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Impact of simultaneous application of mechanical stirring and ultrasound irradiation on nucleation kinetics of borax decahydrate

Generally, crystallization is one of the oldest separation operations in chemical engineering. Owing to the fact that it is conducted at relatively low temperatures, it is also one of the most inexpensive ones. A major advantage of crystallization is that it allows generation of a product of specific properties, but only if the process is controlled properly. Certainly, crystallization will depend on physical and chemical characteristics of the crystallizing substance, but since it lays upon the laws by which transport phenomena is governed, crystallization may significantly depend on the process conditions as well. Since crystallization takes place in suspensions, it is often conducted batch-wise in stirred vessels. Until now, many experiments carried out in single and dual impeller batch crystallizers indicate that mixing parameters can have a major impact on borax decahydrate crystallization kinetics and consequently the product properties (Čelan et al., 2018). Most of these investigations are conducted at the just suspended impeller speed in order to explore how transport phenomena reflects on the characteristics of the final product when the process is performed at optimal energy consumption. In the last years, there has been an increase of interest in the application of ultrasound for the control of crystallization process due to more demanding requirements for crystal purity and defined crystal morphology. Several theories were proposed in order to explain the mechanism by which ultrasound affects crystallization, but an exact one still has not been accurately determined. However, it is known that its influence relies upon the phenomenon of ultrasonic cavitation (Kim and Suslick, 2018). By one of the proposed theories, this event generates high local temperatures (Suslick et al., 1990), some suggest that implosion of the bubble causes the rise of the local pressure values, and other imply that ultrasound cavitation causes segregation of the mixture thus inducing nucleation (Grossier et al., 2007). In any case, previous studies have shown that sonication during crystallization can affect induction time and metastable zone width, the mechanism and rate of nucleation but also the kinetics of crystal growth (Jambrak et al., 2014 ; Kuijpers et al., 2002 ; Lyczko et al., 2002). Sonication also increases the repeatability which is one of its key advantages in comparison to conventional crystallization. Investigation on the influence of ultrasound on crystallization are usually carried in unmixed or in systems mixed by magnetic stirrer. But in crystallizers larger than 0, 5 L, neither ultrasound nor magnetic stirrer are powerful enough to suspend growing crystals and ensure conditions for undisturbed crystal growth. Unlike the majority of papers concerning the topic of sonocrystallization, experiments here were conducted in a mechanically stirred batch cooling crystallizer. The research at hand deals with the investigation of an influence of ultrasound amplitude on nucleation kinetics of borax decahydrate whose applications are so versatile that it can be found in almost all industries. Crystallization has been conducted in a 3 L batch cooling crystallizer. Mother liquor of borax decahydrate has been saturated at 30 °C and it has been cooled down linearly at the rate of 6 °C/h. Cooling profile was controlled by Lauda Proline RP855 C X Edition thermostat. Crystallizer was a baffled, flat bottom vessel in which liquid height was equal to crystallizer diameter, H=dT. Impeller used in this investigation was a straight blade turbine (SBT) which generated radial fluid flow pattern in the vessel. Impeller diameter was equal to 1/3 of the vessel diameter, D=1/3 dT and it was mounted on a shaft at the off-bottom clearance, C, which equaled C/D=1. During the entire process, both mechanical stirring and ultrasound irradiation were simultaneously applied. Ultrasound irradiation was applied by using the Hielscher UP400St homogenizer with a H22L2D sonotrode. All experiments were conducted at the impeller speed which ensured the state of complete suspension (N=NJS). The said impeller speed was determined by Zwieterings 1-2 seconds criterion for all four ultrasound amplitudes applied, A=0, 20, 25 and 30 %. It was essential to determine the change of supersaturation during process time, which is the driving force of the process. Supersaturation was determined based on the values of concentration of mother liquor which was measured off-line by using the potentiometric method. Metastable zone width was determined by using a widely accepted visual method. Based on that data, a dominant nucleation mechanism was determined by Mersmann’s nucleation criteria. Furthermore, for all experimental conditions tested, nucleation rates were calculated and compared. Characteristics of the final product of crystallization, namely crystal size distribution, were determined by laser diffraction using the Horiba LA-300 particle size analyzer. In order to assess the energy requirements of the process, power consumption was determined and it was expressed per unit mass of suspension, P/m. Power consumed by stirring was determined by measuring torque imparted on the shaft (Lightnin Labmaster L1U10F mixer) while the power pertaining to ultrasound irradiation was determined by calorimetric measurements. Obtained results showed that an increase of ultrasound amplitude decreases the metastable zone width and that nucleation commences at lower supersaturation. By using the Mersmanns nucleation criterion, i.e. by plotting the dimensionless supersaturation vs. dimensionless solubility diagram it was found that nucleation started by heterogeneous mechanism and that the rate of nucleation decreased as the ultrasound amplitude grew. An increase of the amplitude also reflected on the properties of the final product. Mean crystal size was larger in the system without ultrasound. Although there was no significant change in mean crystal size in the systems where ultrasound was applied, it was found that the crystal size distribution shifts from bimodal to monomodal as the amplitude rises. The data on power consumption showed that it increases with amplitude but also that the share of the power consumed by ultrasound irradiation is tenfold higher than the one related to stirring. It can be concluded that sonocrystallization is a good choice if monomodal crystal size distribution with high repeatability is required. But with the results of power consumption in mind, if crystal characteristics are not that important, it would be more appropriate to conduct conventional crystallization since its energy requirements are significantly lower.

sonocrystallization, borax decahydrate, stirring, nucleation kinetics

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