Product particle size distribution attrition

Attrition of particulate materials occurs wherever solids are handled and processed. In contrast to the term comminution, which describes the intentional particle degradation, the term attrition condenses all phenomena of unwanted particle degradation which may lead to a lot of different problems. The present chapter focuses on two particular process types where attrition is of special relevance, namely fluidized beds and pneumatic conveying lines. The problems caused by attrition can be divided into two broad categories. On the one hand, there is the generation of fines. In the case of fluidized bed catalytic reactors, this will lead to a loss of valuable catalyst material. Moreover, attrition may cause dust problems like explosion hazards or additional burden on the filtration systems. On the other hand, attrition causes changes in physical properties of the material such as particle size distribution or surface area. This can result in a reduction of product quality or in difficulties with operation of the plant. 

Elutriation is important in most industrial fluidized beds and is generally thought of as a disadvantage. In addition to the small particles which may be present in the initial particle size distribution, fines may be created in the course of operation by the attrition of bed particles. Elutriated particles usually need to be collected and recovered either because they represent the loss of product particles of a given size, because they must be separated from the exhaust gas for environmental reasons, or because of safety concerns there is a considerable risk of a dust explosion with very fine particles and perhaps especially so with many food particulates. Therefore the fluidized bed plant will require ancillary gas cleaning equipment such as a cyclone, filter or electrostatic precipitator to separate the fines from the gas. The loss of a particular size fraction from the bed may change fluidized bed behaviour and it then becomes important to return the fines to the bed continuously. 

Figures 3 and 4 illustrate the particle size distribution and shape of the commercial and experimental microspheres, respectively. In Figure 3, the 100X SEM micrographs demonstrate that a wide range of particle shape and size occur in the commercial materials. The variation in fines content, evident in these photographs, led us to develop an attrition method that was independent of the initial fines content. The experimental products in Figure 4 show a higher degree of sphericity and size uniformity than the commercial samples. Hence, the structure and quality of our microspheres did approximate known FCC catalysts. Also, no significant difference existed between alumina-containing and alumina-free particles. With these results, the first criterion for success was met.

The above definition of the attrition rate considers the bed material as a whole and quantifies solely the production of elutriable material without taking all breakage events or the shrinking of the so-called mother particles into account. More insights into the attrition mechanisms can be obtained from the observation of the change in the particle size distribution as demonstrated by Zenz and Kelleher (1980) and by Lin et al. (1980). An example of one of the results obtained by Zenz and Kelleher (1980) is shown in Fig. 7. 

Moreover, it should be noted that, when one compares different materials test results, the density of the particulate materials must be taken into account. If the gas mass flow and the temperatme are kept constant, then a variation in the solids density will result in a shift of the cut size and thus in the amount of material collected as attrition product. Another point concerns the particle size distribution of the bed material. If the original solid sample is prepared by sieving e.g. or sifting so that the smallest size is significantly larger than the cut size of the gravity separator (Werther and Xi, 1993), one can be fairly sure that the elutriated material is indeed due to attrition. On the other hand, if the cut size of the gravity separator is located... 

The investigation of the gas distributor and the bubbling bed attrition were both almost exclusively carried out in various derivations from the Gwyn-type test facility. Hence the material loss is the only attrition result considered, and the derived model approaches thus exclusively deal with the production of elutriable fines. There is consequently a lack of direct information on the role of attrition in the adjustment of the bed particle size distribution. 

Case B. Suppose, more realistically, that the catalyst undergoes a known, experimentally determined, rate of attrition as a function of particle size (Zenz, 1971 Zenz Kelleher, 1980). The particle loss rate from the cyclone system will now approach and finally equal the rate of production of 0 to 10 micron particles by attrition from all the larger sizes. To maintain reactor inventory, this loss rate will be replaced, at an equal rate, with fresh catalyst. Since the rate of attrition of any size particle depends on its concentration in the stream subjected to the attrition (as finer particles effectively cushion the coarser), and since the loss is replaced with fresh catalyst (containing the coarsest), the bed size distribution will reach a steady state between 10 and 150 microns in which the mean size, as well as all sizes smaller than the largest, will now be decreased from what would have prevailed under conditions of zero attrition. 

Important properties of zeolite adsorbents for a fixed-bed application are adsorptive capacity and selectivity, adsorption-desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal stability, chemical stability, and particle size and shape. Apparent bulk density of zeolite adsorbents is important because it is related to the adsorptive capacity per unit volume and also somewhat to rate of adsorption and desorption. However, more important properties related to the rates and therefore to the actual useful capacity would be the zeolite crystal size and the macropore size distribution. Although the ultimate basis in selecting a zeolite adsorbent for a specific application would be the performance, the price, and the projected service life of a product, these factors depend largely upon the above properties. 

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