CFD Simulation Of Sludge In Anaerobic Digestion Reactor

With the increasing production of the waste sludge, it is necessary to enlarge the volume of sludge treatment and disposal facilities and update it by following modern sustainable development strategy. Due to the complicated composition of sludge and the diversified sludge treatment technology, a good understanding of sludge physic-chemical behavior, especially the hydromechanics properties, is vital to the optimization and efficiency design of sludge treatment technology. The anaerobic digestion is a main method of sludge treatment and disposal, it is necessary to considerate the mass transfer efficiency promotion and energy cost reduction in the digester design. Mixing played a great role in the process of anaerobic digestion as a way to solve the problems of mass transfer difficulty and uneven of physically and biochemical characteristics.As a typical Non-newton fluid, the fluid condition of sludge has great influence on the flow status in the reactors. Several researches show that the power law model is well fitted in describing the flow behavior of sludge with different water content. The flow behavior index n increased from 0.2 to 0.3 with the water content of sludge increased. While the consistency index K decreased significantly with the water content of sludge increased. The consistency index K decreased with the temperature increased, and the flow behavior index n is seemed as less affected by the temperature. The CFD(Computational Fluid Dynamics)model established in this study is based on the rheological parameters obtained in the experiment.The result of the CFD modeling show that the pumping efficiency increased with the growth of rotate velocity. The study of mixing capability shows that the mixing velocity and efficiency will be larger with the growth of the flow behavior index n.The optimization of blade type shows that:(1)The overall velocity of the reactor will become larger with the growth of blade diameter. The pumping efficiency of the digester with a 0.18 m blade is 21.9 times over the digester with a 0.12 m blade. The pumping capability will become larger in a great extent. The mixing efficiency grown 24.26 percent as the dimension of the blade increased from 0.12 m to 0.18 m. At the same time, the power concentration of 0.18 m blade is 1.08 times of the power concentration of 0.12 m blade. In other words, the mixing efficiency decreased by 8 percent.(2)The study of blade combination shows that the pumping efficiency of blade combination is 11 percentage larger than the double blade and both the mixing velocity and efficiency is improved by 15 percent.(3) The study of reactor wall optimization shows that the pumping efficiency of oval wall reactor increased by 11 percent than the circle wall reactor and the mixing velocity also improved by 7.4 percent but the mixing efficiency declined by 6.1 percent.