Marija Ćosić

Effect of Impeller Position on Crystal Growth Kinetics of Borax in Dual Axial Impeller Batch Cooling Crystallizer

Antonija Čelan, Marija Ćosić, Igor Pehnec, Nenad Kuzmanić University of Split, Faculty of Chemistry and Technology, Department of Chemical Engineering R. Boškovića 35, 21 000 Split, Croatia akacunic@ktf-split.hr; akrap@ktf-split.hr; kuzmanic@ktf-split.hr Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, Department of Mechanical Engineering and Naval Architecture R. Boškovića 32, 21000 Split, Croatia ipehnec@fesb.hr

Effect of Impeller Off-bottom Clearance on Crystal Growth Kinetics of Borax in Dual-impeller Batch Cooling Crystallizer

The aim of this work was to experimentally investigate the influence of impeller off-bottom clearance on crystal growth kinetics of disodium tetraborate decahydrate (borax) in a dual-impeller batch cooling crystallizer. Examinations were carried out in a crystallizer of 15 dm3 with liquid height to crystallizer diameter ratio (H/dT) of 1.3. In this research, two pitched blade turbines (PBT) with diameter D/dT=0.33, were mounted on the same shaft with a constant spacing between impellers (s/D=1.0). Mixing was performed at the impeller speed which ensured the state of complete suspension in the system (N=NJS). The impeller off-bottom clearance (C/D) was varied from 0.2 to 1.3. Crystallization was carried out by a controlled cooling of the solution saturated at 30°C. Concentration of the solution was measured in line by potentiometric method, while the changes of crystal size over time were determined by optical microscope and image analysis software. Experimental results indicate that impeller off-bottom clearance influences the change of crystal size over process time. But, regardless of the impeller off-bottom clearance, crystal growth is integration limited. The value of an order of crystal growth rate changes insignificantly with C/D ratio, while the crystal growth rate constant is affected by the change in hydrodynamics. Obtained results are a consequence of the hydrodynamic conditions in the dual-impeller crystallizer. In order to analyze the overall fluid flow pattern, in which an interaction of the flows generated by each of the impellers occurred, photographs of the flows were taken and simulations by VisiMix 2000 Turbulent were made. Taking into account the values of kinetic parameters of crystal growth, characteristic of obtained crystals, power consumption and the overall fluid flow pattern, it is possible to define the impact of impeller off-bottom clearance on borax crystal growth kinetics.

The Investigation of the Influence of Impeller Blade Inclination on Borax Nucleation and Crystal Growth Kinetics

An influence of impeller blade angle on the crystallization kinetics of borax has been investigated in detail. Its importance was studied in a batch crystallizer equipped with a four-blade turbine, whose angle was varied from 30° to 90°. Heterogeneous nucleation mechanism was dominant at all examined conditions, but the rate of nucleation significantly differed. Kinetic parameters of crystal growth were determined as well. Numerical values of order of crystal growth remain almost unchanged regardless of the blade angle. Different values of crystal growth rate constants are the result of the fluid dynamics in the crystallizer, caused by the different impeller blade inclinations. In all examined systems, crystal growth is integration limited. The transition from axial to radial flow, due to different blade angles, reflects on the crystal size distribution, agglomeration ratio, and crystallization yield.