Thermodynamics and Kinetics of Methane Hydrate Formation in Nanoporous Media : Theory and Molecular Simulation

Orateur : Soutenance de Thèse de Jin DONGLIANG

Résumé : Methane hydrate is a non-stoichiometric crystal in which water molecules form hydrogen-bonded cages that entrap methane molecules. Abundant methane hydrate resources can be found on Earth, especially trapped in mineral porous rocks (e.g., clay, permafrost, seafloor, etc.). For this reason, understanding the thermodynamics and formation kinetics of methane hydrate confined in porous media is receiving a great deal of attention. In this thesis, we combine computer modeling and theoretical approaches to determine the thermodynamics and formation kinetics of methane hydrate confined in porous media. First, the state-of-the-art on the thermodynamics and formation kinetics of methane hydrate is presented. Second, different molecular simulation strategies, including free energy calculations using the Einstein molecule approach, the direct coexistence method, and the hyperparallel tempering technique, are used to assess the phase stability of bulk methane hydrate at various temperatures and pressures. Third, among these strategies, the direct coexistence method is chosen to determine the shift in melting point upon confinement in pores. We found that confinement decreases the melting temperature. The shift in melting temperature using the direct coexistence method is consistent with the Gibbs-Thompson equation which predicts that the shift in melting temperature linearly depends on the reciprocal of pore width. The quantitative validity of this classical thermodynamic equation to describe such confinement and surface effects is also addressed. The surface tensions of methane hydrate-substrate and liquid water-substrate interfaces are determined using molecular dynamics to quantitatively validate the Gibbs-Thompson equation. Molecular dynamics simulations are also performed to determine important thermodynamic properties of bulk and confined methane hydrate : (a) thermal conductivity using the Green-Kubo formalism and the autocorrelation function of the heat-flux and (b) the thermal expansion and isothermal compressibility. Finally, some conclusions and perspectives for future work are given.

Télécharger dans mon agenda