Page 371 - Chemical engineering design

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CHEMICAL ENGINEERING
Universal quasi-chemical (UNIQUAC) equation
The UNIQUAC equation developed by Abrams and Prausnitz is usually preferred to the
NRTL equation in the computer aided design of separation processes. It is suitable for
miscible and immiscible systems, and so can be used for vapour-liquid and liquid-liquid
systems. As with the Wilson and NRTL equations, the equilibrium compositions for a
multicomponent mixture can be predicted from experimental data for the binary pairs that
comprise the mixture. Also, in the absence of experimental data for the binary pairs, the
coefficients for use in the UNIQUAC equation can be predicted by a group contribution
method: UNIFAC, described below.
The UNIQUAC equation is not given here as its algebraic complexity precludes its use
in manual calculations. It would normally be used as a sub-routine in a design or process
simulation program. For details of the equation consult the texts by Reid et al. (1987) or
Walas (1984).
The best source of data for the UNIQUAC constants for binary pairs is the DECHEMA
vapour-liquid and liquid-liquid data collection, DECHEMA (1977).

8.16.5. Prediction of vapour-liquid equilibria

The designer will often be confronted with the problem of how to proceed with the design
of a separation process without adequate experimentally determined equilibrium data.
Some techniques are available for the prediction of vapour liquid equilibria (v l e) data
and for the extrapolation of experimental values. Caution must be used in the application
of these techniques in design and the predictions should be supported with experimentally
determined values whenever practicable. The same confidence cannot be placed on the
prediction of equilibrium data as that for many of the prediction techniques for other
physical properties given in this chapter. Some of the techniques most useful in design
are given in the following paragraphs.

Estimation of activity coefficients from azeotropic data
If a binary system forms an azeotrope, the activity coefficients can be calculated from
a knowledge of the composition of the azeotrope and the azeotropic temperature. At
the azeotropic point the composition of the liquid and vapour are the same, so from
equation 8.31:
P
i D
P Ž
i
Ž
where P is determined at the azeotropic temperature.
i
The values of the activity coefficients determined at the azeotropic composition can be
used to calculate the coefficients in the Wilson equation (or any other of the three-suffix
equations) and the equation used to estimate the activity coefficients at other compositions.
Horsley (1973) gives an extensive collection of data on azeotropes.