Attrition, particles strength

A wide variety of catalytic materials are used as slurry-phase catalysts, most being metals supported on high surface area alumina, carbon, and silica (Fig. 2, label 3). Physical properties such as density are important since these catalysts must be suspended in the reaction mix. Since rapid agitation could lead to abrasion and attrition of the catalyst particles, strength is important. 

Neal (14), and Bertolacini (15). Similar standardization groups have since been formed in Japan, United Kingdom, Europe, and the Soviet Union. Twenty-five standards have been developed by the American Society for Testing Materials Committee D-32--Catalysts including tests for attrition, crush strength, particle size distribution, and vibrated apparent packing density (16). 

This is only a problem with moving or fluidized operations. Catalyst particle strength must match specific requirements. Attrition toss is usually determined by some standard test, described in Chapter 7. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

Strength against attrition is particularly important for catalysts in slurry-bed reactors, where physical breakage of the catalyst particles, ultimately to fines, can prevent their use for those reactions. The strength of the high surface area skeletal structures can be contrasted against activated carbon, which readily breaks down due to attrition in these types of environments. For the few environments where attrition is still a problem (e.g., oxidative dehydrogenation of alcohols), the skeletal catalytic material... 

New applications of zeolite adsorption developed recently for separation and purification processes are reviewed. Major commercial processes are discussed in areas of hydrocarbon separation, drying gases and liquids, separation and purification of industrial streams, pollution control, and nonregenerative applications. Special emphasis is placed on important commercial processes and potentially important applications. Important properties of zeolite adsorbents for these applications are adsorption capacity and selectivity, adsorption and desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal and chemical stabilityy and particle size and shape. Apparent bulk density is important because it is related to adsorptive capacity per unit volume and to the rate of adsorption-desorption. However, more important factors controlling the raJtes are crystal size and macropore size distribution. 

The various reduction systems can be classified into knife, hammer, and attrition units, each type producing a characteristic particle. A tramp metal detection system is included in all reduction steps to protect the equipment from serious damage. The shape and integrity of the component particles strongly influence the quality of the resultant particleboard therefore, the optimum in particle preparation is achieved when the desired particle is obtained with no damage to the structure of the wood. Wood failure within the particle will result in a particleboard of lower strength than one formed from intact particles. 

A with macropore diameters of 1000 to 5000 A. The pore volume and the pore size distribution within a porous support determine its surface area. The surface area of supports can range from 0.06 m2/mL (18,300 ftVft3) to 600 m2/mL (1.83 x 108 ffVft3) and above. Higher pore volume catalysts have higher diffusion rate at the expense of reduced crush strength and increased particle attrition. 

To eliminate intraparticle transport limitations, the particle size and average pore size must be carefully controlled during manufacture. Other physical properties that become important in industry have to do with the physical integrity of the catalyst particles. These properties include bulk density, crush strength, resistance to abrasion, and attrition. These properties are very important when working with reactors that contain a large amount of a particular catalyst. Fig. 2 lists a number of chemical and physical properties that affect catalyst performance. 

Powder or particle variables o particle size o size distribution o shape o surface texture o cohesivity o surface coating o particle interaction o wear or attrition characteristic o propensity to electrostatic charge o hardness o stiffness o strength o fracture toughness... 

Mechanical stresses experienced by catalyst particles during handling and use are considerable. The nature of these stresses is discussed in Chapter Six. Properties that relate to stress resistance are crushing strength, attrition loss, and loss on ignition. 

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... 

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. 

All of the various applications require special adsorbent characteristics. The broadest and generally the most significant are the inherent adsorption capacity and selectivity. In many cases, the adsorption and desorption rates or kinetics and pressure drop are also important hence, particle size is important. In addition, nearly every different application has a different set of additional priorities. For example, the main prerequisite for municipal water purification and many other large-scale applications is low cost. Other adverse conditions can complicate adsorbent specifications. For example, properties such as density, color, fluid compatibility, and durability (e.g., attrition resistance, crush strength, and hardness) all may be important. Adsorbents that have the ideal combination of essential characteristics for a specific application may or may not exist. That implies that compromises are frequently necessary. 

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