Fluidized beds fine particles

The freeboard region in a fluidized bed accommodates particles that are being entrained from the dense bed. Entrainment refers to the ejection of particles from the dense bed into the freeboard by the fluidizing gas. Elutriation refers to the separation of fine particles from a mixture of particles, which occurs at all heights of the freeboard, and their ultimate removal from the freeboard. The terms entrainment and elutriation are sometimes used interchangeably. The carryover rate relates to the quantities of the particles leaving the... 

In gas-liquid-solid (three-phase) fluidized beds, solid particles are simultaneously contacted with both gas and liquid. The gas and liquid may flow cocurrently upward, or the liquid may descend, while the gas rises. The liquid usually forms the continuous phase in which the solid particles and gas bubbles are dispersed. The bubbles are larger when the particles are smaller, and bed contraction can occur when gas is introduced into a liquid-fluidized bed of fine particles. Higher pressures lead to smaller bubbles and increased gas hold-ups. 

Smooth fluidization In fine particle A beds, a limited increase in gas flow rate above minimum fluidization can result in smooth, progressive expansion of the bed. Bubbles do not appear as soon as the minimum fluidization state is reached. There is a narrow range of velocities in which uniform expansion occurs and no bubbles are observed. Such beds are called a particulate fluidized bed, a homogeneously fluidized bed, or a smoothly fluidized bed. However, this regime does not exist in beds of larger particles of 13 30 B and D, in these cases bubbles do appear as soon as minimum fluidization is reached. 

This is a well-established technology, and has been replaced today by the fluidization of fine particles (grains of sand, carbonization products, ash) using a controlled flow rate gas stream. As for entrained-bed reactors, they have not yet been employed to treat lignoceilulose wastes. 

Vibration combined with upward flow of air in an aerated bed enables particles to pseudofluidize smoothly. The gas velocity required for minimum fluidization is considerably lower than the minimum fluidization velocity in a conventional FBD. Attrition due to vigorous actions between particle-particle and particle-wall is thus minimized appreciably. Hence, application of fluidized bed can be extended to fragile, abrasive, and heat-sensitive materials. The problem of fine particle entrainment is also avoided. For polydisperse powders, low gas velocity fluidizes the fine particles gently, whereas vibration keeps the coarse particles in a mobile state. 

Studies have shown that certain impurities within the concentrates can lead to the formation of low melting point substances. This leads to particle agglomeration within the fluidized bed. Such particle enlargement in the bed, together with large quantities of very fine material leaving the freeboard, can lead to difficult roaster operation and ultimately to bed de-fluidization. 

Another mechanism for the transport of particles into the freeboard, which may be particularly significant for coarse particle fluidized beds with a wide particle size distribution, is the interstitial velocity in the dense bed. Fine particles at the top of the dense bed will be transported to the bed surface and it will be blown out into the freeboard (Bachovchin et al., 1981). This latter transport mechanism is strongly classifying, but possible fluxes are low compared to the fluxes induced by the bursting bubbles. 

The recycle cyclone, if this is used to reduce earbon losses, must be placed at a sufficient elevation above the relatively dense fluidized bed to provide a sufficiently long standleg to balance the circulation loop pressure profile. The bed elutriation will tend to result in recycled fine particles having low bulk density compared to the dense fluidized bed. Fine recycled particles must be injected into the combustion zone effectively to provide additional carbon conversion. 

In slurry reactors and in fluidized bed reactors, particles under a certain size may give rise to filtration problems. When there are severe heat transfer requirements, one may prefer a fine catalyst in a suspension that is well agitated a stirred slurry reactor, a bubble column or a fluidized bed. 

We consider here an example where two external forces are acting parallel to the direction of the bulk velocity. Mixtures of very fine particles to be separated are often in a dry state this is especially true in the processing of various minerals (e.g. coals). When two particles come into contact, electric charges develop via friction the charge that remains on the particles after "separation of solid-to-solid contacts is called triboelectrification (Inculet, 1984). A fluidized bed of particles is often a convenient method of achieving triboelectrification. One can also predict the polarity of the charge developed, but not necessarily its magnitude. 

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

Fluidized-bed catalytic reactors tend to generate loss of catalyst through attrition of the solid particles, causing fines to be generated. 

Particle Size. The soHds in a fluidized bed are never identical in size and foUow a particle size distribution. An average particle diameter, is generally used for design. It is necessary to give relatively more emphasis to the low end of the particle size distribution (fines), which is done by using the surface mean diameter, to calculate an average particle size ... 

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. 

The fluidized-bed system (Fig. 3) uses finely sized coal particles and the bed exhibits Hquid-like characteristics when a gas flows upward through the bed. Gas flowing through the coal produces turbulent lifting and separation of particles and the result is an expanded bed having greater coal surface area to promote the chemical reaction. These systems, however, have only a limited abiUty to handle caking coals (see Fluidization). 

Fluidized-bed reaction systems are not normally shut down for changing catalyst. Fresh catalyst is periodically added to manage catalyst activity and particle size distribution. The ALMA process includes faciUties for adding back both catalyst fines and fresh catalyst to the reactor. 

Fluid coking (Fig. 4) is a continuous process that uses the fluidized soflds technique to convert atmospheric and vacuum residua to more valuable products (12,13). The residuum is converted to coke and overhead products by being sprayed into a fluidized bed of hot, fine coke particles, which permits the coking reactions to be conducted at higher temperatures and shorter contact times than they can be in delayed coking. Moreover, these conditions result in decreased yields of coke greater quantities of more valuable Hquid product are recovered in the fluid coking process. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

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