Increased production and reduced energy expenditure of massive cultivation tanks of marine microalgae Nannochloropsis oceanica
Microalgae are unicellular, chlorophyllous organisms that form the basis of the food chain in most aquatic environments. These organisms have complex biochemical composition, being able to provide important bioproducts for human and animal consumption. Increasing demand for food and bioproducts to ensure nutritional safety and new drugs can be achieved with the large-scale production of microalgae. However, in order to reach this level, it is necessary to develop new and more productive technologies for cultivating these microorganisms. The massive production of microalgae is still a costly process, which requires a high energy expenditure and a relatively large infrastructure, and is not currently economically viable, especially for the production of low value compounds such as biofuels. The economic viability of a microalgae cultivation system is achieved when high cell densities are obtained in the smallest possible volume and with a minimum energy, water and nutrient expenditure. About one-third of the cost of producing microalgae biomass is related to the energy demand of the mixing / circulation system of the culture tanks. Efficient water column circulation can prevent sedimentation of cells, increase their exposure to incident light, and maximize the nutrient supply for each cell. A very common mixing system in microalgae culture tanks is the injection of air bubbles, which transfers energy from the air bubbles to water, resulting in the circulation (movement) of the water column. The main objective of this thesis was the optimization of the bubbling system in closed systems (photobioreactors) and open (circular tanks). For this, analyzes of bubble size and air flow rate were performed. In the study, numerical modeling techniques were used, using the Computational Fluid Dynamics (CFD) tool and experimental approach. The results of our research are presented as follows: 1) In chapter 1 of this thesis, the effect of air bubble size on the mixing and productivity of two culture tanks was studied: i) closed and vertical cylindrical photobioreactors with 330 L of useful volume; ii) circular open tanks of 1600 L. The initial approach was carried out by applying Fluid Computation Dynamics (CFD) program, which indicated a higher efficiency of smaller
bubbles for the reduction of settling areas (dead zones) and also for the increase of the hydrodynamics of the system. Subsequently experiments were performed on photobioreactors and circular tanks. Air bubble injection systems consisting of a set of nine 1 mm diameter air injectors were compared to standard systems with a single 3 mm diameter injector. Systems with smaller bubbles resulted in production gains of up to 36%, without an increase in energy cost. During the experiments of the first chapter it was verified that significant differences of biomass between the systems with different sizes of air bubbles occurred after 4 to 6 days of experiment. 2) In Chapter 2 we tested the hypotheses that such differences could be caused by i) greater absorption of nutrients, or ii) greater exposure of cells to incident light. Through experiments the incidence of light was measured at various depths of the culture tanks. Likewise, the concentration of dissolved nutrients (ammonia, nitrate and phosphate) was determined throughout the culture. The nutrients were not a limiting factor for cell growth, but the light incidence did, as it decreased dramatically below the 5 cm layer of the tank surface, becoming limiting from 4 to 6 days of culture. It has been suggested that the blending system with smaller bubbles was able to decrease the residence time of the microalgae cells in areas without light of the culture tanks. This result was supported by simulations with lagrangian model, following the microalgae particles during the circulation inside the tank using the CFD tool. 3) In the third chapter different airflows were tested in order to optimize the new mixing system with smaller bubbles. Through experiments with different air flows it was possible to estimate the energy demand of the air bubble injection system. Numerical simulations prior to the experiments with five different flows indicated 3 values that presented the greatest potential for experimental optimization. These three air flow rates were tested in experiments with 330 L photobioreactors. The theoretical analyzes of energy expenditure and experimental biomass production showed that the adjustment of an air flow of 0,024 L min-1 presents the best cost-benefit for the production systems of N. oceanica. However, comparisons of the energy expenditure for the production of microalgae at the different air flows rates and the energy contained in the lipid produced showed that the systems present a negative balance, demanding more optimizations to increase their production.