Isothermal Continuous Stirred Tank Reactor (CSTRs): Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Material balances on individual components are most useful for reactor design:
  • Accumulation(+ or -) = In(+) – Out(+) + Generation by reaction(+ or -)
  • A CSTR is assumed to be gradientless (the concentrations and temperature are the same everywhere in the tank) so the outlet concentrations and temperature are identical to the tank concentrations and temperature.
  • When feed enters a CSTR, it undergoes a step change in concentration.
  • Reaction in a steady-state CSTR takes place at one set of concentration values, and this affects selectivity in a CSTR relative to other type reactors.
  • For an isothermal CSTR, since reaction takes place at the exit concentration (which is lower than the feed concentration), the rate of reaction can be lower than a PFR of the same size.
  • Not all molecules spend the same amount of time in a CSTR; some entering molecules spend almost no time in a CSTR and other spend a long time (much longer than the average residence time). This residence time distribution can affect both rate and selectivity.
  • CSTRs are normally used for liquid phase reactions.
  • CSTRs are used when good mixing is desired, or when a higher selectivity is obtained by carrying reaction out at the same concentration and temperature.
From studying this module, you should now be able to:
  • Describe the differences between a CSTR and a PFR or batch reactor.
  • Explain why a CSTR might be used instead of a PFR.
  • Determine the size of an isothermal CSTR for a single reaction to reach a desired conversion, given the rate constant, order of reaction, inlet reactant concentration, and inlet flow rate.
  • Determine the conversion in an isothermal CSTR for a single reaction, given the rate constant, order of reaction, inlet reactant concentration, and inlet flow rate.

 

For more information on CSTRs, see https://en.wikipedia.org/wiki/Continuous_stirred-tank_reactor

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