Chemical Reaction Engineering II

6. Catalyst Deactivation

Y.H.Yap
1. Introduction Today’s Topics

Non-elementary Heterogeneous External

Reaction Kinetics Reactions Diffusion Effects

Diffusion &
Data Analysis for
Reaction in Design of Reactor
Reactor Design
Porous Catalyst

Catalyst G/L Reaction on

Deactivation Solid Catalyst
Summary

Catalyst How to model decay

Deactivation Mitigation

Mechanisms of catalyst deactivation

Determine the order of decay

Catalyst decay in CSTR

Reactor Design for catalyst decay

1. Introduction Text

• Fogler
– Chapter 10.7: Catalyst Deactivation
1. Introduction

• Fluidized catalytic
cracking unit
• To convert high-boiling
point, high molecular
weight fractions of
crude oil to more
valuable gasoline and
gases
• Better than thermal
cracking because it can
generate higher octane
fuel
Silica-Alumina Cat-Cracking Catalyst (100X)

fresh spent
Silica-Alumina Cat-Cracking Catalyst (400X)

fresh spent
Silica-Alumina Cat-Cracking Catalyst (800X)

fresh spent
Fresh Silica-Alumina Cat-Cracking Catalyst (1700 & 3000X)

fresh spent
Silica-Alumina Cat-Cracking Catalyst (5000X)

fresh spent
 1. Introduction

• 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 Modeling deactivation

• Catalytic deactivation adds another level of

complexity to sorting out the reaction rate law
parameters and pathways
• When modelling the reactions over decaying
catalysts, we can divide into:
• Separable kinetics
• Separate rate law and activity

 r ' A  apast history  r ' A fresh catalyst

• When activity and kinetics are separable, it is possible to

study catalyst decay and reaction kinetics independently
 1. Introduction Modeling deactivation

• And also divide into:

• Nonseparable kinetics
 r ' A  r ' A past history,fresh catalyst

• We only consider separable kinetics

• We define activity as:

 r ' A t  Catalyst used for some time

at  
 r ' A t  0 Rate of fresh catalyst

Activity is a function of history

 r '  apast history  r ' fresh catalyst
 1. Introduction Modeling deactivation

• The rate of disappearance of reactant A on catalyst

that has been used for some time

 r ' A  at  k T  fnC A , CB ,...

• The rate of catalyst decay can be expressed by:

da
rd    pa t  k d T hC A , C B ,...., C P 
dt
Functionality of rate on
Functionality on activity reacting species
concentrations, usually
Specific decay constant independent or linear
 1. Introduction Modeling deactivation

• The functionality of activity term take a variety of

forms:
• First order decay
da
p a   a   kd a at   e  kd t
dt
• Second order decay

p a   a 2 da 1
  kd a 2 a t  
dt 1  kd t
 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
 2. Mechanisms Sintering

• Sintering (aging):
• Loss of activity due to loss of active surface area
resulting 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
 2. Mechanisms Sintering

• Sintering (aging):
• Crystal agglomeration (recrystallization) and growth
of metals (atomic migration)

A. Atomic migration
B. Crystallite migration
 2. Mechanisms Sintering

• Sintering (aging):
• Is usually negligible at temperatures below 40% of
the melting temperature of the solid
• Most common decay rate law:
da
rd    kd a 2
dt
• Integrating with a = 1, t = 0:
1
a t  
1  kd t
• Usually measured in terms of active surface area
1
Sa  Sa0
1 k t
 2. Mechanisms Sintering

• Sintering (aging):
• The sintering decay constant follows the Arrhenius
equation
 Ed  1 1 
k d  k d T0  exp    
 R  T0 T 

• Example: calculating conversion with catalyst decay

in batch reactors
• Reaction is first order
• Decay is second order
A B
 2. Mechanisms Sintering Example

• Sintering (aging):
• Example: calculating conversion with catalyst decay
in batch reactors
• Design equation dX
N A0  r ' A W
dt
• Reaction rate law
r ' A  k ' at C A

• Decay law (for second-order decay)

da
rd    kd a 2
dt
Integrating, with a = 1, t = 0, 1
a t  
1 k t
 2. Mechanisms Sintering Example

• Sintering (aging):
• Example: calculating conversion with catalyst decay
in batch reactors
Stoichiometry N A0
C A  C A0 1  X   1  X 
•

V
• Combining:
dX W
 k ' a t 1  X 
dt V

dX
Let k = k’W/V  kat dt
1 X
 2. Mechanisms Sintering Example

• Sintering (aging):
• Example: calculating conversion with catalyst decay
in batch reactors
X t
• Integrating: dX dt
 0
1 X
k
0

1  kd t

 1  k
ln   ln 1  k d t 
1 X  kd

1
X  1
1  k d t 
k / kd
• You can use the steps for other
type of deactivation
 2. Mechanisms Coking / Fouling

• 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

Carbon on 14% Ni/Al O

 2. Mechanisms Coking / Fouling

• Coking / Fouling:
• Removal of the deposits is called regeneration

C10 H22  C5 H12 + C4 H10 + C on catalyst

• The amount of coke on the surface after time t

follows an empirical relationship:

Ccoke  At n

For East Texas

Ccoke  0.47 t (min) light gas oil
 2. Mechanisms Coking / Fouling 1
a t  
• Coking / Fouling:
1  kd t

• Functionalities between the activity and amount of

coke can be in the form of:
1
1 a  p np
a p A t 1
CC  1
For East Texas
• Or: light gas oil

1Cc 1
ae a
7.6t 1/ 2
1
• Catalysts deactivated by coking can usually be
regenerated by burning off the carbon
 2. Mechanisms Poisoning

• Poisoning:
• Occurs when poisoning molecules become
irreversibly 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 in
gasoline, poisons the catalytic converter
• Consequently, lead has been removed
2. Mechanisms Poisoning

• Pt / Al2O3 on cordierite

1 mm
 2. Mechanisms Poisoning

• Poisoning:
• depends on strength of adsorption of some species
relative to another species
• e.g. Oxygen may be a partial reactant for partial
oxidation but act as poison in ammonia
synthesis

Sulfur
poisoning of
ethylene
hydrogenation
on a metal
 2. Mechanisms Poisoning

• Poisoning:
• We consider poisoning:
• In the form of impurities in the feed
• In packed bed
• By reactants or products
 2. Mechanisms Poisoning

• Poisoning:
• Poison in the feed (impurities):
• Main reaction:

A S  A S
kCA
A  S  B  S  C g   r ' A  a t 
1  K AC A  K BCB
BS  BS

• Poisoning reaction:
da
PS  PS rd    k 'd C pm a q
dt

Why there is an extra concentration term?

 2. Mechanisms Poisoning

• Poisoning:
• Poison in the feed (impurities):
• Progressive decay by poisoning

• Rate of formation of poisoned sites

rP . S  kd CT  CP . S CP
Concentration
Unpoisoned of poison in the
sites gas phase
 2. Mechanisms Poisoning

• Poisoning:
• Poison in the feed (impurities):
• This is equal to rate of removal of total active
sites
dCT
  kd CT  CP. S CP
dt

• Dividing by CT
df CP.S
 kd 1  f CP f 
dt CT

da
rd    a t k d C P Activity depends on the fraction of sites
dt available for adsorption (1-f) !!!
 2. Mechanisms Poisoning

• Poisoning:
• Poisoning in packed bed reactor:

• Initially, only those sites near the entrance will be

deactivated because poison usually present in trace
amounts
• As time continues, the sites near the entrance are
saturated and poison must travel farther
downstream before being adsorbed
• Deactivation move through the packed bed as a
wave front
 2. Mechanisms Poisoning

• Poisoning:
• Poisoning in packed bed reactor:
 2. Mechanisms Poisoning

• Poisoning:
• Poison by either reactants or products:
• Main reaction:

A S  BS  r ' A  k AC An

reactant

• Poisoning reaction:

A S  A S rd  k 'd C Am a q

poison
 2. Mechanisms Poisoning

• 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
 2. Mechanisms Vapourization

• Vapourization:
• Metal loss through direct vaporization is generally
an 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; loss
of noble metals is very expensive.
• Most common types are carbonyls, oxides,
sulfides and halides
 2. Mechanisms Vapourization

• Vapourization:

Formation of volatile nickel tetracarbonyl at the

surface of a nickel crystallite in CO atmosphere.
 2. Mechanisms Vapourization

• Vapourization (examples):
Cat alyt i c P r oces s Cat alyt i c Vapor Comments on Ref.
Solid Formed Deactivation Process
Automotive Pd- RuO 4 50% loss of Ru during 100 h test in Barthol., 1975.
converter Ru/Al 2O3 reducing automotive exhaust.

Me thanation of CO Ni/Al 2O3 Ni(CO) 4 PCO > 20 kP a and T < 425 e to Shen et al., 1981.
Ni(CO)4 formation, diffusion and
decomposition on the support as large
cryst allites.
CO chemisorption Ni catalysts Ni(CO) 4 PCO > 0.4 kPa and T > ue to Pannell et al., 1977.
Ni(CO)4 formation; catalyz ed bys ulfur
compounds.
Fischer-Tropsch Ru/NaY Ru(CO) 5, Loss of Ru during FTS (H 2/CO = 1, 200- Qamar and Goodwin, 1983;
zeolite 250 C, 1 atm) on Ru/NaY zeolite and
Synthesis Ru3(CO)12
Ru/Al2O3 , Ru/Al 2O3; Up to 40% loss while flowing
Goodwin et al., 1986.
Ru/TiO2 CO at 175-2 C over Ru/Al2O3 for 24 h.
Rate o f Ru loss less on titania-supported
Ru and for catalysts c ontaining 3 nm
relative to 1.3 nm. Surface carbo n lowers
loss.
Am monia oxi dation Pt-Rh PtO 2 Loss: 0.05 Š 0.3 g Pt/ ton HNO3; Sperner and Ho hmann,
1976.
gauze recovered with Pd gauze; loss of Pt leads
to surface enrichment with inactive Rh.
HCN synthesis Pt-Rh PtO 2 Ext ensive restructuring an d loss of Hessa nd Phillips, 1 992.
gauze mechan ical stre ngth.
 2. Mechanisms Inactive phase

• Formation of inactive phase:

• Vapor-solid reactions are similar to but not the same
as poisoning; the distinction is the formation of a new
phase altogether in the former process.
• These include:
• Reactions of vapor phase with the catalyst surface
to produce inactive surface and bulk phases
• reaction of CO with Fe to produce iron carbides (some
inactive) during Fischer-T
Fischer-Tropsch
ropsch synthe
synthesis;
sis;
• reaction of
reaction of metall
metallic
ic Fe
Fe to
to FeO
FeO at > 50 ppm
ppm O2 in
ammonia synthe
synthesis;
sis;
• H2O-induced Al migration from the zeolite
zeolite frame-w
frame-work
ork
during regeneration of zeolites.
 2. Mechanisms Inactive phase

• Formation of inactive phase:

• These include:
• Catalytic solid-support or catalytic solid-
promoterr reactions,
promote
• e.g., reactio
reaction
n of Ru metal and Al2O3 to form
inactive
inacti ve surface
surface and bulk Ru alumin
aluminate
atess in auto
emissions control.
• Solid-state transformation of catalytic phases
during reaction
• H2O-induced Al migration
migration from
from the zeolite frame-
work during regeneration of zeolites.
 2. Mechanisms Mechanical

• Mechanical failure may be due to:

• Fracture or crushing of granular, pellet or
monolithic 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,
b eds, and
• erosion (due to collision) of catalyst particles or
monolith coatings at high fluid velocities.
 2. Mechanisms Mitigations

• Six types:
Mechanism Mitigation
Dedicated reactor to regenerate
Poisoning
Purification of feed
Dedicated reactor to regenerate
Fouling / coking
Purification of feed

Sintering / Aging Little we can do, replacement

Vapourized Purification of feed, replacement

Regenerate, purification of feed,
Form inactive phase
replacement

Crush / grind / erode Little we can do, replacement

 2. Mechanisms Decay law

• Six types:

Mechanism With concentration term

Poisoning Yes

Fouling / coking Yes

Sintering / Aging No

Vapourized No

Form inactive phase Yes

Crush / grind / erode No

 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
A B
•

(we need to make it steady state to find out the order of decay)

• Mole balance (No accumulation)

FA0  FA  r ' A at W

• Solving for activity

v0 C A 0  v0 C A v0  C A 0  C A 
a t     
W  r ' A  W kC n
A 
3. Determine order of decay

• First order da
 kd a a t   e  kd t
dt

C An
• Log both side k d t  ln k R  ln
C A0  C A
v0
• First-order decay in a CSTR kR 
Wk
 3. Determine order of decay

• If first order does not fit, we try second order decay

• Mole balance

FA0  FA  r ' A at W

• Solving for activity for second order

da 1
 kd a 2
a t  
dt 1  kd t

1 C A0  C A
a t   
1  kd t k R C An
3. Determine order of decay

• Rearrange
n
C 1 kd
A
  t
C A0  C A kR kR

• Second-order decay in a CSTR

 3. Determine order of decay

• For packed bed:

• For first order reaction, mole balance

dC A
v0   kat C A
dW

• Solving for activity

da
 kd a at   e  k d t
dt

v0  C A0 
a t    n 
Wk  C A 
3. Determine order of decay

• Log both side

v0 C A0
 k d t  ln  ln ln
Wk CA

• First-order decay in a packed bed reactor

 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

• A simple example showing :

• Catalyst decay in fluidized bed modeled as CSTR
• Order of decay is given
4. Catalyst decay in CSTR Fluidized catalytic cracking

• Fluidized catalytic
cracking unit
• We are not using Kunii-
Levenspiel bubbling
model
• Instead we assume
well-mixed reactor and
model the bed as a
CSTR
3.
4. Work
Catalyst
examples
decay in CSTRFluidized
Fluidized
catalytic
catalytic
crackingcracking
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• Mole balance
dC A
V  v0C A0  vCA  rAV
dt
(m3/s)(mol/m3) (mol/m3s)(m3)

• Rate law
 rA  kaCA

• Decay law (first order)

da Remember for poisoning, there is
  k d aC A an extra concentration term
dt
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• Stoichiometry mol/s

FT v0 FA  FB  FC  FI 0 
v  v0 
FT 0 FA 0  FI 0

• 1 mol of A reacted 1 mol of B + 1 mol of C

FB  FC  FA0  FA

v FI 0  2 FA0  FA FI 0  FA 0  FA 0  FA
 
v0 FT 0 FT 0
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• Stoichiometry
FI 0  FA0  FA0  FA FA 0 FA
  1 
FT 0 FT 0 FT 0
v C Av
 1  y A0 
v0 CT 0 v0

1  y A0 C A0
v  v0 y A0 
1  C A / CT 0 where
CT 0
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• From mole balance
dC A
V  v0C A0  vCA  rAV
dt

1  y A0
• Substitute v  v0
1  C A / CT 0

• We get
dC A v0 1  y A 0 
V  v0C A0  C A  kaCAV
dt 1  C A / CT 0
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• Dividing both sides by volume

dC A C A 0 1  y A0 
  C A  kaCA
dt   1  C A / CT 0 

• Therefore, change of concentration with time is:

dC A C A0 1  y A0  / 1  C A / CT 0   ka
  CA
dt  
1
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• Conversion
FA 0  FA vCA  1  y A0  C A 
X   1  1    
FA0 v0C A 0  1  C A / CT 0  C A0 
2
• Space time
V W 50,000kg
     0.02 h
v0 b v0 500kg/m 5000m /h 
3 3

• Previously da
  k d aC A 3
dt
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• Determine concentration, activity and conversion:

• Solve equations 1, 2, 3 simultaneously with ODE
integrator such as POLYMATH or MATLAB ode
solver (e.g. Runge-Kutta)
• We will then get a plot with:
• Concentration
• Activity
• Conversion
• Changing with time
4. Catalyst decay in CSTR Fluidized catalytic cracking
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• What do you see?

• Space time: 0.02h
• Decay time: 0. 5h
• 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
 4. Catalyst decay in CSTR Fluidized catalytic cracking

• 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: dC A
• Concentration dt da
• Activity dt
• conversion X
 5. Reactor Design for Catalyst Decay

• 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
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• 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
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• How do we know what temperature to operate at

particular time?
• For first order reaction (not first order decay)

r   
k0 T0 C A  
a t, T k T CA  
Initial temperature Higher temperature to counter decay

• We neglect any change in concentrations,

k T a t , T   k0 At t = 0, T0

• We want to see how temperature is increased

with time
 E A / R 1 / T0 1 / T 
k0e a  k0
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• How do we know what temperature to operate at

particular time?
• Solve
 E A / R 1/ T0 1/ T 
ln e a  ln 1

 E A  1 1 
     ln a  0
 R  T0 T 

1 EA 1
 ln a 
T R T0
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• How do we know what temperature to operate at

particular time?
• Decay law da  Ed / R 1 / T0 1 / T 
  kd 0e a n

dt
da  Ed  n
  k d 0 exp   ln a a  k d 0 a  n  Ed / E A 
dt  EA 

• from
 E A  1 1 
     ln a
 R  T0 T 
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• How do we know what temperature to operate at

particular time?
• Integrating with a = 1, t = 0:
da  Ed  n
  k d 0 exp   ln a  a  k d 0 a  n  E d / EA 

dt  EA 

• We get time dependence on temperature

 E A  nE A  Ed  1 1 
1  exp    
 R  T0 T 
t
k d 0 1  n  Ed / E A 
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• How do we know what temperature to operate at

particular time?
• For first order decay:  Ed  1 1 
1  exp    
 R  T0 T 
t
k d 0  Ed / E A 

• 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
5. Reactor Design for Catalyst Decay Temperature-time trajectory

Temperature-
time trajectory
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• Work examples (from Tutorial 5 Q5)

• The decomposition of spartanol to wulfrene and CO2 is often
carried 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 occurs
decays with time on stream. The following conversion-time
data were obtained in a differential reactor.

• Assume the order of decay is 2.

 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• Work examples

a) If the initial temperature of the catalyst is 480 K, determine

the temperature-time trajectory to maintain constant
conversion
b) What is the catalyst lifetime?
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• Work examples
• Decay law
da 1
  kd a 2
a t  
dt 1  kd t

• Refer to our note:

k T a t , T   k0
1
k  k0
1  kd t

 E  1 1 
k 0 exp       k 0 1  k d t 
R T T 
  0 
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• Work examples

 E  1 1 
exp       1  k d t
R T T 
  0 
 E  1 1 
exp       1
R T T 
  0 
t
kd

• From the data given:

 84344 
3
k d  1.296  10 exp  
 8.314T 
 5. Reactor Design for Catalyst Decay Temperature-time trajectory

• Work examples

 150 kJ / mol  1 1 
exp     1
 8.314 J / mol.K  480 T  
t
3  84,344 
1.296  10 exp  
 8.314T 
T (K) t (min)
480 0
485 44.3
490 87.3
495 130.4
500 174.9
Plot a graph
 5. Reactor Design for Catalyst Decay Moving bed reactor

• 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 the
reactor into kiln
• Air is used to burn off the
carbon
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Moving bed reactor:

• Regenerated catalysts are
lifted from the kiln by an
airstream and then fed into
a separator
• Catalysts return back into
the 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
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Modelling moving bed reactor at steady state

• Mole balance of A
FA,W  FA,W  W  r ' A W  0
(mol/s) (mol/s) (g)

• Differential form
dX
FA0  r ' A 1
dW
(mol/s)
• Reaction rate

r ' A  at k fnC A , CB ,..., CP 

 5. Reactor Design for Catalyst Decay Moving bed reactor

• Modelling moving bed reactor at steady state

• Decay law
da
  kd a n 2
dt
• Contact time
W g
t
Us g/s

• Differential form
dW
dt  3
Us
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Modelling moving bed reactor at steady state

• Combine 2 and 3

da kd
  an 4
dW Us

• Combine 4 into 1

dX a W  r ' A t  0


dW FA 0
Activity based on W
from da/dW
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Example of moving bed reactor

 5. Reactor Design for Catalyst Decay Moving bed reactor

• Example of moving bed reactor

• Mole balance of
dX
FA0  a (W )( r ' A ) 1
dW

Activity based on W
from da/dW, only for
moving bed
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Example of moving bed reactor

• Rate law
 r ' A  kCA2 2

• Decay law da
  kd a dW
dt dt 
Us

• Combining equations

da kd
  a 3
dW Us

  kd / U s W
a  e
 5. Reactor Design for Catalyst
Catalyst Decay Moving bed reactor

• Example of moving bed reactor

• Combining dX   k d / U s W
FA 0 e kC 2
A0 1  X 
2

dW

• Separating and integrating

X W
FA 0 dX
 1  X   e   k d / U s W
2 2
dW
kC A0 0 0

X k CA2 0U s
 1  e kdW /U s 
1 X FA 0 k d
 5. Reactor Design for Catalyst
Catalyst Decay Moving bed reactor

• Example of moving bed reactor

• Numerical evalua
evaluation
tion
2
X kC U s
 1  e
A0  kdW /U s

1 X FA 0 k d

X

0.6 dm

 6
0.075 mol/dm 
3 2
10,000 g cat .s
1 X mol.g cat .min 30 mol/min min -1
0.72 min

   0.72 min
min 22 kg   
-1
 1  exp
exp     1.24

  10 kg/min 
 5. Reactor Design for Catalyst
Catalyst Decay Moving bed reactor

• Example of moving bed reactor

• Numerical evalua
evaluation
tion

X  55%
 5. Reactor Design for Catalyst Decay Straight-Through Transport Reactor

• Straight-Through Transport Reactor

• Used for reaction systems in which catalyst
deactivates very rapidly
• Commercially is used in the production of
gasoline from cracking of heavier petroleum
fractions where coking occurs very rapidly
 5. Reactor Design for Catalyst Decay Straight-Through Transport Reactor

• Straight-Through Transport Reactor

•Catalyst pellets and reactant
enter 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
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Modelling STTR at steady state

• Mole balance of A over reactor volume
V  Ac z
FA z  FA z  z  rA Ac z  0

• Differential form
dFA
 rA Ac  r ' A  B Ac 1
dz
• In terms of conversion and catalyst activity
dX   B Ac 
   r ' A t  0 a t 
dz  FA 0 
 5. Reactor Design for Catalyst Decay Moving bed reactor

• Modelling STTR
• Residence time
z
t 2
Up
• Substituting in terms of z (i.e. a(t) = a(z/Up))

dX   B Ac   z 
   r ' A t  0 a 
dz  FA 0  U p 
 
dX  B   z 
   r ' A t  0 a 
dz  U g C A 0  U p



FA0  U g Ac C A0
Summary

Separable
How to model decay kinetic 0,1,2
Catalyst
Deactivation Mitigation

Mechanisms of catalyst deactivation

Sintering Vapourization
Poisoning Inactive phase
Fouling/coking Mechanical

Determine the order of decay Try and error

Catalyst decay in CSTR Temperature-Time

trajectory
Reactor Design for catalyst decay Moving Bed Reactor

Straight-Through