Attrition indexes

Data from attrition tests are usually presented as simple numbers called friability or attrition indices. Most of these indices are used as measures in quality control by subjecting the materials to a standard procedure. By comparing the test results with those of known materials, it is possible to give a relative characterization of the tested materials. Examples are given in Sec. 4.3. 

The assessment of the material s strength is commonly based on so-called friability tests or attrition tests (cf. Sec. 4), in which a single particle or a bulk sample of the material is for a certain extent subjected to a specific stress. Data from these tests are usually presented as single numbers called friability or attrition indices. Most of these numbers are derived from a comparison of the material s content of a previously defined fraction of fines before and after the test (British Materials Handling Board, 1987 Davuluri and Knowlton, 1998). 

Such indices may give a ranking of the material s friability but cannot be directly related to the process attrition, which would be the most appropriate assessment. This means, if at given process conditions a material A yields compared to a material B twice as high attrition, the ratio of the respective attrition indices of the materials A and B should be 2. 

Characteristics of attrition and adsorption were investigated to remove CO2 in fluidized hed using activated carhon, activated alumina, molecular sieve 5 A and molecular sieve 13X. For every dry sorbent, attrition mainly still occurs in the early stage of fluidization and attrition indexs(AI) of molecular sieve 5A and molecular sieve 13X were higher than those of activated carbon and activated alumina. Percentage loss of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but that of activated carbon and activated alumina were 8.3% and 8.1%, respectively. Overall attrition rate constant (Ka) of activated alumina and activated carbon were lower than other sorbents. 

For glass beads, the values of Ka were found to be about V12 those for FCC catalyst. For other materials, one should obtain a relative attrition index with respect to either FCC or glass beads and then obtain a value of Ka based on that index. 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

Apparatus. The Attrition Index Analyzer (AIA) is shown in Figure 1. The instrument, a modification of the Forsythe and Hertwig design (16). was developed at Alcoa Laboratories to improve and standardize, within Alcoa, the measurement of attrition resistance for calcined and hydrate aluminas. The instrument and test... 

The data in Figure 5 can be examined from three viewpoints. First, the curves of the Reference and CP-2 samples resemble that of a commercial material (Catalyst I) that has a low Attrition Index (5.3). Second, the CP-2 curve rises at a slower rate than the Reference. This indicates more attrition resistance in the CP-2 alumina formula. Finally, the pseudoboehmite formula produced fines much more rapidly than the CP-2 product. These findings suggest that the rehydratable alumina is a superior binder for some FCC formulations. 

The effect of catalyst slurry and sol age time on the Attrition Indexes of the CP-formulated samples is seen in Table III. There is a trend toward greater attrition as the batch holding time increases for both CP-2 products. Concomitantly, the viscosity range becomes higher, as was observed previously. CP-5 also appears to lose attrition resistance as the age and viscosity of the slurry advance. On the other hand, the holding time does not impact the CP-25 Attrition Index. The CP-2(4) material, made with a sol aged 4 hours, exhibits lower AI values, and, perhaps, a longer useful batch lifetime. 

Table IV gives the Attrition Indexes for the commercial and experimental samples. These values are reported for microspheres that were spray dried before the viscosity of the slurry exceeded 100 cP. The data show, again, that sol age and particle size of the CP alumina affect attrition. Sol age also seems to reduce the influence of the CP-2 as a binder. The Reference (4) has an AI of about 3 which is comparable to the index for CP-2(4). The important point here is that the alumina particles can be incorporated into a standard FCC formula to change catalytic activity without a detrimental effect on attrition resistance.

Figure 6. FCC attrition index vs particle size of CP alumina additive. ...

Attrition index (AI, wt%). A fresh catalyst sample is subjected to high-velocity fluidization for a long time. In this process, wear on the particles occurs as they are... 

There is clearly a need to investigate the mechanism of attrition to relate it to the fracture properties of the solids, and to develop a realistic attrition index , similar to that used for abrasion in cyclones. Such an index would indicate the relative importance of operating conditions and design variables such as inlet velocity, feed solids concentration or cyclone diameter. This could then be used in scale-up to predict (or minimize) the effect of the shape, the particle size distribution or the hardness and strength of the feed solids, if known, may allow such predictions without any experimental tests. Generally, better understanding of attrition and its relation to abrasion may lead to better equipment design and operation. 

The attrition index Ai (Equation 32) proposed by Barletta and Barbosa-Canovas (1993a) was found suitable for studying agglomeration and the effects of agglomerate size and water activity on the attrition kinetics of agglomerated coffee and nonfat milk (Yan and Barbosa-Canovas, 2001a). 

Barletta, B.J. and Barbosa-Canovas, G.V. 1993a. An attrition index to assess lines formation and particle size reduction in tapped agglomerated food powders. Powder Technol. 77, 89-93. 

Attrition Index = (Initial Weight - Remaining Weight) / Initial Weight / Time x 100%... 

For application in fluidization and fluid-particle systems, the attrition index is probably the most important particle characteristic. The particle attrition can affect the entrainment and elutriation from a fluidized bed and thus subsequently dictate the design of downstream equipment. The attrition in a pneumatic transport line can change the particle size distribution of the feed material into a fluidized bed reactor and thus alter the reaction kinetics. Davuluri and Knowlton (1998) have developed standardized procedures to evaluate the Attrition Index employing two techniques, solids impaction on a plate and the Davison jet cup. The two test units used are shown in Figs. 6 and 7. They found that these two test techniques are versatile enough to be applicable for a wide range of materials, such as plastic, alumina, and lime-... 

Equilibrium catalyst attrition index and average particle size distribution (APS) indicate changes in the rate of catalyst attrition. Further analysis of APS for any catalyst that is carried forward into the fractionator, present in the slurry, or which leaves the unit via the regenerator stack can identify problems associated with catalyst quality or cyclone operation. Problems include operation at greater than design feed, catalyst rates or cyclone maloperation. APS is also important in predicting the fluidization properties of the catalyst inventory. 

As well as the need for routine analyses of equilibrium catalyst, regular checks on all batches of fresh catalyst are carried out to check the consistency of particle size, the attrition index, and the activity of the catalyst added to a unit. Excess steam deactivates the catalyst and causes abnormal attrition. High air velocity or maldistribution of air in the regenerator increases catalyst attrition and leads to variations in the carbon content of the equilibrium catalyst. 

Much effort has been made by catalyst manufacturers to improve catalyst atttition resistance and thus reduce the formation of fines (see Catalysts, supported). In the 10-year petiod from 1980 to 1990, most catalyst manufacturers improved the atttition resistance of their catalyst by a factor of at least 3—4. This improvement was achieved even though the catalyst zeoHte content duting this petiod was continually increasing, a factor that makes achieving catalyst hardness more difficult. As an example of the type of atttition improvement that has been achieved, the catalyst atttition index, which is directiy related to catalyst loss rate in a laboratory attrition test, decreased from 1.0 to 0.35 for one constant catalyst grade during 1989—1990 (37). 

The use of selection and breakage functions is much more cumbersome than working with a single index number. This concept has, therefore, been used only forthe description of comminution processes but not for the description of attrition. 

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