Thermodynamic analysis and structural identification of pure and mixed hydrates

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. Three-phase equilibria consisting of vapor, water-rich liquid and solid hydrate were measured for the aqueous solutions containing two gas guest molecules of carbon dioxide and methane. The upper quadruple points at which the four phases of vapor, water-rich liquid, $CO_2$-rich liquid and solid hydrate coexist were also measured to examine the phase boundary of hydrate stability region. However, the stability region of mixed hydrate in the presence of liquid guest molecules was quite different from that of pure hydrate. Three cyclic ethers of THF, propylene oxide and 1,4-dioxane were chosen and their effects on hydrate equilibrium were carefully investigated. It was noted that the organic liquid guest molecules greatly...