Catalysts deactivation functions

However, several assumptions are inherent in this interpretation of the data. First, it is assumed that the change in the observed effect (such as conversion of 850°F+, percentage denitrogenation, etc.) is linear with respect to time. Thus a linear delta-effect per period of time could be established and intermediate data could be adjusted to a MfreshM activity corresponding to that observed at the reference period and at any desired temperature. Second, it is assumed that the intermediate process parameter variations had no adverse effect on the catalyst deactivation function. For example, operation at constant temperature for a given interval of time would produce the same catalyst deactivation as varying temperatures (within limits) over the same interval of time. 

Figure 3 A schematic representation of catalyst deactivation functions for REY and USY catalysts.

Tigrel and Pyle [22] used the following expression for the catalyst deactivation function in a fluidized bed... 

Pure dry reactants are needed to prevent catalyst deactivation effective inhibitor systems are also desirable as weU as high reaction rates, since many of the specialty monomers are less stable than the lower alkyl acrylates. The alcohol—ester azeotrope (8) should be removed rapidly from the reaction mixture and an efficient column used to minimize reactant loss to the distillate. After the reaction is completed, the catalyst may be removed and the mixture distilled to obtain the ester. The method is particularly useful for the preparation of functional monomers which caimot be prepared by direct esterification. 

The appHcations of supported metal sulfides are unique with respect to catalyst deactivation phenomena. The catalysts used for processing of petroleum residua accumulate massive amounts of deposits consisting of sulfides formed from the organometaHic constituents of the oil, principally nickel and vanadium (102). These, with coke, cover the catalyst surface and plug the pores. The catalysts are unusual in that they can function with masses of these deposits that are sometimes even more than the mass of the original fresh catalyst. Mass transport is important, as the deposits are typically formed... 

The presence of other functional groups ia an acetylenic molecule frequendy does not affect partial hydrogenation because many groups such as olefins are less strongly adsorbed on the catalytic site. Supported palladium catalysts deactivated with lead (such as the Liadlar catalyst), sulfur, or quinoline have been used for hydrogenation of acetylenic compound to (predominantiy) cis-olefins. 

A systematic study of differently supported Ru catalysts showed that carbon catalysts provide very high selectivities to higher hydrocarbons (C10-C20) and the CNT-supported catalyst is among the most active systems of all [138]. In parts this is related to the inertness of carbon preventing the formation of hardly reducible mixed metal oxides with the support, such as CoAl204 [139,140], which is, besides coking, the main reason for catalyst deactivation. The carbon surface functionalized with oxygen... 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Table 10.4 illustrates the TPO data for each of the samples tested, standardized in coke (g of coke associated with each peak)/total coke on the catalyst (g). In this table it is observed how the total coke yield of these samples is inversely proportional to the catalyst deactivation severity. Similarly, it is noted that catalytic coke increases with catalyst activity, but it is not a direct function of conversion. 

The approach of this work is to measure product compositions and mass balances in much detail in a time resolved manner and to relate this to the controlling kinetic principles and elemental reactions of product formation and catalyst deactivation. Additionally the organic matter, which is entrapped in the zeolite or deposited on it, is determined. The investigation covers a wide temperature range (250 - 500 °C). Four kinetic regimes are discriminated autocatalysis, retardation, reanimation and deactivation. A comprehensive picture of methanol conversion on HZSM5 as a time on stream and temperature function is developed. This also explains consistently individual findings reported in literature [1 4]. 

The catalyst activity factor (aj) is time-dependent. Several models have been proposed in the literature, depending on the origin of catalyst deactivation, i.e. sintering, fouling or poisoning (8). The following differential equation can represent semi-empirically different kinds of separable deactivation functions. 

In order to elucidate the effect of liquid products on the catalyst deactivation, selectivity versus conversion was investigated. As indicated above particularly interesting was the rapid decrease of conversion over 0.6 g compared to 0.4 g and 0.2 g of catalyst, however a comparison of product selectivities as a function of conversion gave no direct explanation for this rapid deactivation. Very similar... 

Another effect of aromatics is increased carbon formation, which has long been recognized as the primary means of catalyst deactivation in the ATR of hydrocarbons. Using carbon-forming reactions (6)-(8), an equilibrium line for carbon formation as a function of O2/C and S/C ratios can be calculated. Figure 9 shows the results of this calculation for n-Ci4, along with the experimental results for two Ni-based commercial catalysts. 

Fig. 12. Effect of self-poisoning (i hr at 450 C, by the reaction mixture) on hexane/H2 reactions. AT is the temperature increase necessary to achieve the same overall conversion after poisoning over that before poisoning. AT is plotted as a function of the average particle size of various Pt/SiO2 catalysts. From V. Ponec et al, in Catalyst Deactivation, p. 93, Elsevier, Amsterdam (1980).

As the catalyst deactivates, reactor inlet temperature must be increased to maintain the target octane. A typical temperature profile as a function of process stream time is shown in Fig. 7. The temperature increase further... 

With a defined, the functional form for each rate of deactivation da/dt must be determined. The primary cause of catalyst deactivation under reforming conditions is the buildup of coke, which blocks active sites on the catalyst and prevents further reforming reaction. 

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