Model-based versus model-free control designs for improving microalgae growth in a closed photobioreactor: Some preliminary comparisons

Controlling microalgae cultivation, i.e., a crucial industrial topic today, is a challenging task since the corresponding modeling is complex, highly uncertain and time-varying. A model-free control setting is therefore introduced in order to ensure a high growth of microalgae in a continuous closed photobioreactor. Computer simulations are displayed in order to compare this design to an input-output feedback linearizing control strategy, which is widely used in the academic literature on photobioreactors. They assess the superiority of the model-free standpoint both in terms of performances and implementation simplicity.Comment: The 24th Mediterranean Conference on Control and Automation (MED'16), Athens, Greece (June 21-24, 2016

Microalgal reactors: a review of enclosed system designs and performances

One major challenge to industrial microalgal culturing is to devise and develop technical apparata, cultivation procedures and algal strains susceptible of undergoing substantial increases in efficiency of use of solar energy and carbon dioxide. Despite several research efforts developed to date, there is no such thing as “the best reactor system”- defined, in an absolute fashion, as the one able to achieve maximum productivity with minimum operation costs, irrespective of the biological and chemical system at stake. In fact, choice of the most suitable system is situationdependent, as both the species of alga available and the final purpose intended will play a role. The need of accurate control impairs use of open-system configurations, so current investigation has focused mostly on closed systems. In this review, several types of closed bioreactors described in the technical literature as able to support production of microalgae are comprehensively presented and duly discussed, using transport phenomenon and process engineering methodological approaches. The text is subdivided into subsections on: reactor design, which includes tubular reactors, flat plate reactors and fermenter-type reactors; and processing parameters, which include gaseous transfer, medium mixing and light requirements

Optimization of microalgae production in industrial open reactors

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. VIII 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)

Modelling and pH Control in Raceway and Thin-Layer Photobioreactors for Wastewater Treatment

One of the most critical variables in microalgae-related processes is the pH; it directly determines the overall performance of the production system especially when coupling with wastewater treatment. In microalgae-related wastewater treatment processes, the adequacy of pH has a large impact on the microalgae/bacteria consortium already developing on these systems. For cost-saving reasons, the pH is usually controlled by classical On/Off control algorithms during the daytime period, typically with the dynamics of the system and disturbances not being considered in the design of the control system. This paper presents the modelling and pH control in open photobioreactors, both raceway and thin-layer, using advanced controllers. In both types of photobioreactors, a classic control was implemented and compared with a Proportional–Integral (PI) control, also the operation during only the daylight period and complete daily time was evaluated. Thus, three major variables already studied include (i) the type of reactors (thin-layers and raceways), (ii) the type of control algorithm (On/Off and PI), and (iii) the control period (during the daytime and throughout the daytime and nighttime). Results show that the pH was adequately controlled in both photobioreactors, although each type requires different control algorithms, the pH control being largely improved when using PI controllers, with the controllers allowing us to reduce the total costs of the process with the reduction of CO2 injections. Moreover, the control during the complete daily cycle (including night) not only not increases the amount of CO2 to be injected, otherwise reducing it, but also improves the overall performance of the production process. Optimal pH control systems here developed are highly useful to develop robust large-scale microalgae-related wastewater treatment processe

Mathematical Model of a Bubble Column for the Increased Growth of Arthrospira platensis and the Formation of Phycocyanin

The objective of this research was to develop a mathematical model for batch photoautotrophic cultivation of Arthrospira platensis and to validate it against data obtained in experiments. All trials were carried at 30°C, under a light intensity of 60 or 120 µmol m-2s-1. The purpose of the model was to determine the optimal concentration of carbon dioxide, as well as to investigate the formation of phycocyanin. For the experimental conditions in this study, the optimal concentration carbon dioxide (0.8% CO2, v/v) was predicted using the model according to the initial bicarbonate level, the carbon uptake by the microalga, the pH, and the mass transfer process. The use of this optimal value in the gas inlet seems to be a suitable option for maintaining the optimal pH (9.5), thereby eliminating the need for a pH controller in the bioreactor system. According to the simulations, the mass fraction of the phycocyanin formation rate seems to depend on the internal light level. The percentage of adjustment obtained (R2) was ?75%. The velocity of phycocyanin formation was enhanced at intensities up to 120 µmol m-2s-1. However, the actual internal irradiance values were lower than the light compensation point (4.5 µmol m-2s-1), so phycocyanin formation ceased. The mathematical model may facilitate the examination of optimal carbon delivery, as well as the light input, in several A. platensis culture conditions aimed at phycocyanin production

Analysis of mass transfer capacity in raceway reactors

In the present work, a methodology is proposed to determine the mass transfer capacity in existing microalgae raceway reactors to minimize excessive dissolved oxygen accumulation that would otherwise reduce biomass productivity. The methodology has been validated using a 100 m2 raceway reactor operated in semi-continuous mode. The relevance of each raceway reactor section was evaluated as well as the oxygen transfer capacity in the sump to different air flow rates. The results confirm that dissolved oxygen accumulates in raceway reactors if no appropriate mass transfer systems are provided. Therefore, mass transfer in the sump is the main contributor to oxygen removal in these systems. The variation in the volumetric mass transfer coefficient in the sump as a function of the gas flow rate, and therefore the superficial gas velocity in the sump, has been studied and modelled. Moreover, the developed model has been used to estimate the mass transfer requirements in the sump as a function of the target dissolved oxygen concentration and the oxygen production rate. The proposed methodology allows us to determine and optimize the mass transfer capacity in the sump for any existing raceway reactor. Moreover, it is a powerful tool for the optimization of existing reactors as well as for the design optimization of new reactors