Catalyst Deactivation
Short Description
Catalyst Deactivation...
Description
Chemical Reaction Engineering II 6. Catalyst Deactivation Y.H.Yap
1. Introduction
Today’s Topics
Non-elementary Reaction Kinetics
Heterogeneous Reactions
External Diffusion Effects
Diffusion & Reaction in Porous Catalyst
Design of Reactor
Data Analysis for Reactor Design
Catalyst Deactivation
G/L Reaction on Solid Catalyst
Summary
Catalyst Deactivation
How to model decay 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 at r ' A t 0
Catalyst used for some time 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 at k T fnC A , C B ,...
•
The rate of catalyst decay can be expressed by:
r d
da dt
pa t k d T hC A , C B ,...., C P
Functionality on activity Specific decay constant
Functionality of rate on reacting species concentrations, usually independent or linear
1. Introduction •
Modeling deactivation
The functionality of activity term take a variety of forms: •
First order decay
p a a •
da k d a dt
at e
k d t
Second order decay
p a a 2
da k d a 2 dt
1 a t 1 k d 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 gasphase 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 r d k d a 2 dt •
Integrating with a = 1, t = 0:
a t •
1 1 k d t
Usually measured in terms of active surface area
1 S a S a 0 1 k t
2. Mechanisms •
Sintering
Sintering (aging): •
•
The sintering decay constant follows the Arrhenius equation E d 1 1 k d k d T 0 exp R T 0 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
•
Reaction rate law
N A0 '
dX r ' A W dt
r A •
k ' a t C A
Decay law (for second-order decay) da r d k d a 2 dt 1 Integrating, with a = 1, t = 0, a t 1 k t
2. Mechanisms •
Sintering
Example
Sintering (aging): •
Example: calculating conversion with catalyst decay in batch reactors N A0 Stoichiometry 1 X C A C A0 1 X V •
•
Combining:
Let k = k’W/V
dX W k ' a t 1 X dt V dX 1 X
kat dt
2. Mechanisms •
Sintering
Example
Sintering (aging): •
Example: calculating conversion with catalyst decay in batch reactors •
Integrating:
X
t
dX dt k 1 X 1 k d t 0 0
1 k ln ln 1 k d t 1 X k d X 1
1
1 k d t
k / k d
•
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:
C coke At n
C coke 0.47 t (min)
For East Texas light gas oil
2. Mechanisms •
Coking / Fouling
Coking / Fouling: •
Functionalities between the activity and amount of coke can be in the form of: 1 a p np 1 a p A t 1 C C 1 •
For East Texas light gas oil
Or:
ae •
1 a t 1 k d t
1C c
a
1 1/ 2
7.6t
Catalysts deactivated by coking can usually be regenerated by burning off the carbon
1
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
•
1 mm
Pt / Al2O3 on cordierite
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
kC A r ' A a t 1 K AC A K BC B
A S B S C g B S B S
•
Poisoning reaction: P S P S
da k 'd C pm a q r d 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 r P . S k d C T C P . S C P Unpoisoned sites
Concentration of poison in the gas phase
2. Mechanisms •
Poisoning
Poisoning: •
Poison in the feed (impurities): •
•
This is equal to rate of removal of total active sites dC T k d C T C P . S C P dt Dividing by C T df k d 1 f C P dt
r d
da dt
a t k d C P
f
C P . S C T
Activity depends on the fraction of sites 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 poison
r d k 'd C Am a q
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, lowtemperature 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 Solid
Vapor Formed
Comments on Deactivation Process
Ref.
PdRu/Al 2O3
RuO 4
50% loss of Ru during 100 h test in reducing automotive exhaust.
Barthol., 1975.
Me thanation of CO
Ni/Al 2O3
Ni(CO) 4
PCO > 20 kP a and T < 425 e to Ni(CO)4 formation, diffusion and decomposition on the support as large cryst allites.
Shen et al., 1981.
CO chemisorption
Ni catalysts
Ni(CO) 4
PCO > 0.4 kPa and T > ue to Ni(CO)4 formation; catalyz ed bys ulfur compounds.
Pannell et al., 1977.
Fischer-Tropsch
Ru/NaY zeolite Ru/Al2O3 , Ru/TiO2
Ru(CO) 5,
Qamar and Goodwin, 1983; Loss of Ru during FTS (H 2/CO = 1, 200250 C, 1 atm) on Ru/NaY zeolite and Ru/Al 2O3; Up to 40% loss while flowing Goodwin et al., 1986. 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.
Pt-Rh gauze
PtO 2
Sperner and Ho hmann, Loss: 0.05 Š 0.3 g Pt/ ton HNO3; 1976. recovered with Pd gauze; loss of Pt leads to surface enrichment with inactive Rh.
Pt-Rh gauze
PtO 2
Ext ensive restructuring an d loss of mechan ical stre ngth.
Automotive converter
Synthesis
Am monia oxi dation
HCN synthesis
Ru3(CO)12
Hessa nd Phillips, 1 992.
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 solidpromoterr 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 framework 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
Poisoning
Dedicated 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
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
(we need to make it steady state to find out the order of decay)
•
Mole balance
(No accumulation)
F A0 •
A B
' at W
F r A A
Solving for activity
a t
v0C A0 v0C A W r ' A
v0 C A0 C A
n W kC A
3. Determine order of decay •
•
•
First order
Log both side
da k d a dt
k d t ln k R ln
First-order decay in a CSTR
a t e
k d t
C An C A0 C A k R
v0 Wk
3. Determine order of decay •
If first order does not fit, we try second order decay •
Mole balance F A0
•
' at W
F r A A
Solving for activity for second order
da dt
k d a
2
a t
1 C A0 C A a t 1 k d t k R C An
1 1 k d t
3. Determine order of decay •
Rearrange n A
C
C A0 C A •
1 k R
Second-order decay in a CSTR
k d k R
t
3. Determine order of decay •
For packed bed: •
For first order reaction, mole balance
v0 •
dC A dW
kat C A
Solving for activity
da dt
k d a a t
at e k d t v0 C A0
n Wk C A
3. Determine order of decay •
Log both side
v0 C A0 k d t ln ln ln Wk C A
•
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 KuniiLevenspiel 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 cracking cracking
4. Catalyst decay in CSTR •
Fluidized catalytic cracking
Determine concentration, activity and conversion: •
Mole balance
dC A V v0C A0 vC A r AV dt (m3/s)(mol/m3)
•
(mol/m3s)(m3)
Rate law
r A kaC A •
Decay law (first order)
da dt
k d aC A
Remember for poisoning, there is an extra concentration term
4. Catalyst decay in CSTR •
Fluidized catalytic cracking
Determine concentration, activity and conversion: •
Stoichiometry
mol/s
v0 F A F B F C F I 0 F T v v0 F T 0 F A 0 F I 0 •
1 mol of A reacted
1 mol of B + 1 mol of C
F B F C F A0 F A F I 0 2 F A0 F A v0 F T 0 v
F I 0 F A 0 F A 0 F A F T 0
4. Catalyst decay in CSTR •
Fluidized catalytic cracking
Determine concentration, activity and conversion: •
Stoichiometry
F I 0 F A0 F A0 F A F A 0 F A 1 F T 0 F T 0 F T 0 v v0 v v0
1 y A0
1 y A0 1 C A / C T 0
where
C A v C T 0 v0
y A 0
C A 0 C T 0
4. Catalyst decay in CSTR •
Fluidized catalytic cracking
Determine concentration, activity and conversion: •
From mole balance
dC A V v0C A0 vC A r AV dt •
Substitute
•
We get
V
dC A dt
1 y A 0 v v0 1 C A / C T 0
v0C A0
v0 1 y A 0 1 C A / C T 0
C A kaC AV
4. Catalyst decay in CSTR •
Fluidized catalytic cracking
Determine concentration, activity and conversion: •
Dividing both sides by volume
1 y A0 dC A C A 0 C A kaC A dt 1 C A / C T 0 •
Therefore, change of concentration with time is:
dC A C A0 1 y A0 / 1 C A / C T 0 ka C A dt 1
4. Catalyst decay in CSTR •
Fluidized catalytic cracking
Determine concentration, activity and conversion: •
X
Conversion
F A 0 F A F A0
1 y A0 C A 1 1 v0C A 0 1 C A / C T 0 C A0 vC A
2 •
Space time
V W 50,000kg 0.02 h 3 3 v0 b v0 500kg/m 5000m /h •
Previously
da k d aC A dt
3
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: •
Concentration
•
Activity
•
conversion
dC A dt
da dt
X
5. Reactor Design for Catalyst Decay •
How reactors are designed to counteract the effect of catalyst decay: •
Slow decay •
•
Moderate decay •
•
Temperature-Time Trajectory Moving bed reactor
Rapid decay •
Straight-Through Transport Reactor
5. Reactor Design for Catalyst Decay •
•
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? •
•
Temperature-time trajectory
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 k 0 T 0 C A Initial temperature •
a t , T k T C A Higher temperature to counter decay
We neglect any change in concentrations,
k T a t , T k 0 •
At t = 0, T0
We want to see how temperature is increased with time
k 0 e
E A / R 1 / T 0 1 / T
a k 0
5. Reactor Design for Catalyst Decay •
Temperature-time trajectory
How do we know what temperature to operate at particular time? •
Solve
ln e
E A / R 1/ T 0 1/ T
a ln 1
1 1 E A ln a 0 R T 0 T 1 T
E A R
ln a
1 T 0
5. Reactor Design for Catalyst Decay •
Temperature-time trajectory
How do we know what temperature to operate at particular time? •
Decay law
da dt
k d 0 e
E d / R 1 / T 0 1 / T
a
n
E d n da ln a a k d 0 a n E d / E A k d 0 exp dt E A •
from
1 1 E A ln a R T 0 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 :
E d n ln a a k d 0 a n E k d 0 exp dt E A da
•
We get time dependence on temperature
E A nE A E d 1 1 1 exp R T 0 T t k d 0 1 n E d / E A
d
/ 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:
E d 1 1 1 exp R T 0 T t k d 0 E d / 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
Temperaturetime 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 dt
k d a
a t
2
1 1 k d t
Refer to our note:
k T a t , T k 0 k
1 1 k d t
k 0
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 k d
•
From the data given:
84344 k d 1.296 10 exp 8.314T 3
5. Reactor Design for Catalyst Decay •
Temperature-time trajectory
Work examples
150 kJ / mol 1 1 1 exp 8.314 J / mol . K 480 T t 84,344 3 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: •
•
•
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
Moving bed reactor
5. Reactor Design for Catalyst Decay •
Moving bed reactor
Modelling moving bed reactor at steady state •
Mole balance of A
F A,W F A,W W r ' A W 0 (mol/s) •
(mol/s)
Differential form
F A0 •
(g)
Reaction rate r A
'
(mol/s)
dX dW
r ' A
a t k fn C A , C B ,..., C P
1
5. Reactor Design for Catalyst Decay •
Moving bed reactor
Modelling moving bed reactor at steady state •
Decay law
•
•
Contact time
da dt t
k d a n
2
W
g
U s
g/s
Differential form
dt
dW U s
3
5. Reactor Design for Catalyst Decay •
Moving bed reactor
Modelling moving bed reactor at steady state •
Combine
2
and
•
Combine
4
da dW
into
3
k d U s
an
1
dX a W r ' A t 0 dW F A 0 Activity based on W from da/dW
4
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 F A0 a (W )( r ' A ) dW Activity based on W from da/dW, only for moving bed
1
5. Reactor Design for Catalyst Decay •
Moving bed reactor
Example of moving bed reactor •
•
•
Rate law Decay law
r ' A kC A2
da dt
2
k d a
dt
dW
Combining equations
da dW a
e
k d U s
a
k d / U s W
3
U s
5. Reactor Design for Catalyst Catalyst Decay •
Moving bed reactor
Example of moving bed reactor •
Combining
F A 0 •
dX
e
dW
k d / U s W
k C 1 X 2 A 0
Separating and integrating
F A 0
X
1 X
2 A 0 0
k C
dX
2
W
e
k d / U s W
dW
0
X k C A2 0U s 1 e k d W /U s 1 X F A 0 k d
2
5. Reactor Design for Catalyst Catalyst Decay •
Moving bed reactor
Example of moving bed reactor •
Numerical evalua evaluation tion 2 A 0
X k C U s k d W / U s 1 e 1 X F A 0 k d X 0.6 dm 0.075 mol/dm 1 X mol.g cat .min 30 mol/min 6
3 2
10,000 g cat . s 0.72 min min -1
-1 0.72 min min 22 kg 1.24 1 exp exp 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 F A z F A z z r A Ac z 0 •
Differential form
dF A r A Ac r ' A B Ac dz •
In terms of conversion and catalyst activity
B Ac r ' A t 0 a t dz F A 0
dX
1
5. Reactor Design for Catalyst Decay •
Moving bed reactor
Modelling STTR •
Residence time
t •
z
2
U p
Substituting in terms of z (i.e. a(t) = a(z/Up))
z B Ac r ' A t 0 a U p dz F A 0
dX
B z r ' A t 0 a U p dz U g C A 0
dX
F A0 U g Ac C A0
Summary
Catalyst Deactivation
How to model decay
Separable kinetic
0,1,2
Mitigation
Mechanisms of catalyst deactivation Sintering
Vapourization
Poisoning
Inactive phase
Fouling/coking
Determine the order of decay
Mechanical Try and error
Catalyst decay in CSTR
Temperature-Time trajectory
Reactor Design for catalyst decay
Moving Bed Reactor Straight-Through
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