Preparation of Hyflon AD60/PVDF composite hollow fiber membranes for vacuum membrane distillation

Hydrophobic membrane is desired to avoid pore wetting phenomenon in membrane distillation (MD). In the present paper, highly hydrophobic Hyflon AD60/PVDF composite hollow fiber membranes were prepared by coating a copolymer (Hyflon AD60) of tetrafluoroethylene (TFE) and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) on poly(vinylidene fluoride) (PVDF) membranes. The effects of Hyflon AD60 concentration, coating time, heat-treatment temperature and heat-treatment time on the structures and performances of Hyflon AD60/PVDF composite hollow fiber membranes in vacuum membrane distillation (VMD) process were investigated. The results show that the hydrophobicity of PVDF hollow fiber membrane was significantly improved by coating Hyflon AD60. The optimized Hyflon AD60 concentration, coating time, heat-treatment temperature and heat-treatment time were 0.1 wt%, 10–20 min, 40–50 °C and 9 h, respectively. Employing NaCl solution with concentration of 35 g × L⁻¹, under a vacuum of 0.09 MPa and feed temperature of 70 °C, the water flux and salt rejection of the composite hollow fiber membrane in VMD process reached to approximately 10 kg × m⁻² × h⁻¹ and higher than 99.9 %, respectively, and kept stable, indicating that Hyflon AD60/PVDF composite membrane is an excellent candidate for operation in VMD operation.