Introduction Catalyst loss of activity with time-, i.e. “deactivation”. Catalyst have only limited lifetime Also known as Ageing catalyst 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 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 tend to 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. 1-10 year EO MA Formaldehyde Aldehydes Hydrogenations Acetylene Oxychlorination C3 dehydrogenation Fat hardening NH 3 oxidation Time / seconds 10 -1 10 0 TWC 10 1 10 2 10 3 10 4 10 5 1 hour 1 day 10 6 SCR 10 7 10 1 year 8 Batch processes hrs-days .Time-Scale of Deactivation 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Hydrocracking HDS Catalytic reforming FCC Most bulk processes 0. days fluidised-bed reactor. moving-bed reactor minutes . continuous regeneration seconds entrained-flow reactor with continuous regeneration .Tailored Reactor and Process Design Relation between time-scale of deactivation and reactor type Time scale Typical reactor/process type years fixed-bed reactor. slurry reactor. regeneration while reactor is off-line weeks fixed-bed reactors in swing mode. no regeneration months fixed-bed reactor. coking) Thermal Processes and sintering Catalyst loss via Gas Phase .Cause of Catalyst Deactivation Four causes of Catalyst Deactivation Poisoning of the catalyst Deposits on the Catalyst Surface( Fouling. Causes of Catalyst Deactivation . In heterogeneous catalysis the ‘poison’ molecules are absorbed more strongly to the catalyst surface than the reactant molecules. A B A B C D P P P P Modify the nature of active sites C D . the catalyst becomes inactive.Poisoning of a Catalyst Loss of activity due to strong chemisorptions on active sites of impurities present in the feed stream. AsH3. C2H2. Zeolite NH3.As. H2O. HCl Oxidation V2O5 As Ethylene to Ethylene Oxide Ag C2H2 .P Steam reforming Ni/Al2O3 H2S. HCl Catalytic Cracking SiO2-Al2O3. COS. heavy Metals CO hydrogenation Ni. Fe H2S.HCl Methanol Synthesis Cu H2S. Co. S.Poisons of Industrial Catalysts Process Catalyst Poison Ammonia Synthesis Fe CO. CO2. As. Na. Poisons Classification Poisons can be Classified as Selective and Non Selective Reversible or Irreversible Example : Reversible Poisoning is due to Oxygen Compounds (O2. As Ph .CO.CO2) and irreversible Poisoning is connected with non metals such as S. Cl.H2O. Any SO2 present in the exhaust fumes (trace amounts ) would poison the catalyst Once the catalytic converter has become inactive it cannot be regenerated .EXAMPLES OF POISONING OF CATALYSTS Leaded petrol cannot be used in cars fitted with a catalytic converter since lead strongly absorbs onto the surface of the catalyst Cannot use copper or nickel in a catalytic converter on a car instead of the expensive platinum or Rhodium. REASON :. Preventing Poisoning Decrease poison Content in feed E. Cu-Based Methanol Synthesis are strongly poisoned by Sulphur . hydrodesulphurization followed by H2S adsorption to remove sulphur Compounds Catalyst Formulations and Design e.g.g. rd da rd p[a(t )] dt a (t) 1.KINETICS The adjustment for the decay of the catalysts: The reactions are divided into two categories separable kinetics rA' a ( past history ) rA' ( fresh catalyst ) non separable kinetics rA' rA' ( past history . p(a)=a Second Order Decay. fresh catalyst ) Rate of Catalyst decay.0 First Order Decay . p(a) = a2 t . S )C P dC P.S B S r ' A a (t ) Poisoning Re action P S P.S kC A 1 K AC A K B C B da rd dt k d' C pm a q 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 (Ct 0 C P.S ) (C ) rP.S )C P dt p .S k (C to C P. S B . S C g B.S k d (C t 0 C P.S rP. S Main Re action A .Poisoning Impurity P in feed Stream A S A. Typical Stability Profiles in Hydrotreating Initially high rate of deactivation • mainly due to coke deposition Subsequently coke in equilibrium • metal deposition continues II activity coke metals Time-on-Stream III Amount of poisoning Catalytic activity I . Fouling 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 Coke Deposited can be measured TGA or DTA Monitoring 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. .g in catalytic reforming processes Catalyst deactivated by coking can usually be regenerated by burning off the carbon. E. Sintering of Catalyst A loss of active surface area resulting from the prolonged exposure to high gas-phase temperatures Occurs in both supported and unsupported metal catalyst Two models for crystallite growth due to sintering Atomic Migration Crystallite Migration vapour particles migrate coalesce migrating metastable stable surface . Sintering of Alumina upon Heating SBET (m2/g) Sintering Reduction of surface area Tcalc (K) . Experimental analysis of the decay rate is as: da rd p a kd T h Ci dt . as catalyst decays (catalyst activity a decreases).Catalyst Deactivation Separable kinetics rA' freshcatalyst rA" a catalyst history a t rA' t rA' t0 rA' k T f Ci Commercial reactors maintain constant production rate by increasing T (reaction rate constant increases). Change in surface structure through recrystallization or other modes of defect elimination (active site loss).Catalyst Deactivation Sintering (aging) Activity loss by loss of active surface caused by prolonged exposure to elevated gas-phase reaction temperatures. Mechanistically… Crystal agglomeration/growth. da rd kda2 dt a a 1 2 t da kd dt 0 a t 1 1 kdt . reducing internal surface area accompanied by narrowing/blocking of pore cross section. Typically a 2nd order process. Activity is expressed as f(Cc) by one of the following: a e 1Cc .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 1 1 at p p np a 1 Cc 1 A t 1 2 Cc Cc At n where A and n are fouling parameters dependent on the type of gas being processed. but the deactivation function utilized here will refer only to the deactivation chemistry. is represented by This deactivation function is based on the presumed chemical events occurring on the active sites. and can be related to various chemisorption theories.. is the concentration of sites covered with poison the fraction of sites remaining active... Wheeler used a linear relation .Kinetics of Uniform Poisoning Fundamental to his development. in the gas phase inside the catalyst. The overall observed activity changes of a catalyst pellet can also be influenced by diffusional effects. Since C. is the assumption that the catalytic site that has adsorbed poison on it is completely inactive. is not normally measured. called the deactivation or activity function. it must be expressed in terms of the poison concentration. Cp. to which these other effects can then be added. etc. If C.