All impeller designs were composed of three axial impellers with two blades along the perimeter (in-blade angle is 180o) mounted on a common shaft in a staggered 90o arrangement (see Fig. 1 and 2).
The geometry was provided by Melamin in the form of CAD files that contain the reactor cylindrical side walls, the spherical bottom, the heating spiral pipes near the side walls, the vertical baffles, the shaft and the impellers. The source CAD file contained 2749 parts and several geometric operations in ANSYS DesignModeler were performed, in order to arrive in a valid and appropriate for a highquality mesh, geometric model (Fig. 1).
A mesh of 2.2M cells was created, composed mainly by tet elements, except the regions near the walls (impellers, shaft, pipes, reactor walls) where inflation layers of hex and prismatic elements resolving the flow boundary layer (Fig. 2).
For the solution a ReynoldsStress Model of turbulence was employed, capable of predicting the flow swirl effect, combined with a Multiple Reference Frame algorithm to prescribe the clockwise rotating motion of the mechanical parts. The modelling of the mixing was accomplished by the injection of a liquid tracer (at the top of the reactor like in real process where the liquid additive is injected downwards from a pipe), with exactly the same fluid properties as the main fluid and by calculating its concentration in space and time. The efficiency of mixing was measured by a special parameter, Uniformity, U (Fig. 4 & 5) which takes the value of unity when the mixing is complete, i.e. the concentration of the tracer has a uniform value over the whole reactor (higher values indicating rich and lower values poor mixture).
Simulations indicated the best impeller design (Figs. 3 & 4), providing a complete mixing time of approximately 2 [min] (Fig. 5). The CFD calculations revealed for one of the designs that it exhibited the undesired situation where mixing is heavily hindered by the creation of reactor diameter size vortices that strongly recirculate the fluid locally and do not allow it to move all over the reactor height.
Melamin constructed the reactor and it is under normal operation since March 2012 and the mixing performance is practically perfect, as sampling of the products shows that they have the same quality as those of the smaller 12 [m3] reactors. Also, the predicted power consumption was found within 1% of the actual mechanical power input.