PERMEABILITY OF DENSE (HOMOGENEOUS) CELLULOSE-ACETATE MEMBRANES TO METHANE, CARBON-DIOXIDE, AND THEIR MIXTURES AT ELEVATED PRESSURES

Mean permeability coefficients for CH4 and CO2 ((P) over bar(CH4) and (P) over bar(CO2)) in cellulose acetate (CA, DS = 2.45) were determine d at 35 degrees C (95 degrees 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 th e measurements with the pure gases, (P) over bar(CH4) was found to dec rease with increasing pressure, as expected from the ''dual-mode'' sor ption model. By contrast, (P) over bar(CO2), passes through a minimum and then increases with increasing pressure, probably due to the plast icization (swelling) of CA by CO2. The values of (P) over bar(CH4), an d (P) over bar(CO2) 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 (P) over bar(CH4) and (P) over b ar(CO2), obtained with the mixture containing 46.1 mol % CO2 pass thro ugh a minimum and then increase as the total pressure is raised, proba bly also due to the plasticization of CA by CO2. The CO2/CH4 selectivi ty (=(P) over bar(CO2)/(P) over bar(CH4)) of the CA membranes decrease s 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 permeabili ty measurements (''conditioning'' effects) on (P) over bar(CH4) and (P ) over bar(CO2) have also been studied. (C) 1996 John Wiley & Sons, In c.