High velocity attrition nozzles in fluidized beds

Abstract

Gas–solid fluidized beds are used in many industrial applications such as polyethylene production, drying, coating, granulation, fluid catalytic cracking and fluid coking. For some industrial applications, controlling the size distribution of the particles in a fluid bed is extremely important in order to avoid poor fluidization. One method to control the size of the particles in the bed is to use attrition nozzles, which inject high velocity gas jets into the bed creating high shear regions and grinding particles together. The objective of this study was to test different high velocity attrition nozzles and operating conditions in order to determine the effects of fluidization velocity, nozzle size, nozzle geometry, bed material and attrition gas properties on the grinding efficiency. Samples of solids were taken from the bed and analyzed before and after each injection and a grinding efficiency was defined as the new surface area created per mass of attrition gas used. An empirical correlation was also developed to estimate the grinding efficiency, and its predictions were validated using the experimental data. Large diameter nozzles with a Laval nozzle geometry, operating at high upstream pressures and high fluidization velocities, resulted in the highest grinding efficiencies. Gas properties, such as speed of sound and density, had a significant impact on the grinding efficiency.