Catalyst aging and deactivation

Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is also used to determine the composition of catalysts based on weight changes and thermal behavior during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms of catalysts, and to investigate the acid-base properties of solid catalysts using probe molecules. However, these techniques lack chemical specificity, and require corroboration by other characterization methods. 

There were two major objectives of this study. Firstly, the effects of the addition of potassium and/or silicon on catalyst deactivation rates and changes in catalyst properties with TOS were investigated. Secondly, the possible causes of catalyst deactivation were examined by following aged catalyst properties and reactor conditions as a function of TOS for each catalyst. The FTS was carried out in a continuous-flow stirred slurr) reactor to ensure uniformity in catalyst aging and reactor conditions throughout the reactor. The aged catalyst properties examined as a ftinction of TOS were total surface areas, carbon deposits and phase transformations. 

The operation of electrocatalysts in an electrolyte and in the presence of an electric field imposes stringent conditions on maintaining activity. Avoiding catalyst aging or deactivation is of great economic importance for the successful application of electrochemical processes. Deactivation can arise because of ... 

B. Delmon and P. Grange, "Solid State Chemical Phenomena in Aging and Deactivation of Catalysts", in Catalyst Deactivation, eds. Delmon Froment, Elsevier, Amsterdam, 1980, pp. 507-543. 

It is misleading to say that a catalyst is totally unchanged by the reaction it catalyzes. Gradual physical and chemical alterations may take place during catalysis or with usage. Industrial catalysts are slowly deactivated by phenomena that accompany the main catalytic process. Catalyst aging, or deactivation, is indicated by the decrease in catalyst activity with time. It introduces additional complexity to the determination of rate parameters and has to be considered in macrokinetic analysis, that is, in catalytic reactor design. 

It is important to remember that a simulation results are only as good as the mathematical models on which those are based. However, there are still many items that are neglected or treated insufficiently in mathematical models, such as ageing and deactivation of the catalyst and the stability of the products under actual production conditions. Many commercial flowsheeting programs still rely totally on idealistic behaviour. Many programs have only very limited number of reactor types like tube and CSTR. Common multiphase reactors where mass transfer phenomena also plays important role are missing. Also idealized separation models are common. 

When metals are deposited on the catalyst surface, active phase migration to less accessible sites or the loss of metal dispersion could be the result of catalyst aging and consequently deactivation. 

Mowery, D.L., Graboski, M.S., Ohno, T.R. et al. (1999) Deactivation of Pd0-Al203 oxidation catalyst in lean-burn natural gas engine exhaust aged catalyst characterization and studies of poisoning by H20 and S02, Appl. Catal. B 21, 157. 

The oxidation of chlorinated compounds produces hydrogen chloride (HCl) along with the carbon dioxide and water vapor. Some catalyst aging data suggest that HCl exposure over 10,000 parts per million by volume (ppmv) for extended periods may lead to catalyst deactivation over time. 

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 elements of range in value from 0 to 1 and are the ratio of the reformer kinetic constants at time on stream t to the values at start of cycle. At any time on stream t, the deactivation rate constant matrix K(a) is determined by modifying the start-of-cycle K with a. From the catalytic chemistry, it is known that each reaction class—dehydrogenation, isomerization, ring closure, and cracking—takes place on a different combination of metal and acid sites (see Section II). As the catalyst ages, the catalytic sites deactivate at... 

As mentioned in the previous discussions of deactivation, catalyst aging is very composition dependent. Thus, catalyst state at a given time on stream will vary with axial distance in a plug flow reactor. This is shown in Fig. 22. Benzene and methylcyclopentane compositions as a function of time on stream are shown at 20% through the catalyst bed and at the end of the bed. KINPTR predicts the catalyst state gradient in the reactor. 

In commercial aging simulations, KINPTR s deactivation model is used to predict cycle lengths (time between catalyst regenerations) and reactor inlet temperature requirements with time on stream to maintain target reformate... 

Catalyst deactivation may also result from changes in the structure or in the texture of the catalyst. Changes of this kind are usually irreversible and the catalyst cannot be regenerated. This type of deactivation is often called catalyst ageing. 

In Section V, deactivation of catalyst pellets and reactor beds during residuum hydroprocessing is considered. The chemical nature of the metal deposits is described, including a discussion of the physical distribution of these poisons in aged catalysts and reactor beds. Models to predict... 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

The optimization study discussed above suggests the use of a high temperature and a short-reaction time. Because of the heat-up and cool-down time limitations of the autoclaves used, this study was limited to reaction temperatures < 435°C. Verification studies at higher temperatures (>435°C) are required. The present study should be supported by complementary catalyst aging studies to determine the maximum temperature limit below which severe deactivation and aging does not occur. 

Laboratory steam deactivations represent a significant compromise in the effort to simulate equilibrium catalyst. Since hydrothermal deactivation of FCC catalysts is not rapid in commercial practice, deactivation of the fresh catalyst in the laboratory requires accelerated techniques. The associated temperatures and steam partial pressures are often in substantial excess of those encountered in commercial units. In some instances, the effect of contaminant metals is measured by an independent test not affiliated with steam deactivation. In subsequent yields testing, interactions between different modes of deactivation may be overlooked. Finally, single mode deactivation procedures can not reproduce the complex profile of ages and levels of deactivation present in equilibrium catalyst. 

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]. 

Assuming that the metals and other poisons on catalyst are low, we can expect that traditional catalyst steaming will be sufficient to simulate catalyst deactivation. Keyworth et a) [16] recommend to make a composite of several steamings in order to address the age distribution of equilibrium catalyst in a commercial unit. Beyerlein et al [17, 21] critically question the possibility of improving catalyst ageing procedures, which rely onfy on steam treatment at constant temperature for varying times. 

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