Fluidized beds coarse particles

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. 

Spouted beds are used for coarse particles that do not fluidize well. A single, high velocity gas jet is introduced under the center of a static particulate bed. This jet entrains and conveys a stream of particles up through the bed into the vessel freeboard where the jet expands, loses velocity, and allows the particles to be disentrained. The particles fall back into the bed and gradually move downward with the peripheral mass until reentrained. Particle-gas mixing is less uniform than in a fluid bed. 

When heavier refractories are required because of operating conditions, insulating brick is installed next to the shell and firebrick is installed to protect the insulating brick. Industrial experience in many fields of application has demonstrated that such a hning will success-billy withstand the abrasive conditions for many years without replacement. Most serious refractory wear occurs with coarse particles at high gas velocities and is usually most pronounced near the operating level of the fluidized bed. 

Classification The separation of fine particles from coarse can be effected by use of a fluidized bed (see Drying ). However, for economic reasons (i.e., initial cost, power requirements for compression of fluidizing gas, etc.), it is doubtful except in special cases if a fluidized-bed classifier would be built for this purpose alone. 

Fluid bed electrodes consist of a bed of particles supported by a structure such as a coarse sinter and fluidized by an upward stream of electrolyte and two different configurations have been described where the current path is parallel or perpendicular to the direction of fluidization (Backhurst et al., 1969). Such electrodes have been used for electrosynthetic reactions and, in particular, a pilot plant for the reduction of... 

Three different types of furnaces are generally in use for calcination. The shaft furnace is considered to be the most suited for calcining coarse limestone. Furnaces of the rotary kiln type are used for handling materials of mixed particle sizes and lumps which disintegrate during the process. Calcination can be carried out in a fluidized bed-reactor for materials of small and uniform particle size. These furnaces are usually fired with gas, oil or coke in some cases electric heating is resorted to. 

Typically it took about 160 to 200 seconds to inject a pulse of about 455 kg coarse tracer particles into the bed pneumatically from the coaxial solid feed tube. It can be clearly seen from Figs. 38 to 42 that the tracer particle concentration increases from essentially zero to a final equilibrium value, depending on the location of the sampling port. The steady state was usually reached within about 5 minutes. There is considerable scatter in the data in some cases. This is to be expected because the tracer concentration to be detected is small, on the order of 4%, and absolute uniformity of mixing inside a heterogeneous fluidized bed is difficult to obtain. 

Geldart, D., Behavior of Fine Particles in a Fluidized Bed of Coarse Solids, EPRI Report CS-2094, Electric Power Research Institute, Palo Alto, CA. (1981)... 

Kono, H., Attrition Rates of Relatively Coarse Solid Particles in Various Types of Fluidized Beds , AIChE Symp. Ser., 205(77) 96 (1981)... 

Wolny, A., and Opalinski, I., Electric charge neutralization by addition of fines to a fluidized bed composed of coarse dielectric particles, J. Electrostat., 14 279-289(1983)... 

Minimum bubbling velocity timb is defined as the gas velocity at which bubbles first appear in aggregative fluidization. For coarse uniformly-sized particles, for example those in Geldart group B, it is usually the case that M i, = u /- However, very fine non-uniformly sized particles such as those in group A exhibit smooth bed expansion and no bubbling until a gas velocity considerably in excess of the minimum... 

In these devices classification occurs in the space over a fluidized bed from which fine particles are carried away with the air stream. The air velocity must essentially exceed the maximal fluidization velocity of fine fraction. These classifiers operate on the gravitational counterflow separation principle with fluidized bed used to increase particles residence time in the separator. Coarse particles can move horizontally across the bed to the exit, which is another advantage of this device. 

In a later generation of fluidized bed classifiers the feed is poured onto the grid from above (Fig. 3c, d), so crushing is eliminated [9], Several prototypes of such fluidized bed classifiers with cylindrical or conical chambers of diameter 350-1700 mm and throughput of 0.5-10 t/h, were constructed for separation of sand, potassium chloride and ground phosphate particles with sizes from 50 to 1.5 mm [10], The recovery of coarse fraction was 75-92%. 

One disadvantage of the fluidized bed classifiers with cylindrical chambers of large diameters (above 1 m) is formation of dead zones within the fluidized bed far from the axis, where the particles do not move. These zones are eliminated inFB classifiers with chambers of oblong (oval) form (as in Fig. 3d) [11]. Such a device with 40t/h throughput and 160 pm cut size was built in the Karatau phosphate works milling plant. It yielded the fine product coarse fraction cleanness of 86% and separation recovery of 78% [10],... 

Group D comprises coarse particles (dp > 1 mm) which are commonly processed by spouting. When Group D particles are fluidized, the bed expansion is low and the particle mixing is not as good as that for Group A and B particles. 

Satija, S. and Fan, L.-S. (1985). Characteristics of the Slugging Regime and Transition to the Turbulent Regime for Fluidized Beds of Large Coarse Particles. AIChE J., 31, 1554. Stewart, P. S. B. (1968). Isolated Bubbles in Fluidized Beds Theory and Experiment. Trans. Instn. Chem. Engrs., 46, T60. 

Satija, S. and Fan, L.-S. (1985). Characteristics of Slugging Regime and Transition to Turbulent Regime for Fluidized Beds of Large Coarse Particles. AIChE J., 31,1554. 

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