Flash Separations: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • The exiting liquid and vapor streams from a flash separations drum are in equilibrium.
  • The same calculations (mass balances, phase equilibrium, energy balance) are also used for flash separations with more than two components.
  • An energy balance is also necessary to determine the outlet temperature from a flash drum.
  • When pressure of a hot liquid is lowered below its bubble pressure, some of it evaporates. When this is done adiabatically, the temperature decreases to provide the heat of vaporization.
  • The operating line relates the compositions of the two exiting streams from a flash separation.
From studying this module, you should now be able to:
  • Use mass balances and Raoult’s law to determine outlet compositions and flow rates for a flash separation (flash drum) if the outlet temperature is known.
  • Use mass balances, Raoult’s law, and an energy balance to determine outlet compositions and flow rates for an adiabatic flash separation.


Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder