| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 103 === Water resources and environmental pollutions are the growing globally problems and membrane distillation (MD) technology may play an important role for solving these problems. Considering the commercial design to have a higher membrane area per unit volume and easy to scale-up, tube-based membrane modules are more potential for industrial application. Therefore, to establish the basis of its module design for MD is very important for future engineering applications. In this study, the effects of operating conditions such as feed and permeate side temperatures, flow rate and tubular membrane modules characteristics (length and packing density) will be simulated via a numerical solution. In addition, the performances of DCMD with different dimension of modules and different feed conditions will be analyzed experimentally and also to give a comparison with the theoretical simulations.
First, the LEPw of PVDF tubular membranes used in the study was measured and its values decrease from 2.6 to 2.2 bar as solution temperature increased from 25 to 70 ℃. There are 30% difference between the fluxes measured and that by simulated as feed 70 ℃ and permeate side at 30 ℃. It was founded that that tubular membranes in the self-assembled module swing drastically when fluid flows through the module, which may be the main reason to cause such an obvious difference. The effect of salt concentration on the flux was also analyzed by simulation and experiments. For a feed with 15 wt% salt and 50 ℃，the initial flux simulated has a 20 % difference compared to the experimental results. One of the reasons causing the difference is without consideration of concentration polarization in the simulation.
Simulation was also applied to investigate other operation conditions such as flow rate, packing density and module length on flux. Result showed that the increase of Re in feed stream (shell side) from 500 to 3000 will give a 56 % rise in flux. However, under the same Re range in permeate side only 6 % rise was observed. It was also shown that the module operated by countercurrent flow has higher fluxes than that by cocurrent flow. When the length of membranes increased from 20 to 70 cm with feed at 50 ℃ and permeate at 30 ℃, the permeate fluxes decline 39% in cocurrent flow and 10% in countercurrent flow. Local flux analysis showed that along the axial direction the flux decreases quickly at the feed inlet section, then became gently for countercurrent flow. The permeate flux declined 2.7% in countercurrent flow and 8.9% in current flow.
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