GAS SEPARATIONS WITH HIGH-FLUX CELLULOSE ACETATE MEMBRANES P E T E R
K .
GANTZEL
AND
ULRICH MERTEN
Gulf General Atomic Inc.,S u n Diego, Calif. 921 12 Separation of helium from natural gas by permeation through a membrane has been suggested as a competitor to the cryogenic method currently employed. Measurements of permeation rates for helium and other gases indicate that cellulose acetate membranes of the type used for desalination by reverse osmosis may be more suitable for this application than those previously studied. Permeation rates [in units of cm3 (STP)/cm2-sec-cm of Hg] have been obtained for He, 10.6; Ne, 1.9; 02, 0.71; Ar, 0.37, CH4, 0.34, and N z , 0.31; all at 22OC. Higher operating temperatures gave a higher ratio of He/N? flow rates, but no samples gave as good a ratio as that obtained from an "ordinary" membrane of cellulose acetate.
THEhigh-flux
cellulose acetate membranes (Loeb and Sourirajan, 1960) used in desalination by reverse osmosis have been shown (Riley et al., 1964) to consist of a dense layer, perhaps 0.2 pm thick, supported on a highly porous substructure. Since permeation rates through a given membrane material are generally inversely proportional to membrane thickness, these same membranes would be expected to show relatively high transmission rates for gases and might be useful in gas separation processes. Experimental
"Fully dense" cellulose acetate membranes were prepared by casting an acetonic solution of Eastman Chemical Products, Inc., 394-45 cellulose acetate on a glass plate with a doctor blade, and allowing the solvent to evaporate a t room temperature in the laboratory atmosphere. Highflux cellulose acetate membranes of comparable acetyl content and 100-pm nominal thickness were prepared by the procedure of Loeb and Sourirajan (1960), and were then heat-treated in water a t 80°C and dried by quickfreezing and vacuum sublimation a t about -10" C. Permeation measurements were performed with a highpressure cell having an inlet and outlet to pass pressurized gas over the membrane. The gases were used directly out of commercial cylinders, except in one case in which a CaCL drying agent was employed. A porous steel plate covered with filter paper backed the membrane, and flow rates were measured a t atmospheric pressure by water displacement in a graduated cylinder. A gas chromatograph was used to measure the composition of gas mixtures. Results
Table I gives the permeability of a fully dense 41-pmthick cellulose acetate membrane a t 22" C. Results obtained with CaCbdried gas suggest that the observed permeation rates are somewhat dependent on the moisture content of the gas, but the point was not pursued quantitatively. Permeation rates for two samples of high-flux cellulose acetate are also given in Table I, and their temperature dependence is depicted in Figures 1 and 2 . Water flow rates of these samples were 0.8 x lo-' and 1.6 x
1 O F g/cm2-sec-atm, respectively, in reverse osmosis tests conducted before drying. Time-temperature tests at 1000psi total pressure showed unchanged helium flow rate after 43 days a t 47" C and 14 days at 72" C. A gas mixture containing 3070 He, 55% N P , and 15% C3Hs was used a t 100 to 200 psi for determining the permeability to propane. All other results were obtained with the pure gases a t pressures ranging from 50 psi for He to 400 psi for N P . The He and N 2 flow obtained from the He-N2-C3Hs mixture was in agreement with the pure gas measurements, as were results from a mixture containing 55% He, 6.5% Ne, 16.8% Ar, and 21.7'2 CH,. Permeation rates varied essentially linearly with applied partial pressure for total pressures below 400 psi; however, irreversible degradation of He/ N P permeation ratios occurred a t higher total pressures. Discussion
The two high-flux membranes show permeation ratios comparable to those obtained with the dense material and helium permeation rates that correspond to those expected for a 0.5-pm-thick (sample 1) and a 0.13-pmthick (sample 2 ) dense membrane. The characteristics of these dried membranes can thus be reasonably well
Table 1. Permeability of Cellulose Acetate at 22OC Dense-Sample Permeability, Cm3 (STPI-Cm
Gas He Ne 0 2
Ar CH, N, CiHn
Cm2-Sec-Cmof Hg 1.36 x 0.24 x
lo-' io-'
...
0.032 x lo-' 0.014 X lo-' 0.014 X lo-'
