ATTRITION IN FLUIDIZED BED SYSTEMS

Attrition in fluidized bed systems leads primarily to a loss of bed material since the cyclones, which are mostly used for the collection of entrained material, are not able to keep the attrition-produced debris inside the fluidized bed system. The material loss through the cyclone is, therefore, usually taken as the attrition rate. This means that among the attrition modes discussed in Sec. 2, namely fragmentation and abrasion, it is abrasion which is the attrition mode of interest for fluidized bed systems. 

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Newby RA, Vaux WG, Keaims DL. Particle attrition in fluidized-bed systems. In Kunii D, Toei R, eds. Fluidization IV. New York Engineering Foundation, 1983, pp 233-239. 

Reppenhagen, J. Catalyst attrition in fluidized bed systems. PhD dissertation. Technical University Hamburg-Harburg, Aachen Shaker, 1999. 

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. 

In this context Contractor et al. (1989) eonclude that the relative attrition rate depends on the attrition test method used. Knight and Bridgwater (1985) subjected spray-dried powders to a compression test, a shear test, and a test in a spiral classifier. They found that each test gave a different ranking of the materials. Werther and Reppenhagen (1999a) observed this phenomenon as they subjected various types of fluidized bed catalysts to both a cyclone attrition and a jet attrition test, each simulating one of the three major attrition sources in fluidized bed systems (cf. Sec. 5). 

Reppenhagen J, Werther J. The role of catalyst attrition in the adjustment of the steady-state particle sizes distribution in fluidized bed systems. In Kwauk M, Li J, Yang WC, eds. Fluidization X. New York Engineering Foundation, 2001, pp 69-76. 

Attrition in the Overall Fluidized Bed System, Continuous Processes... 

In general there are two ways to minimize attrition. First of all the solid particles should be chosen, treated or produced in such a way that they are as attrition-resistant as possible. On the other hand, the fluidized bed system should be designed in such a way that the effects of the various attrition sources are kept as small as possible. 

In the reclamation of chemically or resin bonded sands, the system employed must be able to break the bond between the resin and sand and remove the fines that are generated. The systems most commonly employed are wet washing and scrubbing for silicate bonded sands, or dry scrubbing/attrition and thermal (rotary drum or fluidized bed) systems for resin bonded sands. 

Since the particle population determines almost all relevant mechanisms in a fluidized bed system, attrition may thus strongly affect the performance of a fluidized bed process. For instance, the elutriation and entrainment effects (cf. Chapter 4 of this book), the heat transfer from bed to inserts (Molerus, 1992) or the conversion and selectivity of reactions (Werther, 1992) are affected either directly or through the bed hydrodynamics by the particle size distribution. Therefore some authors (such as Ray et al., 1987a) even claim that a fluidized bed with attritable materials... 

Considering the influence of the material properties, it must be acknowledged that attrition is a statistical effect, i.e., there are differences in the attrition susceptibility of the individual bed particles, and the attrition stress is acting randomly on them. As a consequence, there is usually not the influence of one particular property evaluated rather there is an evaluation of the materials statistical attritability as a whole. Section 4 summarizes the relevant attrition test procedures. The results of these attrition tests must then be transferred to the actual process by means of a physically sound description of the process conditions (Werther and Reppenhagen, 1999 Boerefijn et al., 2000). This, however, requires a distinction between different regions of the system that apply different types of stress to the solids. For this reason. Sec. 5 first summarizes the attrition mechanisms prevailing in the relevant sources of a fluidized bed system, and Sec. 6 then finally deals with a description of attrition in the entire process. 

However, besides the simple ranking there is quite often even a quantitative prediction of the proeess attrition requested. This requires both an attrition model with a precise description of the process stress and as an input parameter to the model precise information on the material s attritability under this specific type of stress. This calls for attrition/friability tests that duplicate the process stress entirely. As will be elucidated in Sec. 5, the stress in a given fluidized bed system will be generated from at least three sources, i.e., the grid jets, the bubbling bed, and the cyclones. For each there is a corresponding friability test procedure. 

Both devices described above were developed in order to test the friability of FCC catalysts. Nowadays the application of these or similar tests is a common procedure in the development of fluidized bed catalysts. Contractor et al. (1989), for example, used a submerged-jet test to compare the attrition resistance of newly developed VPO catalysts. In fact, such tests can be applied to any type of fluidized bed processes. Sometimes they have to be slightly modified to adapt them to the process under consideration. The drilled plate may, for example, be substituted by a porous plate if only attrition in the bed is of interest. Even temperature and pressure can be adapted. Vaux and Fellers (1981) investigated for example the friability of limestone sorbent that is used for fluidized bed combustion. By surrounding a Gwyn-type test facility with a heating system, they took thermal shock and reaction into account. 

A first approach to finding attrition in fluid beds was made by Zenz (1971). He pointed out that there are various regions in a fluidized bed reactor system in which the stress acting on the bed particles and the corresponding attrition mechanisms are quite different. In the subsequent works (Zenz, 1974 Vaux and... 

In conclusion from the sections above, two main distinctions must be made when attrition in an overall fluidized bed system is considered Primarily it must be recognized that there are several attrition sources in a fluidized bed system with distinctly different attrition mechanisms, which must be described separately. 

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