Deactivation kinetics coke formation

Frequently the kinetic description of catalyst deactivation and coke formation is complicated by instationary reaction conditions prevailing during the respective experiments. In this paper two experimental methods are presented.which enable the determination of such kinetics avoiding this problem 4 Use of a concentration controled continuously operated recycle reactor 4 Experimentation at the thermodynamic equilibrium of the main reaction to determine the coke formation kinetics at well defined operating conditions... 

A fundamental kinetic freunework is developed for deactivation by site coverage, coke grovth and blockage in pores and networks of pores. Diffusional limitations are also accounted for The methodology of kinetic studies of catalyst deactivation by coke formation is discussed by means of a number of practical examples. Finally, the effect of catalyst deactivation on the behavior of reactors is illustrated. 

Kinetic studies of catalyst deactivation by coke formation are not simple. It should be clear by now that the kinetics of the coking reaction(s) are also needed and this calls for techniques which are less familiar than those applied in the kinetic study of reactions not subject to deactivation. 

Catalyst deactivation by coke formation can occur through a more or less reversible mechanism. We have applied a transient approach to model the reversible behavior of the deactivation, and to separate the deactivation from the main reaction kinetics. The deactivation of a Pt-Sn/AbOs catalyst was studied during propane dehydrogenation. The gas composition and temperature were varied during the experiments, which allowed us to model the deactivation by assuming one reversible and one irreversible type of coke. It was found that the deactivation increased with the propene concentration but was independent of the partial pressure of propane. Hydrogen decreased the deactivation rate and could even activate the catalyst by removing reversible coke. 

Anthony and Singh concluded from a kinetic analysis of the methanol conversion to low molecular weight olefins on chabazite that propylene, methane, and propane are produced by primary reactions and do not participate in any secondary reactions, whereas dimethylether, carbon monoxide, and ethane do. Ethylene and carbon dioxide appear to be produced by secondary reactions. It was also shown that the product selectivities could be correlated to the methanol conversion even though the selectivity and the conversion changed with increasing time on stream due to deactivation by coke formation. 

In this case, the process for the production of gasoline from methanol is considered. One of the critical problem in the development of this process is the deactivation of the zeolite catalyst used via coke formation. This deactivation by coke formation in zeolites has been simulated by Guo et al. [12] using a site-bond-site modefwith a two dimensional square lattice. The effect of gaseous pressure on the coke formation was foimd to be similar to that reported in the literature. The authors have not compared their results with kinetic data. 

Kinetic Analysis of Deactivation by Coke Formation Example 5.3.3.A Application to industrial Processes Coke... 

Beeckman and Froment [1979, 1980] and Nam and Froment [1987] formulated the deactivation by coke formation in terms of site coverage, coke growth, and pore blockage. Mechanistic kinetic equations were derived for the rate of change of the coke content with time. An illustration of the application of such equations to deactivation in a fixed bed reactor is given by Froment [1980]. The trends illustrated here on the basis of empirical models are not essentially modified. 

Froment GF. Kinetic modeling of hydrocarbon processing and the effect of catalyst deactivation by coke formation. Catal Rev 2008 50 1-18. 

Although the reaction classes discussed earlier are sufficient to describe the hydrocarbon conversion kinetics, an understanding of the elementary reaction sequence is needed to describe catalyst deactivation. Several of the overall reactions require formation of olefinic intermediates in their elementary reaction sequence. Ultimately, these olefinic intermediates lead to coke formation and subsequent catalyst deactivation. For example, the ring closure reaction... 

Kodama et al. (1980) developed a detailed HDS and HDM model for deactivation of pellets and reactor beds. The model included reversible kinetics for coke formation, which contributed to loss of porosity. Second-order kinetics were used to describe both HDM and HDS reaction rates, and diffusivities were adjusted on the basis of contaminant volume in the pores. The model accurately traced the history of a reactor undergoing deactivation. This model, however, contains many parameters and is thus more correlative than theoretical or discriminating. 

Equations for the kinetic mechanisms of coke formation with the exponential form of the deactivation function are obtained by integrating eqs. (6)—(8) ... 

The limitation to low conversion is the major disadvantage of differential operation. This is not critical if the influence of the catalyst properties on deactivation is studied. If, on the other hand, one is interested in the mechanism and the kinetics of coke formation and in the deactivation of the main reactions, it is necessary to reach higher conversions. A solution to this problem is to combine the electrobalance with a recycle reactor. The recycle reactor is operated under complete mixing, so that the reactor is gradientless. Since in a completely mixed reactor the reactions occur at effluent conditions and not at feed conditions, a specific experimental procedure is necessary to obtain the deactivation effect of coke. 

During experiments performed at constant feed rates, the conversion changes as a consequence of the deactivation of the catalyst. This implies that a single experiment can not distinguish between the influence of coke and that of conversion on the reaction kinetics and the rate of coke formation. 

The parameters a do not differ significantly, so that the rates of formation of the various isomers equally deactivate, except the 2,2-di-Me-butazie formation. Where the corresponding carbcnium ion of all other hexane isomers is tertiary, that for 2,2-di-Me-butane is secondary, and thus its formation is kinetically much less favored. This explains the low concentration of 2,2-di-Me-butane in the reaction mixture at the conversions shown in Table 3b, and implies that its formation needs a stronger acid site. Since coke is preferentially formed on the stronger acid sites, the formation of this hexane isomer will be more deactivated by the coke formation. 

The possibility of obtaining high levels of conversion and the ability to separate the influence of coke formation and of conversion changes on the reaction kinetics, makes this reactor configuration attractive for the study of the deactivation of complex reactions, such as catalytic cracking. 

---