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

RODRÍGUEZ-RUIZ, ANDREA C; PISONI, GERARDO O.; VELEZ, ALEXIS R.

CAMPINAS

Conferencia; Iberoamerican Conference on Supercritical Fluids ? PROSCIBA; 2023

UNICAMP

The family of equations of state (EoS) stemming from the Van der Waals EoS has beenemployed for decades to model phase equilibria in systems of varying natures. This family ischaracterized by being cubic with respect to volume and featuring attractive and repulsiveparameters, both of which depend on the critical constants (Tc and Pc) of the purecomponent. When applying the equation of state to mixtures, a mixing rule for calculatingthese parameters should be used. The Redlich-Kwong, Soave-Redlich-Kwong, orPeng-Robinson EoS can be mentioned as the most widely used within this family, andalthough these equations yield satisfactory results in reproducing equilibrium data, theyencounter difficulties in accurately representing volumetric properties of both purecomponents and mixtures. This problem becomes more pronounced when modeling highlyasymmetric mixtures. The RK-PR EoS belongs to the Van der Waals family of equations ofstate and incorporates a third parameter to adjust the volumetric properties of pure compounds, thus enhancing the accuracy in reproducing these properties. It's important tonote that asymmetric mixtures composed of compounds capable of forming hydrogen bonds(associations) present an additional challenge. Typically, these types of mixtures are modeledusing equations of state designed to account for such associations, but these equations tend tobe more complex than the ones described earlier, making their implementation a morelaborious process. In this context, the objective of this study is to employ the RK-PR EoScoupled with cubic mixing rules to model experimental data for an associative system. Theprimary focus lies in accurately calculating volumetric properties of the mixture, includingdensity and molar volume. For model validation, experimental P-T-x data of bubble points ofthe water + ethanol system under isoplethic and subcritical conditions were used. Thesebubble points were obtained using an isochoric method, where the phase transition pointsfrom liquid + vapor to a single-phase system are detected by the change in slope of theisochore on a pressure vs. temperature graph. Two different values of ethanol molar fractionswere used for the bubble point measurements: 0.4 and 0.6. The experimental setup used forthe isochoric experiments consisted of a stainless-steel constant-volume cell with a capacityof 5.48 ml, sealed at both ends. The experimental transition points obtained provide thevolumetric data, which are subsequently reproduced using the RK-PR EoS for modelingequilibrium data. The isochoric method has proven to be a simple and effective technique fordetecting phase transition points in fluid systems. Furthermore, the RK-PR EoS hasdemonstrated a strong capability for thermodynamic modeling of hydroethanolic mixtures. Itcan be utilized to predict phase envelopes of the water + ethanol system under subcriticalconditions and to predict the volumetric properties of the system in equilibrium withacceptable accuracy.