A high-flux polyimide hollow fiber membrane to minimize footprint and energy penalty for CO ₂ recovery from flue gas

Using a process-guided approach, a new 6FDA-based polyimide - 6FDA-DAM:DABA(4:1) - has been developed in the form of hollow fiber membranes for CO ₂ recovery from post-combustion flue gas streams. Dense film studies on this polymer reveal a CO ₂ permeability of 224 Barrers at 40°C at a CO ₂ feed pressure of 10psia. The dense films exhibit an ideal CO ₂/N ₂ permselectivity of 20 at 40°C, which permits their use in a two-step counter-flow/sweep membrane process. Dry-jet, wet-quench, non-solvent-induced phase inversion spinning was used to create defect-free hollow fibers from 6FDA-DAM:DABA(4:1). Membranes with defect-free skin layers, approximately 415nm thick, were obtained with a pure CO ₂ permeance of 520GPU at 30°C and an ideal CO ₂/N ₂ permselectivity of 24. Mixed gas permeation and wet gas permeation are presented for the fibers. The CO ₂ permeance in the fibers was reduced by approximately a factor of 2 in feeds with 80% humidity. As a proof-of-concept path forward to increase CO ₂ flux, we incorporated microporous ZIF-8 fillers into 6FDA-DAM:DABA(4:1) dense films. Our 6FDA-DAM:DABA(4:1)/ZIF-8 dense film composites (20wt% ZIF-8) had a CO ₂ permeability of 550 Barrers and a CO ₂/N ₂ selectivity of 19 at 35°C. Good adhesion between the ZIF and the 6FDA-DAM:DABA(4:1) matrix was observed. CO ₂ capture costs of $27/ton of CO ₂ using the current, "non-optimized" membrane are estimated using a custom counterflow membrane model. Hollow fiber membrane modules were estimated to have order-of-magnitude reductions in system footprint relative to spiral-wound modules, thereby making them attractive in current space-constrained coal-fired power stations.