Preparation Of Fluoropolymer Microporous Membrane And The Applications In Membrane Distillation

Membrane distillation is a emerging technology by the separation of steam-driven heat,and its commercial development has been limited by the use of membrane for distillation membranes.To date,there is still a lack of dedicated membranes developed for membrane distillation technology with high performance and no delamination.This dissertation proposed to use the nonsolvent thermally induced phase separation（NTIPS）method to fabricate a novel Janus membrane for MD applications.The as-prepared dual-layer membrane consisted of a thin hydrophobic polyvinylidene fluoride（PVDF）top-layer and a relatively thick hydrophilic fluoride-co-polyvinyl alcohol（PVDF-PVA）sub-layer.By adopting a facile one-step co-casting technique,delamination-free dual-layer membrane was obtained.The SEM morphologies and ATR-FTIR crystalline analyses suggested the membrane formation mechanisms,where the hydrophobic top-layer was formed via NTIPS process,resulting in an ultra-thin dense skin with finger-like pores formed beneath;while the hydrophilic sub-layer was induced by TIPS,producing highly porous cellular structure with high degree pore interconnectivity.In direct contact membrane distillation（DCMD）,the effect of different operating conditions on MD performance was tested using as-prepared PVDF/PVDF-PVA dual-layer composite membrane.It was found that the permeation flux increased with increasing flowrate;while the concentration of feed side has little effect on the flux in a certain range and the top-surface facing feed mode was the preferred orientation higher flux than the bottom-surface facing feed mode.As the concentration of the hydrophobic layer PVDF increases,the finger-like pores in the membrane structure becomes shorter and the flux decreases slightly.Coagulating bath temperature affects the mechanism of membrane formation,while has a significant effect on membrane structure changes and membrane flux.The balance between flux performance and mechanical strength was studied by adjusting the total membrane thickness.On the basis of this research,a PVDF/PVDF-PVA hydrophilic/hydrophobic double-layer composite membrane with a concentration of 15 wt%PVDF hydrophobic layer was prepared under a 20°coagulating bath temperature which exhibited stable salt rejection above 99.5%over continuous MD runs and superior permeation flux up to 165kg·m^-2·h^-11 at 80℃,and this result was remarkedly higher than reported MD membranes.Hydrophobic double-layer composite membrane was also used in industrial organic fertilizer concentration,and its contamination resistance were investigated.The permeation flux of the membrane is as high as 87 kg·m^-2·h^-11 at the feed temperature of 70℃.At the same time,deposits on the surface of the membrane are easily cleaned by washing water.Compared with commercially available polytetrafluoroethylene（PTFE）membranes,this membrane exhibits excellent contamination resistance over 15 hours of continuous operation.