Spouted beds depth

As indicated by Eq. (12-62) the superficial fluid velocity required for spouting increases with bed depth and orifice diameter and decreases as the bed diameter is increased. 

Spouting cannot be maintained once the bed height exceeds a certain level, coim monly known as maximum spoutable bed depth, Hm. At Hm, the spout starts to collapse and the bed starts to change from a spouted bed to a fluidized bed. As indicated in Eq. (9.67), the minimum spouting velocity increases with an increase in the bed height. Consequently, (7msp achieves its maximum at Hm. The maximum value of the minimum spouting velocity can be related to the minimum fluidization velocity by... 

For a given solid material, column diameter, and fluid inlet diameter, a maximum spoutable bed depth exists beyond which the spouting action degenerates into poor-quality fluidization. 

A typical spouted bed has a substantial depth, which in the case of a cylindrical vessel is usually at least of the order of two column diameters, measured from the inlet orifice to the surface of the annulus. If the bed is much shallower, the system becomes hydrodynamically different from true spouting. The situation in this respect is similar to that for gas-fluidized beds, where the generally formulated principles of fluidization are not applicable to very shallow beds. 

Thus, the minimum spouting velocity for a given material, column size, inlet size, and bed depth can be obtained by combining Eqs. (7)-(10). Calculation by this method is valid for H/D greater than 1, Rem of 10-100, and Di/Dc less than 0.1. 

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.

The first part of this section briefly summarizes the main findings concerning the effect of the various factors on spouting stability, while the second part deals with methods proposed for calculating the maximum spoutable bed depth. 

The maximum spoutable bed depth was found to decrease with increasing particle size by Malek and Lu (M3), who experimented with four different sizes of wheat (1.2-3.7 mm) in a 6-in. column. On the other hand, Reddy et al. (Rl), who worked with mixed-size materials (alimdum, glass spheres, and polystyrene), also in a 6-in. column, reported that Hm first increases with particle size and then decreases, a peak value being attained at a mean particle size of 1.0-1.5 mm. The observed variation of Hm, correlated by Reddy et al. with mean particle size, is likely to be also influenced by size distribution, which cannot be fully characterized by any particular mean diameter. Nevertheless, the existence of a peak Hm with respect to particle size alone is theoretically predictable from a comparison of the effect of particle size on the gas velocities required for spouting and for fluidizing a given material (Rl). From Eq. (3), the effect of particle size and bed depth on spouting velocity, with all other variables held constant, is as follows ... 

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