Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates

Three-phase equilibria of hydrates from computer simulation. II. Finite-size effects in the carbon dioxide hydrate

In this work, the effects of finite size on the determination of the three-phase coexistence temperature (T3) of the carbon dioxide (CO2) hydrate have been studied by molecular dynamic simulations and using the direct coexistence technique. According to this technique, the three phases involved (hydrate-aqueous solution-liquid CO2) are placed together in the same simulation box. By varying the number of molecules of each phase, it is possible to analyze the effect of simulation size and stoichiometry on the T3 determination. In this work, we have determined the T3 value at 8 different pressures (from 100 to 6000 bar) and using 6 different simulation boxes with different numbers of molecules and sizes. In two of these configurations, the ratio of the number of water and CO2 molecules in the aqueous solution and the liquid CO2 phase is the same as in the hydrate (stoichiometric configuration). In both stoichiometric configurations, the formation of a liquid drop of CO2 in the aqueous phase is observed. This drop, which has a cylindrical geometry, increases the amount of CO2 available in the aqueous solution and can in some cases lead to the crystallization of the hydrate at temperatures above T3, overestimating the T3 value obtained from direct coexistence simulations. The simulation results obtained for the CO2 hydrate confirm the sensitivity of T3 depending on the size and composition of the system, explaining the discrepancies observed in the original work by Míguez et al. [J. Chem Phys. 142, 124505 (2015)]. Non-stoichiometric configurations with larger unit cells show a convergence of T3 values, suggesting that finite-size effects for these system sizes, regardless of drop formation, can be safely neglected. The results obtained in this work highlight that the choice of a correct initial configuration is essential to accurately estimate the three-phase coexistence temperature of hydrates by direct coexistence simulations.

Ab Initio Studies of Structural and Mechanical Properties of NH3, NO, and N2O Hydrates

The investigation on the mechanical properties of clathrate hydrate is closely related to the exploitation of hydrates and gas transportation. In this article, the structural and mechanical properties of some nitride gas hydrates were studied using DFT calculations. First, the equilibrium lattice structure is obtained by geometric structure optimization; then, the complete second-order elastic constant is determined by energy-strain analysis, and the polycrystalline elasticity is predicted. It is found that the NH₃, N₂O, and NO hydrates all have high elastic isotropy but are different in shear characteristics. This work may lay a theoretical foundation for studying the structural evolution of clathrate hydrates under the mechanical field.

Analysis of Influencing Factors and Kinetic Characteristics of Spherical Methane Hydrate Decomposition

Herein, several molecular systems are simulated by molecular dynamics to study the decomposition process and fluctuation-dissipation characteristics of spherical methane hydrates under different conditions. The spherical radius and the movement of the hydrate-liquid water interface during decomposition are measured. Different fitted formulas of the variation of methane numbers are obtained from the decomposition of spherical and bulk methane hydrates. Fluctuation-dissipation characteristics for spherical methane hydrates with different radii are analyzed, which show that increasing the scale of hydrates can increase the relaxation time and slow down the fluctuation process. The variations of the hydrogen bond and hydrogen-bond lifetime are calculated. For hydrate phase water, the peak of the hydrogen-bond lifetime lies between 8 and 10 ps. After complete decomposition, the hydrogen-bond lifetime mainly distributes in 0 and 2 ps and the peak disappears. The effects of temperature, cage occupancy, liquid phase environment, and spherical hydrate scale are explored. The decomposition activation energy for the spherical hydrate with a radius of 20 Å is calculated to be 52.23 kJ/mol. It can speed up the decomposition rate as well as the diffusion of methane and water molecules with a lower cage occupancy. For the effect of the liquid phase environment, it is found that the number of liquid water rarely affects the decomposition. However, when the Na⁺ and Cl^- concentrations change from 0 to 10%, the decomposition time reduces from ∼511 to ∼369 ps, which indicates that there is an obviously positive impact on decomposition.

Molecular Simulations of CO2/CH4, CO2/N2 and N2/CH4 Binary Mixed Hydrates

Abstract

Grand canonical Monte Carlo simulations were performed to study the occupancy of structure I multicomponent gas hydrates by CO2/CH4, CO2/N2, and N2/CH4 binary gas mixtures with various compositions at a temperature of 270 K and pressures up to 70 atm. The presence of nitrogen in the gas mixture allows for an increase of both the hydrate framework selectivity to CO2 and the amount of carbon dioxide encapsulated in hydrate cages, as compared to the CO2/CH4 hydrate. Despite the selectivity to CH4 molecules demonstrated by N2/CH4 hydrate, nitrogen can compete with methane if the gas mixture contains at least 70% of N2.