Residence Time Distribution: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • For most reactors, not all molecules spend the same time in a reactor.
  • Tracer experiments can be used to determine the residence time distribution (RTD) for a reactor to determine how close the reactor is to an ideal reactor.
  • For a PFR (plug flow reactor), all molecules spend the same time in the reactor.
  • For a CSTR, molecules have a large range of residence times, and the largest fraction leave at very short times.
  • As the number of CSTRs in series increases while keeping the total volume constant, the residence time distribution (RTD) starts to approach the RTD for a PFR.
  • Different residence times means different reaction times, and this can affect the product distribution. Thus, the RTD affects the product distribution for polymerization reactors.
  • For a pulse injection of a tracer into a reactor or series of reactors, the concentration of tracer at the outlet is proportional to the RTD.
From studying this module, you should now be able to:
  • Calculate the average residence time from a tracer measurement on a reactor.
  • Explain the shape of residence time distribution (RTD) for a plug flow reactor and a continuous stirred tank reactor.
  • Explain the shape of the RTD for multiple CSTRs in series.
  • Set up and solve material balances for a tracer pulse injection into CSTRs in series.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder