On the limitation of the van der Waals-Platteeuw-based thermodynamic models for hydrates with multiple occupancy of cavities

Phase equilibria molecular simulations of hydrogen hydrates via the direct phase coexistence approach

We report the three-phase (hydrate-liquid water-vapor) equilibrium conditions of the hydrogen-water binary system calculated with molecular dynamics simulations via the direct phase coexistence approach. A significant improvement of ∼10.5 K is obtained in the current study, over earlier simulation attempts, by using a combination of modifications related to the hydrogen model that include (i) hydrogen Lennard-Jones parameters that are a function of temperature and (ii) the water-guest energy interaction parameters optimized further by using the Lorentz-Berthelot combining rules, based on an improved description of the solubility of hydrogen in water.

Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates

The cage occupancy plays a crucial role in the thermodynamic stability of clathrate hydrates and is an important quantity for understanding the CO₂-CH₄ replacement phenomenon. In this work, the occupancy isotherms of pure CH₄, pure CO₂, and their mixture in sI and sII hydrates are studied by GCMC + MD simulations. The adsorption of CH₄ and CO₂ + CH₄ in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated occupancy ratio θ_L/θ_S and the abundance ratio of CO₂ to CH₄ vary with the temperature and pressure, which provide the prerequisite information for the prediction of CH₄ recovery yield at different conditions in the CO₂-CH₄ gas exchange process. The phase equilibria of clathrate hydrates of pure gases and mixtures are explored and the corresponding heat of dissociation and hydration numbers are determined. The current investigation provides new perspectives to understand the mechanism behind the gas adsorption behavior of clathrate hydrates.