Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.1 Supercritical solvents 1425
Figure 21.1.7. The effect of supercritical fluid concentration
on polymer-solvent phase behavior.
components will be liquid. The phase diagram
of systems with solid and supercritical
fluid is shown in Figure 21.1.6. 15 The full
lines are the sublimation, melting, and vapor
pressure curves of the pure components, C 1
Figure 21.1.8 Solubility of naphthalene in supercritical
ethylene.
and C 2. Point Q is the quadruple point, where four phases S 1,S 2, L, and G are in equilibrium.
If the temperature of C 2 lies far below of that A 1 (triple point of C 1), the three-phase region
S 2, L, G curves upward and intersects the gas liquid line to form critical end points U a and
U b. No liquid phase exits between the temperatures of U a and U b. The three-sided region
A 1C 1U b is a region of gas-liquid phase separation. At temperatures between this region and
U a, the solid phase of component C 1 is in equilibrium with C 2 rich gas phase.
The polymer solution behavior is of significant importance due to large number of applications
in the field. The supercritical fluids primarily follow class four behavior. An example
related to the effect of supercritical solvent on phase behavior of such systems is
shown in Figure 21.1.7. More comprehensive coverage of the field can be found in texts 6
and review papers. 16
The unique behaviors of supercritical fluids relate to solubility enhancement and how
the solubility varies with operating conditions. As can be seen from Figure 21.1.8, the solubility
of a solid solute is enhanced several orders of magnitude over the ideal solubility. 17
Furthermore, the solubility decreases with increasing temperature at supercritical pressures
somewhat above the critical, while at supercritical pressures above a “cross-over pressure”,
solubility of a solute increases with temperature. The crossover pressures are different for
different solutes. As depicted in Figure 21.1.9, complete fractionation of solutes solubilized
in the supercritical solvent is possible by slight increase in temperature. 18 Another unique
behavior, called “barotropic effect”, exists when highly compressed dense gas reaches pressures
higher than the incompressible liquid. In such instances, dense gas phase is below the
liquid or the second gas phase.
There are three essential elements that make up the thermodynamic foundations of
supercritical fluids. These elements are experimental and identification techniques for elucidating
the phase behavior, models for dense gases, and computation methods.