Thermodynamic characterization and microscopic analysis of natural gas and mixed hydrates containing guest molecules of low molecular weight

Abstract

Clathrate hydrates are crystalline compounds formed by the physically stable interaction between water and relatively small guest molecules. Under suitable conditions of pressure and temperature, water molecules, via hydrogen bonding, form into polyhedral cavities that stack to give space-filling frameworks. Because of partial cage filling, these crystalline compounds are non-stoichiometric. They can be divided into three distinct structural families I (sI), II (sII), and H (sH), which differ in the combination of cavities of different sizes and shapes. Since each volume of hydrate was known to contain as much as 184 volume of gas, it has been suggested the potential application of crystal hydrate nature to large-scale storage of greenhouse gases and removal of organic pollutants from aqueous solutions. The thermodynamic phase equilibria and structural characteristics of clathrate hydrate provide the fundamental variables contributed to develop the industrial processes. In this study, the phase equilibrium conditions of pure and mixed hydrates were determined to identify the thermodynamic stability region and the microscopic approaches draw the structural characteristics related to actual cage occupancy of guest molecules in hydrate structure In order to understand the characteristics of clathrate gas hydrate, natural gas hydrates from Oregon offshore and Okhotsk sea were investigated in macro and microscopic. In contrast to the researches of laboratory-made gas hydrate, this study has been performed on naturally grown clathrate hydrates from the sea floor. The X-ray diffraction pattern of natural gas hydrate showed that it is ordinary structure I hydrate. $^{13}C$ NMR spectrum was used for natural gas hydrate in order to identify the cage occupancy of guest molecules and determine the hydration number and ensure the hydrate structure. The NMR spectra revealed that natural gas hydrates used in this study contains only methane with no noticeable amount of other ...