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

Abstract

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.