Deactivation kinetics assumptions

For the deactivation kinetics, the following assumptions were made. 

For the analysis of deactivation kinetics, the following assumptions are customarily used (44-46) ... 

This result is dependent on the initial assumptions on the deactivation kinetics. By making different initial assumptions about deactivation steps, several empirical forms of deactivation linear, exponential and hyperbolic decay can be derived (Table 8.4). 

Several factors and assumptions are built into equation (7-97). The most noticeable is the time variation of this term will be important if the rate of the deactivation is rapid compared to the rate of the main reaction, that is if k. The most important assumption in equation (7-97), however, is that the deactivation kinetics are separable hence the activity factor s in the reaction rate term. Finally, since A disappears in both of the reactions in the parallel scheme, the overall rate constant is given by the sum shown above. For the series scheme, of course, some changes will have to be made that, by now, one hopes will be obvious. 

The results discussed earlier for concentration dependent deactivation kinetics were based on Pontryagin s maximum principle (e.g., Koppel 1972) as extended to pseudo-steady state systems (Sirazetdinov and Degtyarev 1967). Here, a weak but more general maximum principle is considered due to Ogunye and Ray (1971). Let the concentrations (conversions) and temperatures be denoted by state variables X and the control variables such as temperatures (which are not state variables) by U and the catalytic activities by A. In general, under the pseudo-steady state assumption one has ... 

As will be seen from the examples to be given here, deactivation kinetics have almost universally been correlated in terms of separable (6) rate factors. More detailed analysis, however, indicates that such assumptions are questionable for surfaces other than those ideal in the Langmuir sense (7). The argument can be developed along the lines employed to derive adsorption isotherms for nonideal surfaces starting with the concept of a subassembly of ideal surfaces distributed according to the heat of chemisorption. The differences in separable and nonseparable... 

Young and Black18 believe, on the basis of the kinetic behavior of their system, that all O D) produced in the primary photolytic step reacts with 02 according to reaction (20). On the other hand, Izod and Wayne, from absolute measurements of [02(1E9+)] and [0(3P)], suggest that only one Oa(1S9+) molecule is produced in reaction (20) for about 500 0(1D) atoms deactivated. The calculation assumes that [0(a/>)] is equal to twice [0(1Z))] formed initially, although it seems improbable that the assumption is greatly in error for the vacuum ultraviolet photolysis of oxygen. For the overall deactivation of 0(1Z>) by 02,... 

Kinetic Considerations. The reaction kinetics are masked by a desorption process as shown below and are further complicated by rate deactivation. The independence of the 400-sec rate on reactant mole ratio is not indicative of zero-order kinetics but results because of the nature of the particular kinetic, desorption, and rate decay relationships under these conditions. It would not be expected to be more generally observed under widely varying conditions. The initial rate behavior is considered more indicative of the intrinsic kinetics of the system and is consistent with a model involving competitive adsorption between the two reactants with the olefin being more strongly adsorbed. Such kinetic behavior is consistent with that reported by Venuto (16). Kinetic analysis depends on the assumption that quasi-steady state behavior holds for the rate during rate decay and that the exponential decay extrapolation is valid as time approaches zero. Detailed quantification of the intrinsic kinetics was not attempted in this work. 

Experimental results supported the assumption that this temperature was necessary to gain the PdZn alloy on the catalyst surface. No catalyst deactivation was detectable during the experiments. At 300 °C full conversion was achieved at a 100 ms residence time [32] and 5% and lower carbon monoxide selectivity. First order kinetics were determined, revealing 7.04 1013 h 1 for the pre-exponential factor and 92.8 kj mol 1 for the activation energy. 

All the previously cited models and works also consider, and some explicitly cite, this assumption—that the catalyst activity varies with time-on-stream (or with coke concentration [12]) in the same manner or with the same deactivation function (VO for all reactions in the network. That is, a nonselective deactivation model is always used. Corella et al. (16) have recently demonstrated that in the FCC process this assumption is not true and that it would be better to use a selective deactivation model. Another work (17) also shows how this consideration, when applied to catalytic cracking, influences the yield-conversion curves. Nevertheless, to avoid an additional complication, we will use in this chapter a nonselective deactivation model with the same a—t kinetic equation and deactivation function (VO for all the cracking reactions of the network. 

Paramagnetic NO and 02 molecules and reactive CO molecules efficiently quench the PL in the order NO > 02 > CO, whereas N20 only weakly affects the PL intensity. This work allowed confirmation of some important steps of the mechanism proposed earlier for the photocatalytic reduction of NO by carbon monoxide on Mo/Si02 (Subbotina et al., 1999). In particular, the NO photoreduction kinetics was consistently described by Subbotina et al. (1999) assuming first that the deactivation rate constant is much smaller than the quenching rate constant and second that the NO molecules efficiently quench the (Mo5+=0-) excited triplet state without chemical interaction (i.e., "physical quenching"), in contrast to the "chemical quenching" by CO molecules to yield C02 molecules. The PL data demonstrate the correctness of those assumptions. 

The assumption of global kinetic control is probably valid for only a handful of catalytic reaction processes. Nevertheless, some typical simulation results of the model of catalyst deactivation under kinetic control are presented here in order to emphasize some of the unique percolation-type aspects of the problem. The overall plugging time 0p, i.e., the time at which the catalyst becomes completely deactivated is shown is Figure 1, where it is plotted versus Z, the average coordination number of the network of pores, (in industrial applications, of course, the useful lifetime of the catalyst is significantly smaller than 0p). Note that as Z increases, (higher values of Z mean a more interconnected catalyst pore structure) 0p increases, i.e., the catalyst becomes more resistant to deactivation. The dependence of normalized catalytic activity (r/rQ) ([Pg.176]

After more than twenty years of research and development, a great deal is known about the kinetics of COIL systems. There are still, however, significant questions that remain to be resolved. One of the more critical issues, the kinetics of I2 dissociation, is still obscure. Efforts to characterize elementary reactions, such as vibrational relaxation of l2(A ), have not removed the ambiguities. Instead, the results highlight the uncertain nature of several rate constants used in the standard kinetic model for COIL systems. The nature of the intermediate state (or states) of I2 involved in dissociation is called into question. Currently accepted rate constants for deactivation of the intermediate appear to be incompatible with the assumption that it is vibrationally excited l2(A ) alone. 

The kinetics of the CO+NO reactions has been studied at 300°C over a fresh and a deactivated bimetallic Pt-Rh/AhOs catalyst. Two kinetic models have been examined including competitive and non-competitive adsorptions of the reactants. The discrimination between these two assumptions has been achieved by using graphic and mathematical methods. From the comparison of kinetic and thermodynamic constants calculated from these methods with those previously obtained on RI1/AI2O3 and on Pt/AljOs, we believe that the kinetic data obtained on the fresh Pt-Rh/Al203 catalyst can be modelled by non-competitive adsorptions of the reactants assuming a preferential adsorption of NO on Rh and CO on Pt. By contrast NO and CO competitive adsorptions can only occur on the deactivated Pt-Rh/Al203 catalyst, which to assume that the active sur ce is mostly composed of Rh. 

The last mentioned assumption allows the use of separable kinetics. Thus the reaction rate model in a deactivating system may be written as two simultaneous equations ... 

Thus, a plot of (k pp) versus the gas pressure [M] yields a straight line with intercept ( a[/iv]) > and slope k /(k [hv]k(E)). Since the photoactivation rate, k [hv] is known from the intercept, the slope permits the determination of the ratio kJk(E). An example of such a Stem-Volmer plot is shown in Figure 5.17 for the isomerization of the previously mentioned allyl isocyanide reaction, C3H5NC —C3H5CN, in which k [hv] k pp is plotted. This results in an intercept of 1.0 and a slope of kJk(E). The quantity of interest, k(E), can be extracted if we know the deactivation rate, k. This is generally taken to be equal to the gas kinetic collision rate constant (strong collision assumption), which is typically about 10 cm3/(molec sec). 

The model of deactivation describes the transformations of two boundary forms of the carbonaceous deposits during the catalytic hydrogenation of CO2. These are the hydrocarbon-formed active deposit (CH) and the graphitic inactive one (C)n. Thus deactivation is based on dehydrogenation of the active deposit into the inactive one that blocks active centers for hydrogenation. The active deposit, a product of polymerization of surface methane precursors (CH ), is simultaneously their consumer and producer. The mass balance of the active intermediates derived from the model assumptions gave the kinetic equation which quantitatively describes the deactivation. 

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