Effect of spinning conditions on the structure and performance of hydrophobic PVDF hollow fiber membranes for membrane distillation

Microporous hydrophobic poly(vinylidene fluoride) (PVDF) hollow fibers were prepared via dry-jet wet phase-inversion method using N,N-dimethylacetamide (DMAc) as solvent, LiCl and PEG-400 as non-solvent additives in the polymer dopes. The effects of various preparation conditions, including the concentration of polymer and additives, bore liquid temperature, air gap, take-up speed and dope extrusion rate on the morphology and properties of membrane were studied. The prepared membranes were characterized through scanning electron microscopy (SEM) observation, gas permeation measurement, and tensile property test. The permeate flux of membrane in both vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD) for desalination was tested. It is found that the permeation property of membrane is mainly determined by membrane porosity, especially effective porosity. The formation of the sponge structure reduced VMD flux more fiercely than DCMD flux. Under the synergetic effect of PEG-400/LiCl, high permeate flux and relatively high mechanical strength of membrane can be simultaneously achieved. There exist the best ranges of bore liquid temperature and of air gap distance for relatively high permeate flux of membrane.