Fundamental Particle Fluidization Mechanism and Handling of Fine Particles in a Rotating Fluidized Bed

Fluidized bed has many advantages such as high heat and mass transfer rates, temperature homogeneity and good mixing property. However, conventional fluidized bed has some limitations; operation at high gas velocity leads to slugging and there exists minimum particle size for the uniform fluidization. Recently, a rotating fluidized bed (RFB) has attracted special interest, since it has a possibility to overcome the limitations. Due to the vessel rotation, the RFB can impart high centrifugal force to particles, leading to achieve uniform fluidization of even fine particles. The RFB also exhibits good particle mixing property with lower elutriation. In this study, we have developed a novel rotating fluidized bed and the fundamental particle fluidization mechanism is analyzed by both experimental and numerical approaches. Handling and processing of fine particles in RFB are also discussed. Introduction Fine powders have become a major interest lately. New functions and high qualities attributed to fine powders are expected in many industries such as pharmaceuticals, agriculture, foods, chemicals, ceramics and electronics. The fluidization is one of the most promising techniques for the fine powder handling. The applications have been extended to a wide variety of processes such as cracking of hydrocarbons, combustion of solid fuels/wastes and roasting of ores as chemical processing, and filtration, drying, wet granulation and coating as physical processing. This is because the fluidization exhibits excellent advantages of high heat and mass-transfer rates, temperature homogeneity and high flowability of particulate materials. However, as pointed out by Geldart in his classification map [1], powders in Group C (fine size and low density) fluidize poorly, exhibiting channeling and other untoward effects when aerated. Therefore, development of a reliable technique to improve the fluidization of cohesive fine powders is strongly required. So far, several devices such as vibration, mechanical agitation, sound, magnetic force, and etc. have been developed to improve the fluidization of cohesive fine powders. However, it is not easy to achieve uniform fluidization and smooth handling of fine powders, despite the use of these devices. To overcome several limitations that the conventional fluidized beds have, we have developed a novel rotation fluidized bed system. The system basically composes of a plenum chamber and a horizontal cylindrical air distributor, which rotates around its axis of symmetry inside the chamber. We have reported that the uniform fluidization of cohesive fine powders could be easily achieved, and several unit operations such as wet granulation and film coating of fine powders were successfully conducted [2-3]. In this study, fundamental fluidization mechanism of a novel rotating fluidized bed has been analyzed by both numerical and experimental approaches. The numerical approach includes a DEM (Discrete Element Method) and a CFD (Computational Fluid Dynamics) modeling. The applications of rotating fluidized bed to handling of cohesive fine powders are also reported. Experimental A schematic diagram of the experimental apparatus is shown in Fig. 1. The rotating fluidized bed composes of a plenum chamber and a cylindrical air distributor (I.D.400×D100mm) made of stainless sintered mesh with 20 micron opening. The horizontal cylinder (air distributor) rotates around its axis of symmetry inside the plenum chamber. There is a stationary cylindrical metal filter (I.D.140×D100mm, opening diameter is 10 micron) inside the air distributor to retain elutriated fine powders. A binary spray nozzle mounted on the metal filter sprays binder mist (mist size is around 7 to 10 micron) onto the powder bed. A pulse air-jet nozzle is also placed inside the metal filter, which cleans up the surface of the metal filter in order to prevent clogging. An air knocker is installed outside the plenum chamber to prevent powder adhesion onto the air distributor mesh and front cover. Pressure taps are mounted on the inlet and exhaust air pipes, so that a manometer measures the pressure drop across powder beds. Figure 2 illustrates the powder flow mechanism in the rotating fluidized bed. In a conventional fluidized bed, air distributor is horizontally mounted and powder samples are introduced onto the air distributor. Powders are lifted up by a vertical airflow (drag force and buoyancy against the gravity force). In a rotating fluidized bed, powder samples are introduced inside the air distributor and are forced to the