Zeroth-order kinetics spherical catalysts

Catalysts with Spherical Symmetry. This analysis is based on the mass transfer equation with diffusion and chemical reaction in spherical catalysts. For zeroth-order kinetics, the molar density of reactant A is equated to zero at the critical value of the dimensionless radial coordinate, iciiticai = /(A). The relation between the critical value of the dimensionless radial coordinate and the intrapellet Damkohler number is obtained by solving the following nonlinear algebraic equation ... 

TABLE 16-2 Effect of the Intrapellet Damkohler Number on the Critical Dimensionless Radius for Radial Diffusion and Pseudo-Homogeneous Zeroth-Order Chemical Kinetics in Porous Catalysts with Spherical Symmetry... 

Problem. Consider zeroth-order chemical kinetics in pellets with rectangular, cylindrical and spherical symmetry. Dimensionless molar density profiles have been developed in Chapter 16 for each catalyst geometry. Calculate the effectiveness factor when the intrapellet Damkohler number is greater than its critical value by invoking mass transfer of reactant A into the pellet across the external surface. Compare your answers with those given by equations (20-50). 

Effectiveness factors for diffusion and zeroth-order chemical kinetics in spherical catalysts, described by equations (20-61) and (20-62), are illustrated in Figure 20-2 and compared with the results for diffusion and first-order irreversible chemical kinetics in the same catalyst geometry, given by... 

Obtain an analytical expression for the effectiveness factor (i.e., E vs. tjcriticai) in Spherical catalysts when the chemical kinetics are zeroth-order and the intrapeUet Damkohler number is greater than its critical value. Use the definition of the effectiveness factor that is based on mass transfer via diffusion across the external surface of the catalyst. 

For a particular experiment in a packed catalytic tubular reactor, the chemical kinetics can be approximated by a zeroth-order rate law where the best value for the zeroth-order rate constant is calculated via the formalism on pages 459 and 460. At what value of the intrapeUet Damkohler number Aa. intrapellet does reactant A occupy 75% by volume of the catalyst if the porous pellets are (a) spherical, (b) long cylinders, and (c) wafer-like ... 

The kinetics of the wear in a spherical aluminosilicate catalyst is shown in Figure 7.57a. This figure shows the relative percentage of mass removed as a function of time, m(f) = [Mq - M(t)]/Afo, where Mq is the initial material mass and M(t) is the residual mass after milling time, t. The three curves labeled 1,2, and 3 represent the abrasion process as a reaction of the zeroth, first, and second order, respectively. Curve 2, representing the exponential dependence m(f) = 1 - exp(-t/x), yields the best fit of the experimental data. The time constant, t, may serve as a measure of the catalyst s resistance to abrasion. 

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See also in sourсe #XX -- [ , ]

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