## 3 Results and Discussion
Advancing to the walls of the reactor, the axial velocity is decreasing, and the flow turns upwards near the baffles of the reactor, where we have positive values for the axial component of the velocity. The zones of liquid with higher values of the axial component of the velocity are larger in the case of the two new impellers than in the case of the single new impeller. This distribution of velocity is the reason why, in the case of using two impellers in tandem, the quantity of liquid with a low load of solid particles decreases from 20% to only 13%. The axial component of the velocity generated by the two impellers in tandem has superior values, and a distribution that generates a better mixing of the two phases for all the investigated sections, than in the case of using only one new impeller, as shown in Figure 5.  
A comparison of the radial profiles of the radial velocity is presented in Figures 6 and 7. Figure 6 shows that the areas with high positive values of the radial component of velocity increase in the case of using two impellers in tandem. For the two new impellers in tandem, Figure 7b, the radial flow increases gradually starting from the wall and a maximum value occurs near the impeller blade tip. This maximum value has the highest value in the case of the lower impeller from the tandem. Figure 7 shows that, overall, for all three investigated sections, the radial component of the velocity generated by the two new impellers in tandem has higher values and better distribution than in the case of the new single impeller. Figures 8 and 9 present the tangential velocity distribution. It can be noticed that the tangential flow for both cases is in the same direction of rotation of the impeller at all the axial levels, as in Figure 9. In both cases, the value of the tangential velocity increases and decrease steeply as going from the axis towards the tip of the blades. The maximum value of the velocity is comparable with the maximum axial velocity at all the axial levels. Figure 4: Axial velocity distribution of liquid phase, single new impeller vs. new impellers in tandem, instantaneous results. Figure 5: Axial velocity distribution of liquid phase at various axial levels, single new impeller vs. new impellers in tandem, instantaneous results: **(a)** above impeller; **(b)** impeller level; **(c)** under impeller; **(d)** axial levels position. Figure 6: Radial velocity distribution of liquid phase, single new impeller vs. new impellers in tandem, instantaneous results. Figure 8: Tangential velocity distribution of liquid phase, single new impeller vs. new impellers in tandem, instantaneous results. Figure 7: Radial velocity distribution of liquid phase at various axial levels, single new impeller vs. new impellers in tandem, instantaneous results: (**a**) above impeller; (**b**) impeller level; (**c**) under impeller. From the analysis of the velocity components distribution presented in Figures 4-9, it is clear that the solution with two new designed impellers operating in tandem generates a much more suitable hydrodynamic field for a better mixing of the liquid and solid phase. That is why only a small amount of the total volume of liquid remains without solid particles inside. Moreover, the better hydrodynamics of the tandem impellers lead to no sedimentation of the solid particles on the lower cover of the industrial reactor. The power needed by the two impellers to operate in tandem was calculated and has the value of 3.45 kW. Although the existing electrical motor generates a power of 3.6 kW, in order for the solution with two impellers in tandem to perform at its best, a replacement of the existing electrical motor is recommended. The new electrical motor should operate at the same rotational speed as the existing one and should provide a power in the range of 4.5 to 5 kW. Figure 10 presents a comparison of the distribution of the velocity vectors, corresponding to the liquid phase, which are coloured as a function of the velocity magnitude of the liquid phase.