(624c) Design of Novel Microalgal Photbioreactor Using Computational Fluid Dynamics

The incessant rise of global CO₂ levels in the environment, depleting fossil fuel reserves and increased human & industrial activities are putting our ecosystem at risk. The search of renewable and sustainable options for generating fuels are on the rise. Microalgae is regarded as a green alternative owing to its several beneficial characteristics such as faster growth rates, CO₂ fixation abilities, etc. Moreover, microalgae has a tremendous potential for efficient CO₂ capture from point sources, biomass production for bio-product generation (e.g. biofuels, cosmetics, fodder and food etc.) and waste-water mitigation. However, one of the major bottlenecks in the microalgae-to-biofuel value chain lies in the growth of microalgae for commercial scale. Microalgae is cultivated in open as well as closed systems. However, the improved performance of a microalgal growth system is generally exhibited in closed photobioreactor which is governed by several physico-chemical factors such as light, pH, mixing, aeration, CO₂ etc. It is generally true that hydrodynamics plays a significant role by improving the mixing of nutrients, gases (improved mass transfer), avoiding the creation of dead zones, improved light penetrability and reduced sheer stress on the biological cells as well as reduced power consumption. In field experiments as well as computer simulation studies, the most common types of PBRs which are thoroughly investigated are bubble column and air-lift reactors. However, there is a scope to enhance flow patterns in PBRs using advanced CFD techniques. Computational Fluid Dynamics (CFD) enables determination of the hydrodynamics of the culture medium and mixing efficiency according to various PBR designs which is otherwise difficult to measure in field experiments. In this work Euler–Lagrangian model has been used to evaluate the turbulence mixing for two phase flow in airlift PBR using commercial CFD software. Three reactor configurations have been proposed with variation in draft tube length and type for assessing its effect on hydrodynamic characteristics. Parameters such as flow rate (Q), mean velocity (u), and mean Turbulent Kinetic Energy (TKE) were characterized for each configuration. The dimensions of the novel design were optimized using iterative design procedure. In all experiments, tap water was used as the liquid phase and was operated in a batch mode, while compressed air (CO₂ rich) was used as the gas phase and was operated in continuous mode. The hydrodynamics properties such as axial dispersion coefficient, mixing time and circulation time were calculated. The experimental results and CFD simulations were thoroughly compared for the novel PBR with that of conventional airlift reactor. In addition to performance evaluation of the novel PBR, a green microalga Chlorella sp. is cultivated in the PBR using air/flue gas and its growth characteristics were determined. The performance of the novel PBR was compared with that of the conventional airlift PBR. The results showed that the novel design had superior liquid circulation properties that resulted in a regular and alternative exposure to light and dark regions of the PBR. Also, CFD led to improved CO₂ mass transfer efficiency, biomass production, nutrient and light utilization profile.