Experimental Measurement and Thermodynamic Modeling of Hydrate-Phase Equilibria for the Ternary C2H6 + NaCl + Water and C3H8 + NaCl + Water Mixtures in Silica Gel Pores

Abstract

We measured hydrate-phase equilibria for the ternary C2H 6 + NaCl (3 wt %) + water and C3H8 + NaCl (3 wt %) + water mixtures in silica gel pores of nominal 6.0, 15.0, and 30.0 nm diameters. We also measured hydrate-phase equilibria for the ternary C 2H6 + NaCl + water and C3H8 + NaCl + water mixtures in nominal 30.0 nm silica gel pores at two different NaCl concentrations of 3 and 10 wt %. The combined effect of pores and electrolytes shifted three-phase H-LW-V equilibrium lines to a higher pressure region for any given temperature depending upon NaCl concentrations and pore sizes. The experimentally measured hydrate-phase equilibria were compared to the calculated results based on the van der Waals and Platteeuw model. The activity of water within silica gel pores saturated with the aqueous electrolyte solutions was expressed by adopting a Pitzer model for electrolyte solutions and a correction term for the capillary effect. With the values for a hydrate-water interfacial tension (σHW) of 39 ± 2 mJ/m2 for C2H6 hydrate and 45 ± 1 mJ/m2 for C3H8 hydrate, the calculation values were in good agreement with the experimental values. The structure of each C 2H6 and C3H8 hydrate formed in silica gel pores saturated with NaCl solutions was found to be identical to that of each C2H6 and C3H8 hydrate in the pure bulk state through nuclear magnetic resonance spectroscopy.