Fabrication Of PVDF Composite Membranes And Their Application In Vacuum Membrane Distillation

In this paper, polyvinylidene fluoride (PVDF) was used as membrane material to make the composite membrane and characterize it. The membrane is used in vacuum membrane distillation. The influences of the main factors were investigated which including polymer concentration, additive concentration, coagulate liquid composition and casting temperature on membrane structure and properties. The factors were observed which including the temperature of feed, the flow rate of the feed, the vacuum degree of cold side and the concentration of feed. And the pollution and cleaning of membrane were studied preliminarily. It provides the technical support for industrial applications of VMD.With the increase of polymer concentration, LiCl concentration and DMAc concentration in coagulation bath, the formation of the membrane surface is more compact. When added the amount of H2O and the casting temperature, the obtained membranes had a porous skin. The max membrane flux can reach at19.93kg/m3h with the solution of12wt.%PVDF concentration,5wt.%LiCl concentration,2wt.%H2O concentration and25℃temperature. The porosity is30%. The membrane has excellent structural performance. Based on the hydrophobic membrane, the composite membrane is made. The porosity of composite membrane is64%. And the surface contact angle is137degrees. The hydrophobic performance is good. The bursting strength of composite membrane is5-7times than the PVDF membrane. It can reach0.4-0.5MPa, improve the tensile strength is10-16times than before. It can reach20-28MPa, It’s suitable for the application in membrane distillation experiment.In VMD, the flux of membrane increased sharply with the increase of the vacuum degree of the cold side, the flux, the temperature of feed. When the concentration of feed increased, the flux decreased. In the vacuum degree of0.06MPa, hot side of circulating flow rate of5.6L/h, and the temperature of hot side is65℃. The flux of composite membrane can reach22.66kg/m h. The effluent conductivity is6.82μS/cm. The rejection rate is above99.7%. To compare with the PVDF hydrophobic membrane, the composite membrane water flux is decreased slightly, but the porosity is substantially improved. The water quality is20μS/cm. And the salt removal rate can reach99%. The composite membrane has excellent performance.After period of time, membranes were contaminated. The downward trend of membrane flux is more obviously. Using different formulas clean the membranes which had been contaminated, experiment testify that cleaning effect was optimization when using0.01mol/L NaOH and2%citric acid. The pollutants of composite membrane can be removal rapidly and effectively. So the flux of membrane can recover at97%.Handle the high salt waste water by vacuum membrane distillation which used polyvinylidene fluoride (PVDF) flat membrane. The conductance of production water is20μ S/cm. The reject rate reaches99%. The practicability and treatment effect is good.