Assessing the quality of SAFT equations for the vapor-liquid equilibrium of pure water

Abstract

The family of eight representative (and most commonly used) SAFT equations of state for water and the cubic-plus-association equation are examined with respect to their ability to predict accurately the vapor–liquid equilibrium (VLE) of water. The problem originates from the fact that all these equations use only the experimental equilibrium liquid densities and pressures for the parameter evaluation. Consequently, when reproducing the VLE calculations once the parameters are evaluated, the equilibrium properties cannot be then determined accurately by the equations, in principle, because the computation involves also the omitted vapor density. In this contribution, the errors of the predicted equilibrium densities, orthobaric pressures, and related properties are determined and discussed. It turns out that even the properties used in the equations’ parametrizations (pressure and liquid density) are not always reproduced with acceptable accuracy, with average errors being around two percent and errors of the individual data points exceeding in many cases even ten percent. Properties not included in the parametrization are then predicted with larger average errors, in general.