直接接觸式薄膜蒸餾─探討抗垢劑對積垢型態及通量之影響

Direct contact membrane distillation─The effect of antiscalants on the scaling morphology and the flux performance for seawater desalination

吳宗鍇(Tsung-Kai Wu)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2015年)

https://doi.org/10.6840/cycu201500545

摘要

薄膜蒸餾(MD)可利用低階能源且可處理較高鹽濃度之進料，然而高鹽濃度之進料容易產生積垢問題，是通量衰退之主因，這應是該程序至今還未被工業界所應用之重要因素之一。本研究以自行調配仿海水溶液作為進料，探討添加不同抗垢劑於進料中對於MD通量、膜面積垢及鹽類阻擋率等影響。
利用批次鹽溶液電導度值隨時間變化，預估抗垢劑(ZDR-82)較佳劑量，與MD通量結果相同。接著添加抗垢劑ZDR-82、ZDR-87及HEDP，於70 ℃之15 wt.%進料中觀測其通量與鹽阻擋率表現，結果顯示ZDR-82對於通量提升效果較不明顯；添加劑量0 ~ 100 ppm之ZDR-87，於50 ppm下整體通量較高，30 ppm者較低，50 ppm者之積垢堆積較鬆散，30 ppm者呈現不規則形狀，而且形成較緻密的積垢層；添加劑量0 ~ 30 ppm之HEDP，30 ppm者之整體通量衰退幅度最小，但鹽阻擋率僅剩56 %，積垢型態則是有大顆粒與小顆粒彼此聚集。於此操作條件下，劑量50 ppm之ZDR-87則有較高之通量。
接著操作於50及70 ℃之15 wt.%進料為連續濃縮觀測其通量與鹽阻擋率表現，除了添加上述所提及之抗垢劑外，還添加了PAA抗垢劑，於50 ℃下操作8小時後，HEDP整體通量表現最高，PAA者最低，HEDP者之積垢則較為分散且只有少部分的積垢互相聚集，PAA者之積垢則彼此緊密的堆積在一起，形成更緻密的積垢層。於70 ℃下，僅添加HEDP、ZDR-87及PAA，結果顯示在連續濃縮2小時後，三者之通量都有大幅度的下降，而添加HEDP者之整體通量較高。
也有探討薄膜清洗程序，操作8小時後利用1 wt.%之檸檬酸沖洗膜面，結果顯示添加HEDP於15 wt.%連續濃縮鹽濃度進料50 ℃下，經沖洗後MD通量雖可回復至初始通量，但通量沖洗後則會迅速的衰減。整體來說劑量10 ppm 之HEDP較適合應用於本論文之所有操作條件，尤其以3.5 wt.%之進料連續濃縮條件下，延緩通量衰減效果較佳。

關鍵字

薄膜蒸餾 ； 積垢 ； 抗垢劑 ； 清洗

並列摘要

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.

並列關鍵字

membrane distillation ； scaling ； antiscalant ； flush

參考文獻

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