Single catalyst particle

Model based on the variation of the active catalyst perimeter. To form the (5,5)-(9,0) knee represented in Fig. 13(c) on a single catalyst particle, the catalyst should start producing the (5,5) nanotubule of Fig. 13(a), form the knee, and afterwards the (9,0) nanotubule of Fig. 13(b), or vice versa. It is possible to establish relationships between... 

These criteria are, of course, a valuable guide in assessing multiplicity and periodic activity for a single catalyst particle. For the sake of comparison a list of values of y, y(3, Lw and is reported for some typical exothermic reactions (cf. Table II). 

Since the critical values of y(3 and Lw are y/3 = 4 and, Lw > 1 respectively, then referring to the results reported in Table II, it seems highly unrealistic to expect multiple steady states and periodic activity for a single catalyst particle resulting from intraparticle heat and mass transfer alone. 

So far, there have been published only a few papers devoted to experimental investigation of multiplicity and oscillatory activity of a single catalyst particle. Observations of multiple steady states and/or oscillations for a single catalyst particle are reported in Table IV. Evidently three types of strong exothermic reactions have been investigated ... 

Experimental Observations of Multiply Steady States and Periodic Activity for a Single Catalyst Particle... 

Fig. 1. Hysteresis loop for H2 oxidation on a single catalyst particle in a packed bed of inactive pellets. Linear gas flow rate w = 1.7 cm/sec. 0 H2 percentage increasing, percentage decreasing (75). (Reprinted with permission from Advances in Chemistry Series. Copyright by the American Chemical Society.)...

Fig. 2. Temperature oscillations of a single catalyst particle. Inlet temperature...

The science of catalysis covers a large spectrum of phenomena. We observe—with some pride and joy—that this volume presents eight topics which, like the rainbow, form an almost systematic and complete sweep of the major classes of topics in catalysis. It spans from the most classical mechanistic study (P. W. Selwood), to a presentation of a hard practical application (M. Shelef et al). As we sweep across, we cover characterization studies of catalyst solids in terms of electronic (G. M. Schwab), surface chemical (H. A. Benesi and B. H. C. Winquist), as well as physicochemical and structural (F. E. Massoth) parameters, chemical reaction mechanisms and pathways (G. W. Keulks et al., and B. Gorewit and M. Tsutsui), and a topic on reactor behavior (V. Hlavacek and J, Votruba), which takes us from the single catalyst particle to the macroscopic total reactor operation. 

Bead-string reactors represent the limit of parallel-passage reactors They contain single-catalyst-particle subunits. Figure 10 gives a schematic representation (25). 

Equations (16) to (19) complete the formal theory of reaction rates in pores. In succeeding sections these formal equations are applied to various special cases. We can apply our theory to either one single isolated pore, or to a single catalyst particle which is a composite of many interconnecting pores. For clarity and simplicity we first consider a chemical reaction occurring in a single pore. 

Beusch, H., Fieguth, P., Wicke, E., 1972b. Unstable behavior of chemical reactions of single catalyst particles. Adv. Chem. Ser. 109, 615-621. 

We now consider the conditions under which it is valid to treat the macroscopic planar diffusion of the reactant S independently of the microscopic spherical diffusion to the microcatalyst particles. In Fig. 2.45, a single catalyst particle of radius R and its associated sphere of action of... 

Single catalyst particles k/ Elementary reactions on surfaces... 

Figure 4.5.24 Effectiveness factors >)overaii. 4po and )ex fora single catalyst particle.

Calculation of Coke Burn-Off within a Single Catalyst Particle Coke bum-off in a catalyst particle is a specific gas-solid reaction as the particle diameter remains constant and only a small part of the solid (the coke deposits with mass utcoke = fWcatfc)... 

For the determination of the strength of single catalyst particles, the relationship between a and external force F is still not clear. As an approximation, a can be considered proportional to F in the measurement of single-particle strength for a variety of catalysts in common use. That is. 

This paper describes a mathematical model for a single catalyst particle in which several chemical reactions take place. The model includes transport restrictions against mass and heat transfer in the interior and in the gas film surrounding the particle, and it accepts a general type reaction rate expression such as a power law expression or a Langmuir-Hinshelwood expression. The model is reduced to a number of coupled second order differential equations - one for each reaction - by use of the stoichiometric coefficients. 

In the following, it is shown how the mathematical model was formulated for the transport restrictions in a single catalyst particle where several chemical reactions take place. It was required that the model does not impose limitations with respect to the rate controlling transport mechanisms and to the actual form of the rate expressions. 

The rate expression for a chemical reaction is in general a function of the concentrations of reactants and products, temperature and pressure. The transport restrictions against mass and heat transport in a single catalyst particle cause a variation in these properties, and hence a variation in the reaction rate. The pressure variation in the catalyst particle is not taken into account, however, because practical experience has shown this effect to be negligible for reactions in ammonia, methanol and hydrogen plants. 

This demonstrates that successful scaleup must duplicate conditions on the microscopic (single catalyst particle) level which results in the same H(kt) upon scaleup, and on the macroscale (flow pattern) which results in the same E (t). Therefore, nonadsorbing tracers provide very useful information with respect to the macroscopic aspect of scaleup. The limiting case of the above... 

Yan Q. Wu J. Modeling of single catalyst particle in cathode of PEM fuel-cells. Energy Convers. Manage. 49 (2008), pp. 2425-2433. 

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