Pinducer Analysis for the Design of a Nucleation Subsystem for Continuous Slug-Flow Crystallization

The reliable and convenient control of crystallization processes has received increased academic and industrial interest. Ultrasonication has been used to aid crystallization, such as accelerating nucleation, reducing crystal size, reducing aggregation, or changing polymorphic ratios [1]–[3]. Most bench-scale continuous sonocrystallization studies are based on configurations involving an ultrasonic probe directly inserted into the solution/slurry or a tube placed within a bath. An alternative multiphase millifluidic tubular crystallizer design combined the advantages of using a probe (focused energy) and a bath (indirect ultrasonication) for continuous nucleation, with the nuclei size tuning by changing the ultrasonic amplitude [4].

To better understand and improve the design of a continuous crystallization process, we employ a needle-tip pressure transducer (“pinducer”) to measure and analyze the “primary effect” of ultrasonication, which is the generation of pressure waves (the term “primary effect” is used as in Ref. [5]). This analysis is used to gain understanding on the crystallization outcome (the “secondary effect”). This separation of the analysis of the primary effect from the secondary effect provides greater insight and understanding than past studies that only reported the crystallization results of applying ultrasonication. The pressure waves of ultrasonication are measured for different equipment configurations, and different positions and orientations of the pinducer relative to the ultrasonication probe. The quantification of the pressure waves as a function of frequency provides insights into bubble dynamics and is used to guide the design of the nucleation subsystem and its positioning relative to other subsystems of the continuous crystallization apparatus. The analysis of the spatial localization of energy dissipation that occurs during ultrasonication informs the design of the focused indirect ultrasonication zone to enhance reproducibility and the control of nuclei size distribution [6]. Although the focus of this study was on a particular continuous crystallizer design, the same procedures for the analysis of physical phenomena via pinducers employed in this study could be applied to provide insights and guidance into other ultrasonication-facilitated crystallization designs.

References

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[3] R. J. P. Eder, S. Schrank, M. O. Besenhard, E. Roblegg, H. Gruber-Woelfler, and J. G. Khinast, “Continuous sonocrystallization of acetylsalicylic acid (ASA): Control of crystal size,” Cryst. Growth Des., vol. 12, no. 10, pp. 4733–4738, 2012.

[4] M. Jiang, C. D. Papageorgiou, J. Waetzig, A. Hardy, M. Langston, and R. D. Braatz, “Indirect ultrasonication in continuous slug-flow crystallization,” Cryst. Growth Des., vol. 15, no. 5, pp. 2486–2492, 2015.

[5] V. S. Sutkar and P. R. Gogate, “Design aspects of sonochemical reactors: Techniques for understanding cavitational activity distribution and effect of operating parameters,” Chem. Eng. J., vol. 155, no. 1–2, pp. 26–36, 2009.

[6] M. Jiang, C. Gu, and R. D. Braatz., “Analysis of focused indirect ultrasound via high-speed spatially localized pressure sensing and its consequences on nucleation,” submitted.