Interactive Self-Study Module: Flash Separations

Department of Chemical and Biological Engineering, University of Colorado Boulder

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Mixtures are flashed to effect partial separations so that one component (or more) is enriched in a component in the vapor phase (relative to the feed).
This is referred to as flash distillation. Only binary mixtures are considered in this module to simplify the explanations,
but multi-component mixtures are used in real systems.

This module is intended for Material and Energy Balances, Thermodynamics and Separations courses.


  • Understand how to apply Raoult's Law
  • Be able to apply the Antoine equation to determine saturation pressure of a single component at a given temperature
  • Be able to apply mass balances to mixtures
  • Be able to do energy balances

After studying this module, you should be able to:
  1. Use mass balances and phase equilibrium equations to determine outlet compositions and flow rates for a flash drum if the outlet temperature is known.
  2. Use mass balances, phase equilibrium equations, and an energy balance to determine outlet compositions and flow rates for an adiabatic flash drum. 

Try to answer these ConcepTests before using this module.




This module uses screencasts and interactive simulations to explain what happens when a feed enters a flash drum and exits as
two streams in vapor-liquid equilibrium. In this module, the outlet conditions are calculated, assuming the temperature and pressure is known,
by using Raoult's law (ideal solution and ideal gas) and mass balances. Example problems are provided to allow the user to test themselves.
We suggest using the learning resources in the following order: 

  1. Watch the screencast that describe the phase diagrams and answer the questions within the screencasts
  2. Use the two interactive simulations to further understand the behavior of the phase diagrams
  3. Use the two quiz interactive simulations to test your understanding by carrying out step-by-step preparation of phase diagrams
  4. Use the three example problem screencasts to test your knowledge by reading the problem statement and try to solve the problem
    on your own and then watch the solution in the screencast.
  5. Answer the ConcepTests


This screencast derives the operating line for binary flash distillation and shows how to use it. 

Flash Distillation Derivation ‎(7 minutes)‎

Important Equations

Interactive Simulations
These simulations were prepared using Mathematica.
To use them, download the free CDF player available here, download the simulation CDF file (click on the images below).
Then, try to predict the behavior when some parameter changes before using a slider to change the parameter.
For most simulations, a screencast is provided to explain how to use the simulation.

Adiabatic Flash Drum with Binary Liquid Feed

This simulation shows an adiabatic flash drum with a liquid feed stream containing a mixture of methanol and water.
The feed mole fraction of methanol, the temperature, and the flash drum pressure can be modified with sliders.
A bar graph shows the moles of water (blue) and methanol (green).

Try to answer these questions before manipulating the simulation:

  1. As temperature increases, how does V/F change and how do the various mole fractions change in the components?
  2. As pressure increases, how does V/F/ change and how do the various mole fractions change in the components?

Adiabatic Flash Drum Simulation Video

Download here

T-x-y and x-y Diagrams for Binary Vapor Liquid Equilibrium in a Flash Drum

The x-y diagram for vapor-liquid equilibrium is shown for a methanol/water stream fed to a flash drum. 

The corresponding T-x-y diagram shows how the points on the x-y diagram are obtained; each point corresponds to a different temperature. 

Use buttons to view the x-y and T-x-y diagrams separately or to view both simultaneously. 

The vapor/liquid ration leaving the flash drum as well as the feed composition can be changed with the sliders. 

Try to answer these questions before manipulating the simulation:

  1. How does changing the L/V ratio affect the temperature of the flash drum and the exiting streams?
  2. What affect does changing the feed mole fraction of methanol have on the exiting streams, if any?

T-x-y and x-y Diagrams for Binary Vapor-Liquid Equilibrium ‎(VLE)‎ in Flash Drum Video

Download here

Quiz-yourself simulations

To be added.

Example Problems

After reading the problem statements below, try to solve the problem before watching the screencast.

Example Problem 1

A 50/50 binary liquid mixture of benzene (1) and toluene (2) is flashed to 1.4 bar pressure and 25% of the liquid vaporizes:

What is the composition of the resulting vapor?

Example Problem 2

A liquid that is 60% component 1 and 40% component 2 is flashed to 1210 kPa. The outlet temperature is 150ºC. The saturation pressures in kPa are:

 (T in ºC)

Calculate the fraction of effulent that is liquid and the compositions of the liquid and vapor phases. Assume the liquid is ideal.

Flash Calculation: Binary Mixture (3.5 minutes)

Flash Calculation: Raoult's Law ‎(6.5 minutes)‎

Example Problem 3

A 30 mol% mehtanol/70  mol% ethanol liquid mixture at 423 K and 20 bar pressure is flashed adiabatically to a pressure of 2 bar.

Calculate the effluent temperature, the fraction of liquid vaporized, and the compositions of the liquid and vapor phases.

Adiabatic Flash of a Binary Mixture (8 minutes)



Try to answer these ConcepTests after using this module as a way to test your understanding. 


Summary of Phase Equilibrium for Immiscible Liquids

  1. The same calculations (mass balances, phase equilibrium) are used for systems with more than two components. 
    Usually an energy balance is also necessary to determine the outlet temperature.
  2. When pressure is lowered on a hot liquid, some of the liquid evaporates, and when this is done adiabatically
    the temperature decreases; the heat of vaporization comes from cooling down the liquid

Prepared by John L. Falconer and Kimberly R. Bourland

Department of Chemical and Biological Engineering, University of Colorado Boulder