Phase equilbria and formation kinetics of clathrate solutions with cyclic ethers

Abstract

Clathrate hydrates (or gas hydrates) are crystalline compounds which are formed by physically stable interaction between water and relatively small guest molecules occupied in the cavities built by water molecules. There are small and large cavities capable of entrapping guest molecules in an open network of host molecules composed of unit crystalline structures. 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 multicomponent gaseous mixture, and in this study, gas separation, especially global warming gases, was tried and verified with measurements of phase equilibria, formation kinetics. Three-phase(hydrate-liquid water-vapor) equilibrium dissociation conditions for simple and mixed hydrates of carbon dioxide, nitrogen, and THF were measured. Equilibrium dissociation pressures were 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. New thermodynamic model was suggested for describing the hydrate promoter. Hydrate formation kinetics of carbon dioxide hydrate with THF, and carbon dioxide and nitrogen gaseous mixture hydrate 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 period of hydrate formation. The kinetic model developed in the current work was based on the adsorption theory and had a few adjustable parameters which represented the hydrate particle growth. Fugacity difference between dissolved and equilibrium...