Kinetics of Coke Formation

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. 

How do the amounts and types of coke deposited on the various metal surfaces vary as a function of time In the present investigation, the resulting coke was obtained during 120-min runs. In the future, shorter and longer runs are needed to determine the kinetics of coke formation and to determine whether one type of coke is a precursor for another type. Possibly both filament and needle cokes act to some extent as a filter for gas phase coke to form eventually amorphous or knobby coke in which metal-containing coke is eventually covered with metal-free 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. 

KEYWORDS catalyst deactivation, coke formation, kinetics of coke formation, diffusional limitations, chemical reactors subject to catalyst deactivation. 

The problem of the kinetics of coke formation is a very Important especially with the Increasing demand for the use of low steam to methane ratios [ 10]. Kinetic rate expressions for the coke formation need to be developed. These rate equations should give the rate of coke formation 1n terms of the partial pressure of the various components and not only 1n terms of the carbon deposition and time it should also take Into consideration pore blockage as well as active site coverage by coke. 

Although Pt and Cu supported on activated carbon catalysts exhibited promising catalytic properties in the hydrodechlorination of I2DCP [3], little work has been done on fundamental research relevant to these catalysts. For instance, adsorption data, which is an essential aspect in catalysis, are hardly available. Catalyst deactivation is a major problem that hampers the application of these catalysts in industry. The effect of coke formation on catalyst deactivation needs to be clarified and the kinetics of coke formation modeled before an industrial process can be designed. 

Columns 5, 6, and 7 represent Sieder-Tate correlation, the latter modified by Whitaker (27), and simulation with Dittus-Boelter heat transfer correlation (15), respectively. The coke formation reactions are not considered in the model since there is very little information on the kinetics of coke formation mechanism, A similar comparison was also made by Sundaram and Froment with main emphasis being given to two dimensional models (24). 

BJ McCoy, B Subramaniam. Continuous-mixture kinetics of coke formation from olefinic oligomers. AIChE J 41 317—323, 1995. 

The introduction of the kinetics of the formation of CO and CO2 into the model modifies the temperature equation (31) and the oxygen equation (33). The slow and fast coke burning equations are unchanged except for a change in the effectiveness factor tjt of Eq. (29) to tjtc given by Eq. (73). A new equation for the conversion of CO is introduced. 

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) ... 

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 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. 

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. 

Experimental Methods for the Determination of Coke Formation and Deactivation Kinetics of Heterogeneous Catalysts... 

To determine the deactivation kinetics by coke formation the knowledge of the stationary kinetics of the system is needed, unless all reaction conditions are independent of time on stream. 

We report die effect on the kinetics of coking and on the activity of the deactivated catalyst when a species with a different propensity to coke formation is added to the feed. Steamed REHY zeolite was used as the catalyst, and feeds containing various... 

Thus, before the rate of coke formation can build up to a steady level, the coke precursors must reach some suitable concentration. At this point, the most logical candidates for coke precursors are the / -resins. Plotting the amount of coke formed as a simple function of the quantity of / -resins produces the curve shown in Figure 5. The / -resins/coke data obtained at 800°, 825°, and 850°F (430°, 440°, and 450°C, respectively) lie approximately on the same curve, while the data obtained at 980°F (530°C) follow a separate curve. At relatively low levels of / -resin formation the coke concentration increases only slowly, but as the /3-resin concentration approaches 14-16%, the amount of coke formed rises rapidly and the two curves converge. In the pyrolysis runs where the reactions were terminated before the / -resin concentration had risen much above 10-12%, a substantial portion of the / -resin producing reaction follows first-order kinetics. However, if the pyrolysis reaction is allowed to continue, the concentration of / -resins levels off at about 16-17%, regardless of any further reaction, and the first-order relationship no longer... 

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. 

The present paper aims at reviewing progress in the modeling of coke formation and catalyst deactivation along the lines set in Table 1, although it stresses to a large extent the kinetic formulation, an area in which considerable progress has been achieved. 

In this work we report the kinetic behavior of a bifimctional catalyst based on a M/H-MFI zeolite with unique shape selectivity and active for enhancing gasoline octane number. As a test reaction the n-octane transformation was used. The catalyst showed to be subject to inhibition as well as deactivation effects other than the effects of coke formation. To test these effects, the presence of inhibitors and poisons in the feedstock, was also studied [9]. 

The nature of the coke and the kinetics and mechanism of coke formation and removal have been studied in some detail [3-7]. Minimisation of coking requires minimal coke formation and maximal coke removal - by gasification of carbon or of intermediates which can lead to carbon. 

H C Beimaert, J R Alleman, G B Marin. A fundamental kinetic model for the catalytic cracking of alkanes on a USY-zeolite in the presence of coke formation. Ind. Eng. Chem. Res. 40, 1337-1347,2001. 

Then, according to Langmulr-Hlnshelwood equilibria and kinetics, the rate of coke formation, r, would be ... 

The models proposed in the literature are usually developed for the prediction of the final constant rate of coke formation [16-18], However, the present model predicts the evolution of the coke deposition from the start to the end of the reaction. The simplest possible equation has been developed, based on previous models [22], A comparison and statistical discrimination of the different kinetic models will be presented in a future paper [25],... 

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