CFD Study On Granule-liquid-gas3-Phase Fluid Dynamics In Aerobic Granular Reactors

The formation process of aerobic granular sludge, which was a complicatedphysical-chemical-biological process, was a biological reunion phenomenon generated bythe self-immobilization mechanism of microorganisms under the hydrodynamic conditions.Currently, study of aerobic granular sludge mainly focused on the biological and chemicalaspects to reveal the formation mechanism and influencing factors, and a fluid dynamicstudy on this field is very limited. It is the lack of knowledge of fluid dynamicscharacteristics and the hydrodynamics control parameters during the sludge granulationprocess that leads to a series of problems in industrial application such as unstablegranulation and poor degradation of pollutants.Based on the sequencing batch reactor (SBR), aerobic granular sludge has beencultivated first in different operation parameters (oxygen and mixer speed) and structureparameters (high aspect ratio). A three-dimensional Eulerian model has then been used tostudy the effect of gas superficial velocity, impeller rotational speed and height anddiameter ratio (H/D) on the hydrodynamics of two-phase flow in SBR. A two-dimensionalmixture model has been used to study the effect of gas distributors, gas superficial velocityand H/D on the hydrodynamics of three-phase flow in SBR finally. The results are listed asfollow:1．Aerobic granules were successfully cultivated in sequencing batch reactors (SBRs)with synthetic wastewaters which organic carbon sources with glucose. After measuring,some key parameters for numerical simulation were obtained. The diameters of bubbles andgranules were measured as2.7mm and0.6mm, respectively. Granule density is1059kg/m~3.2．A three-dimensional computational fluid dynamics (CFD) simulation was performedwith an Eulerian two-phase fluid approach to visualize the2-phase flow pattern in reactors.The results show: a) Pressure behind impeller blades is less than its value in front of blades,in which impeller rotates in clockwise direction. Pressure of air-water mixture in the centerplane of impeller increases with the incensement of gas superficial velocity and impellerrotational speed. b) Impeller rotational speed should be neither too small nor too big. So theoptimal speed should be between800rpm and1200rpm. In addition, a good quantitative agreement was also obtained between the experimental data and simulations. c) IncreasingH/D can make gas residence time longer and make gas distribution more uniform.3．A two-dimensional mixture model was performed d was used to visualize the3-phase flow pattern in reactors. In SBR, the velocity vector of granular is similar with theliquid velocity vector. There are two big circulations in the reactor. The size of circulationsincreased with an increase of H/D. It is also closely connected with numbers of gasdistributors. At the bottom of single gas distributor the distribution of gas hold up isinhomogeneous. Fluid dynamics characteristics in the oval-shape-bottom reactor issubjected to inlet velocity magnitude. But it is not true for the porous aeration which hasstrong adaptability. However, note that the value of gas holdup was not changed while thepattern of reactor varied.