(500c) Monte Carlo Simulation of the Three-Phase Equilibria of Water-Hydrate-Methane

The liquid water-methane hydrate-methane vapor phase equilibria was calculated using Monte Carlo simulation methods for determining fluid and crystal-phase chemical potentials. The potential models used were the TIP4P/Ice potential for the water and the united-atom Lennard-Jones potential for the methane. 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 equilibria was determined in the pressure region of 20-500 bar and the coexistence temperatures differed by 5-15 K compared to experimental values. The pressure-temperature relationship exhibited high level of linearity in a semilogarithmic plot as also observed for experimental data. The equilibrium occupancy of the hydrate follows the Langmuir adsorption theory which is also applied in statistical mechanic theories used to predict hydrate equilibria. Also investigated was the contribution due to the interaction between guests and host molecules on the location and shape of the three-phase equilibria boundary. This was obtained by comparing certain simulations results to those obtained using the original van der Waals and Platteeuw theory and two extensions including guest-guest and guest-host interactions.