Selecao Reatores

8/11/2019 Selecao Reatores

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Chemical reactor selection and

design

Rajesh Kumar Bhagat

B.E.(CHE) 4th year



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



Reactor System

Homogenous

Heterogeneous



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 reactorand at exit

b. Plug flow- flow of fluid through

reactor with order so that only

lateral mixing is possible.



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



Isothermal reactor design algorithm



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



Performance equation



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 tomixed 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.



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



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 equalto slope of curve at intersection of these

two reactor.

• Levenspiel , has proved that after overall

economic consideration equal size

reactors in series are economical.



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 beused.

• 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-2Fig-2

Fig-3

Fig-4

Fig-1



Design for parallel reaction •

When a reactant gives two product(desired, and undesired)simultaneouslywith different rate constant then this iscalled a parallel reaction.

• To keep maximum amount of desiredproduct we can take following steps.

• Ifa1>a2 or the desired reaction is of higher

order then keep reactant concentrationhigh for high product concentration.

• If a1<a2 than for desired reaction keepreactant concentration low.

• For a1=a2 change in reactantconcentration will not affect the productthen, because rate constant k1 and k2 are

different at different temperature so, wecan keep our temperature such thatdesired product will be high or use ofcatalyst would be a option which areselective in nature.





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



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 amaxima 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 diminishand concentration of S will increase.



Residence Time Distribution

• RTD is important factor from the point of viewof real equipment .

• Element of fluid will take different routethrough the reactor and may take differentlength of time to pass through the reactor.

• Ideal reactor design are made by consideringvolume of reactor or time spend by all the

reactant will be same inside reactor.

• Completion of reaction will depend on time ofexposure inside the reactor.

• The distribution of time inside the reactor iscalled 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.



Residence time distribution determination

• RTD can be determined by two

experimental method.(Pulse inputexperiment, 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 agedistribution 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 theconcentration 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.



Holding time and residence time

• Holding time is defined as time needed totreat 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 ofpopper 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 holdingtime and residence time will be equal.



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



Catalytic system

plug flow Reactor

Differential reactor

Integral reactor

mixed flow type

(Fluidized bed reactor)

Performance equation



Catalytic reactor selection parameter and design

• Reaction type

• Reactor type

• Economics

• Rate of deactivation

• Other process

requirement



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.



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.



• 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



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 reactionwill be a problem and less selectivity

• Sintering of catalyst may happen



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 belarge



Fluidized bed catalytic reactor design

Types of fluidized bed catalytic reactor• Bubbling fluidized bed(BFB)- industrial

solid catalyzed reactor generally works asbubbling fluidized bed reactor. Calculationof conversion for bubbling flow variesbetween plug flow to mixed flow.

• Turbulent fluidized bed reactor(TFB)- at

high gas velocity BFB transform in to TFBin this case no distinct bubble of gas willflow and solid movement will be violent.

• Fast fluidized bed- transition from TFBwith very high speed of gas this FFB willformed.

• Pneumatic conveying bed- highest gas

velocity for fluidization are chokingvelocity and after that it will convertedinto pneumatic bed and this reactorpneumatic conveying fluidized bedreactor.

• In all three model TFB, BFB, PCB solidentrain out of bed regularly.



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.



Contact time distribution model

•In BFB faster gas stayed mainly in bubbles and slow moving gas inemulsion, according to this model effective rate constant depend onlength 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



Non-catalytic system

• Heterogeneous fluid-fluid or solid-gas systemwith 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



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 importantResistance 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………



Ignition temperature

For burning of coal minimumignition required so, heatshould be recycled

Plug flow reactor with recyclewill be most suitable reactorfor this system

Mass Transfer resistance andrate equation

Total resistance = film resistance+ ash resistance + reactionresistance

Plug flow reactor



• Know as we know the rate of reaction by

knowing all resistance

•We know the flow type and reactor type isplug flow

• We know feed rate from heat balance of

burning of coal• From performance equation we will get the

volume of reactor



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

http://highwire.stanford.edu/

http://highwire.stanford.edu/



Thank you

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