Ultrasonic micro-reactors are frequently applied to prevent micro-channel clogging in the presence of solid materials. Continuous sonication will lead to a sizeable energy input resulting in a temperature increase in the fluidic channels and concerns regarding microchannel degradation. In this paper, we investigate the application of pulsed ultrasound as a less invasive approach to prevent micro-channel clogging, while also controlling the temperature increase. The inorganic precipitation of barium sulfate particles was studied, and the impact of the effective ultrasonic treatment ratio, frequency and load power on the particle size distribution, pressure and temperature was quantified in comparison to nonsonicated experiments. The precipitation reactions were performed in a continuous reactor consisting of a micro-reactor chip attached to a Langevin-type transducer. It was found that adjusting the pulsed ultrasound conditions prevented microchannel clogging by reducing the particle size to the same magnitude as observed for continuous sonication. Furthermore, reducing the effective treatment ratio from 100 to 12.5% decreases the temperature rise from 7 to 1°C.

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Ultrasonic microreactors are increasingly applied to particle synthesis, crystallization processes, or organic synthesis involving solids due to the clogging prevention offered by ultrasound irradiation. However, the further application of this technology is limited by the scalability of ultrasonic flow reactors. In this work, we combined experiments and numerical simulations for the design of a scaled-up flow reactor. The reactor consists of a perfluoroalkoxy alkane (PFA) tubing immersed in a box to transmit the ultrasound and provide temperature control, to which six Langevin multifrequency transducers are attached. The acoustic pressure distribution was investigated to characterize the effect of ultrasound frequency and the transducer configurations on particle size distribution for the model reaction of barium sulfate synthesis.Operating the scaled-up reactor with six transducers at a frequency of 40 kHz resulted in an acoustic wave distribution with maximum acoustic pressures in the vicinity of the tubular reactor, thus leading to a small particle size distribution and consequently clogging prevention. The productivity of the scaled-up reactor was 14g/h barium sulfate at a remarkably low applied ultrasound power of 0.48W/mL, highlighting the efficient design and the further scale-up potential.

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