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Care must be exercised in applying the UNIFAC method. The specific limitations of
the method are: CHEMICAL ENGINEERING
1. Pressure not greater than a few bar (say, limit to 5 bar)
Ž
2. Temperature below 150 C
3. No non-condensable components or electrolytes
4. Components must not contain more than 10 functional groups.
8.16.6. K-values for hydrocarbons
A useful source of K-values for light hydrocarbons is the well-known “De Priester charts”,
Dabyburjor (1978), which are reproduced as Figure 8.3a and b. These charts give the K-
values over a wide range of temperature and pressure.

8.16.7. Sour-water systems (Sour)
The term sour water is used for water containing carbon dioxide, hydrogen sulphide and
ammonia encountered in refinery operations.
Special correlations have been developed to handle the vapour-liquid equilibria of such
systems, and these are incorporated in most design and simulation programs.
Newman (1991) gives the equilibrium data required for the design of sour water
systems, as charts.

8.16.8. Vapour-liquid equilibria at high pressures
At pressures above a few atmospheres, the deviations from ideal behaviour in the gas phase
will be significant and must be taken into account in process design. The effect of pressure on
theliquid-phaseactivitycoefficientmustalsobeconsidered.Adiscussionofthemethodsused
to correlate and estimate vapour-liquid equilibrium data at high pressures is beyond the scope
ofthisbook.ThereadershouldrefertothetextsbyNull (1970)orPrausnitzandChueh (1968).
Prausnitz and Chueh also discuss phase equilibria in systems containing components
above their critical temperature (super-critical components).

8.16.9. Liquid-liquid equilibria

Experimental data, or predictions, that give the distribution of components between the
two solvent phases, are needed for the design of liquid-liquid extraction processes; and
mutual solubility limits will be needed for the design of decanters, and other liquid-liquid
separators.
Perry et al. (1997) give a useful summary of solubility data. Liquid-liquid equilibrium
compositions can be predicted from vapour-liquid equilibrium data, but the predictions
are seldom accurate enough for use in the design of liquid-liquid extraction processes.
Null (1970) gives a computer program for the calculation of ternary diagrams from vle
data, using the Van-Laar equation.
The DECHEMA data collection includes liquid-liquid equilibrium data for several
hundred mixtures, DECHEMA (1977).