Fluidized bed freeboard

Wen CY, Chen LH. Fluidized bed freeboard phenomena entrainment and elutriation. AIChE Journal 1982 28 117-128. 

A generic multipurpose fluidized bed is illustrated in Figure 2 (1). The soHds are contained in a vessel and gas is introduced into the system via a distributor, which is typically a drilled plate at the bottom of the vessel. A plenum chamber is provided below the distributor plate. The height of the soHds level above the distributor is called the bed height, and the vertical space above the bed height is called the freeboard. A splash zone may exist as a transition between the bed and freeboard. Cyclones, located either in the freeboard or external to the vessel, are used to remove soHds from the gas stream. Diplegs can return entrained soHds directly to the bed. 

Fig. 2. Multipurpose fluidized bed where 1 represents the sheU 2, soHd particles 3, the blower 4, the gas distributor 5, the heat exchanger for fluidizing gas 6, internal heating or cooling 7, external heating or cooling 8, cyclones 9, the soHds feeder 10, soHds offtake 11, Hquid feed 12, the freeboard 13, the...

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

Flue particles ia a fluidized bed are analogous to volatile molecules ia a Foiling solution. Therefore, the concentration of particles ia the gas above a fluidized bed is a function of the saturation capacity of the gas. To calculate the entrainment rate, it is first necessary to determine what particle sizes ia the bed can be entrained. These particles are the ones which have a terminal velocity less than the superficial gas velocity, assuming that iaterparticle forces ia a dilute zone of the freeboard are negligible. An average particle size of the entrainable particles is then calculated. If all particles ia the bed are entrainable, the entrained material has the same size distribution as the bed material. 

The bed level is not weU defined in a circulating fluidized bed, and bed density usually declines with height. Axial density profiles for different CFB operating regimes show that the vessel does not necessarily contain clearly defined bed and freeboard regimes. The sohds may occupy only between 5 and 20% of the total bed volume. 

Morooka, S., Kawazuishi, K., and Kato, Y., Holdup and Flow Pattern of Solid Particles in Freeboard of Gas-Solid Fluidized Bed with Fine Particles, Powder Technol., 26 75 (1980)... 

The fluidized bed occupies the lower part of the vessel, and is supported by a grate containing many openings through which air, entering at the bottom, flows to bring about fluidization. The greater part of the vessel is freeboard, in which lower gas... 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

Generally, it is advantageous to avoid reaction in the freeboard, as much as possible, since the temperature control and near-isothermal conditions observed in the fluidized bed are nearly impossible to achieve in the freeboard region. This is particularly problematic for a complex reaction, since the selectivity is often temperature-dependent. Experiments have shown that the following design features influence the extent of particle entrainment, and, by extension, the likelihood of reaction in the freeboard region ... 

Dust Separation It is usually necessary to recover the solids carried by the gas leaving the disengaging space or freeboard of the fluidized bed. Generally, cyclones are used to remove the major portion of these solids (see Gas-Solids Separation ). However, in a few cases, usually on small-scale units, filters are employed without the use of cyclones to reduce the loading of solids in the gas. For high-temperature usage, either porous ceramic or sintered metal filters have been employed. Multiple units must be provided so that one unit can be blown back with clean gas while one or more are filtering. 

Cyclones can be either placed in the freeboard above the fluidized bed or located outside of the fluidized-bed vessel. There are advantages and disadvantages to each type of placement, and the optimum type of placement depends on what is best for a particular process. 

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