Pores in catalysts

Note that this parameter has the same form as the Thiele number which occurs in the theory of diffusion/reac tion in catalyst pores. 

For the first assumption, the value of Kw for the shift appears to be too high. It must be this high because it is necessary to make C02 appear while both C02 and CO are being consumed rapidly by methanation. The data may be tested to see if the indicated rate appears unreasonable from the standpoint of mass transfer to the gross catalyst surface. Regardless of the rate of diffusion in catalyst pores or the surface reaction rate, it is unlikely that the reaction can proceed more rapidly than material can reach the gross pill surface unless the reaction is a homogeneous one that is catalyzed by free radicals strewn from the catalyst into the gas stream. 

One must understand the physical mechanisms by which mass transfer takes place in catalyst pores to comprehend the development of mathematical models that can be used in engineering design calculations to estimate what fraction of the catalyst surface is effective in promoting reaction. There are several factors that complicate efforts to analyze mass transfer within such systems. They include the facts that (1) the pore geometry is extremely complex, and not subject to realistic modeling in terms of a small number of parameters, and that (2) different molecular phenomena are responsible for the mass transfer. Consequently, it is often useful to characterize the mass transfer process in terms of an effective diffusivity, i.e., a transport coefficient that pertains to a porous material in which the calculations are based on total area (void plus solid) normal to the direction of transport. For example, in a spherical catalyst pellet, the appropriate area to use in characterizing diffusion in the radial direction is 47ir2. 

Equimolar counterdiffusion takes place in catalyst pores when a reaction with a stoichiometry of the form A - B occurs under steady-state conditions. 

A large number of analytical solutions of these equations appear in the literature. Mostly, however, they deal only with first order reactions. All others require solution by numerical or other approximate means. In this book, solutions of two examples are carried along analytically part way in P7.02.06 and P7.02.07. Section 7.4 considers flow through an external film, while Section 7.5 deals with diffusion and reaction in catalyst pores under steady state conditions. 

As progressively higher pressures are used during N2 adsorption, capillary condensation will occur in pores that are increasingly larger. The Kelvin equation illustrates that the equilibrium vapor pressure is lowered over a concave meniscus of liquid, which is why N2 is able to condense in catalyst pores at pressures lower than the saturahon pressure ... 

All molar fractions are considered locally in catalyst pores, the superscript s is omitted for brevity. If the DOC is operated temporarily also under fuel-rich conditions (e.g. during regeneration of the NSRC or DPF in a combined system) the reactions R6-R7, R8-R9 and R11-R14 in Table III (Section VI) should also be considered. 

All molar fractions are considered locally in catalyst pores, the superscript s is omitted for brevity. 

Hunger and Wang provide an account of advances in the characterization of solid catalysts in the functioning state by nuclear magnetic resonance spectroscopy. Examples include investigations of zeolite-catalyzed reactions with isotopic labels that allow characterization of transition states and reaction pathways as well as characterization of organic deposits, surface complexes, and reaction intermediates formed in catalyst pores. 

Wheeler, A. Reaction rates and selectivity in catalyst pores. Adv. Catal. 3, 249 (1951). 

Vladimir Haensel Chemical Characteristics and Structure of Cracking Catalysts A. G. Oblad, T. H. Milliken, Jr., and G. A. Mills Reaction Rates and Selectivity in Catalyst Pores Ahlborn Wheeler Nickel Sulfide Catalysts William J. Kirkpatrick... 

Thus far, however, the influence of diffusion in catalyst pores on the observed rates has been neglected. The method of incorporating this factor was indicated by Wheeler (57). For the exchange experiments, no corrections have to be made. For the equilibration experiments at relatively high temperatures, however, the data appear to be substantially modified. 

With this brief consideration of variations in catalyst pore structure, let us examine the pore structure of two catalysts used in this refractoriness study. One observes in Table V only slight differences between the two cobalt moly catalysts, T and R. They are typified by high surface area, small micropore mode diameters and low macropore volumes. 

Deposition of vanadium in catalyst pores is the only cause for permanent catalyst deactivation. 

The concentrations of hydrocarbons adsorbed in catalyst pores, especially of the heavier products, depend on their partial pressure in the catalyst bed. The partial pressure of the products at the same conversion and selectivity levels depends on the total pressure in the reactor. Therefore, Fischer-Tropsch synthesis at total pressure of 20 bar (tt CO=2) would result in a higher partial pressure of hydrocarbons. The higher partial pressure would lead to the condensation of reaction products which are normally in gaseous phase... 

While the above criteria are useful for diagnosing the effects of transport limitations on reaction rates of heterogeneous catalytic reactions, they require knowledge of many physical characteristics of the reacting system. Experimental properties like effective diffusivity in catalyst pores, heat and mass transfer coefficients at the fluid-particle interface, and the thermal conductivity of the catalyst are needed to utilize Equations (6.5.1) through (6.5.5). However, it is difficult to obtain accurate values of those critical parameters. For example, the diffusional characteristics of a catalyst may vary throughout a pellet because of the compression procedures used to form the final catalyst pellets. The accuracy of the heat transfer coefficient obtained from known correlations is also questionable because of the low flow rates and small particle sizes typically used in laboratory packed bed reactors. 

Ahlborn Wheeler, Reaction Rates and Selectivity in Catalyst Pores.250... 

The effect of mass transfer in catalyst pores was determined by conducting experiments with different mesh sizes of catalyst particles. Three particle sizes (Vs-inch pellets, —14 +20 mesh, and —20 +25 mesh crushed catalyst) were used. The rates observed were in good agreement, indicating that the pore diffusion is not rate controlling under the experimental conditions used in this investigation. 

Radiation catalysis, 13, 55 Raney nickel, uses of, 5, 417 Reaction kinetics, at liquid interfaces, 6, 1 Reaction mechanisms, elucidation of, by intermediates, 5, 311 Reaction rates, in catalyst pores, 3, 249 Reaction systems, complex, structure and analysis of, 13, 203... 

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