Kritische verschijnselen in binaire fluïde mengsels: klassifikatie van fasenevenwichten met de simplified-perturbed-hard-chain theory

I. INTRODUCTION AND THEORY This PhD research is mainly concerned with the global phase behaviour, that is calculated from the Simplified-Perturbed-Hard-Chain equation. This equation distinguishes itself from many other equations of state by a sound theoretical background. We enter the field of the theory of equations of state and their practical application, e.g., in flowsheeting programs. Attention is paid to equations of state in general and especially to the equation of state that results from the Simplified-Perturbed-Hard-Chain Theory, the previously mentioned SPHCT equation. We specifically investigate the possibilities of the SPHCT equation of state in the critical region and extensive research is performed on the various types of binary fluid phase behaviour. This is done according to the classification of Van Konynenburg and Scott. In order to systematically investigate binary fluid phase behaviour Van Konynenburg and Scott developed the concept of the global phase diagram. We need to investigate the mathematical conditions of the various curves and points, that occur in the global phase diagram, among which the tricritical curves, the tetracritical point, the double critical endpoint curves, the Van Laar point, the azeotropical boundary curves, the azeotropical endpoint curves and the mathematical double point curves. The calculation of the global phase diagrams requires a profound knowledge of algorithms to solve sets of nonlinear equations, e.g., the Marquardt-algorithm. For our research we have purchased the symbolic computation program Maple. With Maple we have derived the expressions for the derivatives of the molar Gibbs energy with respect to the composition and of the molar Helmholtz energy with respect to the molar volume and the composition. II. RESULTS AND CONCLUSIONS Several thermodynamic properties that have been calculated from the SPHCT equation are compared with experimental data. For pure fluids we have investigated the vapour pressure curve in the critical region, the temperature dependence of the attractive term of the SPHCT equation, the virial coefficients as a function of temperature and the molar isochoric heat capacity. For binary mixtures we have investigated isothermal binary phase envelopes for mixtures that consist of simple molecules, like methane, ethane, argon and nitrogen, the Henry coefficients and the critical curves in binary hydrocarbon mixtures. It turns out that the global phase diagrams that have been calculated from the SPHCT equation can be different compared to the ones obtained from other equations of state. The existence of a tetracritical point in a global phase diagram has led to the discovery of new types of phase behaviour. They have been numbered according to the classification of Van Konynenburg and Scott and it is proposed to call these types of phase behaviour type VIII, IX, X and XI. More-over we have calculated type VI and VII phase behaviour and a new version of type VII that we propose to call type VII2h. Most of these new types of phase behaviour are based on the possibility to calculate closed critical curves and the socalled high-pressure critical curves from the SPHCT equation.Applied Science