Catalysts friability

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. 

Both devices described above were developed in order to test the friability of fluid-cracking catalysts. Nowadays the application of these or similar tests is a common procedure in the development of fluidized bed catalysts. Contractor et al. (1989), for example, used a submerged-jet test to compare the attrition resistance of newly developed VPO catalysts. In fact, such tests can be applied to any type of fluidized bed processes. Sometimes they have to be slightly modified to adapt them to the process under consideration. The drilled plate may, for example, be substituted by... 

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)... 

Cellular poly(carbodiimides) derived from polymeric isocyanate (PMDI) can be continuously produced using a phospholene oxide catalyst. As the component temperature is increased from 25 °C to 80 °C at a constant catalyst level, foam densities decrease with increasing component temperatures, with an expected corresponding decrease in compressive strength. The foam friability also decreases with increasing component temperature. 

Urethane-Modified Isocyanurate Foams. Urethane-modified isocyanurate foams are prepared by the trimerization of a polyisocyanate in the presence of a polyol, a trimerization catalyst, a blowing agent, and a surfactant. The foams have high flame and temperature resistance. The combined use of an isocyanurate foam and glass fiber not only improves the physical properties, e.g., flexural strength, friability, etc. but it also improves the flame resistance because the char formed from the foam acts as thermal barrier and protects it from flame and heat. This type of composite, therefore, is widely used for buUding applications in the U.S.A. Urethane-modified isocyanurate foam systems have also been used in the SRIM process (26, 36, 37). 

Friability of tablets or granules is tested quite commonly by vibration in a container or on a sieve. Much the same procedure has also been used in testing finer materials like catalysts, bone char or fertilizers54 but no standard exists. Some fertilizer manu-... 

This result suggests that, for this family of structures at least, the value of 1.6 mL g 1 provides sufficient weakness to allow total breakdown and full access to all of the catalyst surface. This inference is also supported by a comparison of results obtained with the best commercial silica gels and with a pyrogenic or "fumed silica (Cabosil) formed by flame hydrolysis of SiCl4. The latter has no pore structure, and no such structural limitations. That the two exhibit similar activities indicates that the silica gel had disintegrated to the level at which nearly all of the surface contributed to the polymerization. Furthermore, once friability of the solid is obtained because the pore volume is sufficiently high, activity can still be influenced by the surface area. However, these are only general trends, and some small exceptions are evident in the data in the table as well. It is the structure itself, rather than any porosity measurement, that determines friability. 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. The trimerization reaction proceeds best at 90-100°C. These temperatures can be achieved using a heated conveyor or a RIM machine. The key to the formation of PUIR foams is catalysis. Strong bases, such as potassium acetate, potassium 2-ethylhexoate, and tertiary amine combinations, are the most useful trimerization catalyst. A review on the trimerization of isocyanates is available (104). 

Friability tests are often used for a comparison of different types of materials to select the most attrition-resistant one (Vaux and Fellers, 1981 Davuluri and Knowlton, 1998). A field where friability tests are of particular importance is catalyst development (Dart, 1974). As an example, Contractor et al. (1989) used a submerged-jet attrition test (described below) in their development of a new generation of fluidized bed VPO-catalyst. 

There are, for example, various drop shatter tests in which the material falls under gravity onto a hard surface or a fixed bed. Such a test was carried out by Zenz and Kelleher (1980), who considered catalyst attrition due to free fall in a CFB downcomer. However, the probably most relevant impact tests are those where pneumatically accelerated particles are impacted onto a target. Yuregir et al. (1986, 1987) pioneered this type of test in their work on NaCl salt. In the meantime, such test devices have found broad industrial application as friability tests. For example. Fig. 8 shows the setup used by Davuluri and Knowlton (1998). It requires approximately 100 grams of material to conduct a test. The velocity at which the solids strike the impact plate was varied from 46 to approximately 144 m/s, but in the upper range a material-specific threshold velocity exists, above which the particles completely shatter. This is for most materials a velocity greater than 76 m/s. If velocities above the threshold... 

The Grace-Davison jet-cup attrition test is often used to test the friability of catalysts (Weeks and Dumbill, 1990 Dessalces et al., 1994). The jet-cup apparatus is sketched in Fig. 9. 

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