Handbook of Solvents - George Wypych - ChemTech - Ventech!

1436 Aydin K. Sunol and Sermin G. Sunol
Entrainer α Entrainer α
Hexadecanol(1) - octadecane(2) - entrainer -
ethane (120 bar, 70°C, y 0 = 15 wt%, a 0 = 2.5,
y (1)+(2) = 30 wt%)
methanol
acetone
hexane
(120 bar, 90°C, y (1)+(2) = 20 wt%)
methyl acetate
benzene
dichloromethane
Separation factor, α =(y 2/x 2)/(y 1/x 2)
2.2
1.6
1.6
1.8
1.2
1.1
Hexadecanol(1) - octadecane(2) - entrainer -
carbon dioxide (120 bar, 70 °C, y 0 = 0.4 wt%,
a 0 = 2.5, y (1)+(2) = 1 wt%)
methanol
acetone
methyl acetate
hexane
Although entrainers provide the aforementioned added advantages, they also bring
forth more complex process flowsheets. The separation of extract from entrainer and
entrainer from the supercritical component are not as easy and as sharp as in the instances
without entrainer. The selection of entrainer is based on thermodynamic, environmental,
and economic considerations. The solute-entrainer and entrainer-supercritical separations
are the key from the processing perspective. 52
21.1.2.4 Reaction rate implication in supercritical solvents
Reactions in supercritical media utilize high pressures. Therefore, the effect of pressure on
reaction equilibrium as well as reaction rate plays an important role in supercritical phase
reactions. 53
The kinetics of the reaction can be explained in terms of the transition-state theory.
According to the theory, the reaction occurs via a transition state species M* and the generic
elementary reaction can be written as:
aA + bB + ............ ↔ M* → Products
The effect of pressure on the rate constant is given as:
where:
* *
⎛∂lnk
x ⎞ ∂lnK
x ∂lnκ V
⎜ ⎟ =
⎝ ∂P
⎠T
∂P
∂P
T
⎛ ⎞
⎜ ⎟
⎜ ⎟
⎝ ⎠
+⎛
⎞
⎜ ⎟ =
⎝ ⎠
Δ
+
RT P T
⎛ ∂lnκ⎞ ⎜ ⎟
⎝ ∂ ⎠
T
kx rate constant in mole fraction units
P pressure
T temperature
K * x mole fraction based equilibrium constant for reaction involving reactants and transition
state
κT isothermal compressibility
ΔV *
activation volume (difference between partial molar volumes of activated complex and
reactants), ΔV * =V * -aVA-bVB- ...
R universal gas constant
The rate constant in the above equation is expressed in terms of pressure independent
units (mole fraction). If the rate constant is expressed in terms of pressure dependent units
(such as concentration), the relevant equation is:
3.4
1.8
1.6
1.4