Study of hydrodynamic characteristics in tubular photobioreactors

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.

Enhancing the flow field in parallel spiral-flow column photobioreactor to improve CO2 fixation with Spirulina sp

A parallel spiral-flow column photobioreactor (PSCP) composed of eight spiral-flow columns, and two pipe headers was designed for scale-up cultivation of microalgae to capture CO₂. To solve the disturbance of spiral flow fields among parallel columns, computational fluid dynamics (CFD) simulation was used to optimize the main structural parameters, such as the number and the height of microalgae solution outlet (MSO), to improve flow field structure and enhance the cells' light/dark cycle. The horizontal velocity in the direction of optical path and the turbulent kinetic energy (TKE) reached the peak values of 0.214 m/s and 5.28 m²/s² when MSO number was four and MSO height was 1.05 m. Meanwhile, the disturbance of the spiral flow field among parallel columns are minimum, and microalgae light/dark cycle frequency was 33.3% higher than that of conventional bubble column photobioreactor. Therefore, the biomass yield and CO₂ fixation rate of microalgae increased by 81.5% and 100.5%, respectively.

How Do Operational and Design Parameters Effect Biomass Productivity in a Flat-Panel Photo-Bioreactor? A Computational Analysis

Optimal production of microalgae in photo-bioreactors (PBRs) largely depends on the amount of light intensity received by individual algal cells, which is affected by several operational and design factors. A key question is: which process parameters have the highest potential for the optimization of biomass productivity? This can be analyzed by simulating the complex interplay of PBR design, hydrodynamics, dynamic light exposure, and growth of algal cells. A workflow was established comprising the simulation of hydrodynamics in a flat-panel PBR using computational fluid dynamics, calculation of light irradiation inside the PBR, tracing the light exposure of individual cells over time, and calculation the algal growth and biomass productivity based on this light exposure. Different PBR designs leading to different flow profiles were compared, and operational parameters such as air inlet flowrate, microalgal concentration, and incident light intensity were varied to investigate their effect on PBR productivity. The design of internal structures and lighting had a significant effect on biomass productivity, whereas air inlet flowrate had a minimal effect. Microalgal concentration and incident light intensity controlled the amount of light intensity inside the PBR, thereby significantly affecting the overall productivity. For detailed quantitative insight into these dependencies, better parameterization of algal growth models is required.

Photo-bioreactor design for microalgae: A review from the aspect of CO2 transfer and conversion

Photobioreactor (PBR) is the most critical equipment for microalgal photosynthetic fixation of CO₂. It provides suitable environmental conditions, such as CO₂, light and nutrients, for microalgal growth. As the major carbon source for microalgae, CO₂ gas is pumped into PBR with the formation of bubbles and formed gas-liquid flow. The gas-liquid flow affects CO₂ and nutrients transmission as well as microalgae cells distribution in PBR, thereby affecting the biochemical reaction of microalgae. While the migration and transport of biochemical reaction products affect the two-phase flow, phase distribution and flow resistance in the PBR in return, thus affecting the transport of light and nutrients. Therefore, microalgal photosynthetic rate is determined synthetically by two-phase flow and the transport of CO₂, light and nutrients in PBR. Deep understanding of gas-liquid two-phase flow, energy and mass transfer coupling with microalgal growth in PBR is the cornerstone for the design of an efficient microalgae PBR.

Optimization of Tubular Microalgal Photobioreactors with Spiral Ribs under Single-Sided and Double-Sided Illuminations

Microalgae can be raw materials for the production of clean energy and have great potential for development. The design of the microalgal photobioreactor (PBR) affects the mixing of the algal suspension and the utilization efficiency of the light energy, thereby affecting the high-efficiency cultivation of the microalgae. In this study, a spiral rib structure was introduced into a tubular microalgal PBR to improve the mixing performance and the light utilization efficiency. The number of spiral ribs, the inclination angle, and the velocity of the algal suspension were optimized for single-sided and double-sided parallel light illuminations with the same total incident light intensity. Next, the optimization results under the two illumination modes were compared. The results showed that the double-sided illumination did not increase the average light/dark (L/D) cycle frequency of the microalgae particles, but it reduced the efficiency of the L/D cycle enhancement. This outcome was analyzed from the point of view of the relative position between the L/D boundary and the vortex in the flow field. Finally, a method to increase the average L/D cycle frequency was proposed and validated. In this method, the relative position between the L/D boundary and the vortex was adjusted so that the L/D boundary passed through the central region of the vortex. This method can also be applied to the design of other types of PBRs to increase the average L/D cycle frequency.

Optimized design of a novel filter brick in denitrification deep-bed filter

Denitrification deep-bed filter has been widely applied in the field of advanced wastewater treatment, yet its efficient operation is highly dependent on the filter bricks-controlled water and air distribution system. Considering the restrictions of existing bricks such as poor hydraulic properties and large non-working area during backwashing, a cuboid novel filter brick with two internal distribution chambers was designed and its hydraulic behaviors under three conditions (air washing, water and air joint backwashing, water washing) were simulated using computational fluid dynamic (CFD) analysis. Results showed that the uniformity of fluid velocity distribution was better than that of the conventional brick under two hydraulic conditions, especially in water and air joint backwashing process with a 10% promotion of water and air distribution uniformity. Furthermore, a 30-day engineering validation test was also carried out to testify the actual performance of the novel filter brick. Better performance was testified in the filters with novel bricks. The present study showed that the novel filter brick had a better uniformity of water and air distribution and smaller dead zone area than those of the conventional brick, implying a good feasibility of application in denitrification deep-bed filter.

Laboratory-scale photobiotechnology-current trends and future perspectives

Phototrophic bioprocesses are a promising puzzle piece in future bioeconomy concepts but yet mostly fail for economic reasons. Besides other aspects, this is mainly attributed to the omnipresent issue of optimal light supply impeding scale-up and -down of phototrophic processes according to classic established concepts. This MiniReview examines two current trends in photobiotechnology, namely microscale cultivation and modeling and simulation. Microphotobioreactors are a valuable and promising trend with microfluidic chips and microtiter plates as predominant design concepts. Providing idealized conditions, chip systems are preferably to be used for acquiring physiological data of microalgae while microtiter plate systems are more appropriate for process parameter and medium screenings. However, these systems are far from series technology and significant improvements especially regarding flexible light supply remain crucial. Whereas microscale is less addressed by modeling and simulation so far, benchtop photobioreactor design and operation have successfully been studied using such tools. This particularly includes quantitative model-assisted understanding of mixing, mass transfer, light dispersion and particle tracing as well as their relevance for microalgal performance. The ultimate goal will be to combine physiological data from microphotobioreactors with hybrid models to integrate metabolism and reactor simulation in order to facilitate knowledge-based scale transfer of phototrophic bioprocesses.

Synergy between flow and light fields and its applications to the design of mixers in microalgal photobioreactors

BACKGROUND

Mixers are usually inserted into microalgal photobioreactors to generate vortices that can enhance light/dark cycles of algal cells and consequently enhance biomass productivity. However, existing mixer designs are usually developed using a trial-and-error approach that is largely based on the designer's experience. This approach is not knowledge-based, and thus little or no understanding of the underlying mechanisms of mixer design for mixing performance of photobioreactors is attained. Moreover, a large pumping cost usually accompanies mixer introduction, and this cost is not favorable for practical applications. This study aims to improve this situation.

RESULTS

In addition to the individual effects of flow and light fields, improving the synergy (coordination) between these fields may markedly enhance the L/D cycle frequency with a lower increase in pumping costs. Thus, the idea of synergy between flow and light fields is introduced to mixer design. Better synergy can be obtained if (a) the vortex core and L/D boundary are closer to each other and (b) the vortex whose core is too far from the L/D boundary is removed. The synergy idea has two types of applications. First, it can facilitate a better understanding of known numerical and experimental results about mixer addition. Second, and more importantly, the idea can help to develop new rules for mixer design. A helical mixer design is provided as a case study to demonstrate the importance and feasibility of the synergy idea. An effective method, i.e., decreasing the radial height of the helical mixer from the inner side, was found, by which the L/D cycle frequency was enhanced by 10.8% while the pumping cost was reduced by 23.8%.

CONCLUSIONS

The synergy idea may be stated as follows: the enhancement of L/D cycle frequency depends not only on the flow and light fields individually but also on their synergy. This idea can be used to enhance our understanding of some known phenomena that emerge by mixer addition. The idea also provides useful rules to design and optimize a mixer for a higher L/D cycle frequency with a lower increase in pumping costs, and these rules will find widespread applications in PBR design.

Strengthening mass transfer of carbon dioxide microbubbles dissolver in a horizontal tubular photo-bioreactor for improving microalgae growth

A CO₂ microbubbles dissolver (CMD) was developed to facilitate dissolving inorganic carbon and strengthening mass transfer in a horizontal tubular photo-bioreactor system (HTPBRS), which enhanced microalgae biomass productivity with flue gas containing 15% CO₂. The influence of pump power on the bubble formation and mixing effect was found to be more obvious than that of gas flow rate. Ceramic shell aerator was more favorable for reducing bubble diameter and enhancing mass transfer than traditional rubber strip aerator. Bubble formation time decreased by 53.4% and mixing time decreased by 68.9% in response to the increased pump power. When the base area ratio of ceramic shell aerator to dissolver in the HTPBRS increased, bubble formation time decreased by 19.6% and mass transfer coefficient increased by 80.9%. The biomass yield of microalgae Chlorella PY-ZU1 with ceramic shell aerator was 30% higher than that with rubber strip aerator in the HTPBRS.

Microalgal cultivation and hydrodynamic characterization using a novel tubular photobioreactor with helical blade rotors

Industrial-scale microalgal cultivation for food, feedstocks and biofuel production is limited by several engineering factors, such as cultivation systems. As a closed microalgal growth system, tubular photobioreactors are the most preferred ones in the mass algae production. In this work, microalgal cultivation and hydrodynamic characterization using a novel tubular photobioreactor equipped with helical blade rotors (HBRs) were investigated, with the aid of computational fluid dynamics and cultivation experiments to evaluate the effect of HBRs on the performance of tubular photobioreactor and growth of the Chlorella sp. The results showed that the use of HBRs in tubular photobioreactors would result in swirl flow and increase of radial velocity and circumferential velocity; it also indicated that the HBRs would enable microalgal cells to move forward helically and to be shuttled alternatively between the light zone and the dark zone. This has led to the faster growth rate of Chlorella sp. and no attachment on the tube surface in the tubular photobioreactor during the whole cultivation cycle. In conclusion, the HBRs could improve the performance of tubular photobioreactors and thus impact positively on the cultivation of microalgae cells for biotechnological industry.

Light/dark cycle of microalgae cells in raceway ponds: Effects of paddlewheel rotational speeds and baffles installation

The aim of this work was to study the light/dark (L/D) cycle in raceway ponds (RWPs) by the computational fluid dynamics (CFD) method via determining the hydrodynamics of culture media and cell trajectories. The effects of paddlewheel rotational speed and flow-deflector baffles installation on the L/D cycle were analyzed. The results indicated that, the L/D cycles of microalgae cells decreased with the increase of the paddlewheel rotational speeds, when the paddlewheel rotational speeds ranged from 5 to 12rpm. In addition, the installation of the flow-deflector baffles in RWPs can greatly increase the light time and the ratio of light time to L/D cycle for microalgae cells. The study provided an effective method to characterize the L/D cycles in RWPs, and may have important implications for designing the effective large-scale microalgae culture system.

Installation of flow deflectors and wing baffles to reduce dead zone and enhance flashing light effect in an open raceway pond

To reduce the dead zone and enhance the flashing light effect, a novel open raceway pond with flow deflectors and wing baffles was developed. The hydrodynamics and light characteristics in the novel open raceway pond were investigated using computational fluid dynamics. Results showed that, compared with the control pond, pressure loss in the flow channel of the pond with optimized flow deflectors decreased by 14.58%, average fluid velocity increased by 26.89% and dead zone decreased by 60.42%. With wing baffles built into the raceway pond, significant swirling flow was produced. Moreover, the period of average L/D cycle was shortened. In outdoor cultivation of freshwater Chlorella sp., the biomass concentration of Chlorella sp. cultivated in the raceway pond with wing baffles was 30.11% higher than that of the control pond.

Effect of Scale on Hydrodynamics of Internal Gas-Lift Loop Reactor-Type Anaerobic Digester Using CFD

This work evaluates the ability of computational fluid dynamics (CFD) to simulate the flow and predict the hydrodynamics of internal gas-lift loop reactor (IGLR)-type anaerobic digester. In addition, it also analyzes if CFD can account for the effects of operating conditions, geometry as well as scale of the reactor. For this purpose, three-dimensional two-phase CFD simulations were performed using CFX for laboratory-scale and pilot-scale IGLR. The CFD predictions were evaluated against experimental data obtained from computer automated radioactive particle tracking (CARPT). The CFD predictions provided good qualitative but only reasonable quantitative comparison. After validation of CFD model, effect of gas flow rate, draft tube diameter, sparger geometry and reactor scale on flow pattern, liquid velocity and dead volume was investigated. Higher gas flow rates did not offer any significant advantage in increasing liquid circulation in the downcomer or decreasing the dead volume. Configuration with draft tube diameter half of tank diameter, equipped with cross sparger showed comparatively better liquid circulation than other configurations. For same superficial gas velocity, increasing the scale increases the magnitude of liquid velocity but fails to match the mixing intensity observed in laboratory scale. Different interphase forces, turbulence models and closures are also evaluated to improve the predictability of CFD models.

Novel flat-plate photobioreactors for microalgae cultivation with special mixers to promote mixing along the light gradient

Novel flat-plate photobioreactors (PBRs) with special mixers (type-a, type-b, and type-c) were designed based on increased mixing degree along the light gradient. The hydrodynamic and light regime characteristic of the novel PBRs were investigated through computational fluid dynamics. Compared with the control reactor without mixer, the novel reactors can effectively increase liquid velocity along the light gradient, the frequency of light/dark (L/D) cycles, and the algal growth rates of Chlorella pyrenoidosa. The maximum biomass concentrations in type-a, type-b, and type-c reactors were 42.9% (1.3 g L(-1)), 31.9% (1.2 g L(-1)), and 20.9% (1.1 g L(-1)) higher than that in the control reactor (0.91 g L(-1)), respectively, at an aeration rate of 1.0 vvm. Correlation analysis of algal growth rate with the characteristics of mixing and light regime shows the key factors affecting algal photoautotrophic growth are liquid velocity along the light gradient and L/D cycles rather than the macro-mixing degree.

Evaluation of power consumption of paddle wheel in an open raceway pond

Open raceway ponds are widely adopted in microalgae cultivation. Paddle wheels consume the most part of power during the process of cultivation in open raceway ponds. The configuration of blades directly determines power consumption for paddle wheels. In this work, power consumption of four blades configurations was determined in a bench-scale open raceway pond of 2.2 m(2). The effect of blades configuration, the influence of filling levels from 5 to 15 cm and influence of rotational speeds from 7 to 15 r min(-1) on shaft power consumption (P(S)), fluid velocity (U(c)) and paddle wheel efficiency (η) was investigated. Results demonstrated that flat blades were the most efficient configuration. Higher culture depth led to larger U(c), more P(s) and larger η, especially when blades were not totally immerged in water. Under the same filling level and rotational speed, the value of P(S) decreases in the order: zigzagged, flat, forward-curved and back-curved, respectively. The zigzagged blades led to a larger U(c) at the culture depth of 5 cm, while flat and forward-curved blades drove a larger U(c) when culture depth was higher than 5 cm. The maximum value of η was 0.50 with flat blades at 11 r min(-1) and 15 cm of culture depth. Empirical correlations of non-dimensional numbers related to operation parameters and blades geometry for four paddle wheel blades were also proposed.

A novel photobioreactor structure using optical fibers as inner light source to fulfill flashing light effects of microalgae

In this work, a novel photobioreactor structure using optical fibers being fixed vertically to culture flow direction as inner light source was proposed to fulfill flashing light effects (FLE) of microalgae, so as to obtain high light efficiency. Three types of optical-fiber photobioreactor fulfilling FLE of microalgae, i.e. air-driven panel, pump-driven panel and stirred tank type, were proposed and a 130 L airlift panel one was practically constructed on which both cold (light profile, liquid velocity) and hot model tests were carried out. Results demonstrated that it could produce uniformed light/dark frequencies being over 10 Hz and microalgae productivity increased by 43% and 38% for Spirulina platensis and Scenedesmus dimorphus respectively, compared with the control. This suggested the structure to be a viable and promising option for future photobioreactors.