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

• Main Authors: Tian-Chao Hua, 花天召
• Other Authors: Ching-Jung Chuang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/96441121796009692273

碩士 === 中原大學 === 化學工程研究所 === 101 === 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.