The melting kinetics of gas hydrate with different cage occupancy and empty cage distribution

Abstract

We present results from a molecular dynamics study of the melting behavior of gas hydrates with empty cages under the specified overheating driving force, to understand how the melting process is dominated by thermodynamics and kinetics, respectively. The melting point of methane hydrates in different motif (cage occupancy and empty cage distribution) was determined firstly by direct simulations, and then the effects of overheating driving force, cage occupancy and empty cage distribution on the melting rate and mechanism were explored. Our results show that the induction time of melting has an Arrhenius relation with the overheating, and the activation energy is used for the distortion of the hydrate skeleton, which is independent of whether the cage is filled or not. The melting of hydrate can be divided into linear stage and final stage. The melting rate increases with the increase of overheating and the decrease of occupancy. In the linear stage, the empty and the filled cages are melted at multiple sites simultaneously if the empty cage distribution is separated, whereas the empty cage is rapidly melted from a single site for the system with concentrated empty cages. The phenomenological equation we proposed can be used to describe the influence of the real-time change of the occupancy and the empty cage distribution on the melting rate and mechanism. With the decrease of the size of guest molecules, the hydrate cage changes from filled-like to empty-like through a multi occupied intermediate state. The species boundary of Lennard-Jones guest molecules that can form empty-like cages is given.