Numerical Simulation Of Gas-Solid Fluidized Bed With Applied Electric Field

Gas-solid fluidized bed with a high mass transfer efficiency of heat transfer, particles tend to advantages of mixing and suitable for large scale continuous production, and therefore is widely used in process industry. However, in the normal operation of the gas-phase polyethylene production, due to the inter-particle friction and collision between the particles and the wall, the occurrence of electrostatic charges is almost unavoidable. The built-up of high equilibrium charges in fluidized systems can resulting in inter-particle cohesion, particle-wall adhesion, wall sheeting and even electrostatic discharges even nuisance discharge resulting in shut down of reactor. It brings many problems for stable and continuous production in industrial fluidized beds. Applied electric field, as a kind of important means of process intensification, often can’turn harm into benefit". But because of the complexity of the electrostatic phenomena, influence factors of diversity as well as the limitations of experimental means, such as the objective conditions, related research of fluidized bed progress on the electrostatic phenomenon and the applied electric field for fluidized bed has been slow.Computational fluid dynamics technology was used for study the fluid mechanics characteristics of the gas-solid fluidized bed. This thesis focuses on the general gas-solid fluidized bed, gas-solid fluidized bed with electrostatic, gas-solid fluidized bed with applied electric field. Firstly, traditional CFD multiphase flow model was studied from mathematics parameters (model, grid, convergence condition, etc.) and physical parameters (drag force model turbulence model). Then electrostatic in gas-so lid fluidized bed was studied. Fox electrostatic model and Grace electrostatic model were compared by simulation. Different electrostatic levels in gas-solid fluidized bed were studied in order to get a better know about how electrostatic influence on the hydromechanics of gas-solid fluidized bed. Finally, the model of two dimensional fluidized bed with applied electric field was established, include the effect of electrostatic produced by own particles and the applied electric field. Coupling applied electric field force model with Two Fluid Model. The electrostatic force from particles itself and electric field force produced by applied electric field was solved independently by method of decoupling. Different kinds of applied electric field of gas-solid fluidized beds were studied. The main results in this thesis can be summarized as follows:Firstly, bubble behavior in the gas-solid fluidized bed was studied from mathematics parameters and physical parameters. It was found that the result was more accurate by soling the whole space than solving symmetric space. The accuracy can be improved effectively by refine the gird where the flow field changes severe. The criterion of convergence condition must satisfy the iterative error less than grid discretization error; the influence of turbulence model is very small; EMMS/bubbling drag force model can’t be directly applied to center jet fluidized bed, the result is almost the same by using Adjust Syamlal O’Brien model and Gidaspow model, the change of bubble size is coincide with theory model of no gas leakage, the bubble size of Syamlal O’Brien model is closest to the experimental value, and consistent with Caram model.Secondly, Fox electrostatic model and Grace electrostatic model were compared. The effect of electrostatic levels on bubble size, bubble detachment time, the distribution of the electric field force and particles were analyzed. The results show that the presence of electrostatic will affect the bubble size and bubble detachment time, with the increase of the electrostatic level, the gas leakage rate increase, the size of bubble decrease, bubble detachment time shorten; Due to the electrostatic, the strong electric field was found in fluidi2ed bed wall and near the distribution plate. The direction of electric field was towards to the center of fluidized bed. The different direction of electric force was exerted on the particle near the distribution plate and the particle near the bed height, it make particle separation more easily. In the center of the fluidized bed, the electric field is mainly horizontal direction, making the particles movement trend to the wall. The wall sheeting happened more frequently; Fox model and Grace model can both successfully applied for fluidized bed. Grace model considering the polarization force, make the electrostatic effect more remarkable than Fox model, the bubble size and bubble detachment time of Grace electrostatic smaller than the value of the Fox model. The difference of Fox model and Grace model was mainly in the bed wall, the area around bubble and the area close to the distribution plate, the simulation results of two electrostatic model are similar.Thirdly, the model of two dimensional fluidized bed with applied electric field was established, included the effect of electrostatic produced by own particles and the applied electric field. Coupling applied electric field force model with Two Fluid Model. The electrostatic force from particles themselves and electric field force produced by applied electric field was solved independently by method of decoupling. The effect of applied electric field on bubble behavior, charge distribution, electric field distribution produced by particles itself and the particle velocity was analyzed. The simulation results show that the bubble will distort by applied DC electric field along the direction of electric field effect, applied cross-flow DC electric field make bubble in the middle and downstream began to break down, the applied co-flow DC electric field makes the bubble become more acute. The separation of positive and negative particles and the electric field around bubble produced by particles themselves were enhanced by applied DC electric field as the vefocity of the particles. The frequency of the applied AC field will affect the growth of the bubble, bubble deformation only can be found in applied the low frequency AC electric field. Applied AC electric field of fluidized bed is less significant. The phase of the applied AC electric field has little effect on bubble growth.