- 學位論文
真空與直接接觸薄膜蒸餾法於酒精水溶液及氨水溶液分離之實驗與模擬探討
Experimental and simulation study of vacuum and direct contact membrane distillation for separating ethanol/water and ammonia/water solutions
摘要
在傳統發酵程序中，當發酵槽內酒精濃度達10~15 Wt%時，酒精產率會明顯下降，常藉由連續性的移除酒精減緩產物抑制現象，使發酵維持在高產率的狀態。另一方面，在許多農業以及工業的排放廢水中，氨(NH3)是其中主要的汙染物。近年來，政府對工業排放廢水中有機揮發性物質及氨氮濃度有逐漸嚴格規範。而薄膜蒸餾程序因單位模組體積下具有相當大之液氣接觸面積及易於操作等特性，在酒精及有機揮發物質分離程序之應用上逐漸受到注意。為進一步了解薄膜蒸餾於有機揮發物質分離之操作特性，並比較真空薄膜蒸餾(VMD)與直接接觸薄膜蒸餾(DCMD)在效能上之差異，本研究分為模擬與實驗兩個部份。模擬方面包含VMD與DCMD的模擬分析，實驗部分則是使用DCMD系統。 在VMD程序中，模擬結果顯示隨著真空端負壓的提升，有機物質之選擇率亦隨之提升，而當進料濃度與操作溫度提高，則有機物之選擇率會有下降的情形。在氨水分離程序中，使用考慮FA之模擬分析較能有效預估氨之選擇率，經模擬預估後可發現操作在氨濃度為0.016M，進料溫度為54.4℃時，溶液pH值從由9.6提升至11.1時，氨之選擇率可由5.45提升至11.84，氨的通量亦大幅提升154%，但pH達11時，進一步提高pH值對分離效能的提升則甚有限。 於DCMD酒精分離程序中，膜材孔徑、酒精濃度與進料溫度皆會影響滲透通量與酒精濃縮因子。實驗結果顯示進料溫度在40℃，酒精濃度範圍在5~15wt%下，孔徑為0.12μm之PTFE膜材其酒精濃縮因子較0.45μm孔徑之PTFE膜材高20~40%。隨著進料溫度與酒精濃度增加，酒精滲透通量隨之提升，而酒精濃縮因子則有下降之趨勢。而在濃度為15wt%時，PVDF膜材會有薄膜被潤濕之現象，導致水與酒精之滲透通量大幅上升。 由DCMD酒精移除程序的通量模擬結果顯示，對於含有揮發性物質進料的水通量預估時，可忽略分子間的相互影響，而在有機物質通量預估上，應考慮分子間的相互作用較能準確預估滲透通量。 經實驗與模擬結果得之，在進料溫度為323~333K，濃度5~15wt%範圍時，VMD程序其酒精滲透通量與濃縮因子皆較DCMD程序高，而當溫度為313K，濃度為10與15wt%時，則DCMD程序之濃縮因子較VMD高。 模擬結果顯示在DCMD操作中使用孔徑較小之薄膜，可提升酒精的選擇率。在進料溫度為60℃，濃度5wt%的條件下，孔徑為0.01μm之膜材其濃縮因子較0.2μm高約22%，而當濃度為15wt%時則高約14%。而在VMD程序中，當膜孔小於0.1μm時，隨著孔徑縮小濃縮因子則會有些微下降之趨勢。
並列摘要
In ethanol fermentation, the major limitation of the continuous process comes from product inhibition, generally inhibition becoming important as ethanol concentration greater than 10 wt % . The integration of a fermenter with membrane distillation has been investigated as a tool for the continuous removal of ethanol from fermentation broths. In addition, membrane distillation has received attention in recent years for the removal of volatile compounds like ammonia for recycling and reuse of industrial wastewater, especially for which has a relatively low level of ammonia. In this study, simulation and experiments are carried out in the study to analyze the effects of operating conditions on the performances of ethanol and ammonia removal from aqueous streams by membrane distillation. Direct contact membrane distillation(DCMD) and vacuum membrane distillation(VMD) two different membrane distillation processes are considered in the study. Comparison between the simulation result in the study and the experimental data from literatures of VMD ammonia separation showed that simulation analysis with considering free ammonia (FA) factor is better to estimate selectivity of ammonia. Simulation showed that the selectivity of ammonia increases with the vacuumed pressured of the permeated side and the pH of the solution in the pH range of 9~11. Under some conditions given in the study, high ammonia selectivity up to 11 is estimated. Experimental results of DCMD with feed of 5~ 15 wt% ethanol and at 40 oC showed that 0.12μm PTFE membrane gives an ethanol enrichment factor 20~40 % higher than that by 0.45 μm PTFE membrane. As the feed temperature and ethanol concentration increase, ethanol flux increases as a result, while lower ethanol enrichment factors are obtained. It is noted that the PVDF membrane will be wetted when the feed has a higher ethanol concentration as 15 wt % . Comparison between the simulation and the measured fluxes in DCMD showed that the effect of interaction between water and ethanol molecules on effective diffusivity of water can be ignored in calculation of water flux. However, the model with considering the interaction between the two molecules will give a better result in simulation of ethanol flux . Experimental and simulation results at the given conditions of 50~60 oC and 5~15 wt % ethanol concentration showed that ethanol flux and ethanol enrichment factor of VMD are higher than the DCMD operation. But, when the feed temperature is at 40 oC and concentration of 10 ~ 15wt%, the DCMD will produce a higher enrichment factor for ethanol. Simulation results of membranes in the pore size range of 0.01~0.2 m showed that the use of membranes with smaller pore size can enhance the selectivity of the ethanol in DCMD. However, the membrane pore size can only give a slight effect on the ethanol enrichment factor under the conditions given in the study.
參考文獻
延伸閱讀
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