Chemical Reactor Analysis and Design

3th Edition

G.F. Froment, K.B. Bischoff†, J. De Wilde

Chapter 5

Catalyst Deactivation

Introduction

1. Transport of reactants A, B, ... from the main stream to the catalyst pellet surface.

2. Transport of reactants in the catalyst pores.3. Adsorption of reactants on the catalytic site.4. Chemical reaction between adsorbed

atoms or molecules.5. Desorption of products R, S, ....6. Transport of the products in the catalyst

pores back to the particle surface.7. Transport of products from the particle

surface back to the main fluid stream.

Steps 1, 3, 4, 5, and 7: strictly consecutive processes Steps 2 and 6: cannot be entirely separated !

Chapter 2: considers steps 3, 4, and 5Chapter 3: other steps

Types of catalyst deactivation

• Solid-State Transformations • Poisoning • Coking

Solid-State Transformations• Prolonged effect of temperature Transition into the different modification (e.g. Alumina)• Presence foreign substances such as gases or impurities (e.g. sodium ions catalyze nucleation)• Texture of catalysts often modified during operation => shift in pore size distribution (e.g. segregation, formation solid solution, migration)• Sintering of metals loaded on a support

Types of catalyst deactivation

Poisoning• Irreversible chemisorption impurity in the feed stream• => Avoidable

Coking• Deposition of carbonaceous residues from reactants, products or intermediates• => Unavoidable

• Solid-State Transformations • Poisoning • Coking

Kinetics of catalyst poisoning

• Metal catalysts: poisoned by a wide variety of compounds “guard” reactors(e.g. poisoning of a Pt hydrogenation catalyst by sulfur)

Liquid-phase hydrogenation of maleic acid (concentration 2.5×10-2 mol) on a platinum catalyst. Variation of relative rate of hydrogenation, rA/r0

A, with degree of coverage by sulfur. After Lama Pitara et al. [1985].

• Acid catalysts: readily poisoned by basic compounds , also by metals in the feed (e.g. hydrotreating petroleum residuum fractions: ppm Fe, Ni, and V in the feed => complete deactivation catalyst after a few months of operation)

Kinetics of catalyst poisoning

Cumene dealkylation.Poisoning effect of (1) quinoline, (2) quinaldine, (3) pyrrole, (4) piperidine , (5) decyclamine, and (6) aniline. After Mills et al. [1950].

Kinetics of catalyst poisoning

• Impurity: - could act like the reactants (or products) - could be deposited into the solid independently of the main chemical reaction and have no effect on it• More often: actives sites for main reaction also active for poison chemisorption => interactions need to be considered => deactivation function

Uniform poisoning:

t

Plt

C

C

CConcentration of sites covered with poison

Fraction of sites remaining active(deactivation or activity function)

To be related to presumed chemical events occurring on the active sites (various chemisorption theories)& diffusional effects

CPl ? => relate to CPs:

CPl = PsPC

reasonable approximation:

Deactivation functions:

• Sites:• Number of sites in a pore:• Particle:

Kinetics of catalyst poisoning

Reaction rate coefficient, krA : ~ number of available active sites

000 11 rAPsrAt

PsPrArA kCk

C

Ckk

Activity decreases linearly with poison concentration

Diffusion limitations: First-order reaction:

rA = ArACk = ArACk]3

1)3[coth(

1

with:

eA

srA

D

kR

3

Account for the effects of poison => substitute krA

ArAPs

Ps

Ps

Ps

A CkCC

CC

r 0

0

0

0

)1(1

13

1)13coth(

Uniform poisoning:0AA rr

Kinetics of catalyst poisoning

Diffusion limitations: First-order reaction (cont.):

ArAPs

Ps

Ps

Ps

A CkCC

CC

r 0

0

0

0

)1(1

13

1)13coth(

1)3coth(3

1)13coth(1300

00

0

PsPs

A

ACC

r

r

zero poison level

Consider two limiting cases:

1) Virtually no diffusion limitations to the main reaction:0 0 :

PsA

A Cr

r 1

0

2) Extreme of strong diffusion limitation: 0 :Ps

A

A Cr

r 1

0

Distorted version of the true deactivation function !

better utilization of the catalyst surface as the reaction is more poisoned

Kinetics of catalyst poisoning

Shell-progressive poisoning:

• Poisoned shell• Unpoisoned core• Moving boundary

Unpoisoned

Poisoned

If boundary moves slowly compared to poison diffusion - or reaction rates => Pseudo steady-state assumption: => Total mass transfer resistance = external interfacial + pore diffusion + boundary chemical reaction in series

Kinetics of catalyst poisoning

Uniform and shell-progressive poisoning:

rA/r0A in terms of amount of poison for uniform [Eq. (5.2.2-10)] and shell-progressive

[Eq. (5.2.3-13)] models. Sh'A → ∞.

---- : uniform poisoning with diffusional limitations on main reaction;

—— : shell progressive;

curve 1 : " = 0, η(0) = 1;

curve 2 : " = 3, η(0) = 0.67;

curve 3 : " = 10, η(0) = 0.27;

curve 4 : " = 100, η(0) = 0.03.

Kinetics of catalyst poisoning

Uniform and shell-progressive poisoning: Effect on selectivity:

Selectivities in multiple reactions for three types of poisoning. From Sada and Wen [1967].

• Solid-State Transformations • Poisoning • Coking

Kinetics of catalyst deactivation by coke formation:

Undesired side reactions Carbonaceous deposits Strongly or irreversibly adsorbed on the active sites

Examples: Many petroleum refining and petrochemical processes: catalytic cracking of gasoil, catalytic reforming of naphtha, and dehydrogenation of ethylbenzene and butene

Coke

Requires catalyst regeneration => Fluidized bed operation

Kinetics of catalyst deactivation by coke formation:

Coke formation in catalytic cracking from hydrocarbons with different basicity.From Appleby et al. [1962].

Coke precursors:

Kinetics of catalyst deactivation by coke formation:

Empirical correlations:

Voorhies [1945]: Coking in catalytic cracking of gas oil:

nC tAC with 0.5 < n < 1

• Widely accepted• Generalized beyond the scope of the original• Completely ignores origin of deactivating agent (coke)

=> Must result from the reactants, the products or some intermediates

=> Rate of coking: must depend on the composition of the reaction mixture, the temperature, and the catalyst activity

=> Treat rate of coke formation simultaneously with that of main reaction

Coke: formed from the reaction mixture itself:

Fundamental rate equations:

Kinetics of catalyst deactivation by coke formation:

Coke formation:

Reaction path parallel to the main reaction:

A R

C

intermediates

Reaction path consecutive to the main:

A → R—intermediates → C

Also in more complex processes:e.g. isomerization of n-pentane on a dual function catalyst:

nC5

PtnC5

Al2O3

iC5

PtiC5

Rate-determining step: adsorption of n-pentene:

Coke formation:

• Starting from component situated before rate-determining step: parallel scheme (even if this component is not the feed component itself)

• Starting from component situated after rate-determining step: consecutive scheme (as if the coke were formed from the reaction product)

Kinetics of catalyst deactivation by coke formation:

nC5

PtnC5

Al2O3

iC5

PtiC5

Rate-determining step: adsorption of n-pentene:

Kinetics of catalyst deactivation by coke formation:

Deactivation functions:

0AA rr

0CA

CA

CortN

CortNMain reaction:

No diffusion limitations: A =

Coke formation:

0CC

CC

Cortdt

dC

Cortdt

dC

No diffusion limitations:0

C

CC r

r

• Sites:• Number of sites in a pore:• Particle:

Main reaction

Coke formation

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage only: Main reaction:

A B Example: Assume: surface reaction rate determining

Steps: A + l Al with CAl = KACACl

Al → Bl

Bl B + l with CBl = KBCBCl

sr

BlAlsrA K

CCkr

follows from site balance

Assume: some species C irreversibly adsorbed on the active site=> competes with A and B for their occupation: ClBlAllt CCCCC

inaccessible

)1( BBAAlClt CKCKCCC

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage only: Main reaction:

A B Example: Assume: surface reaction rate determining

Eliminate the inaccessible Cl :

BBAA

BAAAtsr

A CKCKK

CCKCk

r

1

φA = (Ct - CCl)/Ct with:

often empirical relation,often in terms of coke contentof the catalyst, CC

CA C exp

CA C

1

1

Froment and Bischoff [1961, 1962]

:

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage only: Coke formation:

A B Example: Assume: surface reaction rate determining

Assume C formed from Al by reaction parallel to the main reaction and first order kinetics:

BBAA

ACAtCC CKCK

CKCkr

1

0 φC = (Ct - CCl)/Ct with:

deactivation function coke formation

not necessarily identical to that of main reaction, even when only one and the same type of active site is involved

Coke precursor in most cases:• not really identified• concentration on the catalyst measured as coke by means of combustion

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage only:

Main reaction involves nA sites

An

t

CltA C

CC

Coking reaction involves nC sites

Cn

t

CltC C

CC

If no limit on the available number of sites:

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage only: Coke formation:

A B Example: Assume: surface reaction rate determining

Assume C formed from Bl (reaction product) by reaction consecutive to the main reaction and first order kinetics:

Bl Cl

BBAA

BCBtCC CKCK

CKCkr

1

0

If gradients in concentration of reactants and products: Coke is not uniformly deposited in reactor or catalyst particle Coke profile descending in the pore or in the reactor from the inlet onward for parallel coking Coke profile ascending in the pore or in the reactor from the inlet onward for consecutive coking

Site coverage only: Intraparticle diffusion limitations:

(even under isothermal conditions)

Kinetics of catalyst deactivation by coke formation:

Site coverage only: Intraparticle diffusion limitations:

R

C

0

CAss

CBss

CAs

CBsA

B

parallel coking: Al Cl

consecutive coking: Bl Cl

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage and pore blockage:

• Coke may grow and block pore• Sites no longer accessible: to be considered deactivated

Modeling:

• No preferential location site coverage and pore blockage• Probabilistic approach:

Example: Beeckman and Froment [1979]:

PSA probability site still active

probability site accessible

Deactivation function =

• Structural aspects catalyst involved:• pore diameter• site density

Assumption: Rate-determining step: Site coverage All the coke same size—corresponding to pore diameter Single-ended pore blocked as soon as a coke precursor is formed on a site

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage and pore blockage:

Pore blockage => coke profiles (even if no diffusional limitations)

Local value of deactivation function versus site number for a single-ended pore with a deterministic distribution of sites. Parameter r0

St: curve 1, 0; curve 2, 0.02; curve 3, 0.50; curve 4, 1.00; curve 5, 2.00; curve 6, ∞.From Beeckman and Froment [1979].

Evolution in time

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage and pore blockage:

More general theory [Beeckman and Froment, 1980]:• Two periods to be distinguished:

1) Time required to reach a size sufficient to block the pore => only site coverage and growth occurs

2) Blockage occurs => site density determines deactivation

Pore-averaged deactivation function for main reaction versus time. Parameter σL, number of sites per pore.From Beeckman and Froment [1982].

Kinetics of catalyst deactivation by coke formation:

Deactivation functions: Site coverage and pore blockage inthe presence of diffusion limitations:

Parallel coking:• Concentration gradient emphasizes the effect of blockage• Coke profile not significantly different from that predicted in the absence of diffusion limitations

Consecutive coking:• Concentration gradient and the probability of blockage opposite• Interesting coke profile obtained

Site coverage in a simple-ended pore in the presence of diffusion limitations and blockage. Consecutive coking.Curve 1, 0.122 h; curve 2, 0.0356 h; curve 3, 0.0931 h; curve 4, 0.4453 h; curve 5, 8.42 h.From Beeckman and Froment [1980].

Kinetics of catalyst deactivation by coke formation:

Kinetic studies:

Recycle micro-electrobalance for catalyst deactivation studies [Beirnaert et al., 1994].

Differential operation => no coke profile in the basket

Kinetics of catalyst deactivation by coke formation:

Kinetic studies:

Tapered element oscillating microbalance reactor [Patashnick and Rupprecht (TEOM Series 1500 PMA Reaction Kinetics Analyzer) Thermo Electron Corporation. Environmental Instruments Division, East Greenbush, N.Y. 12061].

Kinetics of catalyst deactivation by coke formation:

Kinetic studies:

Kinetic analysis of main and coking reaction. [Froment, 1982].

Kinetics of catalyst deactivation by coke formation:

Kinetic studies:

Deactivation functions used in the modeling of the deactivation of the US-Y-zeolite catalyst in the catalytic cracking of vacuum gas oil [Moustafa and Froment, 2003].

Mechanistic scheme for coke formation in the catalytic cracking of vacuum gas oil [Moustafa and Froment, 2003].