Experimental characterization of the cohesive behaviour of fine powders by the raining bed test

The paper analyses and discusses the possibility of using the so-called ‘raining bed experiment’ to achieve quantitative characterization of the macroscopic cohesion force that affects fluidization of group A solids. This procedure, originally devised by Buysman and Peersman in 1967 [1], is based on measuring the minimum velocity required to an up-flowing gas stream for an up-flowing gas stream to hold a particulate bed against a porous plate at the top of a column, before bed failure occurs, either as a rain of particles (‘rain-off’) or as a fall of plugs. Rain-off velocities and bed pressure drops relevant to different cuts of glass ballotini, ceramics spheres, alumina, FCC catalyst, Ludox catalyst, silica sand and other powders, with average particle size ranging from 25 to 465 μm, are determined. These data are compared with the corresponding ones obtained by performing fluidization experiments on the same materials. Such a comparison allows a classification of the powders among cohesive and free-flowing solids. Moreover, the raining bed technique is able to provide valuable information about the relationship between macroscopic cohesion and fluidization properties of fine solids. Its development is thus likely to allow evaluating the excess drag force required to overcome interparticle forces in the transition from the fixed to the fluidized state.