Coking rate equations, deactivation models

When the rate equations for coke formation are available, accounting for the effect of deactivation in the modeling of chemical reactors is relatively straightforward an equation for... 

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. 

Another important consideration in the FCC unit model is the deactivation of catalyst as it circulates through die unit Previous work has used two different approaches to model catalyst activity time on stream and coke on catalyst [49]. Since the 21-lump includes discrete lumps for the kinetic and metal cokes, this work uses a coke-on-catalyst approach to model catalyst deactivation. In addition, this work includes a rate equation in the kinetic network for coke balance on the catalyst. The general deactivation function due to coke, coke> is given by Eq. (4.4). 

The problem of diffusional limitations in the main reaction and deactivation by site coverage only was solved by Hasamune and Smith [ref. 22] and further investigated by Kan et al, [ref, 6]. when pore blockage also has to be considered, the problem becomes considerably more complex. As dealt with by Beecknan and Froment [ref, 23] the model equation for the ntain reaction closely resembles the second order differential equation for the diffusion and reaction, but also contains the probability S, Further, the blockage affects the distance available for diffusion Since the deactivation is expressed in terms of the coke content of the catalyst, a differential equation relating the evolution of the coke content with time to the rate of coke formation is required. This differential equation contains P and S Both these quantities are governed by additional differential equations The rate of coke formation contains which is not a constant any lAore, because... 

Equation (2) explains the transient deactivation with a model describing reversible and irreversible coke. We can see that the partial pressure of propane in the reactor does not influence the deactivation. This has also been demonstrated in an earlier study of the same system [8]. This observation is consistent with kinetic models for propane dehydrogenation proposed by Loc et al. [13]. They suggested that the rate-determining step is the dissociative adsorption of propane. From this mechanism it follows that the deactivation will be... 

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. 

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