The Onset of Spouting

The mechanism by which a bed of particulate solids transforms to the spouted state is best explained by following the changes that occur in the pressure drop across the bed, with varying rate of gas flow (Fig. 4). 

At low flow rates, the gas simply passes up without disturbing the particles, the pressure drop rising with flow rate as in a packed bed (along AB). 

At a certain flow rate, the jet velocity becomes sufficiently high to push back the particles from the immediate vicinity of the orifice, forming a relatively empty cavity. The particles surrounding the cavity are compressed against the material above, forming a compacted arch which offers a greater resistance to flow (see Fig. 5a). Therefore, despite the existence of a hollow cavity, the total pressure drop across the bed continues to rise. 

With further increases in gas flow, the cavity elongates to an internal spout (Fig. ob). The arch of compacted material stUl exists above the internal spout, so that the pressure drop across the bed rises further until it reaches a maximum value (Fig. 4, point B). 

As the flow rate is increased beyond point B, the height of the relatively hollow internal spout becomes large in comparison with that of the packed material above the spout. The pressme drop, therefore, begins to decrease along BCD, By point C, enough solids have been displaced from the central core to cause a noticeable expansion of the bed, so that the decrease in AP is arrested over a small region of gas-flow represented by CD (Fig. 5c). 

---

A more detailed study of the sequence of transition to the onset of spouting has been reported by Volpicelli and Raso (Vl), who carried out instantaneous pressure measurements with an electric transducer near the base of the bed at varying gas flow rates in a tw o-dimensional column. They also recorded the development of the internal spout and the particle trajectories with a motion-picture camera. The average pressure, together with records of instantaneous pressures published by these workers, are reproduced in Fig. 6. Their results broadly confirm the mechanism already described (Figures 4 and 5), and provide the following additional information ... 

Referring to Fig. 4, the pressure-drop values of practical interest are those corresponding to B and E, namely the peak pressure drop attained prior to the onset of spouting (AP,a) and the pressure drop at steady spouting (AP,). The former would be encountered when starting up a spouted unit and must be allowed for in designing the gas-delivery system, while the latter would determine the operating power requirement. 

This explanation for the existence of a peak pressure drop is supported by experimental results obtained by Manurung (M7), who measured pressure drops separately across the upper cylindrical part and the lower conical part of the bed contained in a conical-cylindrical column as a function of both increasing and decreasing air flow. It is seen in Fig. 9 that the pressure drop across the upper part of the bed, up to the point at which the spout breaks through, corresponds to that in a packed bed and remains the same irrespective of whether the flow is increasing or decreasing. A peak well before the onset of spouting occurs only in the curve for the lower... 

As noted above, the second quantity necessary for the design of spouted bed equipment is the maximal pressure drop corresponding to the minimal necessary gas flow rate at the onset of fluidization, as the blower must be able to provide this pressure drop during the start-up of the process. 

---