With the current shift from a fossil-based to a biomass-based economy, the study of biorefineries and their main components has gained significant importance. The main components of biorefineries include bioreactors. For many systems, the improvement of mass transfer in and between phases through mixing is the key success factor. So far, many studies have focused on mechanically stirred reactors, but not many on pneumatically stirred systems.
Air-lift reactors (ALR) are widely used in the chemical, biochemical and pharmaceutical industries. ALR inserts allow better flow control. Critical design parameters for ALR with such circular-loops are liquid and gas recirculation. The proper design and placement of these inserts, so-called draft tubes, is essential and has a significant influence on two-phase hydrodynamics as well as on mass transfer in the reactor since the draft tube guides the flow field. In this study, we use computational fluid dynamics (CFD) to characterize the flow of three different internal loop ALR geometries with different internal configurations (single and double draft tubes). Design parameter variations of the studied ALRs will allow the prediction of optimal configurations for bioreactors, e.g. in more efficient biorefinery concepts. No previous CFD studies have been found in literature comparing the flow of single stage and multi-stage internal loop ALRs. Higher mixing intensities were achieved in the upper part of the double stage internal loop ALR, the ratio between the bioreactor and the draft tube has an effect on the downcomer velocity.