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標題: | 以分子動態模擬探討尿素對二氧化碳水合物的熱力學與動力學之影響 Molecular Dynamics Simulations on the Effect of Urea to the Equilibrium and Kinetic Properties of CO2 Clathrate Hydrates |
作者: | Po-Wei Wang 王柏偉 |
指導教授: | 林祥泰(Shiang-Tai Lin) |
關鍵字: | CO2 水合物,尿素,固液界面,質量傳遞,籠狀結構形成, CO2 hydrate,Urea,Solid - liquid interface,Mass transfer,Cage formation, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 本研究利用分子動態模擬(MD)以了解尿素對CO2水合物形成的促進作用。首先我們計算幾種與此系統相關的熱力學性質,包括CO2的溶解度、有無尿素存在下CO2水合物的熔點,確認分子作用力場的可靠性和分子模型的合理性。在模擬中我們觀察到尿素在CO2水合物形成中的動力學促進行為與實驗一致。從一系列的模擬結果,我們歸納出尿素提高CO2水合物生長速率的主因有以下兩種:(1) 尿素增加CO2的質量傳遞速率,以及(2)尿素促進水籠於固液界面處形成速率。當尿素分子存在水相時,單位時間內CO2和水的擴散範圍更廣,且原先在生長界面(固液界面)附近的CO2濃度有最小值的現象會消失,這些都證明了尿素增加CO2在系統內的質量傳遞速率。另一方面,尿素的存在也會增加固液界面處的CO2濃度,同時界面上的尿素分子可催化籠狀結構的形成,從而增進水籠形成速率。我們提出了一個反應-擴散模型,可以定量CO2質傳與表面濃度對水合物生長的影響。同時,從固液界面分析中,我們觀察到了三種尿素促進籠狀結構物形成的可能機制。包含了籠的穩定、促進環的生成與促進籠的生成。 Molecular dynamics (MD) simulations are performed to study the promoting effects of urea on the formation of CO2 hydrates. Various thermodynamic properties, including the solubility of CO2, the melting temperature of CO2 hydrates with and without the presence of urea, are used to validate the force field and simulation models. Under the same subcooling temperature, the kinetic promoting behavior of urea in the formation of CO2 hydrates is successfully reproduced in the simulations. The analysis of the MD trajectory shows that the enhancement of the growth rate of CO2 hydrate upon addition of urea can be attributed to two factors: (1) increased mass transport of CO2 and (2) enhanced cage formation rate at the solid - liquid interface. The increase of mass transport is evidenced by the diminishing of CO2 concentration minimum near the growing front and the higher self-diffusivity of both CO2 and water when urea molecules are present. While the addition of urea reduces the molar concentration of CO2 in the bulk, the presence of urea at the solid - liquid interface increases the CO2 concentration at the interface during growth, resulting in a faster cage formation rate. We observed different ways where the urea molecules catalyzing the cage formation at the growing front, including stabilization of the partial cage, facilitating of ring structure formation, and catalyzation of cage formation. A reaction – diffusion model is purposed to quantify the urea effect on hydrate growth based on these two factors. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/62016 |
DOI: | 10.6342/NTU202001043 |
全文授權: | 有償授權 |
顯示於系所單位: | 化學工程學系 |
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