| Membrane distillation is a new hot spot among separation technology due to its highefficiency and environment friendly. MD can be used for desalination of sea water, separation ofthe industry water and concentration of medicine. It has higher rejection, lower investment costand simpler operation than other separation techniques. Therefore, it is considered as a potentialtechnique in commercial applications.Among the MD materials, PTFE is an ideal material that it possesses good hydrophobicity,heat resistant, strong acid and alkali resistant. This subject mainly focuses on the preparation ofPTFE flat-sheet membrane and its super-hydrophobic modification and application in seawaterdesalination with sweep gas membrane distillation (SGMD).The main research content and conclusions are listed as follows:(1) PTFE flat-sheet membranes were prepared by the “Push-Stretch-Sinter” process. Sixkinds of flat-sheet membranes with different average pore diameter and porosity were producedby controlling the same compression ratio, sintering temperature, time constant, and changingthe draw ratio, draw temperature and setting temperature. The microstructure and pore sizedistribution of samples were researched. The prepared PTFE flat-sheet membranes had themicrostructure of "fibre-node", the porosity was from35%to63%and the average pore size wasfrom0.17μm to0.41μm.(2) The PTFE flat-sheet membrane with super-hydrophobic surface was prepared by theformation of SiO2nanoparticles on the surface of virgin membrane through the sol-gel methodand the modification of perfluorodecyl trimethoxy silane (FAS-17) on the surface of SiO2nanoparticles. The effects of ratio of tetraethyl orthosilicate (TEOS) and trimethyl triethoxysilane (MTES) and the concentration of FAS-17on the hydrophobility and microstructure of themembrane were studied. The results showed that the SiO2nanoparticles adhered and embeddedin the network structures of nodes which caused the decreasing of the pore diameter and porosity.When the ratio of TEOS and MTES and the concentration of FAS-17were1:1and4wt%, respectively, the WCA and rolling angle were154and8. FTIR analysis showed that thechemical groups changed significantly after modification.(3) Using the PTFE as the low surface energy materials, the PTFE flat-sheet membrane withsuper-hydrophobic surface and high porosity were prepared through annealing.The effects of theduration and temperature and cooling time of annealing on the hydrophobility and microstructureof the membrane were studied. The results showed that micro/nano structure appeared on thePTFE flat-sheet membrane surface, and the pore diameter and porosity increased. When theannealing temperature, duration and cooling time were350℃,3min and4h respectively, theWCA and rolling angle were151.94oand8.04. FTIR analysis showed that the chemical groupsalmost remained the same after modification. Hydrophobic properties of membranes wereimproved by changing the surface morphology structure.(4) The effects of membrane distillation flux after super-hydrophobic modification werestudied by120h continuous operation of SGMD on seawater. The results showed that theoriginal PTFE flat-sheet membranes seriously polluted and the water flux decreased along withthe running time. The super-hydrophobic properties of sol-gel modified PTFE flat-sheetmembrane could maintain for a long time. The permeate flux of the modified membranemaintained at about3.65kg/h·m2over the testing period and the salt rejection was more than99.8%. The permeate flux of the annealing modified membrane maintained at about5.32kg/h·m2and the salt rejection was more than99.8%. The super-hydrophobic properties of annealingmodified PTFE flat-sheet membrane could maintain for a long time and the flux was alwayshigher than the original membranes. |
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