DEM Investigation Of The Wall Normal Stress And The Particle Flow Behavior In Rectangular Hopper And Moving Bed

Moving bed has been widely used as reactor and separator due to its advantages of low pressure drop, posibility of working at high temperature and easy coupling of reaction and regeneration of particles. Understanding the variation of particle residence time distribution with particle and equipment properties and inhibiting the cavity formation are necessary for a high performance moving bed. To make these come ture, a deep insight into the particle flow in moving bed is required, but hard to be obtaind due to the complex force field. The particle flow in hopper is determined by a simple force filed and the investigation into this particle flow could provide useful information to understand that in moving bed. In this work, we firstly investigated the wall normal stress and particle flow behavior in steady hopper discharging process by Discrete Element Method, and the force network was analyzed to understand the particle flow. Based on these investigations, we numerically investigated the residence time distribution of particles in moving bed, and proposed a way to inhibit the cavity formation by mixing coarse particles into moving bed. The main achievements in the thesis are as follows:(1) Particle discharging from wedge shaped hopper was simulated, and the effects of hopper and particle geometries on wall normal stress and wall stress ratio were studied. The wall normal stress increases with hopper size because more particles load have to be supported. Near the convergent section, the particle load is hard to be transferred to the vertical wall, making the wall normal stress decrease rapidly. Out of this area, the wall stress ratio is influenced by the hopper scale when the force chains on the wall are formed across the whole hopper length, but independent of hopper half angle, inlet width and particle size distribution.(2) The effect of wall friction coefficient on wall stress ratio in hopper discharging process was investigated. With the increase of wall friction coefficient, the relative density of the force chains attaching the wall increases, which enhances the contribution of the wall to supporting the particles load. The enhanced supporting of one wall due to wall friction weakens that of its neighbouring wall. However, the force chains attaching one wall will exert normal stress on its neighbouring wall. When the wall friction coefficient is sufficiently large, the strength of the relative strong force chains on the wall eventually relies on the internal friction coefficient and the hopper length along the normal direction of the walls, making the wall stress ratio less sensitive to wall friction coefficient. The wall stress ratio is predicted with Walker’s model, in which the effective internal friction angle is replaced by a modified effective internal friction angle to relate to the wall and internal friction coefficients. The prediction is more accurate than with the assumption of critical equilibrium state.(3) Concentric and eccentric discharges of particles from wedge shaped hopper with different particle and hopper properties were simulated and the effect of wall friction coefficient on discharge rate was investigated. In general, increasing wall friction coefficient would decrease discharge rate by retarding particle-wall slip, and the effect is more significant when the wedge shaped hopper and rough particles are used. The friction coefficient in the convergent section has a stronger effect in reducing discharging rate than that in the vertical section. The retarding effect would decrease with the increase of hopper width or the decrease of outlet width while remains unchanged with the variation of particle size distribution.(4) Effects of particle and moving bed properties on particle residence time distribution have been investigated, and were understood by analyzing the velocity distribution and discharge rate. The vertical velocity difference between center and walls gets larger with the wall friction coefficient, while gets larger first and then smaller with the internal friction coefficient. In the horizontal field region, the enhanced field enlarges the vertical velocity difference. The variation of the vertical velocity difference leads to the similar change of the normalized variance. With the increase of half angle, the horizontal velocity of particles in the lower part of moving bed gets smaller, resulting in larger normalized variance. The mean residence time of particles increases with the decrease of discharge rate, and thus increases with the wall and internal friction coefficients and half angle but is independent of the intensity and size of horizontal field.(5) Inhibiting the cavity formation by mixing coarse particles into moving bed was studied. With the mixing of coarse particles or the increase of coarse particle content in moving bed, the field force may contribute less to supporting the particle load transferred to the wall, which inhibits the cavity formation. When the coarse particle have a smooth surface, the particle load transferred to the wall may increase, whch also contributes to inhibiting the cavity formation, especially when the content of coarse particle near the wall is high. This way could also be used to significantly inhibit the cavity formation in a large size moving bed.