Supercritical phase, definition

The supercritical phase is a phase distinct from other fluid phases by virtue of the Gibbs energy. As noted by Moore [11], phase changes are characterised by discontinuous changes in certain properties at some definite temperature and pressiue. Gases and liquids, the fluid phases with which we are most familiar, differ from each other by the degree of condensation and order (the balance of... 

The vaporization line in the phase diagram is a plot of the Clapeyron equation or the Clausius-Clapeyron equation. Notice, however, that this line ends at a particular pressure and temperature, as shown in Figure 6.5. It is the only line that doesn t have an arrow on its end to indicate that it continues. That s because beyond a certain point, the liquid phase and the gas phase become indistinguishable. This point is called the critical point of the substance. The pressure and temperature at that point are called the critical pressure pQ and critical temperature Tc- For H2O, pQ and Tq are 218 atm and 374°C. Above that temperature, no pressure can force the H2O molecules into a definite liquid state. If the H2O in the system exerts a pressure higher than pQ, then it cannot exist as a definite liquid or gas. (It can exist as a solid if the temperature is low enough.) The state of the H2O is called supercritical. Supercritical phases are important in some industrial and scientific processes. In particular, there is a technique called... 

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (the stationary phase), while the other (the mobile phase) moves in a definite direction. A mobile phase is described as a fluid which percolates through or along the stationary bed in a definite direction . It may be a liquid, a gas or a supercritical fluid, while the stationary phase may be a solid, a gel or a liquid. If a liquid, it may be distributed on a solid, which may or may not contribute to the separation process. ... 

This definition cannot be applied directly to mixtures, as phase equilibria of mixtures can be very complex. Nevertheless, the term supercritical is widely accepted because of its practicable use in certain applications [6]. Some properties of SCFs can be simply tuned by changing the pressure and temperature. In particular, density and viscosity change drastically under conditions close to the critical point. It is well known that the density-dependent properties of an SCF (e.g., solubihty, diffusivity, viscosity, and heat capacity) can be manipulated by relatively small changes in temperature and pressure (Sect. 2.1). 

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

The aim of this Chapter is the development of an uniform model for predicting diffusion coefficients in gases and condensed phases, including plastic materials. The starting point is a macroscopic system of identical particles (molecules or atoms) in the critical state. At and above the critical temperature, Tc, the system has a single phase which is, by definition, a gas or supercritical fluid. The critical temperature is a measure of the intensity of interactions between the particles of the system and consequently is a function of the mass and structure of a particle. The derivation of equations for self-diffusion coefficients begins with the gaseous state at pressures p below the critical pressure pc. A reference state of a hypothetical gas will be defined, for which the unit value D = 1 m2/s is obtained at p = 1 Pa and a reference temperature, Tr. Only two specific parameters, Tc, and the critical molar volume, VL, of the mono-... 

Let us now focus upon the critical temperature and consider a few of the definitions that can describe this invariant point. It is important to note that the critical point is defined by the temperature only the value of the critical pressure appears to have a lesser or secondary significance. The critical (or supercritical) fluid region exists at all pressures at or above the critical temperature for a pure substance. Above this critical temperature, there exists only one phase, completely independent of the pressure. That is, no matter how high (or how low) you cause the pressure to be, the one phase wiU not condense to a hquid. 

Phase Diagrams for Supercritical Fluid-Solute Mixtures Table 3.2 Definitions of Phase Transitions Occurring at High Pressures... 

Heterogeneously catalyzed reactions in SCFs must, by definition, involved at least two separate phases, the reaction mixture and the solid catalyst, and frequently may contain more. There has been considerable argument as to whether a singlephase reaction mixture is needed to exploit the advantages of a supercritical fluid, but these arguments really lie out of the scope of this chapter [25]. Suffice it to say... 

The critical point on a phase diagram designates the pressure (pc) and temperature (Tq) at which the vapor and liquid phases of a substance become indistinguishable. By definition, a supercritical fluid (SCF) is above pc and Tc- Generally, the physical properties of an SCF (density, viscosity, and dielectric constant) are intermediate between those of a liquid and a gas, and these properties vary dramatically as a function of temperature and pressure [1,2]. Because of these unique features, there is enormous interest in the use of SCFs as solvents for chemical reactions [3-6]. 

FIGURE 24.1 Definition of the supercritical fluid region in the phase diagram of a pure substance. 

A one-component fluid is loosely defined to be supercritical when its temperature and pressure exceed its critical temperature and pressure, respectively, while it is not far from its critical state. In Fig. la, the region in pressure (P) - temperature (7) phase space is delineated where the fluid is supercritical according to the above definition, namely the right upper quadrant (cross-hatched). 

---