直接接觸式薄膜蒸餾-不同濃度進料和阻垢劑之結垢分析及管狀膜組之模擬

Analysis of scaling in DCMD with synthesized seawater and simulation of tubular membrane distillation

花天召(Tian-Chao Hua)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2013年)

https://doi.org/10.6840/cycu201300828

摘要

薄膜蒸餾至今已經發展了四十餘年，在水資源缺乏及能源成本增加的時代，薄膜蒸餾受到廣泛注意。本研究首先以管膜模組來進行模擬預估，模擬一開始就針對不同進料溫度、進料濃度及進料流速進行模擬，並與文獻上之實驗數據作比較，其預估之誤差值都約小於11%，表示此程式所預估之通量都能與實驗數據值符合，接著分析不同管膜之管長對於通量及效率之影響，當管膜長度增加，其通量會隨之下降，而效率值也是會隨管模膜長增加而減少。
另外，在模擬方面也作了平板膜面上濃度極化之模擬，在高溫70℃，進料溶液為高濃度24wt% NaCl，其膜面濃度高達27.3 wt%，接近NaCl的飽和濃度，因此進料在高溫高濃度的情形下，操作時不僅要注意濃度極化現象，也要考慮膜面積垢之問題。
在膜積垢方面，針對進料端不同溫度及濃度做了SEM-EDS的分析探討，首先從進料端不同溫度的結果顯示，經24小時實驗測試，在高溫70℃下通量衰退了約51%，表示膜面上已有大量積垢，而且此積垢大概都是碳酸鈣之結晶，另外吾人也使用飽和指數(saturation index, SI)及濃度極化來分析膜面上結垢之情形，在高溫70℃時，其碳酸鈣的飽和指數約在144.75，表示膜面已形成碳酸鈣之積垢層，使通量嚴重衰退。
在進料端不同濃度之情形下，在進料端高溫70℃，仿海水淡化之高濃縮液15wt%下(NaCl:14.5wt.%, MgSO4:0.185wt.%, MgCl2:0.115 wt.%, CaCl2:0.08 wt.%, NaHCO3: 0.12 wt.%)，經24小時實驗測試通量衰退了約58%，經SEM-EDS的分析後，其大部分之積垢有氯化鈉、碳酸鈣及硫酸鈣等結晶，而且多為細小且緻密之型態，容易阻塞膜孔，因此通量才會嚴重衰退。
添加抗結垢劑的結果分為兩部分，一是直接添加抗結垢劑，另一是20個小時後再添加清洗，前者添加的劑量為0.1、0.5及1 wt%的檸檬酸，經24小時實驗後，通量都能保持在80~90 kg/m2.hr之間，但在較高劑量的情況下，其鹽阻擋率會下降，表示說檸檬酸特性可能會降低膜面之疏水性，造成膜孔潤濕現象。而後者添加劑量為0.1及0.5 wt%的檸檬酸，在20小時實驗後加入進料桶，其結果顯示通量會回復提升，但鹽阻擋率會大幅下降，這表示說先前膜面之積垢物可能有些已結晶在膜孔裡，經檸檬酸清洗過後而使膜孔潤濕，導致鹽阻擋率下降，因此在添加檸檬酸清洗的過程中，要注意檸檬酸添加之劑量。

關鍵字

薄膜蒸餾 ； 結垢 ； 抗垢劑 ； 檸檬酸

並列摘要

Membrane distillation (MD) has been studied over 40 years, however, there is still very few applications in industries due to the concern of high energy consumption and lack of knowledge in scaling effect of the process. It is important to get a better understanding of the mechanism of membrane scaling and how to limit scaling for MD desalination. In the study, DCMD of tubular membrane module was simulated first and compared with the experimental data from literatures under different feed temperature, feed concentration and feed flow. The differences between both are less than about 11%, which means that the simulation program is feasible to estimate DCMD flux in tubular modules. Then the effect of hollow fiber length on permeate flux and thermal efficiency was also simulated and the results showed that the average flux and thermal efficiency decrease with the increase of hollow fiber length.
In addition, the concentration polarization of flat-sheet DCMD was also concerned in the simulation to predict the solute concentration near membrane surface. The results based on PTFE membrane showed that when the feed solution of imitation seawater has a high salinity as 24wt.% NaCl and a temperature of 70oC, the concentration near membrane surface will accumulated up to 27.3 wt.%NaCl, which is very close to the saturation concentration of NaCl. Therefore, when the feed solution is of high temperature and high salinity concentration, the concentration polarization will cause the problem of membrane scaling.
Experimental results of synthesized seawater solution(NaCl:3.0wt.%, MgSO4:0.185wt.%, MgCl2:0.115 wt.%, CaCl2:0.08 wt.%, NaHCO3: 0.12 wt.%) at 70 oC with PTFE membrane showed that the initial permeate flux reaches 101.3 kg/m2.hr, but after 24 hours operation the flux has a 51% decline due to a scaling layer formed on the membranes surface. The condition of membrane surface scaling was then evaluated by saturation index based on the concentration polarization analysis. At high temperature 70℃, the saturation index of calcium carbonate is about 144.75, which means that scaling layer formed on membrane surface was dominated by calcium carbonate.
Experimental results of feeding 15 wt % synthesized seawater solution at 70 oC showed that the flux has a 58% decline after 24 hours operation. By SEM-EDS analysis, it was found that the scaling layer containing sodium chloride, calcium carbonate and calcium sulfate crystals, and the crystals morphology are in small size and compact structure, which may block pore and results in a significant flux decline.
In order to investigate the effect of adding antiscalant on the DCMD performance, antiscalants were added with two different ways, one is adding antiscalant into the feed before entering the MD cell and , and the other is adding antiscalant into feed after 20 hours MD operation. The former are with the addition of a dose of 0.1, 0.5 and 1 wt% citric acid, and the flux can be maintained at between 80 ~ 90 kg/m2.hr after 24 hours operation. But in the case of higher doses, the salt rejection will decrease, which means that under the higher dosage may cause some membrane pore wetting. The latter with the addition of a dose of 0.1and 0.5 wt % citric acid after 20 hours continuous operation showed the declined flux will be recovered to close to or higher than the initial stage values, but the salt rejection will be substantially reduced. It is indicated that adding antiscalant to remove the preformed scaling layer will cause wetting of membrane pore.

並列關鍵字

citric acid ； humic acid ； scaling ； membrane distillation

參考文獻

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