topic 6- design of multiple reactors part 1 and 2

7/30/2019 Topic 6- Design of Multiple Reactors Part 1 and 2

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TOPIC 6Design for Multiple

Reactions


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Topic covers:

PART 1: week 8

Types of multiple reactionsSelectivity and yield

Parallel reactions

PART 2: week 9

Series reactions


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TOPIC 6Design for Multiple Reactions

4 types of multiple reactions:

1) Parallel reactions

2) Series reactions

3) Complex reactions of parallel and series

4) Independent reactions

CA

BA

2

1

k

k

CBA 21kk

DCB

CA

BA

3

2

1

k

k

k

DCA

CBA

2

1

k

k

DB

CA

2

1

k

k


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Instantaneous Overall

Selectivity

Yield

Example:desired product , rD=k1CA

2CB

undesired product , rU=k2CACB

To maximize the selectivity of D with respect to

U run at high concentration of A and use PFR


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Reaction rates for parallel reactions

A B (-rA)=k1CA

2A C (-rA)=k2CA2

(-rA)nett = k1CA + k2CA

2

UA

DA

U

D

k

k

UDArrr


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Selectivity in Parallel Reactions:

Single Reactant Systems

Desired Reaction:

Undesired Reaction:

D

ADD Ckr

)( UD

U

D

A

U

D

AU

AD

U

DDU

Ckk

CkCk

rrS

DA Dk

D

UA Uk

U

U

AUU

Ckr

Let us examine some reactor operating scenarios to

maximize selectivity.

What is the selectivity ?


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Case 1: D- U >0

High CAfavors D

How can we accomplish this?

)(

A

U

D

U

DDU

UDCkk

rrS

For gas phase reactions, maintain high pressures

For liquid-phase reactions, keep diluents to aminimum

Batch or Plug FlowReactors should be used.

CSTR should NOT be chosen.


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Case 2: D-U < 0

Low CAfavors D


For gas phase reactions,

operate at low pressures.

For liquid-phase reactions,dilute the feed

For ga phase reaction,

inerts can play a role

CSTR is the preferred

)(

AU

D

U

DDU

DUCk

k

r

rS

Reactant concentration

maintained at low level

CAC

A

CA

0


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Case 3:D-

U= 0

Concentration has no effect on selectivity!

Then, how to maximize selectivity?

By choosing the most suitable temperature.

]/)(exp[]/[-exp

]/[-expRTEE

A

A

RTEA

RTEA

k

k

r

rS

UD

U

D

UU

DD

U

D

U

D

DU

(a) If ED > EU

(b) IfEU > ED

Operate reactor at the highestpossible temperature

Operate reactor at the lowestpossible temperature


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Desired Reaction:

Undesired Reaction:

11

BADD CCkr DBADk

11

UBA Uk

22

22

BAUU CCkr

)(

B

)(

A

U

D

U

DDU

2121 CCk

k

r

rS

Selectivity in Parallel Reactions:Two Reactant Systems


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Case 1: 1>2; 1 >2


Use a Batch reactor

Or a Plug Flowreactor for continuous production.

Use high pressure for gaseous systems.

For high SDU, maintain bothA & B as high as possible

bB

aA

U

D

U

DDU CC

k

k

r

rS

Let, a = 1-2; b= 1 -2

Example: A + B D

A + B U


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Case 2: 1>2; 1


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Case 3: 1< 2; 1


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Case 4: 1< 2; 1 > 2


Use a semi-batch reactor whereA is fed slowly

Use Tubular reactor with side streams of A being fed

continuously

Use series of small CSTR with B fedonly to first andA to

each reactor.

a

A

b

B

U

D

U

D

DUC

C

k

k

r

rS For high SDU, maintain concentration of

B high and ofA low

Let, a= 2-1; b= 1 -2

Example: A + B D

A + B U


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17/23

END of TOPIC 6

(part 1)


18/23

PART 2:

Series reactions



19/23

Series reactions

Example:

Series Reaction in a batch/plug flow reactor

Reaction (1) : -r1A=k1CA

Reaction (2): -r2B=k2CB

Mole Balance on every species Species A:

Combined mole balance and rate law for a control volume batch

reactor.

CBA 21kk


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Net Rate of Reaction of A

rA=r1A+0

Rate Law

r1A=-k1ACA

Relative Rates

r1B= -r1A

Integrating with CA=CA0 at t=0 and then rearranging

Series reactions


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Species B:

Net Rate of Reaction of B

Rate Law

r2 B = -k2CB Relative Rates

Combine

Series reactions


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Species C:

CC = CA0 - CB CA

Optimization of Desired product

Series reactions


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topic 6- design of multiple reactors part 1 and 2

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