Deactivation of catalysts
w
w
w
w
w
Types of deactivation
Reaction models
Kinetics
Mass transfer phenomena pellet
Effect on selectivity
Catalysis Engineering - Deactivation
Catalytic reactor design equation
steady state
conversion i
stoichiometric coefficient i
deactivation function
dx i
= i r
d (W Fi )
rate expression
space time
catalyst effectiveness
Coupled with:
Catalysis Engineering - Deactivation
Heat balance - T-profile
Momentum balance - p-profile
Timescale catalyst deactivation
10 -1 10 0
10 1
10 2
10 3
10 4
10 5
10 6
10 7
10 8
HDS
Hydrocracking
Reforming
FCC
EO
Fat hardening
MA
Aldehydes
Hydrogenations
Acetylene
Oxychlorination
SCR
Formaldehyde
C3 dehydrogenation
101
1 hour
Catalysis Engineering - Deactivation
Batch
processes
hrs-days
NH 3 oxidation
TWC
Time / seconds
10-1 10 0
Most bulk
processes
0.1-10 year
102
103
104
105
106
1 day
10 7
10 8
1 year
Deactivation of catalysts
irreversible 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
Poisoning surface
S
Selective poisoning
S
Fouling
active site
Cl
Cl
Cl Cl
Cl Cl
Cl
Cl
Redispersion
&
evaporation
Sintering
Cl Cl
Cl Cl
Carbonfilament growth
Catalysis Engineering - Deactivation
Carbon filament growth
Ni/CaO
H2
CnHm
H H
Catalysis Engineering - Deactivation
Formation of nanotubes
May be reversible
Possible destruction of partices
Preparation of carbon supports
Deactivation
conversion
or
kobs
initial level
process time
k obs = k intr NT
constant
variable
variable
blocking pores
loss surface area
Fouling
loss active sites
Sintering
Catalysis Engineering - Deactivation
Poisoning
Deactivation - depends on?
heat
Fouling
physical blocking surface
by carbon or dust
usually regenerable
Sintering
loss surface area
gradual or catastrophic
irreversible - nonregenerable
feed & process conditions
process conditions
kobs
feed conditions
Poisoning
chemisorption on active
sites
reversible or irreversible
feed & process conditions
Selectivity poisons Modifiers
block side reactions
inhibit consecutive reactions
(kinetics)
kept as secret !
usually found by accident
Catalysis Engineering - Deactivation
What are poisons?
Examples
Surface active
metal or ion
Cu in Ni
Ni in Pt
Pb or Ca in Co3O4
Pb in Fe3O4
High M.W.
product
producer
Strong
chemisorber
Fe on Cu
Fe on Si-Al
from pipes
Bases
acetylenes
dienes
Toxic compounds
(free electron pair)
H2S on Ni
NH3 on Si-Al
Sintering
accelerator
H2 O (Al2O 3)
Cl2 (Cu)
from feed
or product
Catalysis Engineering - Deactivation
Sintering...
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
promoters
melting point
Catalysis Engineering - Deactivation
da
= kam
dt
E
k = k 0 exp a
RT
affects m and Ea
affects Ea
determines Ea
m often 2 (-6)
Example sintering: n-Heptane reforming
m2/g Pt
dp Pt /nm
300
60
50
hydrocracking
50
40
dehydrocyclization
40
200
30
30
20
100
10
0
10 20 30 40 50 60 70 80
time @780 C / h
isomerization
20
10
n-C7
0
10
15
20
time @780 C / h
not 1:1 relation reactivity and metal surface
structure sensitive reactions
edge sites / steps
surface sites
Catalysis Engineering - Deactivation
Mechanisms of sintering
monomer dispersion
2-D cluster
3-D cluster
vapor
particles migrate
surface
coalesce
interparticle transport
migrating
metastable
Secondary
effects
stable
Catalysis Engineering - Deactivation
Deactivation of catalysts
Fouling or self-poisoning
w Parallel reaction
B
A
C
w Series self-poisoning
(FCC, HDM)
w Triangular reaction
(FCC)
Poisoning
w Impurity poisoning
(TWC)
Catalysis Engineering - Deactivation
B
A
C
A
P
B
blocking
Deactivation: global view
Concentration profiles over reactor and in particles:
poisoning versus reaction kinetics
types of reactions
mass transport phenomena
increasing poisoning rate
uniform or homogeneous
poisoning
diffusion limited
poisoning
pore mouth
poisoning
center
poisoning
series poisoning
Catalysis Engineering - Deactivation
Particle deactivation
modelling slab
Homogenous poisoning: Fraction poisoned
Apparent activity:
F=
rpoisoned
runpoisoned
Diffusion model (first order irrev.):
d 2cA
De
= (1 )k B c A
2
dx
Concentration profile:
cosh( (1 )0 x / L)
c A = c A0
cosh( (1 )0 )
unpoisoned Thiele modulus
Catalysis Engineering - Deactivation
Catalyst effectiveness
1st order, irreversible, slab:
1.0
C*
0.8
Effectiveness factor:
0.1
1.0
tanh
0.6
0.4
cylinder
2.0
0.2
0.0
0.0
10.0
0.2
0.4
0.6
0.8
sphere
1.0
z*
Thiele modulus:
=L
slab
kv
Deff
V
1
L= p =
Ap a'
Catalysis Engineering - Deactivation
0.1
0.1
10
Particle deactivation
uniform poisoning
Fraction F of initial activity:
antiselective
nonselective
J poisoned
F=
J unpoisoned
Fraction of initial activity
1.0
0.8
0 =
0.6
large
(1 ) tanh (1 )0
tanh(0 )
=
x =L
Limiting cases:
1
0.4
1. 0 small
small
0.2
0.0
0.0
F (1 )
0.2
0.4
0.6
0.8
Fraction poisoned
1.0
2. 0 large
F
Catalysis Engineering - Deactivation
(1 )
Particle deactivation
modelling slab
Pore-mouth poisoning (sharp interface)
high value Thiele modulus poison
Fraction poisoned
c1
c0
Diffusion model (first order irrev.):
diffusion through completely
deactivated layer L
followed by reaction and diffusion
in the (1-)L layer
L
(1-)L
Catalysis Engineering - Deactivation
Deff (c0 c1 )
L
{A (1 )L} k v c1 tanh {(1 )0 }
(1 )0
Particle deactivation
pore mouth poisoning
Fraction of initial activity:
F=
J poisoned
J unpoisoned
tanh (1 ) 0
1
tanh( 0 )
1 + 0 tanh (1 ) 0
x =L
Fraction of initial activity
1.0
Limiting cases:
0 =
0.8
1. small
0.01
0.6
F (1 )
3
0.4
10
2. large
0.2
100
0.0
0.0
0.2
0.4
0.6
0.8
Fraction poisoned
1.0
1
1 + 0
selective poisoning
Catalysis Engineering - Deactivation
Particle deactivation
more complex: self-poisoning
Series poisoning:
Simultaneous solution diffusion/reaction equations
CA
core poisoning
higher concentration B in center
more coke formation in center
CB
L
Concentration profiles
shell poisoning
high Thiele moduli A and B
Profiles will depend on reactor coordinate (e.g. HDM results J.P. Janssens)
Catalysis Engineering - Deactivation
Particle deactivation
Doraiswamy & Sharma
ln F
Parallel fouling:
Low values Thiele modulus:
inc
re
as
ing
highest residual activity
decreases continuously
But after certain time residual activities for higher Thiele
values are higher
Sometimes one might prefer diffusion limitation conditions
or catalyst activity concentration profiles (TWC)
ln t
Becker & Wei, J. Catal. 46(1977) 372
Series fouling:
Extent of fouling increases continuously with Thiele modulus
Catalyst with least diffusion resistance preferred
Coke deposition effect on diffusivities generally negligible
Catalysis Engineering - Deactivation
Coke formation
Nature:
often aromatic precursors that give deposits of highly condensed
aromatic structures of low hydrogen content (H/C<1)
polymerised and dehydrogenated acetylenes, olefins
Kinetic aspects
Voorhies (1945):
cC = t
with: 0.5 (0.3 - 1)
Define catalyst activity decay function:
=
NT [C *]
NT
= f (cC ),
f (c P )
most convenient
or f (t )
Separate time dependent behaviour and reaction kinetics
(not always possible)
Catalysis Engineering - Deactivation
Kinetic aspects coke deposition
r = C r 0
Reaction kinetics:
unaffected
Examples of catalyst decay functions (see Froment & Bischoff)
C = 1 t
C = exp( t )
1
C =
1+ t
C =
t
C = (1 + t )
Note: use of time instead
of Coke concentration !
Only process time needed
So, for coking kinetics:
rC =
dC
1 dcC
=
Nt dt
dt
Holds for independent coking rates, how self-poisoning?
Catalysis Engineering - Deactivation
Kinetic aspects coke deposition
self poisoning
LHHW kinetics for A->B and coking:
k A0 NT KA C p A
rA =
1 + K A p A + KB p B
Parallel coking A->C
0
k AC
NT KA C p A
rc =
1 + KA p A + KB p B
Series coking B->C
0
kBC
NT K B C p B
rc =
1 + KA p A + KB p B
Relate time to coke concentration and find C
rC =
Catalysis Engineering - Deactivation
dC
1 dcC
=
Nt dt
dt
Coke deactivation function
Deactivation function
t kC0 NT K B pB
C = exp
dt
0 1 + K A pA + K B pB
function of time, concentration and reactor coordinate!
Only time dependent function approach assumes:
uniform deactivation in pellet or reactor
independent of local concentration
lumped parameter valid for determination conditions only
may give serious errors in prediction (Froment & Bischoff)
but most convenient to apply
Catalysis Engineering - Deactivation
