膜孔大小和進料組成對於直接接觸薄膜蒸餾 效能影響之探討

Study on the effects of Membrane pore size and feed composition on the performance of Direct Contact Membrane Distillation

陳喬峰(Chiao-Feng Chen)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2011年)

https://doi.org/10.6840/cycu201100946

摘要

薄膜蒸餾發展至今已四十餘年，在水資源匱乏問題日益嚴重的現今，已有許多國家致力於發展薄膜蒸餾水處理程序，本研究先以程式模擬分析薄膜孔徑分佈對於直接接觸薄膜蒸餾滲透通量之影響，分別建立三種不同的分佈型態(狹窄分佈、常態分佈、小孔洞較多之分佈)，結果顯示，狹窄分佈之滲透通量皆高於小孔洞較多之分佈，其中又以0.2μm之薄膜較為明顯，在薄膜兩端溫度差為40℃時，平均孔徑為0.2μm及0.45μm之條件下，狹窄分佈者之通量分別高約20%以及9.9%，而狹窄分佈僅較常態分佈通量高約2%及0.3%左右而已。
本研究主要部份即自行配製仿海水溶液做為進料溶液，並操作不同之實驗參數探討其滲透通量、鹽類阻擋率變化及結垢情形，所選用的膜材為中原薄膜中心 PTEE 0.12μm以及Millipore PVDF 0.2μm進行實驗，實驗結果顯示 PTEE薄膜操作仿海水進料溶液溫度差為30～70℃時，其第一小時滲透通量可達96.15( kg/m2.hr)，但由於膜表面結垢的生成使得系統經24小時操作滲透通量下降31.63%，而PVDF薄膜因滲透通量較小，因此薄膜結垢的程度較低，且因操作過程伴隨薄膜潤濕現象發生，因此滲透通量幾乎維持在25～30( kg/m2.hr)。研究中亦發現系統操作在低溫度差及高進料流量時，能夠較有效改善薄膜結垢生成。
為了減緩薄膜結垢，本研究於仿海水進料溶液添加檸檬酸，結果顯示檸檬酸能有效抑制膜結垢的生成， PTEE薄膜操作24小時通量僅由112.9 下降至106.03( kg/m2.hr)，下降幅度僅僅5%左右，但因添加檸檬酸會造成薄膜些微的潤濕，因此鹽類阻擋率稍稍下降至99%。
本研究亦探討PTFE及PVDF薄膜經電漿改質後操作仿海水系統其效能之變化，電漿改質功率為100w/2min，雖然實驗初期經電漿改質之薄膜滲透通量有所提升，但無法持續維持改質效果，兩種膜材經電漿改質後鹽類阻擋率有下降的趨勢，且PVDF薄膜較為明顯，原因為當薄膜經電漿改質使其表面較為親水，進料端膜面較易產生潤濕現象，使鹽類阻擋率下降，此時蒸氣分子在此親水層不斷擴散至滲透端，易在膜內形成結垢，造成滲透通量衰退。
最後探討進料溶液為腐植酸溶液時其滲透通量之變化，所配製腐植酸溶液為0.1wt%，實驗中發現滲透通量在實驗操作24小時後仍可維持在70 kg/m2.hr，並不會有通量衰退的趨勢，且將腐植酸加入仿海水中進行操作，其滲透通量降幅與單純仿海水溶液接近，因此可知腐植酸溶液並不是造成薄膜結垢的主因。
希望藉此研究能有助於薄膜蒸餾於海水淡化上的應用，改善目前全球面臨的缺水問題。

關鍵字

薄膜

並列摘要

Because of the growing problem of globally water scarcity﹐much attentions have been payed by applying membrane distillation process for desalination. In order to give a better understanding of membrane distillation performance﹐the first part of this study developed simulation program to analyze the influence of membrane pore size distribution on direct contact membrane distillation permeate flux. Among the three different pore size distribution models (narrow distribution﹑normal distribution and beta distribution) adopted, simulated results showed that the membranes with narrow pore size distribution will give a higher permeate flux, especially for membranes with an average pore size smaller than 0.2 m.
Membrane distillation experiments using imitate seawater as feed solution were conducted in this study to investigate the effect of operation conditions on MD permeate flux﹐salts rejection and fouling phenomenon. Two different membranes, CMT PTFE (0.12μm) and Millipore PVDF (0.2μm) were used in the experiments. Experimental results under the condition of hot feed temperature of 70 oC and the permeate side of 30oC showed that at the initial stage the PTFE membrane provides a high permeate flux as 96.15 kg/m2.hr, but the flux has a 31.63% decline after 24 hours operation due to a fouling layer formed on the membranes surface. Although the PVDF membrane has the same problem of membrane fouling ﹐but the lower permeate flux and the occurrence of partially membrane wetting lead its flux almost remain constant as 25~30 kg/m2.hr during the 24 hr operation. It was also found that the system operated under lower temperature difference and higher feedside flow rate can reduce the membrane fouling effect.
In order to limit membrane fouling in membrane distillation of imitate seawater solution﹐citric acid which is an antiscalant was added into the feed solution for experiments. The results showed that citric acid can inhibit the generation of membrane fouling effectively﹐providing the permeate flux of PTFE membrane only about 5% decline for 24 hrs operation﹒
The effect of plasma modification of membrane on its MD performance with imitate seawater was also analyzed in the study. Although the modification of membrane can give an obvious rise in permeate flux during the initial period﹐but after 24 hrs operation the flux declines to nearly the same as that from untreated membrane. The main reason for such a phenomenon is that the plasma modified surface is rather hydrophilic to cause membrane wetting﹐so the fouling layer form on the membrane and block the pore﹒
In the study﹐the effect of humic acid in imitate seawater solution on MD flux was also analyzed. Experimental results showed that the content of humic acid will not cause obvious decline in MD flux of imitate seawater solution﹒

並列關鍵字

Membrane Distillation ； fouling ； citric acid ； humic acid

參考文獻

中原大學化學工程學研究所, 桃園縣

Alklaibi, A.M. and Noam Lior, “Membrane-distillation desalination: status and potential,” Desalination, 171 (2004) 111-131.

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

Cabassud, C., and D. Wirth, “Membrane distillation for water desalination: how to choose an appropriate membrane ,” Desalination, 157 (2003) 307-314.

Cheng, T., W. Chih-Jung Han, Kuo-Jen Hwang, Chii-Dong Ho, and W. J. Cooper, “Influence of Feed Composition on Distillate Flux and Membrane Fouling in Direct Contact Membrane Distillation,” Separation Science and Technology, 45: (2010) 967–974