Supersonic attrition nozzles in gas-solid fluidized beds

Gas-solid fluidized beds are used in both catalytic and non-catalytic processes, and some of the industrial applications are fluid catalytic cracking, polyethylene production, drying and classification, coating, and granulation. In some applications, the size distribution of the bed particles must be controlled in order to maintain good fluidization, and attrition nozzles can be used for this purpose. Supersonic attrition nozzles are more efficient than subsonic nozzles, and, in this study, different geometries of the Laval nozzle, a convergent-divergent (C-D) nozzle, have been investigated. The geometry of this type of nozzles gives supersonic velocities under the right operating conditions. The attrition or grinding efficiency defined as the new surface area created by mass of attrition gas used, has been experimentally measured under a variety of operating conditions and the supersonic attrition nozzles have been optimized to reduce consumption of the attrition gas. Attrition nozzle pressures used during experimentation varied between 138 and 2550 kPa, and the effects of the gas properties were studied by using different attrition gases, including air, helium, a mixture of helium and nitrogen (0.82:0.18), argon, and carbon dioxide. Depending on shape of the divergent section in the nozzle, the results show that the grinding efficiency changes, and that this efficiency is related to the thrust (F) and equivalent velocity (U-eq) of the supersonic nozzle. This finding applies to different attrition gases, nozzle sizes and geometries. All the experiments were performed using silica sand particles with the same initial size distribution. The mass of the bed of silica sand was kept constant, as well as the fluidization velocity. (C) 2010 Elsevier BM. All rights reserved.