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## Departure functionIn thermodynamics, a ## Additional recommended knowledgeDeparture functions are used to calculate real fluid extensive properties (i.e properties which are computed as a difference between two states). A departure function gives the difference between the real state, at a finite volume or non-zero pressure and temperature, and the ideal state, usually at zero pressure or infinite volume and temperature. For example, to evaluate enthalpy change between two points h(v Departure functions are computed by integating a function which depends on an equation of state and its derivative. ## Departure functions for Peng-Robinson equation of stateThe Peng-Robinson equation of state relates the three interdependent state properties pressure T), one may compute the departure function for enthalpy per mole (denoted h) and entropy per mole (s) :
Where α is defined in the Peng-Robinson equation of state, P is the reduced pressure, _{r}Z is the compressibility factor, and
- κ = 0.37464 + 1.54226ω − 0.26992ω
^{2}
Typically, one knows two of the three state properties ( T), and must compute the third directly from the Peng-Robinson equation of state. To calculate the third state property, it is necessary to know three constants for the species at hand: the critical temperature T, critical pressure _{c}P, and the acentric factor _{c}ω. But once these constants are known, it is possible to evaluate all of the above expressions and hence determine the enthalpy and entropy departures.
## References**^**Kyle, B.G.*Chemical and Process Thermodynamics*, 3rd Ed. Prentice Hall PTR, 1999. p. 118-123.
Categories: Thermodynamics | Fluid mechanics |

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Departure_function". A list of authors is available in Wikipedia. |