engleski

Experimental investigation of impeller position effect on nucleation kinetics of borax decahydrate in dual impeller cooling crystallizer

The production process of crystalline products is governed by two kinetic steps - nucleation and crystal growth. Nucleation kinetics is highly correlated to the width of the metastable zone and with regard to product characteristics it usually has a strong influence on crystal growth. Considering the fact that batch crystallization is usually carried out in suspension, mixing can affect the width of the metastability region, nucleation kinetics and, finally, the properties of the product which include crystal size distribution, purity, morphology, etc. Thus, for a complete understanding of the crystallization process, besides a rigorous description of the crystallization phenomena, the hydrodynamics of the system needs to be analyzed in detail [1, 2]. The majority of the researches dealing with the influence of mixing on crystallization are conducted in a single impeller crystallizer, which is common in laboratory scale investigations, but on an industrial scale, multiple impeller crystallizers are usually employed. It is well known that hydrodynamic conditions in stirred reactor depend on the impeller and crystallizer geometry. The flow fields in these systems are very complex, with vortical structures that are not always well defined and may significantly change with impeller position. The flow complexity greatly increases if a configuration with two impellers is used, especially when applied impellers are of the different type, each having its own flow characteristics. The aim of this work was to investigate the influence of impeller position on the nucleation mechanism and nucleation rate of disodium tetraborate decahydrate (borax) in a dual impeller batch cooling crystallizer. Examinations were performed in a crystallizer of 15 dm3 with a constant impeller to tank diameter ratio (D/dT) of 0.33 and an aspect ratio (H/dT) of 1.3 which required the addition of a second impeller. In this research, two different types of impellers were mounted on the same shaft: axial down pumping pitched blade turbine (PBT) was used as a lower impeller and radial straight blade turbine (SBT) was used as an upper impeller. Mixing was performed at the speed which ensured the state of complete suspension in the system (N=NJS). In order to examine different impeller positions, experiments were carried out at different impeller off-bottom clearances - c/D, in the range of 0.2 to1.5, as well as at different spacing between the impellers - s/D, in the range 0 to1.5. Crystallization was carried out by a controlled cooling of the solution, which was saturated at 30°C, at the rate of 6 °C h-1. Concentration of the solution was measured in line by the means of Na ion- selective electrode, while the onset of nucleation, i.e. the metastable zone width, was detected from the change in concentration values measured over process time. In order to gain a complete insight into the intensity and structure of overall fluid flow in the crystallizer the mixing time was determined by potentiometric method and the photographs of the flows were taken. For all examined positions, during the experiments, torque measurements were performed and power consumption per unit mass was calculated. From the obtained results of metastable zone width, at all examined mixing conditions according to the Mersmann´s nucleation criterion, nucleation in the crystallizer took place by a heterogeneous nucleation mechanism. The values of heterogeneous nucleation rate were calculated by the expression proposed by Mersmann [3]. With regards to the impeller position in the crystallizer, results show that when impeller off-bottom clearance increases in examined range, nucleation rate increases almost by three times. Although the nucleation rate changes significantly, it is not a direct consequence of the impeller clearance but rather of the value of Reynolds number (ReJS). As stated earlier, mother liquor mixing was performed at just suspended impeller speed, which drastically increased with an impeller off-bottom clearance due to the change in the overall fluid flow pattern. A higher value of NJS, resulted in an increase of turbulence in the system, what reflected on the metastable zone width and finally on nucleation rate. When mixing was performed at different impeller spacing, a regular correlation between the values of the nucleation rate and impeller spacing was not found. The lowest value of nucleation rate was obtained at the lowest and the highest examined value of impeller spacing. At minimum impeller spacing, impellers acted as a single impeller but with a changed geometry, with twice the width of impeller blade. On the other hand, when impellers were set apart at position s/D=1.5, completely different hydrodynamic conditions, when compared to the ones at s/D=0, resulted in almost the same value of nucleation rate. To gain insight into intensity of the overall fluid flow at examined mixing conditions systems mixing time was determined as well. It was interesting to notice that the values of mixing time corresponded with the values of metastable zone widths, thus indicating their dependence on the overall fluid flow pattern. Obviously, the obtained results could only be explained if interactions between the flows generated by individual impellers are clearly understood. For that reason the photographs of the flows were taken. This way, it was possible to identify the region in the overall fluid flow structure, in which an interaction of flows, generated by each of the impellers, occurred. By taking into consideration the values of nucleation rate, mixing time, power consumption and overall fluid flow pattern, it is possible to define the impact of impeller position on nucleation kinetics in examined dual impeller system.

Mixing ; impeller position ; crystallization ; nucleation ; borax

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