Study On Treatment Of High Salt Solution By Membrane Distillation Based On PTFE Hollow Fiber Membrane

Membrane distillation (MD) as a new type of membrane separation technology has beenwidely concerned. MD uses a hydrophobic microporous membrane to separate two liquids ofdifferent temperature. The volatile component of hot liquid goes through the membrane pores invapor form under the vapor pressure of two membrane sides, which arrives at the cold side and thenis condensed into the liquid phase, while the other components are insulated in the side of hot liquidby hydrophobic micropore membrane. This is the basic principle of membrane distillation makemixture separation or purification. Membrane material which is suitable for membrane distillationshould have hydrophobic, porous and high temperature resistant. Polytetrafluoroethylene (PTFE)membrane which has good hydrophobic, chemical and temperature stability is an ideal material formembrane distillation. PTFE membrane is divided into flat membrane and hollow fiber membrane.Because of more filling areas per unit volume and self-supporting, the dosage of hollow fibermembrane is more in membrane distillation.In this study, the PTFE hollow fiber membrane was prepared by the paste extrusion and stretchsintering technology and its microscopic structure, pore size, porosity, surface water contact angle,mechanical properties and thermal properties were studied. Using100g/L NaCl solution,200g/LNaCl solution and seawater reverse osmosis water as the feed liquid, the water flux and the saltrejection were investigated in the different conditions of membrane structure parameters andmembrane distillation operation conditions. Then, the desalination performances of vacuummembrane distillation (VMD), direct contact membrane distillation (DCMD), and sweep gasmembrane distillation (SGMD) were compared. Finally the membrane fouling and membranecleaning process of long-term MD experiment were studied inspected.The results are as following:(1) Four kinds of PTFE hollow fiber membranes were prepared by the paste extrusion and stretchsintering technology. The wall thickness and outside diameter changed with cavity diameter and theinner diameter of the extrusion head, as stretch ratio of tensile stage to pore diameter and porosity.SEM images showed that the prepared PTFE hollow fiber membranes had the porous structureformed by the original fiber and junction. The water contact angles of the PTFE hollow fibermembrane were all above125°indicating that showed the PTFE hollow fiber membrane has goodhydrophobic performance. The breaking strength of PTFE hollow fiber membrane increased withthe increase of its thickness, and decreased with the increase of the pore diameter. The PTFE hollow fiber membrane has good thermal stability which melted at about350℃and decomposed at about550C.(2) The membrane modules were made by the PTFE hollow fiber membrane. Using100g/L NaClsolution,200g/L NaCl solution and seawater reverse osmosis water as feeds, the water flux and saltrejection were investigated in different conditions of membrane structure parameters. The waterflux of VMD decreased with the increase of wall thickness of PTFE hollow fiber membrane, andincreased with the increase of pore size and porosity. The salt rejection was above99.9%.(3)The flux decreased as the salt concentration increased, and increased as temperature or vacuumdegree increased. Increasing aeration flux resulted in an increase in flux first, and then fluxdecreased. When the aeration flux reached4m3/m2·h, the water flux was biggest. The salt rejectionwas above99.9%.(4) The desalination performances of SGMD, VMD and DCMD were compared. The water fluxwas VMD> DCMD> SGMD, and the water flux of VMD was biggest of all. In these threeprocesses, the flux increased with the feed liquid temperature and velocity increasing. In SGMD orVMD process, the flux increased as the vacuum degree or sweeping gas velocity increased. But inDCMD process, the flux hardly showed any variation as the water velocity of the permeate sideincreased. The salt rejection of three kinds of MD was over99.9%.(5) Using the VMD experiment dealed with100g/L NaCl solution and seawater reverse osmosiswater run for72hours. In VMD process, the flux gradually decreases as time increased. After72hours, the flux dropped from7.9kg/h·m2(100g/L NaCl) and6.4kg/h·m2(seawater reverseosmosis) to4.2kg/h·m2and2.37kg/h·m2, respectively. Because NaCl and CaCO3deposited onmembrane surface, it caused membrane pores were blocked. After using0.5mol/L hydrochloricacid to clean membrane surface, it improved the contamination of membrane surface, and flux isproduction up to7.5kg/h·m2and5.7kg/h·m2, respectively.