High-pressure Phase Diagrams and Critical Properties of Fluid Mixtures

High-pressure Phase Diagrams and Critical Properties of Fluid Mixtures 

Whenever one looks into modern textbooks of thermodynamics or physical chemistry in order to get information on the phase behaviour of fluid mixtures one might think that most of the inspiring work of van der Waals, Kohnstamm, Roozeboom, Timmermans, Kuenen, and the other pioneers in this field had been completely forgotten and that practically nothing had happened since that time. Only the most simple types of gas-liquid and liquid-liquid equilibria are discussed very rarely are the so-called gas-gas equilibria that were discovered some thirty years ago additionally treated. 

In Section 4 it will be shown that this concept of a continuity between all kinds of phase equilibria in fluid mixtures will aid in understanding the great variety of phase relationships and in giving a certain order to many different types of phase behaviour and critical phenomena in fluid systems. Here practical applications will be equally briefly reviewed. The presentation will be based on some earlier essays and review papers of the author to which the reader is referred for examples, references, and details. A review of these problems has also been given in a book by Rowlinson. Related problems have been treated 

In Sections 2 to 4 critical phenomena will be of primary importance since they make possible a systematic discussion of all types of phase behaviour in fluid mixtures and of the relationships between them. The definition of a critical point for a mixture is essentially the same as that for a pure component at a critical point all intensive properties of two phases in equilibrium become identical. Whereas pure substances are characterized by a critical point for the equilibrium gas-liquid, binary systems exhibit a critical line in the three-dimensional p-T-x space (where x denotes mole fraction), and systems with n components an ( — l)-dimensional critical surface in the ( i + l)-dimensional p-T-Xi-X2. .. Xn-i space for all kinds of fluid-fluid equilibria. 

The following symbols are used T = temperature p = (total) pressure X = mole fraction (of component II if not indicated) c = concentration M = moldm L = liquid phase, G = gaseous phase, S = solid phase CP I, CP II = critical point of the pme component I or II CP = critical point of a mixture UCST (LCST = upper (lower) critical solution temperature A, B, C, D, E, K = critical end point Qx, Qa = quadruple point. 

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Figure 14.7 Schematic representation of the different types of binary (liquid + liquid) phase equilibria, showing the effect of p, T, and x on the two-phase volume. Examples are known for all except figures (k), (o), and (s). Reproduced with permission from G. M. Schneider, High-pressure Phase Diagrams and Critical Properties of Fluid Mixtures , M. L. McGlashan, ed., Chapter 4 in Chemical Thermodynamics, Vol. 2, The Chemical Society, Burlington House, London, 1978.

G. M. Schneider, "High-Pressure Phase Diagrams and Critical Properties of Fluid Mixtures," in Chemical Thermodynamics, A Specialist Periodical Report, vol. 2, The Chemical Society, London, 1978, p. 105. 

High-pressure Phase Diagrams and Critical Properties of Fluid Mixtures 129 3 Results on Selected Systems... 

Schneider, G.M. (1978) High-pressure phase diagrams and critical properties of fluid mixtures, in M.L. McGlashan (ed.). Chemical Thermodynamics, Vol. 2, Specialist Periodical Reports, The Chemic d Society, London, pp. 105-146. 

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