Research PaperOptimised hydrodynamic parameters for the design of photobioreactors using computational fluid dynamics and experimental validation
Introduction
Bubble column photobioreactors (PBRs) which are efficient in growing photosynthetic cells have received enormous attention over the past decade. This design of PBR offers many advantages such as simplicity in the design with no moving parts, simplicity of construction and ease of operation (Bitog et al., 2011). They have suitable heat and mass transfer characteristics and require less operational cost because of their low energy input requirements. From an engineering perspective, the structural configuration and design of the PBRs have a critical role in the flow hydrodynamics which is also important in providing ideal growth conditions for the cells. Thus, the hydrodynamics of bubble column PBRs of various geometries has been investigated both in the laboratory and through simulation studies (Table 1). However, the current PBR designs still need to resolve many hydrodynamic issues within the PBR especially for the process of scaling-up. Many equally important factors should be considered such as light penetration and distribution, gas injection and mixing, the cell species and the cells response to shear stress (Bitog et al., 2011, Michels et al., 2010, Perez et al., 2006). In culturing high density cells in PBRs, the main criteria are good mixing, mass transfer and light utilisation (Chiu, Tsai, Kao, Ong, & Lin, 2009). These factors are closely interrelated such that the penetration and diffusion of light inside the PBRs is affected by the mixing characteristics which in turn are strongly influenced by the gas injection method (Bitog et al., 2011).
In bubble column PBRs, flow hydrodynamics is realised through bubbles which are usually introduced at the bottom section. The bubbles also provide more time for mass transfer and CO2 supply. The reactant gas itself provides the stirring action that is required to conduct gas–liquid and gas–liquid–solid interactions and reaction (Rampure, Kulkarni, & Ranade, 2007). The higher superficial gas velocity generated by air bubbles in PBRs should result in better mixing, but also increase the shear forces which have been long suspected to cause cell death. Tramper, William, Joustra, and Vlak (1986) distinguished three regions where the cells are likely to experience too much stress which may cause the death of the cells. High shear stresses are suspected to occur at the sparger where the bubbles are formed, in the path where the bubbles rise, and at the surface where bubbles break-up. Reducing the shear stress of microalgae cultures in sparged PBRs was the focus of the study by Barbosa, Hadiyanto, and Wijffels (2003) who reported that bubble formation at the sparger was the main cause of cell death. Therefore, finding an optimum air flow rate which will drive significant mixing but not exceed the shear forces that can cause cell death is important.
Various reports have shown different growth rates of algae when cultivated in different growing conditions and the geometry of the PBR cannot be ignored. It is probably one of the most critical factors to consider especially when scaling up is required. Zuzuki et al. (1995) found a linear correlation between the specific death rate and the inverse of culture height; however, they provided no explanation for this. Camacho, Gomez, Sobczuk, and Grima (2000) reported the inverse behaviour, i.e. an increase in culture height caused an increase in death rate. The authors related the effect to cell attachment of bubbles and suggested that a greater height of rise meant that more cells can be captured by the rising bubbles and carried to the surface where they die as the bubbles rupture. However, Barbosa et al. (2003) disputed this claim by stressing that bubble bursting is not the only factor and might not even be the most important factor leading to cell death. Despite the different conclusions drawn, these authors have shown that the sparger site has a major effect on cell damage and the gas entrance velocity should be considered as a possible indication of cell death. However, more work is still required to clarify the influence of sparging on cell death and its scalability in reactor-design for different microalgae strains (Barbosa et al., 2003).
Increasing the culture height may also cause longer cycle times which can decrease photosynthetic efficiency (Janseen, Slenders, Tramper, Mur, & Wijffels, 2001). Also, rapid circulation has been shown to give rise to considerable higher photosynthetic efficiency (Matthijs et al., 1996). Thus, the required growing conditions of photosynthetic cells in the height of bubble column PBRs are limited in scale. According to Miron, Gomez, Camacho, Grima, and Chisti (1999) the PBR diameter is also limited to some extent. For instance, to ensure light penetration inside the PBR, the diameter should not exceed 200 mm or light availability will be severely reduced. However, until now, no standard PBR geometry or working volume has been recommended in terms of the mass production of photosynthetic cells such as algae. Also, the hydrodynamics inside the PBR are strongly influenced by the geometry of the structure which directly affects the light intensity and mixing conditions, thus structure optimisation is not only necessary, but also critical (Yu et al., 2009).
Recently, numerical simulations have been applied to investigate reactor designs which are always guided by the purpose of the production facility, the cell strain and product of interest. Computational techniques have been used to simulate a large variety of engineering and physical systems. Specifically, the simulation approach attempts to imitate the hydrodynamic behaviour of a system and predict the sequences of events which control that behaviour (Oran & Boris, 2001). In PBR design and analysis, numerical simulation approach in studying fluid flow inside PBRs is now widely recognised among design engineers as an effective tool in predicting the complex inherent phenomena inside PBRs especially in cases where using an experimental approach is restricted by technical constraints (Bitog et al., 2011).
The flow hydrodynamics inside a PBR plays a critical role in terms of providing ideal growth environment for cells. The flow affects gas holdup and volumetric mass transfer in the PBRs which are equally important to achieve sufficient CO2 and nutrients for the cells. This becomes complex because the liquid flow is affected by several factors such as the PBR size and design, gas flow velocity. Numerical simulation studies have already attempted to investigate one or more of these factors. For instance, gas hold-up and the volumetric mass transfer coefficients on a phenomenological model for bubble breakup and coalescence in column reactors were investigated by Shimizu, Takada, Minekawa, and Kawase (2000) using computational fluid dynamics (CFD). They proposed a compartment model to describe the bubble movements. Their simulation study did not provide a complete description of bubble behaviours although it gave significant insights into the phenomena occurring in bubble column reactors. A similar study was conducted by Baten and Krishna (2002) where CFD was utilised to investigate bubble characteristics inside a bubble column PBR under homogenous and heterogeneous regimes. Their results revealed that in heterogeneous flow regimes, the larger bubbles are found to concentrate in the central core of the bubble column, while the small bubbles are distributed throughout the column. The holdup of small bubbles was found to be constant in the heterogeneous flow regime. A follow-up study was conducted by Baten, Ellenberger, and Krishna (2003) where the hydrodynamics of internal air-lift reactors was investigated and the experimental results were compared with CFD simulations. A scaling up model was developed from the results but it is only applicable in air-lift reactors. In their scaled-up model, there was a significant reduction in the gas holdup caused by significantly higher liquid recirculation. The authors stressed the need for experimental verification especially when scaling up is involved. Yamashita and Suzuki (2007) studied gas holdup inside the PBR, which is an important parameter for the design and scale up of bubble columns PBRs. Their CFD investigation showed that gas holdup depends on many factors such as gas and liquid velocity, physical properties of gas and liquid, type and arrangement of gas spargers, gas inlet height and the inclination of the column. Drag forces on bubbles in bubble swarms were investigated by Roghair, Annaland, and Kuipers (2009) who focused on the presence of neighbouring bubbles on the drag as a function of the void fraction. They found out that the normalised drag coefficient increases for higher void fractions. They recommended that the effects of preferential horizontal alignment on the averaged drag experienced by the bubbles should be taken into account explicitly in drag closure correlations.
This research has provided a profound understanding of the complex flow characteristics in the PBRs. However, despite of the numerous research attempts, simulation results which can be applied in designing a PBR suitable for the mass production of microalgae is still limited. In this study, a numerical simulation using CFD technique was implemented to investigate hydrodynamics characteristics for cylindrical 30 l bubble reactors. Particle image velocimetry (PIV) was used to validate the initial CFD code and the results were utilised to improve the simulation models. Flow hydrodynamics, as affected by various air flow rates, nozzle diameters and the effect of additional internal baffles were investigated. The hydrodynamic measuring parameters used in the analysis were the volume percentages of dead zones, average circulation time and turbulence intensity. Turbulence intensity measures the ratio of the root-mean-square of the velocity fluctuations to the mean free stream velocity.
The optimum air flow rate and recommended nozzle size diameter appropriate for the 30 l PBR was determined. The effect of adding an internal baffle, in terms of hydrodynamic measuring parameters, was also quantified. The cultivation of microalgae followed a final step towards further validating the CFD technique as a promising tool in PBR designs.
Section snippets
Materials and methods
The flowchart of the study is presented in Fig. 1. PIV was utilised to validate the CFD code and approach in the design of PBRs. After establishing the reliability of the numerical approach, CFD simulations of the PBRs followed where several PBR cases were analysed in terms of their suitability in microalgae production. In the CFD simulation, PBR designs and operating conditions were selected and recommended for the practical cultivation of microalgae. The hydrodynamic parameters which were
Validation of the CFD model
In this study, the average velocity magnitude obtained from PIV and simulation data were visually and quantitatively compared (Figs. 8 and 9; Table 6). In Fig. 8, the PIV results correspond to the average velocity magnitudes obtained from 300 captured images in each test case, which were post-processed using Insight 3G and Tecplot software (TSI Incorporated, Shoreview, MN, USA). In Table 6, the computed velocity magnitude obtained from CFD simulations at 0.001, 0.003 and 0.005 mesh grid sizes
Conclusions
From an engineering perspective, the structural configuration and design of the PBRs have a critical effect in the flow hydrodynamics which in turn are very significant in providing ideal growth conditions for the microalgae cells. In this study, CFD was utilised to investigate the flow hydrodynamics inside a standard and upgraded bubble column cylindrical PBRs. The standard PBR and the upgraded PBR are similar in size and configurations; however, the geometry of the bottom section differs
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