Catalysis of Organic..

Salmi et al. 193of adsorption and the state of hydrogen on the catalyst surface has been the subject ofintensive discussions in the literature (17), but in this case we remained with thesimplest possible model, namely non-competitive adsorption of hydrogen andorganics along with molecular adsorption of hydrogen. Previous comparisons (17)have shown that this simple approach gives an adequate description of the kinetics,simultaneously having the minimum number of adjustable parameters. Sample fitsare provided by Fig. 2. A thorough comparison of experimental data with modelingresults indicated that the model for intrinsic kinetics is sufficient and can be used forsimulation of the reaction-diffusion phenomena in larger catalyst particles.Some simulation results for trilobic particles (citral hydrogenation) are providedby Fig. 2. As the figure reveals, the process is heavily diffusion-limited, not only byhydrogen diffusion but also that of the organic educts and products. The effectivinessfactor is typically within the range 0.03-1. In case of lower stirrer rates, the role ofexternal diffusion limitation becomes more profound. Furthermore, the quasistationaryconcentration fronts move inside the catalyst pellet, as the catalystdeactivation proceeds.Relative concentration10.90.80.70.60.50.40.30.20.1citralcitronellal00 50 100 150 200 250 300time (min)c mol/l0.250.20.150.10.05HydrogenCitronellalCitral00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1xFigure 2. Fit of model to experimental data and concentration profiles inside atrilobic catalyst particle.For “small” catalyst particles used in sugar hydrogenation (slurry reactors), onewould intuitively conclude that the system is safely on the kinetic regime. Thesimulation results obtained for sugar hydrogenation strongly suggest that this is notthe case. The organic molecules are large, having molecular diffusion coefficients ofthe magnitude 4.0·10 -9 m 2 /s. The porosity-to-tortuosity factor can be rather small(