Page 37 - Handbook of Natural Gas Transmission and Processing Principles and Practices

gas behaves as an ideal gas at these conditions. At standard or atmospheric conditions the gas Z
factor is always approximately 1.
Empirical correlations for Z-factor for natural gases were developed before the advent of digital
computers. Although their use is in decline, they can still be used for rapid estimates of the Z-factor.
Chart look-up is another means of determining Z-factor of natural gas mixtures. These methods are
invariably based on some type of corresponding states development. According to the theory of
corresponding states, substances at corresponding states will exhibit the same behavior. The theory
of corresponding states dictates that the Z factor can be uniquely defined as a function of reduced
pressure and reduced temperature. The reduced pressure and reduced temperature are defined as:

(1.4)

where P and T are reduced pressure and reduced temperature, respectively; and P and T are
r
c
r
c
critical pressure and critical temperature of the gas, respectively. The values of critical pressure and
critical temperature can be estimated from the following equations if the composition of the gas and
the critical properties of the individual components are known.

(1.5)

where P and T are the critical pressure and critical temperature of component i, respectively; and
ci
ci
y is the mole fraction of component i.
i
Once critical properties of the mixture are calculated as stated in Eq. (1.5), we can use Eq. (1.4) to
calculate the reduced properties of the mixture.
Table 1.2

Physical Constants for Pure Components (Whitson and Brule, 2000)

37