Chemical Reaction Engineering Heterogeneous reactions

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

Topic 6: Heterogeneous reactions

Ø Gas-solid catalytic reactions Ø Gas-solid noncatalytic reactions Ø Gas-liquid reactions

Heterogeneous catalysis

Heterogeneous catalysis

Concentration and temperature profiles

position

concentration

cAi

cAb

position

temperature

Ti

Tb

External transport

Bulk/Molecular diffusion

Binary ( )( )12

1/ 23 3/ 2

1 5 21 212 2

1 12 1 1 1 2

1 11.86 10: 10 ; :10

T

T M MD gases liquids cm s

P

N D C y y N N

σ

−

− −× +

⎡ ⎤= ⎣ ⎦Ω

= − ∇ + +

Multi-component

1

1

1

1

Nk

k jNk j jk j

j jm T j j k jm Nk

j k jk

Ny yD N

N D C y y N Dy N N

≠

=

=

⎛ ⎞−⎜ ⎟⎜ ⎟⎝ ⎠= − ∇ + =

−

∑∑

∑

Characterization of catalyst pellet

///

( )

g

g

p

p

S surfacearea gmcatalystV void volume gmcatalyst

gmcatalyst pellet volumevoid fraction

f r dr void volumedistribution

ρε

Transport in pore

Bulk Knudson Viscous

( ) 0

1 2

1 1

2 83

Nj j

j k j j kk j jk Kj Kj

Kjj

N y PBp y N y N PRT D D D RT

RTD rM

µ

π

≠

⎛ ⎞− ∇ = − + + ∇⎜ ⎟⎝ ⎠

⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠

∑

Transport in pellet

Flow

jj ej

ej jm

dCN D

dz

D Dετ

= −

=Flow

jj jm

dCN D

dz= −

Reaction and diffusion A→B

bulk phase

cAb

L

catalyst Diffusion

Reaction

z+dz z

dz

A

z

0Adcdz

=

A g A gt dt t

AeA

z dz

AeA

z

Accumulation C A dz C A dz

dCIn D A dtdzdCOut D A dtdz

Generated r Adz dt

+

+

−

−

( ) Ap A eA

CC D rt z zε∂ ∂ ∂⎛ ⎞= −⎜ ⎟∂ ∂ ∂⎝ ⎠

Concentration profiles

Effectiveness factor

Effect of geometry

Nonlinear kinetics

Falsification of the kinetics

Falsification of the kinetics

Multiple reactions

Nonisothermal pellet

Nonisothermal pellet

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

rate

concentration

β= -0.6

0.0 0.2 0.4 0.6 0.8 1.0020406080100120140

rate

concentration

β= 0.6

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

rate

concentration

β=0

0.0 0.2 0.4 0.6 0.8 1.00.00.51.01.52.02.53.03.54.0

rate

concentration

β=0.2

External and internal energy transfer limitations

Multiple steady states and hystersis

1.0 1.2 1.4 1.6

0

50

100

150

200

250

300

G(T

* )/(T

* -1)

Temperature

β= 0.6

Gas-solid noncatalytic reactions

Gas-solid noncatalytic reactions

Gas-solid noncatalytic reactions

Shrinking particle model

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0 Diffusion Control Stokes regime Turbulent regimeReaction Control

Rad

ius

Time

Shrinking unreacted core model

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0 Diffusion Control Gas film AshReaction Control

Rad

ius

Time

Burning of coke from alumina-silica catalyst

Burning of coke from alumina-silica catalyst

Burning of coke

( ) ( ) ( )2 2 1

3 3

1 1 1 1

1 3 1 2 1 1 13 6

s s s

b g b e b

R R Rt X X X XC k C D kCρ ρ ρ⎡ ⎤ ⎡ ⎤= + − − + − + − −⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦

Time required for burning of coke

Gas-solid reaction: General Model – f = 1

Gas-solid reaction: General Model – f = 70

Gas-liquid reactions

Ø  gas purification processes q  CO2 removal from synthesis gas by aqueous

solution of hot potassium carbonate, ethanolamines

q  Removal of H2S by ethanolamines or sodium hydroxide

Ø  production processes q  Air oxidation of aldehydes to acids q  Oxidation of cyclohexane to adipic acid q  chlorination of benzene q  Nitric acid, sulphuric acid

Examples

Two-film model

Gas Liquid

Boundarylayers

Pha

se

inte

rface

Bulk phase

Bulk phase

A

CAb

pAb CAi

pAi

• masstransfer localised in surfacefilms

• CAi&pAi at equilibrium

• continuity of interfacial flux

•  no reaction const. slope

L G

Slow reaction

Gas

Boundarylayers

Pha

se

inte

rface

Bulk phase

Bulk phase

A pAb

CAi

G

L

CAb

CBb

Moderate reaction

Gas Liquid

A

CA0

pA0

CA*

pA*

CB0

A(g) + B(aq) C(aq) L

Fast reaction

Gas Liquid

A

CA0=0

pA0

CA*

pA*

CB0

A(g) + B(aq) C(aq)

L

Fast reaction

Very Fast reaction

Gas Liquid

A pA0

CA*

pA*

CB0

A(g) + B(aq) C(aq)

Rea

ctio

n fr

ont

L

R

Instantaneous reaction

Gas Liquid

A pA0

CA*

pA*

CB0

A(g) + B(aq) C(aq)

Reaction front = Phase interface

L

G