Calculation of liquid water-hydrate-methane vapor phase equilibria from molecular simulations.
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.
Jensen
Lars
L
Center for Energy Resources Engineering, Department of Chemical & Biochemical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
Thomsen
Kaj
K
von Solms
Nicolas
N
Wierzchowski
Scott
S
Walsh
Matthew R
MR
Koh
Carolyn A
CA
Sloan
E Dendy
ED
Wu
David T
DT
Sum
Amadeu K
AK