Frequently Asked Questions (FAQs)

Chapter 8

1. Can X_EB for a non-adiabatic reactor be linear? Under what conditions?

   Yes. Most all cases because deltaC_P (T-T_a) is small wrt the heat of reaction.

2. When doing an energy balance on a PBR, should we ever take into account the properties of the catalyst? (i.e. C_P of the catalyst)

   Only for moving bed reactors and fluidized CSTR's, in which the catalyst is fed to the reactor along with the reactants, operated at steady state and for other reactors operated at unsteady state.

3. For a CSTR ? when determining whether the Q in the energy balance can be reduced to  Q = UA(T_a - T) what would be considered a high coolant flow rate or low UA (range)?

   One must take ratio of the two! Usually when  (UA/mC_P) < 0.1, you can expand exponents in a Taylor series to get Q = UA(T_a - T).

4. For an exothermic reaction with cooling, does the T vs. W graph asymptote at the heat exchange fluid temperature?

   Good point!! Yes it does.

5. How does one approach a CSTR when heat effects are varying through the tank even if well mixed?

   See P13-8, p.865.

6. Will we do any "semibatch" reactor problems where the entering flowrate changes? Would this be helpful toward the end of the reaction?

   Sometimes. Maybe, depends if unwanted side reactions are present.

7. Why doesn't the stirring involved in a  CSTR count towards shaft work (W_S)?

   It does count. It will be a negative value (e.g. -10,000 cal/s) because work is being done on the system. A viscous liquid would increase in temperature with rapid stirring.

8. If dT/dW in PFR energy balance = 0, does it become the energy balance in CSTR?

   No! The term dT/dW will only equal zero if there is a maximum in the temperature down the length (i.e. W) of the packed bed. For a single reaction, the maximum will only occur if the reaction is exothermic and there is heat exchange.

   then
   

9. Why does the curve of G(T) vs. T have a steeper slope for reactions with lower energy of activation?

   A lower activation energy means the reaction is less temperature sensitive. The G(t) curve also depends on t. See Fig. 8-21, p.494.

10. Regarding problem P8-3_C, if you realize what happened to the ammonium nitrate feed 3 minutes after the feed was shut off, what would be the best course of action?

    If there is no chance to add water, run for the door.

11. Calculations in Ch.8 for ignition and extinction temperatures neglect shaft work. How does this approximation affect the results?

    It simply shifts the R(t) curve. It is easily included in the heat removed term

12. What are the effects of inerts on ?

    The inerts do not affect . Delta C_p only involves the reating species. For example

    

13. What are the effects of inerts?

    Inerts will decrease the temperature reise for an exothermic reaction and decrease the temperature drop for an endothermic reaction.

14. Do you include products and inerts in

    ?

    Only include those species which are entering the reactor. Remember