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CHEMICAL ENGINEERING
Chao Seader equation (C S)
The Chao Seader equation gives accurate predictions for light hydrocarbons and
hydrogen, but is limited to temperatures below 530 K; Chao and Seader (1961).
Grayson Stread equation (G S)
Grayson and Stread (1963) extended the Chao Seader equation for use with hydrogen
rich mixtures, and for high pressure and high temperature systems. It can be used up to
200 bar and 4700 K.

Peng Robinson equation (P R)
The Peng Robinson equation is related to the Redlich Kwong Soave equation of state
and was developed to overcome the instability in the Redlich Kwong Soave equation
near the critical point; Peng and Robinson (1970).

Brown K 10 equation (B K10)

Brown, see Cajander et al. (1960), developed a method which relates the equilibrium
constant K to four parameters: component, pressure, temperature, and the convergence
pressure. The convergence pressure is the pressure at which all K values tend to 1. The
Brown K 10 equation is limited to low pressure and its use is generally restricted to vacuum
systems.

8.16.4. Correlations for liquid phase activity coefficients
The liquid phase activity coefficient, i , is a function of pressure, temperature and liquid
composition. At conditions remote from the critical conditions it is virtually independent
of pressure and, in the range of temperature normally encountered in distillation, can be
taken as independent of temperature.
Several equations have been developed to represent the dependence of activity coeffi-
cients on liquid composition. Only those of most use in the design of separation processes
will be given. For a detailed discussion of the equations for activity coefficients and their
relative merits the reader is referred to the book by Reid et al. (1987), Walas (1984) and
Null (1970).

Wilson equation
The equation developed by Wilson (1964) is convenient to use in process design:
n

n 
x i A ik


ln k D 1.0 ln x j A kj 8.38
 n


jD1 iD1  x j A ij 
jD1
where k D activity coefficient for component k,
A ij ,A ji D Wilson coefficients (A values) for the binary pair i, j,
n D number of components.