Handbook of Solvents - George Wypych - ChemTech - Ventech!

1428 Aydin K. Sunol and Sermin G. Sunol
Table 21.1.2. Classification of physicochemical properties that can be measured by
SFC
Equilibrium properties Kinetic and transport properties Other properties
* Fluid phase interactions
- Second virial coefficients
* Solution interactions
- Partial molar volumes
- Solubilities
* Surface interactions
- Adsorption isotherms
* Diffusion coefficients
* Mass transfer coefficients
o Adsorption and desorption rate constants
o Reaction rate constants
* Molecular masses
*Properties determined by SFC; o Properties determined by GC and can be determined by SFC
Due to the inherent limitation of predictive methods, relative importance of experimental
methods become very significant when compared to simple fluids at near ambient
condition. Therefore, the area is a popular review 3,20 topic and is included in most texts 6,21 in
the field.
21.1.2.1.2 Computational aspects
Computation and definition of critical phenomena is essential in prediction, modeling, and
identification of various phases at high pressures. Gibbs presented the definition of critical
state in his pioneering paper “On the Equilibrium of Heterogeneous Substances” in 1876. 22
For mixtures, mechanical and thermal stability criteria are not sufficient to describe the critical
behavior and diffusional (material) stability criteria are required. Various investigators
have transformed Gibbs’ criteria to other variable sets and Reid and Beegle 23 have developed
Gibbs’ criteria in Legendre transforms with suggestions to overcome indeterminacy.
Works of Michelsen 24 and Heidemann 25 combine algorithms with more convenient representation
of Gibbs criteria.
Generation of complete phase diagrams is not a trivial task and may require utilization
of either insight and heuristic guidance or global approaches coupled with Mixed Integer
Non-linear Programming (MINLP) or intelligent search algorithms such as genetic algorithms
and simulated annealing.
The methods used in generation of phase diagrams may either employ the more popular
integral approach that is suitable for design purposes or the more insightful differential
approach that is preferred for generation of phase diagrams. 26 The basic elements of integral
approach are the determination of fluid phase partition coefficient, K, for each component
as well as stability criterion coupled with a computational algorithm.
For determination of fluid phase partition coefficients, K, one may take a symmetric
approach that uses Equation of States (EOS) for all fluid phases or an unsymmetrical approach
that makes use of liquid activity models for the liquid phase retaining the equation of
state models for the gas phase.
For symmetric approach: