engleski

Influence of the fluid flow patterns on borax nucleation mechanism and nucleation rate in a single and dual turbine impeller crystallizer

The aim of this work was to investigate the influence of fluid flow pattern in the crystallizer created by different impellers on the nucleation mechanism and nucleation rate of disodium tetraborate decahydrate (borax) in the process of batch cooling crystallization. Examinations were performed in the crystallizer of 15 dm3 with liquid level to tank diameter ratio (H/dT) of 1, 3 and diameter ratio impeller to tank (D/dT) of 0, 33. The fluid flow patterns in the crystallizer were generated by axial and radial turbine impellers and by their different dual combinations. Impeller speed ensured the state of complete suspension in the system for all configurations used (N = NJS). Crystallization was carried out by cooling of the mother liquor from the saturation temperature of 30°C by the rate of 6 °C h-1. Concentration of solution was measured in line by the means of Na ion-selective electrode, while the onset of nucleation i.e. metastable zone width was detected from the changes of concentration values. Mechanism and nucleation rate were determined according to Mersmann´s nucleation criterion. In order to gain complete insight into the overall fluid flow in the crystallizer the photographs of the flows were taken and simulations by VisiMix 2000 Turbulent package were made. Power consumption over the process time for all impeller configuration used were measured as well. Obtained results indicated that flow pattern developed by single impeller or by interaction of flows generated by each of impellers in dual impeller system significantly influenced the nucleation mechanism and nucleation rates of borax. Different impeller configurations affect the values of just suspended impeller speed, the final product properties and power consumption as well.

crystallization of borax ; mixing ; nucleation ; fluid flow pattern ; single and dual impeller systems.

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