Catalysts aging effects

Catalyst Aging Effects. The yields for most of the catalysts remain relatively stable with catalyst aging. In constant conversion runs such as these, the catalyst temperature is increased to maintain conversion and yield stability can be shown by a plot of yield versus average catalyst temperature. Figure 6 shows results for two amorphous catalysts. 

The catalyst aging effects shown by this fauja-site containing catalyst appear to be a general phenomenon similar effects have been observed in our laboratories with other feedstocks and other zeolite-containing catalysts. Other examples have been reported in the patent literature (2 ). 

Al Ti in the range of 0.9—1.0 appeared optimum for i7j -l,4-polyisoprene yield (20). Other factors such as catalyst preparation temperature, influence of the R group in the alkyl aluminum compound (R Al), and catalyst aging have been extensively studied (16,17). Another variable studied was the effect of... 

In other instances, reaction kinetic data provide an insight into the rate-controlling steps but not the reaction mechanism see, for example, Hougen and Watson s analysis of the kinetics of the hydrogenation of mixed isooctenes (16). Analysis of kinetic data can, however, yield a convenient analytical insight into the relative catalyst activities, and the effects of such factors as catalyst age, temperature, and feed-gas impurities on the catalyst. 

Carbon supported powdered palladium catalysts have been widely used in the chemical industry. In addition to activity and selectivity of those catalysts, the recovery rate of the incorporated precious metal has a major impact on the economic performance of the catalyst. In this study, the effects of catalyst age, oxidation state of the incorporated metal and temperature treatment on the palladium leaching resistance as well as on activity and dispersion of carbon supported palladium catalysts were investigated. 

Similar trends were found for the palladium leaching values when leaching fresh catalysts. However, the overall amount of palladium leached is higher in this case. The results of the leaching tests performed one day after the catalyst preparation show values between 140 and 240 ppm for the reduced and of 30 to 60 ppm for the non-reduced catalysts. A comparison of the decrease of metal leaching over time shows different aging effects for the dry catalysts compared to the wet catalysts of... 

The model includes fundamental hydrocarbon conversion kinetics developed on fresh catalysts (referred to as start-of-cycle kinetics) and also the fundamental relationships that modify the fresh-catalyst kinetics to account for the complex effects of catalyst aging (deactivation kinetics). The successful development of this model was accomplished by reducing the problem complexity. The key was to properly define lumped chemical species and a minimum number of chemical reaction pathways between these lumps. A thorough understanding of the chemistry, thermodynamics, and catalyst... 

The techniques of monomolecular rate theory easily transform measured reaction data into a form where we can analyze apparent kinetics and the effects of intracrystalline diffusion by the use of selectivity data. Time dependency has been eliminated. Since selectivity is extremely reproducible and is independent of short-term aging effects, the number of experimental runs is reduced while data reliability is maintained. For catalyst evaluation at any temperature, it is necessary to determine the equilibrium composition and the straight-line reaction path. With this information any catalyst can be evaluated at this temperature with simply the additional information from a curved-line reaction path. The approach used in the application of monomolecular rate theory to the xylene isomerization selectivity kinetics is as follows. Reference is made to the composition diagram, Figure 1. 

In addition to catalyst pore structure, catalytic metals content can also influence the distribution of deposited metals. Vanadium radial profile comparisons of aged catalysts demonstrated that a high concentration of Co + Mo increases the reaction rate relative to diffusion, lowering the effectiveness factor and the distribution parameter (Pazos et al., 1983). While minimizing the content of Co and Mo on the catalyst is effective for increasing the effectiveness factor for HDM, it may also reduce the reaction rate for the HDS reactions. Lower space velocity or larger reactors would then be needed to attain the same desulfurization severity. 

There appear to be no extrinsic field studies on the familiar amorphous chromia gel catalyst although it is known (24) to have only a slight indication of the zero field anomaly shown by a-Cr203 at TN. A gel catalyst aged in hydrogen has a small positive field effect at 323 K and a small negative effect at 273 K. 

Interesting aging effects are frequently observed in these systems. If the precipitated particles are left in contact with the hydrolysis catalyst and water they appear to reorganize, so that their surfaces become better defined and their sizes become more uniform.15 The process seems quite analogous to the Ostwald ripening 33 much studied by colloid chemists. 

Coke deposition alters catalyst pore size distributions significantly and is an effect to be followed in regard to catalyst aging. 

Subsequent research on this and other systems with various alkyl groups was conducted by Natta (39), Belov et al. (40,41), Patat and Sinn (42), Shilov et al. (43, 44), Chien (45), Adema (46), Clauss and Bestian (47), Henrici-Olive and Olive (48), Reichert and Schoetter (49), and Fink et al. (50, 51). Investigations of kinetics and various other methods have helped to define the nature of the active centers of some homogeneous catalysts, to explain aging effects of solid Ziegler catalysts, to establish the mechanism of the interaction of the catalyst with olefins, and to provide quantitative evidence of some elementary steps (10). 

Unfortunately, the metal level on FCC catalysts is hardly ever in equilibrium and as catalyst deactivation by vanadium does not take place in isolation, but combined with and influenced by hydrothermal deactivation [14t 15], more sophisticated dynamic equations will be needed to describe this behaviour also including the effects of the catalyst age distribution [15,16,17]. 

Figure 6. Effect of alkyl size in tri-n-alkylaluminum on activity of aged catalyst preparations catalysts aged for five weeks at room temperature.

---