Lecture 12

Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of

chemical reactions and the design of the reactors in which they take place.

Lecture 12

Lecture 12 – Tuesday 2/19/2013Multiple Reactions

Selectivity and Yield

Series Reactions

Complex Reactions

2

A DkD

A UkU

A B C

A + B C + D

A + C E

Series: A → B → CParallel: A → D

A → UIndependent: A → B

C → DComplex: A + B →C + D

A + C → E

With multiple reactors, either molar flow or number of moles must be used (no conversion!)

3

4 Types of Multiple Reactions

Instantaneous Overall

Selectivity

Yield

There are two types of selectivity and yield: Instantaneous and Overall.

U

DDU r

rS U

DDU F

FS ~

A

DD r

rY

AA

DD FF

FY

0

~

4

Selectivity and Yield

ABA

BA

U

DUD C

kk

CCkCCk

rrS

2

1

2

21

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

DBA 1k BAD CCkr 21Example: Desired Product:

UBA 2kBAU CCkr 2Undesired Product:

Selectivity and Yield

5

Gas PhaseMultiple Reactions

6

Multiple Reactions

7

Flow Batch

BB

AA

rdVdF

rdVdF

VrdtdN

VrdtdN

BB

AA

A) Mole Balance of each and every species

Multiple Reactions

8

B) Rates

a) Rate Law for each reaction:

b) Net Rates:

c) Relative Rates:

ACAA

BAAA

CCkrCCkr

22

11

AAi

iAA rrrr 211

Multiple Reactions

9

C) Stoichiometry

Gas:

Liquid:

TT

PP

FFCCA

ATA

0

000

0AA FC

Example: A → B → C

(1) A → B k1

(2) B → C k2

Batch Series Reactions

10

1) Mole Balances

Vrdt

dN

Vrdt

dN

Vrdt

dN

CC

BB

AA

AC

AB

AA r

dtdC r

dtdC r

dtdC

V=V0 (constant batch)

Batch Series Reactions

11

2) Rate Laws

B2B1B

A1A

BB1B1

AA1A1

rrrrr

CkrCkr

Laws

Net rates

1r

1r

1r

1r

C2B2

B1A1

Relative rates

Example: Batch Series Reactions

12

A → B → C

(1) A → B

(2) B → C

1) Mole Balances

CC

BB

AA r

dtdC r

dtdC r

dtdC

ttopt

Ci

A B C

OVV

Example: Batch Series Reactions

13

2) Rate Laws

1r

1r

1r

1r C2B2B1A1

Relative:

B2B2

A1A1

CkrCkr

Laws:

Example: Batch Series Reactions

14

3) Combine

Species A:

Species B:

A1AA Ckr

dtdC

tkexpCC 10AA

BB r

dtdC

B2A1B2B1NET BB CkCkrrrr

tkexpCkCkdt

dC10A1B2

B

Example: Batch Series Reactions

15

Using the integrating factor, tkdtkFI 22 expexp.. tkkCk

dttkCd A

B1201

2 expexp

at t = 0, CB=0

BAAC CCCC 0

tktkAC ekek

kkCC 21 11 12

12

0

Example: CSTR Series Reactions

16

ABCWhat is the optimal ?

00 AAA rCC

000 0

BB

BB

rCVrCv

00

000

0

VrvCvC

VrFF

AAA

AAA

1) Mole BalancesA:

B:

Example: CSTR Series Reactions

17

ABC

2) Rate Laws

B2B2

A1A1

CkrCkr

Laws:

1 1

1 2 1 2

0A A A

B A B A B

r r k Cr r r k C k C

Net:

1r

1r

1r

1r C2B2B1A1

Relative:

Example: CSTR Series Reactions

18

ABC

3) Combine

Example: CSTR Series Reactions

19

ABC

dCBd

0

max 1k1k2

Find that gives maximum concentration of B

12

01

11 kkCkC A

B

20

End of Lecture 12

21

22

Supplementary Slides

Blood Coagulation

23

24

25

Notations

26

Notations

Mole Balances

27

28

Mole Balances

29

Mole Balances

30

Results

Many metabolic reactions involve a large number of sequential reactions, such as those that occur in the coagulation of blood.

Cut → Blood → Clotting

Figure A. Normal Clot Coagulation of blood(picture courtesy of: Mebs, Venomous and Poisonous Animals, Medpharm, Stugart 2002, Page 305)31

Blood Coagulation

32

Schematic of Blood Coagulation

Cut

A + B

C

D

E

F

Clot33