薄膜蒸餾(membrane distillation)為近來廣受重視的一種技術，其特色為具有裝置簡潔、低成本、可模組化、高介面面積等多方面之優勢，直接接觸式薄膜蒸餾原理乃是利用薄膜兩側的飽和蒸汽壓差為驅動力促使水蒸氣通過薄膜，進而達到分離的效果，然而，薄膜蒸餾存在所謂的溫度極化現象，對於系統產能有相當顯著的影響，此現象越明顯則產能相對低落。 本研究針對薄膜蒸餾之主要設備進行效率改善的研究，目的為：(1)設計新型紊流促進因子(Turbulence promoter)，以求有效改善系統內部的溫度極化現象進而提升系統產能，並歸納出一經驗公式，描述此型式的紊流促進因子對於通道內部熱對流效應的影響；(2)藉由一維數學模型針對薄膜蒸餾設備的熱量與質量傳送機制進行研究，配合實驗分析以驗證經驗公式與數學模型的正確性；(3)探討設計參數及操作條件對於薄膜蒸餾系統之流體溫度分佈、溫度極化現象、純水透膜通量增加百分率與水力損耗提升百分率的影響。 研究結果顯示，平板型直接接觸式薄膜蒸餾系統之理論值與實驗值的相對誤差總平均為10.07 %，而新型紊流促進因子能夠有效的提升系統透膜通量，在本研究設定的操作條件之中，最高可達到40%的增益。本研究以操作在低體積流率之設備為主，除了可以降低操作成本外，經由新穎的改善策略可提升設備效能並得到令人滿意的熱量及質量傳送效率改善效果。 Membrane distillation (MD) is a membrane technology and it widely uses to remove melt salts from the seawater as referred to desalination. The advantages of membrane distillation in desalination processes can be summarized as follows: (1) it is easy to scale-up the operating device; (2) membrane processes can be easily incorporated with other separation processes; (3) the membrane properties are changeable and can be improved; (4) the separation processes can be carried out continuously. The direct contact membrane distillation (DCMD) design is a MD operation for which liquids directly contact both membrane surfaces. The existence of temperature gradient in a DCMD means that the membrane surface temperatures always contrast with bulk temperatures. This phenomenon, called temperature polarization, may cause a considerable loss of the thermal driving force leading to lower rates of mass flux across the membrane. Attempts to reduce the effect of temperature polarization were made implementing turbulence promoters to improve the flow characteristic. The purposes of this study are (1) to develop the heat transfer correlation for the rough surface channel; (2) to develop a one-dimensional mathematical model and propose a general numerical method for solving this mathematical model in predicting pure water productivity of membrane distillation systems; (3) to study the effect of temperature distributions and temperature polarization on the pure water productivity improvement of the membrane distillation systems. The correlation equation of estimating heat transfer for the turbulence promoter was obtained and the results show that the agreement between the experimental results and the theoretical prediction are fairly good. The new design of turbulence promoter can effectively enhance the mass flux, among the operating conditions set in this study, up to 40% of the gain.
