engleski

Mixing time and nucleation rate in dual axial impeller batch cooling crystalliser

The influence of mixing on crystallisation is often ignored despite the fact that it affects almost every aspect of the process what is especially pronounced in multiple impeller systems where flow complexity is increased. The predominant motivation for this work was to investigate the effect of mixing on nucleation kinetics of borax in the process of batch cooling crystallisation. Crystallisation was conducted in 15 dm3 crystalliser with an aspect ratio (H/dT) equal to 1.3. Such aspect ratio required an installation of a second impeller on the same shaft. Mixing was performed by using two down pumping pitched blade turbines (PBT impellers). Mother liquor was saturated at 30 °C and it was cooled down at 6°C/h. The impact of several mixing parameters, such as impeller speed, size and position, was examined. A key parameter for evaluating the blending effectiveness of stirred tanks – mixing time, was determined by potentiometric method whiles the nucleation mechanism and rate was determined using the Mersmann's nucleation criterion. Obtained results indicate that investigated mixing parameters affect the values of mixing time and consequently the nucleation kinetics. Considering the stated, by choosing proper mixing parameters, it is possible to influence the characteristic of the final product of crystallisation.

Mixing time ; nucleation rate ; PBT impellers ; borax ; crystallisation

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