Airlift Taylor Flow bioreactors as a novel platform to enhance H₂-assisted CO₂ bioconversion processes

The microbial valorization of CO₂ requires the development of novel bioreactor configurations capable of ensuring a cost-effective gas-liquid mass transfer. This study presents the design, characterization and performance evaluation of a novel Airlift Taylor Flow Reactor (ATFR), which integrates the gas-induced liquid recirculation of airlift systems, with the high gas-liquid mass transfer potential of Taylor flow in multicapillary systems. The liquid recirculation flow in the downcomer was quantified using a colour tracer method, and the volumetric oxygen mass transfer coefficient (k_La) was determined via the sulphite oxidation method, both with and without forced recirculation. The results showed that the implementation of forced mechanical recirculation via a centrifugal pump negatively impacted the total liquid recirculation and k_La. A maximum k_La of 891 ± 23 h⁻¹ was achieved at a gas flow rate of 60 L min⁻¹, representing a 75 % increase compared to the same condition with external forced liquid recirculation. The autotrophic growth of Cupriavidus necator showed a direct correlation with k_La, with a 171 % increase in maximum cell concentration and a 67 % increase in the specific growth rate when k_La increased from 183 to 364 h⁻¹. These findings highlight the potential of the ATFR as a promising platform for gas-liquid mass transfer-limited processes, particularly in systems bioconverting poorly soluble gases such as H₂, CH₄, CO and O₂.