Molecular Dynamics Study on the Growth Mechanism of
Methane plus Tetrahydrofuran Mixed Hydrates
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Abstract
Molecular
dynamics (MD) simulations are performed to analyze the
dominating factors for the growth of CH<sub>4</sub> + THF mixed hydrates,
and the results are compared with the growth of single guest CH<sub>4</sub> and THF hydrates. While CH<sub>4</sub> hydrate has a type
I crystalline structure, the presence of THF in the aqueous phase
results in the growth the type II structure hydrate. Compared to THF
hydrates, the presence of CH<sub>4</sub> in the system enhances the
dissociation temperature. The growth rate of CH<sub>4</sub> + THF
mixed exhibits a maximum value at about 290 K at 10 MPa. The growth
rate is found to be determined by two competing factors: (1) the adsorption
of CH<sub>4</sub> at the solid–liquid interface, which is enhanced
with decreasing temperature, and (2) the migration of THF to the proper
site at the interface, which is enhanced with increasing temperature.
Above 290 K, which is about 10 K higher than the dissociation temperature
of pure THF hydrate, the growth of cage can proceed only when a sufficient
amount of CH<sub>4</sub> is adsorbed at the interface. The growth
rate is dominated by the uptake of CH<sub>4</sub> at the interface,
as in the case of pure CH<sub>4</sub> hydrate. Below 290 K, the growth
is not much affected by the presence of CH<sub>4</sub>. Instead, the
growth rate is determined by the rearrangement of THF molecules at
the interface, as in the case of pure THF hydrate