Hydrodynamic modeling of circulating fluidized bed

Fluidization is an operation through which fine granular solids are transformed into a fluid like state through contact with a gas or liquid. Under the fluidized state the gravitational pull on granular solid particles is offset by the fluid drag on them, thus the particles remain in a semi suspended condition. An increase in the gas velocity through a granular solid brings about changes in the mode of gas solid contact in many ways. Among the several flow regimes of fluidization, bubbling fluidization is most popular. Another is high velocity fluidization, which includes both turbulent and fast fluidization.

The transition from bubbling to turbulent fluidization occurs at lower velocity in larger diameter vessels. Fine particles enter the turbulent fluidization at a velocity sufficiently above the terminal velocity of solids, where as coarser particles may enter turbulent fluidization at a velocity less than the terminal velocity of particles.

When the gas velocity of the bubbling bed is increased, a turbulent bed emerges. Further increase of gas velocity results in nothing being retained in the bed, but if solid particles are continuously fed into, bed density can be controlled by solid feed rate. A bed of this nature is called "circulating fluidized bed (CFB)" because a high velocity gas is used to circulate an appropriate quantity of solids particles in this operation.

The fluidized bed reactor is an alternative to the fixed bed reactor. In a fluidized bed reactor, the solids are no longer packed, but the higher gas velocity causes the solids to float and to move relative to one another. The rapid motion of the particles guarantees a high degree of mixing and thus a very uniform temperature distribution without the occurrence of hot spots. Thus, the fluidized bed is a most suitable reactor for highly exothermic and endothermic gas-solid reactions. Typical examples for exothermic processes carried out in fluidized beds are the roasting of metal sulfides, or coal and oil combustion (Reh, 1971). The endothermic group of fluidized bed processes includes the burning of lime, the calcinations of phosphate rock, the decomposition of ferric chloride or ferrous sulfate and the reduction of metal oxides.

Applying even higher velocities creates a significant entrainment of solids out of the reactor. In order to keep the solids inventory with in the vessel constant an external solids recirculation is needed, which led to the development of the circulating fluidized bed (CFB) reactor. This type of reactor additionally requires at least one external cyclone for the gas-solids separation, a return leg for transporting the solids from a high elevation at low pressure to a lower elevation at higher pressure, and a solids feeder to return the solids in to the bottom of the reactor vessel.