Pellet geometry and

The benefits of nonuniform activity distributions (site density) or diffusive properties (porosity, tortuosity) within pellets on the rate of catalytic reactions were first suggested theoretically by Kasaoka and Sakata (Ml). This proposal followed the pioneering experimental work of Maatman and Prater (142). Models of nonuniform catalyst pellets were later extended to more general pellet geometries and activity profiles (143), and applied to specific catalytic reactions, such as SO2 and naphthalene oxidation (144-146). Previous experimental and theoretical studies were recently discussed in an excellent review by Lee and Aris (147). Proposed applications in Fischer-Tropsch synthesis catalysis have also been recently reported (50-55,148), but the general concepts have been widely discussed and broadly applied in automotive exhaust and selective hydrogenation catalysis (142,147,149). 

The density, open porosity, redensification, pellet geometry and grain size of the sintered products are listened in Table 7.8. 

A word of caution must be interjected here at low gas velocities there may be an apparent dependence on the gas velocity because of reactant starvation. Starvation occurs when a sufficient portion of the reactant is consumed by the solid, so that in actual fact, the particle is not contacted with a gas of bulk composition Cao but the gaseous reactant concentration is less than this value. When operating in this region the effective driving force will depend on the gas velocity irrespective of whether the process is mass transfer controlled. In planning experiments care must be taken to avoid this starvation phenomenon whether this criterion is met can be checked by a simple mass balance. All the above considerations were essentially qualitative. The real quantitative verification that a process is mass transfer controlled is by the fact the the actual extent of reaction as a function of time is then predictable from Eq. (2.2.29), where the mass transfer coefficient h y is calculated from the appropriate correlation of the pellet geometry and Reynolds number. (A selection of such correlations was presented in Chapter 2.) Alternatively, mass transfer control may be proved conclusively, if the experimentally measured mass transfer coefficients are found to agree with those predicted on the basis of the appropriate mass transfer correlations. 

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See also in sourсe #XX -- [ ]

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