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Attrition in Fluidized Beds and Pneumatic Conveying Lines 475

all these models, however, the attrition mechanism is treated in a rather superficial way without making a distinction of the different mechanisms prevailing in the different parts of the system.

5.3Steps to Minimize Attrition in Fluidized Beds

In general there are two ways to minimize attrition. First of all the solid particles should be chosen, treated or produced in such a way that they are as attrition-resistant as possible. On the other hand, the fluidized bed system should be designed in such a way that the effects of the various attrition sources are kept as small as possible.

The Bed Material. Only catalytic processes are relevant with respect to modifying the attrition resistance of the bed material. In other processes, e.g., drying, the bed material is the product and cannot be changed. In the combustion of solid fuels, the particle degradation due to attrition enlarges the reacting surface and thus increases the reactivity of the fuel. On the other hand, the lack of attrition resistance is often a major obstacle that hinders the commercialization of fluidized bed catalytic processes.

Catalysts for fluidized bed reactors are preferably prepared by spraydrying to form readily fluidizable spherical particles. A commonly used approach for imparting attrition resistance is to add 30 to 50 wt% of colloidal silica to the catalyst precursor before spray drying. By this procedure the active catalyst is embedded in a continuous, hard skeleton. But the large amount of silica may affect both the catalyst’s activity and selectivity. Contractor et al. (1989) observed a significant selectivity loss for vanadium phosphate catalysts with the selective oxidation of butane to maleic anhydride. As a solution they suggested a new technology that encapsulates the active catalyst in a porous silica shell. The pore openings are large enough for the reacting species to diffuse into and out of the inner region of the particle without any effect on the selectivity. Patience and Mills (1994) reported that this new catalyst type can even be used in a circulating fluidized bed riser.

Kokkoris et al. (1991, 1995) suggested another method to impair catalyst attrition. In a small scale slugging bed they reduced the attrition rate of zeolites by addition of only very small quantities of various fine solid “lubricants.” In the case of graphite the reduction was up to 30%. The fines were assumed to reduce attrition by forming a protective coating that

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facilitates the sliding of the particles against each other. Similar effects were also observed by Wyszynski and Bridgwater (1993) with respect to degradation of granular salt in a powder blender. Even the attritionproduced fines can reduce attrition (Sec. 2). However, in order to apply these effects to industrial processes, it must be ensured that the fine particles have a sufficiently long residence time in the system. This might be possible, because Kato et al. (1994) and Nakagawa et al. (1995) found that fine particles remain much longer in fluidized beds than theoretically expected.

If the bed material is a mixture of different particle types, abrasion of one component can be reduced by the decrease of its surface in relation to the entire material surface in the bed (cf. Sec. 2). A potential field of application is the coal combustion process with the three components coal, ash and limestone.

Design Procedures. Gas Distributor. Perforated plates are often used as gas distributors. Although several investigations (e.g., Seville et al. (1992)) have shown the porous plate to produce the least attrition such distributors cannot be used in technical processes because of their tendency to clog. Bubble-cap distributors are more often used in commercial processes, but up to now there are only a few investigations with respect to their contribution to attrition. In comparison to drilled plates, Blinichev et al. (1968) found a smaller attrition rate of bubble-cap distributors, although Kutyavina et al. (1972) found the attrition to be significant higher.

With respect to grid jet attrition, it is known from Sec. 5.1 that attrition is inversely proportional to the square of the free cross section Ao, which can be achieved by either a lot of small orifices or a few larger ones. A certain distributor pressure drop is generally needed to guarantee uniform gas distribution. This distributor pressure drop immediately implies a certain value of the orifice velocity, uor. According to Eq. (8) the jet attrition rate is proportional to uor3 . A means to reduce the grid attrition is now to avoid direct submersion of the gas jet into the bed. This could be done either by the use of bubble caps or by retracting the jet below the distributor plate level as it is shown in the design suggested by Parker et al. (1976) and Parker and Gwyn (1977) (Fig. 20). The covering pipe surrounding the jet should be sufficiently long such that the gas jet has reached the tube wall and the average velocity in the jet has reduced with the ratio of orifice area to tube area. Caution should be exercised with this design since, if the jet is not completely filling the tube, then the bed will penetrate into the tube and the

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Bubbling Bed. The parameters affecting the fines production rate due to bubble-induced attrition are gas velocity and bed height. Both parameters are normally fixed by the process. It has been mentioned in Sec. 5.1 that internals in the bed might also act as a source of attrition. In this respect it should be mentioned that internals, e.g., tube bundles, should be located in a sufficiently large distance from the distributor to avoid the grid jets impinging on them. The same is valid if for some reason additional gas is injected into the bed region. High jet velocities should be avoided and caution should be exercised with respect to neighboring internals.

Cyclones. According to the model presented above, Eq. (24), a minimum loss rate due to cyclone attrition requires to avoid both high inlet velocities Ue and high solids mass fluxes mc,in at the cyclone inlet. The latter requirement can be fulfilled by locating the cyclone inlet above the transport disengaging height (TDH) (Kunii and Levenspiel, 1991). In addition, an enlargement of the freeboard section will reduce the amount of particles that

are entrained and thus the mass flux, mc,in.

For the special case of circulating fluidized bed systems, which are characterized by high solids loadings µ at the primary cyclone inlet, Molerus and Glückler (1996) have recently suggested a new cyclone separator with an inlet design, which is claimed to reduce the solids attrition. As is shown in Fig. 21, the gas-solid suspension is fed vertically downward with an angle of 30° to the main axis of the cone. The inlet device is shaped in such a way that the feed is accelerated without any direct collision with the cone wall. This cyclone design was tested in a circulating fluidized bed of foamed porous glass beads under rough conditions (gas velocity Ug = 8 m/s, solids loading μ = 6 kg/kg). The assessment of attrition was based on the mass fraction of particles smaller than 100 µm. The respective attrition rate in the new design was found to be significantly smaller than in a conventional cyclone.

6.0ATTRITION IN PNEUMATIC CONVEYING LINES

Pneumatic conveying systems and in particular dilute phase conveying systems are known to create a high stress on particulate solids leading to significant attrition. In contrast to fluidized beds, it is not the material loss which is the main problem. Depending on the application, problems may rather occur in a number of different areas. Attrition may, for example,

Attrition in Fluidized Beds and Pneumatic Conveying Lines 479

Figure 21. Attrition extent caused by a conventional cyclone and by the new cyclone design suggested by Molerus and Glückler (1996).

affect the product quality. In the case of pharmaceutical products, breakage and attrition of tablets must be avoided (Molerus et al., 1989). With respect to agricultural seeds, attrition may cause germination damage (Segler, 1951). A major consequence of attrition is the generation of dust, which may cause problems with handling of the particulate material after conveying and which furthermore requires costly cleaning systems for the conveying gas.

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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includes initial breakage in the acceleration zone and all breakage events that have occurred along the entire length of the pneumatic conveying system. Furthermore, the main parameter, i.e., the gas velocity, is changing with pipeline length due to expansion effects. Moreover, the design of pneumatic conveying lines is very individual and a comparison of results obtained in different conveying systems is almost impossible. Even the wall material plays an important role and has to be considered as a parameter.

6.1Modeling

Despite the little experimental data, there are two models available in the literature. Adams et al. (1992) considered dense phase conveying. They tried to predict the amount of attrition as a function of conveying distance by coupling a Monte Carlo simulation of the pneumatic conveying process with data from single-particle abrasion tests. Salman et al. (1992) focused on dilute phase conveying. They coupled a theoretical model that predicts the particle trajectory with single particle impact tests (cf. Mills, 1992).

6.2Parameter Effects

Segler (1951) has investigated the damage of grains and peas in pneumatic conveying lines in great detail. “Damage” was considered to be breakage of the grains, germination damage, or the removal of the husk. Most experiments were carried out with peas because they are much more sensitive to mechanical damage than the grains.

Segler (1951) found the incidence of grain damage to rise approximately with the cube of the air velocity (cf. Fig. 22). Furthermore, at constant air velocity the damage was found to increase with decreasing solids loading which may be explained by the “cushioning” effect mentioned in connection with attrition in cyclones in Sec. 5.1. Both effects have been confirmed by two other groups, namely by McKee et al. (1995), who studied particle attrition of sea salt in a large-scale dilute pneumatic conveying line (Fig. 23) and by Adams et al. (1992), who investigated the degradation of some unspecified granular material in a dense phase pneumatic conveying loop. Segler (1951) also investigated the effect of the pipe diameter and found a much smaller damage in a 270 mm bore pipeline than in a smaller pipeline with 46 mm diameter.

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Villareal and Klinzing (1994) suggested increasing the gas density in order to reduce the minimum conveying velocity by increasing either the gas pressure or the molecular weight of the gas.

If the conveying velocity is a critical parameter, then the designer of a pneumatic conveying system should also consider the expansion effect caused by the pressure drop along the conveying line. The resulting increase of the gas velocity can be accounted for by increasing the pipe diameter.

Pipeline Routing and Bend Design. Direction changes should be kept to an absolute minimum in order to keep the number of bends that are considered to be the major attrition sources as low as possible. This helps also to reduce the conveying line pressure drop and with it the gas expansion effect.

The bend type is known to have a significant effect on the particle degradation. For example, blinded-tee bends are used by many companies to prevent impact on the outer radial surface. Instead, the primary impact will, in this case, occur on the particulate product itself. This certainly leads to a reduction of bend wear, but the particle-particle impacts will probably cause higher attrition. This has been confirmed by experiments performed at PSRI (Fig. 25). To generate this plot, limestone with an average particle size of 940 microns was passed several times through a pneumatic conveying line consisting of several bends of the same type. All material less than 210 microns was assumed to be fines. The data were plotted as ( Wc−1 − Wci−1 ) vs. N0.5, where Wc is the weight percent of particles > 210 microns. The parameter Wci is the weight percent of particles in the initial material > 210 microns and N is the number of passes through a bend. The blinded-tee is seen to result in a significantly higher attrition rate than the elbow-bends. The short radius elbow is not unexpectedly seen to result in a higher extent of attrition than the long radius version. This latter finding is in agreement with results of Salman (1988), who found damage of 50% of seeds that were conveyed through a bend with r/D = 5. As this ratio was increased to 60 only 1.5% of the seeds were found to be damaged.

Sometimes rubber bends have been successfully used to adsorb some of the impact energy in order to reduce particle attrition (Reed and Bradley, 1991.)

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Figure 25. The effect of bend type on particle attrition (experiments by the PSRI)

(Knowlton, 1996.)

NOTATIONS