Grid jet attrition

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. 

System temperature and pressure affect the momentum of grid jets via the gas density (see Ch. 2). The momentum of the gas jets is p AC/A. When the temperature is increased, the gas density decreases. For the same gas jet velocity this decreases the momentum of the jets and, therefore, decreases the jet penetration and the attrition at the grid. Similarly, when system pressure is increased, gas density increases, gas jet momentum increases and, therefore, the jet penetration and the attrition at the grid are increased. 

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). 

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. 

Zenz, F. A., and Kelleher, E. G., Studies of Attrition Rates in Fluid-Particle Systems via Free Fall, Grid Jets, and Cyclone Impact, J. of Powder Bulk Technol., 4 13 (1980)... 

However, besides the simple ranking there is quite often even a quantitative prediction of the proeess attrition requested. This requires both an attrition model with a precise description of the process stress and as an input parameter to the model precise information on the material s attritability under this specific type of stress. This calls for attrition/friability tests that duplicate the process stress entirely. As will be elucidated in Sec. 5, the stress in a given fluidized bed system will be generated from at least three sources, i.e., the grid jets, the bubbling bed, and the cyclones. For each there is a corresponding friability test procedure. 

As mentioned earlier, one can distinguish three pure and well-defined mechanical stresses on bulk solids material, namely compression, impact, and shear. There are numerous tests that are based on compression and shear, e.g., Paramanathan and Bridgwater (1983), Neil and Bridgwater et al. (1994), Shipway and Hutchings (1993), but they are not further discussed in this chapter because these stresses are usually not relevant to fluidized beds. On the other hand, impact stress occurs whenever particles hit walls or other particles. Attrition caused by impacts can thus be observed, e.g., in grid jets, in the wake of bubbles, in cyclones, or due to free fall. Consequently, there is a great variety of impact tests that try to simulate these particular stresses. 

Zenz FA, Kelleher GH. Studies of attrition rates in fluid-particle systems via free fall, grid jets, and cyclone impact. J Powder Bulk Tech 4 13-20, 1980. 

Because of the random motion of the solids, some abrasion of the surface occurs in the bed. However, this abrasion is very small relative to the particle breakup caused by the high-velocity jets at the distributor. Typically, particle abrasion (fragmentation) will amount to about 0.25 to 1 percent of the solids per day. In the area of high gas velocities at the distributor, greater rates of attrition will occur because of fracture of the particles by impact. As mentioned above, particle fracture of the grid is reduced by adding shrouds to the gas distributor. 

Each nozzle in fact is a tube with a restriction orifice at the bottom. The nozzle pressure drop is taken across the orifice but the nozzle tube is made long enough to contain the expanding gas jet. Since the jet now sweeps the whole cross-section of the tube, it becomes difficult for the catalyst to backflow into the header pipe where it would cause erosion problems. Air grid life is extended. Catalyst attrition is also minimized because, when the gas first contacts the bed of catalyst it is moving much slower than at the orifice (catalyst attrition varies with velocity cubed or worse). 

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