Study Of Hydrodynamic Characteristics In Rectangular Inner Loop Fluidized Bed And Its Performance Optimization Based On CFD

Biological fluidized bed is a novel bioreactor in wastewater treatment which combinesactivated sludge process with biofilm process and employs fluidization technology. On thebasic of its excellent hydrodynamics characteristics, efficient heat and mass transfer as well aswide range of anti-impact loading and sludge loading, it is developed into rectangular innerloop fluidized bed which has advantages on technically economical efficiency(equipments,construction, installation), treatment capacity, engineering aesthetics. Being relativelylow-cost and reaching the standard of effluent under industrial-scale operation proves that thereactor meets the requirement of energy saving and emission reduction.So far it has been20years for researching three-phase fluidized bed on wastewatertreatment, which is still in a developmental stage. Effects by structural parameters(includinginner components)on the flow structure, space and time distribution of multi-phase structure,mass and energy transfer are complicated. Therefore, the design, operation and control of realproject are limited to finite experienced parameters. Based on the Eulerian-Eulerianmulti-phase model in Computational Fluid Dynamics, CFD method, which was adopted tosimulate the fluid mechanics in rectangular inner loop fluidized bed three-dimensionally andvisually, this thesis compared the effects of different operating conditions, structuralparameters and inner components. Thus, it is in favor of optimal designing of fluidized beds,saving cost and providing theoretical support for industrial scale-up of the bioreactors.To improve the fluid performance of the reactor, the cross-shaped inner componentcontributes to lead the multiphase flow vectors from chaos to normalization at the bottom ofthe fluidized bed. Results demonstrated that this inner component could obviously decreasethe head loss caused by the inelastic collision at the bottom. The liquid circulation velocityhad a maximum increase at15.7%and15.0%at the rising area and downcomer respectively.More uniform distribution was achieved at the rising area section, while the peak value ofliquid circulation velocity decreased. The behavior of the cross-shaped baffle optimized thehydraulic condition better under hydrolysis and aerobic condition, and the turbulentdissipation rate had a maximum decrease at31.9%. Numerical analysis indicated that thecross-shaped baffle achieved the optimization of hydraulic condition and energy consumptionof rectangular fluidized bed based on the requirement of reactor for wastewater treatment.Coupling the cross-shaped with funnel-shaped inner components to optimize therectangular inner loop fluidized bed. Results demonstrated that coupling effect on the gasholdup is hanging over and drafting the bubbles by the funnel-shaped one as well as dispersing and cutting the bubbles by the cross-shaped one. The overall gas holdup had apromotion ofU0.063g. The promotion of gas holdup in downcomer was particularly apparent,while the peak value increased by596%. The bubble penetration depth was up to0.3m. Thecoupling effect on the liquid circulation velocity is restricting the disturbance of the fluidabove draft tube by the funnel-shaped one as well as transfering inelastic collision into elasticcollision by the cross-shaped one. The former plays a dominant role that leads to the decreaseof liquid circulation velocity at both of the rising area and downcomer to varying degrees.Nevertheless, it is still within the scope of optimal value of fluidization reported in literatures.Simulation data proved that the coupling of cross-shaped with funnel-shaped innercomponents has a positive effect on improving the mass transfer between gas-liquid phase andthe quality of fluidization which makes rectangular fluidized bed adapt to the intensiveoxygen demand and the requirement of saving energy during industrial wastewater treatmentprocess.The ratio of side length of draft tube to reactor(a/A), distance of draft tube andbottom(Hb), distance of draft tube and liquid level(Ht) were investigated to explore theinherent effects on rectangular fluidized bed by the structural parameters. Resultsdemonstrated that the ratio of side length of draft tube to reactor affects the distribution ofliquid circulation velocity and its peak value by changing the area ratio of the rising area tothe downcomer. Bioreactors are inclined to choose a/A of0.67. While O3fluidized beds tendto choose0.8, however, it requires to optimize the method and position of air inlet toovercome backmixing. Distance of draft tube and bottom affects the size and direction of theliquid circulation velocity at the bottom by changing area ratio of annulus to downcomer. It isinclined to choose Hbof0.96for the reason that it appears high resistance and boundary layerseparation when Hbis too large or too small. Distance of draft tube and liquid level affects thefluid flow above draft tube by changing the ratio of Htto the length of draft tube. The validfluidization volume distinguished by the draft tube decreases when the ratio increases. It tendsto choose Htof0.13based on the demand of low energy consumption although the effects onliquid circulation velocity and gas holdup turn out to be small.