Spouted beds annulus, solids flow velocity

Stable fluidization requires a distribution of particle sizes, preferably in the range of a few hundred microns. Normally a size of 4 mm or so is considered an upper limit, but the coal dryers of Tables 9.15(a) and (b) accommodate sizes up to 0.5 in. Large and uniformly sized particles, such as grains, are dried successfully in spouted beds [Fig. 9.13(f)] Here a high velocity gas stream entrains the solid upward at the axis and releases it at the top for flow back through the annulus. Some operations do without the mechanical draft tube shown but employ a naturally formed central channel. 

The solids movement in a spouted bed is initiated by the interaction between the particles and the high-velocity gas jet, so that particle flow in the spout region shapes the entire solids-flow pattern. While a mutual dependence between the solids flow in the spout and in the annulus is inherent to a spouted bed, it is nevertheless convenient to discuss the flow in the spout and in the annulus separately. 

Fig. 17. Solids flow pattern in the annulus (Thorley et at, Tl). Column diameter, 24 in. air-inlet diameter, 4 in. cone angle, 60° bed depth expanded, 48 in. air flow, 210 cfm minimum spouting condition f vertical component of particle velocity (in./sec) horizontal component of particle velocity (in./sec) data in ovals give solids flow (Ib/sec) based on resultant particle velocity data in rectangles give solids flow (Ib/sec) based on particle velocity at column wall.

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