cre i i unit 5 deactivating catalyst

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CHEMICAL REACTION ENGINEERING-IIUNIT - V

Deactivating Catalysts

B.Manikandan

B.Manikandan CHEMICAL REACTION ENGINEERING-II 1 / 24

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Introduction

Deactivating CatalystsIntroduction - Remember the Rate Equations for Catalytic Reactions !

The previous chapters assumed that the effectiveness of catalysts inpromoting reactions remains unchanged with time.Often this is not so, in which case the activity usually decreases as thecatalyst is used. Sometimes this drop is very rapid, in the order ofseconds;

Sometimes it is so slow that regeneration or replacement is needed onlyafter months of use.In any case, with deactivating catalysts regeneration or replacement isnecessary from time to time.If deactivation is rapid and caused by a deposition and a physical

blocking of the surface this process is often termed fouling.Removal of this solid is termed regeneration.Carbon deposition during catalytic cracking is a common example offouling

C 10

H 22

→ C 5H

12+ C ↓

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Introduction

Introduction

If the catalyst surface is slowly modied by chemisorption on the activesites by materials which are not easily removed, then the process isfrequently called poisoning.Restoration of activity, where possible, is called reactivation.

If the adsorption is reversible then a change in operating conditions maybe sufcient to reactivate the catalyst.If the adsorption is not reversible, then we have permanent poisoning.This may require a chemical retreatment of the surface or a completereplacement of the spent catalyst.Deactivation may also be uniform for all sites, or it may be selective, inwhich case the more active sites, those which supply most of the catalystactivity, are preferentially attacked and deactivated.


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Introduction

Introduction

We will use the term deactivation for all types of catalyst decay, both fastand slow; and we will call any material which deposits on the surface tolower its activity a poison.This chapter is a brief introduction to operations with deactivatingcatalysts.We will consider in turn:

The mechanisms of catalyst decayThe form of rate equation for catalyst decayHow to develop a suitable rate equation from experimentHow to discover the mechanism from experimentSome consequences for design


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Introduction

MECHANISMS OF CATALYST DEACTIVATION

The observed deactivation of a porous catalyst pellet depends on anumber of factors: the actual decay reactions, the presence or absenceof pore diffusion slowdown, the way poisons act on the surface, etc.We consider these in turn.Decay Reactions : Broadly speaking, decay can occur in four ways.

First, the reactant may produce a side product which deposits on anddeactivates the surface.This is called parallel deactivation.Second, the reaction product may decompose or react further to producea material which then deposits on and deactivates the surface.This is called series deactivation.Third, an impurity in the feed may deposit on and deactivate the surface.This is called side-by- side deactivation.


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Introduction

MECHANISMS OF CATALYST DEACTIVATIONDecay Reactions

If we call P the material which deposits on and deactivates the surface,we can represent these reactions as follows:Parallel deactivation:

The key difference in these three forms of decay reactions is that thedeposition depends, respectively, on the concentration of reactant,product, and some other substance in the feed.Since the distribution of these substances will vary with position in thepellet, the location of deactivation will depend on which decay reaction isoccurring.


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Introduction

MECHANISMS OF CATALYST DEACTIVATIONDecay Reactions

A fourth process for catalyst decay involves the structural modication or

sintering of the catalyst surface caused by exposure of the catalyst toextreme conditions.This type of decay is dependent on the time that the catalyst spends inthe high temperature environment, and since it is unaffected by thematerials in the gas stream we call it independent deactivation.


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Introduction

MECHANISMS OF CATALYST DEACTIVATIONDecay Reactions - Pore Diffusion

For a pellet, pore diffusion may strongly inuence the progress of catalystdecay.First consider parallel deactivation. From the previous Chapter we knowthat reactant may either be evenly distributed throughout the pellet(M T < 0.4 and Effectiveness Factor = 1) or may be found close to theexterior surface ( M T > 4 and Effectiveness Factor < 1)Thus the poison will be deposited in a like manner-uniformly for no poreresistance, and at the exterior for strong pore resistance.In the extreme of very strong diffusional resistance a thin shell at theoutside of the pellet becomes poisoned.This shell thickens with time and the deactivation front moves inward.We call this the shell model for poisoning.


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Introduction


On the other hand, consider series deactivation. In the regime of strongpore resistance the concentration of product R is higher within the pelletthan at the exterior.Since R is the source of the poison, the latter deposits in higherconcentration within the pellet interior.Hence we can have poisoning from the inside-out for series deactivation.Finally, consider side-by-side deactivation. Whatever the concentration ofreactants and products may be, the rate at which the poison from thefeed reacts with the surface determines where it deposits.

For a small poison rate constant the poison penetrates the pelletuniformly and deactivates all elements of the catalyst surface in the sameway.For a large rate constant poisoning occurs at the pellet exterior, as soonas the poison reaches the surface.


Introduction

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Introduction


The above discussion shows that the progress of deactivation may occurin different ways depending on the type of decay reaction occurring andon the value of the pore diffusion factor.

For parallel and series poisoning, the Thiele modulus for the mainreaction is the pertinent pore diffusion parameter.For side- by-side reactions, the Thiele modulus for the deactivation is theprime parameter.Nonisothermal effects within pellets may also cause variations indeactivation with location, especially when deactivation is caused bysurface modications due to high temperatures.


Introduction



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Introduction

THE RATE AND PERFORMANCE EQUATIONS


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In terms of nth-order kinetics, Arrhenius temperature dependency, andisothermal conditions, Eqn for the main reaction becomes,

where d is called the order of deactivation, m measures the concentrationdependency and E d is the activation energy or temperature dependencyof the deactivation.


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The Rate Equation from Experiment


Introduction

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The Rate Equation from ExperimentBatch Solids:Determining the Rate for Independent Deactivation

Let us illustrate how to interpret experiments from the variousbatch-solids reactors of Fig.1 and how to manipulate the basicperformance equations for these reactors by testing the t for the simplestof equation forms for independent deactivation.

This represents rst-order reaction and rst-order deactivation which, inaddition, is concentration independent.


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Here we need to develop an expression relating the changing gasconcentration with time. Using time as the one independent variablethroughout the run, the kinetic expressions become

This expression shows that even at innite time the concentration of

reactant in an irreversible reaction does not drop to zero but is governedby the rate of reaction and of deactivation, or


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The batch-batch reactor becomes a practical device when thecharacteristic times for reaction and deactivation are of the same order ofmagnitude.If they are not and if deactivation is much slower, then CA, becomes verylow and difcult to measure accurately.Fortunately, this ratio can be controlled by the expe rime nte r by prop erchoice of W/V.


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Batch-Solids, Mixed Constant Flow of FluidInserting the rate of Eq. 14a into the performance expression for mixed owgives


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Batch-Solids, Mixed Constant Flow of Fluid


Introduction

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Batch-Solids, Mixed Changing Flow of Fluid (to keepC A, xed)

For steady ow in a mixed reactor we have found


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Batch-Solids, Plug Constant Flow of Fluid


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Batch-Solids, Plug Changing Flow of Fluid (to keepC A,out xed


Introduction

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How Pore Diffusion Resistance Distorts the Kinetics ofReactions with Deactivating Catalysts


Introduction



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How Pore Diffusion Resistance Distorts the Kinetics ofReactions with Deactivating Catalysts


cre i i unit 5 deactivating catalyst

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