Thermodynamic study for separating and recovering global warming gases using clathrate

Abstract

Clathrates are non-stoichiometric inclusion compounds formed by a lattice of host molecules, strongly hydrogen bonded under certain conditions of pressure and temperature, which encage low molecular weight gases or volatile liquids in different cavities within the lattice. Applications of clathrates had been tried to several technologies such as desalination, food concentration, bio- material separation, carbon dioxide deposit in ocean floor, and natural gas production from gas hydrate field. It was believed that repeated hydrate formation and dissociation produce highly concentrated gaseous stream from multi-component gaseous mixture, and in this study, gas separation, especially global warming gases, was tried and verified with measurements of phase equilibria, formation kinetics, and heat of dissociation. Three-phase (hydrate-liquid water-vapor) equilibrium dissociation conditions for simple and mixed hydrates of carbon dioxide, nitrogen, and THF were measured. Depending on composition of gas mixture, equilibrium dissociation pressures were varied between two pure lines of carbon dioxide and nitrogen. Equilibrium dissociation pressures were also affected by concentration of added THF. THF was used as a hydrate promoter to increase the hydrate stability: that is, the hydrate promoter THF showed a striking decrease of the equilibrium pressure at a temperature. Equilibrium hydrate phase composition was measured by gas chromatography at two-phase (hydrate-vapor) region, and according to the result, P-x diagrams were constructed for the binary and ternary guest systems. New thermodynamic model was suggested for describing the hydrate promoter. Hydrate formation kinetics of carbon dioxide and nitrogen gaseous mixture with and without THF was investigated. It was observed that hydrate formation occurred in the bulk liquid water if supersaturation existed as a result of the dissolution process and that the formation rate was very fast at the early stage of the whole...