Novel porous poly (vinylidene fluoride) membranes for membrane distillation

Polyvinylidene fluoride (PVDF) membranes were prepared by water immersion of a binary PVDF-solvent solution cast on the outer surface of a porous tubular support. During immersion the free shrinkage of the cast solution was hindered by the support and consequently the nascent membrane was subjected to a stretching action that caused pore formation. A laboratory scale device was realized for casting operation and some preparative parameters were studied to control the deposition of the PVDF solution on the porous support. Membranes were characterized through scanning electron microscope observations, air permeability measurements, ultrafiltration tests and membrane distillation experiments. These latter were carried out with a laboratory unit fed with 3 w/v% aqueous NaCl solution at 69 degrees C. The membranes were housed in a tubular cell with the feed flowing on the shell side and the sweeping air in the membrane lumen. The effect of feed and gas velocity on the membrane performance was investigated and the obtained results were compared with those of commercial tubular membranes tested on the same unit. The influence of casting parameters on the morphology and permeation properties of the membranes was also studied.