Calculation of Liquid Water-Hydrate-Methane Vapor Phase Equilibria from Molecular Simulations

• Main Authors: Jensen, Lars, Thomsen, Kaj, von Solms, Nicolas, Wierzchowski, S., Walsh, M.R., Koh, C.A., Sloan, E.D., Wu, D.T., Sum, A.K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2010
• Subjects: Methane hydrate
• Online Access: https://orbit.dtu.dk/en/publications/50ba02e1-8996-4bf2-b93d-cbc66ee0497f; https://doi.org/10.1021/jp911932q

Monte Carlo simulation methods for determining fluid- and crystal-phase chemical potentials are used for the first time to calculate liquid water-methane hydrate-methane vapor phase equilibria from knowledge of atomistic interaction potentials alone. The water and methane molecules are modeled using the TIP4P/ice potential and a united-atom Lennard-Jones potential. respectively. The equilibrium calculation method for this system has three components, (i) thermodynamic integration from a supercritical ideal gas to obtain the fluid-phase chemical potentials. (ii) calculation of the chemical potential of the zero-occupancy hydrate system using thermodynamic integration from an Einstein crystal reference state, and (iii) thermodynamic integration to obtain the water and guest molecules' chemical potentials as a function of the hydrate occupancy. The three-phase equilibrium curve is calculated for pressures ranging from 20 to 500 bar and is shown to follow the Clapeyron behavior, in agreement with experiment; coexistence temperatures differ from the latter by 4-16 K in the pressure range studied. The enthalpy of dissociation extracted from the calculated P-T curve is within 2% of the experimental value at corresponding conditions. While computationally intensive, simulations such as these are essential to map the thermodynamically stable conditions for hydrate systems.