Calculating critical transitions of fluid mixtures: Theory vs. experiment

Abstract

The progress in predicting critical transitions in fluid mixtures is reviewed. The critical state provides a valuable insight into the general phase behavior of a fluid and is closely linked with the nature and strength of intermolecular interaction. Calculations of critical equilibria have been con fined mainly to binary mixtures. The prediction of binary gas-liquid critical properties was initially limited to empirical correlations. These techniques have been superseded by rigorous calculations of the critical conditions using realistic models of the fluid or equations of state. All of the known types of critical phenomena exhibited by binary mixtures can be, at least, qualitatively calculated. If an optimal combining rule parameter is allowed, continuous gas-liquid properties can be calculated accurately for a wide variety of mixtures. Similarly, the pressure and composition dependence of upper critical solution phenomena can be accurately predicted. Progress has been achieved in predicting discontinuous critical transitions in polar and nonpolar binary mixtures. There is increasing interest in calculating the critical properties of ternary and multicomponent mixtures. Although the techniques applied to binary mixtures often can be directly extended to ternary mixture calculations, calculated critical properties of ternary mixtures indicate that their behavior cannot be considered as a simple extension of binary mixture phenomena. Consequently, ternary critical calculations are likely to provide a superior insight into the phase behavior of multicomponent fluids. IntroductionA critical transition of a fluid is reached when there is no longer any difference in the physical properties between coexisting phases. In the case of a pure fluid, the pressure, temperature and volume of coexisting gas and liquid phases are identical at the gas-liquid critical point. For binary and other multicomponent fluids, the gas-liquid point is also characterized by an equivalence of composition in both phases. Multicomponent fluids can also exhibit critical equilibria between different coexisting liquid phases. It is this latter aspect that generates most of the variety of critical phenomena between mixtures of different component molecules. At very high pressures, the distinction between what constitutes a liquid and what constitutes a gas is not clear, and many workers (Schneider, 1978;McGlashan, 1985) prefer the term "fluid-fluid equilibria'' to embrace both types of phenomena.There are ample examples of the importance of high-pressure critical transitions in chemical engineering processes. The most commonly cited examples include supercritical extraction, distillation, and the enhancement of oil recovery. The location of the critical point determines whether or not retrograde condensation or evaporation (Hicks and Young, 1975) will occur. The vapor pressure curve of a pure liquid ends at the gasliquid critical point, whereas in a binary mixture, at any composition, liquid and vapor coexist in equilibrium between the dew and bu...

Global phase equilibrium calculations: Critical lines, critical end points and liquid–liquid–vapour equilibrium in binary mixtures

“…A comprehensive review on calculation of critical points was published by Sadus [6]. All the methods discussed prescribe how to find the critical temperature and volume (and therefore also the pressure) for a mixture of known composition.…”

Prediction of critical points: A new methodology using global optimization

“…Thus, of course, calculations of the critical points are important in several processes involving fluid phase equilibria at high pressure. Besides, as discussed by Sadus (1994), the study of critical equilibrium is also of considerable practical utility in understanding the phase equilibrium of fluids in general. This author emphasizes that the critical state can be used to determine the global nature of the phase behavior of a mixture, according to the phase behavior classification of van Konynenburg and Scott (1980), for binary mixtures, based on critical transitions.…”

Critical temperatures and pressures for hydrocarbon mixtures from an equation of state with renormalization-group theory corrections

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