Frequently Asked Questions (FAQs)

Chapter 12

1. 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).

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

   Good point!! Yes it does.

3. 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
   

4. 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.

5. Calculations in Ch.12 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