Model curation and validation for a tubular photobioreactor in mixotrophy

Effective scale-up of microalgae cultures in photobioreactors is often limited by low biomass productivity. Even though mixotrophic cultivation may be an effective solution, oxygen balance and concentration gradients are crucial aspects for optimization which have scarcely been investigated. Thus, this Thesis describes the improvement of a model that predicts concentration profiles of substrate, oxygen, carbon dioxide and inert gases in a biphasic tubular photobioreactor for Galdieria sulphuraria. The model can be effectively employed to simulate the reactor with constant light input or real daylight irradiance data. Three different control strategies have been integrated into the model for comparisons. All the Proportional-Integral-Derivative (PID) control logics, that have been designed and tested in silico, resulted in a favorable response of the system in both constant and real light regime, also preventing anoxic conditions in the reactor. These results suggest that the process is appropriate for further optimization in pilot-scale and large-scale reactors. A scale-down experiment to simulate the substrate and oxygen gradients in the tubular photobioreactor has been successfully conducted in a lab-scale stirred tank reactor under constant light conditions. The experiment demonstrated that oxygen concentration in gas phase can be an effective control variable to reach stability and prevent anoxia. The control strategy has been designed beforehand with the aid of the model, and it resulted in a stable and robust control system. Oxygen concentration in gas phase, dissolved oxygen, substrate concentration, and biomass productivity have been compared with the model results, and they are reasonably matching the predictions of the model. Furthermore, it has been proven that the dissolved oxygen gradients achieved with the employed cultivation strategy did not affect the pigment profile of the microalga nor the dark-adapted efficiency of photosystem II. The model that has been improved and the findings presented in this Thesis demonstrate the feasibility of a pilot-scale cultivation of G. sulphuraria, and they will also be important for future developments and optimization of mixotrophic cultivation of other microalgal strains in tubular photobioreactors.