Gibbs and Helmholtz energies of formation of sI clathrate hydrates from CO$_2$, CH$_4$ and water

TL;DR

This study calculates the thermodynamic stability of sI methane and carbon dioxide hydrates using Helmholtz and Gibbs energies, providing insights into hydrate formation and gas exchange processes relevant for energy and climate applications.

Contribution

It introduces a combined computational and literature-based approach to determine thermodynamic energies for hydrate formation and exchange, highlighting the role of molecular occupancy.

Findings

Helmholtz and Gibbs energies for hydrate formation are quantified.

Thermodynamic paths for CH4-CO2 exchange are proposed.

CO2 distinguishes cage sizes, influencing hydrate stability.

Abstract

We determine thermodynamic stability conditions in terms of Helmholtz and Gibbs energies for sI clathrate hydrates with CH$_4$ and CO$_2$ at 278 K. Helmholtz energies are relevant for processing from porous rocks (constant volume), while Gibbs energies are relevant for processing from layers on the ocean floor (constant pressure). We define three steps leading to hydrate formation, and find Helmholtz energy differences from molecular simulations for two of them using grand-canonical Monte Carlo simulations at constant temperature and volume; while the third step was calculated from literature data. The Gibbs energy change for the same steps are also determined. From the variations in the total Helmholtz and Gibbs energies we suggest thermodynamic paths for exchange of CH$_4$ by CO$_2$ in the isothermal hydrate, for constant volume or pressure, respectively. We show how these paths for the mixed hydrate can be understood from single-component occupancy isotherms, where CO$_2$, but not CH$_4$, can distinguish between large and small cages. The strong preference for CH$_4$ for a range of compositions can be explained by these.