| As far as we say MD is applied to the membrane distillation device to separate the materials by using mass transfer driving force causes by steam pressure defference on both sides of the components. According to different applications membrane distillation device can be divided into direct contact membrane distillation(DCMD), vacuum membrane distillation(VMD), sweeping gas membrane distillation(SGMD) and air gap membrane distillation(AGMD). In this article we main study the flat PVDF membrane that used in the direct contact membrane distillation. The research of this article including preparation, modification and separation performance of the PVDF membrane.In the process of preparation of flat PVDF membrane, various factors are investigated such as the coagulation bath, residence time and the dosage of additives.They all have an effect on the structure and performance of the membrane. Through the orthogonal experiment we can get the results as follows: the affect orders of three factors as follows: additive dosage > residence time > coagulation bath temperature. In the end, we get the best preparation of the flat PVDF membrane and its preferred plan as follows: additive dosage is 3%(W/W), residence time is 5s, coagulation bath temperatuer is 50℃.In this article two different methods(i.e., blending modification and surface modification) are used to conduct modification experiment. A、Si O2 blends with Ti O2, Al2O3 blends with Ti O2; glycerin/Ti O2, PMMA/Ti O2 blends with each other. In the blending modification experiments we study the performance of the membrane from above by contronlling the proportion of mixed additives. And the results show that after glycerol blending with Ti O2, the performance of the membrane is good, the best dosage for Ti O2 is 3%(WW), glycerol is 0.4g. b、In the process of surface modification, various factors such as the amount of PEG-600, temperature, the concentration of stearic acid acetone solution, the use of nanometer Ti O2 power particles, the soaking time in stearic acid acetone solution are investigated on the effect of the structure and performance of the flat PVDF membrane. From the result we can get the best preparation method is: the soak time in stearic acid acetone solution is 3h, the concentration of stearic acid acetone solution is 0.05mol/L, the amount of PEG-600 is 6ml, the amount of nanometer Ti O2 power particles is 0.2g, temperature is 50℃.Then ultrafiltration cup, scanning electron microscope(SEM), Fourier infrared spectrometer(FTIR) and contact angle measurement instrument etc. are used to characterize the modification of PVDF membrane and the results show that the flux and intercept rate of modification flat PVDF membrane are bigger than unmodified membrane. At the same time the micro-structure of the film surface is shown to be composed of micro-nano structure which plays an important role in the performance of hydrophobicity, so we say the method of modification is feasible.At last the modified membrane is applied to the direct contact membrane distillation device and we also study the material liquid temperature, the concentration of feed liquid and slurry flow rate etc. on the influence of the distillation process. The results are as follow: the optimal temperature of the material liquid temperature is 50℃; considering the influence of concentration polarization on membrane fouling so we choose 15g/L as the best result; the material liquid flow rate has a significant effect on membrane distillation flux, by comprehensiving various experimental results we select 5L/h as an optimal choice.At the same time, under the optimal experimental conditions(feed liquid temperature is 50℃, the concentration of slurry is 15g/L and the rate of material liquid is 5L/h) we conduct the desalination experiment to analyze the performance of the prepared membrane. The results show that the conductivity of cooling water side has little change and the desalination rate of surface modification flat PVDF membrane can reach 90%. |
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