Influence of morphology of PVDF capillary membranes on the performance of direct contact membrane distillation

The performance of hydrophobic poly(vinylidene fluoride) (PVDF) capillary membranes was evaluated using the direct contact membrane distillation (DCMD) process. The composite capillary PVDF membranes with an admixture of polytetrafluoroethylene (PTFE) particles and a flat PTFE membrane were also investigated. The influences of membrane morphology and hydrophobicity of membrane materials (water contact angle) on the membrane wettability were examined. The SEM investigations revealed that the capillary membranes had sponge-like or mixed sponge-/finger-like structures. The long-term DCMD investigations demonstrated that the amount of wetted pores was gradually increased during the process and, as a result, the process efficiency was significantly decreased. After 1000 h of the DCMD process, the permeate flux decreased by more than a half for the majority of investigated membranes. In spite of that, the salt rejection did not deteriorate, which indicated that the pores located mainly on the membrane surface were wetted. A slower rate of wettability was observed for the membranes with sponge-like structure. The incorporation of PTFE particles into the polymer matrix enhanced the hydrophobicity of the PVDF membrane (water contact angle increased from 88 to 103°), and as a consequence, the membrane wettability was also retarded. Moreover, it was found that the diameter and wall thickness of the investigated capillary membranes changed by 10-20% during the DCMD process.