Coking kinetics

In treating the simultaneous burning of both fast and slow coke, it is convenient for computation purposes to reformulate the slow-coke kinetics in terms of a new effectiveness factor t)t. The total rate of slow coke burning dyjdt)j of Eq. (12) is set equal to r)j(dyjdt)i ... 

IN SITU COKING KINETICS OBTAINED FROM A NEW FLOW THROUGH MICROBALANCE AND REACTION KINETICS MONITORED BY GC... 

Coking Kinetics. These results are described elsewhere (7) in more detail. Nevertheless, for the sake of completeness, a summary is provided here. 

Under these conditions, the evaluation of ki, k2, and M from coking kinetics experiments in the presence of fast-coking species will allow the estimation of these parameters, i.e, the determination of coking kinetics, for the actual feed alone. 

We use the protocol to determine Ri, r, and (f). The majority of the intrinsic coking kinetic data derive from the vibrational microbalance operated in differential mode. In a given microbalance experiment, both the temperature and hydrocarbon pressures remain essentially constant. In this environment, the only quantity that varies significantly with t is Ck. Then... 

Our aim is to predict C, t, r) and C (r, t) from measured or predicted C, . Note that Ck at the end of catalyst life is an initial condition for modeling and control of the catalyst regeneration process. In developing the coking kinetic model for nC reforming, the data lead us to consider that coke can deposit on both active sites and on already-coked sites. This consideration, as will be seen, gives simple explicit expressions for (j) as a function of Ck or t. 

These results, together with those in [3, 6, 7], indicate that coke in paraffin reforming derives mainly from the C5N intermediates. We next use readily available MCP as the model compound for C5N to study coking kinetics. 

The model derived from the above procedure compares quite favorably with experiment, see Fig. 10. We are now ready to combine the model with the coking kinetic model to predict the observed changes in the reforming rate constants during the lifetime of the catalyst. 

The amount of coke that is being deposited on a catalyst has been traditionally followed with conventional microbalances. However, due to the inherent limitations of this equipment, in which it is almost impossible to avoid feed by-pass effect and diffusional effects, this technique has not been very useful to determine coking kinetics as a function of feed composition. A recycle electrobalance reactor has been designed to avoid this undesirable effect . The unit was designed to operate up to 500°C and at atmospheric pressure. 

The kinetic parameters , , A and are used with the modeled thermal history of the source rocks to compute hydrocarbon output, using Eqs. 6.8 and 6.9, and to estimate the quantity of hydrocarbons expelled from the source rocks. The relative contributions of oil, gas, and coke to the total output of hydrocarbons are computed by using the standard three-fraction (oU, gas, and coke) and five-fraction (Cj, C -Cj, Cg-C, C15+, and coke) kinetic models for kerogen types I, II, and III (Tissot et al. 1987 Espitalie et al. 1988). 

Table 1. Influence of the reaction temperature on the coking kinetic parameters.

As illustrated earlier for poisoning, expressions for G and Cp can be derived for the rate of change of y with time if the elementary steps leading to the formation of the coke precursor and the reaction products are given. Thus, Eqs. 5.93 and 5.96 form the basis for the intrinsic coking kinetics. It will become clear why Eq. 5.93 is necessary when the influence of coking on effective diffusivity is considered. 

The coking kinetics for the rate at which the active sites disappear follow directly from the multilayer model. This rate, Rp, is simply the rate of monolayer formation. Thus, we have ... 

We carried out coking kinetic experiments on the Pt-Sn catalyst under different gas compositions and at the temperature of 575 °C (see Table 2.5) (Li et al., 2011b). The catalyst was subjected to coking for 80 min. The amounts of the two types of coke (on metal and on support) were both increasing approximately linearly with time on stream. Therefore, the coking rates could be estimated by averaging coke weights. 

Coke Kinetic coke (produced by reaction scheme) Metal coke (produced by metal activity on catalyst)... 

Coking kinetics has been the subject of various researches reported in the literature, with special emphasis on the mechanism of coke formation, interconversion of the solubility class components during conversion, role of these components in coke formation, influence of structural properties on coking rate and yields, development of correlations, among others. 

Schucker, R.C. 1983. Thermogravimetric determination of the coking kinetics of Arab heavy vacuum residuum. Ind. Eng. Chem. Proc. Des. Dev. 22 615-619. 

Trejo, F., Rana, M.S., Ancheyta, J. 2010. Thermogravimetric determination of coke from asphaltenes, resins and sediments and coking kinetics of heavy crude asphaltenes. Catal. Today 150 272-278. 

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