Thermodinamical consistent method for the calculation of solid-liquid-liquid equilibrium

Póster presentado en el congreso EQUIFASE 99 (Vigo, del 20 al 24 de Junio de 1999). Resumen publicado en el libro de actas del CongresoThe problematic nature of the calculation of phase equilibria is very well known and much literature has been written in this respect. Different aspects have been covered and the treatment of the liquid-liquid and liquid-vapour equilibria has been widely studied, both from the theoretical and the experimental point of view. Recently some efforts have been made to apply the Gibbs energy functions in order to properly determine the parameters of any equation to calculate the activity coefficient of liquids. In this type of work, very useful topological concepts are applied with this objective. Apart from possible inconsistencies in some equations used to formulate the liquid phase activity coefficients the thermodynamic basis for these types of equilibria are correctly used, and the experimental procedures very well established. The case of solid-liquid equilibrium, does not seem to be exactly the same. Many articles have been found in literature where the salting-out effect has been studied, some of them including some mistakes, as has been pointed out in other papers where a systematic procedure to experimentally study these type of systems has been suggested. Nevertheless, very few articles have been found where some type of correlation has been carried out involving the solid phase. Normally, the papers dealing with any type of correlation involving solid compounds (either ionic or non-ionic) only consider the liquid-liquid equilibrium using different equations to formulate the activity of the ions in the liquid phase. Very few papers and a book have been found where the solid phase (normally one pure component) is considered in the equilibrium and where the solubilities are either correlated or predicted. The aim of the present work is to clearly show these problems and suggest a method to calculate and correlate liquid – liquid – solid and liquid – solid equilibrium, in all the regions of the phase diagram in a thermodynamically consistent way, using a unique reference state (pure liquid at the temperature and pressure of the system) for both solid and liquid phases. The method is applied to four ternary systems determined previously at 25 ºC. The results obtained show that the method yields a very good correlation of all the experimental data, with thermodynamic constants that are valid for all the regions of the phase diagram