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

Abstract

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.

Reaction Coordinate of Incipient Methane Clathrate Hydrate Nucleation

“…This corresponds to a point roughly 59.5 K below liquid−hydrate− vapor coexistence for P = 500 bar and the molecular model used. 34 Only 46 of the 200 simulations nucleated and grew hydrate, indicating that we were operating at the edge of current computationally tractable conditions for unbiased, spontaneous nucleation with this model system. The critical nucleus size was estimated in terms of the MCG-1 OP using a basin commitment approach that mimics a proper committor test.…”

Structural and dynamical properties of methane clathrate hydrates from molecular dynamics: Comparison of atomistic and more coarse-grained potential models

“…Indeed, the use of optimal potential models for hydrate systems has always been a challenge, not least in studies of kinetics due to the need to have a good knowledge of the underlying phase diagram in terms of 'driving forces' for nucleation or growth, etc. [29][30][31] For accurate estimation of 'subtleties' in equilibrium and transport properties, such as thermal conductivity, more 'tailored' potential models (especially for host-guest interactions) can be important. 18,19,32,33 In this respect, a number of studies have applied a range of atomistic potential models (e.g., for both water and guests, rigid and flexible, fixedcharge and polarisable, different guest-host combining rules, etc) to demonstrate the variability of results.…”

A novel application of kinetic Monte Carlo method in the description of N2 vapour–liquid equilibria and adsorption

“…A number of schemes for MC simulations use the Metropolis algorithm (Allen and Tildesley, 1989;Frenkel and Smit, 1996) (importance sampling) to generate configurations in the Markov chain, from which the thermodynamic ensemble averages can be determined (Blas, 2001;Gotlib et al, 1997;Jensen et al, 2010;Vega, 1992). This is successful with systems having a uniform distribution of density.…”