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

The existence of a single generalized compressibility-factor relationship for univariant systems has long been denied because reduced variables do not provide a generalized vapor-pressure relationship. The present study demonstrates that a saturation curve depends on the ability of the theorem of corresponding states to correlate the reduced temperature as a function of reduced. pressure and the critical compressibility factor. Saturation curves for both the liquid and the vapor phases are established from compressibility data on a variety of systems, substantiating this conclusion.The compressibility factors of saturated mixtures are correlated with pseudoreduced properties based upon Kay's pseudocritical concept. The results indicate that the compressibility factors of saturated liquid mixtures can be predicted with reasonable precision over the entire two-phase region. For pseudoreduced isotherms of unity and above, the same isotherms apply to both phases and are approximately consistent with published compressibility charts for pure gases.The theorem of corresponding states has been discussed many times in connection with the generalized correlation of PVT data for gases (6, 7, 22, for example). The correlation of compressibility-factor data for the univariant states along the vapor-liquid phase boundary for pure substances has, however, received a relatively s m d l amount of emphasis.Cope, Lewis, and W e h r (6) denied the existence of a simple general reduced equation of state along the saturation curve despite the fact that a single independent variable is sufficicnt to define the state of a pure substance under these conditions. This argument was based I upon conclusive experimental evidence indicating that reduced vapor pressures are not a unique generalized function of reduced temperature. As a result, two reduced-state variables have been used to define the univariant state (8, 20).The present work studies the significance of the critical Compressibility factor Fig. 1 as a correlating parameter for the twophase boundary. The problem of predicting compressibility factors for the saturated vapor and liquid phases is considered for both pure conq)ounds and mixtures. COMPRESSIBILITY FACTORS UNDER SATURATED CONDITIONS Puro CompoundsThe compressibility factor, defined by the equationRT is a stiite property of the system. Equation ( I ) may also be expresscd in reduced form, without affecting the exactness of the functional relationship between Z and the independent variables.With discussion limited to the boundary of a two-phase region, specification of any one of the state variables is sufficient to define the state of the system. If P , is arbitrarily selected as the independent variable, it follows from the principles of pli:ise behavior thrit 17, = .flu',)Conhining these relationships with Equation (2) givesEquation (5) is of interest because it demonstrates the restriction imposed upon the theorem of corresponding states for successful correlation of the rompressibility f:ictor under saturated wntliti...