Transient phenomena in fluidized beds when switching the fluidizing agent from one gas to another

The free-bubbling behaviour of a bed, fluidized with hydrogen, stops temporarily when the hydrogen flow is changed suddenly to an equal volumetric flow of nitrogen; large conglomerates of particles form and channelling occurs. Changing the gas flow in opposite direction causes no disturbance at all. This phenomenon has been studied experimentally with different gases. It is explained how the intensity of the phenomenon depends on both the viscosities and the molecular weights of the gases used.