Poisoning kinetic expressions

The performance of a catalyst will often deteriorate as the catalyst becomes coated with carbon or poisoned with impurities from the process stream. This may be reflected by introducing a parameter t into the kinetic expression, r(c,T r). This parameter might be the time elapsed since the installation of the catalyst or the total volume of reactants passed since its regeneration. The question is considered by Hougen and Watson (1947), who give expressions for the dependence of r on r. The particular form does not concern us here we assume that a satisfactory expression has been developed. We want to know how the control of an adiabatic reactor should depend on r. 

It is clear that B has a poisoning effect (it acts as an inhibitor). Using power law kinetics instead of equation (3.15) will give negative order for component B, and the power law kinetic expression will be empirically valid only over a narrow range of concentrations of components A, B. 

Methanators are usually used in the ammonia production line to guard the catalyst used in the ammonia converters from the poisonous effect of carbon monoxide and carbon dioxide. The present section also includes precise modelling of different types of the industrial methanators using the dusty gas model with the new kinetic expression (Xu and Froment, 1989a) and the results are compared with the results of the simplified models (I,II). 

Previously, for 2-methyl pentane cracking we have used Eley-Rideal kinetic rate expressions to describe the inhibition and poisoning effects of species in the feed, as well as intermediate and product species. In order to utilise such kinetic expressions values for the adsorption equilibrium constants are required. A method for estimating adsorption equilibrium constants has been proposed that uses an integrated form of van t Hoff equation. The heats of adsorption have been calculated using proton affinities and heats of condensation. The entropy of adsorption has been calculated using the Sackur-Tetrode expression. 

The intrinsic rate of the poisoned catalyst could be fitted [390] to the kinetic expression ... 

A comprehensive kinetic model addressing all the findings has not been developed. Some of the reported rate equations consider the self-poisoning effect of the reactant compounds, some other that effect of ammonia, and so on so forth. The reported data is dispersed with a variety of non-comparable conditions and results. The adsorption of the poisoning compounds has been modeled assuming one or two-sites on the catalyst surface however, the applicability of these expressions also needs to be addressed to other reacting systems to verity its reliability. The model also needs of validated adsorption parameters, difficult to measure under the operating conditions. 

Catalysts The major problem with obtaining rate expressions is that most interesting processes employ catalysts to attain high rates and selectivities, and catalytic kinetics depend sensitively on the details of the catalyst chemistry. Aspects such as promoters, poisons, activation, and deactivation play crucial roles in deterrriining catalyst performance. With catalytic processes we expect complex rate expressions and fractional orders of reaction. This was the subject of Chapter 7. 

The mechanism given above places no restrictions on the source of the reversible poison. Consequently, the poisoning can be due not to an adsorption competition between the reactant and a diluent but to an adsorption competition between the reactant and one or more of the reaction products. When this occurs the products will determine the kinetics in the flow type and static systems where appreciable conversion is allowed. Under these conditions the kinetics may be expressed by equations similar to equation (6), and the order will be determined by the magnitude of constants similar to H which depend upon the various velocity constants and adsorption equilibrium constants of the heterogeneous reaction. 

The overall effectiveness factor of a catalyst pellet can be characterized by the ratio of the observed reaction rate to the rate in the absence of poisoning or external mass transfer resistance. It is expressed in the form of a power-law kinetic model for benzene hydrogenation as... 

Often deactivation is expressed in terms of time. This is not the true variable, as it could lead to incomplete predictions. More correctly, the deactivation function has to be expressed in terms of the deactivating agent the coke precursor or the poison, which means that the amount of coke (or poison) on the catalyst site should be known. The determination of a rate equation for the formation of the coke precursor is thus an integral part of the kinetic study of the process. 

In the elucidation of the kinetics of the cracking of cumene on silica-alumina catalyst, the actions of inhibitors (poisons) on the reaction were studied. These inhibitors compete with cumene for cracking sites. Theoretical analysis leads to an expression from which the equilibrium constant for adsorption of inhibitors on cracking sites can be calculated. 

In Eley-Rideal and Langmuir-Hinshelwood type of kinetic rate expressions, the effect of poisons and inhibitors on the reaction rate is accounted for by allowing part of the catalyst surface to become covered with the poisoning compound and so unavailable for desirable reactions. For example, consider the decomposition reaction of A —> B + C that occurs in the presence of an inhibitor, I. If it is assumed that the inhibitor does not participate in the reaction but that it does occupy active catalyst sites and if the surface decomposition is rate controlling then the observed rate of reaction in Langmuir-Hinshelwood terms is given by [13]... 

The implications of the work presented in this paper, are that now it should be possible to move away from lumped component and gas phase power law kinetic rate expressions for FCC to a situation where the major components are accounted for individually. In future work when the reactivity of the adsobed species has been more clearly quantified, a fundamental description of which species will poison an FCC catalyst as well as which are most likely to act as coke precursors should be possible. Thus, prediction of FCC riser and stripper kinetics without excess parameter retuning will be conceivable. 

An alternative approach to the correlation of catalyst deactivation kinetics has been widely employed for coking mechanisms. Here the precise reactions that cause deactivation may be more complex or difficult to identify than in chemical poisoning, and one may not write such precise expressions as equation (3-94) for (—r ). The practice generally has been to relate activity to the time of utilization or time-on-stream , and a number of mathematical forms have been employed. The best known is the Voorhies correlation [A. Voorhies, Jr., Ind. Eng. Chem., 37, 318 (1945)], which relates the weight of coke on the catalyst to a power of time on stream. 

The principles underlying this treatment are capable of extension to cover a greater number of reactants, inhibition by products, poisoning by adventitious impurities, dissociation of reactants upon adsorption (Section 3.2.4) and many other situations. The relevant rate expressions were collected and comprehensively evaluated many years ago by O. A. Hougen and K. M. Watson,and monographs on chemical kinetics ° often contain a fuller presentation than is thought necessary here. 

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