seleção de reatores

1. Chemical reactor selection and
design
Rajesh Kumar Bhagat
B.E.(CHE) 4th year

2. Introduction
Almost all chemical engineering process
contains three operations.
What does chemical reactor design means ?
Unit
operation
(cleaning )
Chemical
reactor
Unit
operation
(separation)
Raw
material
Product

3. Reactor System
Homogenous
Heterogeneous

4. 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.

5. 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

6. Isothermal reactor design algorithm

7. 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

8. Performance equation

9. 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.

10. Multiple reactor system
• 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 be
find

11. 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.

12. Autocatalytic reactor
• When a product will act like a catalyst
then it is called auto catalytic reaction.
• In mixed flow reactor at fixed product
concentration for high yield, efficiency of
reactor will be very low.
• For no recycle for low product
concentration mixed flow reactor will be
preferred and for high conversion plug
flow .
• For optimum efficiency we can use a
recycle or back mixing plug flow reactors.
• For a particular exit concentration a
particular optimum recycle ratio should be
used.
• Optimum recycle ratio introduced to the
reactor feed’s 1/(-r) value should be equal
to average 1/(-r) value for whole reactor.
Plug flow
reactor with recycle
Fig-2
Fig-2
Fig-3
Fig-4
Fig-1

13. 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>a2 or 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 k1 and k2 are
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
selective in nature.

14. 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.

15. Irreversible series-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
• Here the reaction is parallel with 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

16. Case study Product distribution with respect to
contacting pattern
• We will discuss simpler example of
CASE-1 Add A slowly to B
• By contacting A slowly in a beaker
containing B and stirring to consume all
A added , the mixer with very high
concentration in S can be find.
CASE-2 Add B slowly to A
• Now by contacting B slowly to a beaker
containing A, the concentration of R will
be build up inside then after reaching a
maxima R will convert in to S and the
process will be gradual.
CASE-3 Add A and B rapidly
• In this case it will give the behavior of
series reaction , R will increase first and
after reaching a maxima it will diminish
and concentration of S will increase.

17. Residence Time Distribution
• RTD is important factor from the point of view
of real equipment .
• Element of fluid will take different route
through the reactor and may take different
length of time to pass through the reactor.
• Ideal reactor design are made by considering
volume of reactor or time spend by all the
reactant will be same inside reactor.
• Completion of reaction will depend on time of
exposure inside the reactor.
• The distribution of time inside the reactor is
called exit age distribution E, have unite time-1.
• According to RTD fraction of exit stream of age
between t and t+dt is E dt.

18. Residence time distribution determination
• RTD can be determined by two
experimental method.(Pulse input
experiment, and step input experiment )
• In pulse experimental method in a steady
state system we will put a pulse input of
tracer and will plot the graph of this
tracer concentration with time at output.
• This graph will show time variation or age
distribution of tracer concentration with
time.
• Another method of determination of RTD
is by putting a step input (Preferably unite
step input) of tracer.
• Then we can plot the graph between the
concentration versus time graph of tracer.
• The slope versus time graph of this system
will give us residence time distribution .
• Step input method is more accurate than
pulse input method although impulse
input would give the perfect distribution.

19. Holding time and residence time
• Holding time is defined as time needed to
treat one reactor volume.
• Residence time or mean residence time
space time is defined as mean residence
time of flowing material in the reactor.
• From fig when inside popcorn popper,
when popping occurs at back end of
popper then holding time and residence
time will be same.
• When popping occurs in midway or every
where inside the popper then the two
time will be different.
• For unchanging density system holding
time and residence time will be equal.

20. Heterogeneous system
heterogeneous systems are those which consist of two or more than two phase
Apart from temperature pressure and concentration, heat and mass transfer are important
Catalytic systems
Non-catalytic system

21. Catalytic system
plug flow Reactor
Differential reactor
Integral reactor
mixed flow type
(Fluidized bed reactor)
Performance equation

22. Catalytic reactor selection parameter and design
• Reaction type
• Reactor type
• Economics
• Rate of deactivation
• Other process
requirement

23. 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.

24. 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.

25. • For reactor design overall economics should
be considered.
• Like instead of different size of mixed flow
reactor in series, equal size mixed flow reactor
are economically good.
• If catalyst is not very expensive then we may
opt to non-regeneration but for expensive
regeneration must be considered.
Economics

26. Packed bed
• Solid fluid contact will be most efficient
• High amount of catalyst will be used
• Heat transfer will be difficult
• Pressure drop will be high
• Effective for mass transfer control system
• With increase in temperature side reaction
will be a problem and less selectivity
• Sintering of catalyst may happen

27. Fluidized bed
• Industrially most widely used
• Heat transfer are very good
• Pressure drop is low
• Catalyst can easily replaced for regeneration
• Amount of catalyst necessary is less
• Surface area per unite mass of catalyst will be
large

28. Fluidized bed catalytic reactor design
Types of fluidized bed catalytic reactor
• Bubbling fluidized bed(BFB)- industrial
solid catalyzed reactor generally works as
bubbling fluidized bed reactor. Calculation
of conversion for bubbling flow varies
between plug flow to mixed flow.
• Turbulent fluidized bed reactor(TFB)- at
high gas velocity BFB transform in to TFB
in this case no distinct bubble of gas will
flow and solid movement will be violent.
• Fast fluidized bed- transition from TFB
with very high speed of gas this FFB will
formed.
• Pneumatic conveying bed- highest gas
velocity for fluidization are choking
velocity and after that it will converted
into pneumatic bed and this reactor
pneumatic conveying fluidized bed
reactor.
• In all three model TFB, BFB, PCB solid
entrain out of bed regularly.

29. Bubbling fluidized bed
Model for bubbling fluidization
• Dispersion and tank series model
• Hydrodynamic flow model
• K-L model for BFB
• RTD Model
• Contact time distribution model
Bubbling fluidized bed seems like
boiling of liquid and gas bubbles
are moving up with faster velocity
then dispersed gas.

30. Contact time distribution model
• In BFB faster gas stayed mainly in bubbles and slow moving gas in
emulsion, according to this model effective rate constant depend on
length of stay of element of gas in bed.
K= K0tm
here m is a parameter
for first order constant density system concentration at exit will be

31. Non-catalytic system
• Heterogeneous fluid-fluid or solid-gas system
with two or more phase
• heat transfer and mass transfer are important
factor for this model
• Heat may be a product of this model
• Contacting scheme is very important
• Equilibrium solubility (if liquid-liquid system)
• Overall rate scheme
• Many method like shrinking core method of
analysis may be used

32. Reactor selection & design for burning of coal
Reaction type
Burning of coal is a exothermic reaction
C + O2 = CO2 + heat
Reactor selection
For burning of coal contact of air and coal is very important
Resistance to mass transfer will be
1. film above the coal
2. Ash layer with burning of coal
3. Resistance due to chemical reaction
So, very high mass transfer resistance
conti………

33. Ignition temperature
For burning of coal minimum
ignition required so, heat
should be recycled
Plug flow reactor with recycle
will be most suitable reactor
for this system
Mass Transfer resistance and
rate equation
Total resistance = film resistance
+ ash resistance + reaction
resistance
Plug flow reactor

34. • Know as we know the rate of reaction by
knowing all resistance
• We know the flow type and reactor type is
plug flow
• We know feed rate from heat balance of
burning of coal
• From performance equation we will get the
volume of reactor

35. References
Chemical reaction engineering
(octave levinspeil)
Element of chemical reaction engineering
(H. scott fogler)
Chemical reaction design
(Peter harriott)
http://highwire.stanford.edu
http://ocw.mit.com

36. Thank you