Compressibility Factor / Z Factor

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

Ideal gas equation states,
PV = nRT

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

The compressibility of a gas depends on the particular gas as well as temperature and pressure conditions.

Calculating compressibility factor for natural gas

Compressibility charts available in literature are commonly used for calculation of compressibility factor / Z factor. These charts express compressibility values as a function of reduced temperature and reduced pressure. For example, this compressibility factor table for air.
Reduced temperature, T_r = T / T_c
Reduced pressure, P_r = P / P_c
where T_c and P_c are critical temperature and pressure values respectively.

Example of a compressibility chart

Compressibility charts are based on empirical findings. For natural gas, compressibility factor depends on - temperature & pressure conditions, composition of the gas in terms of N₂, CO₂ and H₂S content.