Adsorption of CO$_2$ and CH$_4$ and their mixtures in gas hydrates

TL;DR

This study uses grand-canonical Monte Carlo simulations to analyze how CO₂ and CH₄ gases, alone and mixed, adsorb in gas hydrates, revealing site preferences, thermodynamic stability, and potential pathways for gas exchange.

Contribution

It provides new insights into the adsorption behavior and thermodynamics of CO₂ and CH₄ in gas hydrates, including site-specific interactions and stability conditions.

Findings

Methane sites are equivalent; CO₂ sites vary by size.

Mixtures behave ideally when only large sites are occupied.

Certain CO₂-CH₄ mixtures are thermodynamically more stable.

Abstract

We report results from grand-canonical Monte Carlo simulations of methane and carbon dioxide adsorption in structure sI gas hydrates. Simulations of pure component systems show that all methane sites are equivalent, while carbon dioxide distinguishes between two types of sites, large or small. The adsorbed mixture can be regarded as ideal, as long as only large sites are occupied. A strong preference is demonstrated for methane, when the smaller sites become filled. The molar heat of adsorption of methane decreases with composition, while the molar heat of adsorption for carbon dioxide passes an extremum, essentially in accordance with the observation on the site sizes. The Helmholtz energies of the hydrate with CO$_2$-CH$_4$ gas mixture for temperatures between 278 and 328 K and pressures between 10$^4$ and 10$^9$ Pa indicate that certain mixtures are more stable than others. The results indicate that a thermodynamic path exists for conversion of a pure methane hydrate into a pure carbon dioxide hydrate without destroying the hydrate structure.