Modeling the vapor–liquid equilibrium and association of nitrogen dioxide/dinitrogen tetroxide and its mixtures with carbon dioxide

We have used in this work the crossover soft-SAFT (soft-Statistical Associating Fluid Theory) equation of state to model nitrogen dioxide/dinitrogen tetroxide (NO2/N2O4), carbon dioxide (CO2) and their mixtures. The prediction of the vapor–liquid equilibrium of this mixture is of utmost importance to correctly assess the NO2 monomer amount that is the oxidizing agent of vegetal macromolecules in the CO2 + NO2/N2O4 reacting medium under supercritical conditions. The quadrupolar effect was explicitly considered when modeling carbon dioxide, enabling to obtain an excellent description of the vapor–liquid equilibria diagrams. NO2 was modeled as a self-associating molecule with a single association site to account for the strong associating character of the NO2 molecule. Again, the vapor–liquid equilibrium of NO2 was correctly modeled. The molecular parameters were tested by accurately predicting the very few available experimental data outside the phase equilibrium. Soft-SAFT was also able to predict the degree of dimerization of NO2 (mimicking the real NO2/N2O4 situation), in good agreement with experimental data. Finally, CO2 and NO2 pure compound parameters were used to predict the vapor–liquid coexistence of the CO2 + NO2/N2O4 mixture at different temperatures. Experimental pressure–CO2 mass fraction isotherms recently measured were well described using a unique binary parameter, independent of the temperature, proving that the soft-SAFT model is able to capture the non-ideal behavior of the mixture.