Vapor‐liquid equilibria at high pressures: Calculation of partial molar volumes in nonpolar liquid mixtures

Abstract

To be useful, a thermodynamic treatment of high-pressure vapor-liquid equilibria must describe how the fugacity of each component, in each phase, depends on the temperature, pressure, and composition. In the vapor phase, this dependence i s given by the fugacity coefficient which can be found from vapor-phase volumetric properties a s given by an equation of state. In the liquid phase it is more convenient to express the fugacity of a component as the product of the mole fraction, an arbitrary standard state fugacity and an activity coefficient; the effect of temperature, pressure, and composition on the fugacity of a component in the liquid phase i s determined by the effect of these variables on the activity coefficient. In this work we are concerned with the effect of pressure on the activity coefficient.At low or moderate pressures, liquid-phase activity coefficients are very weakly dependent on pressure and, as a result, it has been customary to assume that, for practical purposes, activity coefficients depend only on temperature and composition. In many c a s e s this is a good assumption but for phase equilibria a t high pressures, especially for those near critical conditions, it can lead to serious error.When the standard state fugacity is defined a t a constant pressure, then for any component i the pressure dependence of the activity coefficient y i i s given exactly by On the other hand, when the standard state fugacity is defined a t the total pressure of the system, Equation (1) must be modified to ( l a )By judicious choice, i t is sometimes possible to use a standard state such that V, = z'io, in which case the activity coefficient is very nearly independent of pressure (17).However, since "i is a function of composition, whereasis not, such a happy choice of standard state can make the right-hand side of Equation (2) very small over only a narrow range of composition. At high pressures in the critical region, Ci is usually a strong function of composition, especially for heavy components where Gi frequently changes sign a s well a s magnitude.Experimental activity coefficients obtained a t P, the total pressure of the system, can be corrected to a constant, arbitrary reference pressure P' by integration of Equation (1):

Influence of Modifiers on Supercritical Fluid Chromatography (SFC) and Supercritical Fluid Extraction (SFE), Part I

“…In section of columns under separation together with complex mobile phase also present analytes of different nature (which are present in initial analyzing mixture). Chueh and Prausnitz [21] [22] have modeled CO 2 with mixtures of some of these compounds (including methanol and ethanol) for the estimation of the critical parameters T c and P c . The most complete collection of SCCO 2 solubility data with multiple solutes is presented in "Solubility in Supercritical…”

Thermodynamic Models for the Prediction of Petroleum-Fluid Phase Behaviour

“…Following London's theory, (Chueh & Prausnitz, 1967) proposed a correlation suitable for mixtures of paraffins which only requires knowledge of the critical volume of the two pure substances i and j (see Eq. (25) in Table 3).…”

Mobile and stationary phases for SFC: Effects of using modifiers

“…[10]). Three important estimation methods for estimating properties of mixed fluids are those of Chueh and Prausnitz [11], Kreglewski and Kay [12] and Lee and Kesler [8]. The first two methods estimate the critical properties, whereas the latter method yields pseudo-critical properties.…”