| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 103 === Membrane distillation is expected to be a potential alternative technology for desalination purposes in the nearest future, especially for high salinity solutions. But the feed with high salt concentration will form large scaling on membrane surface, resulting in a dramatic flux decline and sometimes a progressive wettability of the membrane. In this study several different antiscalants were used in flat-sheet DCMD experiments with synthesized sea water as feed to investigate the effect of adding antiscalants and operating conditions on MD flux, scaling morphology and salt rejection.
Batch experiments by measuring the time-independent conductivity of salt solution after adding the antiscalant(ZDR-82) were carried out first. Experimental results of MD flux showed that the batch test in measuring conductivity is feasible to determine the better dose of antiscalant for MD. Then, three different antiscalants ZDR-82, ZDR-87 and HEDP were used for DCMD experiments. The results for feed with 15 wt.% salt and at 70 "℃" showed that the addition of ZDR-82 in concentration ranging from 0~100 ppm has no obvious effect on the enhancement of flux. After 8hrs MD operations, the scaling morphology by adding 10 and 30 ppm of ZDR-87 is in irregular shape with dense scaling layers. When the dosage was increased to 50 ppm, higher flux was obtained and scaling layer is in a loose structure. The addition of HEDP ranging from 0 to 30 ppm showed that the dosage at 30 ppm can provide the smallest decline on the whole MD flux, but the rejection of salts is reduced to 56 % after 8 hours operation. Under this operation condition, there are coagulation of big and small crystals on the membrane surface. When the MD experiments were carried out with 15 wt.% salt feed but concentrated continuously, results showed that HEDP can provide a higher average flux for 8 hrs operation.
In addition, the cleaning of scaled membrane after 8 hrs MD operation by using 1 wt.% citric acid to flush the membrane surface was also investigated in the study. The result from feed with 15 wt.% salt,10 ppm HEDP and concentrated continuously at 50 "℃" showed that the flux can get back nearly to the initial flux after membrane flushing. However, the flux will quickly decline when MD operation is started again. On the whole, 10 ppm of HEDP is more suitable for all operating conditions of this study, especially for 3.5 wt.% salt feed and concentrated continuously to give a more stable flux.
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