A molecular dynamic simulation on the memory effect of methane hydrate

Abstract

The memory effect is one of the outstanding mysteries to be solved in the field of clathrate hydrates. In this study, a thaw-freeze cycle simulation tracing the evolution of residual hydrate cages was conducted in line with hydrate recrystallization in experiments. It has been demonstrated that the hydrate dissociates layer-by-layer parallel to the exposed surface of the hydrate phase inwards in three steps with different decomposition rates, i.e., firstly, hydrate rapidly decomposes due to its exposure to gas ambience. Secondly, an additional liquid phase mass transfer resistance is introduced for decomposing CH4 molecules into gas phase due to the water reservoirs emerged by decomposition of external hydrate, resulting in a decomposition stable stage until the hydrate phase vanished. Thirdly, gas-liquid two-phase completes separation. The critical nucleus size has been identified as a criterion to explicate the memory effect. The decreasing residual nuclei with increasing temperature and time during dissociation prolong the induction time. These findings can help to better understand the memory effect at the molecular scale.