Example 5.5 - The Effect of Temperature on Rate
We have seen in the homework how to solve the set of design equations that are needed to describe completely the following reactions in a PFR
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The book examines the same reaction set in a PFR and explores the effect of applying the QSSA to the radicals, CH3, C2H5, and H. In the book several graphs were generated that illustrate the steady state approximation does a very good job of capturing the formation of ethylene. The simulation program will generate two of the graphs from the book so you can see them here as well. Our intent here is not to rehash the same QSSA arguments, rather to use the exact solution to gain insight into how sensitive the reaction is to temperature.
THE SIMULATION
Run the simulation and change the temperature from the default value of 925 K. Try increasing and decreasing it by about 0.5 % , i.e., change it by only 5 K. Decreasing the temperature to 920 K decreased the effluent ethylene concentration by 5.3 %. Decreasing the temperature by 10 K changes the final concentration by almost 12%! Why do you think this big of a change occurs? This illustrates how important temperature control is to reactor design and reactor yield.
Increase the temperature. As you do, you will notice that rather quickly the ethane is consumed before the end of the reactor. What temperature is this? Is this an exothermic or endothermic reaction? Why is it important to know this if you had to make sure the ethane feed was consumed in the reaction?
