Mixed-Gas Permeation Study of CO2/CH4 and CO2/N2 in Thermally Rearranged (TR) Polymers

Membranes for CO₂ capture from flue gas emissions must be made from high flux materials that are robust in the face of water vapor and combustion byproducts.  Thermally rearranged (TR) polymers are strong candidates for this application.  These materials have been shown to be resistant to degradation by water vapor at elevated temperatures, resistant to plasticization, and exhibit very high CO₂ permeability.

In this work, results of mixed-gas permeation tests involving CO₂ permeation in TR polymers derived from 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) Hexafluoropropane Dianhydride (6FDA) are presented.  Polyimide HAB-6FDA films were converted to polybenzoxazole (PBO) to varying degrees by following different thermal treatment protocols.  Permeation tests with these films were conducted using CO₂-containing mixtures at temperatures from -15 to 70°C, and pressures up to 50 atm.  The bulk of the results are for 50% CO₂/CH₄ mixtures.  Preliminary results for 50% CO₂/N₂ mixtures are presented as well, and exhibit trends similar to the CO₂/CH₄ results.  Mixed-gas results are compared against pure-gas measurements.  CO₂/CH₄ mixed-gas selectivity at 35°C was 20-30% higher than the simple ratio of pure-gas permeabilities.  Strong dual-mode competitive effects were responsible for a decrease in CH₄ and N₂ permeability in the presence of CO₂, with very little change to CO₂ permeability in the mixture.  Plasticization is not evident in the pure-gas CO₂ permeation data.  However, in the mixed-gas experiments, CH₄ permeability was found to increase gradually with increasing CO₂ pressure.  The degree of plasticization was found to decrease as the degree of thermal conversion to PBO increased.