Attrition Correlation

There is no general correlation available to date to predict the steady state attrition rates for various materials. Zenz and Kelleher (1980) gave a simple correlation to predict steady-state attrition rates for FCC catalyst and glass beads. This is an empirical dimensional equation as given by  

The following table gives the attrition-rate constant (Ka) for FCC catalyst as a function of particle size range (0-d) for upwardly-directed jets. 

For glass beads, the values of Ka were found to be about V12 those for FCC catalyst. For other materials, one should obtain a relative attrition index with respect to either FCC or glass beads and then obtain a value of Ka based on that index. 

Karri (1990) reported that downwardly-directed jets have approximately twice the steady-state attrition rate as that of upwardly directed jets. The attrition rates for upwardly and laterally directed jets are essentially the same. 

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  

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Fluidization, Solids Handling, and Processing 4.1 Attrition Correlation... 

Pressure Drop. The prediction of pressure drop in fixed beds of adsorbent particles is important. When the pressure loss is too high, cosdy compression may be increased, adsorbent may be fluidized and subject to attrition, or the excessive force may cmsh the particles. As discussed previously, RPSA rehes on pressure drop for separation. Because of the cychc nature of adsorption processes, pressure drop must be calculated for each of the steps of the cycle. The most commonly used pressure drop equations for fixed beds of adsorbent are those of Ergun (143), Leva (144), and Brownell and co-workers (145). Each of these correlations uses a particle Reynolds number (Re = G///) and friction factor (f) to calculate the pressure drop (AP) per... 

Chandler [Bull. Br Coal Util. Re.s. A.s.soc., 29(10), 333 (11), 371 (1965)] finds no good correlation of grindabihty measured on 11 coals with roh crushing and attrition, and so these methods should be used with caution. The Bond grindability method is described in the subsection Capacity and Power Consumption. ... 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

Fluidized Bed Tests. These tests have direct relevance to all applications where particles are subjected to conditions of fluidization. Some authors believe that these tests can also to some extent simulate the stress of pneumatic transport. Coppingeretal. (1992) found at least a good correlation with the attrition resistance in dense-phase pneumatic conveying when they tested various powders in a slugging fluidized bed. 

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). 

Although the results from these methods may correlate to some extent, they simulate different essential requirements of the catalysts. Attrition loss primarily relates to handling, transport, loading and screening whereas the crush tests simulate the forces imposed on the catalyst in a fixed bed. The drop test simulates the risk of catalyst break-up during loading and pneumatic transport. 

Response Curves for two commercial and three experimental catalysts are shown in Figure 5. Inspection shows that the period of near linear response was from 45 through 75 minutes. This linear response appears to be associated with a period when the dynamics of the bed are in equilibrium. This behavior demonstrates that the maximum attrition time for this class of samples could be reduced to 75 minutes. The shape of the Response Curve and the rate the collector fines were generated (as represented by the slope of the near linear portion) have the potential to be correlated with the catalyst makeup rate in commercial applications. 

A NIR PAT application was developed to study the Rosenmund drying operation and to determine what factors affected the formation of fines. The NIR data were used to monitor the levels of residual solvent. In parallel with this, samples were obtained at different intervals for particle size analysis using Malvern. A correlation between the two data sets showed that particle attrition occurred primarily once the solvent had been removed down to a certain level (see Figure... 

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