Microbubble Characteristics in a Gas–Liquid–Solid Circulating Mini-Fluidized Bed

A gas–liquid–solid circulating mini-fluidized bed was developed to enhance mass transfer and solid circulation in multiphase microreactors. The system features a 10 mm riser diameter, 370 mm riser height, and 150 mm downcomer, operated with superficial gas velocities of 2.12–12.74 × 10 –3 m/s, superficial liquid velocities of 3.18–16.98 × 10 –3 m/s, and glass beads of 100–170 μm at 3.0 g loading. Bubble measurements reveal a stable bimodal structure, with microbubbles (<1 mm) accounting for m > 95% of the number and millimeter-scale bubbles dominating volume. The maximum gas holdup reached 14.6%, and the dense-phase height exhibited a peak at U G ≈ 6.37 × 10 –3 m/s before decreasing, indicating a transition from microbubble- to millimeter-bubble-dominated flow. Solid circulation rates increased with gas and liquid velocities but decreased with particle size. Quantitative analysis of bubble wake entrainment showed that the wake force exceeded particle apparent gravity by an order of magnitude, confirming its role in enhancing solid transport. These findings provide a hydrodynamic basis for the design and scale-up of microbubble gas–liquid–solid circulating mini-fluidized beds aimed at enhancing gas–liquid mass transfer and particle circulation.