The PVDF Anti-wetting Membrane For Membrane Distillation Is Prepared Through The Synergistic Effect Of Water And Steam

Membrane distillation?MD?is a thermally-driven separation process,in which only vapor molecules could pass through the porous hydrophobic membrane.This separation process is driven by the vapor pressure difference existing between the porous hydrophobic membrane surfaces.However,membrane wetting is a problem that must be solved in the application of hydrophobic membrane processes such as membrane distillation.In order to reduce the risk of membrane wetting,it is very important to improve the hydrophobicity of membrane surface and obtain appropriate pore size.For the above objective,this study focus on the preparation of high hydrophobicity and LEP value membrane for membrane distillation.The main research contents and results as follows:The PVDF hydrophobic membrane was prepared by NIPS,and the effects of exposure time and the content of additive H₂O on the membrane structure and performance were investigated.The exposure time was 5 s,30 s,60 s and 120 s by fixed H₂O content,and the additive H₂O content in the dope solution was 0 wt%,0.83 wt%,1.65 wt%,2.45 wt%,3.24 wt%and 4.28 wt%by fixed exposure time.When the exposure time was 60 s and the H₂O content of dope solution was 4.28 wt%,PVDF membrane with high hydrophobicity and suitable pore size can be obtained.The water contact angle on the top surface of the membrane reached 150.1°,the mean pore size of the membrane was 128.3 nm,the LEP value of the membrane was 113.0 k Pa,and the membrane distillation flux was 33.6 kg/m²?h,which was 1.69 times of the original membrane without H₂O in dope solution.The stable operation time of membrane distillation was 41.6 times of the original membrane without H₂O in dope solution.Through testing the quality of the liquid membrane and testing the cloud point of the dope solution,combined with XRD,FTIR and DSC tests,it was found that after adding the additive H₂O to the dope solution,synergistic effect of the H₂O in the dope solution and the vapor in the air resulted in the gelation of the liquid membrane surface,so that the liquid membrane surface was fixed before immersion in the coagulation bath.At the same time,high viscosity due to high H₂O content was benefit for S-L demixing and PVDF molecular chain crystallization.Therefore,through the synergistic effect of vapor in the air and H₂O in the dope solution,an open network structure was constructed on the membrane surface,which improved the hydrophobicity of the membrane surface.