L -17 Catalyst DeactivationProf. K.K.PantDepartment of Chemical EngineeringIIT Delhi.kkpant@chemical.iitd.ac.in

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 AgeingCatalyst activity is defined as

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 DeactivationCauses of Catalyst DeactivationPoisoning of the catalystDeposits on the Catalyst Surface ( Fouling, coking)Thermal Processes and sinteringCatalyst loss via Gas Phase

Causes of Catalyst Deactivation

Poisoning of a CatalystLoss 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 ClassificationPoisons can be Classified asSelective 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 typeTime scaleTypical reactor/process typeyearsfixed-bed reactor; no regeneration required,

monthsfixed-bed reactor; regeneration while reactor is off-line

weeksfixed-bed reactors in swing mode, moving- bed reactor

minutes - days fluidised-bed reactor, slurry reactor; continuous regeneration

secondsentrained-flow reactor with continuous regeneration

KINETICS

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)=aSecond Order Decay, p(a) = a2

PoisoningImpurity 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

Fouling/ COKING of CatalystPhysical (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 DeactivationFouling/CokingDeposition of carbonaceous material on catalyst surfaceCatalyst 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:

Catalyst DeactivationFouling/CokingDeposition of carbonaceous material on catalyst surfaceCatalyst 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:

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