Thermophysical Properties of Supercritical Fluids

This article presents a general introduction to the physics and dynamics of supercritical fluids and the critical point. It is organized in two parts: In a first section, the nature of the critical point is discussed and its influence on the properties of supercritical fluids is presented; In a second section, we then examine the dynamic processes induced by these peculiar properties, concentrating on heat and mass transfer and more generally on relaxation phenomena. INTRODUCTION Although supercritical fluids have been used in various industrial applications for the past 60 years, it is only in the early 70s that their peculiar thermophysical properties have been understood, following the works of Widom, Kadanoff and Wilson. Hence, the understanding of the physics of critical points is a relatively recent result (around 30 yearsold). If one now considers the dynamic relaxation phenomena in these fluids (and among them heat transfer and hydrodynamics), then the main discoveries are even younger: a mere 15 years, with lots of open questions remaining. Making a short summary of these recent results seems useful to an application-oriented audience: integrating this recent knowledge may indeed help optimize existing processes, and possibly stimulate the design of new approaches heretofore not considered for lack of a better understanding of what is going on near the critical point. The aim of this article is to present a brief review of the present knowledge about the thermophysics and dynamics of supercritical fluids, in qualitative terms. For the more technically-oriented reader, a list of key references is provided. In a first part, general considerations about the thermodynamics of supercritical fluids are reminded. In a second part, the most significant mechanisms governing the dynamics of these fluids are introduced. THE CRITICAL POINT AND SUPERCRITICAL FLUIDS Phase diagram of a pure component Following Gibbs phase rule, a pure component in thermodynamic equilibrium can be observed under the form of three different phases (sometimes more), determined by only two independent state variables. The complete set of accessible equilibrium states is thus a surface in the three-dimensions space of state variables. Such a State Surface is represented in figure 1 with the particular choice of state variables P (pressure), ρ (mass density) and T (temperature). Most state surfaces of classical components have a similar topology. In figure 1, one can also see the projections of the state surface on the planes (P, T) (Pressure,