L -17 Catalyst Deactivation
Prof. K.K.PantDepartment of Chemical Engineering
What is catalyst deactivation?
• Catalysts have only a limited lifetime.
• Some lose their activity after a few minutes, others last for more than ten years.
• The maintenance of catalyst activity for as
long as possible is of major economic importance in industry.
• Catalyst loss of activity with time-, i.e. “deactivation”.
• Also known as Ageing• Catalyst activity is defined as
'-r tAa t ='-r t=0A
•Catalyst deactivation is the result of number
of unwanted chemical and physical changes.
•Decline in activity is due to
- Blocking of the catalytically active sites
- Loss of catalytically active sites due to
chemical, thermal or mechanical processes
Types of Catalyst Deactivation
Three causes for deactivation:
a. Structural changes in the catalyst itself (SINTERING)
. These changes may result from a migration of
components under the influence of prolonged operation
at high temperatures, so that originally finely dispersed
crystallites tend to grow in size.
Ageing : Temperature fluctuations may cause stresses in the catalyst particle, which may then disintegrate into powder with a possible destruction of its fine structure.
b. Poisoning : Essentially irreversible chemisorption of some impurity in the feed stream, which is termed poisoning.
c. FOULING/COKING :Deposition of carbonaceous residues from a reactant, product or some intermediate, which is termed coking.
Cause of Catalyst Deactivation
• Causes of Catalyst Deactivation
– Poisoning of the catalyst
– Deposits on the Catalyst Surface
( Fouling, coking)
– Thermal Processes and sintering
– Catalyst loss via Gas Phase
Causes of Catalyst Deactivation
Poisoning of a Catalyst
• Loss of activity due to strong chemisorptions on active sites of impurities present in the feed stream.
• In heterogeneous catalysis the ‘poison’ molecules are absorbed more strongly to the catalyst surface than the reactant molecules, the catalyst becomes inactive.
• Modify the nature of active sites
Poisons Classification
• Poisons can be Classified as– Selective and Non Selective
- Reversible or Irreversible
Example : Reversible Poisoning is due to Oxygen
Compounds (O2,H2O,CO,CO2) and irreversible
Poisoning is connected with non metals such as
S, Cl, As, Ph
Tailored Reactor and Process DesignRelation between time-scale of deactivation
and reactor type
Time scale Typical reactor/process typeyears fixed-bed reactor; no regeneration required,
months fixed-bed reactor; regeneration while reactor is off-line
weeks fixed-bed reactors in swing mode, moving- bed reactor
minutes - days fluidised-bed reactor, slurry reactor; continuous regeneration
seconds entrained-flow reactor with continuous regeneration
KINETICS
' '-r =a( past history)×-r ( fresh catalyst)A A
' '-r =-r ( past history, fresh catalyst)A A
• non separable kinetics
•The adjustment for the decay of the catalysts:•The reactions are divided into two categories separable kinetics
Rate of Catalyst decay, rd
First Order Decay , p(a)=a
Second Order Decay, p(a) = a2
dar =- =p[a( t) ]d dt
Poisoning
A+S⇔A . SMain Reaction A . S ⇔B . S+C g
B . S ⇔ B+SkC' A-r =a( t)
A 1+K C +K CA A B B
Poisoning Reaction P+S →P.Sda q' mr =- =k C apd ddt
•Impurity P in feed Stream
•Assume rate of removal of gas stream onto catalyst sites is proportional to the Number of sites that are unpoisoned and conc of poison in gas phase i.e
r =k( C -C ) Cto PP.S P.S
dCP.S =r =k ( C -C ) C
d t0 PP.S P.Sdt
( C -C )t0 P.S ( C )p
Fouling/ COKING of Catalyst
• Physical (mechanical) deposition of species from fluid phase onto the catalyst surface which results in activity loss due to blocking of sites and/or pores.
• Common to reactions involving hydrocarbons.
• A carbonaceous (coke) material being deposited on the surface of a catalyst.
Decay by coking
• Coke Deposited can be measured -TGA or DTA -Monitoring the evolution of CO2 and H2O
• Position of Deposited Coke
Catalyst Deactivation• Fouling/Coking
– Deposition of carbonaceous material on catalyst surface– Catalyst activity level is a function of the amount of carbon
deposited on the catalyst surface (Cc):
where A and n are fouling parameters dependent on the type of gas being processed.
– Activity is expressed as f(Cc) by one of the following:
nc AtC
npppc tA1
1C1
1ta
c1Cea
c2C11
a
Catalyst Deactivation
• Fouling/Coking– Deposition of carbonaceous material on
catalyst surface– Catalyst activity level is a function of the
amount of carbon deposited on the catalyst surface (Cc):
1 1a t = =p p np1+C 1+A tc1a=
1+α Cc2
where A and n are fouling parameters dependent on the type of gas being processed.
-Activity is expressed as f(Cc) by one of the following:
-αCc1a=enC =Atc
