Structure and Texture of Catalyst Particles

Manufactured as a powder, the catalyst is formulated into particles, whose shape and size are determined by the end use. Common types, with a brief description of each, are given in Table 1.2, with more details to follow in Chapter 6. 

The first four types, pellets, extrudates. spheres, and granules, are primarily used in packed bed operations. Generally, the larger the particle diameter, the cheaper the catalyst. But this is usually not a significant factor for the process designer. More important are uniform fluid flow, pressure drop, and diffusional effects. 

Uniform fluid flow through the bed is desirable for good utilization of the catalyst and control of the process. To avoid channeling, the bed is packed as evenly as possible. A rule of thumb dictates that the reactor to particle diameter ratio should be from five to ten, with the reactor length at least SO-100 times the particle diameter. This ensures that the flow is turbulent, uniform, and approximates plug flow. For most commercial reactors these criteria are met. Only in the narrow tube reactors found in highly endo- or exothermic processes is there any concern. 

Pellets Made in high-pressure press Shape cylindrical, very uniform, rings Size 2-10 mm diam Use packed, tubular reactors 

Extrudalcs Squeezed through holes Shape irregular lengths circular, star or lobe cross section Use packed, tubular reactors, ebulating beds 

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