Modelling and optimization of algal cultivation in lab scale flat panel Photobioreactors

Microalgae are capable of producing many important chemicals that are used in various industries and can potentially be used as a raw material for biofuels. Optimizing microalgal production is necessary to maximize their usage, because large scale production yields so far are much lower than expected from laboratory measurements. Microalgae accumulate biomass via photosynthesis, therefore raw materials required are water, CO2, light and nutrients such as nitrogen, phosphorus and potassium. Out of them light availability to individual cell is a crucial factor in determining microalgal growth. The aim of the present study is to computationally analyze the impact of operational and design parameters of an air sparged lab-scale flat panel microalgal photobioreactor (PBR) to maximize the overall productivity. Fluid dynamic simulations were performed to characterize the flow profiles of both water and air, which were further used to calculate the traces of microalgal cells. The light intensity profile in the PBR was calculated independently and combined with particle traces to obtain their dynamic light exposure. Specific growth rates of individual cells were then calculated using two different growth models (one instantaneous and one accounting for light history of individual cells) proposed in the literature as a function of light intensity received by cells. Further, productivity was calculated from these specific growth rates. This procedure was repeated for different values of several operational parameters and design parameters. It was observed that varying the air inlet flowrate affected the overall light exposure of cells only in direct comparison between very small and high flowrates, whereas a 50% change in the range of high flowrates had no significant effect. Varying the kinetic parameters of the dynamic growth model only had an effect on overall productivity if the change was in orders of magnitude. Microalgal concentrations, external light intensities and microalgal species strongly determine the amount of light intensity inside the PBR and thus growth rates and productivity. The PBR design was found to have a strong effect on algal growth rates and productivity. Hence, two new alternate designs were proposed by changing the location of the air inlet holes and the internal shape of the PBR. This substantially changes the flow profiles of microalgal cells and thus their light exposure. The new designs show higher productivity than the standard design when calculated with the dynamic growth model. Changing the position of light incidence from front to back wall does not affect the performance significantly. Irradiating light from both sides leads to higher productivities than irradiance from one side for all designs even for standard PBR design, because the light distribution inside the PBR is more homogeneous. In this study a framework has been established for systematically studying the performance of flat panel PBRs. This approach can be used to study various parameters such as different PBR geometries, growth parameter sets, algal species etc