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Show Page 3. SF8C Compared to Conventional F8C A spouted bed differs from a conventional bubbling bed in the way gas is distributed across the bed of solids as well as the resul tant sol ids flow patterns. (Ref. I) In a conventional F8C, gas is distributed uniformly across the entire bed so that particle movement is primarily vertical. Momentum is relatively low and uniform across the bed. Velocity differentials are small. In a SFBC, a large quantity of gas enters at a particular point, i.e. at the spout exit. Momentum is high at the spout and low in between spouts. Velocity differentials are substantial. Using several spouts, a fully predictable solids circulation pattern can be established in the bed. Using horizontal spouts, particle movement is both vertical AND horizontal, thus providing cross circulation. (See figure 2) Mechanics of SFBC In a spouted bed, fluid (usually air) is injected through a larger nozzle(s) into an enclosed area containing small solid particles (usually sand). If the jet velocity and mass flow are high, a dilute central core of fluid is formed in the bed which extends upward to the surface resulting in a "spout". The annular area around the spout receives solid particles falling from the spout which slowly move downward until they are eventually re-entrained and the whole circulation pattern is repeated. (Figure 3) The high velocity upwards of the dilute central core and the slow countercurrent flow of the annulus downward provide a high shear, very turbulent interface which grinds down larger particles thus allowing them to burn and minimizing char. As with all types of FBC, be~.depth can be critical. As the bed depth of a SFBC becomes deeper, the visible spout is smaller at the surface and eventually the spout will disappear altogether. However an internal spout will be there and this may be preferable for some applications to reduce particle carryover, although pressure drops and horsepower usage will be increased. |