Surface modification of particles causes anomalous spouting in a tapered spouted bed*

A spouted bed is an alternative to a fluidized bed to handle coarser particles (Geldart B & D) with an added advantage of a sudden reduction in operating bed pressure drop and improving particle circulation rate. It has three different regimes as defined: spout, fountain & annulus. The spout is the dilute zone forming inside the center of the bed, above the bed surface a fountain is formed and the annulus region is defined in between the spout and vessel wall.

In a usual spouted bed, when fluid is injected at the bottom of the bed, at a particular gas velocity a cavity is formed near the gas entry point. Then with increasing gas velocity internal jet and elongated jets are formed. But in the present study, when we use color coating on the particle surface, a completely different spouting pattern is observed. At a particular gas flow rate, instead of the formation of a cavity, particles from the top of the bed are fluidized and crack propagates downwards. After that, there is a formation of a big gas bubble from the bottom, which subsequently undergoes bursting with the release of pressure. We report such phenomena for a shallow granular bed for the first time. Instantaneous particle dynamics have been characterized by high-speed imaging.