Richard A. Wilsak

Thermodynamic consistency tests based on the Gibbs-Duhem equation applied to isothermal, binary vapor-liquid equilibrium data: data evaluation and model testing

Four thermodynamic consistency tests are discussed along with sufficient background information so that approximations to these otherwise rigorous tests are fully appreciated. The tests are applied to three binary systems, one at low pressures, one at moderate pressures and the last at high pressures. The results provide information on the data and the models used to fit them. These results are highly sensitive to the selection of the models, a conclusion worth noting for those selecting physical property models in process simulators and performing PTx experiments. For example, the choice of an inappropriate equation of state for the representation of vapor-phase properties will introduce thermodynamic inconsistencies that cannot be removed by simply increasing the number of adjustable parameters in the activity coefficient expressions.

Vapor—liquid equilibrium measurements for the n-pentane—methanol system at 372.7, 397.7 and 422.6 K

Abstract P-x-y measurements have been obtained for the n-pentane—methanol system along the 372.7, 397.7 and 422.6 K isotherms. For these isotherms, this system exhibited maximum pressure azeotropic behavior. No other isothermal vapor—liquid equilibrium information exists in the literature for this sytem. The experimental data were correlated with the four-suffix Margules equation. The temperature dependence of the three parameters of this Margules equation was established for temperatures ranging from 372.7 to 422.6 K. However, attempts to extrapolate these relationships to lower temperatures, using excess enthalpy data available in the literature, proved inadequate for the prediction of the isobaric vapor—liquid equilibrium measurements of Tenn and Missen (1963) . Auxiliary expressions have been developed relating azeotropic pressure and composition to temperature. These expressions are valid for temperatures ranging from 303.25 to 422.6 K.

(Vapor + liquid) equilibrium behavior of (n-pentane + ethanol) at 372.7, 397.7, and 422.6 K

Abstract Isothermal (vapor + liquid) equilibrium measurements for (n-pentane + ethanol) have been made at 372.7, 397.7, and 422.6 K. Maximum-pressure azeotropic behavior was observed over the temperature range examined. The experimental results were correlated using the four-suffix Margules equation to represent the excess molar Gibbs free energy of the liquid phase. The three parameters of this model were established at each temperature using Barkers method. The results of this study have been combined with the limited information at atmospheric pressure available in the literature to develop empirical relations for the dependence of azeotropic composition and azeotropic pressure on temperature. These relations extend to the critical locus reported by McCracken et al. (J. Chem. Eng. Data1960, 5, 130). Dew- and bubble-pressures measured by McCracken et al. have been compared with corresponding values resulting from the present study in the temperature range 391 to 422.6 K. In these comparisons, deviations in pressure between the two investigations ranged from 1.2 to 33.2 per cent.

Vapor—liquid equilibrium measurements for the methanol—acetone system at 372.8, 397.7 and 422.6 K

Abstract A static high pressure equilibrium facility has been used to obtain P − x − y measurements for the methanol—acetone binary for the three isotherms 372.8, 397.7 and 422.6 K. These measurements show that maximum pressure azeotropic behaviour exists at each of these temperatures. The data obtained have been correlated satisfactorily using the three suffix Margules equation. A comparison has been made between the information resulting from this study and the high pressure data of Griswold and Wong. Parameters of the three suffix Margules equation have been correlated with temperature over the range 285–425 K using additional vapor—liquid equilibrium and excess enthalpy data available in the literature. These correlations have been used to predict isobaric behavior. Auxiliary expressions have been developed which relate azeotropic pressure and composition to temperature.

Vapor-liquid equilibrium measurements for the N-pentane-methanol-acetone ternary at 372.7 K and their prediction from the constituent binaries

Abstract Vapor—liquid equilibrium measurements for the n-pentane—methanol—acetone ternary system have been obtained along the 372.7 K isotherm. For the measurements of this polar-nonpolar system, the pressure varied from 348.5 to 850.7 kPa. Of all the equations tested, these measurements are correlated best with the four-suffix Margules equation using three adjustable ternary parameters. Nevertheless, the two- and three-parameter forms of the Wilson equation and the NRTL equation are capable of predicting these ternary measurements when the associated parameters are determined from binary information only. The UNIQUAC equation does not perform as well for this sytem primarily because it cannot model the n-pentane—methanol binary behavior satisfactorily. The predictive capabilities of the four-suffix Margules equation and the Peng-Robinson equation of state are poor. However, if an equation of state such as the Peng-Robinson equation of state is to be used, it has been clearly shown that utilizing binary interaction coefficients is a requirement, not a refinement. Finally, this ternary system does not possess a ternary azeotrope at 372.7 K. The maximum pressure is that corresponding to the azeotrope belonging to the n-pentane—methanol binary.