Aeration Device Design Optimization And Carbon Fixation Performance In Microalgae Photobioreactor Based On Bubble Cutting

As one of the key carbon capture utilization and storage（CCUS）technologies to reach the goal of carbon peaking and neutrality,microalgae carbon fixation has achieved a lot of attention from scholars all over the world.Microalgae carbon fixation process is usually completed in a photobioreactor,where CO₂ firstly enters the photobioreactor in the form of bubbles through an aeration device,and then completes the following process of dissolution and photosynthetic CO₂ fixation.In the whole CO₂transfer process,the largest mass transfer resistance exists at the gas-liquid interface.Nowdays,the methods to strengthen mass transfer at the gas-liquid interface are mainly focused on generating microbubbles,ultrasound,turbulence,stirring,etc.,but all at the expense of energy consumption.Cutting bubbles on the way up after detachment can cut large bubbles into small ones without additional energy,increase the gas-liquid contact area and interfacial disturbance,prolong the bubble residence time,and finally enhance CO₂ transfer.Therefore,the application of bubble cutting to the optimal design of aeration device is essential for the improvement of carbon fixation performance of microalgal photobioreactor.In this work,cutting components were installed at designated positions above the aeration holes of the microalgae carbon capture photobioreactor,which could precisely cut the bubbles on the way up after detachment.To achieve the goal of increasing carbon fixation efficiency without increasing energy consumption.Based on this,this work first used the visualization experiment method to analyze the dynamic behaviors of the whole process of a rising single CO₂ bubble in the pre-cutting,mid-cutting,and post-cutting stages.Then,investigated the shape,surface wetting characteristics,location,and arrangement parameters of the cutting components on the effect of dynamic behaviors of bubble cutting.A range of parameters for the design of the structure of the cutting components with the best effect was obtained.Finally,based on the above research results,an aeration device based on bubble cutting in microalgae photobioreactor was designed.The effect of structural design parameters such as the shape of cutting components,surface wetting characteristics,and operational design parameters such as gas flow rate and inlet CO₂concentration on the dynamic behaviors of bubbles and the growth and carbon fixation performance of microalgae were verified in the case of multiple bubbles in photobioreactor.The optimal structure and operational design parameters of the aeration device were obtained.The main conclusions are as follows:（1）The whole process of bubble cutting can be divided into three stages:pre-cutting,cutting,and post-cutting,among which the cutting stage lasted the shortest time and the bubble deformation was the most intense.When the critical point was broken,the central connection neck was broken and the mother bubble was cut into two daughter bubbles.Compared with the uncut mother bubble,the horizontal offset distance of the cut daughter bubbles increased,the rising final velocity decreased by 18.2%,the equivalent diameter decreased to 83%of the mother bubble,the gas-liquid contact area increased by 12.1%,and the residence time increased by 30%,which was beneficial to CO₂ transfer.The higher the gas flow rate,the more obvious the gas-liquid interface fluctuation,and the larger the horizontal offset distance of the daughter bubbles after cutting.（2）Compared with the slice shape,the column-shaped cutting components were prone to problems such as asymmetric cutting of bubbles or reagglomeration after cutting.The cutting success rate of wedge shape was larger,but the daughter bubbles rising velocity was more slowly after cutting and the trajectory was limited.It was only suitable for scenes with low gas flow rate and large gas hole spacing.The slice with hydrophobic surface had the highest maximum contact height of bubbles and the shortest cutting time.But the daughter bubbles appeared to stick to the slice surface and aggregation problems after cutting,which were not conducive to CO₂transfer.The best cutting component shape was slice,with height h_s greater than 2 mm and thicknessδ/d less than 0.48 for hydrophilic surface（glass）cutting component.（3）The best bubble cutting effect was achieved when the location P of the slice was within 7 mm～11 mm and S<1 mm（S/d<0.23）.The bubble cutting effect was better when the slices were arranged side by side than when they were arranged in a cross way.The bubble cutting success rate was higher when the slices were arranged side by side,the number n≤2,and the spacing M<2 mm.The bubble cutting success rate was higher when the slices were arranged side by side and the hydrophobic surface was arranged on the outside（iooi）.But there was a problem of daughter bubbles aggregation after cutting,which was not conducive to CO₂ transfer.（4）The aeration device based on bubble cutting was applied to the microalgae photobioreactor,and the diameter distribution of the generated bubbles was more uniform,and the proportion of small bubbles was higher,which was conducive to CO₂ transfer.The maximum biomass production of microalgae increased by 7%.Placed glass slices with a length of 38 mm,a height h_s of 24 mm,and a thicknessδof 1 mm side by side at the location P=10 mm and S=0 mm above the aeration holes of the aeration device,when the inlet CO₂ concentration was 5%and the gas flow rate was 70 m L/min,the microalgae photobioreactor can obtain the maximum biomass production of 1.531 g/L.