Kinetics of catalyst deactivation by coke formation

download report

Transcript Kinetics of catalyst deactivation by coke formation

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 5
Chapter 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/r0A,
with degree of coverage by sulfur. After Lama Pitara et al. [1985].
Kinetics of catalyst poisoning
• 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)
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:
C t  C Pl

Ct
Concentration of sites
covered with poison
Fraction of sites remaining active
(deactivation or activity function)
Deactivation functions:
• Sites: 
• Number of sites in a pore: ˆ
• Particle: 
To be related to presumed chemical events occurring
on the active sites (various chemisorption theories)
& diffusional effects
CPl ? => relate to CPs: reasonable approximation:
CPl =  P C Ps
Kinetics of catalyst poisoning
Reaction rate coefficient, krA : ~ number of available active sites
k rA

C Ps 
  k rA0 1   C Ps   k rA0
 k 1   P
Ct 

0
rA
Activity decreases linearly with poison concentration
0
Uniform poisoning: rA   rA
Diffusion limitations: First-order reaction:
1
1
rA = k rA C A = [coth(3 )  ]k rA C A

3
R k rA  s
with:  
3 DeA
Account for the effects of poison => substitute krA
coth(3
rA 
0
1   C Ps ) 
1
3 0 1   C Ps
 0 1   C Ps
(1   C Ps )k rA0 C A
Kinetics of catalyst poisoning
Diffusion limitations: First-order reaction (cont.):
coth(3
rA 
0
1   C Ps ) 
1
3 0 1   C Ps
 0 1   C Ps
(1   C Ps )k rA0 C A
0
0
rA 3 1   C Ps coth(3 1   C Ps )  1

0
rA
3 0 coth(3 0 )  1
zero poison level
Consider two limiting cases:
1) Virtually no diffusion limitations to the main reaction:
rA
 1   C Ps
0
rA
2) Extreme of strong diffusion limitation:
0   :
Distorted version of the true deactivation function !
0  0:
better utilization of the
catalyst surface as the
reaction is more poisoned
rA
 1   C Ps
0
rA
Kinetics of catalyst poisoning
Shell-progressive poisoning:
• Poisoned shell
• Unpoisoned core
• Moving boundary
Poisoned
Unpoisoned
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 2 : 
"
curve 3 : 
"
curve 4 : 
curve 1 :
= 0, η(0) = 1;
= 3, η(0) = 0.67;
= 10, η(0) = 0.27;
= 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
Coke
Examples: Many petroleum refining and petrochemical processes:
catalytic cracking of gasoil, catalytic reforming of naphtha,
and dehydrogenation of ethylbenzene and butene
Requires catalyst regeneration => Fluidized bed operation
Kinetics of catalyst deactivation by coke formation:
Coke precursors:
Coke formation in catalytic
cracking from hydrocarbons
with different basicity.
From Appleby et al. [1962].
Kinetics of catalyst deactivation by coke formation:
Empirical correlations:
Voorhies [1945]: Coking in catalytic cracking of gas oil:
CC  At n
with
0.5 < n < 1
• Widely accepted
• Generalized beyond the scope of the original
• Completely ignores origin of deactivating agent (coke)
Fundamental rate equations:
Coke: formed from the reaction mixture itself:
=> 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
Kinetics of catalyst deactivation by coke formation:
Coke formation:
Reaction path parallel to the main reaction:
R
A
intermediates
C
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:
Al2O3
Pt
nC5
nC5
Pt
iC5
Rate-determining step: adsorption of n-pentene:
iC5
Kinetics of catalyst deactivation by coke formation:
Al2O3
Pt
nC5
nC5
Pt
iC5
iC5
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:
Deactivation functions:
• Sites: 
• Number of sites in a pore: ˆ
• Particle: 
N A t or CC 
Main reaction:
N A t or CC  0
No diffusion limitations:
 A = rA rA0
dCC
t or CC 
dt
Coke formation:
dCC
t or CC  0
dt
rC
No diffusion limitations:  C  0
rC
Main reaction
Coke formation
Kinetics of catalyst deactivation by coke formation:
Deactivation functions: Site coverage only: Main reaction:
Example:
A B
Assume: surface reaction rate determining
Steps:
A+l
Al
with
CAl = KACACl
B+l
with
CBl = KBCBCl
Al → Bl
Bl


C


CdC
rA  k sr rCA Al  Bl A  ksr K ACl  CA  B 

Kdt
K 
sr 



follows from site balance
Assume: some species C irreversibly adsorbed on the active site
=> competes with A and B for their occupation: Ct  Cl  CAl  CBl
 CCl
inaccessible
Kinetics of catalyst deactivation by coke formation:
Deactivation functions: Site coverage only: Main reaction:
Example:
A B
Assume: surface reaction rate determining
Eliminate the inaccessible Cl :
Ct  CCl  Cl (1  K ACA  KBCB )
C 

k sr Ct K A A  C A  B 
K 

rA 
1  K AC A  K BCB
with: φA = (Ct - CCl)/Ct
often empirical relation,
often in terms of coke content
of the catalyst, CC :
 A  exp  CC 
A 
Froment and Bischoff [1961, 1962]
1
1   CC
Kinetics of catalyst deactivation by coke formation:
Deactivation functions: Site coverage only: Coke formation:
Example:
A B
Assume: surface reaction rate determining
Assume C formed from Al by reaction parallel to the main reaction
and first order kinetics:
dCCl dC
dCClC
dCC
rC 
kkCCKCAAlC ACkl C K AC ACl
rC  rCrC k C C Al
dt
dt
dt
dt
kC0 Ct K AC C A
rC 
1  K ACA  K BCB
with: φC = (Ct - CCl)/Ct
deactivation function
coke formation
Coke precursor in most cases:
• not really identified
• concentration on the catalyst measured
as coke by means of combustion
not necessarily identical to that of main
reaction, even when only one and the
same type of active site is involved
Kinetics of catalyst deactivation by coke formation:
Deactivation functions: Site coverage only:
If no limit on the available number of sites:
Main reaction involves nA sites
Coking reaction involves nC sites
 Ct  CCl
 A  
 Ct
 Ct  CCl
C  
 Ct



nA



nC
Kinetics of catalyst deactivation by coke formation:
Deactivation functions: Site coverage only: Coke formation:
Example:
A B
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
kC0 Ct K BC CB
rC 
1  K ACA  K BCB
Site coverage only: Intraparticle diffusion limitations:
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
(even under isothermal conditions)
Kinetics of catalyst deactivation by coke formation:
Site coverage only: Intraparticle diffusion limitations:
C
s
CAs
CBs
CAs
A
B
s
CBs
0
R
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:
Deactivation function =
 A  PS
probability site accessible
probability site still active
• Structural aspects catalyst involved:
• pore diameter
• site density
Example: Beeckman and Froment [1979]:
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)
Evolution in time
Local value of deactivation function
versus site number for a singleended pore with a deterministic
distribution of sites. Parameter r0St:
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].
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 in
the 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].