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.