Thermodynamic Equilibria in Xylene Isomerization. 1. The Thermodynamic Properties ofp-Xylene

Abstract

Measurements and calculations leading to the determination of thermodynamic properties for the gaseous and condensed phases of p-xylene (Chemical Abstracts registry number (supplied by the authors) [106-42-3]) are reported. All measurement results reported were obtained with a differential-scanning calorimeter (DSC). The critical temperature was measured by DSC. Saturation heat capacities for the liquid phase between 370 K and 550 K, the critical density and the critical pressure were derived with fitting procedures involving the new DSC results and literature vapor pressures and densities. Results were combined with heat capacities reported in the literature obtained with adiabatic calorimetry and the enthalpy of combustion to derive standard molar entropies, enthalpies, and Gibbs free energies of formation at selected temperatures between 250 K and 550 K. The standard state is defined as the ideal gas at the pressure p ) p°) 101.325 kPa. Standard entropies are compared with those calculated statistically on the basis of assigned vibrational spectra. Results are compared with literature values. Literature vapor pressures, enthalpies of vaporization, virial coefficients, densities, and heat capacities for the condensed and gaseous phases are checked for consistency with the values used in this research.

Thermodynamic properties of the methylpyridines. Part 2. Vapor pressures, heat capacities, critical properties, derived thermodynamic functions between the temperatures 250 K and 560 K, and equilibrium isomer distributions for all temperatures ≥250 K

“…Recently, Selco and Carrick (49) detected a small amount of anharmonicity in several of the fundamental vibrations of the xylenes. The effect of the anharmonicity on calculated standard entropies for the xylenes was shown to be very small; ≤ 0.02· R for temperatures between T = 250 K and T = 550 K. (36)(37)(38) It is probable that similar anharmonicity exists for the methylpyridines, but such small errors (≥ 0.02· R) are not significant in the context of the present paper.…”

“…Second virial coefficients were estimated with the corresponding-states equation of Pitzer and Curl, (32) and third virial coefficients were estimated with the corresponding-states method of Orbey and Vera. (33) This formulation for third virial coefficients was applied successfully in analyses of the thermodynamic properties of pyridine, (34) benzene and methylbenzene, (24) 2-methylaniline, (35) the xylenes, (36)(37)(38) ethylbenzene, (39) and the dimethylpyridines. (40) Uncertainties in the virial coefficients were assumed to be 10 per cent.…”

“…It has been shown (24,(34)(35)(36)(37)(38)(39)(40) that reconciliation of standard entropies T 0 S o m (cal) derived from thermophysical property studies (table 8) with standard entropies T 0 S o m (stat) derived with assigned vibrational spectra and the methods of statistical mechanics can be exploited cent. The method used to estimate the uncertainties was described previously.…”

Possible precursors and products of deep hydrodesulfurization of gasoline and distillate fuels III. The thermodynamic properties of 1,2,3,4-tetrahydrodibenzothiophene

“…The Bartlesville Thermodynamics Group successfully applied this formulation for third virial coefficients in analyses of the thermodynamic properties of a range of compounds in recent years. The list of compounds studied includes benzene and methylbenzene [30], the xylenes [61][62][63], 2-methylaniline [64], pyridine [65], the methyl pyridines [66,67], and the dimethylpyridines [68]. In all the listed cases the agreement between the properties derived using this formulation and those obtained from statistical thermodynamics via accurate spectroscopic measurements has been better than 0.1% over a temperature range of greater than 250 K. Third virial coefficients are required for accurate calculation of the volume of gas for pressures greater than 0.1 MPa.…”

Possible precursors and products of deep hydrodesulfurization of gasoline and distillate fuels IV. Heat capacities, enthalpy increments, vapor pressures, and derived thermodynamic functions for dicyclohexylsulfide between the temperatures (5 and 520) K

“…[25], and third virial coefficients were estimated with the corresponding-states method of Orbey and Vera [26]. This formulation for third virial coefficients was applied successfully in analyses of the thermodynamic properties of a range of compounds in recent years including benzene and methylbenzene [27], the xylenes [28][29][30], 2-methylaniline [31], pyridine [32], the methyl pyridines [33,34], and the dimethylpyridines [35]. In all cases the agreement between the properties derived using this formulation and those obtained from statistical thermodynamics via accurate spectroscopic measurements has been better than 0.1% over a temperature range of greater than 250 K. Third virial coefficients are required for accurate calculation of the volume of gas for pressures greater than 0.1 MPa.…”