(445d) Effect of Mixing, Mass Transfer and Light/Dark Cycles on Microalgae Growth in a Taylor Vortex Photobioreactor

The microalgae Chlorella vulgaris was grown in the annular region of a Taylor vortex photobioreactor. Experiments were carried out under different CO₂ fed concentration, inner cylinder rotation speeds and gas flow rates. The biomass growth rate was significantly enhanced by increasing the inner cylinder rotational speed, whereas only modest increases in biomass growth rates were produced by increasing the gas flow rate. The biomass growth rate was found to be insensitive to the gas mole fraction of CO₂ in the range 8% to 12%. In view of these findings, and in order to definitively establish which factor limits biomass growth, mixing characteristics and volumetric mass transfer coefficients were measured. From these data the characteristic time scales for mixing, interphase mass transfer, and CO₂ consumption were compared, and it was found that microalgae growth in the Taylor vortex reactor is not limited by liquid mixing or by gas-liquid mass transfer rates, and the observed growth rate enhancements with increasing cylinder rotation speed can be attributed to generation of light/dark cycles that improve light utilization efficiency.