- 學位論文
管式直接接觸薄膜蒸餾之模擬及實驗探討
A study on simulation and experiment of tubular direct contact membrane distillation
摘要
薄膜蒸餾近年來逐漸受到重視。因管狀模組在相同體積內可提供較高之有效接觸面積，故管狀模組薄膜蒸餾被廣泛研究。本研究利用Matlab建立管式模組之模擬程式，分析不同操作參數如：進料端與滲透端入口溫度、流量及膜管長度對通量之影響，以預估管式模組之通量，另一方面，利用自行製備之PVDF模組進行實驗，探討不同溫度、流速與進料鹽濃度等對通量之影響，且與模擬結果相比較。 實驗所用之PVDF管膜的液體貫穿壓力（LEP）量測結果顯示，當溫度由25提升至70 ℃，LEP值由2.6降至2.2 bar。而於DCMD實驗操作時發現膜管劇烈擺動，由模擬通量與其實驗值相比較，顯示兩者結果仍有明顯差異，當進料與滲透端溫度分別為70及30 ℃，且流量分別為1及0.1 L/min時，兩者之差距約達30%，推斷乃因膜管擺動所造成之影響。另外也探討進料氯化鈉濃度對通量之影響，進料端與滲透端溫度固定於50及30 ℃，結果顯示當進料鹽濃度為15 wt.%時，初始通量之模擬值與實驗結果兩者差距約20%，模擬程序中未考慮濃度極化應是因素之一。 另外也依據模擬結果進行分析，進料端與滲透端溫度固定於50及30 ℃，在相同的雷諾數變化區間（Re = 500 ~ 3000），進料端流量改變使通量提升56%，而滲透端僅提升6%。比較模組不同流動方向對通量之影響，顯示逆向流動所得之通量皆較大，當進料端與滲透端流量固定於1及0.1 L/min，滲透端為30 ℃時且進料70 ℃時其通量差距約6%，另外也探討膜管長度之影響，當進料端與滲透端溫度分別為50及30 ℃，且管長由20增至70公分時，同向與逆向操作之通量分別下降約39及10%。最後模擬管長為50公分之局部通量，沿著熱進料流動方向，同向與逆向兩者前段（進料入口處）通量皆明顯下降，但於後段時逆向流之下降幅度趨緩，同向流則仍有較大幅度下降，於進料端進出口處逆向流之局部通量差異僅2.7%，而同向流則達8.9%。
並列摘要
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.