Structural Optimization Of Hierarchical Nanofiber Hydrophobic Membranes And Evaluation Of Membrane Distillation Performance

Electrospun nanofibrous membrane(ENM)has been recently developed as a superior alternative candidate for membrane distillation(MD)owing to its unique advantages such as the high porosity,high specific surface area,high hydrophobicity and adjustable pore size.However,the membrane wetting and low water flux in MD are still the main obstacle that restricts the further application of MD membranes.It was proved thatHFC membrane structure with ultra-thin anti-wetting layer and microporous support layer is conducive to improving membrane flux.Based on this work,this study further optimized the design of MD membrane structure,so as to provide guidance and reference for the structural design of high-performance MD membranes.The major research focuses on the following:(1)Fabrication of interweaving hierarchical fibrous composite(iHFC)membranes for high-flux and robust direct contact membrane distillationWe fabricated the interweaving hierarchical fibrous composite(iHFC)membrane comprising the interconnected poly(vinylidene fluoride-co-hexafluoropropylene)(PH)nanofibers and polyethylene terephthalate(PET)microfibers via convectional electrospinning.The hydrophobic PH nanofibers provide an enhanced anti-wetting property and high salt rejection.The PET microfibers could significantly both lower the resistance of mass transfer and improve the heat insulation.The influences of PH/PET ratio and thickness of iHFC membranes on the structure were investigated,and the separation performance of the membrane was evaluated in direct contact membrane distillation(DCMD)system.Results show that the optimized iHFC membrane having the ratio of PH/PET 1.5/0.8 and membrane thickness 80 ?m exhibits both high permeation flux of 65 LMH and stable high performance over 3600 min when operated at 40°C temperature difference.Moreover,the interweaving structure endows the iHFC membrane great mechanical strength and excellent long-term stability.(2)Investigation of wetting phenomena in direct contact membrane distillation using asymmetric tri-layer hierarchical fibrous composite(tHFC)membraneAsymmetric tri-layer hierarchical fibrous composite(tHFC)membranes were fabricated via electrospinning and their long-term wetting behaviors in DCMD were investigated via tracing fluorescence dye diffusion inside the membrane by fluorescence microscope.The tHFC membrane is composed of the top thick and the bottom thin PH nanofibrous dense layers,and the interweaving PET/PH fibrous middle loose layer.The structural parameters of the tHFC membrane including pore size,porosity,thickness,water contact angle,liquid entry pressure(LEP),gas permeability,and thermal conductivity were comprehensively characterized.The impact of temperature and membrane orientation on LEP and membrane wetting behavior of tHFC membranes was comprehensively investigated.Results illustrate that membrane wetting can occur from both sides of tHFC membrane facing to either hot feed solution or cold stream when operated at DCMD.Membrane wetting develops faster at high temperature than at low temperature.Membrane orientation significantly affects the membrane wetting behavior for the asymmetric membrane in DCMD.This study provides a new perspective on membrane wetting and membrane orientation.According to the characteristics of MD process and the principle of mass transfer and heat transfer,the structure of MD membranes was further optimized based on the HFC membrane to improve the interfacial adhesion and structure stability.In addition,it was found that both the membrane orientation and the testing temperature had obvious influence on the membrane wetting for the tri-layer asymmetric membrane,which provided certain reference and guidance for the structure design of DCMD membrane in the future.