Dense-phase fluidized beds entrainment

As discussed in Chapter 9, dense-phase fluidization other than particulate fluidization is characterized by the presence of an emulsion phase and a discrete gas bubble/void phase. At relatively low gas velocities in dense-phase fluidization, the upper surface of the bed is distinguishable. As the gas velocity increases, the bubble/void phase gradually becomes indistinguishable from the emulsion phase. The bubble/void phase eventually disappears and the gas evolves into the continuous phase with further increasing gas velocities. In a dense-phase fluidized bed, the particle entrainment rate is low and increases with increasing gas velocity. As the gas flow rate increases beyond the point corresponding to the disappearance of the bubble/void phase, a drastic increase in the entrainment rate of the particles occurs such that a continuous feeding of particles into the fluidized bed is required to maintain a steady solids flow. Fluidization at this state, in contrast to dense-phase fluidization, is denoted lean-phase fluidization. 

The basic MS-FBC concept incorporates an entrained fluidized bed superimposed on an inert, dense-phase fluidized bed as shown in Figures 1 and 2 for the MSW/DSS application. As currently operated in the MSW/DSS experiments, the dense phase is African iron ore. This dense bed remains in the comb 3tor and its height essentially defines the combustion zone (760-870 C) its high density permits dense-phase turbulent fluidization to be achieved at gas velocities exceeding 9.0 m/sec. 

Entrainer gas flowing through a dense phase fluidized bed of the solid particles. 

Dense phase fluidization Gas fluidized beds are considered dense phase fluidized beds as long as there is a clearly defined upper limit or surface to the dense bed. The dense-phase fluidization regimes include the smooth fluidization, bubbling fluidization, slugging fluidization, and turbulent fluidization regimes. In a dense-phase fluidized bed the particle entrainment rate is low but increases with increasing gas velocity. 

Lean phase fluidization As the gas flow rate increases beyond the point corresponding to the disappearance of bubbles, a drastic increase in the entrainment rate of the particles occurs such that a continuous feeding of particles into the fluidized bed is required to maintain a steady solid flow. Fluidization at this state, in contrast to dense-phase fluidization, is generally denoted lean phase fluidization. Lean phase fluidization encompasses two flow regimes, these are the fast fluidization and dilute transport regimes. 

This term is restricted here to equipment in which finely divided solids in suspension interact with gases. Solids fluidized by liquids are called slurries. Three phase fluidized mixtures occur in some coal liquefaction and petroleum treating processes. In dense phase gas-solid fluidization, a fairly definite bed level is maintained in dilute phase systems the solid is entrained continuously through the reaction zone and is separated out in a subsequent zone. 

It is important to note that the fluidized bed does not conform to Stokes law a dense phase can be maintained at an upward gas velocity at which entrainment of most of the particles would be predicted (207). The upward velocity of fluidized solids is always less than the superficial gas velocity the difference is known as slip velocity (197). 

Before carbonization, the blend may undergo further treatment to enhance the ability of the blend to make better coke or to improve the process economics. For example, blends may contain as much as 10% w/w free moisture that will impede the flow properties and lower the bulk density. Thus, it may be beneficial to preheat (to ca. 200°C [390°F]) the feed before it enters the oven bulk density will be increased and the carbonization time will be reduced by this simple operation of water removal. The abrasion resistance of the coke may also be improved by this treatment. The preheating operation may be carried out by means of an entrained (dilute phase) system or by means of a fluidized-bed (dense-phase) system, but it is important that oxidation of the coal(s) be prevented to avoid adversely affecting the coking properties. 

Fluidization occurs in stationary (segregation of bed and free board), turbulent (circulating fluid bed), and forced dense-phase (transport reactor) mode. All systems require hot gas cyclones either to recycle entrained particles back into the fluid bed or just to separate them from the product gas. 

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