Construction Of Multi-stage Fiber Composite Membrane Based On Carbon Nanotubes And Evaluation Of Membrane Distillation Performance

The composition and structure of membrane materials determine the selective separation performance of the membrane.Optimizing the material selection and structure of the membrane distillation(MD)membrane is expected to greatly improve the flux and stability of the MD process.In this work,two kinds of hierarchical fibrous composite(HFC)membranes were constructed based on carbon nanotubes.Carbon nanotubes(CNTs)were loaded onto the surface of the commercial porous hydrophobic membrane by the spray-coating process,and an HFC membrane was constructed,which was composed of dense CNTs nanofiber active layer and loose microfiber support layer.The effect and mechanism of CNTs on mass transfer and heat transfer of the c HFC membrane during MD were studied systematically.Finally,the structural characteristics and material composition of the ideal high-flux and anti-wetting MD membrane were proposed.The key research contents and the main results of this topic are as follows:(1)Fabrication of ac HFC(Asymmetric structure of carbon nanotube coating on HFC)membrane with asymmetric structure by unilateral spray-coating CNTs and evaluation of its desalination performance.The ac HFC membrane with asymmetric structure was fabricated by spray-coating multi-walled carbon nanotubes(MWCNTs)on the surface of the commercial porous hydrophobic membranes.The effects of CNTs loading,pore size,and material of pristine membrane on the surface structure and separation performance of the ac HFC membrane were investigated.The results show that there is an optimal matching relationship between the optimal CNTs deposition amount and the pore size and surface energy of the pristine membrane.The larger the pore size of the pristine membrane is,the lower the surface energy of the pristine membrane is,and the higher the CNTs deposition amount of the ac HFC composite membrane is when it reaches the highest flux.Compared with the polyvinylidene fluoride(PVDF)membrane and the polypropylene(PP)membrane,the deposition density of CNTs on the surface of the polytetrafluoroethylene(PTFE)membrane is the lowest,the specific surface area of the composite membrane is the largest,and the contribution to the flux improvement of the composite membrane is the greatest,so the PTFE membrane is more suitable for the preparation of high-performance ac HFC membrane.(2)Fabrication of the sc HFC(Symmetric structure of carbon nanotube coating on HFC)membrane with sandwich structure by bilateral spray-coating CNTs and evaluation of its desalination performance.In order to further improve the flux and stability of the MD membrane,on the basis of the ac HFC membrane,we further optimized its membrane structure.Using macroporous PTFE membrane as a support layer,sc HFC membrane with sandwich symmetrical structure was prepared by spray-coating and depositing the same amount of CNTs,on both sides of the sc HFC membrane.The results show that in the process of direct contact membrane distillation when the temperature difference is 40?,the permeation flux of sc HFC membrane can reach 52.3 LMH,and can run stably in MD for more than 50 h,which is significantly better than that of ac HFC.It was also found that the CNTs facing the low-temperature permeation side and the CNTs facing the high-temperature feed side could reduce the temperature difference polarization effect of the membrane and improve the anti-wetting performance of the membrane.In addition,the higher the feed temperature,the better the flux and stability of the sc HFC membrane than that of the ac HFC membrane and PTFE pristine membrane.In this paper,CNTs composite films with asymmetric and symmetrical structures were prepared by spray-coating CNTs on the surface of commercial porous hydrophobic membranes.It was found that the deposition of CNTs on the surface of c HFC films could increase the specific surface area and weaken the temperature polarization effect,and improve the anti-wetting performance of the films.At the end of this paper,the material composition and structural characteristics of the ideal highflux and anti-wetting MD membrane are pointed out,which provides guidance and reference for the preparation of high-performance MD membranes.