Dynamics of internal diffusion during the hydrogenation of 1,5,9-cyclododecatriene on Pd/Al2O3

Dynamic and pseudo-steady state diffusion-reaction models were simulated fo r the three-phase consecutive hydrogenation of 1,5,9-cyclododecatriene on a shell Pd/Al2O3 catalyst in order to examine the time evolution of concentr ation 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, (PH2),= 1.2 MPa) , all the models lead to the same bulk concentration-time curves, but the h ydrocarbon concentration profiles in the pores are dependent on the model. The influence of the diffusion in the inert part of the catalyst on the bul k 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 tha t hydrogen concentration reaches its steady state within a few seconds, whi le the evolution of the organic concentration profiles is slower. Furthermore, the reaction rate has been found to be only affected by the hy drogen diffusion. The diffusion of organics can control the reaction rate o nly for low values of organic concentration and higher pressure in hydrogen . (C) 1999 Elsevier Science B.V. All rights reserved.