New applications of Kahl's VLE analysis to engineering phase behavior calculations

Nitsche, J.M., 1992, New applications of Kahls' VLE analysis to engineering phase behavior calculations. Fluid Phase Equilibria, 78: 157-190. In a little known but profound paper, G.D. Kahl (1967) (Phys. Rev., 155: 78-80) developed a compelling argument questioning the validity of isothermal integration of the van der Waals equation between the spinodal points in accordance with Maxwell's equal-area construction. This work generalizes Kahl's approach and shows conclusively that it is entirely possible to describe vapor-liquid equilibria without lending credence to the thermodynamic properties of unstable states. The analysis is structured around a new free energy parameter Δ(T) that allows adjustment of calculated vapor pressures without alteration of the equation of state itself, as is demonstrated using a novel modification of the Redlich-Kwong equation. The analysis opens the door to greater flexibility in using excess free energy models to devise mixing rules for equations of state. An extension of the theoretical development leads to a new type of VLE procedure whereby unrelated equations for liquid and vapor phases can be combined for essentially exact volumetric description of the pure fluids comprising a mixture. Introduction of a parameter Δ(T) also makes it possible to formulate a conjecture explaining the apparent nonexistence of liquid-solid critical points.