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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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Eq. (4) 5=1.25Pcmp/2.66 = 0.47 Pc1/2p
Equation 4 was used to calculate the Hildebrand solubility parameters for various supercritical fluids. Here p is the density of the supercritical fluid which is related to the pressure and temperature as described earlier.
The enhancement of solvent power obtained by compressing a gas into its critical region can be demonstrated dramatically. An estimate of the solubility of a solid in an SCF solvent can be made using the following expression:171
Eq. (5)
fp*A\

exp
f --dP J RT
\P? ,
where the subscript 2 refers to the solid component, ^ is the sublimation pressure of the solid, v | is the molar volume of the solid, and <j>2 is the fugacity coefficient (a factor that accounts for specific repulsive and attractive interactions that occur between solvent and solid component molecules). On comparing the experimentally observed solubility of naphthalene (y2) at 100 bar and 12C with the solubility calculated of the SCF phase (assumed to behave like ideal gas), it can be found that: y2 observed/j>2 calculated = 16,156.
9.0 CARBON DIOXIDE
Gaseous carbon dioxide is very inefficient as a solvent for liquid and solids, but as its pressure is increased, accompanied by a concurrent increase in density, its extractive power improves. The solubility of organic compounds in carbon dioxide increases with higher density and temperature. Liquid carbon dioxide at 20C and equilibrium pressure of 56 atm is a useful solvent because its density is relatively high at 0.77 g/ml. In 1954, Francis reported the mutual solubilities of liquid carbon dioxide with each of 261 other substances and presented triangular graphs of 464 ternary systems in a single paper J20l The solvent properties are retained as the temperature is increased towards 31C, the critical temperature for carbon dioxide, there are no differences in any of the properties, and in fact two phases no longer exist. Above 31C, there exists only one phase, a supercritical fluid, as illustrated in Ref. 7 and in Fig. 5.
The phase diagram for carbon dioxide is shown in Fig. 5. The features to notice include the positive slope of the solid-liquid boundary (the direction of this line is characteristic of most substances), which indicates that the melting temperature of solid carbon dioxide rises as the pressure is increased. Note also that, as the triple point
(where the three phase boundaries coincide) lies above 1 atm, the liquid cannot exist at normal atmospheric pressures whatever the temperature, and the solid sublimes when left in the open (hence the name dry ice). To obtain the liquid, it is necessary to exert a pressure of at least 5.11 atm. Cylinders of carbon dioxide generally contain liquid and compressed gas; at a temperature of 25C that implies a vapor pressure of 67 atm. When the gas squirts through a restrictive orifice, it cools by the Joule-Thomson effect, so that when it emerges into a region where the pressure is only 1 atm it condenses into a finely divided snow-like solid. Table 2 shows the relationship of a complete range of pressure-temperature-density for SC C02.
Pressure in bar
Figure 5. Phase diagram of supercritical and near critical carbon dioxide, showing the density and pressure relationship. The shaded area represents the supercritical state.
TEMPI PRESSURE (atm) 250.0 300.0 350.0 400.0 450.0 500.0 550.0 600.0 650.0
() 1 150.0 200.0
20.0 | .9062 .9392 .9652 .9869 1.0057 1.0223 1.0373 1.0510 1.0635 1.0752 1.0862
30.0 | .8496 .8930 .9249 .9505 .9721 .9909 1.0076 1.0227 1.0364 1.0492 1.0610
40.0 | .7832 .8427 .8823 .9127 .9376 .9586 .9775 .9941 1.0092 1.0230 1.0358
50.0 | .7033 .7875 .8373 .8734 .9022 .9262 .9470 .9654 .9818 .9969 1.0106
60.0 | a -6098 .7274 .7899 .8329 .8660 .8932 .9163 .9365 .9545 .9707 .9856
|7|
70.0 | | .5155 .6639 .7408 .7914 .8294 .8598 .8854 .9075 .9271 .9446 .9605
80.0 | 3 .4376 .6004 .6909 .7495 .7924 .8264 .8546 .8787 .8998 .9187 .9357
90.0 | .5410 .6417 .7077 .7556 .7931 .8239 .8500 .8728 .8930 .9112

100.0 | .3392 .4888 .5949 .6669 .7194 .7603 .7936 .8217 .8461 .8676 .8869
110.0 | .3036 .4448 .5517 .6279 .6843 .7282 .7639 .7940 .8199 .8427 .8631
120.0 I .2849 .4083 .5128 .5913 .6506 .6971 .7350 .7668 .7942 .8183 .8397
130.0 [ .2660 .3782 .4784 .5575 .6187 .6673 .7071 .7405 .7692 .7944 .8169
140.0 | .2503 .3532 .4482 .5265 .5889 .6390 .6803 .7151 .7450 .7713 .7946
150.0 | .2371 .3321 .4218 .4985 .5611 .6122 .6547 .6907 .7217 .7489 .7730
The Supercritical State
10.0 CANDIDATES FOR SUPERCRITICAL FLUIDS
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