Fluidized-bed seal leg

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

When the solid is one of the reactants, such as in ore roasting, the flow must be continuous and precise in order to maintain constant conditions in the reactor. Feeding of free-flowing granular solids into a fluidized bed is not difficult. Standard commercially available sohds-weighiug and -conveying equipment can be used to control the rate and dehver the solids to the feeder. Screw conveyors, dip pipes, seal legs, and injectors are used to introduce the solids into the reactor... 

Seal legs are frequently used in conjunction with solids-flow-control valves to equ ize pressures and to strip trapped or adsorbed gases from the sohds. The operation of a seal leg is shown schemati-caUy in Fig. 17-19. The sohds settle by gravity from the fluidized bed into the seal leg or standpipe. Seal and/or stripping gas is introduced near the bottom of the leg. This gas flows both upward and downward. Pressures indicated in the ihustratiou have no absolute value but are only relative. The legs are designed for either fluidized or settled solids. 

Cyclone separators are often used in fluidized beds for separating entrained solids from the gas stream (see Chapter 9). Cyclones installed within the fluidized bed vessel would be fitted with a dip-leg and seal in order to prevent gas entering the solids exit. Fluidized systems may have two or more stages of cyclone in series in order to improve separation efficiency. Cyclones are also the subject of erosion and must be designed to cope with this. 

The solution to the problem was to replace the L-valve by a loop seal (Figure 16.3), which is essentially a fluidized bed divided by a vertical baffle except for a gap at the bottom to allow solids to pass between the two side-by-side beds, fluidized by separate air supplies. Hot lime particles fall into the bed nearest to the calciner, flow under the vertical baffle into the second bed adjacent to the cooler driven by the higher pressure in the first leg of the loop. The solids in the second bed then overflow into the cooler. This system operates like two arms of a manometer and there are fluidized solids present at all times in the loop preventing hot gas from passing direct into the cooler. 

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