A macromodel for outdoor algal mass production

Modelado y control de la producción de microalgas en fotobiorreactores industriales

<p>Este artículo presenta una visión general sobre el proceso de producción de microalgas desde un punto de vista de modelado y control de procesos. En primer lugar se exponen las ventajas y el potencial de este tipo de microorganismos, así como los distintos tipos de reactores que se suelen utilizar para su producción. Posteriormente, se analiza el comportamiento dinámico de este tipo de procesos, el cual es muy complejo y cambiante debido a variaciones en las condiciones ambientales tanto diarias como anuales, y se presentan los distintos balances que permiten describir la evolución de las principales variables del sistema. Se exponen distintos tipos de modelos a nivel biológico y a nivel estructural que han sido validados a escala industrial. Tras analizar su comportamiento dinámico, se motivan los distintos problemas de control existentes en este tipo de sistemas y se resume una amplia batería de estrategias de control que han sido evaluadas con éxito en fotobiorreactores industriales. Finalmente, se concluye el trabajo con un balance de los aspectos más importantes expuestos a lo largo del mismo.</p>

Experimental Study of Substrate Limitation and Light Acclimation in Cultures of the Microalgae Scenedesmus obliquus—Parameter Identification and Model Predictive Control

In this study, the parameters of a dynamic model of cultures of the microalgae Scenedesmus obliquus are estimated from datasets collected in batch photobioreactors operated with various initial conditions and light illumination conditions. Measurements of biomass, nitrogen quota, bulk substrate concentration, as well as chlorophyll concentration are achieved, which allow the determination of parameters with satisfactory confidence intervals and model cross-validation against independent data. The dynamic model is then used as a predictor in a nonlinear model predictive control strategy where the dilution rate and the incident light intensity are simultaneously manipulated in order to optimize the cumulated algal biomass production.

Growth modeling of the green microalga Scenedesmus obliquus in a hybrid photobioreactor as a practical tool to understand both physical and biochemical phenomena in play during algae cultivation

In recent years, numerous studies have justified the use of microalgae as a sustainable alternative for the generation of different types of fuels, food supplementation, and cosmetics, as well as bioremediation processes. To improve the cost/benefit ratio of microalgae mass production, many culture systems have been built and upgraded. Mathematical modeling the growth of different species in different systems has become an efficient and practical tool to understand both physical and biochemical phenomena in play during algae cultivation. In addition, growth modeling can guide design changes that lead to process optimization. In the present work, growth of the green microalga Scenedesmus obliquus was modeled in a hybrid photobioreactor that combines the characteristics of tubular photobioreactors (TPB) with thin-layer cascades (TLC). The system showed productivity greater than 8.0 g m^-2 day^-1 (dry mass) for CO₂ -fed cultures, and the model proved to be an accurate representation of experimental data with R² greater than 0.7 for all cases under variable conditions of temperature and irradiance to determine subsystem efficiency. Growth modeling also allowed growth prediction relative to the operating conditions of TLC, making it useful for estimating the system given other irradiance and temperature conditions, as well as other microalgae species.

Attached cultivation for improving the biomass productivity of Spirulina platensis

To improve cultivation efficiency for microalgae Spirulina platensis is related to increase its potential use as food source and as an effective alternative for CO2 fixation. The present work attempted to establish a technique, namely attached cultivation, for S. platensis. Laboratory experiments were made firstly to investigate optimal conditions on attached cultivation. The optimal conditions were found: 25 g m(-2) for initial inoculum density using electrostatic flocking cloth as substrata, light intensity lower than 200 μmol m(-2) s(-1), CO2 enriched air flow (0.5%) at a superficial aeration rate of 0.0056 m s(-1) in a NaHCO3-free Zarrouk medium. An outdoor attached cultivation bench-scale bioreactor was built and a 10d culture of S. platensis was carried out with daily harvesting. A high footprint areal biomass productivity of 60 g m(-2) d(-1) was obtained. The nutrition of S. platensis with attached cultivation is identical to that with conventional liquid cultivation.

Modelling of Microalgae Culture Systems with Applications to Control and Optimization

Mathematical modeling is becoming ever more important to assess the potential, guide the design, and enable the efficient operation and control of industrial-scale microalgae culture systems (MCS). The development of overall, inherently multiphysics, models involves coupling separate submodels of (i) the intrinsic biological properties, including growth, decay, and biosynthesis as well as the effect of light and temperature on these processes, and (ii) the physical properties, such as the hydrodynamics, light attenuation, and temperature in the culture medium. When considering high-density microalgae culture, in particular, the coupling between biology and physics becomes critical. This chapter reviews existing models, with a particular focus on the Droop model, which is a precursor model, and it highlights the structure common to many microalgae growth models. It summarizes the main developments and difficulties towards multiphysics models of MCS as well as applications of these models for monitoring, control, and optimization purposes.

Use of image analysis tool for the development of light distribution pattern inside the photobioreactor for the algal cultivation

Light is one of the important parameters for the growth of photosynthetic microorganisms. In algal photobioreactors, pigmentation of algal cells has additional shading effect which reduces light penetration. Information on the local light intensity inside the photobioreactor is helpful for its efficient designs. Image analysis is based on trichromatic theory and it is used as a tool in studying the light distribution. Digital images of the top view of the photobioreactor were taken and processed using image processing tool in the MATLAB software. This was used to estimate the light intensity distribution in the externally radiating stirred tank photobioreactor across the radial path length. In addition, the effect of light tubes arrangement was studied. This was to find out the effect of light distribution along the periphery of culture suspension. Modified Beer-Lambert's law was found to fit the generated light intensity profile at various cell concentrations and light intensity.

A light distribution model for an internally radiating photobioreactor

Analysis of light energy distribution in culture is important for maximizing the growth efficiency of photosynthetic cells and the productivity of a photobioreactor. To characterize the irradiance conditions in a photobioreactor, we developed a light distribution model for a single-radiator system and then extended the model to multiple radiators using the concept of parallel translation. Mathematical expressions for the local light intensity and the average light intensity were derived for a cylindrical photobioreactor with multiple internal radiators. The proposed model was used to predict the irradiance levels inside an internally radiating photobioreactor using Synechococcus sp. PCC 6301 as a model photosynthetic microorganism. The effects of cell density and radiator number were interpreted through photographic and model simulation studies. The predicted light intensity values were found to be very close to those obtained experimentally, which suggests that the proposed model is capable of accurately interpreting the local light energy profiles inside the photobioreactor system. Due to the simplicity and flexibility of the proposed model, it was also possible to predict the light conditions in other complex photobioreactors, including optical-fiber and pond-type photobioreactors.

Modeling of biomass productivity in tubular photobioreactors for microalgal cultures: effects of dilution rate, tube diameter, and solar irradiance

A macromodel is developed for estimating the year-long biomass productivity of outdoor cultures of microalga in tubular photobioreactors. The model evaluates the solar irradiance on the culture surface as a function of day of the year and the geographic location. In a second step, the geometry of the system is taken into account in estimating the average irradiance to which the cells are exposed. Finally, the growth rate is estimated as a function of irradiance, taking into account photoinhibition and photolimitation. The model interconnects solar irradiance (an environmental variable), tube diameter (a design variable), and dilution rate (an operating variable). Continuous cultures in two different tubular photobioreactors were analyzed using the macromodel. The biomass productivity ranged from 0.50 to 2.04 g L-1 d-1, and from 1.08 to 2. 76 g L-1 d-1, for the larger and the smaller tube diameter photobioreactors, respectively. The quantum yield ranged from 1.1 to 2.2 g E-1; the higher the incident solar radiation, the lower the quantum yield. Simultaneous photolimitation and photoinhibition of outdoor cultures was observed. The model reproduced the experimental results with less than 20% error. If photoinhibition was neglected, and a growth model that considered only photolimitation was used to fit the data, the error increased to 45%, thus reflecting the inadequacy of previous outdoor growth models that disregard photoinhibition. Copyright 1998 John Wiley & Sons, Inc.