使用分子動力學模擬探討甲烷/二氧化碳/氮氣混合水合物的成核與生長;Studying the Nucleation and Growth of CH4/CO2/N2 Mixed Hydrates Using Molecular Dynamics Simulation

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Title:	使用分子動力學模擬探討甲烷/二氧化碳/氮氣混合水合物的成核與生長;Studying the Nucleation and Growth of CH4/CO2/N2 Mixed Hydrates Using Molecular Dynamics Simulation
Authors:	江奕昀;Chiang, Yi-Yun
Contributors:	化學工程與材料工程學系
Keywords:	分子動力學;氣體水合物;結晶;二氧化碳封存;煙氣置換;MD;clathrate hydrates;crystallization;carbon dioxide sequestration;flue gas replacement
Date:	2024-08-20
Issue Date:	2024-10-09 15:29:04 (UTC+8)
Publisher:	國立中央大學
Abstract:	氣體水合物是一種結晶化合物，在低溫和極高壓力下由水形成籠狀結構包覆氣體分子。由於甲烷水合物在全球的大陸棚和海底沉積物中廣泛存在，因此被視為一種極具發展潛力的新興能源。科學家們提出使用二氧化碳取代水合物中的甲烷，可以將二氧化碳以水合物形式封存在海底並將置換出來的甲烷當作能源使用，既可以減緩溫室氣體效應，還能應對能源挑戰。而利用煙氣替代開採甲烷水合物是近年來提出的新方法，煙氣為燃燒發電後的廢氣其主要成份為二氧化碳與氮氣，使用煙氣置換甲烷水合物不僅可以降低分離二氧化碳的成本，氮氣還可以置換二氧化碳難以取代的水合物結構，以提高甲烷回收率。然而目前對混合氣體分子在水合物形成過程中成核情況的了解非常有限。在本研究中使用分子動力學模擬來探討發生在液態水與氣體分子 (甲烷、二氧化碳和氮氣 )系統中形成水合物的成核競爭以及氣體類型和比例對成核速率的影響。研究結果顯示純二氧化碳水合物的成核速率比純甲烷水合物與純氮氣水合物快，與氮氣水合物相比，加入二氧化碳對甲烷水合物有更顯著的促進成核效果。在雙成分甲烷 /二氧化碳水合物中，初始成核為包覆甲烷的 512籠 (由水分子形成的十二個五邊形面 )。在三成分甲烷 /二氧化碳 /氮氣水合物中，觀察到氮氣也有機會存在於初始成核的 512籠中。另外，計算發現水合物形成的比例會與混合氣體比例之間存在線性關係，其中氮氣與甲烷容易形成 512籠，而二氧化碳較容易形成 4151062籠 (由水分子形成的一個正方形面、十個五邊形面和兩個六邊形面 )則有所不同。另外模擬結果顯示當系統沒有固體水合物時，水合物會在系統中某處形成第一個穩定水合物後才開始快速生長而當系統存在固體水合物時，水合物會從固體水合物與液體的界面開始生長且生長的水合物結構與固體水合物相似證明了封存煙氣的可行性。;Clathrate hydrates are crystalline compounds where water forms cage-like structures around gas molecules at low temperatures and very high pressures. Due to the widespread presence of methane (CH4) hydrates in the continental shelves and seabed sediments, they are regarded as a highly promising emerging energy source. A replacement of CH4 by carbon dioxide (CO2) in hydrates, allowing CO2 to be stored as hydrates on the seafloor while the released CH4 can be used as energy, was proposed to both mitigate greenhouse gas effects and address energy challenges. In addition, a novel method involves using flue gas to extract CH4 hydrates was suggested. Flue gas, the exhaust gas from power generation, primarily consists of CO2 and nitrogen (N2). Using flue gas to replace CH4 hydrates can reduce the cost of separating CO2, and N2 can replace hydrate structures that are difficult for CO2 to replace, thus enhancing CH4 recovery rates. However, the current understanding of the nucleation process of mixed gas molecules in hydrate formation is still limited. In this study, molecular dynamics (MD) simulations were used to investigate the nucleation competition of hydrate formation in systems of liquid water and gas molecules (CH4, CO2, and N2) and the effect of gas types and gas ratio on nucleation rates. The results showed that the nucleation rate of pure CO2 hydrates is faster than that of pure CH4 and pure N2 hydrates. Adding CO2 significantly promotes the nucleation of CH4 hydrates compared to N2 hydrates. In a binary CH4/CO2 gas system, the initial nucleation 512 cage (with 12 pentagonal faces formed by water molecules) always encapsulates CH4. In a ternary CH4/CO2/N2 gas system, N2 can also be encapsulated in the initial 512 cage. The proportion of hydrates formed shows a linear correlation with the ratio of mixed gases. CH4 and N2 form 512 cages, while CO2 forms 4151062 cages (with 1 square face, 10 pentagonal face, and 2 hexagonal faces formed by water molecules). Additionally, simulation results indicated that without solid hydrates, hydrate growth only accelerates after the first stable hydrate forms somewhere in the system. In contrast, with solid hydrates present, growth begins at the interface between solid hydrates and the liquid, and the structure of the growing hydrate resembles that of the solid hydrate, demonstrating the feasibility of storing flue gas.
Appears in Collections:	[National Central University Department of Chemical & Materials Engineering] Electronic Thesis & Dissertation

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