Permeation and separation properties of polyimide membranes to olefins and paraffins

Pure and mixed gas permeation experiments for olefins and paraffins of C-2 and C-3 were carried out for several polyimide and other polymer membranes at pressures up to 8 atm and temperatures from 308 to 423 K. The olefins were more permeable to the corresponding paraffins due to their preferential diffusion based on the difference in their molecular size. Polyimide prepared from 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,4,6-trimethyl-1,3-phenylenediamine (TrMPD) displayed relatively high performance: permeability coefficient to propylene, P-C3H6 = 20-40 Barrer (1 Barrer = 1 X 10(-10) cm(3)(STP)/(cm s cmHg)) and ideal separation factor (permeability ratio of pure propylene and propane). alpha(id)(C3H6/C3H8) = 11 at 323 K and 2 atm. Polyimide from GFDA and dimethyl-3,7-diaminodiphenylthiophene-5,5-dioxide, (DDBT) displayed low permeability and high permselectivity: P-C3H6 = 0.8 Barrer and alpha(id)(C3H6/C3H8) = 27 at 323 K and 2 atm. Their performance was much better than that of other polymers such as poly(2,6-dimethyl-1,4-phenyleneoxide), For mixed gas permeation, the separation factor was lower by about 40% than the alpha(id) due to the increase in PC3H8 caused by coexisting propylene.