Spout shapes

The fact that in a steadily spouting bed the spout assumes a stable shape implies the existence of a state of dynamic equilibrium among the various forces acting on the spout-annulus interface. Since these forces arise from the movement of both solids and gas, the shape of the spout provides a valuable clue to an understanding of the entire dynamics of spouted beds. The spout shape has, therefore, received considerable attention. 

Spout-shape observations have usually been made against the flat transparent face of sectional columns, either semicircular or two dimensional. Doubt concerning distortion of the spout in a half-sectional cylindrical column by the flat face has been dispelled by Mikhailik (M13), who made parallel measurements in half and full columns, using the piezoelectric technique mentioned in Section IV,B for the latter. The absolute significance of measurements made in two-dimensional columns remains... 

Lefroy and Davidson (L2), in an extension of the theoretical analjrsis described in Section IV,B, have attempted to explain the spout shape from a consideration of the balance of forces acting on an element of particles in the spout wall (see Fig. 24). and F, are due to percolation of air through... 

Fig. 23. Observed spout shapes (a) diverges continually (b) expands then tapers (c) necks, expands, then tapers slightly (d) necks, then expands (e) necks twice.

Note that with a larger orifice, spout shape changed to type c Tapering of spout dia. near top occurred only for H > 7.9 in. 

Equation (9) is a dimensional equation where SI imits should be used. Equation (9) is good to 5.6% at room temperature. He et al. (1998a) employed a fiber-optic probe to measure the spout diameter in a semicylindrical and a full cylindrical spouted bed and found that the presence of the flat front plate in the semicylindrical bed considerably distorted the spout shape. They found that the McNab equation underestimated the spout diameter in a full cylindrical bed by an average of 35.5%. Under pressure, the McNab equation can also introduce error up to 65.5% at a pressure of 343 kPa (He et al., 1998b). Spout diameter tends to increase with increasing pressure. 

Stirring rods. These are made from glass rod 3-5 mm in diameter, cut into suitable lengths. Both ends should be rounded by heating in the Bunsen or blowpipe flame. The length of the stirring rod should be suitable for the size and the shape of the vessel for which it is employed, e.g. for use with a beaker provided with a spout, it should project 3-5 cm beyond the lip when in a resting position. 

The spouting vessel is conunonly either cylindrical or conical in shape (Fig. 2). With the former it is preferable to have a short conical base tapering down to the inlet orifice, so that the solids in the annulus can easily slide into the gas jet region without forming any dead zone at the base. Most of the work outside the Soviet Union has been carried out in this type of vessel, while spouting in conical vessels has received particular attention in the Soviet Union, and has been extensively discussed in a book by Romankov and Rashkovskaya (R4). The general discussion of spouted bed behavior in this review is in the context of a cylindrical column. 

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. 

Typical fluid bed processors are illustrated in Fig. 1. Fig. lA is a simple fluid bed, usually with a conical shape at the bottom of the chamber, leading to inducing spouted particle flow more or less depending on the angle of the conical chamber. The spray is supplied from the top toward the fluidized bed in agglomeration and into the bed in a tangential direction in coating. Elutriated particles from the fluid bed are... 

At the same time, the more traditional tabletting techniques in the pharmaceutical industry were improved and modernized which led to higher production capacities, better tablet qualities, as well as more variability in product shape and composition. Other agglomeration methods were also introduced such as fluidized and spouted beds, mixers of various kinds, etc. 

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