When small, light solid particles are fluidized by gases it is well known that stable expansion occurs over a finite interval of gas flow beyond the point of minimum fluidization. The existence of such an interval can be predicted from linear stability theory provided the momentum equation for the particles contains a sufficiently large term representing an effective pressure that increases with the concentration of the particles. There is at present some controversy regarding the physical origin of such a term. Some workers attribute it to forces exerted between particles at points of solid–solid contact, while others invoke hydrodynamic mechanisms related to the interaction between the particles and the fluid. In this paper the processes of fluidization and defluidization for fine particles are followed very carefully round complete cycles, starting from zero gas flow and extending to a value at which bubbles appear, then back to zero. The depth of the bed and the pressure drop in the gas traversing it are recorded at each stage, and vertical profiles of the volume fraction of particulate material are determined with a high-resolution gamma-ray densitometer. Similar information is also obtained for sub-cycles extending over more restricted intervals of the gas flow rate. The particles studied are cracking catalyst, with mean diameter 75 μm, and Ottawa sand with mean diameter 154 μm. The results lead to the conclusion that the particle assemblies exhibit yield stresses throughout the range of stable behaviour, and thus are not truly fluidized beds, in the accepted sense. The phenomena observed are such that it is most unlikely that their origin is hydrodynamic. For the particular systems studied we therefore conclude that contact forces are responsible for stabilization.

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