Automated generation of phase diagrams for supercritical fluids from equations of state

M. CISMONDI; M. L. MICHELSEN; M. S. ZABALOY

Barcelona, Spain

Congreso; 11th European Meeting on Supercritical Fluids. May 4 – 7, 2008; 2008

International Society for the Advancement of Supercritical Fluids (I.S.A.S.F)

Phase behavior is of major importance in the analysis and development of applications of supercritical fluids. Different classes of phase diagrams of fluid mixtures are useful to understand mixing/demixing processes, to explore conditions at which such processes occur and to study the influence of temperature and pressure on solubility values. Equations of state are an important type of thermodynamic models. They can be used to model properties of liquid, vapor or supercritical phases, with continuous transitions among such states. Unfortunately, phase equilibrium calculations are not simple. They require iterative procedures, which have no guarantee for convergence, rely on often crucial initial estimates and provide solutions that need to be tested for stability. Special attention has been given in the literature to the development of calculation procedures for two-phase and multiphase flashes, saturation points, phase envelopes, critical points, etc [1]. However, the development of calculation algorithms for phase equilibria has not reached yet a plateau. For instance, a general algorithm for automated construction of phase diagrams for binary mixtures covering all possible situations of fluid-fluid and solid-fluid equilibria for systems of varying asymmetry is, to our knowledge, not yet available. In other words, we not only face the problem of finding a good model for the representation of the experimental reality, but also the problem of finding reliable calculation algorithms to faithfully unveil the reality within the universe of the chosen model. In our opinion, in the scientific community, the level of awareness towards the first of these problems is significantly higher than for the second. During the last years we have focused on the goal of developing tools able to generate phase diagrams in a general and automated way. Such work led to algorithms that apply to a relatively wide variety of phase behavior situations while setting the user free from initializing the calculations. The first part of this presentation reviews such previous contribution, which produced the software GPEC (Global Phase Equilibrium Calculations) [2, 3]. The algorithms are so far restricted to binary systems [4, 5, 6]. They cover, without requiring advance user’s knowledge of the system phase behavior type, the automated location of Critical End Points (CEP), the automated calculation of critical lines and liquid-liquid-vapor (LLV) lines, the integration of the previous items into global-phase equilibrium diagrams, and the automated generation of Pxy and Txy diagrams. The second part of this work discusses and illustrates a new algorithm for the automated generation of complete fluid phase envelopes at specified composition, for binary mixtures. The algorithm is based on the location of key points at the set system composition, before starting off the building of the phase envelope. Such key points include vapor-liquid critical points, liquid-liquid critical points and saturated phases at LLV points. Finally, we discuss our future work in the field of equation of state phase diagrams generation.