Attrition measurement

Many standard test apparatuses have been proposed for comparative attrition tests [57, 58], but all such equipment has been suitable only for comparative studies of different catalysts under consideration for the same process. The attrition measured in large-scale equipment can be far different from the values measured in a test apparatus. 

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]. 

More severe tests are applicable to catalysts used in moving bed reactors. Some testsuse a high velocity stream of gas to cause attrition of the catalyst. In some tests, a small-scale plant which simulates the commercial plant is used. The amount of attrition measured in all these tests is very dependent on the exact procedure used. Reproducibility among laboratories is only likely to be possible if precise details of the method are given. 

Attrition Loss. The tendency of a support body to be reduced to powder is termed susceptibiHty to attrition, and the measurement of such susceptibiHty is termed attrition loss. Attrition can occur when support bodies mb against one another and abrade the surface, such as during calcination in a rotary kiln or sizing on moving screens. 

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. ... 

Garside elal. (1979) measured size distributions of seeondary nuelei and reported their variation with supersaturation. Signifieant inerease of nuelei with supersaturation is observed. Thus the proeess is not simply an attrition event alone, but is also related to the level supersaturation at whieh parent erystal is growing. Jones elal. (1986) also observed anomalous growth of seeondary nuelei in a study of the eontinuous MSMPR erystallization of potassium sulphate with eonsequenees inferred for seeondary nueleation rates. Girolami and Rousseau (1986) demonstrate the importanee of initial breeding meehanism in seeded potash alum bateh erystallization. The number of erystals... 

Air was supplied from a compressor, moisture and particles in the air are removed passing a trap, and air flow rate was controlled by mass flow controller (5850E, Brooks Co.). The dry sorbents after the attrition were collected, then the particle sizes of them were measured by... 

Gas velocity is an important operating condition in the fluidized bed process and it can highly affect the attrition of dry sorbents. Therefore, the weight remaining in the bed with fluidization time for gas velocity of 20.59 cm/s, 25.74 cm/s, and 30.89 cm/s was measured to estimate the attrition of dry sorbent with gas velocity. As shown in Fig. 4, attrition mainly occurred in the early stage of fluidization. The attrition rate with time decreased and the regression equations fit natural log functions. In addition, Fig. 4 shows that the attrition of dry sorbents is highly affected by gas velocity in the fluidized bed process. 

The particles to be removed may range in size from large molecules, measuring a few hundredths of a micrometre, to the coarse dusts arising from the attrition of catalysts or the fly ash from the combustion of pulverised fuels. 

The approach taken here is to employ standard materials characterization tests to measure the materials properties of the granulated product. With this information, the mechanism of attrition, i.e., breakage versus erosion, is determined. The rate of attrition can then be related, semi-empirically, to material properties of the formulation and the operating variables of the process, such as bed depth and fluidizing velocity. 

In order to evaluate the extent of attrition and its impact on the particle size distribution, there is a need of a qualitative and quantitative characterization. This, however, is not as simple as it may seem at first. There are many different properties, parameters and effects that manifest themselves and could be measured. In addition, as will be shown, the choice of the assessment procedure is strongly connected with the definition of attrition which, on its part, depends on the degradation mechanism that is considered to be relevant to the process. Hence there are a lot of procedures and indices to characterize the process of particle attrition. Section 3 deals with those which are relevant to fluidized beds and pneumatic conveying lines. 

In fluidized bed experiments, most authors assume that all attrition products are elutriated. Consequently, they measure either the decrease in bed mass and use Eq. (2) (e.g.,Kono, 1981 Kokkoris etal., 1991, 1995) or the elutriated mass (e.g., Seville et al., 1992, Werther and Xi, 1993). It should be noted that all these authors used a certain particle size as a threshold below which all particles are assigned to be attrition products provided that all initial particles are clearly larger. Breakage events, which lead to particle sizes above the threshold level are, therefore, not considered. The choice of this threshold is very arbitrary and differs between the various research groups. 

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. 

Data from attrition tests are usually presented as simple numbers called friability or attrition indices. Most of these indices are used as measures in quality control by subjecting the materials to a standard procedure. By comparing the test results with those of known materials, it is possible to give a relative characterization of the tested materials. Examples are given in Sec. 4.3. 

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. 

The discrepancies between the exponents found by different authors are not astonishing considering the different methods of measurement and evaluation. For example, Ghadiri et al. (1992a) included the initial breakage in their calculation of the attrition rate, whereas Werther and Xi (1993) measured the attrition rate under steady state conditions (cf. Fig. 2). 

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. 

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. 

Arena etal. (1983) and Pis etal. (1991) also found that Eq. (15) gave a good description of their experimental results. As an example, Fig. 11 shows the results of Pis etal. (1991), which were obtained in a fluidized bed column of 0.14 m in diameter. The distributor had 660 orifices of 1 mm in diameter. Unfortunately, no distinction was made between the measured attrition rate and the influence of the grid jets. However, their influence might be negligible in the present case due to the relatively smalljet velocity. 

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... 

Despite these potential problems, remarkably little work has been published on particle degradation in pneumatic conveying systems. That may be explained by experimental problems that are even much more serious than with fluidized beds. According to the different problems mentioned above, there are more individual measurement techniques and assessment procedures required than with fluidized bed attrition. Usually, the assessment is restricted to the comparison of the particle size distribution before and after conveying. Moreover, there is no steady-state attrition that could be measured. It is only possible to measure an integrated value, which... 

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