Page 39 - Handbook of Natural Gas Transmission and Processing Principles and Practices

in the order of 2%–3%. However, for gas mixtures whose components differ greatly in molecular
weight from 40, this chart provides inaccurate Z-factors (Elsharkawy et al., 2001).
Wichert and Aziz (1972) have developed a correlation to account for inaccuracies in the Standing
and Katz chart when the gas contains significant fractions of acid gases specifically carbon dioxide
(CO ) and hydrogen sulfide (H S). The Wichert and Aziz (1972) correlation modifies the values of
2
2
the pseudo-critical temperature and pressure of the gas. Once the modified pseudo-critical
properties are obtained, they are used to calculate pseudo-reduced properties and the Z-factor is
determined from Fig. 1.3. The Wichert and Aziz (1972) correlation first calculates a deviation
parameter “ε”:

(1.8)

where: A: sum of the mole fractions of CO and H S in the gas mixture; B: mole fraction of H S in
2
2
2
the gas mixture
Then, “ε” is used to determine the modified pseudo-critical properties as follows:

(1.9)

(1.10)

The correlation is applicable to concentrations of CO < 54.4 mol % and H S < 73.8
2
2
mol%. Wichert and Aziz (1972) found their correlation to have an average absolute error of 0.97%
over the following ranges of data: 154 psia < P < 7026 psia and 40°F < T < 300°F.
Methods of direct calculation using corresponding states have also been developed, ranging from
correlations of chart values to sophisticated equation sets based on theoretical developments
(Elsharkawy et al., 2001 and Heidaryan et al., 2010). However, the use of equations of state (EOS) to
determine Z-factors has grown in popularity as computing capabilities have improved. Equations
of state represent the most complex method of calculating Z-factor, but also the most accurate.

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