Yang Fluidization, Solids Handling, and Processing

Attrition in Fluidized Beds and Pneumatic Conveying Lines 455

5.0ATTRITION IN FLUIDIZED BED SYSTEMS

Attrition in fluidized bed systems leads primarily to a loss of bed material since the cyclones, which are mostly used for the collection of entrained material, are not able to keep the attrition-produced debris inside the fluidized bed system. The material loss through the cyclone is, therefore, usually taken as the attrition rate. This means that among the attrition modes discussed in Sec. 2, namely fragmentation and abrasion, it is abrasion which is the attrition mode of interest for fluidized bed systems.

5.1Sources of Attrition

There are several sources that are contributing to the attrition rate of a given fluidized bed process. Gas distributors of fluidized beds are often designed as perforated or nozzle plates. Since a minimum pressure drop is required to obtain a uniform gas distribution over the bed’s cross-sectional area, the open surface area is rather small and the gas jets issuing from the distributor holes have a high velocity. Particles are entrained by these jets, accelerated to high velocities, and impacted onto the fluidized bed suspension at the end of the jets resulting in particle degradation similar to that in jet grinding processes. Above this distributor region, the motion of the bed particles is caused by the rising bubbles and their coalescence. This will result in frequent low velocity impacts between neighboring particles. Another stress situation will occur at the bed surface where bubble eruptions may lead again to high velocity impacts between the particles. Particle-wall impacts may be significant in small scale apparatus, but may probably be negligible on the industrial scale. On the other hand, impacts between bed particles and inserts like heat exchangers tube bundles or baffle plates will, in general, not be negligible. The contribution of the cyclones to the overall attrition rate in the fluidized bed system is naturally correlated to the solids loading of the gas at the cyclone inlet. Attrition will occur primarily in the inlet region where the particles impact on the wall or on other particles that are reflected and in the downward spiral path where the particles impinge onto the walls of the cyclone.

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.

456 Fluidization, Solids Handling, and Processing

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).

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).

30E-9

20E-9

R , kg/s a,j

10E-9

static bed height, m

Figure 6. Influence of the static bed height on jet attrition of spent FCC catalyst in a submerged jet test facility (Dt = 0.05 m, uor= 100 ms-1, dor = 2 mm. (Werther and Xi. 1993.)

Attrition in Fluidized Beds and Pneumatic Conveying Lines 457

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, Ao, number of orifices, Nor) and the operating parameters (i.e., gas density, ρg, volumetric flow rate, vg, superficial gas velocity, Ug, 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, Ra,distr , may thus be related to the jet attrition rate, Ra,j,

Both Ra,distr and Ra,j 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).

Modeling of Jet-Induced Attrition. Werther and Xi (1993) compared the jet attrition of catalysts particles under steady state conditions with a comminution process. They suggested a model which considers the efficiency of such a process by relating the surface energy created by comminution to the kinetic energy that has been spent to produce this surface area. The attrition rate, Ra,j, defined as the mass of attrited and elutriated fines per unit time produced by a single jet, is described by

where K is a parameter which considers properties of the solid material, and ρg is the density of the jet gas.

458 Fluidization, Solids Handling, and Processing

Ghadiri et al. (1992b, 1994, 1995) developed a more fundamental approach. They consider the particles entrained into the jet and relate the production of attrited fines to the attrition rates obtained from single particle impact tests (cf. Sec. 4.3). According to their model, it should be possible to predict jet attrition rates in fluidized beds on the basis of single particle impact tests combined with a detailed description of the jet hydrodynamics.

Experimental Techniques. Jet attrition cannot be investigated in isolation, because there is always some additional attrition of the bubbling bed. For that reason, many authors (e.g., Blinichev et al., 1968; Kutyavina et al., 1972; Arastoopour et al., 1983; Contractor et al., 1989) considered the overall attrition rate resulting from both attrition sources. In order to get direct insights into the mechanisms of jet attrition, it is necessary to separate the jet contribution from the measured overall attrition rate. This can be done in two different ways.

Seville et al. (1992) and Ghadiri et al. (1992a) measured the attrition rates at various static bed heights. As is shown in Fig. 7, they assume a linear increase of the attrition rate with bed height above the jet region. Extrapolating the measured attrition relationship to the jet length calculated

from one of the available correlations yields the attrition rate Ra,distr which is due to the jetting mechanism.

Werther and Xi (1993) used a Gwyn-type test facility with a particular distributor design where a separately fed nozzle was integrated into a porous plate (Fig. 8). At first the bed was only aerated via the porous plate. In this way the contribution of the bubbling bed attrition could be measured without any additional attrition sources. In a second step the facility was operated with a chosen jet gas velocity. In order to maintain the cut size of the gravity separator above the bed at some prescribed constant level they kept the superficial gas velocity by supplying auxiliary air through the porous plate. The resulting attrition rate measured is composed of the contributions of the jets and the bubbling bed. The jet attrition rate is now calculated by subtracting the bubble attrition rate, measured before, from the overall attrition rate. This is certainly an oversimplification since it assumes that the bubble attrition will always be the same regardless of the ratio of jet air mass flow to auxiliary mass flow. However, this method holds fairly well for low jet gas velocities, where the contribution of the jet gas flow to the total gas flow is rather small. For higher jet gas velocities the jet attrition is so large that the contribution of bubble attrition may be totally neglected.

Attrition in Fluidized Beds and Pneumatic Conveying Lines 461

The discrepancies between the exponents found by different authors are not astonishing considering the different methods of measurement and evaluation. For example, Ghadiri et al. (1992a) included the initial breakage in their calculation of the attrition rate, whereas Werther and Xi (1993) measured the attrition rate under steady state conditions (cf. Fig. 2).

Orifice Diameter. Werther and Xi (1993) found the attrition rate per jet to be proportional to the square of the orifice diameter, again in accordance with Eq. (8) (see Fig. 10). The same relationship was found by Zenz and Kelleher (1980) and Contractor et al. (1989) although these latter authors measured the overall attrition rate instead of the jet attrition rate.

Figure 10. Influence of the orifice diameter dor on the jet attrition rate of catalysts (uor = 100 m/s. (Werther and Xi, 1993.)

Distributor Design. If in the case of a given process application, an open surface area Ao is required to create a sufficient high pressure drop for a given superficial fluidizing velocity, then the question arises whether a few large orifices or a large number of small orifices will lead to a lower

distributor attrition rate, Ra,distr. It follows from Eq. (6) for a given volumetric gas flow

462 Fluidization, Solids Handling, and Processing

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

Superficial Gas Velocity. According to Eq. (6) there is a linear relationship between the superficial gas velocity Ug 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.

Bubble-Induced Attrition. Bubble-induced attrition originates from low-velocity interparticle collisions since the bubble rise velocity is of the order of 1 m/s. The energy is, therefore, generally not high enough to shatter particles into fragments. For that reason most laboratory experiments have shown the bubbles to be a minor source of attrition. However, in a deep bed with several meters of height the contribution of bubble-induced attrition may be a significant factor.

The extent of the attrition which is due to the presence of bubbles is conveniently described by the relative production rate of fines

Attrition in Fluidized Beds and Pneumatic Conveying Lines 463

where dWel,bub denotes the mass of fines which is produced by bubbleinduced attrition and which is measured as elutriated material. The

parameter Wbed is the instantaneous bed mass.

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.

Modeling of Bubble-Induced Attrition. Merrick and Highley (1974) have modeled bubble-induced attrition as a comminution process. According to Rittinger’s law of size reduction by abrasion (cf., Perry, 1973), the rate of creation of new surface area S/ t is proportional to the rate of

Since the size distribution of the fines produced by abrasion is approximately constant, the rate of production of new surface can be taken to be proportional to the mass rate of production of fines,

The total rate of input of energy to the fluidized bed is given by the product of the volumetric flow rate of gas (Ug·At) and the pressure drop, which may be expressed as weight of the bed divided by the bed’s cross sectional area, At. However, only part of the input energy is available for bubble formation and thus for comminution. The input energy

(Ug,mf · Wbed · g) is required for keeping the particles in suspension. That part of the rate of input of energy which is remaining for bubble formation

and thus for attrition is then given by [(Ug - Ug,mf ) · Wbed · g]. Insertion into Eq. (14) yields

464 Fluidization, Solids Handling, and Processing

where Kbub is an abrasion rate constant. With a similar approach Ray, et al. (1987a) arrived at the same result.

Vaux (1978), Ulerich et al. (1980) and Vaux and Schruben (1983) proposed a mechanical model of bubble-induced attrition based on the kinetic energy of particles agitated by the bubble motion. Since the bubble velocity increases with bed height due to bubble coalescence, the collision force between particles increases with bed height as well. The authors conclude that the rate of bubble-induced attrition, Rbub, is then proportional to the product of excess gas velocity and bed mass or bed height, respectively,

Experimental Results. Gas Velocity. Merrick and Highley (1974) used a fairly large pilot-scale fluidized bed combustion plant (bed area: 0.91 × 0.46 m2, total height: 3.9 m) in their investigation of coal and limestone attrition in the fluidized bed. The bed was operated at fluidizing velocities in the range of 2 to 8 ft/s and bed heights of 2 to 4 ft. Their results confirmed

the validity of Eq. (15), i.e., the bubble-induced attrition rate Ra,bub turned out to be proportional to (Ug - Ug,mf ) and was independent of the bed height.

Arena et al. (1983) and Pis et al. (1991) also found that Eq. (15) gave a good description of their experimental results. As an example, Fig. 11 shows the results of Pis et al. (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 small jet velocity.

Ray et al. (1987a) investigated the influence of the gas velocity in a 0.1 m diameter bed, which was equipped with a porous plate. Results of their investigation of the velocity effect on the attrition rate of narrow size fractions of limestone with particle sizes between 1.09 and 0.77 mm are plotted in Fig. 12. Similar results were obtained by Xi (1993), who investigated the attrition of fine catalyst particles with a minimum fluidiza-

tion velocity Ug,mf of 0.002 m/s (Fig. 13). As is obvious from Figs. (12) and (13), the attrition rate extrapolates to zero at a fluidizing velocity Ug,min which is significantly larger than Ug,mf. This means that a minimum kinetic

energy or a minimum extent of bubbling is necessary to cause attrition.