The possibility of separating N2 and He by selective permeation through a polymer membrane is being considered. The membrane will be cellulose acetate-butyrate 1 mill thick, operated at 2lC, with upstream and downstream pressure of 10 and 1.0 atm, respectively. The permeabilities to the two gases are reported to be 0.45 10-9 for He and 0.07 10-9 for N2, the units being (cm3 at 0C, 1.0 atm) (cm)/(s)(cm2)(cmHg). In order to get some idea of the separation possible, it is proposed to analyze the steady-state operation of a single stage in which the gases in both upstream and downstream compartments are well mixed, and in which half the feed is withdrawn as permeate (refer to figure). The feed gas will be 30 percent He, 70 percent N2.

(a) Assuming permeabilities independent of pressure and the diffusion of each gas to be unaffected by the presence of the other, calculate the composition of the permeate. (b) How many square feet of membrane will be required to obtain 1 lb mole/h of the helium-enriched permeate?