Numerical and experimental analysis of optimized conical flask photobioreactor structures to improve liquid–gas two-phase distribution and microalgae carbon sequestration

In this paper, light intensity and gas–liquid distribution of microalgae in the original conical flask photobioreactor with central inlet tube bubbling ventilation are studied by applying both numerical simulation and experimental analysis. The different optimized structures of conical flask photobioreactors are proposed to improve light distribution and microalgae carbon fixation. The advanced effects of light intensity and gas–liquid distribution in advanced conical flask reactors are validation by 5 important parameters. First, the Eulerian gas–liquid two-phase model and DO (Discrete Ordinate) light distribution model of the conical flask reactor are developed. Gas distribution and light distribution are analyzed with different conditions of gas inlet velocity. Second, the optimal gas inlet velocity is evaluated with five vital parameters (average velocity, turbulent kinetic energy, gas holdup rate, average light intensity and velocity of the vertical wall). Then, the optimal CO₂ gas inlet concentration is determined by a series of controlled experiment test groups. Finally, based on the numerical simulation and experiment result, three different optimized structures of conical flasks are proposed to prove the effectiveness of optimization, which is named lean-insertion bottle, baffle bottle and cross-nozzle bottle. The results showed that 0.5 m·s⁻¹ gas inlet velocity is the optimal value for the gas–liquid distribution in the 500 ml conical flask reactor. With the increase of CO₂ concentration, the growth rate of microalgae biomass concentration increased first and then decreased after the concentration arrives at 4%. At 4% CO₂ gas inlet concentration, the highest biomass concentration is increased by 57.4% compared with the lowest ones. The gas distribution and light distribution in the three optimized reactors are significantly improved, and among the three optimizations, the PBR with baffle at 0 mm of the gas inlet pipe is the best one. In conclusion, the cultivation condition of microalgae and the structure of algae photobioreactors are both optimized providing a novel beneficial reference to improve the liquid–gas distribution and carbon sequestration rate of microalgae growth in the laboratory.