流動型態對直接接觸式薄膜蒸餾滲透通量之影響

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Title:	流動型態對直接接觸式薄膜蒸餾滲透通量之影響
Other Titles:	Effects of flow patterns on the permeate flux of direct membrane distillation
Authors:	陳威州;Chen, Wei-chou
Contributors:	淡江大學化學工程與材料工程學系碩士班鄭東文;Cheng, Tung-wen
Keywords:	直接接觸式薄膜蒸餾;流動型態;海水;Dusty-Gas model;極化;Direct contact membrane distillation;Flow patterns;Seawater;Dusty-Gas model;Polarization
Date:	2010
Issue Date:	2010-09-23 17:32:26 (UTC+8)
Abstract:	本研究以聚四氟乙烯薄膜(PTFE)利用平板型直接接觸薄膜蒸餾系統蒸餾海水，主要探討系統內流體流動型態對薄膜蒸餾之滲透通量的影響情形，並預測蒸餾海水之理論滲透通量。實驗將以溫度差、進料流量、模組傾斜、曝氣量、改良模組及超音波等六種操作方式影響進料側流體流動型態。在單一液相薄膜蒸餾系統中，隨著溫度差上升則滲透通量有明顯增加，但極化現象也最嚴重。進料流量提高僅增加對流熱傳係數，因此滲透通量提昇很有限。而若將模組傾斜45°時，會由於不穩定自然對流關係促進滲透通量提昇最多。然而在改良模組實驗中，進側側模組之小範圍凹孔會造成流體亂流使得滲透通量明顯提高。至於超音波則在單一液相系統且適當操作時間下較能發揮其效果，但滲透通量提昇有限。在氣液兩相系統中，隨著進料側曝氣量越大則滲透通量提昇越多。而隨著曝氣量的增加，若改變模組傾斜角會使滲透通量變化幅度越大。又因曝氣已能有效減緩極化現象，因此若再增加其它操作條件，如改良模組、超音波，則滲透通量增加的會很有限。藉由Dusty-Gas model並假設海水相當NaCl溶液濃度所預測薄膜蒸餾海水之理論滲透通量，其理論計算的結果與實驗相當符合。結果顯示出溫度極化係數介於0.4~0.6之間；濃度極化係數隨著溫度差或進料流速減少而有明顯的增加，故濃度極化現象為影響滲透通量的主要影響因素。因此為預測海水於薄膜蒸餾的行為所做之假設是為合理的，並可了解極化現象對滲透通量的影響情形，以便為提昇滲透通量而優先預防。 The effects of flow patterns on the permeate flux of direct contact membrane distillation (DCMD) was studied in this work. The operating parameters included temperature difference, feed flow rate, module inclination angle, gas flow rate, module modification and ultrasonic irradiation. The DCMD experiment was conducted in a flat sheet module with using 0.2 µm pore size polytetrafluoroethylene (PTFE) membrane. The pre-filtrated seawater from Tamsui coast area was used as feed. Permeate fluxes were measured under various operating parameters. Theoretical flux prediction model was also presented for single-liquid phase and two phase in DCMD. In the operation of single-liquid phase membrane distillation, the results showed that the permeate flux increased significantly with increasing the temperature difference, but increased slightly with the feed flow rate in presented laminar flow region. As the angle of inclination from the horizontal (flow below membrane) increased, the permeate flux increased, and reached a maximum at 45°, and then decreased. A small area of concave hole on the feed module side which caused turbulence to feed stream allowed flux enhancement significantly. The effect of ultrasonic irradiation enhance on the permeate flux was finite. In the operation of two phase membrane distillation, the permeate flux increased apparently with increasing the air sparging in the feed side. With changing the membrane inclination, the flux enhancement also increased with increasing the gas velocity. However air sparging could already effectively reduce the polarization phenomena, the flux enhancement by other operational parameters was slight. The flux prediction model was derived from the Dusty-Gas model combined with assuming the seawater to the equivalent NaCl solution. The prediction fluxes agreed very well with the experimental results. The theoretical calculations showed that the temperature polarization coefficients were in the range of 0.4 to 0.6 that was reasonable for DCMD operation; and the concentration polarization coefficients increased significantly as the temperature difference increased or the flow rate decreased. The presented flux model can be applied to forecast the effect of polarization on the flux and then prevent it in advance.
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