week 11. design for reactions in series

8/13/2019 Week 11. Design for Reactions in Series

1/18

Reaction Engineering

Design for Reactions in Series

Eko Ariyanto, ST


2/18

Reactor Design

design for multiple reactions-

What do we study:

Parallel reactions

Irreversible Reactions in series

modelling example - CSTR for parallel reactions

Multiple reactions= described by using more than one rate expression


3/18

Irreversible 1storder rxn. in series- product distribution / qualitative discussion -

Reactions proceeded in the presence of light only

A R S1k 2k

2 ways of treating the beaker containing A:

Contents uniformly irradiated

A small stream is withdrawn irradiated - returned


4/18

First case:

1. Initially only A present

light will attack A

R is formed

2. A & R will compete

A - very large excess

A will absorb most of the energy

More R is formed

3. Process continues until R conc. = max

4. After max. rate of decomposition > rate of formation



5/18

Second case:

1. Some A is removed

Exposed to the light

Reacts to completion

S is formed

2. S is returned to beaker

3. In time in the beaker:

Conc. Of A decreases

Conc. Of S increase

R is absen



6/18

First case:

maximum possible formation of R

homogeneous content of the beaker


Second case:

minimum possible formation of R

mixing stream of different compositions - highly reacted + fresh

The mixing of fluid of different compositions =the key of formation of intermediate

A R S1k 2k

Maximum R formed if fluids are not allowed to mix


7/18

Which contacting pattern can give a higher concentration of

intermediate?

Irreversible 1storder rxn. in series- favourable patterns / qualitative discussion -

Max. possible

= less fluid

intermixing


8/18

Batch & PFR

Irreversible 1storder rxn. in series- product distribution / quantitative approach -

A R S1k 2k

AA

A kCdtdCr

RAR

R CkCkdt

dCr 21

RS

S Ckdt

dCr 2

?AC

?S

C

?RC


9/18

AA

A Ckdt

dCr 1

RAR

R CkCkdt

dCr 21

tk

AA eCC 1

0

2112

10

21

kk

e

kk

ekCC

tktk

AR

ARAS CCCC 0

Irreversible 1storder rxn. in series- Batch & PFR / quantitative approach -

tktkAS ekk

ke

kk

kCC 21

12

1

21

20 1


10/18

?max, RC

?, optp

2112

10

21

kk

e

kk

ekCC

tktk

AR12

12

log

,

)/ln(1

kk

kk

k meanoptp

)(

2

1

0

max,

122 kkk

A

R

k

k

C

C

0dt

dCR


maximum conc. of intermediate

corresponding time


11/18

tk

AA eCC 1

0

2112

10

21

kk

e

kk

ekCC

tktk

AR

12

12

log

max)/ln(1

kkkk

kt

mean

)(

2

1

0

max,

122 kkk

A

R

k

k

C

C

Eq. 1

Eq. 3

Eq. 2

Eq. 4

Eq. 5

(1)

tktkAS e

kk

ke

kk

kCC 21

12

1

21

20 1

(2)

(3)

(4)

(5)



12/18


Concentration time plot Relative concentrations plot


13/18

Mixed Flow

Irreversible 1storder rxn. in series- mixed flow / quantitative approach -

A R S1k 2k

reactionbyncedisappearaoutputinput


14/18



VrFF AAA )(0

VCkvCvC AAA 10 v

)1( 10 mAA kCC

Comp. A

mA

A

kC

C

10 1

1


15/18



VrFFRRR

)(0

VrvCvC RRR )(0 v

Comp. R

mmm

A

R

kk

k

C

C

21

1

0 11

VCkCkvC RAR )(0 21

mRAR CkCkC )(0 21

)1/( 10 mAA kCC


16/18



.0

constCCCCASRA

Comp. S

mmm

A

S

kk

kk

C

C

21

2

21

0 11


17/18

maximum conc. of intermediate?max, RC

?max corresponding time

21

,

1

kkoptm

22/1210

max,

1)/(

1

kkC

C

A

R

0m

R

d

dC


mmm

ARkk

kCC

21

10

11


18/18


Concentration time plot Relative concentrations plot

week 11. design for reactions in series

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