Effects of Supercritical CO2 Conditioning on Un-Cross-Linked Polyimide Membranes for Natural Gas Purification

The effects of supercritical CO2 (scCO2) conditioning are examined through changes in gas permeability and solubility for 6FDA-based polyimide membranes. At pressures above the supercritical point, both CO2 permeability and sorption unexpectedly decline, suggesting a structural rearrangement of the polymer facilitated by the presence of a highly soluble species. High-pressure permeability isotherms for He and N2 do not exhibit a similar decline. Upon depressurization, a typical hysteresis is not observed; rather, CO2 permeability remains at reduced levels compared to the original pressurization. Permeability isotherms of the conditioned polymer show a reduction in transport compared to unconditioned samples; however, sorption isotherms show an increase in penetrant solubility following scCO2 conditioning. Pre- and post-CO2/CH4 mixed gas permeation testing at 35 °C shows a reduction in membrane permeability with a corresponding increase in the separation factor after scCO2 conditioning.