Studies on criticality phenomena and collinearity constraints

Typescript (photocopy).This dissertation intends to provide new and efficient tools to improve fluid phase equilibria predictions using equations of state (EOS) and various mixture combining rules (MCR). We developed, and successfully tested, a model to predict critical pressures and temperatures of the members of a homologous hydrocarbon series. This model, based upon the Principles of Corresponding States (PCS) and of Congruence (POC), was further extended to mixtures to yield a new set of power law form MCR, with already built-in binary interaction parameters. We solved the Gibbs' equations for mixture critical points. Binary critical loci predicted from our MCR in the Redlich-Kwong EOS agree well with experimental data and are better than conventional MCR. We provided a thermodynamic proof of collinearity of isochores at temperature extrema (CT) and of isentropes at pressure extrema (CB) of envelopes separating equilibrium phases. Isochoric collinearity can be observed from data; isentropes, however, must be calculated. We developed a piece-wise procedure to calculate thermodynamic properties in the single- and in the two-phase regions, and verified isentropic collinearity at the CB for the experimental data of two binary mixtures (CO₂/CH₄ and ³He/⁴He). We extracted new experimental second virial coefficients of ³He, ⁴He, mixture, and interaction virials (B₃₄) at cryogenic temperatures (3-5) K. These values were compared against quantum mechanical calculations from Lennard-Jones and Exp-6 potential energy functions. Collinearity of isochores was also verified from simulated vapor-liquid equilibrium VLE using the Redlich-Kwong EOS. The major outcome of these studies is that new lines of research can be initiated by exploiting the use of these physical constraints for fluid mixtures in a manner analogous to the critical constraints for pure substances. Tuning mixture parameters to extrema loci (CT/CT) may indicate form and/or limitations of existing MCR for use with any EOS. Finally, since our work involved numerous VLE calculations, we exploited new ways of calculating VLE using the Gibbs' minimization criteria. We designed a new algorithm to improve the convergence rate of phase separation calculations, which rapidly identifies unfeasible flash conditions, avoiding unrealistic P and T in the range of superheated vapor or subcooled liquid