Membrane Fabrication And Application For Ionic Liquid/Water Mixture By Vacuum Membrane Distillation Process

As green solvents,ionic liquids(ILs)have important effects on energy and environment.However,there are a limited number of industrial processes employing ILs due to their high cost and hypotoxicity.How to recycle ILs is very crucial for widening their industrial application.Pressure-driven membrane techniques such as reverse osmosis and nano filtration have been studied for ILs recycling.Compared with such pressure-driven membrane techniques,membrane distillation(MD)is a kind of thermally driven membrane separation process and is unaffected by osmotic pressure.Moreover,MD technique has relatively high flux and almost 100%rejection of non-volatile compounds.Therefore,MD is expected to be an effective method for ILs recycling.However,there is little research about using MD to recycle ILs.In this thesis,the preparation of porous hydrophobic membranes for ILs recycling and the vacuum membrane distillation(VMD)separation performance were investigated,as well as the membrane fouling and mass&heat transfer process.The main results can be summarized as follows:(1)Porous hydrophobic membranes were prepared by carbon tetrafluoride(CF4)plasma modification on hydrophilic polyacrylonitrile(PAN)membrane.The effect of radio frequency power on membrane surface properties,including hydrophobicity,morphology,pore size and porosity,was evaluated.The results showed that membrane water contact angle(WCA)increased from 47° to 132° after modification.Also,the surface mean pore size and porosity increased from 24.6 nm to 150.5 nm and from 7%to 32.3%,respectively.PAN-Cl membrane(the RF power was 80 Wn)had the best stability of VMD performance when treating 20 wt%1-butyl-3-methylimidazolium chloride([Bmim]Cl)aqueous solution.In addition,PAN-C1 membrane possessed good resistances to acid,organic solvent and[Bmim]Cl.(2)The VMD concentration process of[Bmim]Cl aqueous solution,membrane fouling and cleaning strategy were discussed.The results showed that a 20 wt%[Bmim]Cl solution could be concentrated to 65.5 wt%with a total[Bmim]Cl recovery over 99.5%after an optimized VMD concentration process.Moreover,membrane fouling was existed,and it was found that for the hydrophobic PAN-Cl membrane,membrane top layer wetting would occur due to the synergistic effect of surface hydrophilization and deposition of[Bmim]Cl,which not only induced the reductions of the membrane flux and rejection,but also further caused internal pore wetting.(3)Thermodynamic interactions between ILs and membrane surfaces were calculated to elucidate membrane fouling.Based on extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory,effects of membrane surface morphology(e.g.roughness),surface elemental composition,zeta potential and ILs concentration on the interaction energies were studied.The thermodynamic interaction analysis indicated that acid-base(AB)interaction played a critical role in membrane fouling.Additionally,membrane with lower roughness,higher F/C ratio and higher negative zeta potential was favorable for anti-fouling in this system.Finally,a commercial PTFE membrane was used for concentrating 20 wt%[Bmim]Cl to 86.2 wt%.(4)Heat and mass transfer model in VMD process for ionic liquid separation was obtained.The model in term of permeate flux was built by combining physical parameters of IL aqueous solutions with the empirical equation of mass transfer.The predicted values and experimental data tended to agree.Furthermore,the concentration and temperature polarization coefficients(CPC and TPC)were estimated by the predictive model,and the effects of different operating conditions on the coefficients were investigated.The results showed that the CPC serious influenced the flux than that of TPC in high concentration ionic liquid recycling.