Multiple Reactions and Selectivity: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Selectivity can be instantaneous, as represented by the rates of change, or overall, in which the molar flow rate or number of moles is considered.
  • Higher reactant concentration favors the reaction with higher order.
  • Because CSTRs run at the exit concentration of reactants, they favor reactions with lower orders of reaction.
  • Selectivity changes with conversion.
  • Higher temperature favors the reaction with the higher activation energy.
  • A semibatch reactor can yield higher selectivity when the desired reaction is higher order in one of the reactants by starting with that reactant in the reactor.
  • Catalyst can preferentially increase the rate of a desired reaction.
  • The definition of selectivity is not universal and different textbooks and literature articles use different definitions.
From studying this module, you should now be able to:
  • Determine the instantaneous and/or overall selectivity for a given system.
  • Explain how reactant concentrations affect selectivity.
  • Determine how to change temperature to increase selectivity.
  • Apply the concept of selectivity to solve problems in BRs, SBRs, PFRs, and CSTRs.
  • For a given system of reactions, specify what type reactor (or system of reactors) yields the highest selectivity and how the reactor should be operated.
Prepared by Nathan Jarvey and John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder