# Supercritical fluid cleaning - McHardy J.

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Although supercritical fluids can rarely match the solvent properties of conventional solvents such as hydrocarbons, they can be good solvents for highly fluorinated compounds. For example, fluorocarbons are notoriously insoluble in conventional solvents but are very soluble in supercritical carbon dioxide. This exceptional solubility has been exploited by DeSimone and colleagues^14' to replace CFCs in the synthesis of fluoropolymers. The solubility of solids depends on the density of the fluid and hence on the applied pressure; increasing the pressure increases solubility, while reducing the pressure precipitates the solid. This tuning of solubility can be exploited to control the composition and morphology of polymers and similar materials.

7.0 SOLUBILITY ISSUES OF SUPERCRITICAL FLUIDS

For supercritical fluids, the solvent strength of a given fluid is primarily dependent upon its density and pressure. It is much easier to directly measure and control the pressure of a supercritical fluid in a given operation than to measure and control the fluid density.

It is possible, however, to determine a relationship exists between the fluid pressure and its density, thus allowing indirect measurement and control of the density. For an ideal gas, the relationship is simply, PV/RT= 1, where Vis the molar volume (reciprocal of the molar density). From the molecular weight of the gas (M), the mass density, p, can be calculated as M/V. The simple equation breaks down at the high densities characteristic of supercritical fluids, but, the work of Pitzer et al.(15][161 allows the ideal gas law to be extended by introducing a term called the compressibility factor, z. It is a

function of the pressure, temperature, and molecular identity of the fluid. The gas law then becomes PV/RT=z. Pitzer was able to reduce the molecular identity terms of ã to a single number called the acentric factor, m. This factor attempts to account for both molecular size and shape. The value of the acentric factor was taken to be the ratio of the vapor pressure of the substance at 70% of its critical temperature to that at its critical temperature. References 15 and 16 detail the reasons for this choice. Thus, at a given temperature, pressure, and known acentric factor, z can be determined from the tabulated values which are given in Ref. 17. This allows V to be determined from the gas law (V = zRT/P) and thus the mass density to be determined if the molecular weight of the fluid is known. Further, the solvent power (as measured in milligrams of solute per gram of solvent) increases exponentially with the increase in pressure above the critical pressure (greater than 350 bar for carbon dioxide), with relatively little increase in solvent density. At these high pressures (350-700), a rise in temperature causes an increase in the solvent density, because a concurrent increase in vapor pressure of the solute overcomes the decrease in solvent density and leads to a significantly greater solvent power. The solvent power of the supercritical fluid increases exponentially with relatively small increases in temperature or pressure in this high pressure region of 350-700 bar and 50-110°C. During the early 1980s, John Freidrich and Egon Stahl discovered that at comparatively higher pressures and temperature (above 350 bar and 50°C), the solvent power of supercritical fluids is much greater than one would expect from ideal solubility calculations. The actual solubility deviates from the ideal solubility (which can be calculated from the vapor pressure of the condensed phase) by several orders of magnitude because of intermolecular interactions that assist in the solvation of the solute. This surprising but interesting apparent increase in the vapor pressure of a poorly soluble substance can be most easily understood as an “evaporation,” because the solute becomes part of the gas phase. The ratio of the much greater actual solubility to the ideal solubility is called the enhancement factor.

8.0 HILDEBRAND SOLUBILITY PARAMETER

The Hildebrand solubility parameter, 6, is a semi-quantitative entity related to the thermodynamic properties of dense gases (supercritical fluids) and solutions. I181 The solubility parameter in calories per cubic centimeter is calculated from the equation:

Eq.(l) 5 = 1.25Pc1/2Pr/pr(liq)

where Pc is critical pressure in atmosphere, pr is the reduced density and p^liq) is the reduced density of liquid.1191 Reduced density is the ratio of supercritical density at a given pressure and temperature to critical density, pc whereas the critical density is the density of supercritical fluid at its critical pressure and critical temperature.

Eq-(2) pr = p/pc

Based on the semi-empirical calculations as shown in Ref. 18, the r^liq) is given by:

Eq. (3) Pr(liq) = 2.66/pc

From Eqs. 1-3, the following relationship can be derived to relate the Hildebrand solubility parameter with the density of supercritical fluid.

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