Cultivation of micro-algae for biofuel production is not yet commercially viable. Currently, large-scale algal systems produce a wide range of high value products in extremely small niche markets such as dyes, antioxidants and health food. Since algae are more profitable in non-fuel applications, low value products, such as biodiesel, need to be produced at low costs and at large scale to be economically viable. Photobioreactors (PBRs) demonstrate higher biomass productivity (9.08 tons/hr) compared to open pond systems (5.45 tons/hr) primarily due to increased surface area and closely controlled environment. However, one of the major hurdles in employing PBRs as the primary method for cultivation of algae biomass is the lack of consensus on how the geometry and operation of the PBR influences the photosynthetic yield and how it affects scalability. In this study, we model the growth and lipid production kinetics of the algae in conjunction with fluid dynamic effects to predict the yield in a reactor. The study combines an experimentally verified kinetic growth model (Guest et al. 2013), which predicts the photosynthetic kinetics of algae growth, with a computational fluid dynamics (CFD) model that predicts the hydrodynamic behavior and light distribution inside the PBR. The Guest et al. model relates the growth rate of algae to the substrate concentration in culture media and estimates biomass and lipid production based on temperature, pH, light intensity and nutrient consumption. In our study, we use the kinetics and CFD model to compare the performance of conventional flat plate, bubble column and tubular reactors; this methodology can be applied to other reactor geometries. Change in reactor geometry affects the fluid dynamics and light distribution, which affects the mass transfer and algal productivity. The proposed CFD-based approach (Fig.1) models, simultaneously, fluid flow, mass transfer, dispersion phase balances and reactions in the reactor. The reactor geometries are evaluated based on photosynthetic efficiency, volumetric productivity, surface to volume ratio, mass transfer coefficient, extent of photo-inhibition and light intensity distribution.

Coupling fluid dynamics with kinetic modeling to quantify the effects of photosynthetic bioreactor design and operation on yield performance

249th National Meeting (2015)

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