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Catalysis Engineering - Deactivation
Deactivation of catalysts
w Types of deactivationw Reaction modelsw Kineticsw Mass transfer phenomena pelletw Effect on selectivity
Catalysis Engineering - Deactivation
Catalytic reactor design equationsteady state
( ) F-= rFWddx
ii
i hn
stoichiometric coefficient i
catalyst effectiveness
rate expression
conversion i
space time
Coupled with: Heat balance - T-profileMomentum balance - p-profile
deactivation function
Catalysis Engineering - Deactivation
Timescale catalyst deactivation
10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8
10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8
Time / seconds
HDSHydrocracking
ReformingFCC
EO
HydrogenationsAldehydes
AcetyleneOxychlorination
MA
Formaldehyde
NH3 oxidation
SCR
TWC
1 year
Most bulk processes0.1-10 yearFat hardening
1 day1 hour
Batch processeshrs-days
C3 dehydrogenation
Catalysis Engineering - Deactivation
Deactivation of catalystsirreversible loss of activity
Types of deactivation: Fouling: secondary reactions of reactants or products,
coke formation Poisoning:strong chemisorption of impurity in feed Aging: structural changes or sintering (loss of surface
area, high temperature) (Inhibition: competitive adsorption, reversible)
Fouling or self-poisoning often cause of deactivation
Catalysis Engineering - Deactivation
Deactivation types
Fouling
S
Poisoning surface
SSelective poisoning
active site
Sintering
Cl ClClCl
ClCl ClCl Cl
Cl ClCl
Redispersion&
evaporation
Carbonfilament growth
Catalysis Engineering - Deactivation
CnHm
HH
H2
C
C
Carbon filament growthNi/CaO
Formation of nanotubesMay be reversiblePossible destruction of particesPreparation of carbon supports
Catalysis Engineering - Deactivation
Deactivation
conversionorkobs
process time
h= Tintrobs Nkk
constant variable variable
blocking pores loss surface area
loss active sitesFouling
Sintering
Poisoning
initial level
Catalysis Engineering - Deactivation
Deactivation - depends on?
kobs
Sintering loss surface area gradual or catastrophic irreversible - nonregenerable
Fouling physical blocking surface by carbon or dust usually regenerable
Poisoning chemisorption on active sites reversible or irreversible
Selectivity poisons Modifiers block side reactions inhibit consecutive reactions (kinetics)
kept as secret !usually found by accident
process conditions feed & process conditions
feed & process conditionsfeed conditions
heat
Catalysis Engineering - Deactivation
What are poisons?
Surface activemetal or ion
High M.W.product producer
Strongchemisorber
Sinteringaccelerator
Cu in NiNi in Pt
Pb or Ca in Co3O4Pb in Fe3O4
Fe on CuFe on Si-Alfrom pipes
acetylenesdienes
BasesH2S on NiNH3 on Si-Al
Toxic compounds(free electron pair)
H2O (Al2O3)Cl2 (Cu)
from feedor product
Examples
Catalysis Engineering - Deactivation
Loss of active surface due to crystallite growth support active phase
Local heating during preparation (calcination) reduction (fresh or passivated catalyst) reaction (hot spots, maldistribution) regeneration (burn-off coke)
Dependency: time temperature atmosphere affects m and Ea promoters affects Ea melting point determines Ea
Sintering...
-=
RTE
kk aexp0
mkadt
da-= m often 2 (-6)
Catalysis Engineering - Deactivation
Example sintering: n-Heptane reforming
0 10 20 30 40 50 60 70 80
time @780 C / h
0
100
200
300
m2/g Pt
0
10
20
30
40
50
dp Pt /nm
0 5 10 15 20
time @780 C / h
0
10
20
30
40
50
60
%
n-C7
hydrocracking
dehydrocyclization
isomerization
not 1:1 relation reactivity and metal surface structure sensitive reactions edge sites / steps surface sites
Catalysis Engineering - Deactivation
Mechanisms of sintering
particles migrate coalesce
monomer dispersion 2-D cluster 3-D cluster
surface
vapor
interparticle transport
metastable
migrating
stable
q2q2
Secondary effects
Catalysis Engineering - Deactivation
Deactivation of catalysts
Fouling or self-poisoningw Parallel reaction
w Series self-poisoning(FCC, HDM)
w Triangular reaction(FCC)
Poisoningw Impurity poisoning
(TWC)
BA
C
BA
C
A B C
A BP blocking
Catalysis Engineering - Deactivation
Deactivation: global view
center poisoning
uniform or homogeneouspoisoning
diffusion limited poisoning
pore mouth poisoning
Concentration profiles over reactor and in particles:poisoning versus reaction kineticstypes of reactionsmass transport phenomena
increasing poisoning rate
series poisoning
Catalysis Engineering - Deactivation
Particle deactivationmodelling slab
Homogenous poisoning: Fraction poisoned a
0 L
Apparent activity:
Diffusion model (first order irrev.):
Concentration profile:
))1(cosh()/)1(cosh(
0
00
fafa
--
=Lx
cc AA
Dd c
dxk ce
AB A
2
21= -( )a
Fr
rpoisoned
unpoisoned
=
unpoisoned Thiele modulus
Catalysis Engineering - Deactivation
Catalyst effectiveness
1st order, irreversible, slab:
Thiele modulus:
slabcylinder
sphere
0.1 1 10
f 0.1
1
h
0.0 0.2 0.4 0.6 0.8 1.0
z*
0.0
0.2
0.4
0.6
0.8
1.0
C* 0.1
1.0
2.0
10.0
f = Lk
Dv
eff
LV
A ap
p
= =1'
hf
f=
tanhf Effectiveness factor:
Catalysis Engineering - Deactivation
Particle deactivationuniform poisoning
Fraction F of initial activity:
Limiting cases:
1. f0 small
2. f0 large
( )F -1 a
( )F -1 a
( ){ }( )0
0
tanh1tanh)1(
ffaa --
===Lxunpoisoned
poisoned
J
JF
0.0 0.2 0.4 0.6 0.8 1.0
Fraction poisoned a
0.0
0.2
0.4
0.6
0.8
1.0
Frac
tion
of in
itial
act
ivity f0 =
large1
small
antiselective
nonselective
Catalysis Engineering - Deactivation
Particle deactivationmodelling slab
Pore-mouth poisoning (sharp interface)
0 L
Diffusion model (first order irrev.):
diffusion through completelydeactivated layer aL
followed by reaction and diffusionin the (1-a)L layer
Fraction poisoned a
(1-a)L
high value Thiele modulus poison
c0c1
( )L
ccDA eff
a10 - ( ){ } ( ){ }( ) 0
01 1
1tanh1fa
faa-
-- ckLA v=
Catalysis Engineering - Deactivation
Particle deactivation pore mouth poisoning
Fraction of initial activity:
Limiting cases:
1. f small
2. f large
0.0 0.2 0.4 0.6 0.8 1.0
Fraction poisoned a
0.0
0.2
0.4
0.6
0.8
1.0
Frac
tion
of in
itial
act
ivity f0 =
0.01
3
10
100
( )F -1 a
( ){ }( ){ }
( )F
J
Jpoisoned
unpoisoned x L
= =+ -
-
=
11 1
1
0 0
0
0af a f
a f
ftanh
tanh
tanh
F +
11 0af
selective poisoning
Catalysis Engineering - Deactivation
Particle deactivationmore complex: self-poisoning
Series poisoning: A B C
Concentration profiles
L 0
CA
CBhigher concentration B in centermore coke formation in center
core poisoning
Simultaneous solution diffusion/reaction equations
Profiles will depend on reactor coordinate (e.g. HDM results J.P. Janssens)
shell poisoninghigh Thiele moduli A and B
Catalysis Engineering - Deactivation
Particle deactivationDoraiswamy & Sharma
Parallel fouling:Low values Thiele modulus: highest residual activity
decreases continuouslyBut after certain time residual activities for higher Thielevalues are higher
Sometimes one might prefer diffusion limitation conditionsor catalyst activity concentration profiles (TWC)
Series fouling:Extent of fouling increases continuously with Thiele modulus
Catalyst with least diffusion resistance preferred
Coke deposition effect on diffusivities generally negligible
ln F
ln t
increasing f
Becker & Wei, J. Catal. 46(1977) 372
Catalysis Engineering - Deactivation
Coke formation
Nature: often aromatic precursors that give deposits of highly condensedaromatic structures of low hydrogen content (H/C
Catalysis Engineering - Deactivation
Kinetic aspects coke deposition
Reaction kinetics:
Examples of catalyst decay functions (see Froment & Bischoff)
So, for coking kinetics:
Holds for independent coking rates, how self-poisoning?
( )
F
F
F
F
F
C
C
C
C
Cn
t
t
t
t
t
= -
= -
=+
=
= +-
1
11
1
a
a
aa
a
b
exp( ) Note: use of time insteadof Coke concentration !
Only process time needed
r rC= F0
rN
dcdt
ddtC t
C C= = -1 F
unaffected
Catalysis Engineering - Deactivation
Kinetic aspects coke depositionself poisoning
LHHW kinetics for A->B and coking:
Parallel coking A->C
Series coking B->C
rk N K p
K p K pcBC T B C B
A A B B
=
+ +
0
1F
rk N K p
K p K pAA T A C A
A A B B
=
+ +
0
1F
rk N K p
K p K pcAC T A C A
A A B B
=
+ +
0
1F
rN
dcdt
ddtC t
C C= = -1 F
Relate time to coke concentration and find FC
Catalysis Engineering - Deactivation
Coke deactivation function
Deactivation function
function of time, concentration and reactor coordinate!
Only time dependent function approach assumes:uniform deactivation in pellet or reactorindependent of local concentrationlumped parameter valid for determination conditions only
may give serious errors in prediction (Froment & Bischoff)
but most convenient to apply
++
-=F dtpKpKpKNkt
BBAA
BBTCC
0
0
1exp
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