Cage occupancy of methane hydrates from Gibbs ensemble Monte Carlo simulations

摘要

Isobaric-isothermal Gibbs ensemble Monte Carlo simulations of sI methane hydrates in equilibrium with bulk methane are performed to calculate large and small cage occupancies. The OPLS united-atom Lennard-Jones potential, and a variation, are used to model methane, while the TIP4P/Ice model represents water. This model system produces systematically higher total cage occupancies than those of simulations by Henley and Lucia [1] using TIP4P-Ew water and TraPPE-UA methane. We also see higher total occupancies than those observed in prior Raman spectrometry studies by Uchida et al. [2], but find good agreement for large cage occupancies. The simulations provide comprehensive predictions of large and small cage occupancies for temperatures and pressures within the ranges 270-290 K and 20 400 bar, respectively. These predictions demonstrate the advantages of the Gibbs ensemble for the simulation of clathrate hydrates; the ability to control the temperature and pressure, while not constraining the chemical composition of the hydrate phase is the key advantage of this ensemble. (C) 2015 Elsevier B.V. All rights reserved.