Unit 35. Zoned Reactor Models

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.

The first section of Part IV considers reactors that do not satisfy the assumptions of any of the ideal reactor types. It touches upon three approaches to modeling such reactors. One approach is to increase the rigor of the ideal reactor models by changing one of more of the assumptions that define the ideal reactor, but retaining most of the original model. A second approach is to effectively abandon rigor in favor of a quantitatively accurate description of the reactor behavior. The last approach uses statistical methods to describe the performance of a reactor. Section A then concludes by considering changes that are necessary when modeling reactors wherein two phases are involved in the reaction(s) taking place, and presenting with an overview of reactors that are used when the reaction involves two or more phases.

Unit 35 considers zoned reactor models. Sometimes a reactor that is expected to obey the assumptions of an ideal reactor fails to do so, and the failure can be attributed to a physical issue. For example, the baffles in a CSTR might not be sized or positioned properly leading to “pockets” of fluid that are not perfectly mixed with the rest of the fluid in the reactor. This reactor might be modeled as a CSTR in combination with a well-mixed stagnant zone. As a second example, a packed bed reactor might not perform like an ideal PFR because the packing is not properly distributed, causing some fraction of the flow to bypass most of the packed bed. A zoned reactor model might be developed for this system where one zone represents the primary flow through the bed and a second zone represents the bypassing fraction of the flow. In effect, zoned reactor models are multiple reactor networks that are being used to model a single physical reactor.

Learning Resources

• Archive (.zip) - Contains all learning resources listed below for this unit
• Documents to Read:
  • Unit 35 Informational Reading (.pdf)
  • Example 35.1 (.pdf)
  • Example 35.2 (.pdf)
• Videos to Watch (please right-click and save, then play back locally on your computer):
  • Unit 35 Informational Video (.mov)
  • Example 35.1 Video (.mov)
  • Example 35.2 Video (.mov)
• Other Files:
  • Example_35_1.m - MATLAB file used to perform the reactor network calculations and make the plot for Example 35.1
  • Example_35_1_PFR.m - MATLAB file used to perform the PFR calculations for Example 35.1
  • Example_35_2.m - MATLAB file used to perform the calculations for Example 35.2

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 35.1
  • 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_35.m, Activity_35_cstr.m and Activity_35_pfr.m - MATLAB files used in the solution of Activity 35

Practice Problems

1. A tube that is 6 m long with an inside diameter of 7 cm is packed with pellets of solid catalyst. Reaction (1a) takes place within this reactor at a constant temperature of 450 °C and a constant pressure of 5 atm. The reactor will be fed 200 ft³ h^-1 of a gas containing 15% A, 15% B and 70% I (an inert gas). Reaction (1a) is one-half order in A and first order in B. Suppose that the packing in the tube is not uniform, and as a consequence 5% of the bed has a lower density (leading to a rate coefficient of 59.5 mol h^-1 atm^-0.5 m^-3), while the remainder has a higher density (with a rate coefficient of 72 mol h^-1 atm^-0.5 m^-3). Using a zoned reactor model with two equally-sized, well-mixed stagnant zones located 1/3 and 2/3 of the way into the reactor representing the lower density region and modeling the remainder of the reactor as a PFR, calculate the conversion if 7.5% of the flow in the PFR is diverted to each of the well-mixed stagnant zones.

(Problem Statement as .pdf file)