Mixing Process inside a Batch Reactor
Slovenia-based company Melamin d.d. Kočevje, is active in many industry sectors such as the production of paper, paints & lacquers, rubber, fire retardants, as well as in wood processing and civil engineering. In all these sectors, Melamin employs its own technology knowledge and product development procedures and one of the major unit production processes in all the above areas is batch mixing; adding the various (usually liguid, sometimes solid) substances in a vessel/reactor of (usually) cylindrical shape and mixing them by the action or one or more rotating impellers. The geometry of the impeller (blades, stages, type), the number of the blades, combined with the size of the reactor, the existence size and number of baffles around the reactor perimeter, the impeller rotation speed and the injection point and time of the added chemicals in liquid or solid form into the main liquid base, are the most important, but not the only, design parameters that influence the whole process. In all batch mixing application the engineering target is to assure a perfect (usually above 99% rate) mixing of all substances in all sections of the mixture, in the minimum possible time and with a reasonable mixing power consumption.
Melamin recently decided to migrate an existent successful batch mixing process of using 12 [m3] volume vessels into similar vessels of 30 [m3] volume. The idea was to create the large reactor in a modular manner using three 12 [m3] reactors in serial arrangement along the reactor axis, having identical impellers. The major challenge and risk of such “extrapolated” design is the creation of a flow pattern inside the reactor that will result in the “isolation” of the various modular sections of the reactor and inhibit mixing rate.
A CFD model was constructed for the simulation of the mixing process inside the batch reactor of 30 [m3] volume. The aim of the CFD work was to compare three impeller designs, in order to minimize the mixing time required, thus increasing the plant throughput. Another requested result was the required torque/power required by the motor, mounted on the top of the reactor. 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.