| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 97 === -ABSTRACT-
The problem of water shortage becomes increasingly serious globally. There are many processes developed and used for water and wastewater treatment and among these processes membrane distillation(MD) has the advantages that the waste heat or the solar energy may serve to the energy source and high purity water can be obtained. Although MD recently has received extensive attentions, its application in real plant of water treatment is still very few. One of the reasons is the concern of lower permeate rate and energy efficiency as compared with the traditional treatment processes. In order to investigate how to enhance the MD flux, PTFE and PVDF flat-sheet membranes with plasma treatment were used for DCMD experiments in the study under different temperature differences, salt concentrations and fluid hydrodynamics etc.. A theoretical model relating DCMD flux to the membrane physical properties, the hydrophilic layer thickness of membrane after plasma treatment and the fluid hydrodynamics was also developed. In addition, DCMD with tubular modules was also studied theoretically and experimentally in the study.
Experimental results with flat-sheet membranes showed that the plasma treatment of membrane can give a significant enhancement in MD flux due to the hydrophilic layer generated will reduce the mass transfer resistance for vapor transport through the membrane. For the CMT PTFE(0.18μm) membrane operated under a temperature difference greater than 400C, the membrane after 200 W/1min plasma treatment will give about 103.2 % higher flux than that obtained from virgin membrane and its slat rejection is still maintained over 98.3 %. Based on the hydrophilic layer thickness measured by observing color agent penetrating length through the membrane after plasma treatment, the prediction of DCMD flux by the theoretical model developed in the study showed a well agreement with the experimental results.
PTFE, PVDF and PP three different tubular modules were used for experiments in the study. Although the modules are shell-and-tube design, but the quite different packing density between the modules leads it is very difficult to maintain a similar hydrodynamics in the shell side of the three modules. Due to larger temperature drop occurred when hot feed flow through the fine-tube will decrease the effective driving force for membrane distillation, for the modules with PVFD and PP fine-tube membranes the hot feed flows in the shell side can give a higher MD flux than that flows through the tube side. Theoretical model for DCMD in shell-and- tube module was also developed in the study and its prediction agrees with the measured values of PTFE module. Based on the theoretical model, the effects of tubular diameter, tubular length and tube side Reynolds number on the DCMD flux and energy efficiency were also simulated preliminary in the study.
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