Validation of the RK-PR equation of state for the calculation of volumetric properties for hydroethanolic mixtures at subcritical conditions

RODRÍGUEZ-RUIZ, A.C.; PISONI, G. O.; VELEZ, A.R.

Los Cocos, Córdoba

Conferencia; VI Iberoamerican Conference on Supercritical Fluids (PROSCIBA 2023); 2023

IPQA/PLAPIQUI

The family of equations of state (EoS) stemming from the Van der Waals EoS has been employed for decades to model phase equilibria in systems of varying natures. This family is characterized by being cubic with respect to volume and featuring attractive and repulsive parameters, both of which depend on the critical constants (Tc and Pc) of the pure component.When applying the equation of state to mixtures, a mixing rule for calculating these parameters should be used. The Redlich-Kwong, Soave-Redlich-Kwong, or Peng-Robinson EoS can be mentioned as the most widely used within this family, and although these equations yield satisfactory results in reproducing equilibrium data, they encounter difficulties in accurately representing volumetric properties of both pure components and mixtures. This problem becomes more pronounced when modeling highly asymmetric mixtures. The RK-PR EoS belongs to the Van der Waals family of equations of state and incorporates a third parameter to adjust the volumetric properties of pure compounds, thus enhancing the accuracy in reproducing these properties. It's important to note that asymmetric mixtures composed of compounds capable of forming hydrogen bonds (associations) present an additional challenge. Typically, these types of mixtures are modeled using equations of state designed to account for such associations, but these equations tend to be more complex than the ones described earlier, making their implementation a more laborious process. In this context, the objective of this study is to employ the RK-PR EoS coupled with cubic mixing rules to model experimental data for an associative system. The primary focus lies in accurately calculating volumetric properties of the mixture, including density and molar volume. For model validation, experimental P-T-x data of bubble points of the water + ethanol system under isoplethic and subcritical conditions were used. These bubble points were obtained using an isochoric method, where the phase transition points from liquid + vapor to a single-phase system are detected by the change in slope of the isochore on a pressure vs. temperature graph. Two different values of ethanol molar fractions were used for the bubble point measurements: 0.4 and 0.6. The experimental setup used for the isochoric experiments consisted of a stainless-steel constant- volume cell with a capacity of 5.48 ml, sealed at both ends. The experimental transition points obtained provide the volumetric data, which are subsequently reproduced using the RK-PR EoS for modeling equilibrium data. The isochoric method has proven to be a simple and effective technique for detecting phase transition points in fluid systems. Furthermore, the RK- PR EoS has demonstrated a strong capability for thermodynamic modeling of hydroethanolic mixtures. It can be utilized to predict phase envelopes of the water + ethanol system under subcritical conditions and to predict the volumetric properties of the system in equilibrium with acceptable accuracy.