Abstract
In this work, a procedure for calculating critical loci based on the formulation used by Heidemann and Khalil (1980) and continuation method was developed. The proposed method is an easy and efficient way to track the critical locus containing turning points that are commonly found in type III and IV mixtures. It provides a clear geometric picture of the critical curves and the physical phenomena that happen when critical behavior change from one type to another. However, it is still impossible for the proposed continuation method to guarantee the location of all critical points. Furthermore, we have a section of critical locus for which the continuation method fails. Every point is recognized as a solution point, but also as a branch point and an end point. There is no apparent connection between these points. The physical significance of such a phenomena remained to be investigated.The global phase behavior of n-hexane + water was investigated with PRSV equation of state and WS/MHV2 mixing rules. While WS/MHV2 mixing rules could improve LLE correlations over traditional van der Waals mixing rule, unusual critical behaviors were observed in addition to the typical type III critical loci. These extraneous critical points are hard to be observed by traditional Newton’s method or Hicks and Young method. There is no direct evidence that such phenomena do exist. The excess Gibbs free energy data obtained by PVTx measurement reported by Abdulagatov et al., (1994) led to the conjecture that such phase behavior may indeed be real.Our study demonstrated that, due to non-ideality in wide temperature and pressure ranges, the phase diagram and critical loci of aqueous n-alkane mixtures should be used more extensively as the benchmark problem for testing equation of states and mixing rules.