This document discusses reactor design for multiple reactions. It describes types of reactors including batch, semi-batch, plug flow, and continuous stirred-tank reactors (CSTRs). It also covers parameters for reactor design like volume, flow rate, concentrations, kinetics, temperature, and pressure. The document discusses plug flow versus CSTR design and designing for parallel, series, and complex reaction networks. It provides methods for maximizing desired products in multiple reaction systems, including adjusting conditions, choosing proper contacting patterns and reactors, and optimizing space-time or residence time. The document also presents equations for modeling multiple reactions occurring in a CSTR.

1. Design for Mutiple Reactions
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2. Types of reactors
1.Batch- uniform composition everywhere in
reactor but changes with time
2. Semi batch- in semi-batch one reactant will
be added when reaction will proceed
3. Continuous reactor
a.Mixed flow- this is uniformly mixed ,
same composition everywhere, within the
reactor and at exit
b. Plug flow- flow of fluid through reactor with
order so that only lateral mixing is possible

3. Reactor design parameter
Reactor design basically means which type and size of
reactor and method of operation we should employ for
a given conversation
Parameters
• Volume of reactor
• Flow rate
• Concentration of feed
• Reaction kinetic
• Temperature
• pressure

4. Plug flow and mixed flow reactor design
Mixed flow reactor design
Applying mass balance performance
equation for mixed flow reactor
Plug flow reactor design
Performance equation for plug flow reactor

5. Plug flow vs CSTR
•
•
•
•
For any particular duty and for all
positive reaction order the volume of
mixed flow reactor will always be grater
then plug flow
Area under curve in figure is very small
for plug flow as compared to mixed flow
so volume is small for plug flow.
When conversion is small, the reactor
performance is only slightly affected by
flow type. the perforation ratio very
rapidly at high conversion.
Density variation during reaction affects
design, however it is normally of
secondary importance compared to the
difference in flow type.

6. Multiple reactor system
•
•
•
b
e
Number of plug flow reactor
in series are theoretically
same as equivalent volume
of a single plug flow reactor.
Number of mixed flow
reactor of equal size in
series may be used when we
need high conversion and
can’t perform in a single
reactor.
From the given graph, for
first order reaction,
conversion for series of
equal size reactor can
find

7. Mixed flow reactor of different size in series
•
•
•
•
•
From the fig it is clear that for plug flow
reactor volume can be find by dashed
area and for mixed flow whole area.
When we are have to use mixed flow
reactor, then we can use different size
mixed flow reactor so, that over all
volume would be small
To optimized or to find how different size
of mixed flow reactor should used we
have to maximized lower dashed
rectangle.
This optimization gives the slope of
diagonal of the rectangle should be equal
to slope of curve at intersection of these
two reactor.
Levenspiel , has proved that after overall
economic consideration equal size
reactors in series are economical.

8. Design for parallel
reaction
•
•
•
When a reactant gives two product
(desired, and undesired)simultaneously
with different rate constant then this is
called a parallel reaction.
To keep maximum amount of desired
product we can take following steps.
• Ifa1>a2or the desired reaction is of higher
order then keep reactant concentration
high for high product concentration.
If a1<a2 than for desired reaction keep
reactant concentration low.
• For a1=a2 change in reactant
concentration will not affect the product
then, because rate constant k1and k2are
different at different temperature so, we
can keep our temperature such that
desired product will be high or use of
catalyst would be a option which are

9. Reactor design for multiple
reaction
•
•
In multiple reaction reactor design contacting pattern is most important
factor to get a particular product.
In irreversible reaction in series like
•
the mixing of fluid of different composition is the key to formation of
intermediate. The maximum possible amount of intermediate is
obtained if fluid of different composition and different stage of
conversation are not allowed to mixed.
In series of reaction if intermediate reactant is our desired product
than semi batch reactor will be used.

10. Irreversibleseries-parallel reaction
•
•
Multiple reaction that consist of steps in
series and steps in parallel reaction.
In these reaction proper contacting
pattern is very important.
• The general representation of these
reaction are
• reaction is parallel witHere the h respect
to reactant B and in series with A.
Halogenations of alkane is a
example of this kind of
reaction where reaction is
parallel with respect to
halogen

11. Reaction type
•
•
•
•
•
• Chemical kinetics of reaction can be known by knowing
the type of reaction
For reactor selection reaction type will tell us about heat of
reaction either reaction is endothermic or exothermic.
Selectivity is defined as reaction rate ratio for two parallel
reaction.
Catalyst are used to increase reaction rate and selectivity
for a specific reaction.
We can determine what type of catalyst will be used.
Reaction temperature range will be determined.

12. Reactor type
• Reactor may be a plug flow or mixed flow or batch
flow reactor or other.
• Contacting pattern of reaction will be known.
• In case of expensive catalyst and high heat transfer
rate required, mixed flow(fludized bed) reactor are
used.
• For high mass transfer plug flow (packed bed) reactor
will be used.

13.  Parallel rxns (competing rxns)
B
A
C
A + B
A + C
C + D
E
A
D
B + C
E + F
k1
k2
Definition of Multiple Reaction
•  Series rxns (consecutive rxns)
• A k1
B
k2
C
•  Complex rxns (Parallel + Series rxns)
k1
k2
 Independent rxns
k1
k2

14. Maximizing the desired product in series reaction
A
k1
B
k2
C
 In parallel rxns, maximize the desired product
 by adjusting the reaction conditions
 by choosing the proper reactor
 In series rxns, maximize the desired product
 by adjusting the space-time for a flow reactor
 by choosing real-time for a batch reactor

15.  If the first reaction (formation of B) is fast and the reaction to form C is
slow, a large yield of B can be achieved.
 However, if the reaction is allowed to proceed for a long time in a
batch reactor or if the tubular flow reactor is too long, the desired product
B will be converted to C.
 In no other type reaction is exactness in the calculation of the time
needed to carry out the reaction more important than in series reactions.
Maximizing the desired product in series reaction
k1 k2
A B C
Desired Product
 I f t hefirst reaction is slow and second reaction
is fast,it will be extremely difficult to produce species B.

16. Reaction paths for different ks in series reaction
A B C
k1 k2
~1
2
2
1
1
k
k1
k1
k2
k
k1
A C
B
1
 '
2
'
For k1/k2>1, a
Large quantity of B
Can be obtained
For k1/k2<1, a
Little quantity of B
Can be obtained 1st rxn is slow
2nd rxn is fast
'
3
Long rxn time in batch or long tubular reactor
-> B will be converted to C

17. Multiple reactions in a CSTR
Rearranging yields where
After writing a mole balance on each species in the reaction set, we substitute for concentrations
in the respective rate laws.
If there is no volume change with reaction, we use concentrations, Cj, as variables.
If the reactions are gas-phase and there is volume change, we use molar flow rates, Fj as
variables.
q reactions in gas-phase with N different species to be solved
 rj
For a CSTR, a coupled set of algebraic eqns analogous to PFR differential eqns must be solved.
 Fj
V 
Fj0
 Fj  rjVFj0
q
 f j (C1 , C2 ,...,CN ) rij
i1
 rj




T
T0
 T
NN
T
T0
 T
j
q
i1
T0
T
T0
 T
10 1 1 i1 1
CT 0 
F
FN 
C , ,
F
 F1
 FN  r V  V fFN0
CT 0 
F
FN
 C , ,
F
 F1
 Fj  rjV  V  fFj0
, , C
FF
 F FN 
F  F  r V  V r  V  f  1
C