Numerical investigation of a bubble-column photo-bioreactor design for microalgae cultivation

The popularity of photo-bioreactors (PBRs) for the cultivation of microalgae has increased in the last decade because they can provide a suitable environment in terms of light, nutrients, CO_2, and temperature. Among the many types of PBRs, the bubble-column type is very attractive because of its simple design, easy operation, and energy saving. However, despite the availability of PBRs, only a few have been practically used for mass cultivation of microalgae due to the limitations during scaling-up. Because of the limitations of field experiments; such as limited measuring methods, unstable environmental conditions, time consumption and labour; computational fluid dynamics (CFD) was used for quantitative comparisons of PBR designs with the aim of designing a bubble-column PBR that can be economically used for the mass cultivation of microalgae. Four different multiphase models were investigated to select an appropriate modelling technique and achieve the bubble shape and fluid flow made by bubble injection inside the 2-l PBR. In the interests of model accuracy, a 0.005-s step was chosen with a 4-mm CFD mesh size. The results of model verification tests using laboratory experiment and CFD simulation showed that the surface tension factor was 0.048 N m^-1. The CFD model was used to apply the evaluation methods of mixing efficiency which were proposed in the literature for the quantitative comparison of PBR performance. The results could be used as a basis to design the bubble-column PBRs for microalgae cultivation.