Chemical Reactor Analysis and Design

1. Chemical Reactor Analysis and Design 3th Edition G.F. Froment, K.B. Bischoff†, J. De Wilde Chapter 5 Catalyst Deactivation

2. Introduction • Transport of reactants A, B, ... from the main stream to the catalyst pellet surface. • Transport of reactants in the catalyst pores. • Adsorption of reactants on the catalytic site. • Chemical reaction between adsorbed atoms or molecules. • Desorption of products R, S, .... • Transport of the products in the catalyst pores back to the particle surface. • Transport of products from the particle surface back to the main fluid stream. Steps 1, 3, 4, 5, and 7: strictly consecutive processes Steps 2 and 6: cannot be entirely separated ! Chapter 2: considers steps 3, 4, and 5 Chapter 3: other steps

3. Types of catalyst deactivation • Solid-State Transformations • Poisoning • Coking • Solid-State Transformations • Prolonged effect of temperature • Transition into the different modification (e.g. Alumina) • Presence foreign substances such as gases or impurities • (e.g. sodium ions catalyze nucleation) • Texture of catalysts often modified during operation • => shift in pore size distribution • (e.g. segregation, formation solid solution, migration) • Sintering of metals loaded on a support

4. Types of catalyst deactivation • Poisoning • Irreversible chemisorption impurity in the feed stream • => Avoidable • Coking • Deposition of carbonaceous residues from reactants, • products or intermediates • => Unavoidable

5. Solid-State Transformations • Poisoning • Coking

6. Kinetics of catalyst poisoning • Metal catalysts: poisoned by a wide variety of compounds • “guard” reactors (e.g. poisoning of a Pt hydrogenation catalyst by sulfur) Liquid-phase hydrogenation of maleic acid (concentration 2.5×10-2 mol) on a platinum catalyst. Variation of relative rate of hydrogenation, rA/r0A, with degree of coverage by sulfur. After Lama Pitara et al. [1985].

7. Kinetics of catalyst poisoning • Acid catalysts: readily poisoned by basic compounds • , also by metals in the feed (e.g. hydrotreating petroleum residuum • fractions: ppm Fe, Ni, and V in the feed => complete deactivation • catalyst after a few months of operation) Cumene dealkylation. Poisoning effect of (1) quinoline, (2) quinaldine, (3) pyrrole, (4) piperidine , (5) decyclamine, and (6) aniline. After Mills et al. [1950].

8. Kinetics of catalyst poisoning • Impurity: - could act like the reactants (or products) • - could be deposited into the solid independently • of the main chemical reaction and have no effect on it • More often: actives sites for main reaction also active for poison • chemisorption => interactions need to be considered • => deactivation function Uniform poisoning: Concentration of sites covered with poison Fraction of sites remaining active (deactivation or activity function) To be related to presumed chemical events occurring on the active sites (various chemisorption theories) & diffusional effects CPl ? => relate to CPs: reasonable approximation:

9. Kinetics of catalyst poisoning Reaction rate coefficient, krA : ~ number of available active sites Activity decreases linearly with poison concentration Uniform poisoning: Diffusion limitations: First-order reaction: with: Account for the effects of poison => substitute krA

10. Kinetics of catalyst poisoning Diffusion limitations: First-order reaction (cont.): zero poison level Consider two limiting cases: 1) Virtually no diffusion limitations to the main reaction: : better utilization of the catalyst surface as the reaction is more poisoned 2) Extreme of strong diffusion limitation: : Distorted version of the true deactivation function !

11. Kinetics of catalyst poisoning Shell-progressive poisoning: Poisoned • Poisoned shell • Unpoisoned core • Moving boundary Unpoisoned If boundary moves slowly compared to poison diffusion - or reaction rates => Pseudo steady-state assumption: => Total mass transfer resistance = external interfacial + pore diffusion + boundary chemical reaction in series

12. Kinetics of catalyst poisoning Uniform and shell-progressive poisoning:

13. Kinetics of catalyst poisoning Uniform and shell-progressive poisoning: Effect on selectivity: Selectivities in multiple reactions for three types of poisoning. From Sada and Wen [1967].

14. Solid-State Transformations • Poisoning • Coking

15. Kinetics of catalyst deactivation by coke formation: • Undesired side reactions • Carbonaceous deposits • Strongly or irreversibly adsorbed on the active sites • Examples: Many petroleum refining and petrochemical processes: • catalytic cracking of gasoil, catalytic reforming of naphtha, • and dehydrogenation of ethylbenzene and butene Coke Requires catalyst regeneration => Fluidized bed operation

16. Kinetics of catalyst deactivation by coke formation: Coke precursors: Coke formation in catalytic cracking from hydrocarbons with different basicity. From Appleby et al. [1962].

17. Kinetics of catalyst deactivation by coke formation: Empirical correlations: Voorhies [1945]: Coking in catalytic cracking of gas oil: • Widely accepted • Generalized beyond the scope of the original • Completely ignores origin of deactivating agent (coke) Fundamental rate equations: Coke: formed from the reaction mixture itself: => Must result from the reactants, the products or some intermediates => Rate of coking: must depend on the composition of the reaction mixture, the temperature, and the catalyst activity => Treat rate of coke formation simultaneously with that of main reaction

18. Kinetics of catalyst deactivation by coke formation: Coke formation: Reaction path parallel to the main reaction: Reaction path consecutive to the main: Also in more complex processes: e.g. isomerization of n-pentane on a dual function catalyst: Rate-determining step: adsorption of n-pentene:

19. Kinetics of catalyst deactivation by coke formation: Rate-determining step: adsorption of n-pentene: • Coke formation: • Starting from component situated before rate-determining step: • parallel scheme • (even if this component is not the feed component itself) • Starting from component situated after rate-determining step: • consecutive scheme • (as if the coke were formed from the reaction product)

20. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Main reaction • Sites: • Number of sites in a pore: • Particle: Coke formation Main reaction: = No diffusion limitations: Coke formation: No diffusion limitations:

21. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage only: Main reaction: Example: Assume: surface reaction rate determining Steps: follows from site balance Assume: some species C irreversibly adsorbed on the active site => competes with A and B for their occupation: inaccessible

22. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage only: Main reaction: Example: Assume: surface reaction rate determining Eliminate the inaccessible Cl : with: often empirical relation, often in terms of coke content of the catalyst, CC : Froment and Bischoff [1961, 1962]

23. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage only: Coke formation: Example: Assume: surface reaction rate determining Assume C formed from Al by reaction parallel to the main reaction and first order kinetics: with: deactivation function coke formation • Coke precursor in most cases: • not really identified • concentration on the catalyst measured • as coke by means of combustion not necessarily identical to that of main reaction, even when only one and the same type of active site is involved

24. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage only: If no limit on the available number of sites: Main reaction involves nA sites Coking reaction involves nC sites

25. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage only: Coke formation: Example: Assume: surface reaction rate determining Assume C formed from Bl (reaction product) by reaction consecutive to the main reaction and first order kinetics: Site coverage only: Intraparticle diffusion limitations: • If gradients in concentration of reactants and products: • Coke is not uniformly deposited in reactor or catalyst particle • Coke profile descending in the pore or in the reactor from the inlet onward for parallel coking • Coke profile ascending in the pore or in the reactor from the inlet onward for consecutive coking (even under isothermal conditions)

26. Kinetics of catalyst deactivation by coke formation: Site coverage only: Intraparticle diffusion limitations: C s CAs CBs A B CAs s CBs 0 R parallel coking: Al  Cl consecutive coking: Bl  Cl

27. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage and pore blockage: • Coke may grow and block pore • Sites no longer accessible: to be considered deactivated Modeling: • No preferential location site coverage and pore blockage • Probabilistic approach: Deactivation function = probability site still active probability site accessible • Structural aspects catalyst involved: • pore diameter • site density Example: Beeckman and Froment [1979]: • Assumption: Rate-determining step: Site coverage • All the coke same size—corresponding to pore diameter • Single-ended pore blocked as soon as a coke precursor is formed on a site

28. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage and pore blockage: Pore blockage => coke profiles (even if no diffusional limitations) Evolution in time Local value of deactivation function versus site number for a single-ended pore with a deterministic distribution of sites. Parameter r0St: curve 1, 0; curve 2, 0.02; curve 3, 0.50; curve 4, 1.00; curve 5, 2.00; curve 6, ∞. From Beeckman and Froment [1979].

29. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage and pore blockage: • More general theory [Beeckman and Froment, 1980]: • Two periods to be distinguished: • Time required to reach a size sufficient to block the pore => only site coverage and growth occurs • Blockage occurs => site density determines deactivation Pore-averaged deactivation function for main reaction versus time. Parameter σL, number of sites per pore. From Beeckman and Froment [1982].

30. Kinetics of catalyst deactivation by coke formation: Deactivation functions: Site coverage and pore blockage in the presence of diffusion limitations: • Parallel coking: • Concentration gradient emphasizes the effect of blockage • Coke profile not significantly different from that predicted in • the absence of diffusion limitations • Consecutive coking: • Concentration gradient and the probability of blockage opposite • Interesting coke profile obtained Site coverage in a simple-ended pore in the presence of diffusion limitations and blockage. Consecutive coking. Curve 1, 0.122 h; curve 2, 0.0356 h; curve 3, 0.0931 h; curve 4, 0.4453 h; curve 5, 8.42 h. From Beeckman and Froment [1980].

31. Kinetics of catalyst deactivation by coke formation: Kinetic studies: Recycle micro-electrobalance for catalyst deactivation studies [Beirnaert et al., 1994]. Differential operation => no coke profile in the basket

32. Kinetics of catalyst deactivation by coke formation: Kinetic studies: Tapered element oscillating microbalance reactor [Patashnick and Rupprecht (TEOM Series 1500 PMA Reaction Kinetics Analyzer) Thermo Electron Corporation. Environmental Instruments Division, East Greenbush, N.Y. 12061].

33. Kinetics of catalyst deactivation by coke formation: Kinetic studies: Kinetic analysis of main and coking reaction. [Froment, 1982].

34. Kinetics of catalyst deactivation by coke formation: Kinetic studies: Deactivation functions used in the modeling of the deactivation of the US-Y-zeolite catalyst in the catalytic cracking of vacuum gas oil [Moustafa and Froment, 2003]. Mechanistic scheme for coke formation in the catalytic cracking of vacuum gas oil [Moustafa and Froment, 2003].