Post on 10-Oct-2014
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