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Catalyst Deactivation

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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

    Temperature-time trajectory5. Reactor Design for Catalyst Decay

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    Work examples

    From the data given:

    tkTTR

    Ed

    111

    exp0

    dk

    TTRE

    t

    111exp0

    Tkd

    314.8

    84344exp10296.1 3

    Temperature-time trajectory5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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)

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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    Example of moving bed reactor

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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-

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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

    Straight-Through Transport Reactor5. Reactor Design for Catalyst Decay

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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

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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'

    Moving bed reactor5. Reactor Design for Catalyst Decay

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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


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