Design parameters comparison of bubble column, airlift and stirred tank photobioreactors for microalgae production

Abstract

Microalgae are the most propitious feedstock for biofuel production due to their lipid and fatty acid content. Microalgae cultivation shares many features with bioreactors, such as thermal and pH regulation, feeding procedures, and mixing to enhance heat and mass transfers. Aeration and stirring speeds are important parameters to reduce the costs of producing microalgae. In this study, three different photobioreactor types (stirred tank, airlift, bubble column) were characterized and compared for microalgae production. Hydrodynamics, mass transfer, and power consumption were determined for various aeration rates (0.9, 1.2, 1.5 L/min), and stirring speeds (100, 200 rpm), and Chlorella sorokiniana growth performance was compared under the conditions that provided the highest volumetric mass transfer and the lowest mixing time. Photo-bioreactor homogenization was good as indicated by low mixing times (< 10 s). Bubble column had the highest volumetric mass transfer due to its sparger design. Gas holdup and volumetric mass transfer coefficient were found to increase with the air flow rate and stirring speed. For stirred tank, bubble column, and airlift photobioreactors, maximum specific growth rates of C. sorokiniana were 0.053, 0.061, 0.057 h^-1, and biomass productivities were 0.064, 0.097, 0.072 gdw/L.day, respectively. Under the conditions tested, growth was limited by the volumetric mass transfer in the airlift and stirred tank and bubble column was the best option for producing microalgae. These findings pave way for more extensive use of these systems in producing microalgae and provide a basis to compare photobioreactors of different designs.

Keywords: Characterization; Chlorella sorokiniana; Hydrodynamics; Microalgae; Photobioreactor.

Fig. 1

A photograph of the photobioreactor system during the runs. From left to right: ALR, STR (200 rpm), BCR

Fig. 2

Effect of air flow rate (0.9, 1.2, 1.5 L/min), and stirring speed of STR (100 and 200 rpm) on mixing time of photobioreactors

Fig. 3

Bubble size distribution at aeration rate of 1.5 L/min in; (a) ALR, (b) BCR, (c) STR at 100 rpm, (d) STR at 200 rpm

Fig. 4

Effect of air flow rate (0.9, 1.2, and 1.5 L/min), and stirring speed of STR (100 and 200 rpm) on gas holdup of photobioreactors

Fig. 5

Effect of air flow rate and STR stirring speed on volumetric mass transfer coefficient (Air flow rates of 0.9, 1.2, and 1.5 L/min correspond to U_G of 0.30, 0.40, and 0.50 cm/s in STR and BCR, and to U_G of 0.57, 0.76, and 0.95 cm/s in ALR)

Fig. 6

Comparative cell growth profile of C. sorokiniana in photobioreactors. (●): BCR, (□): STR, (○): ALR