(457b) Modeling and Experiments on the Effects of Increasing Flow Baffles on Dead Zone and Growth Performance of Microalgae in a Raceway Photobioreactor

Microalgae for biodiesel production can be grown in an open pond or closed system photobioreactors. Closed system photobioreactors (PBR) are expensive to build and operate, while open system photobioreactors are less expensive to construct per unit volume and directly take advantage of the available sunlight and carbon dioxide (CO2) in the atmosphere. The most common open pond system according to the literature is the raceway photobioreactor (RWP). But limitations of the RWP include poor mixing and dead zones, which are areas with velocity of less than 0.1 m/s. This can lead to low biomass productivity, as compared with the closed system photobioreactor. Common improvements to the RWP to counteract dead zones involve the use of more than one paddle wheel, and introduction of sloping baffles and flow deflectors.

This research work focuses on flow optimization using Computational Fluid Dynamics (CFD) COMSOL Multiphysics to reduce dead zones and improve productivity of the reactor. The effects of variable number of flow baffles, central wall thickness, first channel diameter (distance between the first baffle and central wall), and ratio of first channel diameter to central wall thickness (R­in/Tw) on the percentage of dead zone volume were studied. The results show that, compared to control RWP (RWP with no baffles), dead zone in the RWP modified with four baffles was decreased by 96% while the optimum ratio of R­in/Tw was found to be between 3 and 4.8. To validate the simulation results, experimental results in a pilot –scale 200 liter RWP showed 50% and 40% increase in the productivity and specific growth rate of the microalgae respectively in an RWP with three flow baffles as compared with the control RWP.