Differential heats of condensation and enthalpies of saturated liquid hydrocarbon mixtures

Abstract

By use of saturated-vapor enthalpies determined by accepted procedures in conjunction with differential latent heats of condensation, the saturated-liquid enthalpies are established for the system methane-ethane at 200,400, and 600 lb./sq. in. abs. and for the systems ethane-n-butane and propane-n-butane at the two lower pressures. Where comparison is possible, the liquid-enthalpy values obtained by this method are in general agreement with those previously determined by Edmister and Canjar. The procedure used is thermodynamically rigorous and may be applied to any binary mixture, provided the BenedictRubin-Webb equation of state is valid in the vapor phase and P-V-T-x data are available.-4s operating pressures in the petroleum and related industries increase, the heat effects and the vapor-liquid equilibrium relationships become difficult to predict. Desigris for conditions of elevated pressures are often reduced to questionable estimates and the design engineer is frequently confronted with inconsistencies between #te vapor-liquid equilibrium data and the enthalpy data. The Kellogg equilibrium-vaporization ratios (1 3), based on the Renediet-Rubin-Webb equation of state (4), have helped immeasurably in the prediction of accurate y/x data over wide pressure ranges; however, the determination of the heat effects has remained a problem.At the present time the partialenthalpy approach used in conjunction with the Benedict-Rubin-Webb equation appears to be the most promising method for obtaining precise thermal data on mixtures which will also be consistent with the equilibrium information. This approach has been used by Papadopoulos, Pigford, and Friend (16) and Edmister

Liquid Phase Enthalpy Values for the Methane-Ethane System.

“…The final liquid phase enthalpy data are illustrated on Figure 5 and Table I. The enthalpies presented differ from those given previously (10). The former values were obtained by use of the differential heat of condensation, while the values presented here are based chiefly on the differential heat of vaporization.…”

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In PREVIOUS investigations (8,10,11) saturated liquid phase enthalpies have been calculated for five binary hydrocarbon systems over the pressure range 100 to 600 p.s.i.a. These enthalpy data were calculated from P-V-T-x data and the differential heat of condensation.

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A thermodynamic equation relating equilibrium vapor‐liquid compositions and enthalpy differences in isobaric multicomponent systems

“…For example, the rigorous equation used by Stiehl et al (18) needs information on saturated liquid densities, several sets of equilibrium data at pressure levels in the neighborhood of the pressure of interest to evaluate a partial derivative, and P-V-T data of vapor mixtures. Equation (5) requires only the last item of information.…”

Saturated Liguid Phase Enthalpies at High Pressures. Ethane-n-Heptane System.

“…p.s.i.a. Saturated liquid phase enthalpy data have been presented for other hydrocarbon systems (5,7).…”

“…Once the validity of the Benedict-Webb-Rubin equation (3) was established, the necessary volumetric data for the solution of Equation 1 could be calculated. Also, by using the equation (3) and enthalpy data (7) for the pure components in the ideal gas state (7), the required vapor phase enthalpy data could be calculated. The relationship used was…”