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.

[...] The second proposed fluidization regime is the ‘fast fluidized bed profile’ occurring at high solids circulating rates. Additionally to the acceleration region and the dilute phase appearing in the ‘dilute phase pneumatic transport profile’ a dense phase region and a transition region occur beyond the acceleration region. The acceleration region and the dilute phase region are simulated the same way as for the dilute-phase pneumatic transport profile. For the dense phase region a dilute core and a dense wall region are assumed. [...]

[...] Chapter 3 EXPERIMENTAL ASPECTS A circulating fluidized bed (CFB) has been employed to study the hydrodynamic behavior of gas-solid systems using sand and mustered seeds as bed material. The characteristics of particles used are given in table Table 3.1 Characteristics of Bed Materials Material Mean Particle Umf m/s Ut, m/s Particle Density, seeds 3.1 SET-UP The set-up consists of a compressor air lines provided with orifice meters a riser a cyclone separator with a bag filter a downer along with a solid measuring valve and a solid controlled valve fitted in solid transfer line. [...]

[...] Arena et al (1988) have compared the hydrodynamic behavior for two circulating fluidized beds with fast columns of 0.041 and 0.12 m ID by using fluid cracking catalyst and Glass Beads 70 and 90 m average size respectively as bed solids. Axial profiles of solids volume fraction ) are directly measured by means of quick closing valves and compound to those estimated from pressure drops. Bed blow out experiments are carried out by suddenly turning off solids control valve to find out instantaneous solid mass flux. [...]