Catalyst deactivation intrinsic activity

As the catalyst deactivates, the active surface area decreases. If deactivation is assumed nonselective, the effective rate of reaction is simply the intrinsic rate multiplied by the fraction of surface area still remaining active. This effective rate can then be used in pellet conservation equations to arrive at the global rate,... 

The molybdenum on alumina catalyst was also tested for activity with and without arsenic. Although this catlyst has a much lower intrinsic activity for HDS, the results in Figure 4 show that 3.6% arsenic almost completely deactivates the catalyst. The small amount of activity remaining is that expected for AI2O3 alone. Thus arsenic also deactivates catalysts without cobalt promoters. 

Bertole et al.u reported experiments on an unsupported Re-promoted cobalt catalyst. The experiments were done in a SSITKA setup, at 210 °C and pressures in the range 3-16.5 bar, using a 4 mm i.d. fixed bed reactor. The partial pressures of H2, CO and H20 in the feed were varied, and the deactivation, effect on activity, selectivity and intrinsic activity (SSITKA) were studied. The direct observation of the kinetic effect of the water on the activity was difficult due to deactivation. However, the authors discuss kinetic effects of water after correcting for deactivation. The results are summarized in Table 1, the table showing the ratio between the results obtained with added water in the feed divided by the same result in a dry experiment. The column headings refer to the actual experiments compared. It is evident that adding water leads to an increase in the overall rate constant kco. The authors also report the intrinsic pseudo first order rate-coefficient kc, where the overall rate of CO conversion rco = kc 6C and 0C is the coverage of active... 

Potential pitfalls exist in ranking catalysts based solely on correlations of laboratory tests (MAT or FFB) to riser performance when catalysts decay at significantly different rates. Weekman first pointed out the erroneous conversion ranking of decaying catalysts in fixed bed and moving bed isothermal reactors (1-3). Phenomena such as axial dispersion in the FFB reactor, the nonisothermal nature of the MAT test, and feedstock differences further complicate the catalyst characterization. In addition, differences between REY, USY and RE-USY catalyst types exist due to differences in coke deactivation rates, heats of reaction, activation energies and intrinsic activities. 

During the experimental investigations one should carry out a proper testing of the catalysts, in order to obtain the relevant information with regard to intrinsic activity, selectivity, deactivation, and kinetic behavior. The following guidelines should be applied ... 

The kinetics of reactions in zeolites is conventionally related to the reactant concentrations in the gas phase. Reaction within the pores of zeolites, however, involves adsorption, diffusion of reactants into the pores, reactions of adsorbed species inside the pores, desorption of products, and diffusion of products out of the pores (92). Therefore, intrinsic kinetics based on the concentration of species adsorbed inside the pores is expected to be very useful for catalyst development. TEOM is an excellent technique for measurement of adsorption of reactants under reaction conditions as well as measurement of this adsorption as a function of the coke content (3,88). This technique makes it possible to obtain intrinsic activity of each acidic active site directly and to understand deactivation mechanisms in detail. 

Figure 6. Intrinsic reaction rate in dependence of sulphur poisoning for a) normal catalysts ( ), b) gum deactivated catalysts (A). The dotted line represents the ideal relation for the average intrinsic activity of a normal shell poisoned catalyst, 0 = 0.9. Steam reforming of ethane, H2O/C = 4.0, P = 1 bar. Activities normalized with activity of unpoisoned catalyst.

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Roles of Coke and Metals on Catalyst Deactivation. The model compound activity test (Figure 4) and the diffusivity test (Figure 6) clearly show that both coke and metal sulfides have a responsibility for decreasing intrinsic reaction rate and effective diffusivity. However, the former test suggests that coke and metal sulfides do not independently affect the active sites. As shown in Figure 4, in the third bed, coke rapidly covered more than 70% of the original active sites at the start of the run. However, after the run, the ratio of the coke-covered active sites to the original ones dropped drastically to less than 10%, while that of the metal-poisoned active sites became around 70%. This indicates that the metal sulfides deposit on part of the active sites which coke initially covers and permanently poison them. 

In this review the intrinsic kinetic aspects are dealt with in the first place The progressive coverage of active sites of the catalyst, which affects its activity and the process selectivity, is cast in a mechanistic form. These kinetic aspects are then studied in combination with the influence of the catalyst morphology, first at the pore level, then at the particle level, seen as a network of pores. Next, growth of coke, leading eventually to pore blcx kage and diffusional limitations are introduced The practical application of the models in kinetic studies is given particular attention. Finally, the effect of catalyst deactivation on the behavior of the reactor is discussed. 

Decay is defined simply as the loss of catalyst activity. This activity cannot be recovered without regeneiration of the catalyst. The main types of catalyst deactivation which are related to the decline in the intrinsic rate of reaction can be summarized as follows ... 

One problem in catalysis is gradual loss of activity of the catalyst. There are many reasons underlying the deactivation of heterogeneous metathesis catalysts [42]. The most important causes of catalyst deactivation are (i) intrinsic deactivation reactions, such as the reductive elimination of metallacyclobutane intermediates,... 

In the simplest case, the catalytic activity is proportional to the number of active sites Nj, intrinsic rate constant and the effectiveness factor. Catalyst deactivation can be caused by a decrease in the number of active sites, changes in the intrinsic rate constant, e.g. changes in the ability of surface sites to promote catalysis and by degradation in accessibility of the pore space. When the reaction and deactivation rates are of different magnitudes, the reactions proceed in seconds while the deactivation can require hours, days or months, and moreover the deactivation does not affect the selectivity, the concept of separability is applied. The reaction rates and deactivation are treated by different equations. A quantity called activity, (a) is introduced to account for changes during the reaction. 

In previous sections we have dealt with nonisothermal effects arising from the thermochemistry of the reactions involved. There is another type of thermal effect that appears in the operation of large-scale reactors such as those used in hydrotreating. These reactors are normally subject to slow catalyst deactivation, and constant conversion operation is required in order not to upset subsequent processing units. Here the reactor temperature is used to cope with the loss of intrinsic catalyst activity and the thermal parameters of the main and deactivation reactions, particularly the activation energies, have a great influence on the operation. Further, it has been common practice in many industrial laboratories for many years to evaluate catalyst activity and activity maintenance for such processes in laboratory experiments which are also conducted under constant-conversion conditions. In this procedure, catalyst deactivation effects are manifested in the rate of temperature increase needed to maintain constant conversion, that is, a temperature increase required (TIR). 

Since SSITKA can decouple the apparent rate of reaction into the contribution from the intrinsic activity ( the reciprocal of surface residence time of intermediates) and the nrnnber of active sites ( surface concentration of intermediates), the cause of deactivation of a catalyst during reaction can often be revealed. SSITKA has been used in a number of studies for this purpose. Catalyst deactivation during n-butane isomerization and selective CO oxidation are good examples. Deactivation studies are conducted by collecting isotopic transient data at particular times-on-stream as deactivation occurs. 

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