Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.1 Supercritical solvents 1435
where:
ρ rm i
i
y V
= ∑
V
ci
v
m
where:
λ v
thermal conductivity
ρrm reduced molar density for mixtures
yi mole fraction of component i
Mi molecular weight of component i
Tci critical temperature of component i
Vci critical volume of component i
Zci critical compressibility of component i
The vapor molar volume, V v m can be obtained from the equation of state models as described
above.
21.1.2.2.4 Surface tension
Stability of phase boundaries depends on the surface tension. Surface tension in a supercritical
fluid system is of major importance for drying, surfactant efficacy, and extraction. The
surface tension of a gas increases with pressure and approaches zero at the critical point
while the surface tension of liquid decreases with pressure resulting in dissolution of supercritical
components in the liquid phase. The methods useful in correlating surface tension
include Macleod-Sugden correlation and corresponding states theory. 21
21.1.2.3 Entrainer (co-solvent effects) of supercritical solvents
Entrainers, modifiers, and co-solvents are basically mixed solvent systems and provide another
dimension to supercritical fluid extraction. The entrainers enhance the solubility of
the low volatile substance in the solvent, provide selective solubility in multi-solute instances,
and enhance the sensitivity of the solubility and selectivity to temperature, pressure,
and composition. The entrainers may be reactive and are also useful as slurrying
media. Table 21.1.4 shows representative data on the effect of entrainers on vapor liquid
systems that has been systematically studied by Brunner’s group. Kurnik and Reid 50 as well
as Johnston’s group 51 present data for dense gas-solid systems.
Table 21.1.4. The effect of entrainers on separation factor (Adapted from Brunner 21 )
Entrainer α Entrainer α
Hexadecanol(1) - octadecane(2) - entrainer - nitrous
oxide (120 bar, 70°C, y 0 = 1.5 wt%, a 0 =2,
y (1)+(2) = 2 wt%)
acetone
methanol
dichloromethane
benzene
methyl acetate
hexane
cyclohexane
3
2.4
2.1
2
2
1.9
1.7
Octadecane(1) - salicylic acid phenal ester(2) -
entrainer - nitrous oxide (120 bar, 70 °C, y 0 = 1.5
wt%, a 0 =2,y (1)+(2) = 1.7 wt%)
methanol
dichloromethane
benzene
cyclohexane
acetone
hexane
methyl acetate
1.65
1.5
1.45
1.4
1.4
1.3
1.2