Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.1 Supercritical solvents 1431
ai = αi 0.45724 (R 2 T 2 ci/pci) αi =[1+mi(1-T 1/2 ri)] 2
2
mi = 0.37464 + 1.54226ωi - 0.26992ωi where:
p pressure
R universal gas constant
T temperature
Vm molar volume
b function of Tci and Pci and xi a function of Tci, Pci, xiand kij αi a function of reduced temperature, Tri and accentric factor, ωi mi a function of accentric factor of component i
ωi accentric factor of component i
kij the interaction coefficient of the molecules i and j
Tci, Pci critical temperature and pressure of component i
xi mole fraction of component i
Tri reduced temperature, T/Tci The polymer solutions 39 warrant use of a special class of lattice models such as
Florry-Huggins. For correlation purposes Sanchez-Lacombe 40 method is sufficient but one
may also use Statistical Association Fluid Theory 41 (SAFT) models to obtain a better representation.
21.1.2.2 Transport properties of supercritical solvents
The transport properties of the supercritical fluids fall somewhat in between the gas and the
liquid and also depend on how removed one is from the critical point. Dense gasses have the
solubilizing power of liquids and the mobility of gasses as depicted in Table 21.1.3. There
are quite a few empirical correlations and theoretical models, which are primarily extensions
of corresponding low-pressure liquid and gas counter parts. Similarly, of the classical
experimental methods can be used for measurement of transport properties of supercritical
fluids. A rather brief overview of the methods applicable for supercritical fluids will be presented
since specialized reviews in the area give a good account of the state of the art. 30,42
For engineering purposes, one can use applicable property estimation methods available in
flowsheet simulators 43 such as ASPEN PLUS, PROII, HYSIM, and CHEMCAD. These
methods are discussed in a text classical in the field. 44
Table 21.1.3. Transport properties of gases, liquids, and supercritical fluids
State Condition
Density, g/cm 3
Gas 1 atm, 25 o C 0.6-2×10 -3
Property
Diffusivity, cm 2 /s Viscosity, g/cm-s
1-4×10 -1
Liquid 1 atm, 25 o C 0.6-1.6 0.2-2×10 -5
SC Fluid T c,P c 0.2-0.5 0.5-4×10 -3
SC Fluid T c,4P c 0.4-0.9 0.1-1×10 -3
1-3×10 -4
0.2-3×10 -2
1-3×10 -4
3-9×10 -4
21.1.2.2.1 Viscosity
Both the capillary viscometer (providing about 0.7% accuracy), the theory of which is based
on the Hagen-Poiseuille equation and the oscillating disc viscometer (providing about 0.2%
accuracy) are applicable to experimental determination of viscosity at high pressures and
temperatures.