J.L. Mendoza1*, C.J. Banks1, S.Heaven1, F.G. Acién2, E.Molina2
1 Water and Environmental Engineering Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, UK 2 Microalgae Biotechnology Group, Department of Chemical Engineering, University of Almeria, Almeria, Spain(free)
Open raceways have been proposed as the most feasible option for the production of microalgae for biofuels and wastewater treatment, due to their low cost and power consumption. Although this type of photobioreactor has been known for more than fifty years, its design and operationare being revised according to the new methodologies available. In this work an in-depth analysis of the fluid dynamics and mass transfer in a real-scale (100 m2) raceway reactor is presented. Fluid-dynamic characterization included the influence of design and liquid velocity on the power consumption, and also on the mixing. Mass transfer characterization included measurement of both the oxygen and the CO2 transfer rate in each section of the reactor. The power consumption has been verified to be below 2 W/m3 in an adequate design, including the use of optimally designed deflectors in the bends and sumps. The use of sumps allows enhancement of the mass transfer capacity of the reactor, especially for CO2 supplied from flue gases. The oxygen removal capacity of the entire reactor is very low, thus dissolved oxygen accumulates during culture experiments, and this reduces the yield of the cultures by enhancement of photorespiration. CO2 transferis very low in ‘standard’ conditions, with efficiencies of only 20 % measured when using flue gases as the CO2 source.The efficiency can be increased up to 90 % by enhancing the sump design and operation. Culture experiments performed in real outdoor conditions allowed verification of improvements in the design and operation of this type of photobioreactor, bringing conditions into a suitable range for efficient biomass productivity. It is concluded that although raceway reactors are widely used their design and operation must be carefully analysed to optimise performance.