REAL GASES

Several assumptions were made in formulating the equation of state for ideal gases. Since these assumptions are not correct for gases at pressures and temperatures that deviate from ideal or standard conditions, correction must be made to account the deviation from ideal behavior. The most widely used correction method in petroleum industry is gas compressibility factor, more commonly called the Z-factor. It is defined as the ratio of the actual volume occupied by a mass of gas at some pressure and temperature to the volume the gas would occupy if it behaved ideally. That is,

or

The equation of state is

or

Therefore, the equation of state for any gas becomes

(1)

where, for an ideal gas, Z=1/

The compressibility factor varies with change in gas composition, temperature, and pressure. It must be determined experimentally. The results of experimental determination of compressibility factor normally given graphically and usually take the form shown on Figure 1.

Fig.1. Typical plot of the compressibility factor as a function of pressure at constant temperature.

The shape of the curve is consistent with present knowledge of the behavior of gases, at very low pressure the molecules are relatively far apart and the conditions of ideal gas behavior are more likely to be met. At low pressure the compressibility factor approaches a value of 1.0, which would indicate that ideal gas behavior does in fact occur. Compressibility factors for propane are given on Figure 2.

Fig.2. Compressibility factors for propane.

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