Two-phase transition region

T his chapter presents a new formulation of the equation of state for - -fluids at subcritical and critical temperatures, T < 1. Unlike many other equations of state, this equation of state is defined as a differential equation, and is designed to describe not only the behavior of isotherms on their stable vapor and liquid branches, but also in the two-phase transition region. Interesting insights concerning the nature of metastable and absolutely instable phases are obtained also. 

Condition 2, a very strong constraint, ensures the proper behavior of the slope dP/dp on the phase boundary. Note that when P = Pa, then dP/dp = 0 for all regular points of Equation 1, i.e. Equation 1 yields the correct isothermal behavior in the two-phase transition region of a fluid (see Figures 1 and 2). 

The Differential Equation of State 1 provides not only a good qualitative description of isothermal behavior at subcritical temperatures T < 1, but also yields accurate quantitative representations of experimentally measured data. It describes not only the stable vapor and liquid branches, but also the two-phase transition region, additionally yielding information on the nature of metastable and absolutely unstable phases. A complete and simple description of the vapor-liquid-phase transition and the critical point also is provided by the differential equation of state. 

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