Hydrodynamic Optimization of the Axial Dispersion Bioreactor for Microalgae Production

Abstract

Microalgae are an abundant source of protein, vitamins, and lipids, among other nutrients and high-value products. Effective large-scale mass production is key in the commercialization of microalgae products. The general aim of this study was to characterize the hydrodynamic conditions and algae growth in the Axial Dispersion Bioreactor (ADBR), which is an original and patent-pending invention from the Biosystems Engineering Laboratory of The University of Arizona. The specific objectives of this study were: (1) to determine and compare key hydrodynamic parameters in the ADBR and the conventional bubble column as control, including residence time distribution, mixing time, and oxygen liquid mass transfer coefficient (kLa); and (2) to test if differences in the hydrodynamic conditions would result in significant differences in growth of Arthrospira (Spirulina) platensis between the photobioreactors. The results of the study showed that: (1) the residence time of 1609 s for the ADBR with large square plate holes, 300 rpm, and 3.3 LPM air flow rate significantly exceeded by 11% that of 1445 s for the bubble column with 3.3 LPM air flow rate, signifying greater liquid mixing in the ADBR; (2) the mixing time of 92 s in the bubble column with 4.7 LPM air flow rate significantly exceeded by 53% that of 60 s in the ADBR with small square plate holes, 200 rpm, and 4.7 LPM air flow rate, indicating that complete liquid mixing in the ADBR occurred significantly quicker than in the bubble column; (3) the oxygen liquid mass transfer coefficient (kLa) of 0.22 L/min in ADBR with large square plate holes, 100 rpm, and 4.7 LPM air flow rate significantly exceeded by 205% that of 0.073 L/min in the bubble column with 4.7 LPM air flow rate; (4) the final algae density of 0.225 g DW/L in ADBR significantly exceeded by 16% that of 0.194 g DW/L in the bubble column at the best RTD condition; (5) the final algae density of 0.267 g DW/L in ADBR significantly exceeded by 34% that of 0.199 g DW/L in the bubble column at the best mixing time condition; (6) the final algae density of 0.231 g DW/L in ADBR significantly exceeded by 16% that of 0.199 g DW/L in the bubble column at the best KLa condition; (7) the final algae density of 0.167 g DW/L in ADBR significantly exceeded by 29% that of 0.129 g DW/L in the bubble column at the worst RTD condition; (8) the final algae density of 0.161 g DW/L in ADBR significantly exceeded by 25% that of 0.129 g DW/L in the bubble column at the worst RTD condition; (9) the final algae density of 0.177 g DW/L in ADBR significantly exceeded by 37% that of 0.129 g DW/L in the bubble column at the worst KLa condition; (10) the final algae density of 0.194 g DW/L in the bubble column at the best RTD condition significantly exceeded by 16% that of 0.167 g DW/L in the ADBR at the worst RTD condition; (11) the final algae density of 0.199 g DW/L in bubble column at the best mixing time condition significantly exceeded by 24% that of 0.161 g DW/L in the ADBR at the worst mixing time condition; (12) the final algae density of 0.199 g DW/L in bubble column at the best KLa condition significantly exceeded by 12% that of 0.177 g DW/L in the ADBR at the worst KLa condition. Thus, growth of Arthrospira (Spirulina platensis) was significantly better in ADBR and bubble column with significantly more favorable hydrodynamic characteristics in terms of residence time, mixing time and oxygen liquid mass transfer coefficient.Key words: algae, photobioreactor, hydrodynamic conditions, Spirulina platensis