Computational analysis of hydrodynamics and light distribution in photo-bioreactors for algae biomass production

Microalgae can be directly used in health food or as bio-filters for waste water treatment. They also have numerous commercial applications in cosmetics, aquaculture and chemical industry as a source of highly valuable molecules, e.g., polyunsaturated fatty acids [1]. Moreover, they are increasingly recognized as a promising source for biodiesel production [2]. To realize the full potential of microalgae, optimal operating conditions for their cultivation in photo-bioreactors (PBR) need to be identified in order to maximize productivity, lipid content, and efficiency of photosynthesis. The most important parameters affecting PBR performance are reactor shape, light intensity distribution, algae growth and other metabolic properties.The presented study aims at analyzing sensitivities to these parameters using Computational Fluid Dynamics (CFD) simulations with the COMSOL Multiphysics software. Specifically, flat panel photo-bioreactors with turbulent mixing due to air sparging and one-sided lighting are studied. First, flow profiles of both liquid and gas phases are computed using the Euler-Euler approach for analyzing the air sparging and detecting potential dead zones. Then, light intensity distributions are calculated, based on absorption and light scattering by algae and gas bubbles. Subsequently, the paths of individual algae are traced, and the environmental conditions they are exposed to are recorded over time, in particular aeration and light intensity. Statistical analysis of the particle traces is performed combining the light exposure with an empirical growth model for algae. Results of the above described simulation stages will be presented and discussed.[1] Spolaore et al.: Commercial applications of microalgae, J. Biosci. Bioeng. 101 (2006): 87-96.[2] Bitog et al.: Application of computational fluid dynamics for modeling and designing photobioreactors for microalgae production: A review, Comput. Electron. Agr. 76 (2011): 131-147

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

How Do Operational and Design Parameters Effect Biomass Productivity in a Flat-Panel Photo-Bioreactor? A Computational Analysis

Optimal production of microalgae in photo-bioreactors (PBRs) largely depends on the amount of light intensity received by individual algal cells, which is affected by several operational and design factors. A key question is: which process parameters have the highest potential for the optimization of biomass productivity? This can be analyzed by simulating the complex interplay of PBR design, hydrodynamics, dynamic light exposure, and growth of algal cells. A workflow was established comprising the simulation of hydrodynamics in a flat-panel PBR using computational fluid dynamics, calculation of light irradiation inside the PBR, tracing the light exposure of individual cells over time, and calculation the algal growth and biomass productivity based on this light exposure. Different PBR designs leading to different flow profiles were compared, and operational parameters such as air inlet flowrate, microalgal concentration, and incident light intensity were varied to investigate their effect on PBR productivity. The design of internal structures and lighting had a significant effect on biomass productivity, whereas air inlet flowrate had a minimal effect. Microalgal concentration and incident light intensity controlled the amount of light intensity inside the PBR, thereby significantly affecting the overall productivity. For detailed quantitative insight into these dependencies, better parameterization of algal growth models is required

Computational analysis of hydrodynamics and light distribution in photo-bioreactors for algae biomass production

Microalgae can be directly used in health food or as bio-filters for waste water treatment. They also have numerous commercial applications in cosmetics, aquaculture and chemical industry as a source of highly valuable molecules, e.g., polyunsaturated fatty acids [1]. Moreover, they are increasingly recognized as a promising source for biodiesel production [2]. To realize the full potential of microalgae, optimal operating conditions for their cultivation in photo-bioreactors (PBR) need to be identified in order to maximize productivity, lipid content, and efficiency of photosynthesis. The most important parameters affecting PBR performance are reactor shape, light intensity distribution, algae growth and other metabolic properties.The presented study aims at optimizing these parameters using Computational Fluid Dynamics (CFD) simulations with the COMSOL Multiphysics software. Specifically, flat panel photo-bioreactors with turbulent mixing due to air sparging and one-sided lighting are studied. First, flow profiles of both liquid and gas phases are computed using the Turbulent Bubbly Flow approach for analyzing the air sparging and detecting potential dead zones for different shapes of flat panel PBR. Then, light intensity distributions are calculated inside the PBR, based on absorption and light scattering by algae and gas bubbles. Subsequently, the Particle Tracing module is used to determine the paths of individual algae cells and the environmental conditions they are exposed to are recorded over time, in particular aeration and light intensity. Results of the above described simulation stages will be presented and discussed.[1] Spolaore et al.: Commercial applications of microalgae, J. Biosci. Bioeng. 101 (2006): 87-96.[2] Bitog et al.: Application of computational fluid dynamics for modeling and designing photobioreactors for microalgae production: A review, Comput. Electron. Agr. 76 (2011): 131-147

Single-cell computational analysis of light harvesting in a flat-panel photo-bioreactor

Abstract Background Flat-panel photo-bioreactors (PBRs) are customarily applied for investigating growth of microalgae. Optimal design and operation of such reactors is still a challenge due to complex non-linear combinations of various impact factors, particularly hydrodynamics, light irradiation, and cell metabolism. A detailed analysis of single-cell light reception can lead to novel insights into the complex interactions of light exposure and algae movement in the reactor. Results The combined impacts of hydrodynamics and light irradiation on algae cultivation in a flat-panel PBR were studied by tracing the light exposure of individual cells over time. Hydrodynamics and turbulent mixing in this air-sparged bioreactor were simulated using the Eulerian approach for the liquid phase and a slip model for the gas phase velocity profiles. The liquid velocity was then used for tracing single cells and their light exposure, using light intensity profiles obtained from solving the radiative transfer equation at different wavelengths. The residence times of algae cells in defined dark and light zones of the PBR were statistically analyzed for different algal concentrations and sparging rates. The results indicate poor mixing caused by the reactor design which can be only partially improved by increased sparging rates. Conclusions The results provide important information for optimizing algal biomass productivity by improving bioreactor design and operation and can further be utilized for an in-depth analysis of algal growth by using advanced models of cell metabolism

Special Issue: Applied Computational Fluid Dynamics (CFD)

Many industrial and manufacturing processes exhibit complex and coupled fluid flow phenomena [...

Experimental and Numerical Study on Tapping of Two Liquids through a Single Tap-Hole

The production of industrial metals in pyrometallurgical smelting furnaces is central to modern industry. Tapping of metal and slag from smelting furnaces is a complex and difficult process. Any variations from tap to tap reduce predictability and impact the planning of downstream logistics. Tapping of metal and slag can be generalized as drainage of two immiscible liquids through a particle bed. In the present paper this is studied by both laboratory experiments and numerical modeling of water and oil drainage from a tank. The results show that the numerical model and physical experiment are consistent. This provides confidence that the numerical models can be applied to quantify tapping from metallurgical furnaces.publishedVersio