Influence of membrane skin morphology on CO₂/N₂ separation at sub-ambient temperatures

Matrimid^® hollow fiber membranes with silicone rubber (PDMS) calked fused nodular selective layers display much higher CO₂/N₂ selectivity as well as higher CO₂ permeance than those with isotropic dense selective layers at 253.15 K. Two kinds of asymmetric hollow fiber membranes were successfully spun: fibers with fused nodular skins and fibers with truly dense skins. The skin morphologies were confirmed with SEM images. CO₂/N₂ mixed gas permeation characterization was carried out within the temperature range of 253.15 K to 308.15 K. Permeation results showed that, at 253.15 K, the nodular-skinned fibers displayed CO₂/N₂ selectivity of 90.5, and CO₂ permeance of 63.3GPU after standard PDMS calking; while the dense-skinned fibers displayed CO₂/N₂ selectivity of 52.5, and CO₂ permeance of 16.6GPU. The PDMS calking had negligible effect on the dense-skinned fibers, but it was necessary for the fused nodular-skinned fibers. A hypothesis regarding the introduction of Langmuir sorption sites and a local orientation of polymer chain segments is proposed to explain the better performance of the calked nodular-skinned fibers. This work provides a promising method for improved hollow fiber membranes for the removal of CO₂ from flue gas.