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

• Formation of asymmetric hollow fiber membranes with fused nodular selective layers.
• Nodular skin has high permeance and high selectivity at sub-ambient temperatures.
• Propose a hypothesis of added Langmuir sorption sites.
• Propose a hypothesis of local orientation of polymer chain segments.

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 63.3 GPU after standard PDMS calking; while the dense-skinned fibers displayed CO2/N2 selectivity of 52.5, and CO2 permeance of 16.6 GPU. 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.