Order of deactivation

A kinetic model which accounts for a multiplicity of active centres on supported catalysts has recently been developed. Computer simulations have been used to mechanistically validate the model and examine the effects on Its parameters by varying the nature of the distrlbultons, the order of deactivation, and the number of site types. The model adequately represents both first and second order deactivating polymerizations. Simulation results have been used to assist the interpretation of experimental results for the MgCl /EB/TlCl /TEA catalyst suggesting that... 

Determining the Order of Deactivation. Note that in Figure 6 there is an intrinsic difference in the nature of the curvature... 

It also clearly shows how different is the progress of the reaction with the different orders of deactivation. ... 

The deactivation rate data can be regressed using the above expression (treating the partial pressures of H2 and CO as constants) to find an order of deactivation of m s 3.O however, this approach is based on a questionable assumption (that the value of a at t = > is zero) moreover, this value of m lacks physical meaning. If, however, one uses the generalized power-taw expression advocated by Puentes (ref. 4) in which the activity, a, is substituted by (a-a,), the calculated order of deactivation is 0.9, a value more reasonable and consistent with the observed exponential decrease. 

Figure IB displays relative catalytic activity (RA) - in terms of pseudo first-order rate constants, corrected for coke content, related to the fresh, sulfided catalyst vs carbon content. The individual HDS, HVD and CNH activities all decrease with increasing carbon content, the order of deactivation being HYD < HDS < CNH. (The results for relative HDN activities followed closely those of CNH, and are not shown). Relative activities fall off less sharply as coke content increases. Because of the limited set and scatter of the data, a definitive deactivation correlation could not be obtained. Best fit curves to the data were constructed from a power-deactivation equation in C (1), and are shown by the solid curves in Fig. IB.

Generally as the Thiele modulus of main reaction is smaller than unity, the order of deactivation is equal to unity and, therefore the apparent deactivation rate constant can he extracted from the slope of -In Xg(t) versus t Krishnaswamy and Kittrell (ref. 10) have shown that the relationship between the... 

Decimal (not-whole) numbers are normally obiained for d (apparent and overall order of deactivation). This fact occurs in the analysis of all runs. With the work of Corclla ei iil. (13), Airs (6), or Astarita (7,8), ihis fact can be explained. Note that the gas-phase and the catalyst surface are continuous mixtures. Other approaches and models such as diffusion/percolaiioa theory through a pore network of the FCC catalyst, can explain the values of d found hereof 1.4 to 2.8,... 

Different orders of deactivation will be used in the modeling. A simplification will be that the activity function for all components is the same. Using the reaction scheme given in figure 2 and the reaction rates as described by equation 1, in combination with a plug flow reactor model, mass balances can be derived for the different lumped groups, resulting in ... 

The approach followed in deactivation studies is often different from the one outlined here. The alternate approach does not consider a coking rate equation and uses an empirical time-related deactivation function for the particle or bed O = /( called activity [Szepe and Levenspiel, 1971 Wojchiechowski, 1968]. Linear, hyperbolic or exponential functions of time were used. Deriving the activity O with respect to time gives the corresponding rates of change of the activity and defines a so-called order of deactivation, from which it has been attempted to get some insight into the mechanism of deactivation—an attempt... 

A general equation (9.225) and (9.226) is thus obtained, from which many empirical forms used in describing deactivation can be derived. Often deactivation is considered to be irreversible, a =0 and the initial activity is set to unity, ao=l. Then by varying the order ( ) of deactivation, different forms of the empirical functions can be obtained as special cases. 

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