Catalytic pellets, transport-limited

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. 

Madon and Boudart propose a simple experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions [R. J. Madon and M. Boudart, Ind. Eng. Chem. Fundam., 21 (1982) 438]. The experiment involves making rate measurements on catalysts in which the concentration of active material has been purposely changed. In the absence of artifacts from transport limitations, the reaction rate is directly proportional to the concentration of active material. In other words, the intrinsic turnover frequency should be independent of the concentration of active material in a catalyst. One way of varying the concentration of active material in a catalyst pellet is to mix inert particles together with active catalyst particles and then pelletize the mixture. Of course, the diffusional characteristics of the inert particles must be the same as the catalyst particles, and the initial particles in the mixture must be much smaller than the final pellet size. If the diluted catalyst pellets contain 50 percent inert powder, then the observed reaction rate should be 50 percent of the rate observed over the undiluted pellets. An intriguing aspect of this experiment is that measurement of the number of active catalytic sites is not involved with this test. However, care should be exercised when the dilution method is used with catalysts having a bimodal pore size distribution. Internal diffusion in the micropores may be important for both the diluted and undiluted catalysts. 

Reactive depletion of CO from catalyst sites leads to much higher rates of secondary olefin hydrogenation reactions as pellets and reactors become limited by the rate of arrival of fresh reactants at catalytic sites. We have simulated intrapellet CO depletion experimentally by continuously decreasing the space velocity of mixtures with a H2/CO ratio (3 1) higher than the stoichiometric consumption value (—2.1 1). This reactant ratio was chosen because it corresponds to the relative rates of H2 and CO transport in stoichiometric mixtures through FT liquids at 473 K. As a result, the resulting axial gradients that occur in the catalyst bed as H2 and CO reactants are consumed resemble those that develop within transport-limited catalyst pellets. 

The initial increase in C5+ selectivity as x increases arises from diffusion-enhanced readsorption of a-olefins. At higher values of CO transport restrictions lead to a decrease in C5+ selectivity. Because CO diffuses much faster than C3+ a-olefins through liquid hydrocarbons, the onset of reactant transport limitations occurs at larger and more reactive pellets (higher Ro, 0m) than for a-olefin readsorption reactions. CO transport limitations lead to low local CO concentrations and to high H2/CO ratios at catalytic sites. These conditions favor an increase in the chain termination probability (jSr, /Sh) and in the rate of secondary hydrogenation of a-olefins (j8s) and lead to lighter and more paraffinic products. 

C. Changes in the Liquid Composition Within Transport-Limited Catalytic Pellets... 

Ensuring the Absence of Transport Limitations at the Catalyst Pellet Scale. Several criteria have been developed for verifying that concentration and temperature gradients, internal or external from the catalyst pellet, can be neglected. Section 2.2.1 deals with external transport limitations and, hence, with steps 1 and 7 from the catalytic cycle discussed above, whereas Section 2.2.2 focuses on steps 2 and 6, that is, on internal transport limitations. Apart from the calculation of these criteria, also a few experimental tests are at hand to verify the absence of transport limitations (see Section 2.2.3). It must be noted, however, that the result of such experimental tests may depend on the reaction order and not necessarily lead to a conclusive interpretation. As a result, it is recommended to double check the outcome of the experimental verification by calculating the corresponding criteria. 

It should be noted that the quality of the products discussed in this section is mainly defined by catalytic activity and catalyst pellet efficiency, the latter being a measure of mass transport limitations within the particle. Depending on the... 

However, the behavior of the catalysts measured in this work is different. At temperatures above 400 K the catalytic activity becomes limited, in agreem t with the Thiele theory. However, the apparent activation energy gradually decreases from 94 to 6 kJ/mol, rather than to 50 kJ/mol, which implies that the apparent activation energy of diffusion is exhibited. Nevertheless, the size of the wider pores in the pellet does appear to affect strongly the activity. Therefore, it is impossible that merely external diffusion limitation, that is, diffusion from the bulk of the gas flow to tiie external surface of the catalyst body, is rate-determining. Since the catalyst spheres had the same diameter, the activity of all catalysts should be equal if external transport is determining the activity. As the concentration of reactants inside the particle is nearly zero, the pore size should be of no importance. However, this is in contradiction with the measurements. 

The support has an internal pore structure (i.e., pore volume and pore size distribution) that facilitates transport of reactants (products) into (out of) the particle. Low pore volume and small pores limit the accessibility of the internal surface because of increased diffusion resistance. Diffusion of products outward also is decreased, and this may cause product degradation or catalyst fouling within the catalyst particle. As discussed in Sec. 7, the effectiveness factor Tj is the ratio of the actual reaction rate to the rate in the absence of any diffusion limitations. When the rate of reaction greatly exceeds the rate of diffusion, the effectiveness factor is low and the internal volume of the catalyst pellet is not utilized for catalysis. In such cases, expensive catalytic metals are best placed as a shell around the pellet. The rate of diffusion may be increased by optimizing the pore structure to provide larger pores (or macropores) that transport the reactants (products) into (out of) the pellet and smaller pores (micropores) that provide the internal surface area needed for effective catalyst dispersion. Micropores typically have volume-averaged diameters of 50 to... 

Figure 2. Stationary concentration (reactant) and temperature profiles inside and around a porous catalyst pellet during an exo thermic, heterogeneous catalytic fluid-solid reaction (a) without transport influence, (b) limited only by intraparticle diffusion, (0 limited by lntcrphase and intraparticle diffusion, (d) limited only by interphase diffusion (dense pellet)...

When testing catalytic properties, it is of utmost importance that other phenomena than those occurring at the catalyst s active sites do not become a limiting factor. Only then the observations can be directly related to the catalyst properties. Two types of other phenomena are likely to affect the observations, that is, the mass and heat transport phenomena at the catalyst pellet scale and the reactor flow pattern nonidealities at the reactor scale. 

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