In the last few decades, an interest in batch crystallization had increased significantly due to the numerous advantages of this separation method. Even though the number of scientific papers dedicated to the subject is continuously increasing, only a few of them deal with the impact of mixing on the process of crystallization. Additionally, the number of those researches in which mixing is performed by dual or even a multi-impeller system is almost negligible. Since mixing ensures favorable conditions for mass and energy transfer, an impact of this operation on every aspect of a crystallization process should be investigated. For this reason, the aim of this work was to find how the off-bottom clearance influences mechanism and crystal growth kinetics.
Batch crystallization of disodium tetraborate decahydrate (borax) by controlled cooling was conducted in a crystallizer of 15 dm3, which was equipped with four baffles. Saturated solution was prepared at 30°C and was cooled at the rate of 6 °C/h. Since liquid height to vessel diameter ratio (H/dT) equaled 1.3, mixing was performed by using two impellers of a standard diameter (D/dT=0.33). Radial SBT impeller was mounted above an axial PBT impeller on a single shaft (PBT-SBT dual impeller system). Impeller spacing (S/D) was kept constant and equal to S/D=1 while the off-bottom clearance was varied in the range of 0.2 to 1.3. Impeller speed, at which crystallization was conducted, ensured the state of a complete suspension (N/NJS=1). Solution concentration was monitored continuously by Na-ion selective electrode. Linear crystal size over process time was determined by microscopic analysis of the slurry samples with Motic Images Advanced 3.2 software. In order to explain obtained results, it was also necessary to analyze the fluid flow generated by impellers. For this purpose, the overall fluid flow pattern was photographed according to the method developed by S. Ibrahim and A. W. Nienow.
Results of this investigation indicate that the change of off-bottom clearance reflects on the values of linear crystal size over process time and consequently on the crystal growth kinetics. It was found that the overall crystal growth coefficient, kg, increases with an increase of impeller off-bottom clearance. A deviation from this trend is only present when the flow generated by the lower impeller is transformed due to the vicinity of the crystallizer bottom. An analysis of numerical values of the order of overall crystal growth process, g, did not imply a regular dependence of this parameter on the off-bottom clearance. To determine the crystal growth mechanism, a model proposed by Mersmann was used. By this model, in all tested conditions, crystal growth was integration limited.