Flow regime transition and mass transfer intensification of microbubbles in graded fixed‐bed: Experiments and modeling

Particle size gradation in fixed‐bed reactors has been extensively studied, but almost exclusively with millimeter‐scale inlet bubbles. Whether these principles hold for sub‐millimeter (microbubble) scale, comparable to or smaller than the bed pore size, remains unclear. Herein, high‐speed photography and CO 2 ‐NaOH absorption were used to investigate the effects of particle gradation on microbubble dynamics and mass transfer. Results show that when the bed pore size exceeds the bubble size, the flow transitions from plug flow to bubbly flow, suppressing coalescence and raising the outlet microbubble number fraction to 84.9%. This transition enhances k L a by 50% over a mono‐sized bed, via a threefold increase in interfacial area to 279.7 m 2 /m 3 . Empirical correlations for bubble size and k L a were established with prediction errors within ±20%. This work demonstrates that particle gradation offers an energy‐free strategy for unlocking the mass transfer potential of microbubbles in fixed‐bed.