Improvement of real-scale raceway bioreactors for microalgae production using Computational Fluid Dynamics (CFD)

In this work, the design and hydrodynamic of a 500 m2 raceway bioreactor were optimized using Computational Fluid Dynamics (CFD). First, the influence of the bend type (traditional, isle partition and baffle partition) and the liquid velocity was studied, after that the dynamic behavior of the optimal configuration was analyzed. A monophasic analysis employing the Finite Element Method (FEM) with COMSOL Multiphysics™ allows to confirm that the utilization of deflectors in the baffle partition bend type provide the best performance in terms of fluid velocity, reduction of dead zones, shorter residence time and a suitable cell Reynolds number. A multiphasic analysis using the Finite Volume Method (FVM) with ANSYS Fluent was performed to complete the analysis, it considering the geometry and rotation speed of the paddlewheel. Different angular velocities from 14 to 20 rpm were studied. Results shows that at 18 rpm average liquid velocity of 0.38 m·s−1 were achieved and stabilized at 240 s, the pressure drop in the overall system stabilizes at 700 Pa at 330 s, power consumption in the rage of 30 W·m−3, a maximum turbulence intensity of 0.4 and vorticity greater than 9 s−1 in areas adjacent to the paddlewheel being determined. In addition, it was determined that more than 14% of the total volume is useful for mass transfer with the atmosphere as well as to water loss by evaporation. This study provides a robust start point for improving large-scale raceway reactors, a highly relevant issue because these are the most used reactors in large-scale microalgae production.