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.