Unit 39. Gas-Liquid Reactions

This website provides learning and teaching tools for a first course on kinetics and reaction engineering. In the preceding parts of the course, the reacting fluid was always treated as if it was homogeneous, and only ideal reactor types were considered. The knowledge gained to this point is sufficient for reaction engineering for many commercial processes. Nonetheless, there are situations where the reactor does not conform to one of the ideal types and/or the rates are affected by the kinetics of physical processes in addition to the chemical reaction rate. Part IV of the course surveys a few such situations. It does not provide an in-depth analysis of any of them, but the information provided should serve as a good foundation for further study.

This, final section of the course, considers situations where the kinetics are affected by factors other than the rate of the chemical reaction. This topic was introduced as part of the discussion on performing kinetics experiments in Part II of the course, specifically in Unit 12. The perspective and objective there was to ensure that these other factors were sufficiently small in magnitude so that they could be ignored. In Section A of this part of the course, specifically in Unit 37, it was noted that when two phases are present in a reactor, concentration and temperature gradients may exist near the interface between phases, and the design equations must properly account for those gradients. Section B of Part IV introduces a few ways that this can be accomplished. Once again, a limited, introductory presentation is offered with comprehensive treatment left for a second, more advanced course on kinetics and reaction engineering.

Unit 38 described ways to model reaction kinetics in heterogeneous catalytic systems that involve two phases. Whenever two phases are present and involved in the reactive process, interfacial concentration and temperature gradients can affect the apparent kinetics. Unit 39 offers an overview of systems where a reaction involves one component in a liquid phase and a second component in the gas phase, with the reaction taking place in the liquid phase. The approach is actually quite similar to the approach used for heterogeneous catalysts. One difference is that more than one set of dimensionless quantities are used to characterize gas-liquid system. In one case the quantities are defined with the perspective that the occurrence of the reaction increases the apparent rate of gas absorption into the liquid while in the other, the perspective is more like heterogenous catalysis: the gradients reduce the apparent rate of reaction.

Learning Resources

• Archive (.zip) - Contains all learning resources listed below for this unit
• Documents to Read:
  • Unit 39 Informational Reading (.pdf)
  • Example 39.1 (.pdf)
• Videos to Watch (please right-click and save, then play back locally on your computer):
  • Unit 39 Informational Video (.mov)
  • Example 39.1 Video (.mov)
• Reference Files:
  • Unit 39 Definitions, Nomenclature and Equations (.pdf)
• Other Files:
  • Example_39_1.m, Example_39_1_bulk.m and Example_39_1_film.m - MATLAB files used to perform the calculations for Example 39.1

Teaching Resources

• Archive (.zip) - Contains all teaching resources listed below for this unit
• Sample Class
  • Online Pre-class Quiz (.pdf file)
  • Redacted slides to make available to students before class
    • Keynote 09
    • PowerPoint
  • Other files to make available to students before class
    • Worksheet for Activity 39.1
  • Class Lesson Plan (.pdf)
  • Complete slides to make available to students after class
    • Keynote 09
    • PowerPoint
  • Other files to make available to students after class
    • Activity_39_1.m, Activity_39_1_bulk.m and Activity_39_1_film.m - MATLAB files used in the solution of Activity 39

Practice Problems

1. The solution to Example 39.1 states that if the presence of a liquid film is not included in the analysis, the predicted conversion will be 99%. Verify this claim.

2. Suppose that the presence of the gas film was not included in the analysis for Example 39.1 (but the liquid film was included). What conversion would be predicted.

3*. Sometimes, when the rate of reaction is small, the amount of reaction that takes place in the liquid film may be ignored because the volume of the liquid film is so small compared to the volume of the bulk liquid. If this assumption is made in the analysis of Example 39.1, what conversion is predicted?

4*. Sometimes, when the rate of reaction is very large (essentially instantaneous), the concentration of A will become equal to zero somewhere within the film. When this happens, the concentration of B will also equal zero at the same point in the film. Suppose that the rate coefficient for the reaction in Example 39.1 is so large that the reaction is effectively instantaneous. At what distance into the liquid film will the concentrations of A and B become equal to zero?