Precipitation of salicylic acid with and without US-assistance. Study on supersaturation discharge, conversion, and yield

The paper investigates the precipitation of salicylic acid from a highly supersaturated solution (β_in = 46.3) in three reactor types: Koflo STM, STR, and STR+DT, operating in both silent and ultrasound-assisted (sonoprecipitation) modes. It examines process intensification in terms of conversion, supersaturation discharge, and yield by adjusting operating conditions, including turbulence distribution (via reactor type selection) and turbulence intensity induced either by mixing (ε_mix) or by a combination of mixing and ultrasound assistance. Across all systems, pure crystalline salicylic acid was obtained, as confirmed by FT-IR and XRD analyses. Results show that the distribution of unit power inputs and the residence time distribution (RTD) significantly influence both reaction and precipitation kinetics, regardless of ultrasound use. In the silent STM, increasing ε_mix improved conversion but reduced the mean residence time, negatively affecting yield due to incomplete supersaturation discharge. Conversely, in the silent STR, higher ε_mix impaired reaction kinetics, limiting conversion, but also reduced outlet supersaturation, promoting nucleation. The impact of ultrasound assistance was complex and not straightforward compared to silent systems, with final outcomes resulting from overlapping and often opposing effects. In US-assisted reactors, increasing ε_US decreased induction time and led to both improved supersaturation discharge and higher yield.