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Catalyst Deactivation

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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 ac

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