Research On VMD Process Based On Rough Surface Membrane

Membrane distillation(MD),a combination of membrane technology and traditional distillation processes,is a low-energy and high-rejection process of liquid separation.The process was based on hydrophobic microporous membrane as a barrier,the hot side of the material to be treated was in contact with the hydrophobic membrane,and the volatile components were transferred to the other side of the membrane after vaporization,so that the separation effect was achieved.In the recent years,preparations of high properties porous membrane have been more and more.Especially,increasing attentions have been absorbed on preparation of highly hydrophobic or superhydrophobic membranes.At present,the research on membrane distillation process mainly was focused on the influence of operating parameters and membrane thickness,pore size and other structural parameters on the process,and the study ofthe surface structure of hydrophobic microporous membrane during membrane distillationhas not been reported.In this paper,the vacuum membrane distillation(VMD)process of low concentration brine was studied,the effects of membrane surface structure parameters,membrane thickness,porosity,pore size and tortuosity on VMD process flux were investigated,establish the corresponding transfer process model.VMD transfer process was divided into four steps:(1)Convective transfer in the boundary layer.For different flow states,the convective transfer coefficient was obtained in different states.(2)The volatile components were vaporized on the surface of the rough membrane.This process was the control step for the total transfer process.The average width of microscopic bulge was introduced as the membrane surface structure parameters,the vaporized liquid on the surface of the rough film wsa subjected to micro-treatment.The evaporation mass transfer rate of the droplets on the surface roughness of the membrane have been obtained.(3)The vapor transmembrane transfer.There were three basic transfer mechanisms of the gas transfer in the process of transmembrane mass transfer,and itwas found that they can not be neglected.Therefore,the dusty-gas model(DGM)is introduced to deal with the rational relationship between the Kundsen diffusion,molecular diffusion and viscous flow transfer mechanism.The transmembrane mass transfer coefficient was obtained.(4)The vapor was collected on the downstream side of the membrane.The pressure of downstream side of membrane was low,mass transfer resistance was small,could be directly ignored.Under the condition of certain operating conditions,the hydrophobic membrane with different rough surface structure was used for VMD experiment,and the influence of membrane surface structure parameters on VMD process was obtained.The flux of VMD process was increased first and then decreased with the decrease of the average width of the surface of the rough membrane.When the average width of the bulge was 15?m and the contact angle was 150°,the flux was the largest.Experimental results verified the reliability of the model and determined the model parameters.