| There are much volatile organic compounds (VOC) in industrial and domestic wastewater at present. It is difficult to treat VOC which can endanger environmental safe because of toxity and low biochemical degradation. At the same time, VOC is an important chemical raw material, and its recovery has important economic value. Therefore, the treatment of VOC is a research hotpot recently. The membrane separation technology of pervaporation, which have the advantages of low mass in phase transition, efficiency, low power consumption with simple equipment and low technological scale up effect, are preferred ways of environmental protection. In this paper, several kinds of PTFE-PDMS composite pervaporation membranes were prepared using PTFE micro-powder and PDMS membrane materials. PTFE-PDMS pervaporation membrane structures, solution-diffusion properties, permeation performance for dilute chloroform or ethanol organic aqueous solution and the mass transfer resistance in permeation were investigated. The conclusions were as follows:Firstly, PTFE-PDMS pervaporation flat-sheet membranes prepared with different ratioes of PTFE into PDMS. The structures and characterizations of PTFE-PDMS pervaporation membranes were studied by SEM-EDAX, FT-IR, XRD, DSC, TG and water contact angle method. The properties of membrane swelling behavior and permeation were discussed. The experimental results showed that PTFE-PDMS membranes were dense and no phase separations due to the good compatibility between organophilic PTFE particles and organophilic PDMS. The PTFE particles are only physically blended with the PDMS polymer matrix. With an increase of PTFE, the crystallinity, the water contact angle and thermal stability of the PTFE-PDMS membranes were enhanced, but the swelling degree in chloroform or ethanol aqueous solutions were reduced. And the mechanical strength was increased greatly at first and then decreased with an increase of PTFE. The examinations showed that the PTFE filled PDMS membranes exhibited striking advantages in flux and separation factor as compared with unfilled PDMS membranes. For chloroform aqueous solutions, separation index was best when the content of the PTFE micro-powder additive in PDMS composite membrane was30wt.%. For the30%PTFE-PDMS membrane, the total fluxes and the separation factor can reach31.9g/(m2·h) and3215respectively at the condition of feed temperature50℃, permeate pressure2.2mmHg, feed concentration620mg/L, feed flow400mL/min. But for ethanol aqueous solutions, the PTFE additive in PDMS composite membrane was10wt.%. For the10%PTFE-PDMS membrane, the total fluxes and the separation factor can reach52.2g/(m2·h) and10respectively at the condition of feed temperature60℃, permeate pressure120mmHg, feed concentration10wt.%, feed flow1600mL/min.Then, on the basis of solution-diffusion theory, the sorption and diffusion behaviors of pure solvents such as chloroform, ethanol and water in the composite membranes were investigated. The results showed that at the same temperature, the solubility w∝,c and w∞,W of chloroform and water in PTFE-PDMS membranes decreased respectively with increasing PTFE content, but the w∞,E of ethanol first increased then decreased. With an increase of solubility temperature, w∞,c for PTFE-PDMS membranes with same proportion unchanged, but w∞,E and w∞,W increased. The order of solubility is w∞,c> w∞,E> w∞,W, which proved that PTFE-PDMS membrane was water repellent. The diffusion coefficients of chloroform, ethanol and water in the composite membranes have a complex change, and the order is DC> DE>DW. Adding PTFE in PDMS increased permeation coefficient Pc, PE and PW of chloroform, ethanol and water respectively, and the ideal separation selectivities for both chloroform to water and ethanol to water, αs,C and αs,E, increased.Next, two kinds of PTFE-PDMS pervaoparation membranes, PTFE-PDMS/PET and PTFE-PDMS/PVDF pervaoparation composite membranes were fabricated, in which PET non-woven fabrics and PVDF UF membrane were as the support layers. The pervaporation performances of PTFE-PDMS membranes, PTFE-PDMS/PET composite membrane for dilute chloroform aqueous solutions and PTFE-PDMS/PVDF composite membrane for ethanol aqueous solutions were investigated. The effects of feed concentration, feed temperature and permeate-side vacuum on the PV performance of the composite membrane were studied. The examinations showed that the two kind of composite membranes exhibited a similar trend. The difference was that the permeation fluxes of PTFE-PDMS/PET membrane for chloroform aqueous solutions were lower than that of PTFE-PDMS/PVDF, and the separation factor of PTFE-PDMS/PET membrane for chloroform aqueous solutions were higher than that of PTFE-PDMS/PVDF. For the permeation of PTFE-PDMS/PET for ethanol aqueous solution, total and ethanol flux and separation factor all increased with feed concentration increasing. With an increase of temperature, total and ethanol flux and separation factor all increased. Total, water and ethanol flux and separation factor all increased lightly with feed flow increased. That proved there is no obvious mass transfer boundary layer when the PTFE-PDMS/PET membrane was used to permeate ethanol aqueous solution. After that, mass transfer process of permeation for chloroform aqueous solution on the basis of solution-diffusion theory and resistance in series model was analyzed. With an increase of chloroform content in feed, chloroform permeation coefficient, KC,t, increased linearly, but water permeation coefficient, Kw,t, decreased linearly. With an increase of feed flow, boundary layer thickness was reduced gradually, so chloroform permeation coefficient in boundary layer, Kb, increased gradually and mass transfer Resistance, Rb, decreased gradually. When Re for feed flow is133, Rb was29times over Rm, which proved that chloroform mass transfer process was controlled by Rb, When Re for feed flow is5330, Rt was closed to Rm and Rb can be ignored, which proved that chloroform mass transfer process was controlled by Rm.Finally, PTFE-PDMS/PVDF hollow fiber pervaporation membrane and external pressure membrane module were prepared. The effects of active layer thickness, feed concentration, feed temperature, permeate-side vacuum, feed flow and operating time on the PV performance of the composite membrane were studied. The results showed that with an increase of active layer thickness, total flux decreased but separation factor increased. When feed concentration increased, total flux increased linearly but separation factor decreased. Both total flux and separation factor increased with permeation pressure decreased. With feed flow or feed temperature increased, both total flux and separation factor increased. Total flux decreased but separation factor increased lightly after11operating days. The suitable pervaporation condition is:active layer thickness13.8μm, low feed concentration (less than350ppm), permeate pressure2.2mmHg, Re1932and feed temperature60℃. |
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