realgas

Real gas refers to any gas that does not obey the ideal gas law PV = nRT exactly. In real gases, intermolecular forces and finite molecular size cause deviations from ideal behavior, especially at high pressures or low temperatures. These deviations are described by more complex equations of state and by the virial expansion, which relate pressure, volume, and temperature for real substances.

The most common real-gas models are cubic equations of state, such as van der Waals, Redlich-Kwong, Soave-Redlich-Kwong, and Peng-Robinson. These equations include parameters to account for molecular size and attractive forces, fitted to experimental data. The compressibility factor Z = PV/(nRT) measures deviation from ideal behavior; Z differs from 1, typically being less than 1 at moderate conditions due to attractions and greater than 1 at higher pressures due to repulsion.

Real-gas behavior is especially important near the critical point and in phase behavior calculations for mixtures. Critical constants Tc, Pc, and Vc characterize substances and are used to parameterize equations of state. For mixtures, mixing rules and more advanced EOS or activity-coefficient models are used to predict phase equilibria and properties across compositions.

Applications include natural gas processing, liquefied natural gas design, hydrocarbon refining, and supercritical carbon dioxide processes. While real-gas models improve accuracy, they are approximations that rely on fitted parameters, and different models may be used to cross-check results in engineering calculations.