Preparation and properties of flat-sheet membranes from poly(vinylidene fluoride) for membrane distillation

The flat-sheet membranes from poly(vinylidene fluoride) were prepared by the phase inversion process. The effects of the casting solution composition, exposure time prior to coagulation and temperature of the coagulation bath on properties of prepared membranes were investigated. LiCl was used as a modifying agent. The membrane structure was studied by scanning electron microscopy. For all prepared membranes an asymmetric structure, sometimes without a dense skin layer, was observed. The porosity of prepared membranes before the drying process varied from 72-88%. After drying the membranes become hydrophobic. A contact angle of water droplet on the membrane surface was 107 degrees. The nitrogen permeability varied from 12-2,205 m(3)/m(2) d, depending on the preparation conditions. The maximum pore size, LEP(W) and mechanical properties were also determined. The membrane distillation process of 1-2% aqueous NaCl solution was applied as a final test of membrane performance. The permeate flux up to 233 dm(3)/m(2) d was achieved at the temperature of the feed and permeate of 333 K and 293 K, respectively. A chloride elimination in the permeate higher than 99% was reached.