11 Equations of State in the Vapor-Liquid Critical Region P. T. EUBANK
Downloaded by NANYANG TECHNOLOGICAL UNIV on June 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch011
Texas A&M University, College Station, TX 77843
Equations of state (ES) may be divided between those that are analytic and those that are not. Analytic equations of the form P(ρ,T,[Z]) cannot provide an accurate description of thermodynamic properties in the critical region whether for the pure components or their mixtures. Scaled ES are non-analytic in the usual P (ρ,T) coordinates but assume analyticity inμ(ρ,T)for pure components. The choice of variables for a scaled ES for a mixture is not welldefined although Leung and Griffiths (1) have used P(T,[μ]) with success on the He-He system. Phase diagrams are simplier in such coordinates as the bubble-point surface and dew-point surface col lapse into a single sheet. Analytical equations operating outside the critical region generally correlate P-ρ-T-[Z] data with greater difficulty as one of the components is exchanged for a compound of greater acentricity or, particularly, polarity. Disclaimers usually accompany classical equations to discourage use with highly polar compounds. Steam is a good example--both the ES and correlation procedures used to produce steam tables differ considerably from those used with hydrocarbons. Scaled ES operating in the critical region, where intermolecular forces are not so important, do not incur additional difficulty with highly polar compounds. i
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I. Correlation of Fluid Properties with Analytic ES Angus (2) has recently surveyed modern ES including 1. virial-type 2. extended BWR including Strobridge, Bender, Gosman-McCartyHust, and Stewart-Jacobsen 3. Helmholtz free energy equations as used by Pollak and by Keenan-Keyes-Hill-Moore for water/steam 4. orthogonal polynomials as in the NEL steam tables of 1964 5. spline functions as used by Schot in 1969 for steam 6. the Goodwin ES (non-analytic) 7. the Kazavchinskii ES. 231
Storvick and Sandler; Phase Equilibria and Fluid Properties in the Chemical Industry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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The form of the ES p l u s the number and type of c o n s t r a i n t s to be p l a c e d upon i t s constants depends on the compound and the r e g i o n of reduced pressure and temperature over which the equation i s to be used. Thirst, we w i l l examine compounds t h a t are not h i g h l y p o l a r — i . e . , d i p o l e moment
