Phase equilibria and formation kinetics of clathrate solutions containing electrolytes and organic substances

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. The thermodynamic model for the equilibrium calculation of the hydrate-forming system is developed using the van der Waals and Platteeuw model and the Soave-Redlich-Kwong equation of state incorporated with the Huron-Vidal mixing rule. The fitted parameters are two Kihara energy and size parameters, ε and σ, only. The continuity between hydrate-ice-vapor and hydrate-aqueous liquid-vapor three-phase curves at the lower quadruple point is proved and the fugacity relationship between the ice and pure liquid water is newly derived through the proof of that continuity. The equilibrium calculation is performed in all possible phase boundaries, H-I-V, $H-L_1-V$, $H-I-L_1$, $H-L_2-V$, $H-L_1-L_2$, $L_1-L_2-V$, and $I-L_1-V$ ($L_1$ for water-rich and $L_2$ for guest-rich liquid phase), of various hydrate-forming systems and covers to a very low temperature region. Upper and lower quadruple points of each system are obtained. The thermodynamic model described in this work is capable of the calculation of the hydrate dissociation pressure as well as compositions of all coexisting phases and also applicable to the inhibition effect of methanol in the hydrate formation system. Calculated results have good agreements with extensive experimental data reported in the literature. Temperature-pressure phase equilibria of R22 (chlorodifluoromethane) and carbon dioxide clathrate hydrate formation systems containing various organic substances, such as sucrose, glucose, fructose and lactic acid, and several electrolytes, such as NaCl, KCl and $MgCl_2$ were determined experimentally in the three-phase, hydrate-aqueous liquid...