Gas and vapor sorption and permeation in poly(2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole-co-tetrafluoroethylene)

The solubilities of N-2, CO2, CH4, C2H6, C3H8, and C3F8 and permeabilities of N-2, O-2, CO2, CH4, C2H6, and C3H8 were determined in a glassy, amorphous fluoropolymer prepared from 80 mol % 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and 20 mol % tetrafluoroethylene (TFE), commercially known as Hyflon AD 80. This polymer exhibits lower increases in hydrocarbon gas and vapor solubility with increasing penetrant critical temperature than conventional hydrocarbon polymers. On the basis of a best fit of the natural logarithm of solubility vs critical temperature, Hyflon AD 80 should have much lower solubility for high molar mass hydrocarbon compounds (e.g., n-decane) than conventional hydrocarbon polymers. Pure gas CO2/CH4 separation properties of this polymer are comparable with those of some hydrocarbon polymers considered for natural gas purification. When exposed to a feed stream containing a mixture of CO2 and CH4, the polymer exhibits a CO2 permeability of approximately 250 barrers and a CO2/CH4 mixed-gas selectivity of 10.6 at 1.6 atm CO2 partial pressure. The mixed gas selectivity decreases minimally as CO2 partial pressure increases to 10.6 atm. The mixed gas selectivity is also maintained when moderate amounts of toluene and n-hexane are present in the CO2-CH4 feed stream. Diffusion coefficients, calculated from pure gas permeability and solubility coefficients, suggest membrane plasticization at higher pressures of CO2 and C2H6. The polymer also exhibits reversible hysteresis in C3H8 permeability with pressure.