Numerical study on the transport phenomena for microalgae cultivation in a foam bed photobioreactor
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Seyyed Javad Heydari,1,* Arezou Jafari,2 Seyyed Mohammad Mousavi,3
2. Tarbiat Modares University
3. Tarbiat Modares University
- Introduction: Microalgae are an engaged green feed for bulk materials, like the chemicals, energy or feed constituents. For decreasing the production costs of microalgal biomass, a modern bubble column photobioreactor has been created that this plan is based on the usage of liquid foams as a growth bed in order to microalgal cells. Liquid foam bed photobioreactors (LFB-PBR) have various benefits over common. This advantages including: Increase contacting time between liquid /gas phases the great gas holdup of liquid foams caused on decreasing light efficacy path. Due to this little irradiance path, high cell density will be gained, which results in the decrease of biomass separation and purification. High interphase surface presented via liquid foams, improves the mass transfer rates. Enhancing mass transfer rates and the little hydrostatic pressure in the bioreactor caused a reduction in energy needs. Furthermore, in LFB-PBR, CO2 biofixation yield may be achieved because of the high residence time of the CO2 phase in the column. The low pressure depletion in LFB-PBR also caused to reduction construction requirements. So, this research focused on the simulation of a liquid foam bed photobioreactor (LFB-PBR) using computational fluid dynamics (CFD) to obtain the maximum possible high-density culture (HDC). It is worthy to note that mixture method is applied as the multiphase model and COMSOL Multiphysics 5.5® was used.
- Methods: In CFD simulation the basic equations included continuity equation, momentum balance and species reaction equations are mathematically presented. In addition, in this research, a model for characterizing light transfer is presented. All equations were computed in Comsol multiphysics software 5.5 (64bit), which calculated partial differential equations through finite element method (FEM). Coupling of hydrodynamic and mass transport was applied, and then another one way coupling from concentration field to radiative model was used. Suitable algae species chlorella sorokiniana and chlorella vulgaris were selected. The light attenuation in LFB-PBR was modelled with Lambert-Beer law. CFD simulations were carried out on a 24-logical super processor system and 96 GB of random access memory. Three-dimensional hexahedral meshes of the LFB-PBR were created and the grid-dependency test was checked, and finally the grid of 10504 elements with 0.28 mm size was used for all the simulations
- Results: The accuracy of CFD simulations was verified by comparing with experimental data reported by Agnes Janoska (2018, Dissertation in Wageningen university). Concentration profiles of algae biomass, gasses species (oxygen and carbon dioxide), as well as velocity of dispersed and continues phases were compared with experimental results. Furthermore, light scattering profile versus total time of a batch system of LFB-PBR was obtained. It can be seen that the CFD predictions are matching the experimental data within 80%. According to the obtained results, in the downward of column carbon dioxide is consumed and oxygen is released. Increasing of radial profiles related to the oxygen concentration and algal biomass inside the reactor was observed, while concentration of carbon dioxide was decreased, these finding are in accordance with photosynthetic reaction. Hence, in the layer close to the glass wall, the reaction conversion efficiency was the highest and the main reason for these changes is related to the photoinhibition phenomenon in the microalgae cultivation process. Ccomputational results also show values of dispersed phase volume fraction (liquid phase) swiftly decreases along the height of photobioreactor and in Z=20cm reduction percentage is more than 90%, additionally biomass density is reached to 30 g/L while light intensity was about 1500 μmol m-2 s-1.
- Conclusion: Prediction of gas holdup and dispersed phase volume fraction (liquid phase) is of great importance in determining liquid circulation, mixing, mass and light transfer in LFB-PBR and it is of critical importance in photobioreactors design, besides; recirculation rate are basic operational factors for the foam bed photobioreactor, determining its yield and energy requirements. In conclusion, LFB-PBR is an engaged bioprocess for microalgae biomass production.
- Keywords: CFD, Microalgae, Photobioreactor, LFB-PBR, Transport equations
