Two Phase Flow And Carbon Biofixation Characteristics In Helical Tubular Photobioreactor For Microalgae Cultivation

Overexploitation and utilizationge of non-renewable fossil energy resources hasbeen causing emissions of CO₂from combustion and resulting in greenhouse effect,which does harm to the ecological environment. Exploration of carbon capture andstorage technologies, with environment-friendly and economic benefits is the urgenttask in emission reduction area. Among the carbon capture and storage methods,biological carbon fixation is considered as the most efficient, economical and safestcarbon storage approach. It can promote the natural carbon cycle and reduce the CO₂content in atmosphere efficiently. Compared with other plants, microalgae grow rapidlyand absorb more CO₂. Besides, it can be cultured in waste water and absorb carbon,nitrogen, phosphide and heavy metal.Among the photobioreactors for microalgae culture, helical tubular photobioreactoris an excellent photobioreactors due to its large illumination area and potential of easycontrol. Currently, performances of photobioreactors and optimization of cultureconditions have been extensively investigated, while transfer characteristics of energyand mass in photobioreactors have been rarely reported. The characteristics andinteraction of gas-liquid flow, microalgae growth and biofixation in the helical tubularphotobioreactor have not been investigated yet. However, they are significant to solvethe limitation of energy and mass transfer in photobioreactors for algae cultivation,which can improve the light utilization and carbon fixation efficiency.In the present study, effects of CO₂concentration, gas flow rate and illuminationintensity on the characterictics of biochemical reaction, light utilization and carbonfixation of Chlorella pyrenoidosa FACHB-9were investigated. In addition, thecharacteristics of carbon biofixation and metabolism of algae in a flat bioreactor underdifferent operation conditions were analyzed. Based on the results, a helical tubularphotobioreactor with CO₂distributor was designed. In this photobioreactor, effects ofgas-liquid flow on mass transfer, growth and CO₂biofixation of Chlorella pyrenoidosaFACHB-9were experimental and numerical studied. The main results weresummarized as follows:â‘ Characteristics of absorption spectrum and carbon source capture ofChlorella pyrenoidosa FACHB-9were experimental investigated. The results revealedthat CO₂rather than carbonate easily permeated across cell membrance. After that it was converted to carbohydrate by photosynthesis. Chlorella pyrenoidosa FACHB-9cultured in CO₂supply could accumulate more energy than in carbonate medium.Besides, Simultaneous thermogravimetric analysis was proved to be an effectiveapproach to characterize the biomass with high sensitivity, specificity and reliability.â‘¡Growth kinetics and performances of carbon biofixation under differentCO₂concentrations, gas flow rates and illumination intensities were analyzed. Lightintensity affected the photons capture and absorption of Chlorella pyrenoidosa FACHB-9due to the light transfer and pathway. Appropriate increase in light intensity broughtbenefit to growth and carbon fixation of Chlorella pyrenoidosa. However, lightsaturation or photoinhibition might occur at excessive light intensity, which inhibitedthe growth and carbon fixation of Chlorella pyrenoidosa. Experimental results indicatedthat the optimal performances of carbon fixation efficiency, light utilization andbiomass growth of algae in the photobioreactor were obtained at the CO₂concentrationof10%（v/v）, gas flow rate of50mL/min and illumination intensity of9W/m2. Besides.â‘¢A helical tubular photobioreactor system was designed. It is excellent inlarge illumination area and stable gas concentration. Three kinds of CO₂distributorswere designed and their performances were studied. It was found T tube was good CO₂gas distributor with the lowest energy consumption. The bubbles and flow pattern couldbe easily adjusted.â‘£Influences of gas flow rate on characteristics of gas-liquid phase flow andbiochemical reaction of Chlorella pyrenoidosa were investigated in the helical tubularphotobioreactor system working in a gas-driven mode. The results were compared witha straight tubular photobioreactor. The effects of gas flow rates on flow pattern,biochemical reaction and carbon biofixation of Chlorella pyrenoidosa were analyzedfor the two types of photobioreactors. It was found that when the gas flow rate increasedfrom400mL/min to1200mL/min, the flow pattern in the helical tubularphotobioreactor changed from plug flow to bubble flow and then slug flow. However,the flow pattern in the straight tubular photobioreactor exhibited two other kinds of flowpattern while the bubble flow was absent. Meanwhile, variations of CO₂absorptionefficiency, photosynthetic rate and light utilization efficiency of algae cultured in boththe photobioreactors demonstrated parabolic trends with the gas flow rate. The biomassof algae obtained in the helical tubular photobioreactor was3.88times higher than thatin the straight tubular photobioreactor. In addition, CO₂absorption efficiency andphotosynthetic rate of microalgae cultured in the helical tubular photobioreactor increased by28%and23%, respectively, compared that in the straight tubularphotobioreactor. The effect of gas flow rate on combustion enthalpy and maincomponent of the microalgae was obviously. This may due to the illumination receivedby microalgae cell, which resulted from the flow pattern in helical tubularphotobioreactor. The combustion enthalpy of cells varied with the component of starch,suger or cellulose change.⑤Influences of liquid flow rate on characteristics of gas-liquid phase flow andbiochemical reaction of Chlorella pyrenoidosa were investigated in the helical tubularphotobioreactor system. The results indicated with increasing liquid flow rate from302mL/min to1508mL/min, the diameter of bubbles in the helical tubular photobioreactordecreased gradually, during which the flow pattern changed from slug flow to plug flow,finally to bubble flow. The same phenomenon was observed in the straight tubularphotobioreactor, but it was not so obvious as that in the helical tubular photobioreactor.Compared the patterns between the two bioreactors, it was found that under the sameliquid flow rate, the length of gas bubble in the helical tubular photobioreactor wassmaller than that in the straight tubular photobioreactor. Meanwhile, the performancesof both the photobioreactors descended with increasing liquid flow rate. This can beattributed to the decreased hydraulic retention time of culture medium. The biomass ofalgae obtained in helical tubular photobioreactor was as high as3.35times than that inthe straight tubular photobioreactor. In addition, CO₂absorption efficiency andphotosynthetic rate of microalgae cultured in the helical tubular photobioreactorincreased by20%and28%, respectively. The combustion enthalpy of cells varied withthe component change.â‘¥The numerical simulation results revealed that the dissolution and transferof CO₂in helical tubular photobioreactor have a good performance when the gas flowrate was800mL/min. Further incease of CO₂inlet flow rate can not promote thedissolution and transfer of CO₂. Therefore, the best CO₂flow rate for the present systemis800mL/min, which is in accordance with the experimental results. In addition, CO₂bubbles with different sizes under the flow rate of800mL/min did not show obviousdifferences in dissolution and transfer, which validated the CO₂distributer in theexperiment. As time goes, CO₂enters the helical photobioreactor. The CO₂will rise inthe helical tubular photobioreactor due to buoyancy, leading to a higher CO₂conceteration in the upper tube. Under such flow condition of CO₂and water, there is anon-uniform distribution of CO₂concentration. In the tube, the liquid flow is smooth at the location where the CO₂bubbles did not reach. The reach of CO₂bubbles willintroduce turbulence in the flow. That is beneficial to the CO₂dissolution and transferand then enhance the growth of algae.