Diffusion and Reaction in Porous Catalysts: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
ConcepTest 1 Answer — for more details see this screencast
Key points from this module:
  • The rate of reaction in a porous catalyst pellet is the rate that reactant diffuses into the pellet.
  • When comparing several porous catalyst spheres that have the same rate constant (for a positive-order reaction) and the same diameter, the sphere with the highest reactant concentration in the sphere forms the most product.
  • For a positive-order reaction, the effectiveness factor is highest for the catalyst with the highest concentration of reactant in the catalyst.
  •  At the center of a catalyst pellet or cylinder, \(dC_A/dr\) = 0, where \(C_A\) is the reactant concentration and \(r\) is the radius.
  • The concentration of reactant at the catalyst surface may be less than in the bulk because bulk diffusion may also limit reaction.
  • As the Thiele modulus increases, the effectiveness factor decreases.
  • For a given catalyst, smaller particles have higher effectiveness factors.
From studying this module, you should now be able to:
  • Calculate the Thiele modulus and effectiveness factor for a catalyst pellet.
  • Explain what a Thiele modulus represents and what an effectiveness factor represents.
  • Explain what the concentration profile looks like in a catalyst pellet for fast and slow reactions (or for slow and fast diffusivities).
  • Calculate the rate of reaction in the pellet, given a concentration profile in a catalyst pellet.

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