Catalyst sizes

Sphere> pellet > trilobe > hollow extrudate > wagon wheel/ minilith The definitive catalyst size selection will be a compromise between high reaction rate (small partiele, exotic shape), low pressure drop (large particle, exotic shape), large crushing strength... 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

Hydrocarbon Catalyst size Temp. Conversion Ratio... 

Pore diffusion can be increased by choosing a catalyst with the proper geometry, in particular the pellet size and pore structure. Catalyst size is obvious (r if pore diffusion limited for the same total surface area). The diameter of pores can have a marked influence on r) because the diffusion coefficient of the reactant Da witl be a function of dp if molecular flow in the pore dominates. Porous catalysts are frequently designed to have different distributions of pore diameters, sometimes with macropores to promote diffusion into the core of the catalyst and micropores to provide a high total area. 

This result means that the number of active sites per unit mass of catalyst is not constant since it depends on the particle size. Again, the term nj can be considered to be a constant property for a given catalyst, prepared by the same technique, for all catalyst sizes. Then... 

As discussed for TWC models (Section II), a DOC model can be used for catalyst sizing and system design. Figure 26 shows a validation plot comparing model prediction with measured data over the ESC (European Stationary Cycle) excellent agreement is observed. Good agreement has also been obtained with this model over other test cycles (York et al., 2005). 

Fig. 36. Predicted effect of catalyst size and engine out NOx on tailpipe NOx emissions.

In a second and possibly alternative stage of the kinetic investigation, laboratory experiments are performed over the same catalyst as for the microreactor tests, but now in the form of small monolith samples with volumes of few cubic centimeter. Flow rates, as well as catalyst size, are thus typically increased about by a factor of 100 with respect to the microreactor kinetic runs. This experimental scale provides data either for intermediate validation of the intrinsic kinetics from stage one, or directly for kinetic parameter estimation if runs over catalyst powders are omitted. 

The pure compound rate constants were measured with 20-28 mesh catalyst particles and reflect intrinsic rates (—i.e., rates free from diffusion effects). Estimated pore diffusion thresholds are shown for 1/8-inch and 1/16-inch catalyst sizes. These curves show the approximate reaction rate constants above which pore diffusion effects may be observed for these two catalyst sizes. These thresholds were calculated using pore diffusion theory for first-order reactions (18). Effective diffusivities were estimated using the Wilke-Chang correlation (19) and applying a tortuosity of 4.0. The pure compound data were obtained by G. E. Langlois and co-workers in our laboratories. Product yields and suggested reaction mechanisms for hydrocracking many of these compounds have been published elsewhere (20-25). 

Figure 11. Effect of catalyst size and effectiveness factor in different reaction systems...

Studies undertaken with petroleum feedstocks to elucidate an understanding of hydrodemetallation reactions have yielded ambiguous and in some cases conflicting results. Comparison of kinetic phenomena from one study to the next is often complicated. Formulation of a generalized kinetic and mechanistic theory of residuum demetallation requires consideration of competitive rate processes which may be unique to a particular feedstock. Catalyst activity is affected by catalyst size, shape, and pore size distribution and intrinsic activity of the catalytic metals. Feedstock reactivity reflects the composition of the crude source and the molecular size distribution of the metal-bearing species. 

Fig. 36. Calculated effect of catalyst size and average pore diameter on Boscan demetallation kinetics (Bridge and Green, 1979).

Tamm et al. (1981) have suggested that metals are primarily responsible for the initial deactivation. They report that the length of the initial deactivation period is directly related to the concentration of metals in the feedstock and the total run length or catalyst lifetime. Tamm et al. (1981) also observed that the initial deactivation period responds to process variables that influence the metal deposition pattern in catalysts. Lower hydrogen pressure and smaller catalyst size result in a greater loss of initial... 

Octane additives were composed of 25 wt % zeolite in a silica-alumina binder, spray-dried to conventional FCC catalyst size, and treated at 1450°F in 100% steam at 1 atm for five hours before testing. 

Fig. 10. Influence of WS2 catalyst size on aniline points of prehydrogenation product fractions.

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