Application of the Benedict‐Webb‐Rubin equation of state to argon

Abstract

The coefficients of the Benedict-Webb-Rubin equation of state have been developed for argon. By employing these coefficients, the volumetric behavior of argon has been predicted with an average deviation of 0.241 % for five hundred ninety-seven smoothed and experimental data points in the superheated region.At temperatures below the critical, two sets of Go's, one for the liquid and one for the vapor, were needed to relate the vapor pressure to the densities of saturated argon. However, consistent fugacities for the saturated vapor and liquid argon could not be predicted with these Co values. Therefore, another set of C, ' s was developed by equating the pure component vapor and liquid fugacities along the vapor pressure curve. These values were used to test the applicability of the equation of state to predict derived thermodynamic properties.The original BWR expression for calculating isothermal pressure effects on enthalpy was modified to include explicitly the temperature dependence of the coefficient C,. Vapor-liquid equilibrium relations for the argon-nitrogen system predicted by the standard BWR procedure were compared with experimental data.The empirical equation of state of Benedict, Webb, and Rubin ( 3 ) was originally developed to correlate and predict the fugacities of light hydrocarbon mixtures. Since its publication, it has been used extensively to represent the volumetric and phase relations of light hydrocarbons and some nonhydrocarbons. Several papers (3 to 5, 8,13) have been published describing success in predicting high pressure phase behavior of multicomponent systems. AS the BWR technique gained widespread use, more and more shortcomings were reported (8, 1 3 ) . Most of the difficulties have been observed in predicting P-V-T relationship within the critical region, and predicting vaporliquid equilibrium ratios at low temperatures. As a result, it is widely believed that the BWR equation is inadequate for systems at low temperatures. Attempts have been made to modify the equation and the combining rules for mixtures in order to use it for low temperature predictions. The limitations of the BWR method and suggested modifications have been summarized by Canjar ( 8 ) , Ellington ( 1 3 ) , and Lin (20). The application of the BWR approach to the cryogenic systems nitrogen-methane (26) and nitrogen-carbon monoxide ( 2 4 ) at temperatures below 200"R. established that temperature is not a limiting condition for use of the technique. The results of the present work extend the application of the BWR method to another cryogenic material, namely, argon.As was recently stated by Thodos ( 2 5 ) , argon is a good substance to test the validity of an equation of state. Its molecule is monatomic and therefore its properties should resemble those expected from a simple fluid as defined by Pitzer ( 2 3 ) . Furthermore, its relatively high atomic weight provides a good test for any equation employing a high power series in density. Beattie and Bridgeman (2) showed that such an equation can apply to argon.Th...

The P-V-T Behavior of Nitrogen, Argon, and Their Mixtures

“…The Strobridge equation was then used to compute values of Z and P for each composition using the constants of Table XII and equation (11), and these values were compared to the experimental results. Calculated values of Z and P were also determined using the Benedict-Webb-Rubin equation with constants for argon from Zudkevitch et al [28], constants for nitrogen from Hirschfelder et al [8], and constants for the mixtures determined using the combining rules as listed in the latter [8].…”

Nitrogen constants for the Benedict-Webb-Rubin equation of state

“…The argon constants (14) used in this investigation are listed in Table III. The mixture combining rules (8) used are as follows: a Lorentz combination for S0; linear squareroot combinations for A0, Co, and y, and linear cube root combinations for a, b, c, and a.…”

“…The equation subsequently has been applied to a number of other substances (9,11,13,14). As has been discussed elsewhere (9), it is not possible to determine a unique set of constants for this equation that will correlate accurately liquid-vapor phase behavior as well as gas phase P-V-T behavior over a wide range of temperature and pressure.…”

Conformal solution correlation for the nitrogen‐oxygen‐argon system

“…These equations represent mixture averages over the p and v constants a b(16) in the LennardJones equations for the various pairs in the mixture(17) ThePTP ( p a b ) and PTP ( V a h ) are pair-type-probability functions which permit weighting of the individual pairs as necessary to secure agreement between prediction and experiment (10). The PTP function can be represented symbolically a s The W, and Wb are factors which give numerical representation of the size and shape effects of species a and b.…”