Improving CO2/CH4 and O2/N2 separation by using surface-modified polysulfone hollow fiber membranes

Membrane technology for industrial gas separation was begun in the 1980s and has shown significant advantages over conventional techniques such as cryogenic distillation and solvent absorption in terms of energy consumption and operational cost. The main objective of this work is to develop a simple yet effective coating technique to modify the surface properties of polysulfone (PSF) hollow fiber membranes for enhanced separation of CO2/CH4 and O-2/N-2. The hollow fiber membranes made of different polymer concentration (15-35 wt%) were coated with different materials (polydimethylsiloxane (PDMS) or polyether-block-amide (Pebax) or PDMS followed by Pebax) through a simple dip-coating method before subjecting to pure gases testing and instrumental characterization. Results showed that the multilayer coated membranes exhibited better performance compared to the single-layer PDMS-coated and single-layer Pebax-coated membranes. It is also found that the Pebax concentration could affect the performance of the multilayer coated membranes in which the gas permeance decreased while its selectivity decreased with increasing Pebax concentration from 1 to 3 wt.%. Further increase in Pebax concentration from 3 to 9 wt% however negatively affected selectivities as high Pebax concentration tended to cover the membrane surface with tighter polymer chain packing which led to lower free volume. Experimental results also showed that the optimized multilayer coated membrane (3 wt.% Pebax) could achieve the highest gas pair selectivity, recording 34.28 and 6.65 for CO2/CH4 and O-2/N-2 separation, respectively. As a comparison, the membrane coated with 1 wt% Pebax only showed selectivity of 29.47 and 6.07, respectively. The findings of this work demonstrated that multilayered coating could be performed on the outer layer of hollow fiber membranes and is not limited to the flat sheet membranes as previously reported in the literature, provided a good combination of two coating solutions were used.