Permeation of gases at high pressures

The permeation of He, N2, CH4, C2H6, C2H4, CO2, and N2O through polyethylene membranes has been studied at pressures up to 60 atm and at temperatures between −10° and 60° C. The experimental temperature range encompassed the critical temperature of the last four penetrants. The mean permeability coefficient, P, for C2H6, C2H4, CO2, and N2O exhibited an exponential dependence on pressure at reduced penetrant temperatures below and near unity. This dependence decreased with increasing reduced temperature. The values of P for CH4, which were obtained at reduced temperatures above unity, were essentially constant over the entire pressure range. Finally, the P's for N2 and He, which were measured at the highest reduced temperatures, appeared to decrease slightly with increasing pressure. Possible causes for the observed behavior are discussed, and a new correlation for the pressure dependence of the permeability coefficients is presented. The separation of two different N2O-CO2 mixtures by selective permeation was studied at 27 atm and 30°C. The degree of separation was measured as a function of “stage cut” and was also calculated for both constant and pressure-dependent permeability coefficients. The agreement between theoretical and experimental results was not satisfactory, probably because of interactions between the components of the gas mixture during permeation.