Key equations for Process Engineers Process Design Thermodynamics

Compressibility factor

Compressibility factor of gases is a thermodynamic property which is used to modify the ideal gas equation for real gases.

For Ideal gases, Ideal gas equation states,
PV = nRT

This equation can be modified for real gases, to be expressed as,
PV = ZnRT, here Z is the compressibility factor for the given gas.

The compressibility factor of a gas depends on the particular gas as well as temperature and pressure conditions.
Various equations of state can be used for calculation of compressibility factor of a gas as a function of temperature and pressure. Using different equations of state, various equations are derived for compressibility factor calculation. The constant parameters used in these equations are empirically determined for a gas.

Compressibility charts which are available in literature are also very commonly used for calculation of compressibility factor. These charts express compressibility factor values as a function of reduced temperature and reduced pressure.
Reduced temperature, Tr = T / Tc
Reduced pressure, Pr = P / Pc
where Tc and Pc are critical temperature and pressure values respectively.

Some correlations such as Pitzer correlations, use the gas reduced properties such as reduced temperature and reduced pressure to approximately calculate compressibility factor. EnggCyclopedia's compressibility factor calculator is based on such correlations.

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