| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 104 === Methane hydrate is a nonstoichiometric crystalline compound composed of water and methane at low temperatures and high pressures. Molecular Dynamics (MD) simulation has been a simulation tool used to unveil the molecular level details of methane hydrate, in this research, we use MD simulation to examine two important thermodynamic properties of methane hydrates, dissociation temperature and heat of
dissociation, and its self-preservation phenomenon simulated on the basis of the combination of Tip4p-Ice force field for water and OPLS-AA for methane. The self-preservation phenomenon shows anomalously slow methane dissociation rate on 242 K to 271 K. Self-preservation phenomenon was probably caused by amorphous quasi-liquid layer on the surface of dissociated methane hydrate decomposed from methane hydrate.
Our results supported that the TIP4P-Ice and OPLS-AA force fields could reproduce good evaluation comparing with the experimental dissociation temperatures and heat of dissociation of methane hydrate. We discovered that there was also an anomalously slow dissociation rate on 255 K to 265 K in this research, and there is flat density peak on the surface showing in density profile, and the F4 value of surface water drop down to about 0.4, and the parallel diffusivity is smaller than vertical diffusivity of surface for two degree. These structural properties might result in anomalously slow dissociation rate.
Molecular dynamics simulation can evaluate key thermodynamic properties of methane hydrate with qualitatively accuracy, and also reproduce the similar trend of self-preservation
during dissociating process, but the mechanism still need further study.
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