A method for the study of performance of a single spherical particle with nonuniform catalytic activity

Abstract

In recent years, there has been increasing interest in porous catalysts which contain nonuniform activity. For example, Kasaoka and Sakata ( 1968), Villadsen ( 1976), Corbett and Luss (1974), Shadman-Yazdi and Petersen (1972), and Becker and Wei (1977u, b) have recently developed theoretical models to describe the performance of catalysts with nonuniform activity. Some catalysts contain nonuniform activity as a result of preparation methods employed, while others develop this feature because of poisoning. The deliberate inclusion of nonuniform activity in porous catalysts provides a means of tailoring a catalyst to a specific process to improve activity or selectivity, or to reduce the rate of poisoning.This note develops the theoretical basis for an experimental study of the effect of catalyst activity distribution on selectivity. The reaction examined is the deuterium exchange with neopentane on supported platinum, It is a series reaction with many steps and resembles industrially important reactions such as the isomerization of certain olefins (see Haag and Pines, 1960) and the hydrogenolysis of polynuclear aromatics (see Sullivan et al., 1964;Germain, 1969). It was used as a test reaction in previous experimental studies of laboratory flow reactors (Zahner, 1970) and diffusion effects in porous catalysts (Dwyer et al., 1968;Ernst and Wei, 1975) because of several desirable features: negligible heat of reaction, single step surface exchange mechanism, and first order in neopentane , THEORYIn a mass spectrometer, neopentane undergoes fragmentation such that the major peak corresponds to a t-butyl fragment, and the parent neopentane peak is negligible. Kemball (1954) has provided a means of following the course of the deuterium-neopentane exchange reaction via the deuterium exchanged t-butyl fragments. Thus, there are nine deuterated products that must be monitored during the course of the reaction.For brevity, the model will be developed for the singlestep exchange of methane: The model can be readily extended to the reaction of neopentane. The following assumptions will be made: a large excess of Dz is used so that q is approximately equal to 1, and all reverse reactions are insignificant; the reaction occurs within a spherical catalyst particle containing the activity distributionwhere k, is the catalytic activity at the particle surface

Effects of radially nonuniform distributions of catalytic activity on performance of spherical catalyst pellets

“…Indeed, this has been demonstrated to be the case by Shadman-Yazdi and Petersen (1972) and by Corbett and Luss (1974) who modeled the performance of spherical catalyst particles under isothermal conditions for the series reactions: A + B + C. They showed that selectivity is a strong function of the distribution of activity within the catalyst particle. Similar conclusions were reached by Ernst and Dougherty (1978) who modeled the reaction of deuterium exchange with neopentane over nonuniformly activated catalyst pellets.…”

Optimal catalyst activity profiles in pellets. 1. The case of negligible external mass transfer resistance

“…This recognition has been both theoretical (cf. Kasaoka and Sakata, 1968; Minhas and Carberry, 1969; Shadman-Yazdi and Petersen, 1972;Corbett and Luss, 1974; Smith and Carberry, 1975;Smith, 1976;Villadsen, 1976;Becker and Wei, 1977;Nystrom, 1978;Ernst and Daugherty, 1978) and experimental (cf. Michalko, 1966;Summers and Hegedus, 1978;Hegedus et al, 1979).…”

A chemical probe for catalyst nonuniformity

“…The deuterium-neopentane exchange (DNE) reaction was employed in the study (Dwyer et al, 1968;Ernst and Daugherty, 1978;Ernst and Wei, 1975). The theory is based upon the work of Wei and Prater (1962) and Wei (1 962a, b).…”

“…The theory is based upon the work of Wei and Prater (1962) and Wei (1 962a, b). Ernst and Daugherty (1978) showed that the highly coupled system of rate equations which describe the behavior of Dexchanged hydride species such as neopentane can be uncoupled if the following transformation is made. Wang and Varma (1980) solved the intraparticle diffusionreaction problem for a catalyst pellet that has an activity profile described by the equation k ( x ) = m z ( 1 -x )~.…”