Preparation And Characterization Of The Electrospinning Composite Membranes For Membrane Distillation

The current study aimed at solving the low flux and poor hydrophobicity of the hydrophobic membranes applied in the membrane distillation, the preparation process of electrostatic spinning of nano-fiber membranes was investigated. The preparation parameters and spinning solution component were improved and the effect of support materials and nano-particles addition to the electrospinning nano-fiber membranes were investigated. The prepared membranes were applied to the membrane distillation(MD) process. The results showed that:(1) The research on different materials of the membrane supporting layer indicated that the looser the support layer was, the higher the gas flux and permeability were. The electrospinning polyvinylidene fluoride(PVDF) membranes spined on all supporting materials were high hydrophobic and the highest contact angle was 149.5°, and it completely meet the hydrophobic requirement in MD process. The electrospinning membranes had high porosity and the highest porosity was up to 92.6%. The diameter of the nano-fibers and mean pore size increased with increasing the thickness of supporting layers in the experiment. In the MD process, the highest permeate flux of the electrospinning composite membrane was up to 50.29 kg/(m2·h) with 60°C temperature difference between both sides of membrane. All of the membranes had excellent salt rejection, higher than 99.98%.(2) The effect of SiO2 nano-particles to the structure and performance of the electrospinning poly(vinylidene fluoride co- hexafluoropropylene)(PVDF-HFP) membranes was investigated. The electrospinning nano-fiber membranes with different concentration of PVDF-HFP had great hydrophobicity and porosity were higher than 88%. The permeate flux of the membranes decreased with the increase of PVDF-HFP concentration. With increasing the amount of SiO2, the membrane thickness increased and the porosity decreased. All the electrospinning membranes had high hydrophobicity with contact angle higher than 140°. The mean pore size increased at first and then decreased with increasing the amount of SiO2, but the permeate flux of the membranes were on the contrary. When the content of SiO2 increased to 10%, the most concentrated pore size distribution and the highest permeate flux was get.(3) Moreover, the separation surface membrane of PVDF-HFP and PVDF-HFP/SiO2 respectively on the top of the PVDF membrane through the electrostatic spinning technology were constructed. The structure and performance of the dual-layer composite membranes were investigated. Both of the nano-fiber membranes had great hydrophobicity with contact angle of about 140°. The thickness of the dual-layer composite membranes increased with increasing the spinning time. The porosity of the electrospinning layer of all composite membranes was higher than 87%. The mean pore size and the flux decreased with increasing the spinning time. The MD experiment indicated that both of the dual-layer membranes had excellent salt rejection.(4) The experiment of the stability of the electrospinning nano-fiber membranes in MD process was carried out. The results indicated that the PVDF membrane had the highest permeate flux, however, the flux kept decreasing in the first 48 h and conductivity of product water increased obviously at the later period, but the salt rejection of the membrane reached to 99.97%. The membrane flux of PVDF-HFP membrane was lower but more stable. The PVDF-HFP/SiO2 membrane with 10% SiO2 contend showed that the permeate flux stabilized after a 10% decrease at the beginning, and the salt rejection was 99.99%.