Dynamics of internal diffusion during the hydrogenation of 1,5,9-cyclododecatriene on Pd/Al 2O 3

Abstract

Dynamic and pseudo-steady state diffusion–reaction models were simulated for the three-phase consecutive hydrogenation of 1,5,9-cyclododecatriene on a shell Pd/Al 2O 3 catalyst in order to examine the time evolution of concentration profiles inside the catalyst pellet: a model accounting only for the dynamics of the active layer and a model taking also into account the inert part of the catalyst were compared. In the conditions of the semibatch experiments ( T=433 K, p H 2 =1.2 MPa), all the models lead to the same bulk concentration–time curves, but the hydrocarbon concentration profiles in the pores are dependent on the model. The influence of the diffusion in the inert part of the catalyst on the bulk concentrations becomes nonnegligible only when the external liquid volume (out of the catalyst) is reduced. The transient evolution of the concentration profiles in the pores show that hydrogen concentration reaches its steady state within a few seconds, while the evolution of the organic concentration profiles is slower. Furthermore, the reaction rate has been found to be only affected by the hydrogen diffusion. The diffusion of organics can control the reaction rate only for low values of organic concentration and higher pressure in hydrogen.