Effects Of Gas-liquid Flow And Mass Transfer On Microalgae Growth In Photobioreactor

Energy utilization and carbon sequestration of microalgae has become a researchhotspot at home and aboard. Open raceway pond is the most common open-typecultivation system, which has some characteristics like low cost and easy to large-scalecultivate. However, the progress of raceway pond was restricted by microalgaephotosynthesis efficiency, mass transfer and light availability, reducing the efficiency ofproduction. The internal gas-liquid two-phase transfer performance of photobioreactorwas the main factor to influence the growth and carbon sequestration of microalgae.In this thesis, Chlorella pyrenoidosa(CP) was selected to study the growthcharacteristics of microalgae, the characteristics of gas-liquid two-phase transferperformance and enhancement of transfer performance. The influence of gas flow, lightintensity, light/dark cycle etc. on the stratification of microalgae, the characteristics ofgrowth and metabolism was studied systematically. Visual experiments have beencarried out to study the gas-liquid two-phase flow behavior in the reactor. Effects ofaeration pore diameter, gas flow and CO2concentration on bubble dynamic behaviorwere investigated. And the effect of gas on the CP solution stratification and flotationwas also studied. Pulse aeration was proposed to enhance the mass transfer, and visualexperiment was also performed to investigate the dynamic behavior of bubble underpulse aeration. Meantime, the effect of interval of pulse aeration on growthcharacteristics of CP was investigated.The main research contents and results were as follows:①The optimal growth performance of CP cells were achieved at a gas flow rate2000mL/min and a light intensity of160μmol·m-2·s-1with a photoperiod of16:8.While the maximal lipid accumulation could be obtained at a light intensity90μmol·m-2·s-1and a photoperiod of16:8.②Light conditions influence the growth, stratification and sedimentation of CP,and there was a relationship between the stratification and content of biomacromolecule(lipid and carbohydrate). Namely, the upper layer has higher lipid content, and the lowerlayer has higher carbohydrate content.③The bubble diameter and velocity both increased with increasing pore diameter,and the diameter and velocity in CP suspension were smaller than in pure water. Bubblediameter and velocity also increased with increasing gas flow rate. However, bubble diameter and velocity decreased with continuous increasing CO2concentration.④Under gas flow rate of20mL/min, reducing the pore diameter would intensifystratification of CP. The CP cell concentration of upper layer first increased and thendecreased with increasing the gas flow. Higher CO2concentration would also intensifythe stratification.⑤Compared with continuous aeration, proper pulse aeration interval increasechlorophyll content of CP cell. Thus, higher biomass could be obtained with improvedphotosynthetic efficiency and CO2fixation capacity. The maximal growth rate andbiomass of1.47g/L were obtained under aeration interval of12s, which was2.63timesthan continuous aeration. The increase of bubble diameter and velocity would beinhibited by plus aeration, and optimum average bubble diameter and velocity for CPgrowth were1.036mm and7.73cm/s.