Attrition testing

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). 

Bemrose, C.R. and Bridgwater, J., 1987. A Review of Attrition and Attrition Test Methods. Powder Technology, 49, 97-126. 

Unfortunately, the basic physical mechanisms that control the attrition process are still poorly understood. As a consequence, particular test methods are used to evaluate the degradation tendency of the materials or to predict the rate of attrition for a given process. There are a lot of procedures using widely different devices and operations. Some of them observe the degradation of only one individual particle, whereas others treat a considerable amount of material. The particles are subjected to stress systems which range from well-defined ones like impact or compression, to those which are similar to the more or less randomized stresses occurring in natural processes. Section 4 attempts to summarize the huge variety of attrition tests in a systematic way. 

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. 

There is no attrition test method that is accepted as being superior to others. However, there are two highly standardized and widely used test... 

The large number of experiments that are termed attrition tests can be divided into two major fields of application, namely tests of material friability and experiments to study attrition phenomena. They will be separately discussed in the following subsections. The various test devices will be discussed afterwards. 

Friability tests can be used for various purposes. They are widely used in quality control. Here, samples of produced material are subjected to a more or less arbitrary but well defined stress. The attrition extent is assessed by comparison with a standard value and a decision is reached whether the material meets the standard. Moreover, friability tests are often used for comparison of different materials to select the most attrition-resistant one. This is a usual procedure in the case of catalyst development. For example, Contractor et al. (1989) tested anew developed fluidized bed VPO-catalyst in a submerged-jet attrition test (described below). Furthermore, the specific attrition rate of a material in a certain process can be roughly estimated by friability tests. In this case the stress must be similar to that occurring in the process and the obtained degradation extent must be compared with those of other materials from which the process attrition rate is known. 

The most commonly used philosophy is to design a test facility that simulates the relevant process stress. Examples of such test facilities are the various jet attrition test devices which are used in fluidization technology. 

The Grace-Davison jet-cup attrition test is often used to test the friability of catalysts (e g., Weeks and Dumbill, 1990 Dessalces et al., 1994). The respective jet-cup apparatus is sketched in Fig. 5. The catalyst sample is confined to a small cup, into which air is tangentially added at a high velocity (about 150 m/s). Some authors (e.g., Dessalces et al., 1994)... 

Figure 5. Schematic drawing of the Grace-Davison jet-cup attrition test. (After Weeks and Dumbill, 1990.)...

Bemrose, C. R. and Bridgwater, J. Powder Technology 49 (1987) 97. A review of attrition and attrition test methods. 

Many standard test apparatuses have been proposed for comparative attrition tests [57, 58], but all such equipment has been suitable only for comparative studies of different catalysts under consideration for the same process. The attrition measured in large-scale equipment can be far different from the values measured in a test apparatus. 

After subjecting a sample containing 20 wt% Ni+V to an attrition testp the fines generated by attrition testing (about S wt% of the sample), as well as the extrudates before and after attrition, were analyzed by a number of techniques to deternine the chemical changes responsible for the high attrition loss,... 

Cold flow attrition tests on the sorbent showed that sorbent attrition would be very high, leading to extremely high annual sorbent costs. 

ASTM fluidized bed and jet cup attrition tests can determine the attrition properties of pow-dered/pelleted catalysts. 

The catalyst is made by impregnating the beads with aqueous solutions of salts of some rare earth metals and of salts of the desired precious metals such as Pt, Pd and Rh these impregnated beads are then dried and calcined. The distribution of precious metals over the bead radius must be achieved with care, to balance the mass transfer requirements with the poison resistance requirements (Figs. 24-26). The distribution of the active component over the pellet radius can be measured by an Energy Dispersive X-ray (EDX) scan on an individual pellet. However, since in the application a relatively broad distribution in diameters occurs, special procedures have been developed to determine some kind of average distribution of the active components over the pellet radius. The most common procedure is the attrition test, in which a known mass of pellets of known diameter distribution is immersed in a liquid that neither dissolves the active components nor the carrier. The pellets are stirred for a defined time, and are separated from the attrited powder. The powder mass is determined, and its chemical composition analyzed by sensitive methods. 

Catalyst particles, if properly selected and installed according to specifications, should have sufficient strength to resist failure due to fracture. However, crushing and attrition tests are run on fresh catalysts. Changes during process operations result in gradual deterioration of mechanical properties, perhaps unevenly, through the bed. Consequences of this are... 

The shear cells as used in testing yield strength of solids may also be used for testing friability. As large strain is required in order to produce significant attrition, the annular shear cells (which permit infinite strain) are usually used. A variant on the annular shear cell for attrition testing is available commercially from Ajax Equipment in Manchester. 

The Rolling Drum Dust Generator described in a recent BMHB review54 is a variation on the same idea except that it uses a tumbler similar to those used in attrition tests for the dust generation and a cascade impactor to sample the dust cloud. 

The Zr/AbOa and La/AbOa systems were used for the preparation of washcoated supports. Special washcoating procedure has been developed. The properties of washcoated supports are given in Table 2. The attrition test procedure includes the measurement of weight loss of the sample by abrasive powder in the air flow. Testing conditions were the following ... 

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