Structure Desinging And Regulation A High Property Pervaporation Membrane And Studying On Its Desalination Performance

With the development of society and economy,the problems of fresh water shortage and pollution are becoming more and more serious.Desalination treatment of seawater,brine,and various types of saline wastewater can solve these problems effectively.The mainstream desalination technologies are the methods consisting of thermal evaporation and membranes technology at present.For example,thermal methods contain multi-stage flash evaporation（MSF）,multi effect evaporation（MED）and mechanical thermal compression（MVR）and so on;And membrane methods,like reverse osmosis membranes（RO）,membrane distillation（MD）and pervaporation（PV）and so on.The thermal desalination is limited in fossil fuels rich areas because of high investment of equipment and energy consumption.For RO technology,although the head position of seawater desalination market is occupied,but at high feed concentration,high osmotic pressure of the concentrated salt water causes the difficulty of RO operation,thus increases its energy consumption.MD has good tolerance to high brine,but the problems of humidification,blocking of membrane pores and membrane contamination lead this technology is difficult to operate stably in long-time.All the above technologies have limitations in handling the high concentration saline water.PV membrane,introducing a dense hydrophilic layer onto porous substrates,greatly inhibits the occurrence of the phenomena of wetting,blockage of membrane pores,and membrane pollutants.Meanwhile,by using low quality heat sources（such as plant waste heat,multi-effect utilization）,the PV energy consumption can be further reduced.Therefore,PV displays promising prospect in desalination field.However,several factors delayed the application of PV desalination membrane:Firstly,the swelling effect of the polymer separation layer makes the membrane having less resistant to concentrated salt water;Secondly,the high gas transport resistance of support layer hinders the performance improvement of its composites;Finally,lacking the preparation technology of high-performance composites in large-area.In response to solve the issues,this paper conducted the following research:1.Optimizing the cross-linking agent and regulating structure of the polyvinyl alcohol（PVA）separation layer to make it was resistant to high concentration salt water.On the basis of acid-catalyzed thermal crosslinking PVA/cross-linking agent films,the polymers containing coexisting groups of sulfonic acid and carboxylic acid（AA-AMPS and P（SS-MA））were selected via evaluating the dispersion effect of PVA/cross-linking agent solution and its compatibility of blended films.This was because the electrostatic repulsion between the ionized-SO₃^-prevented the condensation of the molecular chain effectively,even in acidified cross-linker solution.Then,a PVA cross-linking films（C-PVA）which could treat concentrated brine was designed by controlling its cross-linking density and PV performance.2.Optimizing the electrospinning support layer and constructing ultra-thin separation layer composites.By comparison the property of composites which were fabricated using non-solvent phase transformed（NIPS）polyvinylifluoride（PVDF）and electrospun polyacrylonitril（PAN）nanofiber as substrates and C-PVA as separation layer,the pressure between two substrates and C-PVA layer were calculated for proving the PAN porous structure could tremendously alleviate the accumulation of water vapor at the interface of substrate/separation layer.Then,a transition layer of nanofiber cellulose（NC）was added onto PAN to solve the problem of PVA solution infiltration,constructing C-PVA/NC/PAN composites successfully（C-PVA was the ultra-thin separation layer,～0.70μm）.When desalting with 70°C,3.5 wt%NaCl solution,the membrane had>99.9%NaCl rejection rate and the water flux reached to 153.4±3.4kg/m²·h.3.Regulating a polysulfone（PSF）membrane with whole sponge but skin-free structure for preparing its composites.Due to the complexity and time-consuming process of PAN electrospinning membrane,NIPS method was used to regulate PSF membrane structure with high permeability by introducing non-solvent phase into casting solution and increase coagulation bath temperature,then prepared C-PVA/PSF composite membrane.The composites had a water flux reaching to 124.8±3.2 kg/m²·h when treated with 70℃,3.5 wt%NaCl solution,surpassing the PV performance by the reported PSF based composites.Moreover,the designed PSF substrate with rough surface had excellent interface bonding force with C-PVA layer,preventing the layer shed from substrate effectively.4.Developing a method for rapid preparation the high-performance composite membrane in large-scale.1)By comparing the structure of different substrates and determining the electrospinning-like,high porosity polytetrafluoroethylene（PTFE）membrane was selected as the base;2)Using photo crosslinking instead of acid-catalyzed thermal crosslinking to short the crosslinking time of separation layer from 15min to 60 s;At the same time,introducing ZIF-8 nanoparticles inside PVA layer to increase its free volume and amorphous region and decrease the mass transfer resistance of water molecules.In vacuum mode for desalting 80℃,3.5 wt%NaCl solution,PVA/ZIF-8/PTFE membrane achieved>300 kg/m²·h water flux.Comparing with PTFE MD,this membrane maintained stable flux,good quality of producing water and excellent anti-pollution ability for treating 80℃,10 wt%NaCl+1 wt%humic acid solution.3)Spraying combined with photo cross-linking technology solved the problem of PVA film forming at PTFE base,and realized the preparation of PV composites in large-area,providing a favorable support for its industrial application.