매립지가스 수송 및 저장을 위한 이산화탄소-메탄 혼합가스 하이드레이트의 상평형 및 분광학적 해석에 관한 연구

Abstract

The recent quest to replace fossil fuels with renewable and sustainable energy sources has increased interest on utilization of landfill gas. It is further augmented due to environment concerns and global warming caused by burning of conventional fossil fuels, energy security concerns and high cost of crude oil, and renewable nature of these gases. Gas hydrates are non-stoichiometric inclusion compounds, which are formed by the physically stable interaction between water and relatively small guest molecules occupied in the cavities built by water molecules. They have been classified into three distinct structures I, II and H by the difference in the cavity shape and size of hydrates. These structures are arranged into well-defined three-dimensional crystalline solids. In this study, three phases, H-Lw-V, equilibrium behaviour for the carbon dioxide-methane and carbon dioxide-methane-tetrahydrofuran (THF) mixed hydrate systems are observed at several isothermal conditions. For the carbon dioxide-methane hydrate system, the three phase dissociation pressures decrease with increasing the concentration of carbon dioxide at all the considered isothermal conditions. At all equilibrium conditions, the concentrations of carbon dioxide in the hydrate phase are always higher than that in the vapor phase. However, it is interesting to note that, for the carbon dioxide-methane-THF hydrate system, the three phase dissociation pressures decrease with increasing the concentration of carbon dioxide at 290.15 K, whereas it is almost the same regardless of the concentration of carbon dioxide-methane mixtures at 283.15K. This may be caused by the difference in occupancy and stability of guest molecules in the small cages of sII hydrates. X-ray powder diffraction (XRPD) and Raman spectroscopy are used to investigate the crystal structure of hydrates and occupation of guest molecules in the cages of hydrates. The hydrates formed from the carbon dioxide-methane mixtures are found to be sI hydrate, whereas the carbon dioxide-methane-THF hydrates form sII structure. For the sI structure of carbon dioxide-methane hydrates, both guest species of carbon dioxide and methane can occupy both small and large cages of the hydrate frameworks. However, the small guest molecules of carbon dioxide and methane occupy only the small cages of the sII hydrate, as all the large cages are occupied by THF molecules of 5.56 mol%.