Solid-Supercritical Fluid Phase Diagrams

As shown in figure 3.1e, this phase behavior is very similar to the previously described type-III system. But in type-V phase behavior, there is no region of liquid immiscibility at temperatures below the LCST. 

Solid-SCF mixtures constitute a very large and important subset of binary mixtures. For these types of mixtures, the normal melting temperature of the solid is greater than the critical temperature of the SCF. In this section we describe the two schematic solid-SCF phase diagrams that depict solid-SCF equilibria to very high pressures (Diepen and Scheffer, 1948a Streett, 1976 McHugh, 1981). 

Shown in figure 3.12a is the P-T-x diagram for the type of solid-SCF system described in the previous paragraph. The phase behavior depicted in figure 3.12c is observed if a P-x diagram is experimentally determined at Ti, a temperature below the critical temperature of the lighter component Tc,- At low pressure solid-vapor equilibria are observed until the three-phase SLV line is intersected. Three equilibrium phases exist at this pressure a pure solid, a liquid, and a gas. 

If the overall mixture composition is less than that of the liquid phase, then a vapor-liquid envelope is observed as the pressure is further increased. The vapor-liquid envelope eventually intersects the pressure axis at the vapor pressure of pure component 1, since Ti is less than the critical temperature of 

If the operating temperature is now increased to T, the phase behavior shown in figure 3.15d occurs. At Ty the mixture critical point pressure of the vapor-liquid envelope occurs precisely at the same pressure at which the three-phase SLV line is intersected. Hence, a vapor-liquid mixture critical point is observed in the presence of excess solid. This vapor-liquid mixture critical point in the presence of excess solid is the lower critical end point (LCEP) (Diepen and Scheffer, 1948a). If the temperature is increased slightly above the LCEP temperature, only solid-SCF phase behavior is observed at all pressures, since the three-j)hase (SLV) line ends at the LCEP. 

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Figure 1. Solid-liquid-gas-supercritical fluid phase diagram. TP = triple point, CP - critical point, Pc = critical pressure, Tc = critical temperature.

Chapter 14 describes the phase behavior of binary mixtures. It begins with a discussion of (vapor -l- liquid) phase equilibria, followed by a description of (liquid + liquid) phase equilibria. (Fluid + fluid) phase equilibria extends this description into the supercritical region, where the five fundamental types of (fluid + fluid) phase diagrams are described. Examples of (solid + liquid) phase diagrams are presented that demonstrate the wide variety of systems that are observed. Of interest is the combination of (liquid + liquid) and (solid 4- liquid) equilibria into a single phase diagram, where a quadruple point is described. 

The presence of a solid phase adds considerable complexity to the fluid phase diagram [14, 16]. We discuss here only two cases, one for Type-I fluid phase behavior including a solid phase well below the critical line, and one in which the solid phase reaches into the supercritical regime. F or a full discussion of the latter case, see Peters [16]. 

If, however, we increase p at constant T, the supercritical fluid will change to a solid. In the phase diagram of H2O, the coexistence curve for ice VII and liquid shown in Fig. 8.4 extends to a higher temperature than the critical temperature of 647 K. Thus, supercritical water can be converted to ice VII by isothermal compression. Refs. [162] and [9]. 

The discovery of supercritical fluids occurred in 1879, when Thomas Andrews actually described the supercritical state and used the term critical point. A supercritical fluid is a material above its critical point. It is not a gas, or a liquid, although it is sometimes referred to as a dense gas. It is a separate state of matter defined as all matter by both its temperature and pressure. Designation of common states in liquids, solids and gases, assume standard pressure and temperature conditions, or STP, which is atmospheric pressure and 0°C. Supercritical fluids generally exist at conditions above atmospheric pressure and at an elevated temperature. Figure 16.1 shows the typical phase diagram for carbon dioxide, the most commonly used supercritical fluid [1]. 

PCB phase diagrams with carbon dioxide Effect of temperature and pressure on extraction of PCB and polyaromatic hydrocarbons Combination of solid-phase carbon trap with supercritical fluid chromatography for PCB, pesticides, polychlorodibenzo-p-dioxins and polychlorofurans... 

Solubilities of meso-tetraphenylporphyrin (normal melting temperature 444°C) in pentane and in toluene have been measured at elevated temperatures and pressures. Three-phase, solid-liquid-gas equilibrium temperatures and pressures were also measured for these two binary mixtures at conditions near the critical point of the supercritical-fluid solvent. The solubility of the porphyrin in supercritical toluene is three orders of magnitude greater than that in supercritical pentane or in conventional liquid solvents at ambient temperatures and pressures. An analysis of the phase diagram for toluene-porphyrin mixtures shows that supercritical toluene is the preferred solvent for this porphyrin because (1) high solubilities are obtained at moderate pressures, and (2) the porphyrin can be easily recovered from solution by small reductions in pressure. 

Figure 3.32 Phase diagram illustrating the domains of the solid (S), gaseous (G) and liquid (L) phases as a function of pressure (p) and temperature (T). tp is the triple point, at which three phases co-exist, cp is the critical point, which forms the end of the vapour pressure curve (between tp and cp). The area in the top right corner indicated by SF represents the domain of supercritical fluids.

Nature presents a large number of atomic and small molecular species that might be discussed as biosolvents. Table 6.1 lists some of these, together with their freezing and normal (i.e., at 1 atmosphere) boiling points. It is important to note another contribution of pressure to physical properties. The physical properties of the substances listed the Table 6.1 are described by a phase diagram that relates the state of a material (solid of various types, liquid, or gas) to temperature and pressure. Above a critical point in the phase diagram, the substance is a supercritical fluid, neither liquid nor gas. Table 6.2 shows the critical temperatures and pressures for some substances common in the solar system. 

The pressure-temperature phase diagrams also serve to highlight the fact that the polymorphic transition temperature varies with pressure, which is an important consideration in the supercritical fluid processing of materials in which crystallization occurs invariably at elevated pressures. Qualitative prediction of various phase changes (liquid/vapor, solid/vapor, solid/liquid, solid/liquid/vapor) at equilibrium under supercritical fluid conditions can be made by reference to the well-known Le Chatelier s principle. Accordingly, an increase in pressure will result in a decrease in the volume of the system. For most materials (with water being the most notable exception), the specific volume of the liquid and gas phase is less than that of the solid phase, so that... 

Figure 2 A schematic of a phase diagram for water showing solid, liquid and gas phases relative to the supercritical fluid region.

Fig. 4.28 Generalized phase diagram. Only a supercritical fluid exists above the critical temperature (T ) and critical pressure (p ), which has properties intermediate between liquid and gas. (Note water is unusual in having a negative slope for the solid-liquid phase equilibrium line.)...

PHASE EQUILIBRIA. A useful solvent for supercritical extraction, especially in food processing, is carbon dioxide, which has a critical point of 31.06 C and 73.8 bars (1070 Iby/in. ). The phase diagram for pure COj (Fig. 20.16) shows the equilibrium regions of solid, liquid, and gas and the conditions under which a supercritical fluid exists. In the supercritical region there is no distinction between liquid and gas and no phase transition from one to the other the supercritical fluid acts like a very dense gas or a light, mobile liquid. 

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