title: 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.
authors.affiliation: Center for Energy Resources Engineering, Department of Chemical & Biochemical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
authors.forename: Lars
authors.initials: L
authors.lastname: Jensen
authors.forename: Kaj
authors.initials: K
authors.lastname: Thomsen
authors.forename: Nicolas
authors.initials: N
authors.lastname: von Solms
authors.forename: Scott
authors.initials: S
authors.lastname: Wierzchowski
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Walsh
authors.forename: Carolyn A
authors.initials: CA
authors.lastname: Koh
authors.forename: E Dendy
authors.initials: ED
authors.lastname: Sloan
authors.forename: David T
authors.initials: DT
authors.lastname: Wu
authors.forename: Amadeu K
authors.initials: AK
authors.lastname: Sum