Representation of hydrate phase equilibria in aqueous solutions of methanol and electrolytes using an equation of state

In this work, the method for the calculation of hydrate phase equilibria proposed by Zuo and Stenby (Zuo, Y.-X.; Stenby, E. H. SEP 9816. J. Sec. Petr. Eng., submitted) has been extended to represent incipient equilibrium hydrate formation conditions in aqueous solutions containing methanol and electrolytes. To predict hydrate formation conditions, the parameters of the extended Furst-Renon electrolyte equation of state (Furst, W.; Renon, H. AIChE J. 1993, 39, 335-343) have been reevaluated. Interaction parameters between water and methanol have been determined to match experimental hydrate formation data of methane in the water-methanol solutions. The interaction parameters between methanol and ions are associated with cationic Stokes and anionic Pauling diameters. Furthermore, cationic Stokes diameters in methanol are assumed to be a function of temperature. Coefficients of the temperature dependence have been determined by two methods. In method I, the cationic Stokes diameters in methanol are equal to those at 298.15 K. In method II, the coefficients are adjusted so that the predictions match the experimental hydrate formation data of the 80% methane + 20% carbon dioxide mixture in the presence of methanol and electrolytes. The proposed methods have been applied to calculate hydrate formation conditions for a number of systems containing methanol and/or electrolytes. Good agreement has been reached between the calculated hydrate formation temperatures (or pressures) and the experimental data. The overall temperature deviations obtained by methods I and II are within 0.56 and 0.70 K, respectively.