Sources of Attrition

Equipment for solids transport which is usually needed in fluidized bed processes will also add to the overall attrition rate. In screw feeders or rotary valves, the particles can be crushed or sheared between the vanes and the housing. The contribution of pneumatic conveying lines is discussed in Sec. 6. 

In circulating fluidized beds two main attrition sources, namely the riser and the return leg, may be distinguished. Although a lot of information is available about solids flow patterns and flow structures inside the circulating fluidized bed risers, no systematic investigations have been found in the open literature on the influence of riser geometry and flow conditions inside the riser on attrition. With respect to attrition occurring in the return leg, the work of Zenz and Kelleher (1980) on attrition due to free fall may be mentioned (cf. Sec. 4.3). 

In most commercial fluidized bed processes, the bed is much higher than the jet penetration length. There are several parameters that affect attrition in the jetting region, namely the design parameters of the distributor (i.e., orifice diameter, dor, open surface area, Aa, number of orifices, Nor) and the operating parameters (i.e., gas density, pg, volumetric flow rate, vg, superficial gas velocity, t/g, orifice velocity, uor). It holds 

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 

Both Rajdistr andR are introduced here as fines production rates with the dimension kg/s, which is in contrast to the definition of the overall attrition rate defined in Eqs. (2) and (3). 

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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). 

Optimizing the fresh catalyst physical properties including particle density, PSD, and attrition resistance is critical to maintaining acceptable fluidization and resulting circulation of the catalyst inventory. Excessive attrition of the catalyst will lead to nonuniform fluidization and disrupt circulation. Potential sources of attrition include ... 

The largest source of attrition and nucleation is the collisions between the suspended particles and the blades of the circulation pump. 

Particle Attrition. Distributor jets are a potential source of particle attrition. Particles are swept into the jet, accelerated to a high velocity, and smash into other particles as they leave. To reduce attrition at distributors, a shroud or larger-diameter pipe is often added concentric to the jet hole, as shown in Figure 15. The required length of the concentric shroud is given by the relation... 

Note List is not complete, due to the passage of time and the gradual attrition of memory and other sources of. historical data. 

Nozzle (atomizing) air can contribute significantly to product movement and can also be a source of a significant increase in product attrition. 

Another source of elutriated carbon are the fines produced by the attrition of the carbon surface by abrasion. This mechanism for attrition has been studied by Massimilla and coworkers (38-41),... 

The external validity refers to applicability and generalization and is outlined in the section, Applying the Results. The remainder of this section focuses on critically appraising the quality— that is, the internal validity—of individual trials. The internal validity is determined by how well the trial ensures that the known and unknown risk factors are equally distributed between the treatment and control groups. To ensure validity, the conduct of the trial should minimize systematic bias and random error as much as possible to provide results that are as accurate and close to the truth as possible. Four sources of bias are possible in trials of health care interventions selection bias, performance bias, attrition bias, and detection bias. Bias can result in an overestimation or underestimation of the effectiveness of a drug therapy and mislead the reader. While it is beyond... 

Centrifugal pumps. Probably the most common source of particle attrition problems is the centrifugal pump. The high shear forces inherent to these pumps, particularly in the eye of the impeller, make some crystal damage... 

Martian atmospheric Ne has proven difficult to measure in Martian meteorites, but Martian atmospheric He has proven impossible. However, it has been possible to make telescopic measurements of at least the He elemental abundance (Krasnopolsky et al. 1994). As in the case of the Earth, the abundance, and presumably the isotopic composition, of He in the Martian atmosphere is an equilibrium between gain and attrition. Like the Earth, the attrition is from loss from the top of the atmosphere. The situation may be similar to the Earth, where radiogenic " He and primordial He are degassing from the interior. Unlike Earth (Pepin and Porcelli 2002, this volume), cosmogenic He is probably the dominant source of the atmospheric inventory (Krasnopolsky et al. 1994). 

Attrition of rubber carpet backing contributed significantly to elevated concentrations of Cd in household dust sampled in an urban setting in Illinois, U.S. by Solomon and Hartford (1976), although this source of Cd was absent from British homes sampled by Harrison (1979). In some areas, carpet backing may be a source of Zn, in addition to other household sources of Zn such as rubber, paints, and fillers used in linoleum (Fergusson and Kim, 1991). 

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