Deactivating catalysts rate equations

A batch reaction is carried out with a slurried catalyst that, deactivates gradually. The rate equation is... 

Prior to the kinetic experiments, possible deactivation phenomena of the catalytic system were checked by recycling experiments with prenal and citral as substrates. These results provide not only important hints on the form of the rate equation, but also on which reaction is convenient for long-term investigations in the loop reactor. After the reaction, the aqueous and organic phases were separated and the catalyst phase was reused without further purification. Results on the hydrogenation of prenal are shown in Fig. 7. The reaction rate clearly decreases if the catalyst phase is reused. According to GC analysis and H-NMR studies, this can be attributed to the fact that the product of the reaction, prenol, is highly soluble in water. Consequently, a simple phase... 

Deactivation of the catalyst phase due to the fact that prenol is not fully withdrawn from the aqueous phase is taken into account by an inhibiting term in the denominator of the rate equation. Since a significant deactivation was... 

The progressive drop in conversion suggests that the catalyst deactivates with use. Find rate equations for the reaction and for the deactivation which fit these data. 

Catalyst deactivation is assumed to take place by a poisoning mechanism only. The deactivation of catalyst by thiophene will serve as a model deactivation reaction. Weng et al. (40 analyzed the deactivation data and proposed a rate equation, linear in concentration of poison, x, and activity 0 ... 

Using the resin, asphalt (R+AT) and aromatics (AR) separated from an atmospheric rcsid oil (ARO) as fc stocks, we have investigated the effects of catalytic coke additive coke (Cgdd) on the cracking activity of a commercial FCC catalyst in a fixed bed (FB) and a rixed fluid bed (FFB) pilot units. Correlations between catalyst activity (a) and coke on catalyst (Q.) have been developed. A catalyst deactivation model, which is useful in modeling of cracking reaction kinetics, has been derived through rate equations of coke formation. 

For the evaluation of the kinetic equation for the deactivation kinetics the asumption was made, that for a deactivated catalyst the same kinetic modd is valid and the rate constants do not change with time on stream. Therefore reaction kinetics and ci eac-... 

The predominance of kinetic studies have assumed uniform sites on the catalyst surface. However, it has long been recognized that many catalyst surfaces exhibit non-uniform sites. Boudart and Djega-Mariadassou [3] have discussed the kinetics of non-uniform surfaces and conclude that "a non-uniform surface behaves catalytically. .like a uniform surface..", and that "rate equations are similar for a given mechanism on a uniform or non-uniform surface". This result justifies "the common practice of neglecting non-uniformity of catalytic surfaces in kinetic studies". However, it appears that uniform catalyst sites catmot adequately explain catalyst deactivation phenomena. The objective of the present study was to explain deactivation in terms of a model based on a variable activation energy site distribution on the catalyst. 

Measurements of rates at different concentrations and temperatures lead to empirical rate equations. If. however, a series of catalysts is to be compared, each must be expressed at a specified temperature and concentra tion. Initial rates at zero process time are the most reliable. In the case of deactivation or surface conditioning by the reactant, rates should be measured at some standard process time. This must be established carefully and rates interpolated, but never extrapolated, from rates measured over the complete range. 

The double-line arrow indicates that catalyst decay is second order). The Langmuir-Hinshelwood rate equation, for control by the surface reaction and with q a(t) instead of q-r to account for progressing deactivation, is... 

Often deactivation is expressed in terms of time. This is not the true variable, as it could lead to incomplete predictions. More correctly, the deactivation function has to be expressed in terms of the deactivating agent the coke precursor or the poison, which means that the amount of coke (or poison) on the catalyst site should be known. The determination of a rate equation for the formation of the coke precursor is thus an integral part of the kinetic study of the process. 

Derivation of a rate equation is based once again on the theory of complex reactions, considering deactivation as an independent route leading to coke on the catalyst surface. 

Equation (4-158) for CSTR deactivation was derived on the basis of a time-on-stream model for catalyst decay, equation (4-157). Suppose, however, that it is necessary to account for the concentration dependence of the decay rate, as given in equation (4-156). Derive an equation on this basis for (Ca/CaJ as a function of time-on-stream. 

A series of Cu- and Cu-Ln-ZSM-5 (Ln= Ce, Sm) were prepared by ionic-exchange of ZSM-5 with various copper salts. These catalysts were subjected to TG-DSC measurements coupled with mass spectrometry analysis in the presence of a 30 ml min flow of NO. The fitting of these experimental curves with rate equations led to the determination of the kinetic parameters. The correlation of these parameters with catalytic, FTIR, and XPS data provided a model of deactivation of these catalysts in NO decomposition. 

All the balances have accumulation, convection, axial dispersion, and reaction terms. The equations include liquid holdup, Bi, and superficial liquid velocity, w. Langmuir-type rate equation, for the main reaction, Equation 15.4, included also an activity correction term a. Kst and in Equations 15.5-15.7 indicate the adsorption parameters for stearic acid and heptadecene, respectively. Equation 15.4 corresponds to a monomolecular transformation of stearic acid via the adsorption of the reactant to the main product. Adsorption terms for stearic acid and heptadecene were used, since both of these compounds contain functional groups enabling adsorption on the active sites of the catalyst Reaction rates were assumed not to be limited by heptadecane adsorp-UoiL Thus, the adsorption term of heptadecane was n ected. In line with the experimental observations indicating catalyst deactivation. Equation 15.4 (Table 15.2) was modified to incorporate the decrease in catalyst activity. In particular, the activity was assumed... 

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