An Accurate Thermodynamic Model to Characterise Dissociating N₂O₄ at Vapour-Liquid Equilibrium States

A new thermodynamic model is presented, capable of accurately representing the vapour-liquid equilibrium pressures and densities, and liquid phase densities and enthalpies of dissociating dinitrogen tetroxide (N₂O₄ ⇄ 2NO₂). The model is based on the Peng-Robinson equation of state coupled with advanced mixing rules. The -required but non-measurable- critical coordinates of the pure components forming the reactive mixtures are optimized, within a variability range defined in a previous study, to fit experimental vapour-liquid equilibrium data. The optimized parameters are then validated by comparing calculated thermodynamic properties with available experimental data in the subcritical region. The negligible impact of the higher temperature reaction 2NO₂ ⇄ 2NO + O₂, within the vapour-liquid equilibrium region where the optimisation is performed, is also proven. The resulting model is finally compared with the currently most accurate available equation of state, showing comparable results when considered both the scatter in available experimental data and the relative simplicity of the proposed equation of state. In particular, the proposed model demonstrates the satisfactory capability of a cubic equation of state to accurately reproduce both saturation pressures and saturation densities without requiring volume translation.

The online version contains supplementary material available at 10.1007/s10765-025-03565-x.