For the mixed turnings, the difference in Bo between the introduction and displacement of the tracer is much more pronounced (table ). For the introduction of toluene, the Bodenstein numbers are in a range comparable to the coarse magnesium turnings. In contrast, when pure THF is used (displacement), the measured Bo are more like those of fine magnesium turnings alone. Standard deviations are also in the range of fine respectively coarse turnings, depending on the introduction or the displacement of the tracer. Probably, the slight difference in density of the solvents is decisive for the differences in the results obtained. The lighter toluene rises through the THF due to buoyancy, leading to shorter residence times but higher axial dispersion. When the toluene tracer is displaced from the magnesium bed by using pure THF, the THF displaces the toluene, only driven by the pump pressure. The axial dispersion is less pronounced (larger Bo) and larger residence times are observed. The broad particle size distribution of the mixed magnesium turnings leads to even further axial dispersion. For these reasons, wide particle size distributions (mixed turnings but also coarse turnings alone) cause deviations of the RTD between the two different ways of generating the tracer signal. The broader the particle size distribution, the more pronounced this effect is. The displacement of the tracer toluene by pure THF with pump D leads to residence times above the hydrodynamic residence time independent of the bed composition (table ). The average residence time of one component can be higher than the ideal hydrodynamic residence time calculated on the basis of reactor volume and (theoretical) volumetric flow rate, if either the real reactor volume is larger or the real volumetric flow rate is smaller than assumed, or if there is more backmixing or short-circuiting flow of one component.