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Physical Characteristics of Solid Catalysts

Although a few very active solid catalysts are used as fine wire mesh or other finely divided form, catalysts are mostly porous bodies whose total surface is measured in m /g. These and other data of some commercial catalysts are shown in Table 17.8. The physical characteristics of major importance are as follows. [Pg.564]

Specific surface. Solid spheres of 0.1 mm dia have a specific surface of 0.06 m /mL and an activated alumina one of about 600 m /iiiL. Other considerations aside, a large surface is desirable because the rate of reaction is proportional to the amount of accessible surface. Large specific surfaces are associated with pores of small diameters and are substantially all internal surface. [Pg.564]

Pore diameters and their distribution. Small pores himt accessibility of internal surface because of increased resistance to diffusion of reactants inwards. Diffusion of products outward also is slowed down and may result in degradation of those products. When the catalyst is expensive, the inaccessible internal surface is a liability. A more or less uniform pore diameter is desirable, but the distribution usually is statistical and only molecular sieves have nearly uniform pores. Those catalyst granules that are extrudates of compacted masses of smaller particles have bimodal pore size distribution, between the particles and within them. Clearly a compromise between large specific surface and its accessibility as measured by pore diameter is required in some situations. [Pg.564]

Effective diffusivity. Resistance to diffusion in a catalyst pore is due to collisions with other molecules and with the walls of the pore. The corresponding diffusivities are called bulk diffusivity and Knudsen diffusivity Many data and correlations of the former type exist the latter is calculable from the following formula (Satterfield, 1970, p. 42)  [Pg.564]

This equation applies to uniform cylindrical pores whose length equals the thickness of the catalyst through which the diffusion takes place. The actual diffusivity in common porous catalysts usually is intermediate between bulk and Knudsen. Moreover, it depends on the pore size distribution and on the true length of [Pg.564]


Kinds of Catalyzed Organic Reactions 563 Physical Characteristics of Solid Catalysts 564 Catalyst Effectiveness 565... [Pg.771]


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