- 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.
- 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
- 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.
- 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.
- The Damköhler (Da) number
- 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,
-rA0, [e.g. -rAo=kCAoCBo]
into the definition .
- Is Da always indicative of certain conversion?
Yes, for irreversible reactions.
- 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%.
- 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.
- In the text it states that for developing the design equation for
a PBR when X
<< 1 you can use the relationship
For what values of X
is this valid?
- 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
The same is true for a PFR
If you increase
and you increase X.
- Is the CSTR conversion equation always the same no matter the order
No! This equation is only for first order.