Abstract
Vertical photobioreactors (PBR) with cylindrical cross section, namely air-lift reactors (ALR) and bubble column reactors (BCR), are often chosen both for bench-scale and industrial scale microalgal cultivation. It was common belief that ALR was the most favorable configuration in terms of light conversion efficiency (LCE) and/or photosynthetic productivity than BCR because of the regular cyclic flow pattern achieved inside the PBR. In the present study, we simulated the flow patterns in both ALR and BCR by means of computational fluid dynamics (CFD) and clarified the effects of such flow pattern on the LCE and productivity. Simulation results, obtained from the open-source CFD suite OpenFOAM, showed good agreement both for the flow velocity and the mixing time observed in the actual PBR using high-speed photography and conductivity pulse response, respectively. Subsequently, Lagrangian particle tracking was conducted on the simulation results to highlight the main fluid-flow patterns and to calculate the local flashing-light frequency, which was necessary in order to estimate the overall light conversion efficiency of the PBR. The BCR was characterized by a highly random fluid pattern with macroscopic, low-frequency circular loops while the ALR was characterized by numerous swirling flows localized inside the draft tube in addition to the main recirculation between the inner and outer portions of the tube. Finally, image analysis was used to correlate the numerical calculations with the light conversion efficiencies attained in a Haematococcus pluvialis culture that had been illuminated with flashing light.