Alleviating mass transfer limitations in industrial external-loop syngas-to-ethanol fermentation

•Detailed analysis on gas transfer rate in industrial syngas fermentation.•CFD reveals mass transfer limitation topography in external-loop gas-lift reactors.•Bubbles smaller than 2 mm are key to reach required mass transfer capacities.•The produced ethanol minimizes bubble size and increases gas hold-up.•Transport limitations in air–water systems can be alleviated in syngas fermentation.