(401ac) Improved Polyethersulfone Substrates for Composite Membranes in CO2 Separation

Pilot-scale nanoporous polyethersulfone (PES) substrates were fabricated by vapor- and non-solvent-induced phase inversion steps successively. N-methyl-2-pyrrolidone (NMP) and water were employed as solvent and non-solvent, respectively. 2-Methoxyethanol (2-ME) was incorporated into the casting solution as a pore-forming additive because of its high affinity with water, leading to an interconnected spongy structure in the bulk of the substrate. The 14-inch wide PES substrates were fabricated successfully by using a continuous roll-to-roll casting machine. Two hydrophilic polymers, hydroxylated PES (PES-OH) and polyvinylpyrrolidone (PVP), were incorporated for modifying the pristine substrate both hydrophilically and morphologically. The surface morphologies and gas permeation resistances of the PES membranes were characterized by scanning electron microscopy (SEM) and a gas permeation unit, respectively. The pristine PES substrate, with a pore size of 41 nm and a surface porosity of 12%, showed a CO₂ permeance of about 9200 GPU (1 GPU = 10^-6 cm³ (STP)·cm^-2·s^-1·cmHg^-1) and a CO₂/N₂ selectivity ~ 0.9 (Knudsen diffusion). The addition of hydrophilic polymers reduced the gas permeation resistances of PES substrates because of the more open morphology. By incorporating appropriate amounts of PES-OH and PVP in the casting solutions, modified PES substrates showed CO₂ permeances of 14100 GPU and 21000 GPU, respectively, indicating a significant reduction of gas permeation resistance. The developed PES substrates were applied for the preparation of composite membranes for CO₂ separation. Amine-containing selective layers, with a thickness of approximately 175 nm, were coated on the developed pristine PES substrate, PES-OH modified substrate, and PVP-modified substrate to prepare 3 composite membranes, respectively. From the gas transport measurements by using a simulated flue gas, these 3 composite membranes showed CO₂ permeances of 785 GPU, 836 GPU, and 843 GPU, and CO₂/N₂ selectivities of 150, 148, and 160, respectively. Based on the resistance-in-series model, the separation performance improvements could be attributed to the reduced mass transfer resistances of the modified substrates. In addition, a thinner selective layer can be coated on the hydrophilic polymer modified PES substrates due to the enhanced hydrophilicity, which can further improve the separation performances of the composite membranes in the future. The developed fabrication process has demonstrated the potential for the mass production of the suitable PES substrate for composite membranes in CO₂ separation.