Effectiveness factor plot spherical catalyst

Effectiveness factor plot for spherical catalyst particles based on effective diffusivities (first-order reaction). 

Effectiveness factor plot for nth-order kinetics— spherical catalyst particles... 

This relation is plotted as curve Bin Figure 12.11. Smith (66) has shown that the same limiting forms for are observed using the concept of effective dififusivities and spherical catalyst pellets. Curve B indicates that, for fast reactions on catalyst surfaces where the poisoned sites are uniformly distributed over the pore surface, the apparent activity of the catalyst declines much less rapidly than for the case where catalyst effectiveness factors approach unity. Under these circumstances, the catalyst effectiveness factors are considerably less than unity, and the effects of the portion of the poison adsorbed near the closed end of the pore are not as apparent as in the earlier case for small hr. With poisoning, the Thiele modulus hp decreases, and the reaction merely penetrates deeper into the pore. 

Figure 12-5 (a) Effectiveness factor plot for nth-order kinetics spherical catalyst particles (from Mass Transfer in Heterogeneous Catalysis, hy C. N. Satterfield, 1970 reprint edition Robert E. Krieger Publishing Co., 1981 reprinted by permission of the author), (b) First-order reaction in different pellet geometries (from R. Aris, Introduction to the Analysis of Chemical Reactors, 1965, p. 131 reprinted by permission of Prentice-Hall, Englewood Cliffs, NJ)... 

Figure 12.7 Effectiveness factor plots for sphere and slab geometries for first- and zero-order kinetics. The Thiele modulus is given by = RyJfor spherical catalysts or by...

In Figure 7.4 the effectiveness factor is plotted against the Thiele modulus for spherical catalyst particles. For low values of 0, Ef is almost equal to unity, with reactant transfer within the catalyst particles having little effect on the apparent reaction rate. On the other hand, Ef decreases in inverse proportion to 0 for higher values of 0, with reactant diffusion rates limiting the apparent reaction rate. Thus, decreases with increasing reaction rates and the radius of catalyst spheres, and with decreasing effective diffusion coefficients of reactants within the catalyst spheres. 

A plot of the effectiveness factor as a function of the Thiele modulus is shown in Figure 12-5. Figure l2-3(a) shows t) as a function of the Thiele modulus < )j for a spherical catalyst pellet for reactions of zero, first, and second order. Figure 12-5(b) corresponds to a first-order reaction occurring in three differently shaped pellets of volume Vp and external surface area Ap, and the Thiele modulus for a first-order reaction, < >], is defined difierently for each shape. When volume change accompanies a reaction (i.e., 0) the corrections shown in Figure 12-6 apply to the effectiveness factor for a first-order reaction. 

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