Fabrication of defect-free Matrimid® asymmetric membranes and the elevated temperature application for N2/SF6 separation

N-2/SF6 mixtures have been widely used in gas-insulated equipment. Highly efficient separation approaches are urgently required to recycle SF6 from the mixtures during equipment maintenance. Gas separation with glassy polymeric membranes is regarded as a promising approach due to the great size difference between SF6 and N-2 molecules. However, the common membranes are quite low permeable for N-2/SF6 mixtures. In this research, the dry-wet phase inversion was customized with sectionalized air gap, i.e., dried air at upper section and humidified air at lower section, to manufacture Matrimid (R) asymmetric membranes in bulk with an ultrathin and defect-free selective layer. After optimization, the defect-free selective layer could be thinned to 80 nm. The pure gas test at 25 degrees C revealed that J(N2) was increased to 2.58 GPU, and alpha(N2/SF6) was approximately 41, very close to the value in the literature [J. Membr. Sci., 2014, 452, 311]. Without the restriction from the usual PDMS coating layer for defect-blocking, the elevated temperature operation was able to be employed to further enhance gas permeation. At 128 degrees C, the selectivity alpha(N2/SF6) was maximized to 115, and the relevant J(N2) is 9.0 GPU. At 200 degrees C, the temperature close to the allowable upper limit, J(N2) was increased to 15.9 GPU, and the relevant alpha(N2/SF6) was 97. Furthermore, the mixed-gas test demonstrated the favorable long-term stability of the customized membranes, even under the extreme running condition with pressure up to 2.0 MPaG and temperature up to 200 degrees C. With high efficiency and throughput under elevated temperature operation mode, the Matrimid (R) asymmetric membrane with ultrathin and defect-free selective layer is a promising approach for the separation and recycle of SF6 from the N-2/SF6 mixtures.