Simulation of algae growth in a bench-scale bubble column reactor

Modeling and Simulation of Photobioreactors with Computational Fluid Dynamics—A Comprehensive Review

Computational Fluid Dynamics (CFD) have been frequently applied to model the growth conditions in photobioreactors, which are affected in a complex way by multiple, interacting physical processes. We review common photobioreactor types and discuss the processes occurring therein as well as how these processes have been considered in previous CFD models. The analysis reveals that CFD models of photobioreactors do often not consider state-of-the-art modeling approaches. As a comprehensive photobioreactor model consists of several sub-models, we review the most relevant models for the simulation of fluid flows, light propagation, heat and mass transfer and growth kinetics as well as state-of-the-art models for turbulence and interphase forces, revealing their strength and deficiencies. In addition, we review the population balance equation, breakage and coalescence models and discretization methods since the predicted bubble size distribution critically depends on them. This comprehensive overview of the available models provides a unique toolbox for generating CFD models of photobioreactors. Directions future research should take are also discussed, mainly consisting of an extensive experimental validation of the single models for specific photobioreactor geometries, as well as more complete and sophisticated integrated models by virtue of the constant increase of the computational capacity.

Model-Based Prediction of Perceived Light Flashing in Recirculated Inclined Wavy-Bottomed Photobioreactors

Microalgae biomass production rate in short light-path photobioreactors potentially can be improved by mixing-induced flashing light regimes. A cascade photobioreactor features a thin liquid layer flowing down a sloping, wavy-bottomed surface where liquid flow exhibits peculiar local recirculation hydrodynamics, potentially conducive to an ordered flashing light regime. This article presents a model-based analysis of the frequency distribution of perceived irradiance in said wavy-bottomed photobioreactor. The model combines a Lagrangian description of the motion of individual cells, in turn derived from the hydrodynamic parameters of the photobioreactor extracted from an experimentally validated Computational Fluid Dynamic model, with a simplified description of the irradiance field across the culture thickness, down to the spectral analysis of perceived irradiance. The main finding of the work is that the wavy bottomed photobioreactor provides a ‘robust’ spectral excitation to the circulating microalgae up to 3 Hz frequency, while in flat panels and bubble columns excitation decays evenly at a 24 db/octave rate. This analysis paves the way to improving the light flashing performance of the wavy-bottomed photobioreactor with respect to geometry (cavity size and installation inclination) and operation (flow rate).

Prediction of maximum algal productivity in membrane bioreactors with a light-dependent growth model

Algal productivity in steady-state cultivation systems depends on important factors such as biomass concentration, solids retention time (SRT), and light intensity. Current modeling of algal growth often ignores light distribution in algal cultivation systems and does not consider all these factors simultaneously. We developed a new algal growth model using a first principles approach to incorporate the effect of light intensity on algal growth while simultaneously considering biomass concentration and SRT. We first measured light attenuation (decay) with depth in an indoor algal membrane bioreactor (A-MBR) cultivating Chlorella sp. We then simulated the light decay using a multi-layer approach and correlated the decay with biomass concentration and SRT in model development. The model was calibrated by delineating specific light absorptivity and half-saturation constant to match the algal biomass concentration in the A-MBR operated at a target SRT. We finally applied the model to predict the maximum algal productivity in both indoor and outdoor A-MBRs. The predicted maximum algal productivities in indoor and outdoor A-MBRs were 6.7 g·m^-2·d^-1 (incident light intensity 5732 lx, SRT approximately 8 d) and 28 g·m^-2·d^-1 (sunlight intensity 28,660 lx, SRT approximately 4 d), respectively. The model can be extended to include other factors (e.g., water temperature and carbon dioxide bubbling) and such a modeling framework can be applied to full-scale, continuous flow outdoor systems to improve algal productivity.

Models of microalgal cultivation for added-value products - A review

Microalgae are considered a promising feedstock for biorefineries given that their chemical composition - rich in carbohydrate and lipid - can be directed towards the co-production of various value-added fuels and chemicals. Production of microalgal biomass for biorefinery purposes requires the identification and establishment of optimal cultivation systems, a crucial yet complicated task due to the numerous factors (e.g. media composition, light, temperature) that simultaneously regulate biomass growth and intracellular composition. Modelling these biological processes, taking into account a single or multiple growth-limiting factors, offers a valuable tool to simulate, design and optimise the dynamics of microalgae cultivation. This review provides an overview of existing models developed to describe microalgal growth processes at the macroscopic scale (also termed black-box models) and discusses their formulation in detail. The black-box kinetic modelling frameworks are compiled into single-factor (6 formulations) and multiple-factor (32 formulations - further divided into non-interactive, additive, and interactive) growth kinetic models, as reported in more than 80 studies, for the prediction of biomass growth as a function of major operational factors such as media composition (e.g. nutrient concentration) and environmental factors (e.g. transient light and temperature). In addition, the review focuses on those models that further account for the production dynamics of two microalgal intracellular products with renowned potential as biorefinery substrates: carbohydrate and lipid molecules. Models of microalgal cultivation dynamics offer a robust engineering tool to understand the natural yet complex responses of microalgae to their growing environment and can help - if used appropriately - to optimise microalgae cultivation and increase the economic viability and sustainability of microalgal systems.

Advanced integration of fluid dynamics and photosynthetic reaction kinetics for microalgae culture systems

Background

Photosynthetic microalgae have been in the spotlight of biotechnological production (biofuels, lipids, etc), however, current barriers in mass cultivation of microalgae are limiting its successful industrialization. Therefore, a mathematical model integrating both the biological and hydrodynamical parts of the cultivation process may improve our understanding of relevant phenomena, leading to further optimization of the microalgae cultivation.

Results

We introduce a unified multidisciplinary simulation tool for microalgae culture systems, particularly the photobioreactors. Our approach describes changes of cell growth determined by dynamics of heterogeneous environmental conditions such as irradiation and mixing of the culture. Presented framework consists of (i) a simplified model of microalgae growth in a culture system (the advection-diffusion-reaction system within a phenomenological model of photosynthesis and photoinhibition), (ii) the fluid dynamics (Navier-Stokes equations), and (iii) the irradiance field description (Beer-Lambert law). To validate the method, a simple case study leading to hydrodynamically induced fluctuating light conditions was chosen. The integration of computational fluid dynamics (ANSYS Fluent) revealed the inner property of the system, the flashing light enhancement phenomenon, known from experiments.

Conclusion

Our physically accurate model of microalgae culture naturally exhibits features of real system, can be applied to any geometry of microalgae mass cultivation and thus is suitable for biotechnological applications.

Multiphysics simulation of algal growth in an airlift photobioreactor: Effects of fluid mixing and shear stress

A multiphysics model has been developed to predict the effects of fluid mixing and shear stress on microalgal growth in an airlift photobioreactor. The model integrates multiphase flow dynamics, radiation transport, shear stress, and algal growth kinetics using an Eulerian approach. The model is first validated by comparing its predictions with experimental data, and then the radiation transport and algal growth kinetics submodels are added to predict biomass accumulation under different flow conditions. The simulations correctly predict biomass growth curves for a wide range of superficial gas flow rates and demonstrate that biomass productivity increases with increased gas flow rate due to better light delivery to microorganisms. However, at the higher gas flow rates considered, shear stress on microorganisms inhibits biomass growth. Lastly, it is shown that the Eulerian approach used here provides a less cumbersome computational approach and provides better predictions than the circulation time and Lagrangian approaches.

Improvement on light penetrability and microalgae biomass production by periodically pre-harvesting Chlorella vulgaris cells with culture medium recycling

To improve light penetrability and biomass production in batch cultivation, a cultivation mode that periodically pre-harvesting partial microalgae cells from suspension with culture medium recycling was proposed. By daily pre-harvesting 30% microalgae cells from the suspension, the average light intensity in the photobioreactor (PBR) was enhanced by 27.05-122.06%, resulting in a 46.48% increase in total biomass production than that cultivated in batch cultivation without pre-harvesting under an incident light intensity of 160μmolm(-2)s(-1). Compared with the semi-continuous cultivation with 30% microalgae suspension daily replaced with equivalent volume of fresh medium, nutrients and water input was reduced by 60% in the proposed cultivation mode but with slightly decrease (12.82%) in biomass production. No additional nutrient was replenished when culture medium recycling. Furthermore, higher pre-harvesting ratios (40%, 60%) and lower pre-harvesting frequencies (every 2, 2.5days) were not advantageous for the pre-harvesting cultivation mode.

Simulating cyanobacterial phenotypes by integrating flux balance analysis, kinetics, and a light distribution function

BACKGROUND

Genome-scale models (GSMs) are widely used to predict cyanobacterial phenotypes in photobioreactors (PBRs). However, stoichiometric GSMs mainly focus on fluxome that result in maximal yields. Cyanobacterial metabolism is controlled by both intracellular enzymes and photobioreactor conditions. To connect both intracellular and extracellular information and achieve a better understanding of PBRs productivities, this study integrates a genome-scale metabolic model of Synechocystis 6803 with growth kinetics, cell movements, and a light distribution function. The hybrid platform not only maps flux dynamics in cells of sub-populations but also predicts overall production titer and rate in PBRs.

RESULTS

Analysis of the integrated GSM demonstrates several results. First, cyanobacteria are capable of reaching high biomass concentration (>20 g/L in 21 days) in PBRs without light and CO2 mass transfer limitations. Second, fluxome in a single cyanobacterium may show stochastic changes due to random cell movements in PBRs. Third, insufficient light due to cell self-shading can activate the oxidative pentose phosphate pathway in subpopulation cells. Fourth, the model indicates that the removal of glycogen synthesis pathway may not improve cyanobacterial bio-production in large-size PBRs, because glycogen can support cell growth in the dark zones. Based on experimental data, the integrated GSM estimates that Synechocystis 6803 in shake flask conditions has a photosynthesis efficiency of ~2.7 %.

CONCLUSIONS

The multiple-scale integrated GSM, which examines both intracellular and extracellular domains, can be used to predict production yield/rate/titer in large-size PBRs. More importantly, genetic engineering strategies predicted by a traditional GSM may work well only in optimal growth conditions. In contrast, the integrated GSM may reveal mutant physiologies in diverse bioreactor conditions, leading to the design of robust strains with high chances of success in industrial settings.

Photobioreactors for microalgal cultures: A Lagrangian model coupling hydrodynamics and kinetics

Closed photobioreactors have to be optimized in terms of light utilization and overall photosynthesis rate. A simple model coupling the hydrodynamics and the photosynthesis kinetics has been proposed to analyze the photosynthesis dynamics due to the continuous shuttle of microalgae between dark and lighted zones of the photobioreactor. Microalgal motion has been described according to a stochastic Lagrangian approach adopting the turbulence model suitable for the photobioreactor configuration (single vs. two-phase flows). Effects of light path, biomass concentration, turbulence level and irradiance have been reported in terms of overall photosynthesis rate. Different irradiation strategies (internal, lateral and rounding) and several photobioreactor configurations (flat, tubular, bubble column, airlift) have been investigated. Photobioreactor configurations and the operating conditions to maximize the photosynthesis rate have been pointed out. Results confirmed and explained the common experimental observation that high concentrated cultures are not photoinhibited at high irradiance level.

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.

Modeling algal growth in bubble columns under sparging with CO2-enriched air

A theoretical model for predicting biomass growth in semi-continuous mode under sparging with CO(2)-enriched air was developed. The model includes gas-to-liquid mass transfer, algal uptake of carbon dioxide, algal growth kinetics, and light and temperature effects. The model was validated using experimental data on growth of two microalgal species in an internally illuminated photobioreactor: Nannochloropsis salina under gas flow rates of 800 and 1200 mL min(-1) and CO(2) enrichments of 0.5, 1, and 2%; and Scenedesmus sp. at a gas flow rate of 800 mL min(-1) and CO(2) enrichments of 3 and 4%. Temporal algal concentration profiles predicted by the model under semi-continuous mode with harvesting under the different test conditions agreed well with the measured data, with r(2) values ranging from 0.817 to 0.944, p<0.001. As demonstrated, this model can be beneficial in predicting temporal variations in algal concentration and in scheduling harvesting operations under semi-continuous cultivation mode.

Analysis of light regime in continuous light distributions in photobioreactors

Maximum photobioreactor (PBR) efficiency is a must in applications such as the obtention of microalgae-derived fuels. Improving PBR performance requires a better understanding of the "light regime", the varying irradiance that microalgal cells moving in a dense culture are exposed to. We propose a definition of light regime that can be used consistently to describe the continuously varying light patterns in PBRs as well as in light/dark cycles. Equivalent continuous and light/dark regimes have been experimentally compared and the results show that continuous variations are not well represented by light/dark cycles, as had been widely accepted. It has been shown that a correct light regime allows obtaining photosynthetic rates higher than the corresponding to continuous light, the so-called "flashing light effect" and that this is possible in commercial PBRs. A correct PBR operation could result in photosynthetic efficiency close to the optimum eight quanta per O(2).

Kinetic modeling of phototrophic biofilms: The PHOBIA model

A kinetic model for mixed phototrophic biofilms is introduced, which focuses on the interactions between photoautotrophic, heterotrophic, and chemoautotrophic (nitrifying) functional microbial groups. Biofilm-specific phenomena are taken into account, such as extracellular polymeric substances (EPS) production by phototrophs as well as gradients of substrates and light in the biofilm. Acid-base equilibria, in particular carbon speciation, are explicitly accounted for, allowing for the determination of pH profiles across the biofilm. Further to previous models reported in literature, the PHOBIA model combines a number of kinetic mechanisms specific to phototrophic microbial communities, such as internal polyglucose storage under dynamic light conditions, phototrophic growth in the darkness using internally stored reserves, photoadaptation and photoinhibition, preference for ammonia over nitrate as N-source and the ability to utilize bicarbonate as a carbon source in the absence of CO(2). The sensitivity of the PHOBIA model to a number of key parameters is analyzed. An example on the potential use of phototrophic biofilms in wastewater polishing is discussed, where their performance is compared with conventional algal ponds. The PHOBIA model is presented in a manner that is compatible with other reference models in the area of water treatment. Its current version forms a theoretical base which is readily extendable once further experimental observations become available.

Is there an optimal time for laparoscopic cholecystectomy in acute cholecystitis?

BACKGROUND

Laparoscopic cholecystectomy (LC) is safe in acute cholecystitis, but the exact timing remains ill-defined. This study evaluated the effect of timing of LC in patients with acute cholecystitis.

METHODS

Prospective data from the hospital registry were reviewed. All patients admitted with acute cholecystitis from June 1994 to January 2004 were included in the cohort.

RESULTS

Laparoscopic cholecystectomy was attempted in 1,967 patients during the study period; 80% were women, mean patient age was 44 years (range, 20-73 years). Of the 1,967 LC procedures, 1,675 were successful, and 292 were converted to an open procedure (14%). Mean operating time for LC was 1 h 44 min (SD +/- 50 min), versus 3 h 5 min (SD +/- 79 min) when converted to an open procedure. Average postoperative length of stay was 1.89 days (+/- 2.47 days) for the laparoscopic group and 4.3 days (+/- 2.2 days) for the conversion group. No clinically relevant differences regarding conversion rates, operative times, or postoperative length of stay were found between patients who were operated on within 48 h compared to those patients who were operated on post-admission days 3-7.

CONCLUSIONS

The timing of laparoscopic cholecystectomy in patients with acute cholecystitis has no clinically relevant effect on conversion rates, operative times, or length of stay.

Analyzing and modeling of photobioreactors by combining first principles of physiology and hydrodynamics

Mixing in photobioreactors is known to enhance biomass productivity considerably, and flow dynamics play a significant role in the reactor's performance, as they determine the mixing and the cells' movement. In this work we focus on analyzing the effects of mixing and flow dynamics on the photobioreactor performance. Based on hydrodynamic findings from the CARPT(Computer Automated Radioactive Particle Tracking) technique, a possible mechanism for the interaction between the mixing and the physiology of photosynthesis is presented, and the effects of flow dynamics on light availability and light intensity fluctuation are discussed and quantitatively characterized. Furthermore, a dynamic modeling approach is developed for photobioreactor performance evaluation, which integrates first principles of photosynthesis, hydrodynamics, and irradiance distribution within the reactor. The results demonstrate the reliability and the possible applicability of this approach to commercially interesting microalgae/cyanobacteria culture systems.

Optimisation of cultivation parameters in photobioreactors for microalgae cultivation using the A-stat technique

Light availability inside the reactor is often the bottleneck in microalgal cultivation and for this reason much attention is being given to light limited growth kinetics of microalgae, aiming at the increase of productivity in photobioreactors. Steady-state culture characteristics are commonly used for productivity optimisation and for cell physiology studies in continuous cultures, and are normally achieved using chemostat cultivations. In the present study, we investigated the applicability of a new and dynamic cultivation method called acceleration-stat (A-stat) to microalgae cultivations where light is the limiting substrate. In the A-stat, the dilution rate is increased at a constant rate. This acceleration rate should be a compromise between a short cultivation time, in order to make it a fast process, and the metabolic adaptation rate of the microorganism to changes in the environment. Simulations of the A-stat were done with different acceleration rates to have an indication of the best rate to use. An A-stat was performed in a pilot plant bubble column (65 l) with Dunaliella tertiolecta as a model organism, and results showed that a pseudo steady state was maintained throughout the experiment. From this work, it was concluded that the A-stat can be used as a fast and accurate tool to determine kinetic parameters and to optimise any specific type of photobioreactor.