Electrostatic Characteristics Of Particles In Fluidized Bed And Its Influence Mechanism On Fluidization Characteristics

In a gas-solid fluidized bed where highly insulating substances is used as bed particles,the particles inevitably carry electrostatics due to the uninterrupted contact and separation between the particles as well as between the particles and the wall.The generation of electrostatics can cause a series of problems.For example,due to the accumulation of electric charges,the particles may agglomerate,leading to dead zones and channeling,affecting the hydrodynamic characteristics of the fluidized bed.The accumulation of electrostatics also may cause the particles to adhere to the inner surface of the fluidized bed reactor and melt-bond in the form of a sheet.If the sheeting is detached on the air distribution plate,it will cause blockage of the air distribution plate,which will cause accidental shutdown.Therefore,to explain the mechanism of electrostatics' influence on fluidization characteristics of fluidized bed is significantly important for the design and operation of fluidized bed reactor.However,the current research on electrostatics in fluidized bed is still relatively scattered and lacks systematic and deep understanding.In this paper,a visual two-dimensional fluidized bed system of plexiglass was designed and constructed.Particles with different physical parameters were used as bed materials.The effects of static electricity generation and static electricity on gas-solid flow during gas-solid fluidization were studied experimentally.The magnitude and direction of the electrostatic force of the particles and the potential distribution in the fluidized bed were numerically simulated,revealing the electric field distribution in the current-charged bed and its influence on the gas-solid flow.The main research contents and conclusions of this paper are as follows:(1)Experimental study on particle charge in fluidized bed The experimental study on fluidized bed operating parameters and bed material properties' influence on characteristics of the fluidized bed charge were carried out.The experimental results show that if the fluidized velocity is constant,the average charge density of the particles increases with the increase of fluidization time,and reaches saturation after fluidization for a certain period of time.As the fluidization velocity increases,the particle saturated charge density increases.The fluidized velocity determines the intensity of gas-solid flow in the fluidized bed.The more intense the fluidization of the particles become,the higher the charge generation rate is,and the higher the average charge density of the particles is when the charge in the fluidized bed reaches saturation.The change of the initial height of static bed will also cause the change of the average charge density of the particles.The difference between the severities of the gas-solid flow in different regions of the fluidized bed will lead to the difference between the degrees of particles charge in different regions of the fluidized bed.The experiments compared the particle charging characteristics of glass beads(GB),linear low density polyethylene(LLDPE)and corn powder(CP)with different particle sizes.The experimental results show that under the same experimental conditions,GB particles and CP particles carry a positive charge after fluidization,while LLDPE particles carry a negative charge after fluidization.Under the same particle size conditions,the average charge density of the CP particles and the LLDPE particles is higher than that of the GB particles when the charge reaches a saturated state.For particles of the same material with different particle sizes,when the charge reaches saturation,the average particle density of the small particle size is higher than that of the large particle size.This indicates that the interaction between the particles and the wall of the fluidized bed plays a decisive role in the sign of the final charge of the particles.The higher the work function of the insulator material is,the easier it is to obtain electrons and carry negative electrons.The lower the work function,the easier it is to lose electrons and carry positive electricity.In addition,due to the difference in specific surface area caused by the difference in particle size and apparent structure,the particle of small size carries more charge when the charge reaches saturation.In the case of the same particle size,the CP particles and the LLDPE particles carry more charge than the GB particles when the charge reaches saturation.(2)Experimental study on the effect of fluidized bed static electricity on gas-solid flow The experiment used GB particles as bed material,recorded the whole process of bubble injection into the fluidized bed by high-speed camera,and analyzed the change rule of the area,the aspect ratio,and the rate of rise of single bubble in its rise process in the fluidized bed with critical fluidization state under different charge densities by digital image analysis(DIA).The bubble experiment results show that as the charged density of the fluidized bed increases,the aspect ratio of the single bubble becomes smaller and the area becomes larger during the ascending process.Under the influence of static electricity,the resistance encountered by the bubbles in the bottom layer of the bed becomes smaller and the resistance encountered in the upper layer of the bed becomes larger.The velocity of the bubble increases at its initial rising period,and the velocity becomes smaller as the bubble rises to the upper layer in the bed.In addition,as the degree of charge in the fluidized bed increases,the bubbles split as they approach the surface of the bed.The charge of the particles causes a change in the force of the particles,which in turn affects the gas-solid flow characteristics in the fluidized bed.(3)Numerical simulation of gas-solid flow characteristics of electrostatics in fluidized bed The Euler-Eulerian method and Eulerian-Lagrangian method were used to analyze the causes of bubble characteristics changes in the current-carrying bed.The Eulerian-Eulerian method uses the potential scalar equation derived from Maxwell's equations to establish an electrostatic model,and implants the electrostatic model into the two-fluid model(TFM)to simulate the bubble experiment.The simulation analyzes the electrostatic force of the solid phase according to the potential gradient distribution and the electrostatic force distribution.The Eulerian-Lagrangian method uses Coulomb's law to establish an electrostatic force model,and implants the electrostatic force model into the CFDDEM model.The force analysis of the particles is used to study the cause of bubble deformation in the charged fluidized bed.The simulation results of the full-scale simulation of the bubble experiment by the Eulerian-Eulerian method show that the charge phase of the solid phase causes the aspect ratio of the bubble to become smaller and the volume to become larger.The phenomena that the bubble tip particles fall or bubble split are not found in the simulation results.In addition,by analyzing the potential distribution of the fluidized bed in the simulation results,the influence between the electric field and the bubble is mutual,and the bubble is affected by the static electricity,at the same time affects the potential distribution of the fluidized bed;The simulation results of the small-size fluidized bed bubble injection process by the Eulerian-Lagrange method show that the particle charge causes the particles at the edge of the bubble to be subjected to the electrostatic force directed to the inside of the bubble,causing the particles to fall in the upper half of the bubble,and the bubble volume becomes smaller,indicating that the influence of electrostatics on the force and motion of charged particles around the bubble is the main cause of the deformation of the bubble.The electrostatic force distribution in the bipolar-charged system is irregular,and the particles carrying the different electric charges are easy to form particle pairs due to the opposite attraction,which weakens the change of gas-solid flow characteristics caused by the interaction between particles.In addition,this thesis also compares the simulation results of the Eulerian-Eulerian method and the Eulerian-Lagrangian method.By analyzing the principle of the two methods,the difference of principles between the two methods is discussed.In the study of the advantages and disadvantages of gas-solid flow electrostatic phenomena,and combining the advantages of these two methods,a new hybrid method is proposed.The hybrid method solves the gas-solid flow and the electrostatic interaction between the particles by the Eulerian-Lagrangian method,and calculates the potential of each time step by the Eulerian-Eulerian method.According to the simulation results of the new hybrid method,in the mono-charged system,the region of the emulsion phase away from the boundary of the bubble and the bed has the highest potential,and the region near the bubble and the boundary of the bed has a lower potential.The direction of the electrostatic force of the particles is along the direction of the potential from high to low,which causes the particles at the edge of the bubble in the mono-charged system to have a tendency to move toward the inside of the bubble and ultimately affect the gas-solid flow characteristics of the fluidized bed.The new hybrid method achieves a fullscale analysis of the electrostatic problem of gas-solid flow,and promotes the mechanismal understanding of the influence of static electricity on gas-solid flow characteristics.