Kinetics of Coking

Consider a simple reaction A B with the conversion of adsorbed /4 into adsorbed S on a single site as the rate-determining step. The steps may be written  

Suppose now some component that will ultimately lead to coke is also adsorbed and competes for active sites  

but not Ca- This coke precursor is generally strongly adsorbed and not found in the gas phase, so that Cci cannot be referred to a measurable quantity in the gas phase. Then there are two possibilities, starting from Eq. S.3.b-6 to eliminate Cl from Eq. 5.3.b-4. The first is to write Eq. 5.3.b-6 as follows  

Since neither Cq or C, can be measured, some empirical correlation for Ca/C, has to be substituted into Eq. 5.3.b-7 to express the decline of o in terms of the deactivation. The ratio CaJC, could be replaced by some function of a measurable quantity. 

The first approach, leading to Eq. S.3.b-7, was followed in the early work of Johanson and Watson [16] and Rudershausenand Watson [17]. In thetominology of Szepe and Levenspiel [18], Eq. S.3.b-7 would correspond to a deaaivation that is not separable, but Eq. 5.3. b-9 to a separable rate equation. 

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Pt-Re Pt-Sn none Kinetics of coking during hydrocarbon reforming. 

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. 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

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... 

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. 

Are there other metal treatments beside alonizing (or aluminizing) that are beneficial Chromizing might be tested. Will any of these treatments minimize both the kinetics of coking and also the rate of surface corrosion ... 

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 kinetics of coking for the reforming process has been investigated in a gravimetric reactor. Coking after the initial start-up period obeys the following rate law ... 

For industrial catalytic processes, It is iBiportant to understand cataly.<%t deactivation and the role that coke plays. Most models assume an activity-coke relationship which is probably true when coke is deposited only on active sites and when the window of conditions Is kept small or constant. However, in practice significant differences occur in many of the variables including the nature and type of catalyst and feed. The present study was undertaken to elucidate some of these problems for the teformlng process by providing a better understanding of the kinetics of coking. 

Kinetics of Coke Combustion during Temperature-Programmed Oxidation of Deactivated Cracking Catalysts... 

The aging and combustion kinetics of coke deposited on an HZSM-5 zeolite-based catalyst in the MTG process have been studied. The kinetic study of coke combustion in air was carried out at 500-550°C in a differential scanning calorimeter, by following the evolution of the combustion products with on-line FTIR analysis. The results provide evidence for limitations on coke reactivity that can be attributed to the combined effects of several circumstances (e.g. bad oxygen-coke contact and heterogeneous distribution of coke within the zeolite crystal). The need is demonstrated for a thermal aging treatment which equilibrates reproducibly the coke prior to combustion. The aging of coke is also limited by a peculiar coke deposit in the microporous stmcture of the zeolite. 

In this work these two factors were studied with the aim of determining (1) the effects of the coke H/C ratio on combustion and kinetics and (2) the role of the porous stmcture of the HZSM-5 zeolite in limiting combustion. In this study, coke equilibration conditions for reproducible regeneration were sought and the kinetics of coke combustion were determined. Aging and combustion of coke were studied in a differential scanning calorimeter, where the gas-solid contact in a fixed-hed reproduces the operational conditions of the regeneration step in the industrial reactor. 

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. 

Gasification Kinetics of Coke Deposited on Silica-Alumina. Within the temperature range 1400 to 1600°F and in the presence of excess steam, the gasification reaction of coke deposited on the silica-alumina cracking catalyst closely followed first-order kinetics with respect to unreacted carbon (Figure 1). First-order rate constants were calculated from the slopes of these plots (Table III), and yielded an activation energy of 55.5 Kcal/mole. 

T Hattori, R L Burwell, Jr., Role of carbonaceous deposits in the hydrogenation of hydrocarbons on platinum catalysts. Journal ofPhysical Chemistry 83 241-249, 1979. C A. Querini, S C Fung, Temperature programmed oxidation technique kinetics of coke-02 reaction on supported metal catalysts. Applied Catalysts A 117 53-74, 1994. 

Determination of Oxidation Kinetics. The reaction rate between oxygen and coke depends upon the oxygen partial pressure, the composition, morphology and location of coke, and the catalytic effect of catalyst components and impurities. Therefore, the study of the kinetics of coke oxidation provides information regarding coke characteristics. Additionally, it is necessary to... 

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. 

A simulation model for the reaction-regeneration steps in the transformation of methanol into hydrocarbons has been proposed and used for predicting the behaviour of a laboratory fixed bed pseudoadiabatic reactor. Kinetic models for both the main reaction and deactivation have been used, which take into account the attenuating role of water on both the zero time kinetics and the deactivation by coke deposition. The kinetics of coke combustion and the relationship between activity and coke content have been used for the design of the regeneration. The activity-coke content relationship is different in the reaction and regeneration steps. 

A mathematic model of the decoking process would be a helpful tool to determine how to perform the process both rapidely and safely. Such a model should be based on experimental data of the kinetics of coke bum-off. In addition, mass and heat transfer have to be considered. Up to now, such models are based on several simplifications, e. g. on the assumption of external mass transfer control for the bum-off rate (2). Therefore, systematic investigations on the kinetics both of coke formation and of coke bum-off were done (3 - 5). The major results are presented in this paper. 

The kinetics of coke bum-off were determined by systematic experiments in different lab-scale reactors. The experiments and theoretical considerations clearly indicate that for carbon loads of 10 gc/lOOgcat and temperatures above 400°C the effective rate of carbon bum-off is strongly influenced by pore diffusion. The external gas-solid mass transfer can be neglected for temperatures below 800 C, i. e. for temperatures of industrial relevance (T<550°C ). Based on the experimental results, the technical process of the regeneration was modeled. 

The conversion of simple organic molecules (e.g. methanol, ethanol or ethylene) can also be monitored by the use of combined TG-DTA. For instance such an analysis, applied to ethylene conversion on the acid form of ZSM-5, enabled the transformation to be interpreted in terms of five different reaction steps [25]. Another example of thermal analysis application to the study of the development of a catalyzed reaction is the use of isothermal TG for investigating the kinetics of coke deposition in inner or external zeolitic sites and its subsequent removal by oxidation in air [25]. 

To prove that direct reduction, that is, the Boudouard reaction [Eq. (6.5.15)], comes to a standstill for T < 1000 °C, we have to inspect the kinetics of coke gasification in more detail. 

Intrinsic Kinetics of Coke Burn-Off The intrinsic kinetics of deactivation and regeneration of a reforming catalyst have been studied by Kern et al. (Kern, 2003 Kem and Jess, 2005 Ren et al., 2002). The main results are ... 

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