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Y.H.Yap
Chemical Reaction Engineering II
6. Catalyst Deactivation
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Todays Topics
Non-elementary
Reaction Kinetics
Heterogeneous
Reactions
External
Diffusion Effects
Diffusion &
Reaction in
Porous Catalyst
Design of Reactor Data Analysis forReactor Design
Catalyst
Deactivation
G/L Reaction on
Solid Catalyst
1. Introduction
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Mechanisms of catalyst deactivation
How to model decay
Summary
CatalystDeactivation
Determine the order of decay
Catalyst decay in CSTR
Mitigation
Reactor Design for catalyst decay
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Text1. Introduction
Fogler
Chapter 10.7: Catalyst Deactivation
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Fluidized catalytic
cracking unit To convert high-boiling
point, high molecular
weight fractions of
crude oil to morevaluable gasoline and
gases
Better than thermal
cracking because it cangenerate higher octane
fuel
1. Introduction
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Silica-Alumina Cat-Cracking Catalyst (100X)
fresh spent
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Silica-Alumina Cat-Cracking Catalyst (400X)
fresh spent
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Silica-Alumina Cat-Cracking Catalyst (800X)
fresh spent
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Fresh Silica-Alumina Cat-Cracking Catalyst (1700 & 3000X)
fresh spent
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Silica-Alumina Cat-Cracking Catalyst (5000X)
fresh spent
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So far, we have always assumed that the activity of
catalysts remained unchanged with time Usually the activity decreases as catalysts is used
Catalysts are mortal
The decrease (in active sites) can be:
Rapid
Over a period of time
For deactivated catalysts, regeneration or
replacement is necessary from time to time Catalysts deactivation could be:
Uniform
Selective
But they are probably partially preventable
1. Introduction
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Catalytic deactivation adds another level of
complexity to sorting out the reaction rate lawparameters and pathways
When modelling the reactions over decaying
catalysts, we can divide into:
Separable kinetics
Separate rate law and activity
When activity and kinetics are separable, it is possible to
study catalyst decay and reaction kinetics independently
1. Introduction
catalystfresh'historypast' AA rar
Modeling deactivation
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And also divide into:
Nonseparable kinetics
We only consider separable kinetics We define activity as:
1. Introduction
catalystfreshhistory,past'' AA rr
Modeling deactivation
0''
trtrta
A
A Catalyst used for some timeRate of fresh catalyst
Activity is a function of history
catalystfresh'historypast' AA rar
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The rate of disappearance of reactant A on catalyst
that has been used for some time
The rate of catalyst decay can be expressed by:
1. Introduction Modeling deactivation
PBAdd CCChTktapdt
dar ,....,,
Specific decay constant
Functionality of rate on
reacting species
concentrations, usually
independent or linear
,...,fn' BAA CCTktar
Functionality on activity
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The functionality of activity term take a variety of
forms: First order decay
Second order decay
1. Introduction Modeling deactivation
aap
2aap
tkdeta akdt
dad
2ak
dt
dad
tk
tad
1
1
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2. Mechanisms
Six types:
Mechanism How
Poisoning Chemical
Fouling / coking Mechanical
Sintering / Aging Thermal
Vapourized Chemical / Thermal
Form inactive phase Chemical / Thermal
Crush / grind / erode Mechanical
Although there are six mechanisms, there are only three causes
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2. Mechanisms
Sintering (aging):
Loss of activity due to loss of active surface arearesulting from prolonged exposure to high gas-
phase temperatures. Can be lost by:
Crystal agglomeration (recrystallization) and growth
of metals (atomic migration)
Narrowing or closing of pores inside the catalyst
pellet
Sintering
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2. Mechanisms
Sintering (aging):
Crystal agglomeration (recrystallization) and growthof metals (atomic migration)
Sintering
A. Atomic migration
B. Crystallite migration
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2. Mechanisms
Sintering (aging):
Is usually negligible at temperatures below 40% ofthe melting temperature of the solid
Most common decay rate law:
Integrating with a = 1, t = 0:
Usually measured in terms of active surface area
Sintering
2akdtdar dd
tkta d 11
tkSS
daa
1
1
0
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2. Mechanisms
Sintering (aging):
The sintering decay constant follows the Arrheniusequation
Example: calculating conversion with catalyst decay
in batch reactors
Reaction is first order
Decay is second order
Sintering
TTR
ETkk ddd
11exp
0
0
BA
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2. Mechanisms
Sintering (aging):
Example: calculating conversion with catalyst decayin batch reactors
Design equation
Reaction rate law
Decay law (for second-order decay)
Sintering
Wr
dt
dXN AA '0
AA
Ctakr ''
tk
tad
1
1
Example
2akdt
dar dd
Integrating, with a = 1, t = 0,
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2. Mechanisms
Sintering (aging):
Example: calculating conversion with catalyst decayin batch reactors
Stoichiometry
Combining:
Sintering
X
V
NXCC AAA 11
00
XtakV
W
dt
dX 1'
Example
dttkaX
dX
1Let k = kW/V
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2. Mechanisms
Sintering (aging):
Example: calculating conversion with catalyst decayin batch reactors
Integrating:
Sintering Example
t
d
X
tk
dtk
X
dX
00 11
tkk
k
X d
d
1ln
1
1ln
dkkdtkX
/1
11
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You can use the steps for othertype of deactivation
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2. Mechanisms
Coking / Fouling:
Common to reactions involving hydrocarbons:
Results from carbonaceous (coke) material being
deposited on the surface of the catalyst
Or it could be through blocking of pores
Coking / Fouling
Carbon on 14% Ni/Al2O3
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2. Mechanisms
Coking / Fouling:
Removal of the deposits is called regeneration
The amount of coke on the surface after time t
follows an empirical relationship:
Coking / Fouling
n
coke AtC
For East Texas
light gas oil(min)47.0 tCcoke
10 22 5 12 4 10 on catalystC H C H + C H + C
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2. Mechanisms
Coking / Fouling:
Functionalities between the activity and amount ofcoke can be in the form of:
Or:
Catalysts deactivated by coking can usually be
regenerated by burning off the carbon
Coking / Fouling
1
1
p
CC
a
For East Texas
light gas oil
1
1
npptAa
16.7
12/1
t
acCea 1
tk
tad
1
1
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2. Mechanisms
Poisoning:
Occurs when poisoning molecules becomeirreversibly chemisorbed to active sites, thereby
reducing the number of sites available for the main
reaction.
The poisoning molecule may be reactant, product
or impurity in the feedstream
Example:
Lead, which is used as antiknock component ingasoline, poisons the catalytic converter
Consequently, lead has been removed
Poisoning
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1 mm
Pt / Al2O3 on cordierite
2. Mechanisms Poisoning
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2. Mechanisms
Poisoning:
depends on strength of adsorption of some speciesrelative to another species
e.g. Oxygen may be a partial reactant for partial
oxidation but act as poison in ammonia
synthesis
Poisoning
Sulfur
poisoning of
ethylene
hydrogenation
on a metal
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2. Mechanisms
Poisoning:
We consider poisoning: In the form of impurities in the feed
In packed bed
By reactants or products
Poisoning
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2. Mechanisms
Poisoning:
Poison in the feed (impurities): Main reaction:
Poisoning reaction:
SASA
gCSBSA SBSB
BBAA
AA
CKCKkCtar
1'
SPSP qmpdd aCk
dt
dar '
Poisoning
Why there is an extra concentration term?
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2. Mechanisms
Poisoning:
Poison in the feed (impurities): Progressive decay by poisoning
Rate of formation of poisoned sites
PSPTdSP CCCkr .. Unpoisoned
sites
Concentration
of poison in the
gas phase
Poisoning
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2. Mechanisms
Poisoning:
Poison in the feed (impurities): This is equal to rate of removal of total active
sites
Dividing by CT
PSPTdT CCCk
dt
dC.
Pd Cfkdtdf 1
T
SP
CCf .
Pdd Cktadt
dar Activity depends on the fraction of sites
available for adsorption (1-f) !!!
Poisoning
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2. Mechanisms
Poisoning:
Poisoning in packed bed reactor:
Poisoning
Initially, only those sites near the entrance will bedeactivated because poison usually present in traceamounts
As time continues, the sites near the entrance aresaturated and poison must travel fartherdownstream before being adsorbed
Deactivation move through the packed bed as a
wave front
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2. Mechanisms
Poisoning:
Poisoning in packed bed reactor:
Poisoning
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2. Mechanisms
Poisoning:
Poison by either reactants or products: Main reaction:
Poisoning reaction:
SBSA n
AAA Ckr '
SASA qmAdd aCkr '
Poisoning
reactant
poison
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2. Mechanisms
Poisoning:
Restoration of activity is called reactivation If adsorption is reversible, a change of operating
conditions might be sufficient
Just like regeneration in the fluidized bed If not, that is called permanent poisoning, can be
mitigated by:
Chemical retreatment of surface
Replacement of spent catalysts
Poisoning
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2. Mechanisms
Vapourization:
Metal loss through direct vaporization is generallyan insignificant route to catalyst deactivation
even at high reaction temperatures.
Metal loss through formation of volatile
compounds can be significant over a wide range
of reaction conditions including mild, low-
temperature conditions.
Deactivation is almost always irreversible; lossof noble metals is very expensive.
Most common types are carbonyls, oxides,
sulfides and halides
Vapourization
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2. Mechanisms
Vapourization:
Vapourization
Formation of volatile nickel tetracarbonyl at the
surface of a nickel crystallite in CO atmosphere.
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2. Mechanisms
Vapourization (examples):
Vapourization
Catalyt ic Process Catalyt icSolid VaporFormed Comments onDeactivation Process Ref.
Automotiveconverter
Pd-
Ru/Al 2O3
RuO4 50% loss of Ru during 100 h test inreducing automotive exhaust.
Barthol., 1975.
Methanation of CO Ni/Al 2O3 Ni(CO)4 PCO> 20 kP a and T < 425 e toNi(CO)4formation, diffusion anddecomposition on the support as largecrystallites.
Shen et al., 1981.
CO chemisorption Ni catalysts Ni(CO)4 PCO> 0.4 kPa and T > ue toNi(CO)4formation; catalyz ed by s ulfurcompounds.
Pannell et al., 1977.
Fischer-Tropsch
Synthesis
Ru/NaYzeoliteRu/Al2O3 ,Ru/TiO2
Ru(CO)5,
Ru3(CO)12
Loss of Ru during FTS (H2/CO = 1, 200-250 C, 1 atm) on Ru/NaY zeolite andRu/Al2O3; Up to 40% loss while flowing
CO at 175-2 C over Ru/Al2O3for 24 h.Rate of Ru loss less on titania-supportedRu and for catalysts c ontaining 3 nmrelative to 1.3 nm. Surface carbon lowersloss.
Qamar and Goodwin, 1983;
Goodwin et al., 1986.
Ammonia oxidation Pt-Rhgauze
PtO2 Loss: 0.05 0.3 g Pt/ ton HNO3;recovered with Pd gauze; loss of Pt leadsto surface enrichment with inactive Rh.
Sperner and Hohmann,1976.
HCN synthesis Pt-Rhgauze
PtO2 Ext ensive restructuring and loss ofmechanical strength.
Hess a nd Phillips, 1992.
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2. Mechanisms
Formation of inactive phase:
Vapor-solid reactions are similar to but not the sameas poisoning; the distinction is the formation of a new
phase altogether in the former process.
These include:
Reactions of vapor phase with the catalyst surfaceto produce inactive surface and bulk phases
reaction of CO with Fe to produce iron carbides (some
inactive) during Fischer-Tropsch synthesis;
reaction of metallic Fe to FeO at > 50 ppm O2 inammonia synthesis;
H2O-induced Al migration from the zeolite frame-work
during regeneration of zeolites.
Inactive phase
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2. Mechanisms
Formation of inactive phase:
These include: Catalytic solid-support or catalytic solid-
promoter reactions,
e.g., reaction of Ru metal and Al2
O3
to form
inactive surface and bulk Ru aluminates in auto
emissions control.
Solid-state transformation of catalytic phases
during reaction H2O-induced Al migration from the zeolite frame-
work during regeneration of zeolites.
Inactive phase
2 h i h i l
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2. Mechanisms
Mechanical failure may be due to:
Fracture or crushing of granular, pellet ormonolithic catalyst forms due to a stress
attrition, the size reduction and/or breakup of
catalyst granules or pellets to produce fines,
especially in fluid or slurry beds, and
erosion (due to collision) of catalyst particles or
monolith coatings at high fluid velocities.
Mechanical
2 M h i Mi i i
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2. Mechanisms
Six types:
Mechanism Mitigation
PoisoningDedicated reactor to regenerate
Purification of feed
Fouling / coking
Dedicated reactor to regenerate
Purification of feed
Sintering / Aging Little we can do, replacement
Vapourized Purification of feed, replacement
Form inactive phase Regenerate, purification of feed,replacement
Crush / grind / erode Little we can do, replacement
Mitigations
2 M h i D l
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2. Mechanisms
Six types:
Mechanism With concentration term
Poisoning Yes
Fouling / coking Yes
Sintering / Aging No
Vapourized No
Form inactive phase Yes
Crush / grind / erode No
Decay law
3 D i d f d
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3. Determine order of decay
We use try and error to find the order of reaction
that fits the data Consider at steady-state in CSTR(we need to make it steady state to find out the order of decay)
Mole balance
Solving for activity
BA
WtarFFAAA
'0
(No accumulation)
n
A
AA
A
AA
kC
CC
W
v
rW
CvCvta 00000
'
3 D i d f d
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3. Determine order of decay
AA
n
ARd
CC
Cktk
0
lnln
First order
Log both side
First-order decay in a CSTR Wk
vkR
0
ak
dt
dad tkdeta
3 D t i d f d
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3. Determine order of decay
If first order does not fit, we try second order decay
Mole balance
Solving for activity for second order
WtarFFAAA
'0
n
AR
AA
d Ck
CC
tkta
0
1
1
2akdt
dad tk
tad
1
1
3 D t i d f d
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3. Determine order of decay
tkk
kCCC
R
d
RAA
n
A
1
0
Rearrange
Second-order decay in a CSTR
3 Determine order of deca
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3. Determine order of decay
For packed bed:
For first order reaction, mole balance
Solving for activity
AA Ctka
dW
dCv 0
ak
dt
dad tkdeta
n
A
A
C
C
Wk
vta 00
3 Determine order of decay
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3. Determine order of decay
A
Ad
C
C
Wk
vtk 00 lnlnln
Log both side
First-order decay in a packed bed reactor
3 Determine order of decay
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3. Determine order of decay
There are basically two types of questions
The one shown in lecture note (as just shown) Given the plant data, see how activity changes
with time
Or like in Tutorial 5 question 6
However for most of the problems we deal with,
order of decay will be provided
4 Catalyst decay in CSTR
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4. Catalyst decay in CSTR
A simple example showing :
Catalyst decay in fluidized bed modeled as CSTR Order of decay is given
Fluidized catalytic cracking4 Catalyst decay in CSTR
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Fluidized catalytic
cracking unit We are not using Kunii-
Levenspiel bubbling
model
Instead we assumewell-mixed reactor and
model the bed as a
CSTR
Fluidized catalytic cracking4. Catalyst decay in CSTR
3 Work examples Fluidized catalytic cracking
Fluidized catalytic cracking4 Catalyst decay in CSTR
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3. Work examples Fluidized catalytic crackingFluidized catalytic cracking4. Catalyst decay in CSTR
Fluidized catalytic cracking4 Catalyst decay in CSTR
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Determine concentration, activity and conversion:
Mole balance
Rate law
Decay law (first order)
VrvCCvdt
dCV AAA
A 00
AA kaCr
AdaCk
dt
da
Fluidized catalytic cracking4. Catalyst decay in CSTR
Remember for poisoning, there is
an extra concentration term
(m3/s)(mol/m3) (mol/m3s)(m3)
Fluidized catalytic cracking4 Catalyst decay in CSTR
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Determine concentration, activity and conversion:
Stoichiometry
1 mol of A reacted 1 mol of B + 1 mol of C
00
00
0
0
IA
ICBA
T
T
FF
FFFFv
F
Fvv
AACB FFFF 0
0
00
0
2
T
AAI
F
FFF
v
v
0
000
T
AAAI
F
FFFF
Fluidized catalytic cracking4. Catalyst decay in CSTR
mol/s
Fluidized catalytic cracking4 Catalyst decay in CSTR
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Determine concentration, activity and conversion:
Stoichiometry
00
0
0
000 1T
A
T
A
T
AAAI
F
F
F
F
F
FFFF
00
0
0
1vC
vCy
v
v
T
AA
0
00
/1
1
TA
A
CC
yvv
where
0
00
T
AA
C
Cy
Fluidized catalytic cracking4. Catalyst decay in CSTR
Fluidized catalytic cracking4 Catalyst decay in CSTR
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Determine concentration, activity and conversion:
From mole balance
Substitute
We get
VrvCCvdt
dCV AAA
A 00
0
00
/1
1
TA
A
CC
yvv
VkaCC
CC
yvCv
dt
dCV AA
TA
AA
A
0
0000
/1
1
Fluidized catalytic cracking4. Catalyst decay in CSTR
Fluidized catalytic cracking4. Catalyst decay in CSTR
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Determine concentration, activity and conversion:
Dividing both sides by volume
Therefore, change of concentration with time is:
AATA
AAA kaCCCC
yC
dt
dC
0
00
/1
1
A
TAAAA
CkaCCyC
dt
dC
000 /1/1
Fluidized catalytic cracking4. Catalyst decay in CSTR
1
Fluidized catalytic cracking4. Catalyst decay in CSTR
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Determine concentration, activity and conversion:
Conversion
Space time
Previously
00
0
000
0
/1
111
A
A
TA
A
A
A
A
AA
C
C
CC
y
Cv
vC
F
FFX
h02.0
/hm5000kg/m500
kg000,5033
00
v
W
v
V
b
Fluidized catalytic cracking4. Catalyst decay in CSTR
2
AdaCk
dt
da 3
Fluidized catalytic cracking4. Catalyst decay in CSTR
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Determine concentration, activity and conversion:
Solve equations 1, 2, 3 simultaneously with ODEintegrator such as POLYMATH or MATLAB ode
solver (e.g. Runge-Kutta)
We will then get a plot with:
Concentration
Activity
Conversion
Changing with time
Fluidized catalytic cracking4. Catalyst decay in CSTR
Fluidized catalytic cracking4. Catalyst decay in CSTR
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y gy y
Fluidized catalytic cracking4. Catalyst decay in CSTR
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What do you see?
Space time:
Decay time:
The assumption of quasi-steady state is valid
But catalyst decay in less than an hour
Fluidized bed would not be a good choice to carry
out this reaction
We will see what other strategies can be used to
mitigate the decay
y gy y
0.02h
0. 5h
Fluidized catalytic cracking4. Catalyst decay in CSTR
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The steps will be the same for other type of reactors,
but we might need to change the following: mole balance equation
Order of reaction (rate law)
Order of decay
Stoichiometry
to get differential equations of:
Concentration
Activity
conversion
y gy y
dt
dCA
dt
da
X
5. Reactor Design for Catalyst Decay
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g y y
How reactors are designed to counteract the effect
of catalyst decay: Slow decay
Temperature-Time Trajectory
Moderate decay
Moving bed reactor
Rapid decay
Straight-Through Transport Reactor
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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In many large-scale reactors, catalyst decay is slow
But constant conversion is necessary
So that downstream processes are not upset
How to maintain constant conversion? We can replace the catalysts
But if turnaround is not due or cost ineffective
Increase the feed temperature slowly
Therefore keeping the reaction rate constant
p j yg y y
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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How do we know what temperature to operate at
particular time? For first order reaction (not first order decay)
We neglect any change in concentrations,
We want to see how temperature is increased
with time
AA
CTkTtaCTkr ,00
0, kTtaTk
0
/1/1/
00 kaek TTREA
p j yg y y
Initial temperature Higher temperature to counter decay
At t = 0, T0
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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How do we know what temperature to operate at
particular time? Solve
1lnln /1/1/ 0 ae TTREA
0ln11
0
a
TTR
EA
0
1ln
1
Ta
R
E
TA
p j yg y y
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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How do we know what temperature to operate at
particular time? Decay law
from
nTTREd aek
dt
dad /1/1/
00
Ad EEnd
n
A
dd akaa
EEk
dtda /
00 lnexp
aTTR
EA ln11
0
yg y y
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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How do we know what temperature to operate at
particular time? Integrating with a = 1, t = 0:
We get time dependence on temperature
Ad EEnd
n
A
dd akaa
E
Ek
dt
da /00 lnexp
Add
dAA
EEnk
TTR
EnEE
t/1
11exp1
0
0
Decay constant at temperature T0
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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How do we know what temperature to operate at
particular time? For first order decay:
However, in many industrial reactions, decay rate
law changes as temperature increases
Initial stage: fouling of acidic sites
Slow coking linear regime
Accelerated coking exponential increase in T
Add
d
EEk
TTR
E
t
/
11exp1
0
0
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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Temperature-
time trajectory
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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Work examples (from Tutorial 5 Q5)
The decomposition of spartanol to wulfrene and CO2 is oftencarried out at high temperatures. Consequently, the
denominator of the catalytic rate law is easily approximated as
unity, and the reaction is first order with an activation energy of
150 kJ/mol. Fortunately, the reaction is irreversible.
Unfortunately, the catalyst over which the reaction occursdecays with time on stream. The following conversion-time
data were obtained in a differential reactor.
Assume the order of decay is 2.
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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Work examples
a) If the initial temperature of the catalyst is 480 K, determine
the temperature-time trajectory to maintain constantconversion
b) What is the catalyst lifetime?
Temperature-time trajectory5. Reactor Design for Catalyst Decay
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Work examples
Decay law
Refer to our note:
2akdt
dad tk
tad
1
1
0, kTtaTk
0
1
1k
tkk
d
tkkTTR
Ek d
1
11exp 0
0
0
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Work examples
From the data given:
tkTTR
Ed
111
exp0
dk
TTRE
t
111exp0
Tkd
314.8
84344exp10296.1 3
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Work examples
T
TKmolJ
molkJ
t
314.8
344,84exp10296.1
11
480
1
./314.8
/150exp
3
T (K) t (min)
480 0
485 44.3
490 87.3
495 130.4
500 174.9Plot a graph
Moving bed reactor5. Reactor Design for Catalyst Decay
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For significant decay, we can use moving bed reactor
Example: Fluidized catalytic cracking Fresh catalysts enter from
top
Moves through the bed as
compact packed bed
Catalysts are coked
continually as it moves
Catalysts exit from thereactor into kiln
Air is used to burn off the
carbon
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Moving bed reactor:
Regenerated catalysts arelifted from the kiln by an
airstream and then fed into
a separator
Catalysts return back intothe reactor
The reactant flows rapidly
through the reactor relative
to the flow of the catalyst
If feed rate of catalyst and reactants do
not vary with time, the reactor is
operating at steady state
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Modelling moving bed reactor at steady state
Mole balance of A
Differential form
Reaction rate
0',, WrFF AWWAWA
AA rdW
dXF '0
PBAA
CCCktar ,...,,fn'
1
(mol/s) (g)(mol/s)
(mol/s)
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Modelling moving bed reactor at steady state
Decay law
Contact time
Differential form
n
dakdt
da
sUWt
g/s
sU
dWdt
2
3
g
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Modelling moving bed reactor at steady state
Combine and
Combine into
n
s
d aU
k
dW
da
2 3
4
4 1
0
0'
A
A
F
trWa
dW
dX
Activity based on W
from da/dW
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Example of moving bed reactor
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Example of moving bed reactor
Mole balance of
)')((0 AA
rWadW
dXF 1
Activity based on W
from da/dW, only for
moving bed
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Example of moving bed reactor
Rate law
Decay law
Combining equations
2' AA kCr
ak
dt
dad
aUk
dWda
s
d
WUksdea
/
2
3
sUdWdt
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Example of moving bed reactor
Combining
Separating and integrating
220/
0 1 XkCedW
dXF AWUk
Asd
X WWUk
A
A dWeX
dX
kC
Fsd
0 0
/
22
0
0
1
sd UWkdA
sA ekF
UkC
X
X /
0
2
0 11
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Example of moving bed reactor
Numerical evaluation
sd UWkdA
sA ekF
UkC
X
X /
0
2
0 11
1-
cat
23
cat
6
min72.0
.g000,10
mol/min30
mol/dm075.0
.minmol.g
dm6.0
1
s
X
X
24.1
kg/min10
kg22min72.0exp1
1-
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Example of moving bed reactor
Numerical evaluation
%55X
Straight-Through Transport Reactor5. Reactor Design for Catalyst Decay
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Straight-Through Transport Reactor
Used for reaction systems in which catalystdeactivates very rapidly
Commercially is used in the production of
gasoline from cracking of heavier petroleum
fractions where coking occurs very rapidly
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Straight-Through Transport Reactor
Catalyst pellets and reactantenter together and are
transported very rapidly through
the reactor (usually travel at
same velocity) Bulk density of catalyst pellets
are significantly smaller than in
moving-bed reactors
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Modelling STTR at steady state
Mole balance of A over reactor volume
Differential form
In terms of conversion and catalyst activity
zAV c
0 zArFF cAzzAzA
tatrF
A
dz
dXA
A
cB 0'0
1cBAcA
A ArArdz
dF'
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Modelling STTR
Residence time
Substituting in terms of z (i.e. a(t) = a(z/Up))
pU
zt
p
A
A
cB
U
zatr
F
A
dz
dX0'
0
p
A
Ag
B
U
zatr
CUdz
dX0'
0
00 AcgA
CAUF
2
Summary
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Mechanisms of catalyst deactivation
How to model decayCatalystDeactivation
Determine the order of decay
Catalyst decay in CSTR
Sintering
Mitigation
Poisoning
Fouling/coking
Vapourization
Inactive phase
Temperature-Time
trajectory
Reactor Design for catalyst decay Moving Bed Reactor
Straight-Through
Trans ort Reactor
Mechanical
Try and error
Separable
kinetic 0,1,2
Summary
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Look at batch reactor example in sintering section
See the steps for how to work out the change
of conversion with time
Do it for other systems
Steps:
Design equation mole balance
Rate law
Decay law
Stoichiometry
Combine and derive
L k h h d f d i d i d