Calculating the Phase Behavior of Gas-Hydrate-Forming Systems from Molecular Models

We describe a calculation of the phase behavior of the methane−water system, including the structure-I hydrate phase, starting from a model of the intermolecular forces in the system and using Monte Carlo simulations and theory. The approach we use differs from previous calculations of methane hydrate phase behavior in that it does not treat the water molecules in the hydrate as a static or harmonic lattice, i.e., thermal fluctuations are fully incorporated. Our approach is quite general, but we illustrate it here for perhaps the simplest model capable of forming stable hydrate-like structures. This is a mixture of network-forming associating hard spheres and nonassociating hard spheres. With this model as a reference system, we add dispersion forces and dipole−dipole interactions as perturbations. Monte Carlo simulations were used to determine the solid-phase properties of this model, and the fluid phases were treated using an accurate thermodynamic perturbation theory. Our results show that this simple molecular model is able to describe the phase diagram in qualitative agreement with the experiment. In particular, it correctly describes the region of stability of a single-phase methane structure-I hydrate and its dependence on temperature, pressure, and composition.