Control And Experimental Study Of The Electrostatic Potential Distribution In Gas-solid Fluidized Beds

Due to the friction and the collision among the particles,gas and the wall,the electrostatic charges is thus occurred,and then accumulated in insulated system in ethylene polymerization fluidized bed reactors.The occurrence of electrostatic charges can influence the motion of the particles and fluid in the fluidized bed, besides the continual accumulation of charges on system components may cause agglomeration,sheeting and even nuisance discharge resulting in shut down of reactor, which hampers the desired steady state process in industry.Because of the complexity of the particles and gas behavior in the fluidized bed reactor,the occurrence and accumulation of the electrostatic charges is difficult to estimate.Therefore,it is of profound theoretical and practical significance to systematically investigate the electrostatics potentials distribution based on the experiment and theoretical research in the gas-solid fluidized bed.It is a great help to better understand the electrostatics phenomena and keep the electrostatic level under control.The experiment is carried out in cold-model fluidized bed in the laboratory, based on an on-line technique to measure electrostatic potential.This thesis focus on electrostatic potentials distribution,electrostatic level control and establishment of the distribution model,carefully investigated the mechanism of electrostatics distribution in the fluidized bed and try to control it.Firstly,the influence to electrostatic potentials profiles under different velocities or with the injection of granules into the fluidized bed is systematically investigated.Then the electrostatic behavior in a polyethylene gas-solid fluidized bed with different charge inducing agent added is experimentally experimented.And the electrostatic behavior in core-wall fluidized bed system was studied too,which provides the primary study to the electrostatic level control.Afterwards,a primitive model to describe the electrostatic potential distribution in the fluidized bed is established based the previous research.The main results in this thesis can be summarized as following:Firstly,the influence to the axial electrostatic potentials profiles with the injection of granules into the fluidized bed is investigated.An impact factor of granules(F_b) is proposed to study the influence of particles size and the weight percent of the residual catalyst to the electrostatic potentials.Meanwhile,experiment is carried out in fluidized bed under different superficial velocities,to investigate the effect of superficial velocities to the electrostatic potentials in the system.A charge mechanism is proposed that the electrostatic potential distribution was controlled by not only the bubble behavior but also the particles concentration in the fluidized bed. The more acute bubbles behavior the more electrostatic charges accumulated on the particles lead to higher electrostatic potentials in the fluidized bed.Besides the more charges on the particles,the higher concentration of the particles results in the higher electrostatic potentials in the fluidized bed.Thus,the distribution of electrostatic potentials is the overall result of the two effects.Secondly,the electrostatic behavior in a polyethylene gas-solid fluidized bed with different charge inducing agent added is studied.A(δU-χ_i) theory is proposed for the system.Affecting mechanism of charge inducing agents is discussed based on the experimental results and the electronegativity of corresponding compounds.It is found that the electronegativity(χ_i) of the metal ion plays an important role in determining the charge polarity on the LLDPE particles and the change in the electrostatic potentials after contact and separation with the charge inducing agent particles.It can be helpful to choosing charge inducing agent in industrial application.Thirdly,in the newly designed fluidized bed,with gas in core or wall zone separately,the electrostatic potential distribution can be controlled by changing and controlling the fluid hydrodynamics behavior in the bed.It is concluded from the experimental result that in core-wall fluidized bed system,the positive charged zone can be related to the upper particles cycle in the bed and negative charged zone can be related to the down particles cycle.The positive charged zone increases with the superficial velocities when the bed is fluidized with only gas in the core zone.And the negative charged zone increases with the superficial velocities when the bed is fluidized with only gas in the wall zone.Fouthly,a primitive model to describe the electrostatic potential distribution in the fluidized bed is established based the previous research.The fluidized bed can be regard as an electrified body with uniform volume electric charge density.Based on the Possion Equation and the boundary conditions,the primitive mathematical model is discussed from the electrostatic mechanism and physical theory to better understand the electrostatic phenomena in the fluidized bed.