Strengthening Energy And Mass Transfer With Diaphragm Microporous Aerators To Promote CO₂ Fixation By Microalgae

Flue gas CO₂ bio-fixation by microalgae has the advantages of fast sequestration rate,high potential,strong environmental adaptability and high biomass value,which is a hot spot for international frontier research and will provide a green and sustainable technology route option for China’s carbon neutral strategic goals.Traditional raceway pond photobioreactor has problems such as large bubble generation diameter,poor gas-liquid mixing and mass transfer,fast light attenuation inhibiting photosynthesis,and low frequency of microalgal light/dark cycles,and existing aerators are difficult to be applied in engineering scaling due to structural and cost problems.A kind of microporous diaphragm aerator was developed in this study to generate micron-sized bubbles to prolong CO₂ residence time and promote photosynthetic carbon sequestration by Arthrospira cells.The influences of aerator structural parameters and dynamic changes of bubble aggregation or fragmentation on flow field distribution,mixing and mass transfer,light attenuation characteristics and flashlight frequency were revealed.CFD（Computational Fluid Dynamics）software coupled with Population Balance Model（PBM）were used to investigate effect of dynamic changes of bubble aggregation and fragmentation on flow field distribution and mixing and mass transfer in a raceway pond with microporous diaphragm aerators installed.Bubble distribution and microalgal flashlight effect within the flow field were regulated and optimized.The average turbulent kinetic energy in raceway pond was highest（128.31 cm2/s2）at inclination angle θ = 11o,while the inhomogeneity of turbulent energy distribution increased with the shortening of bubble movement distance whenθ > 11o.When bubble generation diameter increased from 0.4 mm to 2.5 mm,average solution velocity and mixture turbulent kinetic energy increased and then decreased,reaching peaks（10.28cm/s and 70.15 cm2/s2,respectively）at 1 mm;bubble aggregation height decreased and aggregation range became larger.As initial bubble flow rate increased from 0.0314 m/s to 0.1257m/s,microalgal flashlight frequency increased and then decreased,reaching a maximum value of0.47 Hz at 0.1005 m/s;gas volume fraction and turbulent dissipation rate gradually increased.The growth rate of Chlorella increased by 22% to 0.71 g/L/d and the photosynthetic oxygen release rate increased by 13% to 31.07 nmol/m L/min after structural optimization.CFD software coupled with discrete ordinates radiation model and embedded with userdefined function CO₂ mass transfer model and bubble scattering model were used to solve radiation transfer equation.Light attenuation processes in the microalgae-CO₂ bubble–culture medium system under different microalgae concentration and bubble characteristics were dissected.As bubble generation diameter increased from 0.1 mm to 1.6 mm,ratio of light zone length increased from 37.95% to 42.64%,and microalgal average flashlight cycle（T）decreased and then increased,reaching a minimum value of 1.81 s at the bubble diameter of 0.8 mm.As initial CO₂ volume fraction increased from 0.02 to 0.2,flashlight frequency of microalgae cells decreased from 0.55 Hz to 0.29 Hz and the light zone time ratio φ decreased from 36.90% to18.40%.When biomass concentration increased from 0 to 0.4 g/L,ratio of light zone length in the reactor decreased from 81.13% to 20.00%.The decrease trend in light zone length became smaller with an increase in microalgae concentration and eventually leveled off.As incident intensity increased from 50 W/m2 to 260 W/m2,ratio of light zone length increased by 104% to 47.02%;microalgal flashlight frequency（f）first increased and then decreased,reaching a maximum value of 0.63 Hz at 200 W/m2.A novel microporous fibrous-diaphragm aerator（MDA）was proposed in which CO₂ gas was dispersed into millions of microporous holes on the fiber membrane to produce micron-sized bubbles,in order to reduce bubble generation diameter and increase gas-liquid contact area to strengthen mixing and mass transfer in raceway pond photobioreactor.When inclination angle θof internal support increased from 0° to 45°,bubble generation time and generation diameter first decreased to valley bottoms of 4 ms and 0.45 mm at 22° and then increased,mass transfer coefficient firstly inreased to the peak（2.6 h-1 when θ = 22°）and then decreased.Bubble generation diameter and generation time increased as aeration pore size（6-126 μm）increased,and mass transfer coefficient increased and then decreased,reaching a maximum of 2.6 h-1 when the aeration pore size was 28 μm.Compared to common strip aerator,bubble generation time and generation diameter decreased by 50% and 60%,while mixing time decreased by 22% and mass transfer coefficient increased by 40%,leading to increased actual photochemical efficiency（by80%）and increased biomass yield（by 38.5%）of Arthrospira cells with 99% CO₂ aeration through the fibrous-diaphragm aerator.The diaphragm aerators after numerical simulation and experimental optimization were applied in a industrial 660 m2 raceway pond in Inner Mongolia.Food grade CO₂（≥99.9%concentration）purified from coal chemical tail gas and a certain amount of air CO₂（⁴00 ppm CO₂ concentration）were mixed to formulate a volume concentration of 2.5-10% CO₂,and then went into photobioreactor through MDA to strengthen CO₂ dissolution and diffusion,improving photosynthetic growth and carbon sequestration characteristics of Arthrospira.An increase（9.78%）in maximum photochemical efficiency of Arthrospira cells promoted biomass growth by15.5% in the MDA-equipped raceway pond.When CO₂ concentration increased from 0.04%（air）to 10 %,the openness of active reaction centers ψ0 gradually increased;electron transfer energy flux and number of active reaction centers per unit area（ET0/CSm）first increased and then decreased.The number of active reaction centers per unit cross section（RC/CSm）and the driving force of photosynthetic electron transfer（DFCS）were maximum at were 628 and 2.72,respectively,under an optimal CO₂ concentration of 7.5%.Appropriate increase in CO₂ concentration can promote diffusion of CO₂ molecules into microalgae cells to save the energy consumed in transmembrane transport for more photosynthetic carbon sequestration,but too high CO₂ concentration can cause acidification of intracellular environment and affect the activity of key enzymes and reaction centers for carbon sequestration.