Elutriation rate

Special attention has to be paid to a definition of attrition rates in the case of continuous processes where fresh solid material is continuously added. This is particularly the case in heterogeneously catalyzed fluidized bed processes where fresh make-up catalyst must be added to compensate for attrition losses. The fresh catalyst may contain elutriable fines which add to the measurable elutriation rate thus leading to an apparently higher attrition rate. 

These difficulties are avoided in Gwyn s (1969) design (Fig. 3). Here, the attrition products are not kept inside the system but it is rather assumed that they are elutriated. In the enlarged diameter top section, gravity separation defines the limiting diameter of the elutriable particles. The attrition rate is assumed to be given by the elutriation rate. The steady-state elutriation rate can, therefore, be used as a friability index. 

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. 

Stojkovski, V, and Kostic , Z. Empirical Correlation for Prediction of the Elutriation Rate Constant. Thermal Sci., 1 (2003) 43-58. 

The fraction mc e/mF lost by elutriation is determined by the elutriation rate constant K(R), the particle size distribution p(R) of carbon in the bed, and the carbon loading WQ (Eq. 10). A first approximation for this loss may be obtained by assuming that that particles of feed diameter d shrink to an elutriable size d, approximately 300 ym from Figure 4, and are then blown out of the bed. For this simple model... 

Here x, is the mass fraction of particle-size fraction i in the bed material and x( is the elutriation rate constant for this fraction... 

Elutriation—the selective removal of fines by entrainment—has been studied by several workers (K25, L6, M23, 08, T7, W2). Most of this work has been reviewed by Kunii and Levenspiel (K24) and Leva and Wen (in D5). However, Merrick and Highley (M13) point out that early correlations for elutriation rate constant are inaccurate for very fine particles, since they assume m, proportional. od% (rather than toc/p) and hence predict that the elutriation rate constant reduces to zero as the particle size reduces to zero. Thus, care must be taken when such empirical equations are applied to a fluid bed containing fine particles. 

Gas velocity Increases attrition and elutriation rates (major effect) Decreases coalescence for inertial growth Has no effect on coalescence for noninertial growth, unless altering bed moisture through drying Increases granule consolidation and density... 

Using Equation (CS12.19), calculate the outlet solids size distribution Estimate the elutriation rate from Equation (CS12.17). 

Figure 13.3-5 Generalized correlation of elutriation rate coefficients for two-component systems after Leva [4]). 

Roaster freeboard diameter, for a given gas volume flow, determines particle elutriation and this increases proportionally to the product gas volume. At Zincor, particle elutriation rates oftrai reached +90% of the feed. The design figure is 70% elutriation. 

Zincor is limited in roaster concentrate feed rate by the off-gas system capacity. Dust handling systems cannot handle the quantity of calcine dust if the dust elutriation rate exceeds 85% of the feed at 10% over design feedrates. 

The elutriation rate constant K cannot be predicted from first principles and so it is necessary to rely on the available correlations which differ significantly in their predictions. Correlations are usually in terms of the carryover rate above TDH, K. Two of the more reliable correlations are given below. 

First calculate the elutriation rate constants for the three size ranges under these conditions from the Zenz and Weil correlation [Equation (7.46)]. The value of particle size X used in the correlation is the arithmetic mean of each size range ... 

In most cases, experimental results are given either in terms of the total entrainment flux in kg/(m s) at the height h above the distributor or in terms of an elutriation rate constant K, defined as the ratio of the instantaneous rate of removal of solids of size dpi based on the cross-sectional area A to the fraction of the mass of the bed material with size dpi. ... 

For fine particles with a terminal velocity below the gas velocity in the freeboard, the elutriation rate constant K increases with decreasing particles size. While there is a general consensus about this when... 

Figure 4 Elutriation rate constant versus the particle size for fines and superfines for (a) limestone and (b) a-alumina. (Data from Smolders and Baeyens, 1997.)...

While there is an influence of the entrainment of flnes on the carryover of coarse material, no influence of the fines content on the elutriation rate constant of the fines can be found (Taneda et al., 1998). This is an assumption implicitly already made in Eq. (3). 

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