Chemical Reaction Engineering Lecture 6: Complex Reactions

Jayant M. Modak Department of Chemical Engineering Indian Institute of Science, Bangalore

Topic 3: Complex systems

! Analysis of “Simple complex” systems ! Kinetics of complex systems

"  Chain reaction "  Catalysis "  Polyermization

! Lumping analysis

Complex systems - Examples

! Large number of reactions and reactants

3 8 3 6 2

3 8 2 4 4

3 8 2 4 2 6 3 6

Thermal cracking of alkanesC H C H HC H C H CHC H C H C H C H

Cracking of crude to petrol

Metabolic network insidecell

! +! ++ ! +

Complex systems - Examples

! Chain reactions

3 4

2

( )

Thermal decompositionCH CHO CH CO

Auto oxidationR H O ROOH

Polymerizationstyrene poly styrene

! +

"" + !

!

Complex systems - Examples

! Catalytic reactions

Thermal decompositionC12 H22O11 + H2O

acid! "!! C6 H12O6 + C6 H12O6

Ammonia synthesis12

N2 +32

H2Fe! "! NH3

Yield – conversion diagram

Polymer weight distribution

Catalytic reaction kinetics

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0

0.5

1.0

1.5

2.0

2.5

3.0

0.061 0.132 0.263 0.526 0.789

rate

pEthylene

pO2

Complex reactor behavior

Cracking of ethane to ethylene

! New questions "  Are all products useful? "  How to monitor the reaction? "  Is conversion of ethane the only criteria for design?

26 24 2CHCHH!+

26 38 42CHCHCH!+

24 222CHCH!+

Parallel reactions

2 6 2 4 2

2 6 38 42CHCHHCHCHCH! +!+

A1 !A3

"A2

Series reactions

2 6 2 4 2

2 4 22 2CH CHHCH CH! +!+ A1 ! A2 ! A3

Complex (Series-parallel) reactions

2 6 2 4 2

2 6 3 8 4

3 8 2 4 4 8 4

2CH CHHCH CHCHCHCH CHCH

! +! ++ ! +

A1 !A3

"A2

A2 + A3 ! A4

Independent Reactions

2 6 2 4 2

3 8 3 6 2

CH CHHCH CHH! +! +

A1 ! A2

A3 ! A4

Desired and Undesired Reactions

A1 ! A2

A1 ! A3

A1 ! A2 ! A3

26 24 2CHCHH!+

26 38 42CHCHCH!+

Yield

Desired reaction A1 ! A2 r1

Undesired reaction A1 ! A3 r2

OverallYield Y2 =

Exit molar flowrateof desired productInlet molar flowrateof reactant

Selectivity

Desired reaction A1 ! A2 r1

Undesired reaction A1 ! A3 r2

InstantenousSelectivity s2 =

r1

r1 + r2

OverallSelectivity !S2 =

Exit molar flowrateof desired productExit molar flowrateof all products

Selection of reactor type and conditions

CSTRC2

C10 ! C1

=r1

r1 + r2

PFR

C2

C10 ! C1

=1

C10 ! C1

r1

r1 + r2C1

C10

" dC1

Desired reaction A1 ! A2 r1

Undesired reaction A1 ! A3 r2

Complex systems - selectivity

1 0 -1

se

lect

ivity

(C1)

C10-C1

q2-q1

Complex systems – series reactions

0 10

0.0

0.2

0.4

0.6

0.8

1.0

!=0.1 A1

A2

A3

Concentration

Time

A1 ! A2 r1 = k1C1

A2 ! A3 r1 = k2C2 " = k2 / k1

Concept of yield-conversion diagram

A1 ! A2 r1 = k1C1

A2 ! A3 r1 = k2C2 " = k2 / k1

Concept of rate determining step

0 10

0.0

0.2

0.4

0.6

0.8

1.0

!=0.1 A1

A2

A3

Concentration

Time

A1 ! A2 r1 = k1C1

A2 ! A3 r1 = k2C2 " = k2 / k1

Concept of rate determining step

0 10

0.0

0.2

0.4

0.6

0.8

1.0

!=5

A1

A2

A3

Concentration

Time

A1 ! A2 r1 = k1C1

A2 ! A3 r1 = k2C2 " = k2 / k1

Concept of quasi-equilibrium approximation

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0 k1=1, k-1=0.5, k2=k-2=1

C1

C2

C3

concentration

time

A1! A2 r1 = k1C1 ! k!1C2

A2 ! A3 r1 = k2C2 ! k!2C3

Concept of quasi-equilibrium approximation

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0 C1

C2

C3

conntration

time

k1=1, k-1=0.5, k2=k-2=10

A1! A2 r1 = k1C1 ! k!1C2

A2 ! A3 r1 = k2C2 ! k!2C3

Concept of quasi-equilibrium approximation

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0 C1

C2

C3

C1 (e) C2 (e)

conntration

time

k1=1, k-1=0.5, k2=k-2=10

A1! A2 r1 = k1C1 ! k!1C2

A2 ! A3 r1 = k2C2 ! k!2C3

Concept of quasi-steady state approximation

0 10

0.0

0.2

0.4

0.6

0.8

1.0

!=5

A1

A2

A3

Concentration

Time

A1 ! A2 r1 = k1C1

A2 ! A3 r1 = k2C2 " = k2 / k1

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

1 10 50 !

C 3/C

10

k1t

k2

Concept of quasi-steady state approximation

A1 ! A2 r1 = k1C1

A2 ! A3 r1 = k2C2 " = k2 / k1

Chemical Reaction Engineering Lecture 6: Complex Reactions

Jayant M. Modak Department of Chemical Engineering Indian Institute of Science, Bangalore

Chain reactions

! Combustion reactions ! Decomposition reactions ! Autooxidation!! Polymerization!

Chain reactions – decomposition of acetaldehyde CH 3CHO! CH 4 + CO

CH 3CHO! CH 3• + CHO•

CH 3• + CH 3CHO! CH 3CO

• + CH 4

CH 3CO• ! CH 3

• + CO2CH 3

• ! C2H6

r = kCCH3CHO3/2

Polymerization

! Chain polymerization of "  Ethylene (X=H), vinyl chloride (X=Cl) "  Styrene (X=C6H5) etc

!  Initiator I (!-!)!

CH2 = CHX (RX )

Initiation C6 H5COO ! OOCC6 H5 " 2C6 H5COO •

# • +RX "#! RX •Propogation #! RX • +RX "#! (R) ! RX •

#! (R) j!1 ! RX • +RX "

#! (R) j ! RX •

Termination#! (R) j!1 ! RX • + • XR ! (R)i!1 ! #"

#! (R) j!1 ! RX ! XR ! (R)i!1 ! #

Polymerization

Initiation I k0! "! 2# k0 = 10$4 $10$6

# + M ki! "! R1

Propogation R1 + Mkp! "! R2 kp = 102 $104

Rj$1 + Mkp! "! Rj

Termination Rj + Rika! "! Pi+ j ka = 106 $108

Polymerization

Species Appearance disappearanceI k0 I

! r0 = 2 fk0 I ri = ki!M

R1 ri kp MR1 + ka R1 Rj"Rj kP MRj kp MRj + ka Rj Ri"Pj

ka

2Rj# i Ri"

Polymerization

Initiation rate ri = 2 fk0 I

Total radicals !Rj = "0 =ri

ka

#

$%&

'(

1/ 2

Monomer consumption rM = kp M"0

Radical concn Rj =ri

kP M

#

$%&

'(1

1+ ri / rM

#

$%&

'(

j

polymer generation rPj= Rj j )1( ) ka

2ri

kP M

#

$%&

'(

Polymerization

0 500 1000 1500 2000 2500 3000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

wei

ght f

ract

ion

number of monomers

0.04 0.02 0.08

Initiator

0 500 1000 1500 2000 2500 3000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

wei

ght f

ract

ion

number of monomers

360 180 60

time (min)

Polymer weight distribution

Chemical Reaction Engineering Catalytic reactions

Jayant M. Modak Department of Chemical Engineering Indian Institute of Science, Bangalore

Catalytic reactions

Solid

Gas

(2nd Liquid)

Liquid

Catalyst

Heterogeneous catalysis

Catalytic reactions

H2O2 soluti

on 25°C

stable over months

>320°C uncontrolled,

thermal decomposition in

seconds

25°C controlled, catalytic

or enzymatic

decomposition in seconds

Example: Hydrogen peroxide decomposition

2H2O2 2H2O + O2

Catalytic reactions Efficiency of Phthalic Acid Anhydride Production

Non-catalytic Oxidation of naphthalene in fluid phase

with MnO2+HCl (1872), Chromic acid (1881),

Oleum (1891)

Catalytic Oxidation of o-Xylene in the gas phase

on V2O5-catalyst

Yield: 5-15%

Yield: 75-87%

Catalytic reactions Efficiency of nitrogen fixation

catalytic processes

Catalytic reactions Product spectrum from partial oxidation of propene

substrates catalysts products

propene + oxygen

acrolein

acrylic acid

acetone

propylene oxide

acetic acid

1,5-hexa- diene

benzene

Catalytic reactions

Steps during the course of the reaction

External diffusion Internal diffusion Adsorption on the active sites Surface reaction forming the products Desorption of the products Internal diffusion External diffusion

!

" # $

% &

'

reaction: substrate A product P

fixed bed reactor

Boundary layer

Lumping analysis

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0 A1

A2

A3

Concentrations

Time

A10=1, A20=0, A30=0

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0A10=1, A20=0, A30=0

A1+A2

A3

Â1

Â2

Concentrations

Time

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0A10=0.5, A20=0.5, A30=0

Concentrations

Time

B

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0A10=0.5, A20=0.5, A30=0

A1+A2

A3

Â1

Â2

Concentration

Time

Lumping analysis

0.0 0.1 0.2 0.3 0.4 0.5

0.6

0.8

1.0Â1=A1+A3, Â2=A2

1,0,0 0,0,1

Concentration

Time

Initial A1,A2,A3