Home >Documents >L24Catalyst Deactivation

L24Catalyst Deactivation

Date post:14-Dec-2015
Category:
View:229 times
Download:2 times
Share this document with a friend
Description:
Catalyst deactivation
Transcript:
  • IntroductionCatalyst loss of activity with time-, i.e. deactivation. Catalyst have only limited lifetimeAlso known as Ageingcatalyst activity is defined as

    Catalyst deactivation is the result of number of unwanted chemical and physical changesDecline in activity is due toBlocking of the catalytically active sitesLoss of catalytically active sites due to chemical, thermal or mechanical processes

  • Types of Catalyst Deactivation

    Catalysts frequently lose an important fraction of their activity while in operation.Three causes for deactivation:

    a. Structural changes in the catalyst itself. These changes may result from a migration of components under the influence of prolonged operation at high temperatures, for example, so that originally finely dispersed crystallites tendto grow in size.

    Important temperature fluctuations may cause stresses in the catalyst particle, which may then disintegrate into powder with a possible destruction of its fine structure.

    b. Essentially irreversible chemisorption of some impurity in the feed stream,which is termed poisoning.

    c. Deposition of carbonaceous residues from a reactant, product or some intermediate, which is termed coking.

  • Time-Scale of DeactivationMost bulk processes0.1-10 yearBatch processeshrs-days

  • Tailored Reactor and Process DesignRelation between time-scale of deactivation and reactor typeTime scaleTypical reactor/process typeyearsfixed-bed reactor; no regenerationmonthsfixed-bed reactor; regeneration while reactor is off-lineweeksfixed-bed reactors in swing mode, moving-bed reactorminutes - daysfluidised-bed reactor, slurry reactor; continuous regenerationsecondsentrained-flow reactor with continuous regeneration

  • Cause of Catalyst DeactivationFour causes 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 of Industrial Catalysts

    ProcessCatalystPoisonAmmonia SynthesisFeCO, CO2, H2O, C2H2, S,PSteam reformingNi/Al2O3H2S,As,HClMethanol SynthesisCuH2S, AsH3, HCl

    Catalytic CrackingSiO2-Al2O3, ZeoliteNH3, Na, heavy MetalsCO hydrogenationNi, Co, Fe H2S, COS, As, HClOxidationV2O5AsEthylene to Ethylene OxideAgC2H2

  • 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

  • EXAMPLES OF POISONING OF CATALYSTSLeaded petrol cannot be used in cars fitted with a catalytic converter since lead strongly absorbs onto the surface of the catalystCannot use copper or nickel in a catalytic converter on a car instead of the expensive platinum or Rhodium. REASON :- Any SO2 present in the exhaust fumes (trace amounts ) would poison the catalystOnce the catalytic converter has become inactive it cannot be regenerated

  • Preventing Poisoning

    Decrease poison Content in feedE.g. hydrodesulphurization followed by H2S adsorption to remove sulphur Compounds

    Catalyst Formulations and Designe.g. Cu-Based Methanol Synthesis are strongly poisoned by Sulphur

  • KINETICSThe adjustment for the decay of the catalysts:The reactions are divided into two categoriesseparable kinetics

    non 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

  • Initially high rate of deactivation mainly due to coke depositionSubsequently coke in equilibrium metal deposition continuesTypical Stability Profiles in Hydrotreating

  • Fouling 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 poresCommon to reactions involving hydrocarbons

    A carbonaceous (coke) material being deposited on the surface of a catalyst

    Coke Deposited can be measuredTGA or DTAMonitoring the evolution of CO2 and H2O

    Position of Deposited Coke

  • Preventing of Coking

    Optimum catalyst composition

    Equilibrium must be in between rate of coke production and rate of coke removal

    Coking can be reduced by running at elevated pressure and hydrogen-rich streams.E.g in catalytic reforming processes

    Catalyst deactivated by coking can usually be regenerated by burning off the carbon.

  • Sintering of CatalystA loss of active surface area resulting from the prolonged exposure to high gas-phase temperaturesOccurs in both supported and unsupported metal catalystTwo models for crystallite growth due to sinteringAtomic MigrationCrystallite Migration

  • Sintering of Alumina upon HeatingSinteringReduction of surface area

  • Catalyst DeactivationSeparable kinetics

    Commercial reactors maintain constant production rate by increasing T (reaction rate constant increases), as catalyst decays (catalyst activity a decreases).Experimental analysis of the decay rate is as:

  • Catalyst DeactivationSintering (aging)Activity loss by loss of active surface caused by prolonged exposure to elevated gas-phase reaction temperatures.MechanisticallyCrystal agglomeration/growth, reducing internal surface area accompanied by narrowing/blocking of pore cross section.Change in surface structure through recrystallization or other modes of defect elimination (active site loss).Typically a 2nd order process;

  • 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:

  • Kinetics of Uniform PoisoningFundamental to his development, is the assumption that the catalytic site that has adsorbed poison on it is completely inactive. If C,, is the concentration of sites covered with poison the fraction of sites remaining active, called the deactivation or activity function, is represented byThis deactivation function is based on the presumed chemical events occurring on the active sites, and can be related to various chemisorption theories. The overall observed activity changes of a catalyst pellet can also be influenced by diffusional effects, etc., but the deactivation function utilized here will refer only to the deactivation chemistry, to which these other effects can then be added.Since C,, is not normally measured, it must be expressed in terms of the poison concentration, Cp, in the gas phase inside the catalyst. Wheeler used a linear relation

    *

Popular Tags:

Click here to load reader

Embed Size (px)
Recommended