Molecular Dynamics Study on the Growth Mechanism of Methane plus Tetrahydrofuran Mixed Hydrates

Molecular dynamics (MD) simulations are performed to analyze the dominating factors for the growth of CH4 + THF mixed hydrates, and the results are compared with the growth of single guest CH4 and THF hydrates. While CH4 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 CH4 in the system enhances the dissociation temperature. The growth rate of CH4 + 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 CH4 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 CH4 is adsorbed at the interface. The growth rate is dominated by the uptake of CH4 at the interface, as in the case of pure CH4 hydrate. Below 290 K, the growth is not much affected by the presence of CH4. Instead, the growth rate is determined by the rearrangement of THF molecules at the interface, as in the case of pure THF hydrate.