Simulation Study On The Stirring Process Of An Anaerobic Reactor In A Biogas Project

Anaerobic reactors are the core structures of biogas engineering,and continuous stirred tank reactors(CSTR)are the main type of reactors in this field.The core stirring process can enhance material mass and heat transfer,increase contact between fermentation raw materials and microbial populations,promote biodegradation of raw materials,and improve biogas yield.Therefore,the research on the CSTR stirring process has important theoretical and engineering value.However,due to the inherent characteristics of high concentration agricultural waste materials,as well as the multidimensional and complex fluid flow in the reactor,it is difficult to fully analyze the rheological characteristics of the materials in the reactor solely using theoretical analysis or laboratory scale small-scale research.This thesis takes the CSTR anaerobic reactor and its stirring device,which is the main body of a large-scale biogas project in Hubei Province,as the research object.CFD technology is used to simulate and analyze the internal flow field state of the reactor.The method of combining unidirectional flow and two-phase flow simulation is used to optimize the design of the stirring device,providing operational guidance for practical engineering.The simulation results of this study indicate that:(1)Under the current stirring conditions of the project(stirring speed of 10r/min,agitator distance of 5m from the bottom),the simulation results of the single-phase flow model found that the overall flow velocity in the reactor was low,the proportion of high-speed zone was small,and only existed near the stirring blades,with a speed of about 1.8m/s;A medium speed zone where the rotating outer diameter rapidly decreases to 1.2m/s;The low speed zone between the middle and bottom of the reactor,as well as between the two agitators,accounts for a large proportion,with a speed of only about0.2m/s.The two-phase flow model simulation found that the material flow at the bottom of the reactor was poor and the flow rate was low,which was likely to cause uneven material mixing,so optimization was needed.(2)Single factor adjustment of mixing speed and mixer height from the bottom.When the height of the stirrer from the bottom remains unchanged and the speed increases to15r/min and 20r/min,the overall flow rate of the reactor increases by about 0.3m/s and0.8m/s,respectively.The speed distribution state is similar to the initial condition,but the proportion of high speed zone and medium speed zone increases,and the increase is more obvious when the speed is 20r/min,and the flow pattern is better.By comparison,it was found that only increasing the rotational speed did not significantly increase the flow rate at the bottom of the reactor,and the flow state of the materials did not significantly improve;When the agitator is moved down 1m without changing the speed,a certain high-speed zone appears at the bottom of the reactor,accounting for about 10%.The bottom flow rate increases,improving the flow state of the material and the mixing effect.However,the overall flow rate does not increase,which is equivalent to the overall spatial distribution of the velocity moving down.The results showed that the single factor adjustment of speed and agitator height from the bottom did not significantly improve the flow field and material flow pattern in CSTR.(3)Optimization of the composite factors of stirring speed and agitator height from the bottom.When the speed is increased to 20r/min and the agitator is moved down 1m,the overall speed of the reactor increases significantly,with a significant increase in the proportion of high-speed and medium speed zones,reaching a maximum speed of around2.7m/s.The proportion of low-speed zones is very small and only exists in the middle of the reactor near the bottom,with a speed of around 0.4m/s;The flow rate in most areas at the bottom of the reactor is increased,and the material flow pattern is significantly improved.The simulation results of single-phase flow and two-phase flow show that the mixing flow field under the composite optimization condition has a high velocity,significantly improves the bottom material flow pattern,and has a good overall manifold,which can be used as the optimization scheme of the mixing device.