Flux enhancement in membrane distillation by fabrication of dual layer hydrophilic–hydrophobic hollow fiber membranes

For the first time, co-extrusion was applied for the fabrication of dual layer hydrophilic–hydrophobic hollow fibers especially for the direct contact membrane distillation (DCMD) process. The effect of different non-solvents on the morphology of the PVDF membranes was investigated and it was found that weak coagulants such as water/methanol (20/80, w/w) can induce a three-dimensional porous structure on PVDF membranes with high surface and bulk porosities, big pore size, sharp pore size distribution, high surface contact angle and high permeability but rather weak mechanical properties. Hydrophobic and hydrophilic clay particles were incorporated into the outer and inner layer dope solutions, respectively, in order to enhance mechanical properties and modify the surface tension properties in the membrane inner and outer layers. Different membrane characterizations such as pore size distribution, gas permeation test, porosity and contact angle measurements were carried out as well. Ultimately, the fabricated hollow fibers were tested for the DCMD process and flux as high as 55 kg/(m2 h) at 90 °C was achieved in the test. This performance is much higher than most of the previous reports, indicating that the application of dual layer hydrophilic–hydrophobic hollow fibers may be a promising approach for MD.