Shallow beds are not plug flow reactors – Analysis of kinetic data in the presence of axial dispersion effects

Abstract

Laboratory-scale shallow beds filled with catalyst particles are frequently used in catalyst screening, activity tests and kinetic measurements. Often the experimental data are interpreted with plug flow or differential reactor models. Residence time measurements have shown large deviations from ideal plug flow behavior in shallow beds, indication the presence of axial dispersion effects, i.e. is backmixing and diffusion in the bed. The behavior of laboratory-scale bed reactors was investigated by theoretical considerations of the axial dispersion model and numerical simulations in the Damköhler-Péclet space. The results revealed that large errors in kinetic parameters can appear when the dispersion model is replaced by the plug flow model. This key effect was quantified using the ratio (F) of two Damköhler numbers: the real one, and the one wrongly estimated on experimental data affected by dispersion problems and interpreted with ideal plug-flow approach. Thus, the effect of back-mixing is evaluated varying a single parameter of the kinetic model, namely the rate constant. The conclusions are valid for power-law, Langmuir-Hinshelwood and Eley-Rideal kinetics.