Deactivation kinetics kinetic formulation

Ion-molecule association reactions and the collisional deactivation of excited ions have been the subjects of recent reviews.244-246 Several systematic studies have been performed in which the relative deactivating efficiencies of various Mf species have been determined. By applying the usual kinetic formulations for the generalized reaction scheme of equation (11.31), and assuming steady-state conditions for (AB+), an expression for the low-pressure third-order rate coefficient can be derived ... 

The present paper aims at reviewing progress in the modeling of coke formation and catalyst deactivation along the lines set in Table 1, although it stresses to a large extent the kinetic formulation, an area in which considerable progress has been achieved. 

The data required for a kinetic formulation of the deactivation of the main reaction are probably best collected in a differential reactor. Some extrapolation to zero time is required when the reaction rate of the main reaction cannot be observed at zero coke content. The procedure can be hazardous with very fast coking, of course. In their study of butene dehydrogenation, Dumez and Froment [1976] were able to take samples of the exit stream of stabilized operation of the fixed bed reactor after 2 minutes, while the observations extended over more than 30 minutes. 

The manner in which the rates of reactant consumption and of formation of each individual product varies with reactant concentrations and temperature affords information that is useful in two ways first, as providing the basis for the reactor design if the reaction is to be operated on a significant scale, and second, and more to the immediate point, to give a framework within which a reaction mechanism can be formulated. Whatever the practical difficulties of obtaining this information, and they can be considerable with microporous catalysts and those undergoing rapid deactivation, it is essential for mechanistic analysis. It cannot be stressed to strongly that no formulation of a reaction mechanism can be accepted as plausible until shown to be consistent with experimentally determined kinetics. The corollary is however equally important the mechanism cannot be deduced from kinetic measurements alone, because many different mechanisms can lead to the same kinetic expression. 

In what follows, the effect of coking on the rates of reaction is expressed quantitatively. Generally only empirical correlations have been used for this purpose. What is needed, however, for a rational design accounting for the effect of the catalyst deactivating agent on the reactor behavior, is a quantitative formulation of its rate of formation. Such a kinetic equation is by no means easy to develop. 

Beeckman and Froment [1979, 1980] and Nam and Froment [1987] formulated the deactivation by coke formation in terms of site coverage, coke growth, and pore blockage. Mechanistic kinetic equations were derived for the rate of change of the coke content with time. An illustration of the application of such equations to deactivation in a fixed bed reactor is given by Froment [1980]. The trends illustrated here on the basis of empirical models are not essentially modified. 

In addition, the reactors used for upgrading and refining heavy petroleum are complicated to model and design. The composition and properties of the heavy petroleum that is converted in reactors are such that the reaction system can involve various phases, types of catalysts, reactor configuration, reaction conditions, catalyst deactivation, and so on, making the development of a model a challenging task. Moreover, hundreds of components are present in heavy petroleum that undergo different reaction pathways and compete for the active sites of catalysts, which increases the complexity for the formulation of the kinetics and reactor models. 

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