Liquid Circulation in Airlift Loop Reactors: Suppressed by Small Bubble Accumulation and Driven by Large Bubble Momentum

Gas holdup and liquid circulation velocity are key hydrodynamic parameters for airlift loop reactors (ALRs). Classical theory posits that liquid circulation velocity in ALRs is predominantly determined by the gas holdup differential between riser and downcomer zones. Nevertheless, this difference gradually decreases and even vanishes as the superficial gas velocity rises or small bubbles increase, rendering the traditional model no longer applicable. Poor understanding of bubble size distribution (BSD) effects on hydrodynamics has created challenges for reactor design and scale-up under wide superficial gas velocities and in inorganic electrolyte systems. The present study systematically investigates these phenomena over 0–0.2 m/s superficial gas velocities in pure water and sodium sulfate solutions (0.1, 0.5 mol/L) with various gas distributors, via comprehensive measurements of bubble size, gas holdup, and liquid circulation velocity. Leveraging salt-induced bubble coalescence inhibition, distinct initial bubble populations were generated, enabling the establishment of empirical correlations for gas holdup and circulation velocity based on BSD evolution. Experimental results show that large-scale bubbles significantly promote liquid circulation, whereas small bubbles impede it, which is verified in CFD simulations with strictly regulated bubble sizes. This work provides critical hydrodynamic data for aerated electrolyte systems and proposes a scalable modeling framework intrinsically linking BSD to reactor performance, deepening mechanistic understanding of ALR hydrodynamics.