Prediction of gas-solid equilibrium using equations of state

The correlation of experimental data of solubility of solids in supercritical solvents using equations of state (EOS) is a well known and relatively simple approach. Normally, cubic EOS with quadratic mixing rules (or modifications of them) may be satisfactorily used for fitting purposes and much research is being done in order to improve the predictive capabilities of present models. Usually, models are assessed in terms of goodness of fit of solubility data; however, in spite of the pressure and temperature conditions of supercritical extraction processes, the critical behavior predicted by models used for correlating data is rarely analyzed. In this work, the analysis of critical lines in the presence of solid-fluid equilibria is proposed as an additional test of models because it gives the correct topology of predicted phase diagrams. It is shown that multiphase equilibria is normally predicted in ranges of interest for supercritical extraction. Multiphase equilibria is not obvious when a stability analysis is ignored, inducing possible modeling pitfalls. In addition, multiphase behaviour affects the prediction of stable solid-fluid equilibria. Examples and phase diagrams are discussed in detail using a modified van der Walls EOS and quadratic mixing rules.