| Membrane distillation technology has attracted more and more researchers'attention due to its advantages of low energy consumption,good separation effect,simple operation and easy control.Due to the demand for high membrane distillation flux,the Janusmembrane with high mass transfer and low heat transfer has been intensively researched and developed.The preparation methods of Janusmembrane can be roughly divided into layer-layer preparation method and chemical asymmetric modification method,but most of the preparation methods have disadvantages of complex process and high cost,which hinder practical application.In this paper,enhanced hydrophilic/hydrophobic Januspolyvinylidene fluoride(PVDF)composite membranes were prepared by the solution phase inversion method(NIPS)with the advantages of simple operation process and easy structure control.The performance of direct contact membrane distillation(DCMD)was investigated.First of all,the hydrophilic/hydrophobic JanusPVDF flat composite membrane was prepared by using PVDF as the membrane forming polymer and coating PVDF casting solution on the surface of the hydrophilic cellulose non-woven fabric.The influence of the content of dioctyl phthalate(DOP)in the PVDF casting solution on the structure and performance of the flat composite membrane was studied.The results showed that the increase of the DOP content induced the phase separation process of PVDF casting solution,which further brought about the promotion of membrane porosity and pore size,liquid entrance pressure and breaking strength gradually decreased.The obtained hydrophilic/hydrophobic JanusPVDF flat composite membrane was used in the DCMD process,and the influence of the process parameters on the feed side and permeate side on the DCMD process and the stability test were mainly studied.The results showed that the hydrophilic layer in the flat composite membrane can effectively promote the mass transfer and condensation of water vapor in the DCMD process,thereby increased the membrane distillation flux.Under the same DCMD conditions,the flux of Janusmembrane was 28.39%higher than that of pure PVDF membrane,and the salt rejection were maintained above 99%.Increased the feed temperature,feed flow rate,permeate flow rate,and reduced the permeate temperature can effectively increase the membrane distillation flux of DCMD.Janusflat composite membrane maintained a stable permeate flux(10.25 kg·m-2·h-1)within 14 hours of operation,which proved that JanusPVDF flat composite membrane had good DCMD performance stability.Secondly,hydrophilic polyvinyl alcohol(PVA)and hydrophobic polyester(PET)fibers with different proportions were woven into hollow braided tubes by two-dimensional braiding technology,and the enhanced hydrophilic/hydrophobic JanusPVDF hollow fiber composite membrane was prepared by concentric dry-wet spinning technology.The effect of the ratio of PVA/PET fiber in the hollow braided tube and the membrane forming process on the performance of JanusPVDF hollow fiber composite membrane were systematically studied.The results showed that with the reduce in the proportion of PVA fiber filaments in the hollow braided tube,the water absorption of the braided tube gradually reduced from 340.54%to 272.97%,and the thickness of the separation layer of composite membranes increased from283.2?m(PVA braided tube in the inner membrane)up to 342.5?m(PET braided tube in the inner membrane).The performance of JanusPVDF hollow fiber composite membrane in DCMD process was further studied.The results showed that in the DCMD process,increased the feed temperature,feed flow rate,permeate flow rate and reduced the permeate temperature can effectively increase the membrane distillation flux.Under the same DCMD conditions,the membrane distillation flux of JanusPVDF hollow fiber composite membrane was 39.56%higher than that PVDF hollow fiber membrane,and the salt rejection were maintained above 99%. |
PDF Full Text Request