Frequently Asked Questions (FAQs)

Chapter 5

1. What specifically causes a CSTR in series to have a higher conversion than a CSTR in parallel?

   The CSTR is always operating a the lowest concentration, the exit concentration. When say two CSTRs are in series, the first operates at a higher concentration, therefore the rate is greater, therefore the conversion is greater. The second reactor in series builds on the conversion in the first reactor. The conversion in the parallel scheme is the same as the conversion to the first reactor to the series scheme. See Example 5-2.

2. When are reactors in parallel used since it seems as though reactors in series would always achieve higher conversion?

   The PBRs in parallel are ued when there would otherwise be a large pressure drop in one long reactor or identically several PBRs connected in series.

3. Is it possible to have a pressure drop for a liquid phase reaction, as is possible for a gas phase reaction?

   You can have a pressure drop in liquid phase systems, but it does not affect the reaction rate because liquids are virtually incompressible and therefore the concentration does not change with pressure.

4. Since two equal CSTR in series give a higher conversion than two in parallel, are reactors in parallel ever used to increase conversion?

   Not for a CSTR, only a PFR/PBR when there is a significant pressure drop.

5. The Damköhler (Da) number
   1. How is the Damköhler (Da) number defined for a reaction
      (A + B C) when the reaction is first order in A, first order in B, but second order overall?

      Just substitute the rate law evaluated entrance to the reactor, -r_A0, [e.g. -r_Ao=kC_AoC_Bo] into the definition .

   2. Is Da always indicative of certain conversion?

      Yes, for irreversible reactions.

   3. How does defining an extra variable, the Damk�hler number, save us time and confusion, as opposed to solving without it? When should it be used?

      It serves as rule of thumb. When Da < 0.1 then X < 10% and when Da >10 then X >90%.

6. In the chapter we are given total cycle times excluding reaction for a batch polymerization. Is there a similar standard for a PBR when the catalyst must be removed and are there similar standards for flow systems that experience coking on the walls and need cleaning eventually.

   The onstream time for flow systems is much much greater than the down time for cleaning and repair.

7. In the text it states that for developing the design equation for a PBR when X << 1 you can use the relationship

   

   Neglecting X

   

   For what values of X is this valid?

   

8. We understand that for a CSTR, the conversion X increases as residence time increases. We were unsure as to what the relationship is for a PFR?

   The same is true for a PFR If you increase and you increase X.

9. Is the CSTR conversion equation always the same no matter the order of reaction?

   No! This equation is only for first order.