Research Of Square Internal Loop Fluidized-bed Structure Parameter Alteration And Internals Reinforcement Based On CFD

As the mass transfer enhancing reactor,internal loop fluidized-bed is currently applied and developed in environmental engineering,chemical engineering,pharmaceutical industry,mining industry,food engineering,energy engineering.However,reactors with specific fluid characteristic are required differently according to various application fields and objects.The factors of internal loop fluidized-bed(contour structure,structural parameter,internals,aeration zone,operating condition,etc.)can alter the fluid characteristics(fluid pattern,phases distribution,energy and mass transfer efficiency,etc.).In order to apply fluidized-bed in wide engineering fields,it is difficult to produce large amounts of reactors with different sizes to acquire sufficient data and recur the nonlinear effect result from the change of structure parameters by physical experiments.These disadvantages most probably restrict the large-scale application of internal loop fluidized-bed.The team of this research has studied the internal loop fluidized-bed for nearly thirty years,accumulating operational experience by several successful projects,with some un-explained phenomena observed.The optimization of fluidized-bed still possesses the great potential future.Based on these concerns,this thesis established a platform based on CFD(Computation Fluid Dynamics),synthetically considering the effect of reactor shapes and internal composites on the fluid flow regulation,either in the whole scope or in different functional areas,which was desired to find the numerical expression which can't be achieved by physical experiments.Under the premise of effective cognitive of internal loop fluidized-bed and the aid of commercial CFD software,Eularian multiphase model,Realizable k-? turbulence model and Schiller-Neumann drag model were set up.This study analyzed gas holdup and liquid circulation velocity at different operating conditions from the discretization perspective,as well as the further attempted on energy consumption and fluid energy transfer efficiency with the rate and distribution of liquid kinetic energy.Through factors-controlling method,the effect of three groups of structural parameters,ratio of rising area to drop area(Ad/Ar),ratio of annular area to drop area(Ab/Ar)and ratio of distance between draft tube and liquid surface to the height of tube(Ht/Hdt)on square internal loop fluidized-bed's flow performance was investigated.The results indicated that the effect of three structural parameters on liquid velocity was significant,nonetheless,the effect on the rate and distribution of gas holdup was no nsense.Ad/Ar mainly influenced the rate and distribution of fluid velocity,structural resistance was obvious when the ratio was too small,and the bottom countercurrent aeration appeared when the ratio was too large.Ab/Ar played a dominant role in liquid flow velocity and direction at the bottom of fluidized-bed,which created sludge accumulation by dispersive liquid velocity vector distribution in the mixed zone if the ratio was too large,while caused energy loss by resistance as it was too small.Ht/Hdt majorly acted on effective rectification effect of the liquid at the top of drag tube,however energy dissipation by liquid velocity vector anisotropy was brought out with a large ratio.Within the scope of the numerical simulation,three kinds of structural parameters were set as 1.25,1.2 and 1/8 in sequence,at which square internal loop fluidizedbed may achieve optimal flow performance.Moreover,after comparing the effect of these three structural parameters on fluidized-bed flow characteristic,the influenced order during engineering design was Ad/Ar>Ab/Ar>Ht/Hdt.On the basis of optimization of structural parameters,internal composites,cross-shaped internal at the bottom of the reactor and funnel-shaped internal at the top of reactor,were firstly used for optimization of structural design for internal loop fluidized-bed.The results demonstrated that internal composites reduced the liquid circulation velocity,decreased liquid kinetic energy and cut down the possible impact of velocity gradient on the microorganisms.Internal composites within coupling effect can effectively improve overall gas holdup(up to maximum 26.5% in simulation),and realized anoxic zone for SND process in drop area simultaneously.In addition,it was also found that cross-shaped internal played an important role in realizing fluidization steady and quick stable starting process for the reactor.This thesis iteratively calculated discretization by numerical simulation software of macroscopic fluid,exported gas holdup and liquid velocity(rate and spatial distribution)in the internal loop fluidized-bed.The work provided the possibility for solving the numerical expression of fluid mechanic characteristics,which is influenced by multiple structural parameters.Besides,it ranged the parameters for reactor structure optimization,which realized further mechanical optimization by the implementation of internal composites.Moreover,it proved that with the coupling effect of reactor mechanical structure and internal composites,the flow of fluid could be restrained and guided,resulting in structural control.The proper use of features like fluid field distribution,fluid power/vector ordering,strengthening mixing and separation,et al.may greatly serve the promotion of fluidized-bed reactor,strongly improving the aerobic biological wastewater treatment process as well as energy conservation and consumption reduction.