Molecular Dynamics Study of Clathrate-like Ordering of Water in Supersaturated Methane Solution at Low Pressure.

Using molecular dynamics, the evolution of a metastable solution for "methane + water" was studied for concentrations of 3.36, 6.5, 9.45, 12.2, and 14.8 mol% methane at 270 K and 1 bar during 100 ns. We have found the intriguing behavior of the system containing over 10,000 water molecules: the formation of hydrate-like structures is observed at 6.5 and 9.45 mol% concentrations throughout the entire solution volume. This formation of "blobs" and the following amorphous hydrate were studied. The creation of a metastable methane solution through supersaturation is the key to triggering the collective process of hydrate formation under low pressure. Even the first stage (0-1 ns), before the first fluctuating cavities appear, is a collective process of H-bond network reorganization. The formation of fluctuation cavities appears before steady hydrate growth begins and is associated with a preceding uniform increase in the water molecule's tetrahedrality. Later, the constantly presented hydrate cavities become the foundation for a few independent hydrate nucleation centers, this evolution is consistent with the labile cluster and local structure hypotheses. This new mechanism of hydrogen-bond network reorganization depends on the entropy of the cavity arrangement of the guest molecules in the hydrate lattice and leads to hydrate growth.

Influence of N2 on Formation Conditions and Guest Distribution of Mixed CO2 + CH4 Gas Hydrates.

In this contribution, a method based on a solid solution theory of clathrate hydrate for multiple cage occupancy, host lattice relaxation, and guest-guest interactions is presented to estimate hydrate formation conditions of binary and ternary gas mixtures. We performed molecular modeling of the structure, guest distribution, and hydrate formation conditions for the CO2 + CH4 and CO2 + CH4 + N2 gas hydrates. In all considered systems with and without N2, at high and medium content of CO2 in the gas phase, we found that CO2 was more favorable in occupying clathrate hydrate cavities than CH4 or N2. The addition of N2 to the gas phase increased the ratio concentration of CO2 in comparison with the concentration of CH4 in clathrate hydrates and made gas replacement more effective. The mole fraction of CO2 in the CO2 + CH4 + N2 gas hydrate rapidly increased with the growth of its content in the gas phase, and the formation pressure of the CO2 + CH4 + N2 gas hydrate rose in comparison to the formation pressure of the CO2 + CH4 gas hydrate. The obtained results agreed with the known experimental data for simple CH4 and CO2 gas hydrates and the mixed CO2 + CH4 gas hydrate.

Ozone storage capacity in clathrate hydrates formed by O3 + O2 + N2 + CO2 gas mixtures.

Ozone storage capacity in clathrate hydrates formed from gas mixtures of O3 + O2 + N2 + CO2 was studied. It was found that in such system the amount of ozone included in the hydrate phase can be at least several times higher than for the experimentally described O3 + O2 + CO2 gas hydrates. The most promising thermobaric conditions and gas phase compositions for the formation of ozone containing hydrates from gas mixtures which include nitrogen are suggested on the basis of the obtained results.