Percolation of gas

The presence of the draft tube in the reactor allows to achieve other desirable features. In conventional spouted beds, the spout (jet) is in continuous contact with the annular solids over the entire height of the bed and, to achieve a stable operation, it must be able to support the surrounding annular bed. To provide this support, the spout must be at a greater pressure than the annulus. Because of this pressure difference, there is continuous percolation of gas into the annular region and, if the bed is too deep, the spout can easily collapse. The bed height at which this transition occurs is termed the maximum spoutable height (Mathur and Gishler, 1955). The presence of the draft tube removes this limitation. 

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