Study On Fabrication And Functional Properties Of PVDF Electro-spun Hydrophobic Nanofiber Membranes For Membrane Distillation

In this study, the novel polyvinylidene fluoride(PVDF) electro-spun nanofiber hydrophobic membranes based on the support of non-woven fabric were prepared by electrospinning process to improve the flux and hydrophobicity. During 35g/L NaCl solution desalination process through direct contact membrane distillation(DCMD), the experimental results showed that the fabricated membrane had high permeate flux and excellent hydrophobicity. The following conclusions can be drawn:(1)The effects of different solvent ratio on the surface tension, the viscosity and the conductivity of the polymer dope were investigated. It was found that the addition of acetone can cause the decline of surface tension, the viscosity and the conductivity of the dope, reduce the number of beads in the obtained membrane and increase the fiber diameter and the membrane thickness. The membrane porosity reached 83% when DMF and acetone were mixed at a ration of 9:1, and the contact angle of membrane can reache as high as 142.8°.(2)With the mixed ratio of DMF and acetone at 9:1,the effects of polymer concentration on the properties of spinning dope and the morphology and the characteristics of the resultant membranes were investigated in detail. It was found that PVDF content increasing declined the surface tension, the conductivity of the dope, reduced the number of beads in the membrane and increased the fiber diameter. In the DCMD test, for the membrane prepared from 12wt% PVDF polymer dope, the permeate flux can reach 62.38 kg/m2 h with the hot feed temperature and cold distillate temperature of 80 ℃ and 20 ℃, respectively, the permeate conductivity was as low as 8 μS/cm, and the membrane salt rejection was more than 99.99%.(3) The effects of different operation parameters of spinning machine on the morphology and the properties of the PVDF electrospun nanofibers hydrophobic membrane were also investigated. It was found that when the voltage was as low as 16 kV, the flow rate of polymer dope would decline because of plaque formation for a lack of enough electric field power; improving the voltage, the fibers would became denser, but it would not cause great difference on the membrane hydrophobicity; an increase of polymer dope flow rate would result in the increase of the fiber diameter and the membrane whole thickness. The receiving distance hardly affected the morphology of the resultant membrane, while a farther receiving distance would make the fibers less dense. Higher the rotational speed was, lower the permeate flux of the membrane would be, and the appropriate rotational speed in the experiment range was about 10 r/min.The stability experiment was continuous running for 150 h, after which the flux of MD has descreased 30%, but the conductivity of permeate wat er was lower than 15.After three times of successive MD concentration and washing experients, the initial flux of ENHMs showed high recovery rates, reached 98% and 96%, respectively.