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Deactivation Modelling

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  • Catalysis Engineering - Deactivation

    Deactivation of catalysts

    w Types of deactivationw Reaction modelsw Kineticsw Mass transfer phenomena pelletw Effect on selectivity

  • Catalysis Engineering - Deactivation

    Catalytic reactor design equationsteady state

    ( ) F-= rFWddx

    ii

    i hn

    stoichiometric coefficient i

    catalyst effectiveness

    rate expression

    conversion i

    space time

    Coupled with: Heat balance - T-profileMomentum balance - p-profile

    deactivation function

  • Catalysis Engineering - Deactivation

    Timescale catalyst deactivation

    10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8

    10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8

    Time / seconds

    HDSHydrocracking

    ReformingFCC

    EO

    HydrogenationsAldehydes

    AcetyleneOxychlorination

    MA

    Formaldehyde

    NH3 oxidation

    SCR

    TWC

    1 year

    Most bulk processes0.1-10 yearFat hardening

    1 day1 hour

    Batch processeshrs-days

    C3 dehydrogenation

  • Catalysis Engineering - Deactivation

    Deactivation of catalystsirreversible loss of activity

    Types of deactivation: Fouling: secondary reactions of reactants or products,

    coke formation Poisoning:strong chemisorption of impurity in feed Aging: structural changes or sintering (loss of surface

    area, high temperature) (Inhibition: competitive adsorption, reversible)

    Fouling or self-poisoning often cause of deactivation

  • Catalysis Engineering - Deactivation

    Deactivation types

    Fouling

    S

    Poisoning surface

    SSelective poisoning

    active site

    Sintering

    Cl ClClCl

    ClCl ClCl Cl

    Cl ClCl

    Redispersion&

    evaporation

    Carbonfilament growth

  • Catalysis Engineering - Deactivation

    CnHm

    HH

    H2

    C

    C

    Carbon filament growthNi/CaO

    Formation of nanotubesMay be reversiblePossible destruction of particesPreparation of carbon supports

  • Catalysis Engineering - Deactivation

    Deactivation

    conversionorkobs

    process time

    h= Tintrobs Nkk

    constant variable variable

    blocking pores loss surface area

    loss active sitesFouling

    Sintering

    Poisoning

    initial level

  • Catalysis Engineering - Deactivation

    Deactivation - depends on?

    kobs

    Sintering loss surface area gradual or catastrophic irreversible - nonregenerable

    Fouling physical blocking surface by carbon or dust usually regenerable

    Poisoning chemisorption on active sites reversible or irreversible

    Selectivity poisons Modifiers block side reactions inhibit consecutive reactions (kinetics)

    kept as secret !usually found by accident

    process conditions feed & process conditions

    feed & process conditionsfeed conditions

    heat

  • Catalysis Engineering - Deactivation

    What are poisons?

    Surface activemetal or ion

    High M.W.product producer

    Strongchemisorber

    Sinteringaccelerator

    Cu in NiNi in Pt

    Pb or Ca in Co3O4Pb in Fe3O4

    Fe on CuFe on Si-Alfrom pipes

    acetylenesdienes

    BasesH2S on NiNH3 on Si-Al

    Toxic compounds(free electron pair)

    H2O (Al2O3)Cl2 (Cu)

    from feedor product

    Examples

  • Catalysis Engineering - Deactivation

    Loss of active surface due to crystallite growth support active phase

    Local heating during preparation (calcination) reduction (fresh or passivated catalyst) reaction (hot spots, maldistribution) regeneration (burn-off coke)

    Dependency: time temperature atmosphere affects m and Ea promoters affects Ea melting point determines Ea

    Sintering...

    -=

    RTE

    kk aexp0

    mkadt

    da-= m often 2 (-6)

  • Catalysis Engineering - Deactivation

    Example sintering: n-Heptane reforming

    0 10 20 30 40 50 60 70 80

    time @780 C / h

    0

    100

    200

    300

    m2/g Pt

    0

    10

    20

    30

    40

    50

    dp Pt /nm

    0 5 10 15 20

    time @780 C / h

    0

    10

    20

    30

    40

    50

    60

    %

    n-C7

    hydrocracking

    dehydrocyclization

    isomerization

    not 1:1 relation reactivity and metal surface structure sensitive reactions edge sites / steps surface sites

  • Catalysis Engineering - Deactivation

    Mechanisms of sintering

    particles migrate coalesce

    monomer dispersion 2-D cluster 3-D cluster

    surface

    vapor

    interparticle transport

    metastable

    migrating

    stable

    q2q2

    Secondary effects

  • Catalysis Engineering - Deactivation

    Deactivation of catalysts

    Fouling or self-poisoningw Parallel reaction

    w Series self-poisoning(FCC, HDM)

    w Triangular reaction(FCC)

    Poisoningw Impurity poisoning

    (TWC)

    BA

    C

    BA

    C

    A B C

    A BP blocking

  • Catalysis Engineering - Deactivation

    Deactivation: global view

    center poisoning

    uniform or homogeneouspoisoning

    diffusion limited poisoning

    pore mouth poisoning

    Concentration profiles over reactor and in particles:poisoning versus reaction kineticstypes of reactionsmass transport phenomena

    increasing poisoning rate

    series poisoning

  • Catalysis Engineering - Deactivation

    Particle deactivationmodelling slab

    Homogenous poisoning: Fraction poisoned a

    0 L

    Apparent activity:

    Diffusion model (first order irrev.):

    Concentration profile:

    ))1(cosh()/)1(cosh(

    0

    00

    fafa

    --

    =Lx

    cc AA

    Dd c

    dxk ce

    AB A

    2

    21= -( )a

    Fr

    rpoisoned

    unpoisoned

    =

    unpoisoned Thiele modulus

  • Catalysis Engineering - Deactivation

    Catalyst effectiveness

    1st order, irreversible, slab:

    Thiele modulus:

    slabcylinder

    sphere

    0.1 1 10

    f 0.1

    1

    h

    0.0 0.2 0.4 0.6 0.8 1.0

    z*

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    C* 0.1

    1.0

    2.0

    10.0

    f = Lk

    Dv

    eff

    LV

    A ap

    p

    = =1'

    hf

    f=

    tanhf Effectiveness factor:

  • Catalysis Engineering - Deactivation

    Particle deactivationuniform poisoning

    Fraction F of initial activity:

    Limiting cases:

    1. f0 small

    2. f0 large

    ( )F -1 a

    ( )F -1 a

    ( ){ }( )0

    0

    tanh1tanh)1(

    ffaa --

    ===Lxunpoisoned

    poisoned

    J

    JF

    0.0 0.2 0.4 0.6 0.8 1.0

    Fraction poisoned a

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    Frac

    tion

    of in

    itial

    act

    ivity f0 =

    large1

    small

    antiselective

    nonselective

  • Catalysis Engineering - Deactivation

    Particle deactivationmodelling slab

    Pore-mouth poisoning (sharp interface)

    0 L

    Diffusion model (first order irrev.):

    diffusion through completelydeactivated layer aL

    followed by reaction and diffusionin the (1-a)L layer

    Fraction poisoned a

    (1-a)L

    high value Thiele modulus poison

    c0c1

    ( )L

    ccDA eff

    a10 - ( ){ } ( ){ }( ) 0

    01 1

    1tanh1fa

    faa-

    -- ckLA v=

  • Catalysis Engineering - Deactivation

    Particle deactivation pore mouth poisoning

    Fraction of initial activity:

    Limiting cases:

    1. f small

    2. f large

    0.0 0.2 0.4 0.6 0.8 1.0

    Fraction poisoned a

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    Frac

    tion

    of in

    itial

    act

    ivity f0 =

    0.01

    3

    10

    100

    ( )F -1 a

    ( ){ }( ){ }

    ( )F

    J

    Jpoisoned

    unpoisoned x L

    = =+ -

    -

    =

    11 1

    1

    0 0

    0

    0af a f

    a f

    ftanh

    tanh

    tanh

    F +

    11 0af

    selective poisoning

  • Catalysis Engineering - Deactivation

    Particle deactivationmore complex: self-poisoning

    Series poisoning: A B C

    Concentration profiles

    L 0

    CA

    CBhigher concentration B in centermore coke formation in center

    core poisoning

    Simultaneous solution diffusion/reaction equations

    Profiles will depend on reactor coordinate (e.g. HDM results J.P. Janssens)

    shell poisoninghigh Thiele moduli A and B

  • Catalysis Engineering - Deactivation

    Particle deactivationDoraiswamy & Sharma

    Parallel fouling:Low values Thiele modulus: highest residual activity

    decreases continuouslyBut after certain time residual activities for higher Thielevalues are higher

    Sometimes one might prefer diffusion limitation conditionsor catalyst activity concentration profiles (TWC)

    Series fouling:Extent of fouling increases continuously with Thiele modulus

    Catalyst with least diffusion resistance preferred

    Coke deposition effect on diffusivities generally negligible

    ln F

    ln t

    increasing f

    Becker & Wei, J. Catal. 46(1977) 372

  • Catalysis Engineering - Deactivation

    Coke formation

    Nature: often aromatic precursors that give deposits of highly condensedaromatic structures of low hydrogen content (H/C

  • Catalysis Engineering - Deactivation

    Kinetic aspects coke deposition

    Reaction kinetics:

    Examples of catalyst decay functions (see Froment & Bischoff)

    So, for coking kinetics:

    Holds for independent coking rates, how self-poisoning?

    ( )

    F

    F

    F

    F

    F

    C

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