The effect of temperature on the minimum fluidization conditions of industrial cohesive particles

Abstract

In order to understand the factors responsible for changes in the fluidization behaviour of industrial particles at high temperatures, an experimental campaign was performed using a 140 × 1000 mm heated gas fluidized bed. Five powder cuts sieved out of the same mother powder covering Group B, A and C of Geldart's classification were investigated over a range of temperatures from ambient to 500 °C. The results show that the mean size distribution affects significantly the fluidization behaviour of the materials investigated. In particular, significant differences were observed in the fluidization behaviour of the coarsest samples (Group B-A) and finest samples (Group A-C). The minimum fluidization conditions were compared with the prediction of the Ergun equation. The comparison was satisfactory only when accounting for the experimental values of the bed voidage. In fact, the non-monotonic trend of the minim velocity for fluidization with increasing temperature cannot be explained only with the effects of temperature on the bed fluid dynamics. But several others are the observed effects on the fluidization behaviour due to the temperature rise that can be ascribed to the enhanced interparticle forces: 1) the increase of the peak of pressure drops, close to the minimum for fluidization, in the fluidization curve at increasing gas velocities; 2) the increase for the finest samples of the hysteresis in the fluidization curves, considering the fluidization and defluidization branches of the curve; 3) a greater tendency of the bed to expand homogeneously; 4) the increasing difference between the parameters of the Richardson-Zaki equation found with a fitting procedure on the experiments and those found using the Richardson-Zaki correlations and the theoretical terminal velocity. Furthermore, in the cases where larger interparticle forces were expected, the X-Ray facility allowed to identify different internal structures within the bed. Mostly vertical channels but also, in the case of the finest powder tested, horizontal channels.