Thermodynamic and spectroscopic analysis of pure and mixed gas hydrates

Abstract

Gas hydrates have been applied to many industrial fields such as desalination, food concentration, selective separation of targeted component, carbon dioxide sequestration into the deep ocean and exploitation of natural gas hydrates. In this study, thermodynamic and spectroscopic analysis of pure and mixed gas hydrates was intensively investigated, especially focused on application to energy and environmental systems. The research scope of this study can be divided into three major fields. The first one is two-phase equilibria of $CH_4$ hydrates, the second one is phase behavior and structure identification of the mixed chlorinated hydrocarbon hydrates and the third one is hydrate phase equilibria in porous media. For thermodynamic analysis hydrate phase behaviors of multi-phase and multi-guest systems were carefully measured and compared with thermodynamic modeling. For spectroscopic analysis the structure and cage occupancies of the pure and mixed hydrates were determined through NMR and Raman spectroscopy. First, the aqueous solubilities of $CH_4$ in the two-phase (hydrate (H)-liquid water $(L_W)$) region, which is very close to the deep sea floor condition, were measured at various ranges of temperature and pressure. The hydration number determined via Raman spectroscopy at 10.0 MPa and 274.15 K was found to be 6.00 that is a little higher than 5.75 of the ideal one. The solubility of $CH_4$ in liquid water largely increased with a small increase of temperature, but slightly decreased with increasing pressure in the two-phase $(H-L_W)$ region. This solubility behavior was experimentally confirmed to be completely different from that occurring in the three-phase $(H-L_W-V)$ boundary. The present results might be valuable as the fundamental data for estimating the amount of in situ gas hydrate and understanding the unique feature of hydrate formation/dissociation mechanism and the hydrate stability in the deep ocean sediments. Second, thermodynamic feasibi...