利用分子動力學模擬探討二氧化碳水合物在純水和海水中的性質

Abstract

氣體水合物是由客體氣體分子和水，在高溫低壓下所形成的白色結晶固體。由於甲烷水合物豐富地存在世界各地的大陸棚以及海底沉積物中，因此甲烷水合物被視為一種極具有發展潛力的新興能源。近幾年有科學家提出水合物的新應用，將溫室氣體二氧化碳封存在水合物中，以減緩溫室效應，因此二氧化碳水合物的研究越來越被重視。 在本研究中，我們使用分子動力學模擬方法研究二氧化碳水合物在純水和海水中的熱力學性質，本研究中模擬的水合物相關性質包括二氧化碳在水相中的溶解度、二氧化碳在水相中的擴散係數、二氧化碳水合物的熔點、融解熱以及水籠子填滿率。結果顯示在溶解度的研究方面，模擬值比實驗值大約30%；在擴散係數的模擬中，模擬值大約為實驗值的一半；在60大氣壓下，熔點的模擬值為283.5K，和實驗值283.1K非常接近；在60atm，283K下的，二氧化碳水合物融解熱的模擬值約為45.30kJ/mol CO2，比實驗值小了約13%；在填滿率的模擬，大水籠子的填滿率較小水籠子高，並且溫度約接近熔點，大水籠子填滿率越高。 加入海水後，結果顯示氯化鈉的存在降低了二氧化碳在水中的擴散係數和溶解度，和實驗上所觀察到的一致，對二氧化碳水合物則是有抑制劑的作用，也實驗所觀察到的雷同。

Clathrate hydrate are non-stoichiometric crystalline compounds composed of water and gas molecules, such as methane and carbon dioxide, in which guest gas molecules are trapped inside the hydrogen bonded cages composed of host water molecules. Methane hydrate is regarded as a potential energy resource because of its significant amount of methane gas inside the hydrate deposits in nature. Some scientists propose a new method to exploit methane from methane hydrate and to trap CO2 inside hydrate simultaneously, which can make use of methane and lower the concentration of CO2, the green house gas. As a result, the studies of CO2 hydrate draw lots of attention recently. Molecular Dynamics simulation is a useful tool to study gas hydrate under microscopic view. In this work, we use molecular dynamics simulation to study the properties of CO2 hydrate in water and 3.5 wt% sodium chloride solution. We measure solubility and diffusivity of CO2 molecules in water. In addition, we measure melting point, dissociation heat and growth occupancy of CO2 hydrate. The simulation result of solubility is larger than experimental result by about 30%, and simulation diffusivity is about half of the experimental result. The simulation melting point under 60 atm is 283.5 K, which is close to the experimental result of 283.1K. The simulation dissociation heat under 60 atm is 45.30 kJ/ mol CO2, which is smaller than experimental result by about 13%, and the variation is caused by the intrinsic property of water force filed. The occupancy of big water cages is larger than that of small water cage, and the occupancy of big cage is larger as temperature is higher. The existence of sodium chloride decrease solubility and diffusivity of CO2 in water phase, which is in agreement with experimental result. In addition, sodium chloride acts as an inhibitor in our simulation, as same as in experiments.