(257e) Use of Computational Fluid Dynamics (CFD) Simulation and Image Analysis Tool for Modelling Light in a Microalgal Photobioreactor

Light, one of the most important parameter for the growth of microalgal cells, is converted into chemical energy by the process of oxygenic photosynthesis. Microalga is a valuable feedstock for the production of industrially important metabolites. The energy for algal growth is obtained from the visible light within the photosynthetically active radiation (PAR) ranging from 400 to 700 nm wavelength. The high density of microalgal cultures is usually attained by cultivation in the closed photo-bioreactors (PBR) as compared to open systems due to better process control, mixing, light distribution etc. However, there is a need for optimizing the cultivation processes that should support both autotrophic and mixotrophic cultivation. Light in particular is known to directly influence the algal biomass production and product yield (Kumar, Sirasale, & Das, 2013). The conventional algal PBRs suffers from self-shading effect which is caused when microalga attains high cell density, thereby reducing the light penetration inside PBR and leading to development of dark zones. This is mainly because gradient of incident light intensity varies along the radius of PBR. Thus, local light intensity distribution at any given point inside the PBR is dependent on the path length, light penetration and absorption coefficient that becomes much acute especially during scaling-up of the process. Existing PBR designs do not have the desired efficiencies, and lead to high material and energy costs. Moreover, predicting the hydrodynamics (fluid flow pattern) of a PBR in laboratory experiments is difficult and cumbersome. Therefore, novel PBR designs need to be developed and benchmarked against existing designs.
Light utilization in the microalgal PBR’s is usually measured by incident light intensity on the surface of the PBR or the irradiating surface area per unit culture volume. Average light intensity received by the microalgal cells is used for measuring the light utilization or photosynthetic efficiency of photosynthetic microorganisms. Efficient mixing allows cells to continuously pass through various zones inside a PBR such as complete dark, photo-limiting (cell growth increases with the light intensity), saturating and photo-inhibition (increase in light intensity leads to growth inhibition) and gives them flashing light effect. Despite several simulation studies on PBRs, a comprehensive analysis and evaluation of light penetration in PBRs has been limited.
The present study discusses use of CFD simulation for designing a novel airlift PBR with improved hydrodynamics of fluid flow. The novel PBR will be used for the cultivation of microalgae. The designed reactor performance in terms of light distribution will be compared with the conventional bubble column PBR. Since the application of multiphase CFD is complicated due to difficulty in describing the variety of interactions in these systems. The hydrodynamics of the reactor will be simulated using CFD and later will be used for microalgal cultivation. The present work describes the use of image analysis technique for the measurement and tracking of cell growth at different growth phases of a live bio-culture in the indigenously designed novel PBR. The light penetrability of incident light intensity was assessed in a Chlorella sp. culture throughout a batch experiment. The light penetration profile could be generated following modified Beer-Lambert’s law by spectrophotometric method. Digital images of the top view of the PBR’s were taken and processed using image processing tool in the MATLAB software. This was used to estimate the light intensity distribution in the externally radiating column PBR across the radial path length. Light gradient pattern inside the algal suspension was assessed by combining fluid dynamics simulations of three-dimensional turbulent single-phase fluid flow with statistical particle tracking and signal analysis. (Perner-Nochta & Posten, 2007). The experimental results and CFD simulations exhibited a good fit. Also, the light penetration was mapped with the image analysis tool for a better understanding of light availability and detection of dark zones inside a working PBR.