Bubble‐column and airlift photobioreactors for algal culture

Abstract

AbstractBubble columns and airlift photobioreactors can be useful for culturing phototrophic organisms requiring light as a nutrient. Light availability in bubble columns and airlift devices is influenced by aeration rate, gas holdup, and the liquid velocity (mixing and turbulence). The photosynthetically generated oxygen also needs to be removed, as excessive dissolved oxygen suppresses photosynthesis. Oxygen removal capacity is governed by the magnitude of the overall gas–liquid mass‐transfer coefficient, kLaL. This work characterizes the relevant hydrodynamic and mass‐transfer parameters in three air‐agitated reactors: bubble column, split‐cylinder airlift device and concentric draft‐tube sparged airlift vessel. The reactors are then evaluated for culture of the microalga Phaeodactylum tricornutum. All reactors were about 0.06 m3 in working volume, and the working aspect ratio was about 10. Data were obtained in tap water for a base‐line comparison and in Mediterranean seawater, as a potential medium for algal culture. A theoretical relationship was developed and proved between kLaL and the aeration rate. In addition, a method based on mechanistic relationships was proved for predicting the liquid circulation velocity and kLaL in airlift reactors. Existing correlations applied satisfactorily to gas holdup and kLaL data obtained in the bubble column. Aqueous solution of sodium chloride (0.15 M) closely resembled seawater in terms of its hydrodynamics and oxygen transfer behavior. Under the conditions tested, all three reactors attained a biomass concentration of about 4 kg·m−3 after ∼260 h. The mean maximum specific growth rate was 0.022 h−1 in all cases at a power input of 109 W·m−3.