(82b) Hydrophobic and Hydrophilic Membrane for DCMD: Effect of Non-Solvent Additives and Operation Condition on the Performance

Dual-layer composite membrane is a new design for direct contact membrane distillation (DCMD) with membrane performance potentially superior to that of single layer porous membranes. Using dry-wet phase inversion technology, novel dual-layer hollow fiber membranes were fabricated in current research. The outer layer was made from polyvinylidene fluoride (PVDF) with polyvinylpyrolidone (PVP), glycerol or ethanol as non-solvent additive, while the inner layer consists of PVDF and polyvinyl alcohol (PVA) blend. Effects of PVA/PVDF blending ratio and non-solvent additive type on the morphological, mechanical and separation characteristics of the composite membranes were investigated. It was found that increasing the PVA content has no obvious effect on the macrovoids formation, but increase the pore size located inside the spongy section. The DCMD tests showed that the change in the composition of inner layer material led to the variation of the flux and rejection coefficient. The higher PVA content gave rise to higher hydrophilicity of inner layer, and hence better wettability by water, which was confirmed by elementary analysis. But enhancing the PVA content in the inner layer was related with lower flux and lower rejection. Considering the higher thermal conductivity coefficient of water, the decreasing trend of flux with improved inner layer hydrophilicity was attributed to the transport of water trapped inside the hydrophilic porous structure. The lower rejection was probably due to the relatively hydrophilic interface serving as the distillate side of the hydrophobic layer. It was also found that the pattern of the relation between flux/rejection and liquid flow rate were different between lumen side and shell side. The findings in current work suggests: 1) the surface energy of the inner layer is important to the performance of the dual-layer hydrophobic/hydrophilic DCMD membrane; 2) the transport resistance in the inner layer should not be neglected.