Circulating fluidized beds high-velocity

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. 

A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

The flow structure within circulating fluidized beds is very complex and exhibits axial as well as horizontal non uniformities as it is shown in Fig 9 Unless the solids holdup is very low and the gas velocity very high... 

Donsi, G., and L. Sesti Osseo. Gas Solid Flow Pattern in a Circulating Fluid Bed Operated at High Gas Velocity," in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 696-701. Somerset, Pennsylvania (1993). 

Arena, U., Cammarota, A., and Pistane, L. High velocity fluidization behavior of solids in a laboratory scale circulating fluidized bed, in Circulating Fluidized Bed Technology (P. Basu, ed.), pp. 119-128. Pergamon Press, 1986. 

Dry, R. J. Radial concentration profiles in a fast fluidized bed, Powder Technol. 49,37-44(1987). Dry. R. J., and White, C. C. Gas residence time characteristics in a high-velocity circulating fluidized bed of FCC catalyst, Powder Techn. 58, 17 (1989). 

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