| Membrane distillation (MD) is a highly efficient, energy-saving separation technology. Thevapor goes through the membrane pores from hot side to the cold side and is condensed intoliquid water, and other component is blocked in the feed at the hot side by the hydrophobicmembrane. The mass transfer driving force is the vapor pressure difference. MD can be used fordesalination, process of the industry water and concentration. MD have higher salt rejection,lower investment cost, simpler operation, compared with other separation techniques. Therefore,it is considered as a technique having a potential commercial applications.The key of MD process is the high temperature resistant porous hydrophobic membranematerials. Polytetrafluoroethylene (PTFE) is an ideal material for membrane distillation, withexcellent hydrophobicity, chemical and temperature stability. In this paper, PTFE hollow fibermembranes were produced by Push-Expanded-sintering method. Using the preparedmembranes for desalination to process NaCl solution and seawater. The PTFE hollow fibermembranes of different thickness (0.40~0.70mm) can be prepared by changing the size ofextrusion device (inner diameter:0.80~1.10mm, outer diameter:1.60~2.30mm), with the samereduction ratio. The PTFE hollow fiber membranes of different bubble point (0.085~0.130MPa)can be prepared by changing the stretching ratio (1.7~2.8). The membrane modules weredesigned and prepared for submerged vacuum membrane distillation (SVMD). The effect ofcharacteristics of membrane, operating conditions and characteristics of membrane modules onthe performance of SVMD were studied. The results show that flux can be increased with thedecreases of the thickness. Flux does not monotonically change with the increase bubble point, apeak flux can be observed when the bubble point of the membrane is0.100MPa. Raising thetemperature and reducing the vacuum can increase the flux. Flux does not monotonically changewith the increase of air blowing intensity, a peak flux of8.87kg/m2·h can be observed when theair blowing intensity is6m3/h·m2, when the temperature of the feed solution is75℃. A low fluxcan be obtained, when the concentration of NaCl increases. There is little effect caused by characteristics of membrane modules on flux. Throughout the entire process, the maximum fluxcan up to10.86kg/m2·h with a salt rejection of99.9%.The stability of PTFE hollow fiber membranes was tested by a continuous VMDexperiment of800hours. The results show the PTFE hollow fiber membranes show an excellenthydrophobicity after the experiment of800hours. During the test, flux stabilized at around4kg/m2·h after100hours, with a salt rejection of99.9%. The results of flux recovery test of PTFEhollow fiber membranes show that the microstructure of the PTFE hollow fiber membranes willchange after the experiment of800hours. A simple pickling and drying can make flux recoveryafter the PTFE hollow fiber membranes were contaminated.A VMD prototype and a solar multi-effect air gap membrane distillation (AGMD) prototypewere designed, basis on the result of experimental studies. The area of the membrane is1m2. Aconcentration of3%sodium chloride solution was processed. The flux of VMD prototype can upto10kg/m2·h and the average flux of solar multi-effect AGMD prototype is2~3kg/m2·h with asalt rejection more than99.9%, when the temperature of the feed solution is75℃. The solarmulti-effect AGMD shows a good application potential with the advantage of low energyconsumption (the energy consumption is around9kW·h/t). |
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