Uniform annular cavitation field and superior mixing characteristics of a tube-in-tube continuous-flow ultrasonic reactor.

Straight-tube ultrasonic reactors (ST-USR) are widely used in chemical, environmental, and materials engineering. However, the uneven distribution of the cavitation field, coupled with the resultant poor mixing performance and the absence of effective temperature regulation, compromises the consistency of the final product. To address these issues, a 20 kHz temperature-controllable continuous-flow tube-in-tube ultrasonic reactor (TIT-USR) was developed, featuring a 2.5 mm annular cavitation channel between a central heat exchange tube and an outer vibrating glass tube. High-speed video and sonochemiluminescence imaging confirmed TIT-USR's uniform radially annular cavitation field (bubble clouds rotating at 5.6 r/s) without sparse zones. An axial dispersion model (Pe = 9.63) verified improved mixing and residence time distribution of TIT-USR over ST-USR. The dimensionless residence time distribution exhibited remarkable consistency across the entire range of flow rates tested (1.77-14.12 mL/min), which indicates that the mixing characteristics are predominantly governed by the uniformly distributed annular cavitation field. In the continuous sonochemical degradation of 10 mM chloroform at 7 mL/min flow rate and 30 s residence time, TIT-USR achieved a maximum HCl(aq) yield (7.6 μmol/min) and a higher concentration (1.08 μmol/mL vs. ST-USR's 0.86 μmol/mL), confirming TIT-USR's higher cavitation Intensity. In the continuous preparation, TIT-USR produced smaller, more uniform roxithromycin nanocrystals (9 nm, PDI 0.21 vs. ST-USR's 10.5 nm, PDI 0.42) and vitamin E nanoemulsions (150 nm, PDI 0.18 vs. ST-USR's 151 nm, PDI 0.26) under the same other conditions. These results prove that the uniform and effective annular cavitation field enables consistent nanoparticles and nanoemulsions.