OPTIMAL CATALYST ACTIVITY PROFILES IN PELLETS: SINGLE PELLET THEORY AND EXPERIMENTS

The interaction of chemical and transport rate processes occurring within porous pellets, has led to a variety of engineering problems dealing with the design of catalysts for optimum performance. The optimization of active catalyst distribution within the support pellet can significantly enhance catalyst performance. Recently, for some specific cases, Varma and co-workers have determined the optimal catalyst activity profile which maximizes the effectiveness factor. In this dissertation, the theory of optimal catalyst activity distributions is generalized, and verified experimentally for a specific case.; Firstly, the generalization of theory of optimal catalyst activity profiles is carried out to include the most general case of a nonisothermal catalyst pellet, where an irreversible reaction takes place in the presence of external transport resistances. It is found again that the optimal catalyst activity profile is given by a Dirac-delta function; i.e., all the active catalyst should be deposited at a specific position within the pellet. The effects of physicochemical parameters on the optimal location are analyzed in detail. The possibility of enhancing catalytic effectiveness is also discussed.; Secondly, an effort was made to experimentally verify the theory of optimal catalyst activity profiles, by using CO oxidation on supported platinum catalyst as a test reaction. The study involved preparing a catalyst in which platinum was deposited in form of an annular ring, and then testing its performance under diffusion-influenced reaction conditions. The experimental results were matched satisfactorily with predictions of the diffusion-reaction model. It was found that catalyst performance with internally deposited platinum was sensitive to small variations in the activity profile.; Thirdly, the case of an isothermal finite cylindrical catalyst pellet, with finite external mass transfer resistance is considered, where the reaction-diffusion problem involves two spatial dimensions. It is shown that, for given physico-chemical parameters, unlike the case of one-dimensional geometries, there are infinitely many optimal catalyst activity profiles. For a selected family of activity profiles, the computations of optimal catalyst location are illustrated, for a bimolecular Langmuir-Hinshelwood reaction rate expression. It is also observed that the numerical effort can be substantially reduced, by viewing the problem from fundamental principles of mass transfer.