Coking, catalyst deactivation from defined

In contrast to Fig. 12a, the spectrum of a coke from the same process shown in Fig. 12c, surprisingly, strongly resembles the signals of the well-defined species [Fe(H20)Cl5] (49), the simulated spectrum of which is also included Fig. 12d. Alternative structures would show quite different vibrational spectra. The strongest band, at 386cm is assigned to the Fe-OH2 torsional mode. The presence of this species indicates another cause of catalyst deactivation. This species was probably the result of traces of moisture in the HCl recycle gas stream, which can lead to dew point corrosion and hence to the formation of [Fe(H20)Cl5] species, which may dominate the whole IINS spectrum of this type of coke. 

Froment and Bischoff (1961, 1962) examined the effect of catalyst decay and reactor performance when coke is produced from both products and reactants. TTiey showed a Voorhies type law holds over certain operating ranges and defined a deactivation function as the fraction of active sites remaining active on the catalyst. They related this function, to the coke content, Cc, by the following two empirical relationships which are equivalent at low coke concentrations ... 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

A major problem in the catalytic hydrodesulfurization of residual oils is the deactivation of the catalyst by metal-containing asphaltenic species in the feed. As can be seen from the results of a typical desulfurization experiment presented in Fig. 1, the catalyst shows a rapid initial decline which is attended with a fast build-up of coke on the catalyst. At a relatively low catalyst age 0, as defined in Section IV, a stationary coke level is reached. In contrast, the deposition of the inorganic remnants of the hydro-cracked asphaltenes (mainly vanadium and nickel sulfides) continues and gradually clogs the pores in the outer zone of the catalyst particles, as confirmed by electron microprobe analyses of spent catalyst samples (see Fig. 2). This causes a slow further loss in desulfurization activity over a longer period of time. Ultimately, the catalyst becomes totally inactive for desulfurization because the - still active - inner core has become completely inaccessible to the sulfur-bearing molecules. 

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