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Role of mass transfer and kinetics in the hydrogenation of rapeseed oil on a supported palladium catalyst

1994; Elsevier BV; Volume: 116; Issue: 1-2 Linguagem: Inglês

10.1016/0926-860x(94)80294-7

1873-3875

E. Santacesaria, Paolo Parrella, Martino Di Serio, Gianfranco Borrelli,

Surface Chemistry and Catalysis

The partial hydrogenation of a rapeseed oil in the presence of a supported palladium catalyst was studied. In the temperature range 60–120°C, at which kinetic data were collected, we observed that diffusion limitations are always operative, in particular intraparticle diffusion limitations. The contribution of the mass transfer to the reaction rates was carefully evaluated by determining the mass transfer parameters according to independent routes. In spite of the masking effect of diffusion, kinetic laws suitable to describe the hydrogenation of trienic, dienic and monoenic compounds were identified. A Langmuir-Hinshelwood kinetic equation describes well the behaviour of the unsaturated molecules in the reaction, while the hydrogen reaction order was indirectly shown to be zero. A 3/2/1 ratio between the adsorption constants of trienic, dienic and monoenic compounds was assumed, corresponding to the different probability of a double bond interaction with the palladium surface. The kinetic constants obtained by fitting experimental data gave quite similar results for the polyunsaturated fatty acids and very different for the monounsaturated ones, revealing an intervention of two distinct reaction mechanisms. In the case of polyunsaturated fatty acids, double bond migration with the formation of conjugated dienes would occur before hydrogenation. Conjugated dienes are more quickly hydrogenated than isolated or single double bonds. In a second part of the paper, a more general kinetic model is developed based on a complex scheme of simultaneous and consecutive reactions where hydrogenation reactions occur together withcis-trans isomerizations. A kinetic equation with the corresponding parameters for thecis-trans isomerization of the C18 monoenic species was determined from experimental data. Subsequently, this equation was used with the same parameters in the general kinetic model to describe the behaviour of all the othercis-trans isomerization reactions. This assumption has turned out to be reasonable. The reliability of the more general kinetic model, of the mass transfer and kinetic equations for hydrogenation processes, of the kinetic equation forcis-trans isomerizations, and of the parameters appearing in the model was verified on the basis of all the experimental data collected. A brief discussion on the factors influencing the catalyst selectivity concludes the paper.