Crystallization behavior of salts during membrane distillation with hydrophobic and superhydrophobic capillary membranes

Superhydrophobic nano-composite polypropylene capillary membranes were fabricated for membrane distillation and crystallization by coating the surface with TiO2 and fluoro-silane compounds. The membrane distillation performance was investigated during direct contact membrane distillation and vacuum membrane distillation, and the crystallization behavior of these membranes was observed in real-time. Crystallization on hydrophobic and superhydrophobic membranes differed, with crystal growth occurring as isolated 'pillars' on hydrophobic membranes, while crystal deposition and growth remained largely uniform on superhydrophobic membranes. The uniformity of crystal deposition for both membranes increased with the introduction of humic acid to the feed due to the presence of a humic layer across the membrane surface. The rate of deposition was found to be lower for superhydrophobic membranes, and deposited solutes were more easily removed. Crystal nucleation and growth behavior were consistent when using flat sheet membranes, indicating similar crystallization mechanisms for both membrane modules. (C) 2014 Elsevier B.V. All rights reserved.