Physical constants of pure fluids: estimation methods

The critical point is a singular point in the phase diagram of a pure fluid, located at the end of the saturation curve. It is defined by three constants: a characteristic temperature, pressure and volume.

The critical temperature is the upper limit of the liquefaction temperature. The critical pressure is the upper limit of the saturation vapor pressure, and the critical volume is the limit common to the mass volumes of saturated liquid and saturated vapor.

Knowledge of the critical properties of fluids is essential for the study and operation of many processes that use fluids at high temperatures and pressures. For example, in supercritical extraction, which promises to be more energy-efficient than traditional separation processes such as distillation and liquid-liquid extraction, it is important to know the operating conditions near the critical point in order to optimize solvent power. In addition, critical properties are important for predicting thermodynamic and transport properties using the principle of corresponding states. Thus, critical temperature, critical pressure and critical volume or density are pure fluid constants commonly used as reduction parameters for thermodynamic quantities. This application is important in simulations or process studies, when properties such as vapor pressures or enthalpies of vaporization need to be known over a wide range of experimental conditions.