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DESIGN INFORMATION AND DATA
8.16.3. Equations of state
An equation of state is an algebraic expression which relates temperature, pressure and
molar volume, for a real fluid.
Many equations of state have been developed, of varying complexity. No one equation
is sufficiently accurate to represent all real gases, under all conditions. The equations of
state most frequently used in the design of multicomponent separation processes are given
below. The actual equation is only given for one of the correlations, the Redlich Kwong
equation, as an illustration. Equations of state would normally be incorporated in computer
aided design packages; see Chapter 11. For details of the other equations the reader should
consult the reference cited, or the books by Reid et al. (1987) and Walas (1989). To
selection the best equation to use for a particular process design refer to Table 8.11 and
Figure 8.4.

Redlich Kwong equation (R K)
This equation is an extension of the more familiar Van der Waal’s equation. The Redlich
Kwong equation is:
PT a
P D ð 8.37
V b T 1/2 V V C b
2 2.5
where a D 0.427 R T /P c
c
b D 0.08664 RT c /P c
P D pressure
V D volume

The R K equation is not suitable for use near the critical pressure (P r > 0.8), or for
liquids; Redlich and Kwong (1949).

Redlich Kwong Soave equation (R K S)
Soave (1972) modified the Redlich Kwong equation to extend its usefulness to the critical
region, and for use with liquids.

Benedict Webb Rubin (B W R) equation
This equation has eight empirical constants and gives accurate predictions for vapour and
liquid phase hydrocarbons. It can also be used for mixtures of light hydrocarbons with
carbon dioxide and water; Benedict et al. (1951).

Lee Kesler Plocker (L K P) equation
Lee and Kesler (1975) extended the Benidict Webb Rubin equation to a wider variety of
substances, using the principle of corresponding states. The method was modified further
by Plocker et al. (1978).