Coke formation rate constant

The authors used the Amoco model and compared it with a model developed by Lee and Groves (1985). They parameterized the Lee model to match the more complicated Amoco model by adjusting the heat of reaction and coke formation rate constant. They also studied non-linear multivariable control of process variables that showed much inteiactioa... 

As the catalyst ages, the rate constants decrease since the number of active sites is reduced by coke formation ... 

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. 

Note that as a first approximation the effect of hydrogen is not taken into account, which implies that the model will hold only for a limited range of hydrogen pressures. As a driving force for the reaction we use the gas-phase concentration, Cq, of the coke precursor Q, is the equilibrium constant of adsorption of Q on the catalyst surface. The rate constant for coke formation, kc, depends on the amount of coke present on the surface ... 

In Figure 8 we have gathered data for the extent of coke formation as a function of temperature for a series of experiments and model calculations with the CoMo/A O catalyst. Although the rate constants of coke formation increase with temperature, the net effect of VLE and accelerated kinetics again lead to a pronounced maximum. The arguments discussed above also hold here. 

The continuous decrease in the propane dehydrogenation activity, with constant HD formation rate can have different origins. Calculations indicate that diffusion limitations may play a role in the propane dehydrogenation, but not in the H2-D2 reaction, when the amount of coke in the pores of the catalyst is high. Different structure sensitivity for the two reactions might also contribute to this effect. Somorjai [8] showed that the H2-D2 reaction is structure sensitive. For the propane dehydrogenation, on the other hand, fiiloen et al. [9] found that only one Ft atom is necessary for the reaction to proceed. 

Two regions of coke formation are observed in the first one the amount of coke increases rapidly at the constant rate (r-) but it falls abruptly in the second region (r ] ... 

The paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

A patented process has been developed for the production of electrode binder pitch from petroleum-based materials. Carbon anodes produced from the petroleum-based pitch and coke have been used successfully on a commercial scale by the aluminum industry. One stage of the process involves the pyrolysis of a highly aromatic petroleum feedstock. To study the pyrolysis stage of the process a small, sealed tube reactor was used to pyrolyze samples of feedstock. The progress of the reaction is discussed in terms of the formation of condensed aromatic structures, defined by selective solvent extraction of the reaction product. The pyrolysis of the feedstock exhibits a temperature-dependent induction period followed by reaction sequences that can be described by first-order kinetics. Rate constants and activation energies are derived for the formation of condensed aromatic structures and coke. 

Kinetics of Coke Formation. On the basis of the x-ray diffraction data, the QI can be considered equivalent to coke and for the remainder of the discussion the term coke will be used in place of QI. The first-order rate equation was applied to the data for coke formation. The plots of these data in Figure 3 are similar to the curves produced with the / -resin results. A temperature-dependent induction period is obtained, followed by a reaction sequence that shows a reasonable fit with the first-order kinetic equation. Rate constants calculated from the linear portion of each curve are plotted in the Arrhenius equation in Figure 4. From the slope of the best straight line for the data points in Figure 4, the activation energy for coke formation is found to be 61 kcal. 

Equation (49) is applicable, for example, to coke growth through a polymerization process, the rate of which is independent of the degree of polymerization. Equation (50) is correct, e.g., if coke formation is limited by diffusion of reactants through the coke coverage. The rate constants k and v (or w) may be, in principle, interconnected if the disappearance of active sites is caused by formation of the coke precursor. However, we do not take into consideration any interconnection between these rate constants. 

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... 

The model proposed assumes reversible formation of amorphous coke (Ccm) and simultaneous formation of whiskers (Ccw) on the catalytic surface. The amount of monolayer coke at infinite time (Cc ax) depends on the operating conditions (e.g. temperature) and is governed by values of the rate constant for... 

The catalyst in each reactor section can be unloaded without mixing and its coke content determined by a highly sensitive TPO technique [4], using a modified Altamira temperature-programmed unit (Model AMI-1). In this modification, the gas exiting the reaction cell enters a methanator where CO2 and CO are converted to methane over a Ru catalyst with a constant supply of hydrogen. The methane formation rate is measured by an FID detector. 

Equations (2) and (3) were fit to experimental data using nonlinear regression to obtain values of the first-order reaction rate constants and the stoichiometric coefficients at each temperature. The conversion data from the 400°C thermal run and the best fit of the kinetic model are shown in Figure 1. It is interesting to note that at the time of incipient coke formation ( 60 minutes) the asphaltene and maltene data deviate from predicted first-order behavior. From this we concluded that both asphaltenes and maltenes were participating in secondary coke-forming reactions. Further separation of the maltenes into resins (polar aromatics) and oils confirmed this to be true and showed that it was the resin fraction that was involved in coke formation. 

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