Atomistic Molecular Dynamics Simulations for the Growth Mechanism of Methane and Carbon Dioxide Hydrates and the Replacement Mechanism
Date Issued
2011
Date
2011
Author(s)
Tung, Yen-Tien
Abstract
The key factors that affect the growth of methane and carbon dioxide hydrates from pure or aqueous NaCl solutions are identified using molecular dynamics simulations. The three-phase molecular models consisting of methane/carbon dioxide gas, liquid water, and solid hydrate phase are used in this study. The melting temperatures of pure methane and carbon dioxide hydrates are found to be in good agreement with experiment over a wide range of pressures. The growth rate of clathrate hydrate is found to be dominated by (1) the temperature and pressure driving forces (2) the solubility of gas in the liquid phase, and (3) the mobility of water molecules. From our simulation results, the growth rate of methane hydrate increases with the pressure (the solubility of gas) below the equilibrium temperature of clathrate hydrate. In addition, the mobility of water is affected by and the pressure and the presence of NaCl. The growth rate of carbon dioxide hydrate decreases with the pressure and the low growth rate of methane hydrate is found when the NaCl is added.
The mechanism of in-situ methane recovery and carbon dioxide sequestration in methane hydrate is uncovered using molecular dynamics simulations. Our results suggest that in situ conversion of methane hydrate to carbon dioxide hydrate without melting is possible. The two-phase molecular models consisting of liquid carbon dioxide and solid methane hydrate phase are used in this study. Depending on the distance to the interface, there are two replacement mechanisms (1) the swapping of methane and carbon dioxide molecules (2) co-occupation of methane and carbon dioxide molecules. The carbon dioxide spontaneously enters into the cage through the an opening of broken hydrogen bond. The break of hydrogen bond is caused by the fluctuation of water in the unstable cages, the collision of gas molecules, or the hydrogen bonding interaction between the solute and water.
Subjects
clathrate hydrate
crystallization mechanism
molecular dynamics simulation
Type
thesis
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