Distributor attrition rate

Equation (11a) reveals that the decisive quantity for the distributor attrition rate is the open surface area A rr Obviously, with respect to attrition it is unimportant whether A0 originates from a few large or from many small orifices. 

It should be noted here that the above definition of the attrition rate considers the bed material as a whole. More insights into the influence of elements of the fluidized bed apparatus, e.g., of the cyclone or of the gas distributor may be obtained from the observation of the change in the particle size distribution as has been demonstrated by Zenz and Kelleher (1980) and Lin et al. (1980). 

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... 

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. 

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. 

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]. 

Figure 23 shows a comparison of the experimental data depicted in Fig. 22 with the calculation from the model equation (27). The required attrition rate constants Cj, K, and Q that describe the materials susceptibility to attrit in the respective regions have been determined by the corresponding attrition tests as described in Sec. 4.3. Cj has been determined from exactly that Gwyn-type test facility that is shown in Fig. 14 and was set to zero in the case of the porous plate distributor has been measured in a 200 mm ID Gwyn-type test apparatus, and Q has been determined from exactly that cyclone attrition-test procedure that is described in Sec. 4.3.4 using the equipment sketched in Fig. 11. The parameters me,in and dpc were measured in the apparatus sketehed in Fig. 21 under the assumption that mc n may be...

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... 

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. 

Attrition resistance Rate of attrition (Forsythe method) <0.1-0.5 wt. %/hr Avoidance of catalyst loss Maintenance of optimum size distribution Very flne particles produced by attrition (cf. Table IV) Reduction of gas velocity injected through distributor... 

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