Chapter 4: Stoichiometry

Frequently Asked Questions (FAQs)

1. Why is the limiting reactant our basis of calculation?

   If the limiting reactant is not chosen as the basis of calculations, one could calculate a NEGATIVE concentration. See Example 4-2.

2. In solving problems for this class, is there ever a case where you need more steps than the mole balance, rate law, stoichiometry, and combining? (When do you deviate from this algorithm?)

   We will always use this basic algorithm and then just add to these steps to it, e.g. the energy balance. We will not deviate from these first four steps.

3. Please clarify the method for deriving the rate law expressed with partial pressures.

   When studying catalytic reactions the rate law is developed in terms of partial pressure,

   e.g. .

   To rewrite the rate law just use ideal gas law to relate to concentrations C_A and C_B

   

   and then write concentration in terms of conversion.

   Writing partial pressures in terms of conversion

   

   

   

4. How accurate is the perfect mixing assumption in dealing with CSTRs? It seems kind of far fetched that the entire CSTR is at the same concentration as the exit.

   True but these are ideal CSTRs, and non ideal reactors and the perfect mixing assumption is discussed and modeled in the PRS. Once we understand ideal reactors (perfectly mixed), we can easily model non-ideal reactors.

5. In a PFR or CSTR reactor, wouldn't the reaction still be taking place in the pipe that the products leave through? Why does the reactor just magically stop occurring when the contents leave the reactor? Is there a good way to model a CSTR that is not perfectly mixed?

   It does continue to react to some extent. It depends on the temperature in the pipe! However, these are ideal reactors. Different models for Non-ideal reactors are discussed in the PRS but we must understand ideal reactors first.

6. Can you compare space time for a flow reactor to the time spent in a batch reactor for the purposes of measuring conversion?

   Sometimes. See Lecture 6 on the CD-ROM.

7. Is the assumption that there are no radial gradients a good one for most tubular reactors? When is it not valid?

   It is quite a good assumption for turbulent flow. It is not valid for Laminar flow.

8. When is the compressibility factor in the ideal gas law not equal to one? Will w ever encounter this?

   You are only interested in the ratio Z/Z_o being the same, we don't care if Z doesn't equal one.

9. I am having trouble understanding the concept that when epsilon is negative, change in P will be less (i.e. higher pressure) than for epsilon=0.

   If epsilon is greather than 1, density is greater than if epsilon=0 because

   A larger density results in a smaller change in pressure.