McCabe-Thiele Diagrams: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • McCabe-Thiele diagrams can be used to quickly determine the number of equilibrium stages required to reach desired distillate and bottom compositions.
  • The optimum feed stage is the first stage below the point where the q-line intersects the two operating lines.
  • Partial equilibrium stages do not exist, always round up to the next stage.
  • McCabe-Thiele diagrams are generally faster than numerical methods, but only provide an estimate.
  • The temperature is higher at the bottom of the column than at the top.
  • A constant molar overflow rate is often assumed when trying to determine the number of stages in a distillation column.
  • For each stage of a distillation column, the liquid and vapor streams leaving that stage are assumed to be in vapor-liquid equilibrium.
  • Feed quality q: q < 0 is superheated vapor; q = 0 is dew point vapor; 0 < q < 1 is a liquid vapor mixture; q = 1 is bubble point liquid; q > 1 is a subcooled liquid
From studying this module, you should now be able to:
  • Describe the meaning and application of reflux ratios, distillate compositions, and bottoms compositions for a distillation column.
  • Determine the number of equilibrium stages and output flow rates for a distillation column using McCabe-Thiele diagrams.
  • Develop feed lines and operating lines based upon the mass balances and equilibrium data for a system.
  • Determine the optimal feed location for a distillation column.
Prepared by Nathan Jarvey and John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder