| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 103 === Tetrahydrofuran (THF) is an effective promoter of methane hydrates. In this work, the stability limit (aqueous-hydrate coexisting condition) of THF/CH4+THF hydrates and the THF solubility in water were reproduced using molecular dynamics (MD) simulations. After that, the growth/nucleation mechanism of THF/CH4+THF hydrates were investigated and discussed the roles of CH4 and THF in these reaction. In the research of THF hydrate growth, the rate of growth of THF hydrates is found to exhibits a maximum value when the liquid phase THF concentration is about 0.3 to 0.8 times (depending on temperature) of the THF concentration in the hydrate phase. The maximum growth rate of THF hydrate is a result of two competing effects: the adsorption of THF molecules to the growing interface, which is the limiting step at low THF concentrations, and the desorption/rearrangement of THF molecules at the interface, limiting step at high THF concentrations. The dominating factors for the growth of CH4+THF mixed hydrates are analyzed 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 (increasing with the pressure). 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: the adsorption of CH4 at the solid-liquid interface (such as CH4 hydrate growth), which is enhanced with decreasing temperature, and the migration of THF to the proper site at the interface (such as THF hydrate growth), which is enhance 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 sufficient amount of CH4 is adsorbed at the interface. Below 290 K, the growth is not much affected by the presence of CH4.
The nucleation of methane, tetrahydrofuran (THF), and methane+tetrahydrofuran hydrates were also investigated in this work. Our simulation results of the nucleation of CH4 hydrates and CH4+THF hydrates supported the blob hypothesis (BH). Comparing with different systems, the CH4 molecules in the aqueous solution were found to supply the regular cages and the THF molecules were found to enhance the formation of blobs (decrease the induction time) by supplying enough guests.
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