| Membrane distillation?MD?seawater desalination is a new membrane separation technology,which provides a potential high-efficiency and low-cost seawater desalination process.However,membrane fouling and membrane wetting are the key problems that hinder the practical application of seawater desalination by membrane distillation.the effective way to solve this problem is to construct superhydrophobic membrane surface.In order to improve the hydrophobic and mechanical properties of the membrane and reduce the production cost of the membrane at the same time,PVC and nano graphite were doped in the process of PVDF membrane preparation,and the flat composite membrane was prepared by non solvent phase separation method.The double-layer composite membrane was prepared by coating immersion phase separation method,and the hollow fiber membrane was modified.The self-cleaning experiment,membrane distillation desalination experiment and membrane distillation treatment of high concentration brine experiment were carried out by using the composite membrane.The main results are as follows:1.The PVDF/PVC composite membrane was prepared by non-solvent phase separation method.The effects of the ratio of PVDF to PVC,the thickness of the composite membrane,the type of casting agent,the concentration of casting solution,scraping temperature and solvent evaporation time on the hydrophobicity of PVDF/PVC composite membrane were investigated.The results show that the concentration of casting solution has the greatest effect on the hydrophobicity of PVDF/PVC composite membrane.When the concentration of casting solution is 4wt%,the contact angle of PVDF/PVC composite membrane can reach152o,and the surface energy is less than 8 mN·m-1,which is significantly higher than that of PVDF?90±3o?and PVC?75±3o?.The results of scanning electron microscope and atomic force microscope show that there are nano-sized crystal particles on the surface of the composite membrane,and other factors have little effect on the hydrophobicity of the membrane.2.By doping nano-graphite into PVDF/PVC,the effect of nano-graphite on the mechanical properties and hydrophobic properties of the composite film was studied.Due to the poor film-forming performance when the concentration of nano-graphite is4wt%PVDF/PVC,and the mechanical properties of PVDF/PVC composite film are better when the concentration of casting solution is 12wt%,so the concentration of nano-graphite is chosen to be 12wt%PVDF/PVC.The results show that nano-graphite doping accelerates the phase transition of the film and changes the crystal form of PVDF.The doping of nano-graphite induces the appearance of irregular particles on the surface of the film,which increases the surface roughness of the composite film,thus improving the hydrophobicity of the film.When the concentration of casting solution is 1.0wt%nano-graphite in the12wt%PVDF/PVC composite film,the contact angle of the film increases from 108oto 137o,and the surface energy is 18.97mN·m-1.Although the nano-graphite doped PVDF/PVC composite film maintains good mechanical properties,its hydrophobicity does not reach superhydrophobicity.3.PVDF/PVC bilayer composite membrane was prepared by coating-immersion phase separation method to enhance the mechanical properties of the composite membrane.The composite membrane prepared with the casting solution concentration of 12wt%PVDF/PVC was used as the base membrane,and then the PVDF/PVC coating with the casting solution concentration of 4wt%was coated at 40?.The contact angle of the PVDF/PVC bilayer film was up to 150.4o.Scanning electron microscope showed that the surface of the PVDF/PVC bilayer film was covered by cross-linked micro-nano microspheres.The results of mechanical properties show that the maximum tensile force of PVDF/PVC double-layer composite film is2.34N,which is 19.4%higher than that of PVDF/PVC base film?1.96N?.The preparation of PVDF/PVC bilayer films can improve the mechanical properties of the films to some extent and obtain the superhydrophobic properties of the surface coatings.4.In order to further improve the hydrophobicity and mechanical properties of the composite film,the PVDF/PVC base film was coated with PVDF/PVC coating with 4wt%nano-graphite casting solution at 40?.The contact angle of nano-graphite doped PVDF/PVC bilayer composite film can reach 153.7o,which is higher than that of PVDF/PVC bilayer composite film.The results of scanning electron microscope showed that the layer of micro-nano microspheres on the surface of nano-graphite doped PVDF/PVC bilayer films was more obvious,and the size of nano-microspheres on the surface of the films was smaller.The results of mechanical properties show that the maximum tensile force of nano-graphite doped PVDF/PVC double-layer composite film is 2.39N,which is 21.9%higher than that of PVDF/PVC base film?1.96N?.5.In the self-cleaning experiment,water droplets were added to the surface of the inclined film of sprayed graphite powder,and the surface cleaning of the film was observed.The results show that PVDF/PVC composite film,nano-graphite doped PVDF/PVC composite film,PVDF/PVC double-layer composite film and nano-graphite doped PVDF/PVC double-layer composite film all have good self-cleaning properties.After the self-cleaning experiments of composite films and double-layer composite films,the static contact angle is basically unchanged,but still shows good hydrophobicity,indicating that the four composite films have long-lasting self-cleaning properties.6.Using PVDF hollow fiber membrane as substrate membrane,PVDF/PVC coating with4wt%casting solution and PVDF/PVC coating with 4wt%nano-graphite were coated on the surface of substrate membrane by coating-immersion phase separation method,and their hydrophobic properties were tested.The results showed that the contact angle of PVDF/PVC coating with 4wt%casting solution was 149.9o.The contact angle of the hollow fiber membrane coated with 4wt%PVDF/PVC coating solution doped with nano-graphite is 151o,which keeps good hydrophobicity.The results of scanning electron microscope showed that the micro-nano microspheres of hollow fiber membrane coated with 4wt%PVDF/PVC coating solution doped with nano-graphite were more obvious and the surface was rougher.Therefore,the hollow fiber membrane coated with PVDF/PVC coating with 4wt%nano-graphite-doped casting solution was selected as the membrane for membrane distillation experiment.7.The effects of PVDF hollow fiber membrane and surface modified PVDF hollow fiber membrane on permeability,membrane surface fouling and thermal efficiency were compared between?SGMD?seawater desalination experiment and treatment of high concentration brine?333K saturated NaCl solution?.The results of scanning electron microscope showed that the membrane fouling of surface modified PVDF hollow fiber membrane was less in the process of seawater desalination by membrane distillation and the treatment of high concentration salt water,and the permeation flux of surface modified PVDF hollow fiber membrane was higher than that of PVDF hollow fiber membrane after running for about 13 hours in the process of seawater desalination by membrane distillation,and that of surface modified PVDF hollow fiber membrane was higher than that of PVDF hollow fiber membrane after running 60min or so during the treatment of high concentration brine.Compared with the thermal efficiency,the thermal efficiency of surface modified PVDF hollow fiber membrane in the process of seawater desalination by membrane distillation and the treatment of high concentration salt water was 49.2%and 47.96%respectively,which was 20.9%and 36.75%higher than that of PVDF hollow fiber membrane.That is to say,the surface modified PVDF hollow fiber membrane can improve the effect of treating seawater and high concentration brine. |
PDF Full Text Request