| Summary: | The doctoral thesis has be developed in the framework of the research project "Control and optimization of biomass production with microalgae as a source of renewable energy" (DPI2014-55932-C2-1-R), which is focused on the modeling and control of the combined process of microalgae production and wastewater treatment with industrial reactors. This research project is a continuation of a previous project entitled "Modelling, Control and Optimization of Photobioreactors", where significant results were obtained in the field of modelling and automatic control of microalgae production in closed tubular photobioreactors. The current project continues in the same line, with the application of modelling and control techniques for the optimal production of biomass, but now focused on open photobioreactors, which are the most used worldwide.
This thesis aims to improve the knowledge regarding to open reactors characterization, light distribution and utilization by microalgae, mass transfer and oxygen accumulation, as well as the use of control strategies to improve this technology. Both, raceway and thin-layer reactors are considered in this thesis. The information obtained from this thesis is being applied into the European Project Horizon 2020 SABANA focused to the development of microalgae based biorefineries for the improvement of agriculture and aquaculture sectors.
The experimental work has been developed at three different locations: (i) “Las Palmerillas” Experimental Station (Almería, Spain) where experiments related with the improvement of open raceway reactors was performed, (ii) “Algatech” Experimental Station (Třeboň, Czech Republic) where experiments related with the evaluation of thin-layer reactors were performed, and (iii) “IFAPA” Experimental Station (Almería, Spain) where experiments related with the modelling of thin-layer cascade reactors were carried out.
The major contributions on this thesis can be summarized such as:
1. Characterization and improvement of open raceway reactors
Previous works demonstrated that dissolved oxygen accumulation affects to photosynthesis activity. This thesis demonstrates that dissolved oxygen accumulation limits the biomass productivity in raceway reactors if the mass transfer capacity is not improved. Although oxygen is desorbed to the air in the channel and the paddlewheel, this is not enough to remove the oxygen produced by photosynthesis when high biomass productivity is achieved, thus being necessary to include a sump, which adequately designed and operated, contributes to avoid oversaturation of oxygen. Therefore, the mass transfer capacity in the sump must be optimized to compensate the oxygen production rate in the system. Moreover, the influence of gas flow on the mass transfer coefficient was also determined, obtaining a calibrated empirical model. Using this model, it is possible to properlyregulate the air flow in the sump and thus, the reactor operation can be optimized. Full information is available in (Barceló-Villalobos et al., 2018).
To improve the productivity of microalgae reactors in order to optimize the light pattern at which the cells are exposed to into the reactors must be optimized. For that, the first step is to know the real light pattern taking place in raceway reactors. This thesis demonstrates that microalgae cells are adapted to local irradiance because of the unfavourable cell movement pattern in raceway reactors. It has also been demonstrated how the light regime at which the microalgae cells are exposed to in a raceway reactor is far from the optimal one required to optimize the performance of microalgae cultures through light integration. Photosynthesis rate measurements were performed along different seasons at different daytime by using different light/dark cycles. These assays confirmed that no light integration exists at 0.15 m water depth. Moreover, it has also been confirmed that the cells are adapted to the local irradiance inside the reactor. Full information is available in (Barceló-Villalobos et al., 2019a).
Regarding control strategies, a selective control strategy proposed previously by Pawlowski et al., 2015, has been used to control pH and dissolved oxygen simultaneously. In this control, the pH value is prioritized over the dissolved oxygen value since it has a critical influence on the process performance. This thesis demonstrates the correct functionality of this selective control approach in a semi-industrial raceway (100 m2) operated in semi-continuous mode. Furthermore, the oxygen mass transfer model already developed (Klal sump model) in the present thesis, has been validated in a simulation stage to demonstrate that it is possible to adjust the mass transfer capacity of the system close to the optimal value by controlling gas injections. It is shown that it is possible to reduce gas inflow actuations and control oxygen accumulation in the system by using a feedback control strategy. Finally, it has also been demonstrated that when the dissolved oxygen reference goes down respect to the initial reference (250% Sat), the necessary gas flow is higher (full information is available in Barceló-Villalobos et al., 2019c; Barceló-Villalobos et al., 2019d)
2. Characterization and improvement of thin-layer reactors
It has been demonstrated that although thin-layer reactors are currently more productive than raceway reactors, their productivity can also be improved if the operating conditions are optimized close to the optimal culture values. This is the first step in optimizing and scaling-up this type of reactor for industrial applications. This thesis demonstrates the influence of variations of culture parameters (irradiance, temperature, pH and dissolved oxygen) on the performance of a microalgae culture. Different assays were done to analyse the system parameters in terms of position inside the reactor and time of the daylight cycle. Results demonstrate that average irradiance and temperature to which the cells are exposed are mainly a function of time, whereas pH and dissolved oxygen concentrations also showed relevant gradients depending on their position inside the reactor.
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Furthermore, it has also been demonstrated that the existence of culture parameters gradients reduces the performance of the cultures (using two different methodologies: chlorophyll-fluorescence and net photosynthesis rate methods). Moreover, the influence of culture conditions on Scenedesmus almeriensis cell performance was modelled. Full information is available on (Barceló-Villalobos et al., 2019b).
The performance of pilot scale thin-layer reactors located in Algatech (Trebon) has been also evaluated. Temperature and dissolved oxygen production have been analysed and modelled at three different pilot-scale thin-layer cascade reactors (small, medium, and large). Different assays were developed to analyse: (i) the variation of culture conditions, (ii) oxygen mass balance and (iii) model the oxygen production. Temperature is a stable parameter along the channel and through the day. On the other hand, dissolved oxygen increases along the channel through the day as it is expected by photosynthesis process. The modelling of oxygen production has been done by using light integration is here reported. Temperature and dissolved oxygen measurements were done along the thin-layer cascade reactor along the day. It was demonstrated that it is more accurately to use the integrated average irradiance parameter than the average irradiance concept, to demonstrate the effective light use into the culture. Full information is available on (Barceló-Villalobos et al., in review).
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