An investigation to revitalize the separation performance of hollow fibers with a thin mixed matrix composite skin for gas separation

Hollow fibers with a thin zeolite beta-polysulfone mixed matrix selective layer and enhanced selectivity for He/N2 and O2/N2 separation have been developed in this study. It is believed that the heat treatment and the coating processes combine to bring out the separation properties of zeolite beta imbedded in the polymer matrix. The fibers were initially formed by using the dual-layer co-extrusion and dry-jet wet/wet spinning technology. Owing to the loose nature of the mixed matrix skin, the as-spun fibers show Knudsen diffusion even after silicone rubber coating. The effectiveness of heat treatment and surface coating in healing the hollow fiber has been investigated. SEM pictures show that heat treatment at above Tg can significantly densify the loose mixed matrix layer and produce hollow fibers with a thin mixed matrix selective layer of around 1.5–12 μm. However, heat treatment at above Tg does not necessarily produce defect-free hollow fiber membranes. Three types of silicone rubber coatings were applied and compared. It is found that, after a two-step coating, the ideal selectivity of mixed matrix fibers increases to surpass that of hollow fiber with a neat polysulfone outer layer. The sealing of the mixed matrix outer layer instead of the caulking of the inner layer is confirmed to be the cause for the selectivity increment. The reaction between the two chemicals in the two-step coating is contributive to the efficiency of this coating method. In addition, it is also observed that the thickness of the mixed matrix composite outer layer influences permeance and selectivity. The activation energies (Ep) for O2 and N2 calculated from the permeance or permeability at different temperatures indicate that the addition of zeolite beta in the polysulfone has reduced the energy barrier for O2 permeation.