Thermodynamics of liquid‐liquid equilibria including the critical region

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Abstract
To obtain good representation in the liquid‐liquid critical region, a semitheoretical correction is added to a conventional expression (e.g., van Laar or NRTL) for the excess Gibbs energy of a binary or ternary system along the coexistence curve. This correction is an exponential function of a suitable distance from the critical point; it is significant in the critical region but not elsewhere. In the correction function, the preexponential factor is determined from stability considerations. In the exponential argument, two parameters are determined from theoretical power laws. While the coordinates of the critical point must be known (or estimated), no phase‐equilibrium data in the critical region are required to determine parameters. The correction presented here provides an excellent approximation for phase equilibria along the coexistence curve close to and remote from critical conditions. Further, the correction facilitates simultaneous representation of isothermal vapor‐liquid and liquid‐liquid equilibria including the liquid‐liquid critical region.

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