Molecular design of the morphology and pore size of PVDF hollow fiber membranes for ethanol-water separation employing the modified pore-flow concept

In this study, we have established the fundamental science and engineering of fabricating poly(vinylidene fluoride) (PVDF) asymmetric hollow fiber membranes for ethanol-water separation and elucidated the complicated relationship among membrane morphology, pore size, pore size distribution and separation performance using the concept of the modified pore-flow model proposed in our previous work. The variation of bore-fluid composition, air-gap distance and take-up speed results in membranes with various morphologies ranging from large-finger-like macrovoid to nearly perfect macrovoid-free structures. Interestingly, an increase in air-gap distance or take-up speed not only effectively suppress the formation of macrovoids but also results in the reduction of membrane pore size and the narrowing of pore size distribution, hence leading to the enhancement of membrane performance. The permeation flux is found to be mainly controlled by the overall porosity and the contribution of large pore sizes of the membrane, while the selectivity or separation factor is greatly determined by membrane pore size and pore size distribution, which is consistent with the modified pore-flow model proposed in our previous works. The newly developed PVDF asymmetric hollow fiber membranes demonstrates remarkable high fluxes of 3500-8800gm^-2h^-1 and reasonable ethanol-water separation factors of 5-8 compared to existing polymeric-based pervaporation membranes.