Ternary Phase Diagrams: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • One phase exists outside of the phase envelope, and two distinct liquid phases are in equilibrium for compositions inside the phase envelope.
  • Tie lines connect two liquid phases that are in equilibrium.
  • For any two compositions on a ternary phase diagram, all mixtures of these two compositions lie on a straight line connecting the two compositions.
  • The triangular phase diagrams are at constant temperature and pressure, but temperature has a larger effect on the liquid-phase equilibrium than pressure.
  • The point on the phase envelope boundary at which the one phase switches to the second phase is the plait point, which is found by following the tie lines up until a tie line is tangent to the phase envelope boundary.
  • Ternary phase diagrams could be drawn with mole fractions or mass fractions.
From studying this module, you should now be able to:
  • Given mole fractions of each component, locate that composition on a ternary phase diagram.
  • Given phase equilibrium data for a ternary system, locate the phase envelope on the phase diagram and draw tie lines.
  • For a mixture composition in the two-phase region on a ternary diagram, use the lever rule (or a mass balance) to determine the amount of each phase.   

See also: Hunter-Nash Method for Liquid-Liquid Extraction

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