CFD Simulation And Structure Optimization Of Light UASB Reactor

With the gradual growth of the world’s population,the discharge of sewage is getting higher and higher,and environmental problems are becoming more and more serious.The effective treatment of sewage is becoming more and more important.Anaerobic biotechnology is one of the mainstream of the high concentration sewage treatment technologies.At the same time,the rapid development of industry has led to the discharge of more and more sulfur-containing wastewater.Some scholars have found that photosynthetic bacteria have a promoting effect in the treatment of sulfur-containing wastewater,and based on this,they have designed a combination of anaerobic biotechnology and photosynthetic bacteria,light UASB(Up-flow Anaerobic Sludge Blanket,UASB)reactor,but in actual operation the start-up period is long and the reactor is found to have air leakage.In order to solve the existing problems,in addition to studying the biochemical mechanism of the light UASB reactor,It is also necessary to analyze the internal flow field.In this paper,the laboratory UASB reactor was used as the object,and the computational fluid dynamics software Fluent was used to perform three-dimensional numerical simulation on it,and conducts simulation research and proposes optimization for the reactor with different rising speed and the three-phase separator with different structural parameters,the main results were as follows:(1)A steady-state simulation of single-phase flow in the light UASB reactor was carried out.The results showed that although the reactor distributes water through three water distribution holes with the same spacing under the water distribution pipe,the water distribution was not uniform:the area near the water inlet has a higher flow rate,and the flow velocity in the area farther from the inlet was lower,and the gap between the two continues to decrease as the height increases until it was basically the same;The flow velocity in the area close to the wall of the toroidal column reactor was lower,and the closer to the middle of the inner and outer walls,the higher the flow velocity.(2)The liquid-solid two-phase flow simulation of the light UASB reactor was carried out,and the flow field when the rising velocity u=0～6 m/h was analyzed.The change characteristics of the sludge bed when the reactor was started were observed,which also explained the unevenness of the water distribution in the reactor;The increase of rising velocity u can increase the volume of the turbulent flow zone and reduce the volume of the push flow zone,and cause vortexes in the middle of the reactor;The sludge distribution under the lower rising velocity was more uniform and higher The increase of rising velocity also can increase the height of the sludge blanket.When the actual reactor is started,it can be considered to increase the rising velocity first so that the sludge bed can be quickly stirred,and then the rising velocity can be appropriately reduced to make the mud-water mixing more uniform;Under comprehensive consideration,the flow field in the reactor was the best when u=1.774m/h.(3)Carrying out the RTD experiment,it was found that the height and change trend of the sludge blanket are basically consistent with the numerical simulation results,and the relative error was less than 7.14%.The results of data processing show that as the rising velocity u increases,the V_d in the reactor gradually decreases from 12.54%to 7.71%,and suddenly increases to 20.92%when u=5.321 m/h.With the increase of u,the value of the standardized varianceσ_θ~2 of the RTD curve also reduced first from 0.23 to 0.18,and then increased to 0.24.The flow pattern in the light UASB reactor was analyzed by the series model and the axial diffusion model.With the increase of u,N increased first from 4.39 to5.70 and then decreased to 4.22,and D/μL decreased from 0.13 to 0.10 and then increased to 0.14;light UASB reactor has both plug flow and complete mixing flow.According to the experimental results,comprehensively considering factors such as V_d,σ_θ~2,N,D/μL,and the height of the sludge blanket,it was concluded that the reactor has the best processing capacity when u=1.774 m/h.The RTD experimental results are compared with the simulation results,and it was found that the experimental results are in good agreement with the simulation results,indicating that it was feasible to use the simulation instead of the experiment.(4)The three-phase separator of the light UASB reactor was used as the research object.Through the gas-liquid two-phase numerical simulation study of the three-phase separator area with different lower baffle inclination angles and the width of the return crack,the optimal size was that the lower baffle inclination angle was 50°and the width of the return crack was 3 mm.Then the gas separation effect was investigated by numerical simulation.The results show that the gas separation effect was greater than 90%after stable operation,which has a certain guiding significance for the design of the light UASB reactor in the future.