Improvement of the Van Der Waals equation of state

The Van Der Waals equation of state has been modified in order to improve its accuracy. The introduction of a temperature dependence of the attraction term allows reproducing accurately pure-compound vapour pressures. The modification of the mixing rules, with the infinite-pressure excess-free-energy term expressed by a NRTL-like model, allows reproducing accurately phase equilibria of polar and apolar systems. Finally, the introduction of a volume correction improves markedly the calculated liquid densities, without changing the phase-equilibrium conditions. The improved equation is accurate enough to be applied for design purposes.