INTRINSIC CATALYST DEACTIVATION

The authors would like to draw attention to the importance of considering the features distinctive to various schemes of generation of coke precursors. The catalyst deactivation may follow different kinetics according to the mechanism of blockage, and the intrinsic pecularities of the kinetics are to be accounted on predicting the apparent deactivation of large catalyst grains. 

The phenomena relevant to HDM catalyst deactivation are intrinsic reaction kinetics, restrictive intraparticle diffusion and (changing) catalyst porous texture. 

Preliminary HDM catalyst deactivation simulations using the reaction kinetics of model compound vanadyl-tetraphenylporphyrin indicate that reliable metal deposition profiles and catalyst life-time predictions can be made provided that intrinsic reaction kinetics and restrictive intraparticle diffusion are introduced in the catalyst deactivation model. 

For most reaction systems, the intrinsic kinetic rate can be expressed either by a power-law expression or by the Langmuir-Hinshelwood model. The intrinsic kinetics should include both the detailed mechanism of the reaction and the kinetic expression and heat of reaction associated with each step of the mechanism. For catalytic reactions, a knowledge of catalyst deactivation is essential. Film and penetration models for describing the mechanism of gas-liquid and gas-liquid-solid reactions are discussed in Chap. 2. A few models for catalyst deactivation during the hydrodesulfurization process are briefly discussed in Chap. 4. 

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. 

Metal deposition in hydrotreating of heavy oils is one of the most important phenomenon causing catalyst deactivation. Present work focuses on the modeling of hydrodemetallisation catalyst deactivation by model compound vanadyl-tetraphenylporphyrin. Intrinsic reaction kinetics, restrictive diffusion and the changing catalyst porous texture are the relevant phenomena to describe this deactivation process. The changing catalyst porous texture during metal depositon can be described successfully by percolation concepts. Comparison of simulated and experimental metal deposition profiles in catalyst pellets show qualitative agreement. 

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,... 

To summarize, transient kinetic experiments are an established and valuable tool in the investigation of heterogeneously catalysed gas phase reactions. For liquid-phase systems, transient studies are much more rare than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration (Figure 8.11) we will consider three-phase continuous hydrogenation of an organic compound leading to two products over a metal catalyst on a structured support (knitted silica). 

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. 

A vast effort has been expended over the years in investigation of these types of deactivation as they are encountered in catalytic reactions and catalysts of technological importance. The uninitiated are often amazed at the fact that many reaction-system process designs are dictated by the existence of catalyst deactivation, as are process operation and optimization strategies. In some cases the deactivation behavior is so pronounced as to make detailed studies of intrinsic kinetics of secondary importance. 

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. 

It must be stressed that a is the only parameter to be estimated in Model 4 as T was kept equal to 0.56. Parameter a was estimated as 10-5. Figure 4.4.1 jjresents experimental normalized CO conversion plotted against predicted normalized values considering only the validation data set. It can be observed that the use of catalyst deactivation improved model performance, indicating an important role played by catalyst deactivation. However, the intrinsic reaction rate was not yet used for parameter estimation as exponents i, m, n, p values were considered the ones reported in the literature (see Table 4.1.1), consequently, this creates an alternative for model improvement. 

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