P̄/P̄) of the CA membrances decreases with increasing total pressure and, at constant pressure, decreases with increasing CO2 concentration in the feed mixture. The effects of exposing the CA membranes to high-pressure CO2 prior to the permeability measurements (“conditioning” effects) on P̄ and P̄ have also been studied. © 1996 John Wiley & Sons, Inc.