The heterogeneous nucleation of methane hydrate in the presence of ice (hexagonal ice-Ih) was studied at different initial methane concentrations (from 3 to 10 mole %) at 250 K and 50 MPa using molecular dynamics simulations. The ice growth rate was observed to decrease with CH4 concentration. The newly grown ice shows stacking faults with formation of layers of hexagonal and cubic ice. The growing ice produced an accumulation of methane molecules near the advancing interface and promoted formation of partial hydrate cages at the ice surface. These partial cages have a short lifetime, but the transient hydrate-like order frequently propagated away from the surface leading to nucleation near but usually not on the surface. A growing ice front thus works as a catalyst-promoting agent and initiates a faster rate of hydrate nucleation compared to the homogeneous system. Under the conditions of the simulation, the nucleation time of methane hydrate was reduced by approximately half in the presence of ice and the catalytic effect of ice has been seen in the form of hydrate order fluctuations from the enhanced ice front. The space-time correlation (Van Hove Function) highlights the diffusional behavior of self and distinct parts of hydrate-like order parameters, which distinguish respectively the roles of molecule diffusion from order propagation through different molecules in the nucleation mechanism. The great majority of runs showed nucleation occurring near the ice interface or in the bulk, while nucleation on the surface was observed only in the cases (4/40) where the growing ice front was cubic.

Hydrate-like order propagation promotes nucleation of CH4 hydrate in the presence of ice: A molecular dynamics study

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