Kinetics of Catalyst Poisoning

This is the result of an essentially irreversible chemisorption of some impurity in the feed stream. 

This is the term used when the deactivation is the result of the deposition of carbonaceous residues from reactants, products or intermediates. 

This chapter discusses the local (i.e., up to the particle size) effects of deactivation by poisoning and by coking. The effect on the reactor scale is dealt with in Chapter 11. 

Liquid-phase hydrogenation of maleic acid (concentration 2.5x10 mol) on a platinum catalyst. Variation of relative rate of hydrogenation, rVA. with degree of coverage by sulfur. After Lamy Pitara et al. [1985]. 

Cumene dealkylation. Poisoning effect of (1) quinoline, (2) quinaldine, (3) pyrrole, (4) piperidine, (5) decyclamine, and (6) aniline. After Mills et al. [1950]. 

Metal catalysts are poisoned by a wide variety of compounds, as is evidenced by Fig. 5.2.a-l. The sensitivity of Pt-refonning catalysts and of Ni-steam reforming catalysts is well known. To protect the catalyst, guard reactors are installed in industrial operation. They contain Co-Mo-catalysts that transform the sulfur 

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Bukhavtsova, N.M. and Ostrovskii, NAi., Kinetics of catalyst poisoning during capillary condensation of reactants, Kinet. Catal., 43, 81—88, 2002. 

Many industrial processes are mass-transfer limited so that reaction kinetics are irrelevant or at least thoroughly disguised by the effects of mass and heat transfer. Questions of catalyst poisons and promoters, activation and deactivation, and heat management dominate most industrial processes. 

The hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraqumone (THEAQ) at the oxygen in the presence of a palladium supported catalyst is a key step in the industrial production of hydrogen peroxide. In industrial plants, the performance of the catalyst slowly decreases because of deactivation. Two types of catalyst poisoning are operative, a reversible one, related to the presence of water, and a permanent one, probably due to the condensation of two or more anthraquinone molecules on the palladium surface. The kinetic data obtained from laboratory runs are used to simulate the performance in industrial plants. 

A practical approach to quantitative kinetics of catalyst systems subject to decay or poisoning is to model the catalytic reaction as though no loss occurred but, unless the loss is compensated by make-up, to make the total amount of catalyst material time-dependent in accordance with the loss reaction. Often, little is known about the exact cause and mechanism of loss, and an empirical loss rate is the best one can come up with. In the rare instances in which the exact loss reaction has been identified, a more detailed modeling is possible, as a specific example will illustrate. 

The situation described by the above considerations in all probability corresponds to that responsible for the second-order kinetics of catalyst decay observed in the cracking of small molecules on most catalysts. The ions formed in such reactions are probably too small and too simple to allow a significant rate of monomolecular elimination of saturated fragments to form the unsaturated site poisoning species. Rather, pairs of adjacent small ions seem to disproportionate and produce di-ions which stick to the surface and irreversibly deactivate two sites per event... 

PlOC-4 The kinetics of self-poisoning of Pd/Al203 catalysis in the hydrogenolysis of cyclopentane is discussed in J. Catal, 54, 397 (1978). Is the effective diffu-sivity used realistic Is the decay homographic The authors claim that the deactivation of the catalyst is independent of metal dispersion. If one were to determine the specific reaction rate as a function of percent dispersion, would this information support or reject the authors hypotheses ... 

B. J. Wood, W, E. Isakson and H. Wise, Kinetic Studies of Catalyst Poisoning during Methanol Synthesis at High Pressures, Ind. Eng. Chem. Prod. Res. Dev., 19(1980) 197-204. 

The present results clearly suggest that well-characterized singlecrystal samples can serve as models of practical, working catalysts. The ultrahigh vacuum apparatus described herein will be used further to study the pressure dependence of reaction kinetics, and in particular, the systematics of catalyst poisoning in a quantitative fashion (using the molecular beam doser in conjunction with AES). 

Equation (7.17) to equation (7.19) suggest that for a PtRu alloy catalyst there are two catalyst sites that can be occupied. Carbon monoxide adsorbs onto platinum, while the hydroxide ions adsorb onto ruthenium. Therefore, to understand the rates of reaction mathematically, the coverage of platinum and ruthenium by molecules should be considered. Enback and Lindbergh [64] developed a steady-state model to simulate the reaction kinetics of CO poisoning in the presence of a PtRu/C catalyst. The reaction kinetic parameters were obtained from fitting the model predictions to the experimental measurements and are listed in Table 7.2. The following equation set was used to describe their mathematical model ... 

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