Phase Equilibrium


Phase equilibrium can be used to control the concentration of components. It is also a concept widely used in various types of separation processes. In this chapter, we will learn to collect information from existing diagrams, including variations of phase diagrams for pure components and diagrams summarizing vapour-liquid equilibrium conditions for multicomponent systems. We can use the information to find the phase(s) of components in our processing equipment and the mole fractions of components in each phase.

Learning Objectives

By the end of this chapter, you should be able to:

Interpret phase diagrams for relevant thermodynamic information (e.g. composition, temperature, pressure…)
Apply the definition of intensive and extensive properties to process variables
Describe the relationship between variables of state for gases
Describe gas behaviour using the ideal gas law and compressibility factor for equations of state
Estimate the vapour pressure of pure compounds at a given temperature using Antoine’s equation
Apply properties of ideal mixtures to describe vapour-liquid separations
Read Txy and Pxy diagrams for binary mixtures
Find bubbles points and dew points on the diagrams
Calculate the mole fraction of substances in the vapour and liquid phases


As you’re going through this chapter, here are some important terms for you to take note of:

  • physical properties
  • pressure
  • manometer
  • gauge pressure
  • absolute pressure
  • temperature
  • heat transfer
  • absolute temperature
  • extensive variables
  • intensive variables
  • phase transitions
  • intermolecular forces
  • critical point
  • triple point
  • phase diagram
  • supercritical fluid
  • reduced temperature/pressure
  • thermodynamic system
  • surroundings/environment
  • boundary
  • Gibb’s Phase Rule
  • degrees of freedom
  • compressible/incompressible
  • ideal gas law
  • molar volume
  • Dalton’s law
  • Amgat’s law
  • partial pressure
  • mole fraction
  • vapour temperature
  • vapour pressure
  • Antoine equation
  • vapour-liquid equilibrium
  • Raoult’s law
  • Henry’s law
  • volatile
  • solubility
  • Txy/Pxy diagram
  • ideal mixture
  • binary mixture
  • bubble point
  • dew point
  • saturated liquid
  • saturated vapour


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Foundations of Chemical and Biological Engineering I Copyright © 2020 by Jonathan Verrett is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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