Studies about an Equation of State for Pure Associated Fluids: Temperature Dependent Co-Volume Accounting a Physically Consistent Repulsive Term

Studies related to the development of equations of state (EOS) to represent thermophysical properties of pure compounds are considered as important tools for engineers to design and optimize industrial equipment and processes. Furthermore, these tools also contribute to amplify the researchers’ knowledge related to molecular interaction types, in attempting to predict and correlate both energetic and volumetric effects existing in the compounds. From several equations of state existing, the cubic plus association (CPA) EOS are employed in the calculations of thermophysical properties of compounds, in which the molecular interactions occurring are the association type. In spite of good representation of these properties, it is possible to improve the predictive and correlative capability of the CPA EOS by substitution of terms whose physical meaning can be better. In this way, modifications of the cubic plus association equation of state are proposed: the original repulsive term is replaced by the Carnahan-Starling repulsion term; the attractive term is changed to an attraction term similar to the Peng-Robinson EOS. Furthermore, both attraction and repulsion terms are taken to be temperature dependent when alpha and beta functions are employed in calculations. All implementations make the equation of state non-cubic in relation to volume. Vapor pressure and liquid molar volumes of 1-alkanols (C1 to C10) and water were correlated to experimental data using this non-CPA–EOS format, and good agreement is observed