Cyclone attrition rate

Consequently, it is very difficult to evaluate the cyclone attrition rate from the measured elutriation rate. In order to study the cyclone attrition mechanism in detail it is necessary to study the cyclone in isolation. This can be achieved by feeding a cyclone batch-wise and directly without any additional equipment that could contribute to attrition. 

For the special case of an isolated cyclone, which is fed with a mass flux mc in of material sufficiently large to be sent into the catch of the cyclone, the measured mass flux in the loss of the cyclone, mc loss is solely due to attrition inside the cyclone. In such a case the cyclone attrition rate Ra c may be defined by... 

According to the model described above the cyclone attrition rates Ra c obtained under steady-state conditions have been plotted against the square of the cyclone inlet velocity in Fig 18. Although the number of experiments is not large the general relationship between Rac and Ue predicted by Eq. (19) is confirmed. 

Figure 18. Influence of inlet velocity Ue and solids loading ju on the steady-state cyclone attrition rate. (Reppenhagen and Werther, 1997.)...

As is seen from this latter figure, an increase of the solids loading results in a decrease in the cyclone attrition rate. This may be due to a cushioning effect of the increased solids concentration which is well known in comminution processes. This cushioning effect may be interpreted as a decrease in the efficiency T] of the abrasion process. If we assume r] to be a function of jJ and in its simplest form to follow an exponential function,... 

Hence it can be concluded that for each operating condition the fines concentration in the material tends to a characteristic value of which the accumulation of fines is balanced by the release of fines. When this characteristic concentration is reached, the loss rate is at steady state, i.e., it is equal to the production rate of fines. Reppenhagen and Werther (1999a) suggested that we take this steady-state value as a characteristic value for both the assessment of a material s attritabil-ity and the study of cyclone attrition mechanisms in dependence on the various influencing parameters. For this purpose, they defined the cyclone attrition rate as... 

Figure 19 Influence of the cyclone inlet velocity on the cyclone attrition rate at different solids loadings measured by Reppenhagen and Werther (1999a) in a 90 mm ID cyclone. Material spent FCC catalyst cyclone inlet velocity solids-to-gas loading ratio. 

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. 

It should be noted here that the above definition of the attrition rate considers the bed material as a whole. More insights into the influence of elements of the fluidized bed apparatus, e.g., of the cyclone or of the gas distributor may be obtained from the observation of the change in the particle size distribution as has been demonstrated by Zenz and Kelleher (1980) and Lin et al. (1980). 

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. 

According to Eq. (19) the cyclone attrition will be proportional to the square of the gas inlet velocity. The production rate of attrited fines, which is identical here to the mass flux in the cyclone loss, is then given by... 

Cyclones. According to the model presented above, Eq. (24), a minimum loss rate due to cyclone attrition requires to avoid both high inlet velocities Ue and high solids mass fluxes mc m at the cyclone inlet. The latter requirement can be fulfilled by locating the cyclone inlet above the transport disengaging height (TDH) (Kunii and Levenspiel, 1991). In addition, an enlargement of the freeboard section will reduce the amount of particles that are entrained and thus the mass flux, mc in. 

Zenz, F. A., and Kelleher, E. G., Studies of Attrition Rates in Fluid-Particle Systems via Free Fall, Grid Jets, and Cyclone Impact, J. of Powder Bulk Technol., 4 13 (1980)... 

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). 

In complete agreement with Eq. (20), the attrition rate for each material is proportional to However, the absolute value of the attrition rate depends on the properties of the individual material, which are summarized in the rate constant K. Moreover, the authors found that this constant depends not only on the type of material but also on the surface mean diameter of the solids that enter the cyclone. Hence they suggested that we subdivide the attrition rate constant into... 

Figure 23 shows a comparison of the experimental data depicted in Fig. 22 with the calculation from the model equation (27). The required attrition rate constants Cj, K, and Q that describe the materials susceptibility to attrit in the respective regions have been determined by the corresponding attrition tests as described in Sec. 4.3. Cj has been determined from exactly that Gwyn-type test facility that is shown in Fig. 14 and was set to zero in the case of the porous plate distributor has been measured in a 200 mm ID Gwyn-type test apparatus, and Q has been determined from exactly that cyclone attrition-test procedure that is described in Sec. 4.3.4 using the equipment sketched in Fig. 11. The parameters me,in and dpc were measured in the apparatus sketehed in Fig. 21 under the assumption that mc n may be...

Zenz FA, Kelleher GH. Studies of attrition rates in fluid-particle systems via free fall, grid jets, and cyclone impact. J Powder Bulk Tech 4 13-20, 1980. 

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). 

Case B. Suppose, more realistically, that the catalyst undergoes a known, experimentally determined, rate of attrition as a function of particle size (Zenz, 1971 Zenz Kelleher, 1980). The particle loss rate from the cyclone system will now approach and finally equal the rate of production of 0 to 10 micron particles by attrition from all the larger sizes. To maintain reactor inventory, this loss rate will be replaced, at an equal rate, with fresh catalyst. Since the rate of attrition of any size particle depends on its concentration in the stream subjected to the attrition (as finer particles effectively cushion the coarser), and since the loss is replaced with fresh catalyst (containing the coarsest), the bed size distribution will reach a steady state between 10 and 150 microns in which the mean size, as well as all sizes smaller than the largest, will now be decreased from what would have prevailed under conditions of zero attrition. 

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