Overflow and Underflow Standpipes

Standpipes are commonly grouped into overflow and underflow types, as illustrated in Fig. 8.19. An overflow standpipe is structured with solids overflow from the top of a 

In general, the outlet of an overflow standpipe is immersed into a bed in order to provide an adequate hydrostatic head or seal pressure in the standpipe, as shown in Fig. 8.19(a). The overflow pipe can be operated in either a moving bed or fluidized bed mode. Usually, solids occupy only a fraction of the pipe near the standpipe outlet. The solids flow rate in the pipe depends on the overflow rate from the fluidized bed. Consequently, no valve is needed at the pipe outlet. In the overflow standpipe system where solids transfer from one fluidized bed to the other, the pressure drops across the lower fluidized bed, the grid (distributor), the upper fluidized bed, and the height of solids in the standpipe. For a given standpipe system and given particles, the pressure distribution profile depends on the gas velocity. The difference in the pressure distribution reflects the difference in solids concentration, 

The pressure drop in the overflow and underflow standpipes can be predicted using Eqs. (8.64) through (8.67) considering either the moving bed flow or the suspension flow. The pressure drops across the fluidized bed and grid or distributor can be evaluated, respectively, by Eqs. (9.7) and (P9.7) given in Chapter 9. 

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Typical overflow and underflow standpipes are shown in Figure 8.15, where they are used to continuously transfer solids from an upper fluidized bed to a lower fluidized bed. For solids to be transferred downwards against the pressure gradient gas must flow upward relative to the solids. The friction losses developed by the flow of the gas through the packed or fluidized bed of solids in the standpipe generates the required pressure gradient. If the gas must flow upwards relative to the downflowing solids there are two possible cases (i) the gas flows upward relative to the standpipe wall and (ii) the gas flows downwards relative to the standpipe wall but at a lower velocity than the solids. 

Figure 8.19. Schematic diagrams and pressure distributions (a) Overflow standpipe (b) Underflow standpipe (from Knowlton, 1986).

Figure 8.15 (a) Overflow and (b) underflow type standpipes transporting solids from low... 

There are two basic types of standpipe configurations, the overflow standpipe (Fig. 6A) and the underflow standpipe (Fig. 6B). The overflow standpipe is so named because the solids overflow from the top of the fluidized bed into the standpipe, and there is no bed of solids above the standpipe. In the underflow standpipe, the solids are introduced into the standpipe from the underside, or bottom, of the bed or hopper, and a bed of solids is present above the standpipe. With this definition, a cyclone dipleg is classified as an overflow standpipe because there is no bed of solids above the entrance to the dipleg. 

The primary standpipes in FCC units are underflow fluidized bed standpipes, not overflow fluidized bed standpipes. Standpipes in FCC units can be either completely vertical, completely angled, or a combination of vertical and angled sections. Nearly all FCC units incorporate two standpipes in their loop systems. The solids flow rate around the system is controlled by a slide valve or a cone valve in each standpipe. 

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