碩士 / 中原大學 / 化學工程研究所 / 100 / Membrane distillation (MD) has been studied over 20 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 membrane scaling and how to use antiscalants to limit scaling for MD desalination. In the study, direct contact membrane distillation experiments using high salt concentration solution as feed were conducted to investigate the effect of operation conditions on permeate flux﹐salts rejection and scaling phenomenon. In addition, energy cost and membrane area requirement for a three-stage direct contact membrane distillation process with a desalination capacity of 100 CMD were also simulated in the study.
Three different membranes, CMT PTFE (0.12μm), Millipore PTFE (0.45μm) and Millipore PVDF (0.2μm), were used in the experiments. Experimental results of feeding 15 wt % NaCl solution at 70 oC showed that the initial permeate flux reaches 76.2 kg/m2.hr, but after 24 hours operation the flux has a 64% decline due to a scaling layer formed on the membranes surface.
In order to limit membrane scaling in membrane distillation of high salt concentration solution, citric acid and hydrochloric acid as antiscalant were added into the feed solution for experiments. Although both the antisaclants can limit the rate of scaling formation, citric acid is seen to give a better performance in flux than that by hydrochloric acid, and after 24 hours operations, the salt rejection with the condition of adding hydrochloric acid will decrease to 80%. Experiments with continuous concentration of 15 wt % NaCl solution indicated that when the salt concentration in circulated stream increased to about 20~21 wt % will give a fast decline in flux. Under the critical NaCl concentration the addition of antiscalants will only give a slightly effect on enhancing flux.
Base on the fluxes part from experiments and the other part from theoretical calculations, a three-stage process designed for 100 CMD seawater desalination was simulated. The flat-sheet membrane module each unit has an area of 0.72 m2 and waste heat is assumed to be available for heating the seawater feed to 90 oC. Simulated results showed that the energy consumption of pumps for per unit volume of water produced are 5.36 MJ/m3 for 70 % water recovery ( 35, 25, and 10 % for each stage). When the water recovery is increased to 80 % (40, 30 and 10 %, respectively), the energy consumption is 6.14 MJ/m3. The membrane areas requirement evaluated for the former is 120 m2 , while the latter is 138 m2. If the waste heat can only provide the feed solution to 70 ℃, simulation of two-stage MD process for 80 % recovery showed the pumping energy and the membrane modules requirement are 10.14 MJ/m3 and 333 units, respectively. When the seawater feed at 70 oC is further heated to 90 oC by another thermal source before entering the 1st stage MD modules, simulation results for three stage MD process showed that the thermal and pumping energy required increases to 1095.61 MJ/m3 for 80 % water recovery, but the membrane modules required is only 193 units.
| Identifer | oai:union.ndltd.org:TW/100CYCU5063083 |
| Date | January 2012 |
| Creators | Yi-Zhe Li, 李一哲 |
| Contributors | Ching-Jung Chuang, 莊清榮 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 150 |
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