Cyclones, attrition

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... 

Modeling of Cyclone Attrition. A very simple model of cyclone attrition may be formulated in analogy to the models discussed with respect to jet attrition and bubble-induced attrition (Reppenhagen and Werther, 1997). 

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... 

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 16. Experimental set-up for the isolated investigation of cyclone attrition (cyclone ID = 0.09 m). (By 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,... 

Figure 19. Influence of the solids loading p on cyclone attrition. (Reppenhagen andWerther, 1997.)...

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. 

Figure 7 Changes in the particle size distribution of a FCC catalyst material that was subjected to cyclone attrition. (Zenz and Kelleher, 1980.) The respective experimental procedure is described in Sec. 5.3.

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). 

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... 

For the reasons explained above there is only a limited amount of work published in the open literature on cyclone attrition. In fact there are results from only... 

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. 

From this model equation (27), it is obvious that the attrition effects in the overall system depend not only on the prevailing attrition mechanisms but also on the solids transport in the freeboard which determines the solids flow into the cyclone and thus the amount and the particle size distribution of the material that is subjected to cyclone attrition. Hence in a second step of the overall modeling, these transport effects must be taken into account, which has in a first approach been simply done by measuring the necessary data. 

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...

From a previous work (Reppenhagen and Werther, 1999a), the results of a long-term cyclone attrition experiment are available for comparison with the above-derived approach for the abrasion-induced shift in the PSD. In this experiment, a batch of catalyst has been fed 600 times to an isolated 90 mm ID cyclone operated at an inlet gas velocity of M(,m = 18m/s and a solids loading of P(,c = 0.3. Figure 28 shows the measured and the calculated PSD after the experiment in comparison to the initial one. 

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