直接接觸式薄膜蒸餾─氯化鈉與碳酸鈣積垢形態及其對通量影響之分析

Direct contact membrane distillation─Analysis of scaling morphology and its influence on the flux performance for sodium chloride and calcium carbonate systems

黃奕達(Yi-Da Huang)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2014年)

https://doi.org/10.6840/cycu201400684

摘要

在水資源愈趨缺乏的現今，海水淡化是一重大發展目標，而薄膜蒸餾(MD)在海水淡化中漸被重視，因其相較於RO逆滲透，除了有較好的產水品質之外，最重要的是MD可以操作於較高濃度條件下之進料。然而操作於高濃度的進料下，膜面上積垢所造成的通量衰退，是MD尚未廣泛地被工業界所應用的最大因素。故本研究各別針對海水中的氯化鈉及碳酸鈣鹽對於薄膜蒸餾通量的影響進行探討。首先建立模擬程式來對於不同進料溫度、濃度及流速進行通量預估，也討論濃度極化效應後對通量的影響，在較高濃度15 wt.%與24 wt.%的進料下，模擬時有無考慮濃度極化之最大通量差距可分別高達7.4%及15.6%。
在實驗方面則各別對於氯化鈉與碳酸鈣鹽進料改變操作參數來觀測其通量表現，並配合SEM-EDS分析膜面積垢情形。於較高溫70℃下3 wt.%的氯化鈉進料，8小時操作內通量可維持恆定且僅有輕微膜積垢的跡象；而較高濃度15 wt.%與24 wt%，在實驗進行6個小時後開始有較明顯通量衰退的情形發生，分別下降了10% 及36%，其膜面已有大量氯化鈉積垢出現且型態趨於緻密，而在低流速下積垢情形更為嚴重。

利用海水以及海水淡化回收40%濃縮液中的鈣離子與碳酸氫根離子濃度配置MD實驗進料液，於高溫70℃時，其通量的衰退幅度較低溫50℃下明顯，且於高溫下其膜面形成之積垢為較小的圓球varterite晶型；而在低溫時，則為針狀的aragnite晶型。透過測量進料槽中pH值的變化，可知溫度與濃度的提高，會縮短達到最小pH值的時間，即有較快碳酸鈣的沉澱析出速率。研究中也估算溶液及膜面的飽和度指數(Saturation Index, SI)，在高溫70℃時，濃縮液其碳酸鈣於膜面的飽和指數約為246.37，代表膜面已有碳酸鈣顆粒的出現。若以HCl調控進料槽中的pH值，當pH=6時，能夠使其膜面SI值為1，可以有效的減緩膜面的碳酸鈣沉積。文中也將氯化鈉與碳酸鈣混合液作為實驗進料，可發現，其通量初期的衰退的趨勢與僅碳酸鈣溶液為進料時相近，透過SEM-EDS分析，可初步判定膜面的積垢大多為碳酸鈣。
另外，本研究也針對如何清洗膜材，使其恢復通量來做探討，於氯化鈉進料液實驗中，可以透過通入清水沖洗膜面，膜面上大部分的氯化鈉鹽會回溶於水中，因此可回復至初始通量。於碳酸鈣進料液中，利用清水則無法將難溶於水的碳酸鈣沉積沖洗掉，故通量無回復的跡象；然而利用0.1 wt.%之檸檬酸漂洗，則可以與碳酸鈣反應形成可溶於水的檸檬酸鈣Ca_3 (C_6 H_5 O_7 )_2，通量則可回復接近至初始通量。

關鍵字

薄膜蒸餾 ； 氯化鈉 ； 碳酸鈣 ； 結垢 ； 清洗

並列摘要

Due to the growing problem of globally water scarcity nowadays﹐much attentions have been payed applying desalination process. Membrane distillation (MD) is expected to be a potential technology for desalination as compared to reverse osmosis, not only the better water quality that it can produce , but it also can provide a higher water recovery. However, there is still very few applications for MD in industries .One of the major reasons is the lack of knowledge in scaling effect of the process. In this study, we analyzed the scaling morphology of sodium chloride and calcium carbonate salts and its effect on the MD flux, respectively. We also used the simulation program and experiments to study the effects of feed temperature , feed concentration and feed flow rate etc. on the flux. Simulated results showed that the difference in fluxes predicted with and without considering concentration polarization can up to 7.4% and 26.5% for feed concentration at 15 wt% and 24 wt% , respectively.
Direct contact membrane distillation experiments were carried out with a flat sheet module to observe the flux performances of NaCl and CaCO3 salts feed solution. The membrane scaling was examined by SEM-EDS analysis.Results showed the flux is almost unchanged and it have just a minor scaling problem after 8 hours operation for feed of 3 wt% NaCl solution at 70 oC. However, as the feeds with 15 wt% and 24 wt% NaCl concentration, 10% and 36% flux decline were observed and the membrane surface have deposited a dense scaling layer.
On the other hand, based on the calcium and bicarbonate ions concentrations in seawater and that in concentrate stream from RO desalination process with 40 % water recovery, we also prepared the feeds for DCMD experiments.The scaling at 70℃is formed by sphericalsmall precipitates, , but it has the needle-like rode structure when operation is at 50℃. Through detecting the time dependent pH valuein the feed stream, it indicates that more faster CaCO3 precipitation rate will occur at higher feed temperature and salt concentration. In this study, we also calculated the saturation index(SI) at the membrane surface. The SI value for CaCO3is about 246.37 for concentrated solution at 70℃, and it means the membrane surface have large amount deposit on it. When the feed was conditioned to pH=6, the SI value evaluated at the membrane surface is equal to 1 and experimental results showed that there is almost no decline in flux under such condition.When the feed solution is prepared by mixing NaCl and CaCO3 salts, its MD behavior is similar to the feed only with CaCO3salt.
Besides, we have also discussed how to clean the membrane to make it recover the flux performance. The scaling by NaClon the membrane surface can completely flushed by water, so we can see the apparently flux recovery. However, using fresh water to flush the membrane have not significant effect for CaCO3 scaling. butthe flux can recover close to the initial fluxif the scaled membrane is flushed with 0.1 wt% citric acid solution.

並列關鍵字

sodium chloride ； calcium carbonate ； scaling ； flush ； membrane distillation

參考文獻

Alklaibi, A. M., N. Lior, “Membrane distillation desalination: Status and potential,” Desalination, 171(2005) 111-131.

Bird, R., W. E. Stewart and E, N. Lightfoot, “Transport phenomena,” 2nd ed. WILEY, New York (2005).

Bandini, S. and A. Saavedra. "Vacuum Membrane Distillation : Experiments and modeling", AlChE J., vol.43, 398-408, (1997)

Charfi, K., M. Khayet, M. J. Safi, “Numerical simulation and experimental studies on heat and mass transfer using sweeping gas membrane distillation,” Desalination, 259(2010) 84-96.

Chen, G., X. Yang, R. Wang and A. G. Fane, “Performance enhancement and scaling control with gas bubbling in direct contact membrane distillation,”Desalination, 308 (2013) 47–55.