Prediction of the Three-Phase Coexistence Conditions of Pure Methane and Carbon Dioxide Hydrates Using Molecular Dynamics Simulations
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Clathrate hydrates are solid crystals that consist of three-dimensional networks of hydrogen-bonded water molecules forming well-defined cages within which small “guest“ molecules are needed in order to stabilize the structures. More than 130 different molecules can form hydrates when mixed with water at relatively low temperatures and high pressures, including methane, ethane, propane, iso-butane, carbon dioxide, nitrogen and hydrogen. The accurate prediction of thermodynamic properties of clathrate hydrates has gained much attention due to the relevance of clathrate hydrates to many industrial applications. For example, hydrates play a major role in the problem of flow assurance in the oil and gas industry. They are also being considered for use in gas transport and separation applications. In addition, the existence of methane hydrates in large quantities in nature makes them a potential energy source. In this work, Molecular Dynamics (MD) simulations have been used in order to determine the Hydrate – Liquid water – Guest coexistence line for methane and carbon dioxide hydrates. The direct phase coexistence method was used where slabs of the three constituent phases were separately equilibrated and then brought in contact at the conditions under investigation. In order to account for the stochastic nature of the hydrate growth and dissociation processes, many long, independent simulations at different conditions of temperature and pressure were conducted while avoiding bubble formation phenomena. This allowed for performing a statistical averaging of the results to identify the three-phase coexistence temperature at different pressures. Also, the erroneous use of dispersion tail corrections was investigated. For methane hydrates, where the Lorentz-Berthelot combining rules for the two force fields used gave accurate predictions for the solubility of methane in the aqueous phase, this approach yielded predictions that are in good agreement with experimental data. A correction to the Lorentz-Berthelot cross-interaction energy parameter was applied in the case of carbon dioxide hydrates to obtain accurate predictions of the solubility of carbon dioxide in the aqueous phase, which in turn resulted in equally accurate and consistent predictions of the three-phase coexistence temperature. Therefore, it was shown that both the water-water and water-guest interactions play an important role in the application of this methodology to the study of clathrate hydrate systems. For systems where the water-guest interactions can accurately predict guest solubility in water, the predictions of the three-phase coexistence are as accurate as the water force field used to predict the melting of ice. It was also shown that the methodology cannot be directly applied to low pressures for carbon dioxide hydrates, where a liquid-like layer of carbon dioxide is adsorbed at the water surface. Several possible causes for this deficiency are suggested, including the possible effect of box anisotropy and box size fluctuations at low pressures.
direct phase coexistence
carbon dioxide hydrate
carbon dioxide solubility
Costandy, Joseph GN (2015). Prediction of the Three-Phase Coexistence Conditions of Pure Methane and Carbon Dioxide Hydrates Using Molecular Dynamics Simulations. Master's thesis, Texas A & M University. Available electronically from