Umsatzprofile in Fließbett-Reaktoren

Abstract

The axial conversion profiles for hydrogenation of ethylene as model reaction, were experimentally estimated at the wall regions as well as in the centre of bench scale fluidized bed reactors. The temperature range was between 125 and 200°C. The catalyst was palladium on aluminum-silicate carriers with particle diameters of 80–100, 100–120 and 120–150 microns. The flow rate was relatively high, 3–10 times as much as the flow rate at the fluidization point. The measured overall conversions for the whole bed were smaller than the ones calculated by means of the axial mixing coefficients (evaluated from the difference of the second moments of the distributions of residence times) and the reaction velocity constants (measured in small laboratory reactor) using a simple (one-zone) diffusion model. The difference increases with the flow velocity. The axial conversion profiles at the wall and in the centre of the bed show significant differences. At the wall the profiles are fairly steep and in the centre they are rather flat. The quantitative evaluation of these profiles by separate diffusion models for both of these regions (two-zone diffusion model), made it possible, to calculate the local mean residence times and the local mixing coefficients for these regions. The local mean residence times are smaller and the local axial mixing coefficients are larger in the centre of the bed than those at the wall. The difference increases with the flow velocities.