Optimization of nanocomposite membrane for vacuum membrane distillation (VMD) using static and continuous flow cells: Effect of nanoparticles and film thickness

In this study, the effects of incorporation of nanoparticles (NPs), including 7 nm TiO2, 200 nm TiO2, and hydrophilic and hydrophobic SiO2 with a mean diameter in the range of 15-20 nm, and their concentrations on the membrane properties and vacuum membrane distillation (VMD) performance were evaluated. The effect of membrane thickness and support materials were also investigated. The membranes were characterized extensively in terms of water contact angle, liquid entrance pressure of water (LEPw), scanning electron microscopy, surface roughness, and pore size. While the best polyvinylidene fluoride (PVDF) nanocomposite membranes with 200 nm TiO2 NPs were obtained at 2 wt% NPs concentration, the optimal NPs concentration was 5% when 7 nm TiO2 was integrated. Using a nanocomposite membrane containing 2 wt% TiO2 - 200 nm NPs, a VMD flux of 3.2 kg/m(2)h and a LEPw of 34 psi were obtained with 99.99% salt rejection. Furthermore, it was observed that decreasing the membrane thickness would increase the portion of the finger-like layer in the membrane and reduce the spongy-like layer when hydrophilic NPs were used. Using continuous flow VMD, a flux of 3.1 kg/m(2)h was obtained with neat PVDF membranes, which was higher than the flux obtained by the static VMD with the same membrane at the same temperature and vacuum pressure. The fluxes of both static and continuous flow cells VMD increased with temperature. Furthermore, it was evident that the continuous flow cell VMD at 2 L per minute yielded higher flux than the static cell VMD at any given temperature, indicating strong effects of turbulence provided in the continuous flow cell VMD.