DETERMINATION OF THE TENSILE STRENGTH OF A PARTICULATE SOLID BY THE RAINING BED METHOD

Based on an experiment first devised to investigate the stability of bubbles in a fluidized bed (Buysman and Peersman, 1967), an experimental technique is presented, capable to determine the tensile strength of a powder bed subjected to interparticle gas flow.As a major difference with the usual fluidization procedure, in the Raining Bed Method (RBM) the particulate solid is held against a porous plate located on top of the column, supported by air flowing upwards from its bottom. The air flow rate is gradually reduced until the fall of the bed (rainoff) occurs at the moment at which the equilibrium condition between fluid drag, gravity and interparticle cohesion is attained. Determination of the superficial velocity and of the pressure drop across the bed at the moment of particle fall gives a valuable insight into the behaviour of systems affected by interparticle forces, in that it provides a criterion for distinguishing cohesive solids from cohesionless ones. Coupled with other experimental data, it eventually leads to quantitative determination of the tensile strength of the bed.The paper illustrates the main features of the RBM technique; although still under development, it seems capable to support the effort of developing a better representation of the fluidization properties of cohesive materials.