Departure Functions: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • The departure function is the difference between the thermodynamic property (internal energy, enthalpy, entropy, etc.) for a real gas and an ideal gas at the same conditions such as temperature and pressure.
  • An equation of state can be used to calculate a departure function.
  • Steam tables can be used to calculate a departure function for water. Any table of properties or graph of properties for a pure component can be used to calculate the departure function for that component. Property data are used to calculate departure functions by determining the value of the state function at sufficiently low pressure that ideal gas behavior is observed and at the same temperature as the state of interest. The ideal gas value at the high pressure is then calculated. For example, the ideal gas enthalpy does not change with pressure, whereas the change in the ideal gas entropy is calculated by: \[\Delta S^{ig} = -R \hspace{1mm} ln \frac{P_2}{P_1}\]
From studying this module, you should now be able to:
  • Explain what a departure function is.
  • Calculate U, H, S, and G using the PVT relations in the EOS spreadsheet.
  • Calculate changes in state variables using departure functions and ideal gas heat capacities.
  • Explain why ideal gas heat capacities can be used for real fluid calculations.
  • Use the departure function approach to solve first and second law problems for real fluids.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder