Syimi-Topic 6 Week 8.ppt

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Types of multiple reactions

Selectivity and yieldParallel reactions

Reference: Fogler and Levenspiel

TOPIC 6

Design for Multiple Reactions



Types of multiple reactions

4 types of multiple reactions

Parallel reactions

Series reactions

Complex reactions of parallel and series

Independent reactions

CA

BA

2

1

k

k

CBA 21 k k

DCB

CA

BA

3

2

1

k

k

k

DCA

CBA

2

1

k

k

DB

CA

2

1

k

k



Selectivity and yield

Instantaneous Overall

Selectivity

Yield

Example:

desired product , rD=k1CA2CB

undesired product , rU=k2CACB

To maximize the selectivity of D with respect to U run at high concentrationof A and use PFR



Parallel reactions

Reaction rates for parallel reactions

A B (-r A)=k 1C A

A C (-r A)=k 2 C A2

(-r A)nett = k 1C A + k 2 C A2

U A

D A

U

D

k

k

U D A r r r



Selectivity in Parallel Reactions:

Single Reactant Systems

Desired Reaction:

Undesired Reaction:

1

1α

AD C k r

2

1

2

1̀

α

α

A

A

U

D

DU

C k

C k

r

r S

D A k

1

1α

U A k 2

2α

22

α AU

C k r

Let us examine some reactor

operating scenarios to maximize

selectivity.

What is the

selectivity ?



Case 1:

1

-

2

>0

High C A favors D

How can we accomplish this?

)(

2

1 21 αα

A

U

DDU C

k

k

r

r S

• For gas phase reactions, maintain high

pressures

• For liquid-phase reactions, keep

diluents to a minimum

• Batch or Plug Flow Reactors should be

used.



Case 2:

1

-

2

< 0

Low C A favors D

How can we accomplish this?• For gas phase reactions,

operate at low pressures.

•

For liquid-phase reactions,dilute the feed

• CSTR is the preferred

reactor.

)(2

1

21 αα

AU

D

DU C k

k

r

r S

Reactant

concentration

maintained at l owlevel

CA

CA

CA

0



Case 3:1-

2 = 0

Concentration has no effect on selectivity!

Then, how to maximize select iv i ty?

By choosing the most suitable temperature.

]/)(exp[]/[-exp

]/[-exp

2

1RT E E

A

A

RT E A

RT E A

k

k

r

r S

U D

U

D

U U

DD

U

D

DU

(a) If ED > EU

(b) If E U > E D

• Operate reactor at the highest possible temperature

•

Operate reactor at the lowest possible temperature



Desired Reaction:

Undesired Reaction:

11

1 β α

B AD C C k r DB A

k

1

11 β α

U B A k

2

22 β α 22

2 β α

B AU C C k r

2

1

2

1

2

1

β

β

α

α

B

B

A

A

U

D

DU

C

C

C

C

k

k

r

r S

Selectivity in Parallel Reactions:

Two Reactant Systems



Case 1: 1>2; 1 > 2


• Use a Batch reactor

• Or a Plug Flow reactor for continuous production.

•Use high pressure for gaseous systems.

For high SDU, maintain both A & B as high as possible

b

B

a

A

U

D

DU C C k

k

r

r S

2

1

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

Example: A + B D

A + B U



Case 2: 1>2; 1 < 2


• Use a semi-batch reactor where B is fed slowly.• Use Tubular reactor with side streams of B being fed c

• Use series of small CSTR’s with A fed only to the first

small amounts of B to each reactor.

For high SDU, maintain

concentration of A highand of B low

b

B

a

A

U

D

U

D

DUC

C

k

k

r

r

S

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

Example: A + B D

A + B U



Case 3: 1

<2

; 1

<

2


• Use a CSTR.

• Use a Tubular reactor with large recycle ratio.

• Use diluted feed

• Use low pressure for gaseous reactions.

For high SDU, maintain bothconcentration of A and B

low.b

B

a

AU

D

U

D

DUCCk

k

r

r S

Let, -a = 1-2; -b = 1 - 2 Example: A + B D

A + B U



Case 4: 1<2; 1 > 2


• Use a semi-batch reactor where A is fed slowly

• Use Tubular reactor with side streams of A being fed continuously

• Use series of small CSTR with B fed only to first and A to each reactor.

a

A

b

B

U

D

U

D

DUCC

kk

r r S

For high SDU

, maintain

concentration of B high and of

A low

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

Example: A + B D

A + B U

Syimi-Topic 6 Week 8.ppt

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