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Catalysis and Catalysts - Activity, Selectivity and Stability Catalyst Selectivity Synthesis gas applications CH 4 CH 3 OH C n H 2n+2 C n H 2n C n H 2n+1.

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Presentation on theme: "Catalysis and Catalysts - Activity, Selectivity and Stability Catalyst Selectivity Synthesis gas applications CH 4 CH 3 OH C n H 2n+2 C n H 2n C n H 2n+1."— Presentation transcript:

1 Catalysis and Catalysts - Activity, Selectivity and Stability Catalyst Selectivity Synthesis gas applications CH 4 CH 3 OH C n H 2n+2 C n H 2n C n H 2n+1 OH (n = 1 - 6) H 2 / CO Ni Cu Cu + Co Fe, Co

2 Catalysis and Catalysts - Activity, Selectivity and Stability Examples of Catalyst Deactivation Time (h) k 1.85 (g  cm -3  h -1  %S ) S-344 (660 K) S-324 (655 K) a HDS Time (h) Methanol Yield (g  cm -3  h -1 ) p GHSV T = 70 bar = h -1 = 515 K b CO + 2 H 2 CH 3 OH Time (h) r (rel) c FCC Methanol Synthesis

3 Catalysis and Catalysts - Activity, Selectivity and Stability Catalytic Reforming (Gasoline Production) Conversion (% olefins/initial paraffins) Time (h) % W % Re % Ru % Ir Pt only p H p HC LHSV T = 1.35 bar = 0.10 bar = 1 h -1 = 745 K 2 C 12 H 26 C 12 H 24 + H 2 Catalyst Pt (0.2%) / Al 2 O 3 d Deactivation due to coke deposition Alloying quite successful

4 Catalysis and Catalysts - Activity, Selectivity and Stability Time-Scale of Deactivation Hydrocracking HDS Catalytic reforming EO Hydrogenations Aldehydes Acetylene Oxychlorination MA Formaldehyde NH 3 oxidation SCR Fat hardening Time / seconds TWC year1 day1 hour C 3 dehydrogenation FCC Most bulk processes year Batch processes hrs-days

5 Catalysis and Catalysts - Activity, Selectivity and Stability Deactivation of catalysts irreversible loss of activity Types of deactivation: Poisoning:strong chemisorption of impurity in feed (Inhibition: competitive adsorption, reversible) Fouling: secondary reactions of reactants or products, ‘coke’ formation Thermal degradation: sintering (loss of surface area), evaporation Mechanical damage Corrosion/leaching Fouling or ‘self-poisoning’ often cause of deactivation

6 Catalysis and Catalysts - Activity, Selectivity and Stability Types of Deactivation

7 Catalysis and Catalysts - Activity, Selectivity and Stability What are poisons? Surface active metal or ion High M.W. product producer Sintering accelerator Cu on Ni Ni on Pt Pb or Ca on Co 3 O 4 Pb on Fe 3 O 4 Fe on Cu Fe on Si-Al from pipes acetylenes dienes H 2 O (Al 2 O 3 ) Cl 2 (Cu) from feed or product Examples Strong chemisorber Bases H 2 S on Ni NH 3 on Si-Al ‘Toxic compounds’ (free electron pair)

8 Catalysis and Catalysts - Activity, Selectivity and Stability Time-on-Stream Amount of poisoning activity coke metals Catalytic activity IIIIII Typical Stability Profiles in Hydrotreating Initially high rate of deactivation mainly due to coke deposition Subsequently coke in equilibrium metal deposition continues

9 Catalysis and Catalysts - Activity, Selectivity and Stability Influence of Pore Size on Vanadium Deposition Hydrotreating of Heavy Feedstock

10 Catalysis and Catalysts - Activity, Selectivity and Stability Carbon Formation on Supported Metal Catalyst

11 Catalysis and Catalysts - Activity, Selectivity and Stability Carbon Filaments due to CH 4 Decompostion 873 K, Ni/CaO catalyst

12 Catalysis and Catalysts - Activity, Selectivity and Stability Sintering of Alumina upon Heating T calc (K) S BET (m 2 /g) Sintering Reduction of surface area

13 Catalysis and Catalysts - Activity, Selectivity and Stability Sintering of Supported Catalysts particles migrate coalesce monomer dispersion 2-D cluster3-D particle surface vapour interparticle transport metastable migrating stable Dependent on: carrier properties temperature composition of bulk fluid …. Predictable?

14 Catalysis and Catalysts - Activity, Selectivity and Stability T Hüttig and T Tamman Sintering is related to melting T Hüttig : defects become mobile T tamman : bulk atoms become mobile T melting T Hüttig T tamman Al 2 O Cu CuO CuCl

15 Catalysis and Catalysts - Activity, Selectivity and Stability Deactivation due to Mechanical Damage  during transport, storage, packing, use –loading in barrels, unloading, packing of reactor –in reactor: weight of column of particles –attrition in moving systems (fluid beds, moving beds)  during start-up, shut-down –temperature variations (thermal shocks) –chemical transformations sulphiding, reduction regeneration: high T, steam

16 Catalysis and Catalysts - Activity, Selectivity and Stability Corrosion / Leaching - Examples  Alumina –dissolves at pH > 12 and pH < 3, so close to these pH-values corrosion and leaching –use carbon instead at very low or very high pH  Sulphiding of oxides in the presence of H 2 S  Liquid-phase catalysis –in heterogenisation of homogeneous catalysts activity was due to the leached compounds rather than the solid phase –in solid-catalysed fat hydrogenation traces of the Ni catalyst appear in the product; with Palladium this is not the case

17 Catalysis and Catalysts - Activity, Selectivity and Stability Influence of Deactivation on Reaction Rate conversion or k obs process time ‘constant’‘variablevariable blocking of pores loss of surface area loss of active sites Fouling Sintering Poisoning initial level

18 Catalysis and Catalysts - Activity, Selectivity and Stability Deactivation - depends on?

19 Catalysis and Catalysts - Activity, Selectivity and Stability Stability too low; What to do?  Understand the cause of deactivation  Take logical measures –at catalyst level –sound reactor and process design –good engineering practice

20 Catalysis and Catalysts - Activity, Selectivity and Stability Catalyst Level  improvement of active phase or support –e.g. use titania instead of alumina in SCR  optimisation of texture –use wide-pore catalyst in HDM to prevent pore blocking  profiling of active phase –e.g. egg-yolk profile will protect active sites against poisoning and fouling if these are diffusion-limited and the reaction is not  reduce sintering by structural promoters or stabilisers  make catalyst more attrition resistant –encapsulation of active material in porous silica shell increases attrition resistance without influencing activity

21 Catalysis and Catalysts - Activity, Selectivity and Stability Tailored Reactor and Process Design Relation between time-scale of deactivation and reactor type Time scaleTypical reactor/process type yearsfixed-bed reactor; no regeneration monthsfixed-bed reactor; regeneration while reactor is off-line weeksfixed-bed reactors in swing mode, moving-bed reactor minutes - daysfluidised-bed reactor, slurry reactor; continuous regeneration secondsentrained-flow reactor with continuous regeneration

22 Catalysis and Catalysts - Activity, Selectivity and Stability Different Engineering Solutions allowing for Regeneration Propane dehydrogenation - deactivation by coke formation

23 Catalysis and Catalysts - Activity, Selectivity and Stability Good Engineering Practice  Feed purification for removal of poisons –upstream reactor –poison trap inside reactor on top of catalyst (if flow is downward) –overdesign of reactor if catalyst itself is poison trap  Optimisation of reaction conditions –use of excess steam in steam reforming decreases coke deposition –catalyst deactivation in selective hydrogenation of CCl 2 F 2 strongly increases above 500 K  operate below 510 K  Optimisation of conditions as function of time-on-stream –compensate for activity loss by increasing T with time

24 Catalysis and Catalysts - Activity, Selectivity and Stability Examples


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