| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 105 === 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.
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