(62e) Numerical Simulations of Hydrodynamic Behaviors in Conical Spouted Beds

Due to the vigorous solids motion and intimate gas-solids contact, conical spouted beds have been commonly used for drying suspensions, solutions and pasty materials. Many other applications have also been under research and development, such as catalytic partial oxidation of methane to produce synthesis gas, incineration of waste-materials, coating of medical tablets, coal gasification and liquefaction, pyrolysis of sawdust or mixtures of wood residues. Compared to other multiphase systems, such as fixed/packed beds, fluidized beds, circulating fluidized beds (co-current upflow) and co-current downflow fluidized beds (downers), numerical simulations of spouted beds have received little attention, especially on conical spouted beds. Based on two-fluid CFD model, Lu et al. (2001, 2004), He et al. (2004) and Duarte et al. (2004) simulated both cylindrical spouted beds and conical spouted beds under stable spouting by assuming the bed is fully fluidized, with a flat (Lu et al., 2001, 2004; He et al., 2004) or parabolic gas velocity profile in the inlet, with the simulation results compared with the experimental data of He et al. (1994a, 1994b) from a cylindrical spouted bed and San Jose et al. (1998) from a conical spouted bed. Kawaguchi et al. (2000) simulated the cylindrical spouted beds using discrete element method (DEM) with particle motion being traced discretely by solving Newton's equation of motion for each particle. It is well known that, for a cylindrical multiphase system, as the fluid velocity is increased, the pressure drop across the bed increases before the minimum fluidization velocity is reached, the multiphase system is operated as a packed/fixed bed, the pressure drop of the bed can be described by the Ergun equation. Once the minimum fluidization velocity has been exceeded, particles are in dynamically suspended condition, with the pressure drop across the bed remaining a constant equal to the effective weight of the bed per unit area within a wide range of fluid velocities. Because of the difference of the suspended state between a packed bed and a fluidized bed, approaches used to simulate packed beds and fluidized beds are quite different. For spouted beds, the bed structure is quite different from those of fluidized beds and packed beds. At stable spouting, a spouted bed consists of three unique regions, a spout in the center, a fountain above the bed surface and an annulus between the spout and the wall. The spout and the fountain are similar to fluidized beds with particles fully suspended, while the annulus can be treated as a packed bed or partially fluidized bed. At partial spouting, there are only two distinct regions, an internal spout that is similar to fluidized beds and is surrounded by packed particles in the annulus. Thus, original codes used for simulating fluidized beds or packed beds in commercial software such as Fluent are unsuitable for the simulation of spouted beds. Furthermore, it was found that the maximum ratio of the spouting to the fluidization pressure drop at stable spouting is about 0.64-0.75 (Mathur and Epstein, 1974), indicating that particles are not completely suspended even at stable spouting. The inclined wall as well as the base of the bed will exert some kind of supporting force on particles. To properly simulate spouted beds, the gravity term in the vertical momentum equation for the particle phase in commercial software needs to be modified when applied to the annulus region. At the same time, the radial momentum equation for the granular phase should be modified, especially for conical spouted beds, because of the existence of the inclined wall. This paper present the simulation results of a conical spouted bed at partial spouting and stable spouting states using the commercial Fluent code modified by the user-defined functions (UDFs) to account for the packed bed region and the inclined wall structure. The simulation result is then compared to experimental data on the distribution of the static pressure, vertical solid velocity, vertical gas velocity, as well as the height of the internal spout, which were generated recently from a cold model unit in our laboratory.