The Numerical Simulation Of Mixing Process In Fermentation Tank And Parameter Optimization

Anaerobic digestion of sludge becomes the most commonly used method of sludge treatment process at present. Due to the poor mixing conditions of fermentation tank, some phenomena exit in the tank, such as uneven distribution of sludge, layered, accumulation and so on, which influence the production of biogas. In the process of anaerobic digestion of sludge, the optimal design of the stirrer is an important link in the industrial process. Using computational fluid dynamics (CFD) software to study fluid flow and mixing in the stirring reactor has showed its advantages. In this paper, the numerical simulation will be used to study flow state and suspended state of sludge in fermentation tank, which has important significance to improve the effect of anaerobic fermentation of sludge.In this paper, the fermentation tank in a sludge treatment plant is regarded as the research object. Based on FLUENT software, the solid-liquid two-phase flow of stirring in the fermentation tank is simulated with standard κ — ε turbulence model, Eulerian-Eulerian multiphase model and multi-reference flame method. The result shows that sludge is uneven distributed in fermentation tank, and it is layered at the top of tank. Sludge accumulation appears at the bottom. The flow of fluid circulation speed is not enough to make the sludge evenly suspended in the fermentation tank.The influences of some factors, such as stirrer rotation speed, the length of the blade and the distance between impeller and bottom. The results show that the increase of rotation speed and length of blade is helpful for the velocity of the fluid and the uniform distribution of solid phase. When the length of the blade increases to a certain value, the distribution of solid phase hardly changes. Meanwhile, the increase of rotation speed and length of blade increase the pressure and power of the stirrer, which multiply with the change of parameters. Within a certain range, the increase of distance between impeller and bottom can improve suspension of sludge at the top area of fermentation tank, but when it increases to a certain value, the form of flow field changes and the sludge concentration no longer increases. The power of stirrer decreases with the increase of distance from bottom, but changes a little.The orthogonal experiment method is used to optimize three factors. The results indicate that the most important parameter to solid concentration is the length of blade, and the others’ order is the distance between impeller and bottom and the rotation speed. The optimized parameters are the rotation speed 626r/min, the distance between impeller and bottom 1.6m and the length of blade 425mm. Through the analysis of energy saving benefits of several different schemes, the following conclusions can be obtained: the mixing time decreases with the increase of power, after mixing, the sludge concentration fluctuates around 7% and the range is very small, when the rotation speed is 326r/min, the blade length is 360mm and the height from the bottom is 1.6m, the energy consumption of stirrer is lowest.