week 7. design for single reaction

8/13/2019 Week 7. Design for Single Reaction

1/20

Reaction EngineeringDesign for Single Reactions

Eko Ariyanto, ST


2/20

Reactor Design overview -

Many ways for processing a fluid:

Single batch reactor

Mixed flow reactor

Chain of reactors .

Factors to be considered in our choice:

reaction type

planned scale of production

cost of equipment & operations

safety ..

Which scheme do wechoose?

No formula to give us theoptimum set up!!!

eng. judgement

experienceeconomics


3/20

Reactor Design design for single reactions-

What do we study:

Single reactors (compare the size)

Multiple reactor systems

Recycle reactor Autocatalytic reactors

Single reactions = described by using one and only one rate expression coupled with the stoichiometry and equilibrium expressions


4/20

Single reactions design batch reactor -

Advantages:

small instrumentation cost

flexibility of operation (shut down easily)

Disadvantages:

high labour and handling cost

considerable shutdown time to empty, clean and refill

poorer quality control of the product

Well suited for producing small amounts of material & different

products using one piece of equipment


5/20

dV r dX F A A A )(0

PFR

dt dX

N V r A A A 0)(

Batch

t C A 0 dX A( r A )0 X A

A X

A

A A r

dX C vV

00

0

0 A

For constant density systems:

identical performance equation

the same V is needed to do the job

Single reactions design batch / plug reactor -


6/20


7/20


8/20

Single reactions design- CSTR (mixed) / plug reactor -

C A 0n 1

m C A 0n 1 p

C A 0

n V

F A 0

mC

A 0

n V

F A 0

p

X A1 A X A

1 X A

n

m

1 A X A1 X A

n

dX A0 X A

p

0

0

0 A

A

F VC

vV

Dividing (1) / (2) and

0 A


9/20


C A 0n 1

m C A 0

n 1 p

X A1 X A n

m

1 X A 1 n 1n 1

p

C A 0n 1

m C A 0n 1 p

X A

1 X A

mln(1 X A ) p

0 A

1n 1n


10/20


m p

V mV p

X A1 X A n

m

1 X A 1 n 1n 1

p

Plot for various X A and comparethe performance

For equal quantities (F A0 ) ofthe same feed (C A0 ):


11/20


Graph observations:

For all positive reaction orders themixed reactor is always larger than

plug flow The ratio of volumes increases with

reaction order

For small X - reactor performanceslightly affected

The performance increases veryrapidly with high conversion

Density variation during reactionaffects design

p

m

p

m

V V


12/20

Single reactions design- general reactor performance -

m p shaded areahatched area

Mixed flow always needs a larger volume that plug flow


13/20

Single reactions design- plug reactor systems -

X 1, X 2,... X N

Consider: N plug flow reactors connected in series Conversion of component A leaving reactor 1,2,3N

V F A 0

dX Ar A X Ai

X Af

V i F A 0

dX Ar X i 1

X ifor i th

reactor

V F 0

V i F 0i 1

N

V 1 V 2 ...V N F 0

dX

r X 0 X 1 dX

r X 1 X 2

... dX

r X N 1 X N dX

r X 0 X N

N reactors in series

N plug reactor in series conversion = single plug reactor conversion


14/20

Example 1 : 3 plug flow reactors in 2 parallel branches

Series of plug flow reactors

What fraction of feed should go to branch D?

Conversion should be the same in each branch


15/20

Solution Example 1 :

Series of plug flow reactors

Branch D: two reactors in series: V D 50 30 80l

Conversion must be identical = V/F must be identical

V F

D V F

E

F D F E

V DV E

8040

2

2/3 of F must be fed to branch D

F

FD

FE


16/20

Single reactions design- mixed reactor systems -

X 1, X 2,... X N

Consider: N mixed reactors connected in series Conversion of component A leaving reactor 1,2,3N

Assumptions: negligible density changes

t


17/20

Single reactions design- mixed reactor systems -

X 1, X 2,... X N

Consider: N mixed reactors connected in series Conversion of component A leaving reactor 1,2,3N

Assumptions: negligible density changes

t


18/20

Single reactions design- mixed reactor systems / 1 st order rxn. -

)(

)(0

0

0

A

Ai Af A

A

A

r

X X C

F VC

For mixed reactor, in general:

for i th

reactor )()( 10

0

0

Ai

iiii r

X X C F

C V

0

f(conc)

i

ii

i

iii kC

C C kC

C C C C C 10100 )/1()/1(


19/20


i

iii kC

C C 1 ii

i k C

C 11

ii V all in same

N

i N

N

N N

k C

C

C

C

C

C

X C

C 1...

1

1 12

1

1

00

N reac tors N i

N k

C 0C N

1/ N

1


20/20


1

/1

0

N

N ireactors N

C

C

k

N N

C C

k p0ln

1

N

Series of N mixed

Plug flow

p

N

p

N

V V

R A

week 7. design for single reaction

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