#### 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