Particle attrition

Good gas distribution is necessary for the bed to operate properly, and this requites that the pressure drop over the distributor be sufficient to prevent maldistribution arising from pressure fluctuations in the bed. Because gas issues from the distributor at a high velocity, care must also be taken to minimize particle attrition. Many distributor designs are used in fluidized beds. The most common ones are perforated plates, plates with caps, and pipe distributors. 

Particle Attrition. Distributor jets are a potential source of particle attrition. Particles are swept into the jet, accelerated to a high velocity, and smash into other particles as they leave. To reduce attrition at distributors, a shroud or larger-diameter pipe is often added concentric to the jet hole, as shown in Figure 15. The required length of the concentric shroud is given by the relation... 

A salient feature of the fluidized bed reactor is that it operates at nearly constant temperature and is, therefore, easy to control. Also, there is no opportunity for hot spots (a condition where a small increase in the wall temperature causes the temperature in a certain region of the reactor to increase rapidly, resulting in uncontrollable reactions) to develop as in the case of the fixed bed reactor. However, the fluidized bed is not as flexible as the fixed bed in adding or removing heat. The loss of catalyst due to carryover with the gas stream from the reactor and regenerator may cause problems. In this case, particle attrition reduces their size to such an extent where they are no longer fluidized, but instead flow with the gas stream. If this occurs, cyclone separators placed in the effluent lines from the reactor and the regenerator can recover the fine particles. These cyclones remove the majority of the entrained equilibrium size catalyst particles and smaller fines. The catalyst fines are attrition products caused by... 

The significance of this novel attempt lies in the inclusion of both the additional particle co-ordinate and in a mechanism of particle disruption by primary particle attrition in the population balance. This formulation permits prediction of secondary particle characteristics, e.g. specific surface area expressed as surface area per unit volume or mass of crystal solid (i.e. m /m or m /kg). It can also account for the formation of bimodal particle size distributions, as are observed in many precipitation processes, for which special forms of size-dependent aggregation kernels have been proposed previously. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

Designing a model fluidized bed which simulates the hydrodynamics of a commercial bed requires accounting for all of the mechanical forces in the system. In some instances, convective heat transfer can also be scaled but, at present, proper scaling relationships for chemical reactions or hydromechanical effects, such as particle attrition or the rate of tube erosion, have not been established. 

Particle attrition not due to thermal or chemical reaction effects (i.e., mechanical attrition) occurs much more rapidly in the grid region of fluidized beds than in the bulk of the bed. This is due to high-velocity gas... 

There can also be substantial particle attrition in cyclones in fluidized-bed systems because particles are accelerated at the inlet of the cyclone and impacted against the cyclone wall. Although there is little information on particle attrition in cyclones in the literature, it has been reported (Sishtla) that increasing system pressure decreases the attrition rate in cyclones operating with coal char. The mechanism by which this occurred was not determined. 

Shrouds are generally placed around grid holes to reduce the velocity at the gas-solids interface and reduce particle attrition. Shrouds simply consist of short pipes centered over the smaller grid holes which have been selected in size and number to operate at a hole velocity defined by Eq. (9). 

If properly sized and installed, particle attrition is reduced by a factor (Karri, 1990) calculated from ... 

If excessive particle attrition is expected, it is a common practice to place a shroud/nozzle around a grid hole as discussed in Sec. 3.5. For properly sized nozzles, one can derive from Eq. (17), particle attrition is reduced by a factor ... 

Example 2. For the conditions of Example 1 of perforated plate design, estimate the submerged jet height and particle-attrition rate in the fluidized bed. 

Particle attrition rate will be reduced by a factor calculated from Eq. (16)... 

In order to evaluate the extent of attrition and its impact on the particle size distribution, there is a need of a qualitative and quantitative characterization. This, however, is not as simple as it may seem at first. There are many different properties, parameters and effects that manifest themselves and could be measured. In addition, as will be shown, the choice of the assessment procedure is strongly connected with the definition of attrition which, on its part, depends on the degradation mechanism that is considered to be relevant to the process. Hence there are a lot of procedures and indices to characterize the process of particle attrition. Section 3 deals with those which are relevant to fluidized beds and pneumatic conveying lines. 

Grid Jets as a Source of Attrition. Jet attrition affects only a limited bed volume above the distributor, which is defined by the jet length. As soon as the jet is fully submerged its contribution to the particle attrition remains constant with further increasing bed height. Figure 6 shows some respective experimental results by Werther and Xi (1993). The jet penetration length can be estimated by various correlations, e.g., Zenz (1968), Merry (1975), Yates et al. (1986) or Blake et al. (1990). 

Internals. Commercial fluidized beds are often equipped with internals, eg., baffle plates and heat exchanger tubes. Due to their interaction with the rising bubbles they will certainly cause particle attrition. Unfortunately, no systematic investigations are available in the open literature which could serve as practical guidelines. 

Sometimes rubber bends have been successfully used to adsorb some of the impact energy in order to reduce particle attrition (Reed and Bradley, 1991.)... 

Figure 25. The effect of bend type on particle attrition (experiments by the PSRI)...

British Materials Handling Board, Particle Attrition, Trans Tech Publications Series on Bulk Mat. Handling, p. 5 (1987)... 

In a fluidized bed reactor, entrained particles leaving in a dilute phase stream are conventionally and desirably either partially or wholly condensed into a bulk stream and returned to the bed via a centrifugally driven cyclone system. At equilibrium, or when steady state operation is attained, any particle loss rate from the cyclones, as well as the remaining bed particle size distribution, are functions of (a) the rate of any particle attrition within the system and (b) the smallest particle size that the cyclone system was designed to completely collect (i.e., with 100% efficiency), or conversely the largest size which the system cannot recover. These two functions result in an interdependency between loss rate and bed particle size distribution, eventually leading to an equilibrium state (Zenz Smith, 1972 Zenz, 1981 Zenz Kelleher, 1980). 

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