Analysis and design of photobioreactors (PBRs) for the production of biomass and high-value metabolites derived from microalgae.

IGNACIO NIIZAWA; JOSUE MIGUEL HEINRICH; FAUSTO ADRIAN BOTTA; IGNACIO FERRERO; ALEJANDRO RAUL TROMBERT; HORACIO ANTONIO IRAZOQUI

Berlín

Conferencia; 19th European Biomass Conference & Exhibition; 2011

The term ?microalgae? includes microscopic algae (eukaryotic organisms) and cyanobacteria (oxygenic photosynthetic bacteria). Microalgal biomass has recently received considerable attention as it is regarded as a potential source of biofuels. Compared with vascular plants, microalgae show a higher photosynthetic efficiency. A PBR is a system that uses light ?natural or artificial- to grow algae via the photosynthetic mode of cultivation.The purpose of the work is to present and apply a methodology which provides PBRs mathematical models that permit an a priori and rigorous design of reactors. Thus, from laboratory data, the scale-up can be performed with no limitations in the size or shape of a large scale application. This modeling approach of PBRs is derived from biochemical reaction engineering principles and radiative energy transport fundamentals. The first reaction of photosynthesis depend on light quantity (intensity) and quality (specific wavelengths) and these parameters are critical because at a certain level algae experience light saturation. The analytic procedure is illustrated by presenting the modeling of a batch annular PBR that is used in order to obtain the kinetic parameters. The design procedure is illustrated by presenting the modeling of the same PBR that is used in a predictive mode. In order to formulate the photosynthetic reaction rate properly, the radiative transfer equation applied to a participating, reactive and heterogeneous medium is presented. The approach followed in this work includes the following steps: a) the construction of a bench-scale PBR capable of working with different irradiation levels and at mixing regimes as close to perfect as possible; b) the validation of perfect mixing regime by performing input-response experiments -in this way we ensure that we have measured an intrinsic rate, free of the interference of possible imperfect mixing effects-; c) the physical and mathematical modeling of the radiation field in the PBR and the development of its solution algorithm; d) the proposal of a plausible mechanism for the photosyntethic process on which to base an intrinsic kinetic model; e) the operation of the PBR within a defined range of experimental conditions to obtain accurate laboratory data by means of the adopted analytical procedures; and vi) validation of the kinetic model against experimental data and regression of the model parameters by means of a Genetic Algorithm.We employed as model microorganisms Scenedesmus quadricauda and Chlorella sp. The production of microalgae was carried out in a bench scale annular PBR. We used several inorganic synthetic media for microalgae culture. The PBR has been supplied with CO2 as the culture carbon source. The effect of different operating conditions on the reactor performance has been explored. The operating variables chosen for this purpose were the radiation intensity reaching the reactor, the alternation of light/darkness periods of time, the CO2 concentration in the feed stream, temperature, pH and mixing conditions. The reactor filled with the corresponding medium was sterilized in situ employing germicidal UV radiation during 30 min. The reactor was inoculated with the microalgae suspension (300 mL aprox.) in late exponential/early stationary phase of growth in the same culture medium but without external supply of CO2. In order to follow the time evolution of the cultures we have drawn samples from the reactor every 12 h during 3 weeks and we measured the pH, counted the individual cells in the microscope with a Neubauer chamber, measured the optical density of the sample at 540 nm and determined the total biomass (mg/L) employing a gravimetric technique (total suspended and total volatile solids determination). We also measured the total lipid contents using the method of Bligh y Dyer on previously disrupted cells. For cell disruption we have used different procedures including freezing at - 20°C, mortar grinding and homogenization, autoclave treatment during 15 min at 121°C and acidic hydrolysis with HCl 10 N during 1 hour at 90 °C. The relevance of this contribution is to obtain an intrinsic kinetic model that could be used for scaling up purposes in microalgae production.