Compressibility factor

The compressibility factor, Z, is the ratio of a gas’s actual pressure–volume behavior to the ideal-gas prediction at the same temperature and amount of gas. It equals one for an ideal gas and departs from one when intermolecular effects matter.

Equation of state: A mathematical relation connecting a substance’s pressure, volume, temperature, and amount. The compressibility factor rewrites a gas equation of state as a dimensionless correction to ideal behavior.
Pseudoreduced properties: Gas properties scaled by pseudocritical values for applying corresponding-states methods to mixtures. They locate gas mixtures on generalized compressibility charts.
Molar volume: The volume occupied by one mole of a substance. Using molar volume makes Z independent of the sample’s amount.
Van der Waals equation: A cubic equation of state correcting ideal-gas behavior for molecular attraction and finite molecular volume. It predicts Z from a simple model rather than treating the factor as measured data.
Ideal gas law: The equation PV = nRT relating pressure, volume, temperature, and amount for an ideal gas. It supplies the baseline whose predicted pressure–volume behavior Z corrects.
Standing–Katz chart: A chart estimating natural-gas compressibility factors from pseudoreduced pressure and temperature. It provides engineering estimates of Z where a simple ideal-gas calculation fails.
Reduced temperature: A temperature expressed as a ratio to a substance’s critical temperature. It is a key coordinate for estimating Z through corresponding states.
Peng–Robinson equation of state: A cubic equation of state widely used to estimate fluid properties and phase equilibria. It offers a practical alternative for calculating Z across gas and liquid conditions.
Virial equation of state: A power-series equation of state whose coefficients encode intermolecular interactions. Its density expansion expresses Z through virial coefficients.
Natural gas: A naturally occurring gas mixture composed mainly of methane and other hydrocarbons. Its measured volume and density depend on Z, especially at pipeline pressures.

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