Core-annular solids transfer

Figure 11 allows us to draw some important conclusions about the true nature of the CFB fluid mechanics. From it we can infer that the overall flow structures are as shown in Figure 13. A core-annular flow structure dominates, with particles carried up in the central core and travelling down at the column walls. Along the height of the unit there is a net particle transfer from core to annulus which creates the decrease in overall bulk density with height. Superimposed upon the internal structure is a net flux through the unit which, depending upon the particles, gas velocity, solids flux, and exit employed, may be large or small compared to the net internal circulation. Typically, it is desirable that it be small to assure temperature uniformity. However, in reactions where plug flow of solids is desirable, this may not be the case.

The concept of a core-annulus structure has led many groups to report the thickness of the outer annular wall layer. This thickness is then used in core-annulus reactor models (Sec. 9 below) and in heat transfer models (Sec. 7). Wall layer thicknesses have been based on radial profiles of either particle velocity or solids flux, with the radial position at which the time mean value is 0 taken to define the boundary. However, as shown by Bi et al. (1996) and Fig. 12, the location where the time mean velocity is 0 differs from that where the time mean flux is 0. This arises because fluctuations of local instantaneous voidage are strongly correlated with fluctuations of local instantaneous particle velocity. The most meaningful wall layer thickness is based on the point at which the time mean particle flux is 0. This thickness is well correlated (Bi et al., 1996) by... 

The annular wall layer thickness may shrink or grow with height. With a constricted exit, the thickness passes through a minimum part way up the column (as in Fig. 12), indicating a net transfer of solids inward from the wall region to the core in the upper part of the riser, while there is a net outward transfer in the lower part of the riser as solids descend along the outer wall. 

Emulsion Models To simulate the core-annulus strueture, the cross section in emulsion models is divided into an inner dilute core region where particles are transported upwards, and a denser annular region where partieles descend along the wall, as in Fig. 26, but without the clusters. The thickness of the solid layer along the vertical heat transfer surfaces is often approximated as uniform. However, for membrane wall heat transfer surfaces, the annulus layer tends to be thicker at the fin than at the tube crest (Grace, 1990 Golriz 1992). 

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