Research On Membrane Formation Mechanism And Morphology Regulatory Of HDPE/EVA Co-blending Hydrophobic Microporous Membrane

Membrane distillation(MD)is a kind of new membrane technology,which use hydrophobic microporous membrane as the medium,and separate by the difference vapor pressure on both sides of the membrane.Compared with other separation technology,it has advantages of high interception rate,low energy etc.It has broad prospects in seawater desalination,pure water preparation,and heat-sensitive material enrichment.In this paper,we blend hydrophobic microporous membrane by HDPE and EVA via TIPS to improving blend membrane stability in VMD process,and improving the efficiency and high separation performance of VMD.Firstly,we determined thermodynamic non-equilibrium phase diagram of HDPE/EVA blend system,discussed he thermodynamic mechanism of phase separation and the crystallization kinetics of phase separation in this system.The results showed that:The non-equilibrium thermodynamic phase diagrams of HDPE/EVA-Phenyl ether co-blending polymer/diluent system were typical weak intreraction system phase diagram.The additional of EVA promoted the crystallization process of HDPE/EVA co-blending system by L-L phase separation system,and produced aperture large cell structure.Secordly,based on the above theory results,effect of the phase separation thermodynamic and kinetic parameters such as polymer weight ratio,polymer concentration and cooling bath temperature on the morphology and structure of membranes were discussed.The decrease of the EVA content in the blending system,the phase separation of HDPE/EVA blend membrane from L-L phase separation to S-L phase separation structure and L-L phase separation coexisting phase.The structure of microporous membranes become cellular by dendritic.The increase of polymer concentration also makes the S-L phase separation advantage is more obvious,and the membrane pore size is smaller.When the co-blending membrane thickness is 125?m,unusually large hole will not produce which because of the incomplete deaeration and dust.The stability of membrane used for VMD increased significantly.The best membrane preparation conditions of HDPE/EVA hydrophobic microporous membrane were obtained as follows.The HDPE/EVA polymer weigh ratio was 3:1,the polymer concentration was 30%,the co-blending membrane thickness was 125?m,the scraping speed was 1.5 m·min-1,and the cooling bath temperature was 25?.At this point,the appearance of HDPE/EVA co-blending hydrophobic membrane is good,and the internal structure is like cellular.It also have good hydrophobic performance and mechanical properties,porosity up to 68.8%.Finally,the co-blending hydrophobic membrane applied in VMD process.The effect of polymer weight ratio,polymer concentration,feed temperature and feed velocity on VMD process were discussed.The biggest permeate flux of HDPE/EVA hydrophobic microporous membranes were 13.01 kg·m-2·h-1,when the weigh ratio was 3:1.At the same time,with the increase of the polymer concentration,the permeate flux of co-blending hydrophobic membrane was falling.When the polymer concentration was 20wt%,the permeate flux increase to 24.67kg·m-2·h-1,and When the polymer concentration was 30wt%,the permeate flux increase to 20.49kg·m-2·h-1.It has poor mechanical property and short stable operation time,so we choose the 30wt%of polymer concentration.Because of the effect of temperature polarization and concentration polarization,the permeate flux of co-blending hydrophobic membrane was growthing by feed temperature and feed velocity.The best membrane preparation conditions of HDPE/EVA hydrophobic microporous membrane were obtained as follows:the HDPE/EVA polymer weigh ratio was 3:1,the polymer concentration was 30%,feed temperature was 85?,feed velocity was 30L·h-1,0.095MPa vacuum degree,the permeate flux of HDPE/EVA hydrophobic microporous membranes were 20.49 Kg·m-2h-1?The co-blending hydrophobic membrane could be stable operation in 30h.The rejection rate of VMD prcesses were all above 99.99%.