Using the slightly lighter toluene as the tracer and introducing the tracer into the reactor cartridge, effects and countereffects on the axial dispersion and the Bodenstein number by means of the pump system or the jogging motor are easier detectable, since axial dispersion is already pronounced due to the small difference in density. On the other hand, by displacing the toluene tracer from the reactor cartridge with the denser pure THF a stable displacement and therefore recording of actual residence time distribution and mean residence time should be possible. Hennico et al. obtained a significant effect of viscosity on the axial dispersion coefficient at large Reynolds numbers , but since the particle Reynolds numbers of the systems studied here were low (0.11 for fine magnesium turnings, 0.19 for coarse magnesium turnings), the influence of viscosity of THF or toluene on the flow behaviour was neglected. Larger Bodenstein numbers were registered for the fine magnesium turnings (see table ). This is particularly related to the denser magnesium packing and the smaller voids in the magnesium packing (ε=0.51 for fine vs. ε=0.58 for coarse magnesium turnings). The broader particle size distribution of the coarse turnings results in an increased axial dispersion and thus lower Bo. Compared to a packed bed of fine magnesium turnings, larger voids or flow paths might occur. Since the crosssection of the flow channel has an influence on the flow velocity at a constant volumetric flow rate, lower flow velocities might be achieved with coarse magnesium turnings. According to the definition of the Bodenstein number (equation 5), smaller flow velocities and larger axial dispersions cause a reduction in the Bodenstein number. For Bo >100, only small deviations from ideal plug flow occur, but already from Bo ≥7 one can speak of flow behaviour similar to a plug flow. For the fine magnesium turnings higher Bodenstein numbers and thus a more pronounced plug flow behaviour was registered. On average, pump D achieved the highest Bodenstein numbers, followed by pump C and pump A. This is remarkable because pump D is a pump type with comparatively strong pulsation, depending on the set stroke volume and speed (RPM). Pump A also exhibits a short, albeit significant, deviation from constant volume flow when switching between the alternating syringes. Pump C, on the other hand, generates an almost pulsation-free volumetric flow rate (figure ). Standard deviations of the obtained Bodenstein numbers show the following correlations: • Higher deviations are obtained when displacing the tracer compared to the introduction of the tracer.