Jet attrition

Friability tests can be used for various purposes. They are widely used in quality control. Here, samples of produced material are subjected to a more or less arbitrary but well defined stress. The attrition extent is assessed by comparison with a standard value and a decision is reached whether the material meets the standard. Moreover, friability tests are often used for comparison of different materials to select the most attrition-resistant one. This is a usual procedure in the case of catalyst development. For example, Contractor et al. (1989) tested anew developed fluidized bed VPO-catalyst in a submerged-jet attrition test (described below). Furthermore, the specific attrition rate of a material in a certain process can be roughly estimated by friability tests. In this case the stress must be similar to that occurring in the process and the obtained degradation extent must be compared with those of other materials from which the process attrition rate is known. 

The most commonly used philosophy is to design a test facility that simulates the relevant process stress. Examples of such test facilities are the various jet attrition test devices which are used in fluidization technology. 

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). 

Figure 6. Influence of the static bed height on jet attrition of spent FCC catalyst in a submerged jet test facility (Dt = 0.05 m, uor= 100 ms 1, dor = 2 mm. (Werther andXi. 1993.)...

Assuming there is no interaction between the individual jets, the entire attrition in the jet region can be interpreted as the sum of the contributions of the individual jets. The overall attrition rate of the distributor, Radistr, may thus be related to the jet attrition rate, R... 

Modeling of Jet-Induced Attrition. Werther and Xi (1993) compared the jet attrition of catalysts particles under steady state conditions with a comminution process. They suggested a model which considers the efficiency of such a process by relating the surface energy created by comminution to the kinetic energy that has been spent to produce this surface area. The attrition rate, RaJ, defined as the mass of attrited and elutriated fines per unit time produced by a single jet, is described by... 

Ghadiri et al. (1992b, 1994, 1995) developed a more fundamental approach. They consider the particles entrained into the jet and relate the production of attrited fines to the attrition rates obtained from single particle impact tests (cf. Sec. 4.3). According to their model, it should be possible to predict jet attrition rates in fluidized beds on the basis of single particle impact tests combined with a detailed description of the jet hydrodynamics. 

Experimental Techniques. Jet attrition cannot be investigated in isolation, because there is always some additional attrition of the bubbling bed. For that reason, many authors (e.g., Blinichev et al., 1968 Kutyavina et al., 1972 Arastoopour et al., 1983 Contractor et al., 1989) considered the overall attrition rate resulting from both attrition sources. In order to get direct insights into the mechanisms of jet attrition, it is necessary to separate the jet contribution from the measured overall attrition rate. This can be done in two different ways. 

Figure 8. Distributor design suggested by Werther and Xi (1993) to separate the jet attrition and bubble-induced attrition.

Orifice Diameter. Werther and Xi (1993) found the attrition rate per jet to be proportional to the square of the orifice diameter, again in accordance with Eq. (8) (see Fig. 10). The same relationship was found by Zenz and Kelleher (1980) and Contractor et al. (1989) although these latter authors measured the overall attrition rate instead of the jet attrition rate. 

Figure 10. Influence of the orifice diameter on the jet attrition rate of catalysts...

Superficial Gas Velocity. According to Eq. (6) there is a linear relationship between the superficial gas velocity U and the jet velocity uor. With Eq. (8) it follows that the grid jet attrition rate will be proportional to the cube of the superficial gas velocity. 

Again, as in the case of jet attrition, attention must be paid in the experimental determination of Ra bub to the isolation of the attrition that is due to bubbles. There are basically two ways to do this. The one is to use a porous plate distributor in order to avoid any grid jets. The other is the procedure suggested by Ghadiri et al. (1992a) which is depicted in Fig. 7 the measurement of the production rate of fines at different values of the static bed height permits to eliminate the grid jet effects. 

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). 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

The jet attrition rate / aj defined as the mass of attr-ited fines per unit time produced by a single jet, is proportional to the jet gas density, the square of the orifice diameter, and to the cube of the jet exit velocity [64] ... 

The attrition rate constant K describes in the first place material properties which may be influenced by, for example, the manufacturing process of the catalyst. The attrition rate also depends on whether the jet issues into a prefluidized bed or into a nonaerated bed. The attrition effect of an upward jet equals that of a horizontal jet, whereas the attrition effect of a downward jet is significantly higher. For the prediction of the attrition effect of a multihole gas distributor, a model has been developed that is based on a single jet attrition measurement [62]. 

These findings may also explain the observed influence of the fines content on the attrition propensity of a given material Forsythe and Hertwig (1949) already noticed a reduction of the degradation of FCC catalysts in jet attrition tests due to the presence of fines. They themselves supposed some kind of cushioning effect that limits the force of collision impact and thus limits the degradation of the coarse particles. The effect of fine particles is of strong interest because they are produced by attrition, so attrition inhibits itself if the fine particles remain in the system. 

Friability tests are often used for a comparison of different types of materials to select the most attrition-resistant one (Vaux and Fellers, 1981 Davuluri and Knowlton, 1998). A field where friability tests are of particular importance is catalyst development (Dart, 1974). As an example, Contractor et al. (1989) used a submerged-jet attrition test (described below) in their development of a new generation of fluidized bed VPO-catalyst. 

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). 

Werther J, Xi W. Jet attrition of catalyst particles in gas fluidized beds. Powder Technol 76(l) 39-46, 1993. 

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