Development and mathematical modeling of an airlift-driven raceway reactor for microalgae cultivation

A new airlift-driven raceway reactor configuration for microalgae cultivation was developed in this research. A preliminary liquid circulation model was proposed to predict the liquid circulation velocity in the reactor system based on theoretical energy balance. Theoretical analysis of the energy requirements for the traditional paddlewheel--driven raceway reactor and the proposed airlift-driven raceway reactor showed that the mixing energy requirement in a paddlewheel-driven raceway could be reduced by as much as 60--80% with the proposed airlift configuration. Two prototype versions (20 L and 23 L) of the airlift--raceway reactor were fabricated and tested. The feasibility of microalgae cultivation in these prototype airlift-raceway reactors was tested with two model microalgae species, Scenedesmus sp. and Nannochloropsis salina, under a range of CO2 supply rates.;Fresh water microalgae, Scenedesmus sp. were cultivated in the 23-L airlift--driven raceway reactor under artificial lighting and laboratory conditions, in batch and continuous modes. In continuous mode, the maximum volumetric biomass productivity was 0.19 dry g L-1 day-1.;Marine microalgae Nannochloropsis salina were tested in the 20-L and 23-L versions of the airlift--driven raceway reactor under artificial lighting and outdoor sunlight, in batch and continuous modes. In continuous modes, the maximum volumetric biomass productivity were 0.072 dry g L-1 day-1 and 0.077 dry g L-1 day-1 in laboratory and outdoor conditions, respectively.;As a part of this research, a mathematical model for predicting biomass growth in the airlift-raceway design was developed. This model included supply and transfer of CO2 and the synergetic effects of light, CO 2, nitrogen, and temperature. Biomass concentrations predicted by the proposed model with two test species, Nannochloropsis salina and Scenedesmus sp., agreed well with the temporal trend of the experimental data, for both indoor and outdoor conditions, with r 2 ranging from 0.96 to 0.98, p < 0.001.;Based on cultivation energy requirements, biomass productivities per unit power input in the proposed reactor configuration under laboratory conditions (0.30 to 0.69 dry g W-1 day-1) were comparable to or better than those reported in the literature for different photobioreactors (0.10 to 0.51 dry g W-1 day-1). Under outdoor conditions, a maximum net energy gain of 9.3 W m-3 was achieved with Nannochloropsis salina based on the energetic value of algal biomass. The maximum CO2 utilization efficiency (33%) demonstrated with the airlift-driven raceway configuration with Scenedesmus sp. (0.25 CO2 to air ratio) under laboratory conditions was comparable to or better than those reported in the literature for various bioreactors (2--35%).