Ester Crosslinkable Polyimide Hollow Fibers for CO2 Removal From Natural Gas

Despite its importance as one of the most important fuel sources in the world, raw natural gas often contains a high level of carbon dioxide (CO₂).  Removal of the CO₂ to a level <2%(mol) is required to avoid problems such as pipeline corrosion, additional compression cost and reduction of gas heating value. Compared to traditional amine scrubbers to remove CO₂ from natural gas, membrane separation technology offers lower capital cost, higher energy efficiency and less environmental restriction.

Notwithstanding intrinsically attractive separation performance, conventional polymeric membranes suffer from plasticization induced by high CO₂ partial pressures, which reduces CO₂/CH₄ separation efficacy significantly. Researchers have demonstrated that covalent ester-crosslinking can improve the plasticization resistance by controlling the segmental chain mobility in the polymer; however, only relatively thick selective skin layers and lower separation productivity have been reported to date. Furthermore, a typical natural gas feed contains a certain amount of heavy hydrocarbon impurity, such as aromatic contaminants. Those contaminants can severely undermine the separation productivity due to the antiplasticization. While a pre-treatment utilizing filters or adsorbents can reduce the high level hydrocarbon impurities, it also increases separation cost and can hardly remove impurities completely. On the other hand, a robust crosslinked hollow fiber membrane will require less pre-treatment to achieve an effective separation even in the presence of high level contaminants.

This study will address the challenges of developing a more productive hollow fiber membrane with enhanced plasticization and antiplasticization resistance. By using a novel high-performance polyimide, the skin layer thickness of crosslinked hollow fibers was first optimized by engineering the spinning process to improve CO₂ permeance without losing CO₂/CH₄ selectivity. Ester-crosslinking was then utilized to stabilize the highest productivity hollow fibers against CO₂ plasticization. The crosslinked hollow fibers were subjected to a CO₂ partial pressure up to 400 psi to probe their CO₂ plasticization resistance. Finally, the highly productive crosslinked fibers were characterized by using a hydrocarbon contaminant content up to 1000 ppm to simulate extremely challenging natural gas feed conditions.