ROLE OF MASS-TRANSFER AND KINETICS IN THE HYDROGENATION OF RAPESEED OIL ON A SUPPORTED PALLADIUM CATALYST

The partial hydrogenation of a rapeseed oil in the presence of a suppo rted palladium catalyst was studied. In the temperature range 60-120-d egrees-C, at which kinetic data were collected, we observed that diffu sion limitations are always operative, in particular intraparticle dif fusion limitations. The contribution of the mass transfer to the react ion rates was carefully evaluated by determining the mass transfer par ameters according to independent routes. In spite of the masking effec t of diffusion, kinetic laws suitable to describe the hydrogenation of trienic, dienic and monoenic compounds were identified. A Langmuir-Hi nshelwood kinetic equation describes well the behaviour of the unsatur ated molecules in the reaction, while the hydrogen reaction order was indirectly shown to be zero. A 3/2/1 ratio between the adsorption cons tants of trienic, dienic and monoenic compounds was assumed, correspon ding to the different probability of a double bond interaction with th e palladium surface. The kinetic constants obtained by fitting experim ental data gave quite similar results for the polyunsaturated fatty ac ids and very different for the monounsaturated ones, revealing an inte rvention of two distinct reaction mechanisms. In the case of polyunsat urated fatty acids, double bond migration with the formation of conjug ated dienes would occur before hydrogenation. Conjugated dienes are mo re quickly hydrogenated than isolated or single double bonds. In a sec ond part of the paper, a more general kinetic model is developed based on a complex scheme of simultaneous and consecutive reactions where h ydrogenation reactions occur together with cis-trans isomerizations. A kinetic equation with the corresponding parameters for the cis-trans isomerization of the C18 monoenic species was determined from experime ntal data. Subsequently, this equation was used with the same paramete rs in the general kinetic model to describe the behaviour of all the o ther cis-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 for cis-trans isomerizations, and of the par ameters appearing in the model was verified on the basis of all the ex perimental data collected. A brief discussion on the factors influenci ng the catalyst selectivity concludes the paper.