Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Abstract

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature. The free energy calculations from MD suggest that the methane storage capacity may be increased when methane molecules are placed inside the large cage of structure H (sH) hydrate. However, methane storage in pure methane sH clathrates with multiple methane occupancies are only of theoretical interest as the required pressure is about 0.5 GPa or higher. It has also been demonstrated that methane may replace tert-butyl methyl ether (TBME) as the large guest species in the large cage.