Fabrication And Manipulation Of Superhydrophobic And Anti-fouling Membrane For Seawater Desalination

Membrane distillation?MD?seawater desalination is a new technology which is driven by thermal energy.The desalination process relies on the selection of permeable membranes to produce water.In the process of operation,pollutants would degrade the performace of the membrane itself,resulting in the reduction of the water yield of the system.Therefore,the performance of the membrane is the key to ensure the efficiency and stable operation of membrane distillation.Generally speaking,the membrane used for membrane distillation should meet the following conditions:?1?low mass transfer resistance to meet the requirements of water yield;?2?low thermal conductivity to reduce heat losses;?3?superhydrophobicity to prevent leaking.In addition,the membrane should have good thermal stability and good anti-fouling properties in order to achieve efficient and stable operation of seawater desalination.In the literature,modification of the film itself for a low thermal conductivity and the resulted reduction of the energy consumption in seawater desalination have not yet been reported.The hydrophobic membrane with pollution resistance for real water desalination has not yet been reported.These problems seriously affected the commercial application of membrane-type desalination.In addition,the membrane-type seawater desalination technology is still in the experimental stage.When the system runs for a long time,the salt may reach the saturation on the membrane surface and precipitate out,and then block the membrane pores and reduce the membrane flux.Therefore,it is very important to optimize operating parameters and heat and mass transfer in the membrane components.To address above problems,this paper focuses on the following aspects:Firstly,a composite PVDF/SiO₂ membrane with low thermal conductivity for seawater desalination is fabricated.The thermal conductivity of the membrane decreased from 0.08199Wm^-1K^-1to 0.04487 Wm^-1K^-1,but the permeability only decreased slightly.The performance of the composite film is better than that of the doped membrane made with aerogel powder reported in the literature.In addition,the composite membrane can further improve permeability by reducing the thickness of SiO₂ aerogel membrane.Secondly,a graphite oxide?GO?-polyvinylpyrrolidone?PVP?/PVDF membrane with high moisture permeability and high hydrophobicity was prepared by vacuum filtration.Because of the excellent selective permeability and excellent mechanical strength of graphene,as well as the greatly reduced thickness of the skin layer,it is possible to apply these membranes in the desalination process.Thirdly,membrane fouling caused by wetting of membrane pores is the main reason that hinders the wide application of membrane separation in the industry.Under the background of this research,the mechanism and causes of membrane wetting,the preparation of anti-fouling membranes,the influences of operating conditions on film properties are analyzed in detail.A model for superhydrophobic surface in mass transfer is built and is used to analyze the effects on permeation and anti-fouling properties.It is found that the superhydrophobic structure can alleviate fouling effects.Fourthly,by electrostatic spinning,superhydrophobic and porous PVDF membrane is used for real seawater desalination.After the characterization of the surface morphology,the deposition and congestion of contaminants on the surface of the membrane are explored,explaining the reason why membrane flux declines in real desalination processes.So far,this paper reveals two core problems that limit the commercial operation of seawater desalination:membrane materials and membrane fouling.The main factors affecting membrane fouling and their mutual influence are analyzed.Super-hydrophobic membranes are prepared.Its anti-fouling mechnism in real seawater is investigated,which provides reference for the commercial applications of MD type seawater desalination.