Adiabatic Flash Drum with Binary Liquid Feed

A high-pressure, hot, liquid mixture of methanol and water is fed into an adiabatic flash drum (or vapor-liquid separator). Because the flash drum pressure is below the bubble pressure, some of the liquid evaporates and the temperature decreases because energy is needed to evaporate the liquid. Thus, a vapor-liquid mixture in equilibrium exits the drum. You can vary the feed mole fraction of methanol, the feed temperature and flash drum pressure with sliders. This is a continuous process, but calculations are presented for 10 moles of feed. Material balances, an energy balance and Raoult's law for vapor-liquid equilibrium are used to determine the amounts of liquid and vapor exiting the drum and the mole fractions in each phase.


  • [Snapshot]
  • [Snapshot]


An overall and component mole balance are performed:
where is the number of moles, the superscripts , and refer to the feed, liquid and vapor streams and the subscript refers to a component (methanol or water).
Overall and component energy balances with the reference state and :
where is enthalpy (kJ).
The enthalpies of each stream are calculated using heat capacities (kJ/(mol K)) and heat of vaporization (kJ/mol):
The flash drum has a single equilibrium stage, so the exiting liquid and vapor streams are at the same temperature, .
Saturation pressure of the components in vapor-liquid equilibrium is calculated using the Antoine equation:
where , and are Antoine constants for each component, and is in bar.
Raoult's law is used for the exit streams to find the vapor-liquid equilibrium compositions:
where and are the liquid and vapor mole fractions.
The sum of the mole fractions times their saturation pressures is the total pressure .
The screencast video at [1] explains how to use this Demonstration.
[1] Adiabatic Flash Drum with Binary Liquid Feed [Video]. (Dec 8, 2016) www.colorado.edu/learncheme/separations/AdiabaticFlash.html.
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