Catalyst Deactivation

June 6, 2019 | Author: Joson Chai | Category: Catalysis, Chemical Reactor, Chemical Kinetics, Chemical Reactions, Unit Processes
Share Embed Donate


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  at    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  at  k T  fnC  A , C  B ,...



The rate of catalyst decay can be expressed by:

r d   

da dt 

  pa t   k d  T hC  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 

at   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 

 kat dt 

2. Mechanisms •

Sintering

Example

Sintering (aging): •

Example: calculating conversion with catalyst decay in batch reactors •

Integrating:

 X 



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:

ae •

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

' at 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



' at 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  A0  C  A •



1 k  R



Second-order decay in a CSTR

k d  k  R



3. Determine order of decay •

For packed bed: •

For first order reaction, mole balance

v0 •

dC  A dW 

  kat C A

Solving for activity

da dt 

 k d a a t  

at   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



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 



/ 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



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



 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

View more...

Comments

Copyright ©2017 KUPDF Inc.
SUPPORT KUPDF