Acoustic cavitation-enhanced gas–liquid mass transfer during catalytic cyclohexane oxidation in a gas–liquid–solid fluidized bed reactor

Liquid-phase catalytic oxidation of cyclohexane produces precursors for adipic acid and caprolactam, but its performance reflects coupled reaction and multiphase-transport effects. This study examines ultrasonic irradiation (20 kHz; 0–120 W/L) in a gas–liquid–solid fluidized bed reactor using O 2 over Co–Mn/SiO 2 at superficial gas velocities of 0.01–0.05 m/s and temperatures of 393–423 K. At 403 K, 80 W/L increased the measured volumetric gas–liquid mass transfer coefficient, kLa, from 0.018 to 0.061 s -1 (3.4-fold); the point estimates of gas holdup and Sauter mean bubble diameter were consistent with increased interfacial area, although their independent measurement uncertainties were not quantified. At the same temperature, conversion increased from 8.2% without ultrasound to 19.8% with ultrasound (KA-oil selectivity 79.5%); at 423 K and 80 W/L, conversion reached 21.6% with 76.3% selectivity. These conversion gains cannot be assigned to the bulk kLa increase alone: kLa and the single-endpoint pseudo-first-order conversion index describe different quantities, and particle-scale transport or catalytic effects remain plausible but unverified explanations. A modified empirical Sherwood-type correlation fitted the dataset with a mean absolute deviation of 9.5% (92% of points within ±10%), while CFD provided qualitative, not independent quantitative, support for near-horn bubble-size and turbulence trends. The 80 W/L condition is therefore a local optimum for the measured mass-transfer and ultrasound-specific energy metrics, not a demonstrated process-wide optimum. Overall feasibility remains conditional on uncertainty characterization for bubble properties, kinetic validation, full process-energy accounting, catalyst-attrition economics, and formal process-safety assessment.