Hydrodynamic Analysis of the Novel 360 Air-Loop Series Bioreactor (360-ASBR) for Microalgae Production

Abstract

Microalgae are vital microorganisms due to their role in oxygen production, marine ecosystems, and as a source of various high-value products such as vitamins, biofuel, livestock feed, cosmetics, and more. Within some applications, challenges arise in large-scale microalgae production, characterized by high costs and growth rates that fall short of meeting commercial production demands. Therefore, there is still a need for further research to be conducted on microalgae strains and cultivation systems to improve costs and production rates. The focus of this study was to research a novel, patent-pending 360 Air-Loop Series Bioreactor (360-ASBR) and compare it to a Bubble Column Bioreactor (BCBR) that serves as the control. The specific objectives were to (1) determine and compare the hydrodynamic variables of the 360-ASBR and the BCBR including mixing time, residence time, and oxygen liquid mass transfer coefficient (KLa) and (2) determine the effect of the amount and placement of spargers in the BCBR. The results of this study showed that: (1) the mixing time of 198 seconds for the high flow rate and 247 seconds for the low flow rate for the ASBR significantly exceeded that of 25.1 seconds and 38.7 seconds respectively for the BCBR (2) the average residence time of 829 seconds was not significantly less than 1048 seconds for the BCBR at the high flow rate whereas the average residence time of 698 seconds for the ASBR was significantly less than 1053 seconds for the BCBR at the low flow rate, (3) the KLa of 0.012 s-1 for the ASBR was not significantly higher than 0.011 s-1 for the BCBR at the high flow rate whereas 0.009 s-1 for the ASBR significantly exceeded the KLa of 0.002 s-1 for the BCBR at the low flow rate, and (4) the mixing times of 25.1, 25.0, and 29.5 s for each sparger arrangement at the high flow rate and 38.7, 29.8, and 39.2 s for each sparger arrangement at the low flow rate did not result in significant differences. The principal conclusions drawn from the results were as follows: (1) the ASBR and the BCBR had statistically indistinguishable average residence time and KLa at the high flow rate; (2) the BCBR yielded shorter mixing time than the ASBR at both high and low flow rates; and (3) the placement and amount of spargers did not affect the mixing time results in the BCBR. The longer mixing time for the ASBR could be explained by its defined flow path, serving to constrain its bulk liquid mixing within the relatively narrow confines of such a flow path. On scale-up, such defined flow paths are expected to serve the ASBR well in that it ensures and preserves the integrity of its bulk flow patterns independently of the bioreactor volume. By contrast, the BCBR, lacking such defined flow paths, is known in industry and literature to suffer a significant reduction in its mixing efficiency on scale-up.