Solids flow regions

Figure 45. Three major solids flow regions in 3-m model derived from force probe signal. 

A solid flow region where the material behaves like a dry compacted powder. Slow rearrangements allow for the packed spheres to yield without creating any large separation. 

Total pressure drop for horizontal gas/solid flow includes acceleration effects at the entrance to the pipe and fric tional effects beyond the entrance region. A great number of correlations for pressure gradient are available, none of which is applicable to all flow regimes. Govier and Aziz review many of these and provide recommendations on when to use them. 

Solids Circulation Pattern. Yang et al. (1986) have shown that, based on the traversing force probe responses, three separate axial solids flow patterns can be identified. In the central core of the bed, the solid flow direction is all upward, induced primarily by the action of the jets and the rising bubbles. In the outer regions, close to the vessel walls, the solid flow is all downward. A transition zone, in which the solids move alternately upward and downward, depending on the approach and departure of the large bubbles, was detected in between these two regions. 

In correlating the data, the solid exchange rate between the two regions, Wzl was assumed to be constant. The tracer concentration data were analyzed statistically and the solids circulation rates are reported in Table 2. The positive fluxes indicate that the net solids flow is from bubble... 

When conjugate heat transfer through solid particles in the tube is to be included, the energy balance must be solved in the solid particles, in addition to the fluid flow regions. The energy balance for a solid region is defined by ... 

Experience has shown that a concave-downward (Fig. 17-10f) gas distributor is a better arrangement than a concave-upward (Fig. 17-10-e) gas distributor, as it tends to increase the flow of gases in the outer ortion of the bed. This counteracts the normal tendency of the gas to ow into the center of the bed after it exits the gas distributor. In addition, the concave-downward type of gas distributor tends to assist the eneral solids flow pattern in the bed, which is up in the center and own near the walls. The concave-upward gas distributor tends to have a slow-moving region at the bottom near the wall. If solids are large (or if they are slightly cohesive), they can build up in this region. 

The degradation ribbon at the merger of the flows occurs because of the crosschannel flow of material from the region between the solid bed and the screw root to the melt pool. As shown by Fig. 6.35, this flow is relatively large. As previously stated, the flow occurs because of pressure-induced flow and the dragging of fresh material under the solid bed by the backwards motion of the screw root. This process is consistent with the physics presented for screw rotation. The flow fields developed for a barrel rotation system would not create the low-flow region such as shown in Fig. 6.37. 

We may wish reaction to occur in the center of the bed so that both ends can be used as heat exchange regions to heat up reactants. This can be done by matching the gas and solids flow rates however, this is inherently an unstable system and requires proper control. A second alternative, shown in Fig. 26.8c,... 

The key to solving these problems is to design the vessel for a mass flow pattern. This involves consideration of both the hopper angle and surface finish, the effect of inserts used to introduce gas and control the solids flow pattern, and sizing the outlet valve to avoid arching and discharge rate limitations. In addition, the gas or liquid must be injected such that the solid particles are uniformly exposed to it, and flow instabilities such as fluidization in localized regions are avoided. 

Figure 8.23. Stagnant regions in an L-valve for three solids flow rates (from Knowlton, 1986).

These factors lead to localized particle accumulation in the wall region. The particle accumulation alters the large-scale motion in the gas-solid flow, which in turn affects the cluster size and motion. 

Figure 10.15. Evolution process of wavelike solids flow structure inferred from the solids concentration measurements and visual observations (from Jiang et al., 1993) (a) Particle interaction yielding a thin, dense, wavy solids layer in the wall region (b) Solids accumulation and descent of enlarged solids layers (c) Shaping as a bluff solids layer (d) Solids layer bursting (e) Solids sweeping to the core region.

Wwoo Asymptotic limit of the solids flow rate in the wall region... 

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