The hydrodynamic characteristics of the different inserts were evaluated with qualitative residence time measurements. For that, a tracer (Methylene Blue) was injected while water was pumped with a flow rate 𝑉 ̇= 100 mL min -1 through the reactor. By following the tracer injection with a camera, specific flow patterns as well as the time for decolorization could be evaluated. Frames taken at certain times for the different inserts are shown in Figure . Two different flow patterns can be distinguished: The grids and the hollow sphere inserts hardly show any dead zones but short-circuit flows can be observed. For these inserts, the tangential inlet causes a circulating flow and with this an improved convection. Contrary to this, pronounced dead zones can be observed for the spiral and the columns inserts. For the hollow sphere, the dye is only visible at the outlet after 60 s, while for the other inserts a stronger coloring can be observed at this time. The spiral insert shows the most intense coloring. This qualifies the hollow sphere insert as the one with the narrowest residence time distribution, while the broadest residence time distribution is observed for the spiral insert. Within the more open structures, i.e. the grid and the hollow sphere, the desired rotational flow patterns can be observed, which is most pronounced for the hollow sphere insert. For the spiral and the columns inserts the rotational flow patterns are disturbed by the design of the inserts. In the spiral insert the flow follows the predetermined path, only minor cross mixing over the spiral walls can be observed. Within the column insert no defined flow pattern can be observed. A broad liquid front moves from the inlet to the columns, spreads across the whole insert and eventually flows to the outlet in the middle of the insert.