Effect of Cage Occupancy on Stability and Decomposition of Methane Hydrate.

Gas hydrates usually contain a certain number of empty cages that will both affect the hydrate stability and reduce the gas storage capacity. In this work, by MD simulations, we found that the hydrate stability is related to the cage occupancy, the empty cage types, and especially the distribution of empty cages. With the decrease of overall occupancy, the stability of hydrate becomes worse. Under the same overall occupancy, the more concentrated the empty cages are, the more unstable the hydrate is and hence the faster it decomposes. The methane molecules may migrate between distorted cages during the decomposition, resulting in a temporary increase in the stability of hydrate. Hydrates with different empty cage distributions show different decomposition mechanisms: when empty cages are concentrated, the melting rate is fast first due to the rapid decomposition of empty cages, but the remaining filled cages will reduce the melting rate; when empty cages are separated on the contrary, the early melting is slow because of the high local occupancy, and the following melting will be accelerated because of the high melting surface area. It indicates that the empty cage distribution plays a controlling role in hydrate decomposition kinetics at different stages.