Coke formation kinetic mechanism

Equations for the kinetic mechanisms of coke formation with the exponential form of the deactivation function are obtained by integrating eqs. (6)—(8) ... 

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. 

SAPO-34 has a high activity for the MTO reaction, but unfortunately there is fast deactivation caused by coke formation (79-81). Thus, detailed knowledge of the kinetics (82) and mechanism of the reaction (83-85) is important and necessary for process development. The MTO reaction serves as a case study illustrating the use of TEOM for measurement of mass changes during reactions with fast deactivation. The following problems were addressed by means of this technique ... 

Besides activity and selectivity, stability is crucial in catalysis applications. Catalyst deactivation can have a kinetic origin. For instance, deactivation might occur by a serial reaction mechanism in which an intermediate can undergo a reaction to form a substance that is a poison for the active catalyst sites. Frequently encountered examples are oligomerization and coke formation. 

An analysis of the rate of CO, CO2 and H2O evolution during TPO of industrial and laboratory coked cracking catalysts has provided information on the mechanism and energetics of coke combustion. The mechanism has been deduced from previously reported studies on amorphous carbon oxidation [8], while rate parameters have been calculated from non-linear regression simulations of the TPO spectra. The rate of water vapour formation has not been analysed due to re-adsorption expected to affect the apparent kinetics. "Soft" and "hard" coke have been identified in the spectra, and oxidation activation energies of each compared. A further outcome of this work is the proposal that coke deposition on cracking catalysts proceeds from "soft" to "hard" coke via a series of dehydrogenation or dehydration steps. 

Catalyst deactivation by coke formation can occur through a more or less reversible mechanism. We have applied a transient approach to model the reversible behavior of the deactivation, and to separate the deactivation from the main reaction kinetics. The deactivation of a Pt-Sn/AbOs catalyst was studied during propane dehydrogenation. The gas composition and temperature were varied during the experiments, which allowed us to model the deactivation by assuming one reversible and one irreversible type of coke. It was found that the deactivation increased with the propene concentration but was independent of the partial pressure of propane. Hydrogen decreased the deactivation rate and could even activate the catalyst by removing reversible coke. 

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. 

Catalyst may be useful for either activity or selectivity, or both. Another important issue is the catalyst stability. A catalyst with good stability will change very slowly over the course of time under the conditions of use. Indeed, it is only in theory that the catalyst remains unaltered during the reaction. Actual practice is far from this ideal, as the progressive loss of activity could be associated with coke formation, attack of poisons, loss of volatile agents, changes of crystalline structure, which causes a loss of mechanical strength. Due to the extreme importance of catalyst deactivation, the kinetic aspects of this phenomenon will be treated in a separate chapter. 

Table 2 shows the EPMA results for the Ni-E-SBA catalyst. Carbon contents of the samples are in good agreement with the results of the SEM image. Carbon contents increased and Cl/Ni ratio decreased with the reaction temperature. The predominant mechanisms for deactivation of the catalysts were thought to be the irreversible adsorption of HCl for low reaction temperature (300 C) and coke formation on the catalysts for high temperature (500"C), respectively. Results of kinetics for dechlorination of TCEa will be reported. 

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). 

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. 

The first two schemes correspond to cases when coke precursors arise from intermediates appearing before the slow surface step, while schemes (2c) and (2d) model the formation of blocking agents out of intermediates generated beyond the limiting step. Each of these mechanisms may bring to specific pecularities of the deactivation kinetics, depending on the power... 

It is important that the kinetic thermodynamic analysis, unHke a simple equilibrium thermodynamic analysis, of conjugate processes allows more correct conditions of the reversal of some channels of the stepwise trans formations to be obtained and new practically significant catalytic systems to be created, even though the mechanism of the catalytic action is not fuUy understood. We shall consider now some simple examples of this analysis of the processes of catalyst coking, involvement of Hght molecules (CO2, CFI4, etc.) into reactions with heavy parafSns, and so forth. 

The critical issues associated with manipulating this mechanism include volatile yield in the combustor (the distribution between volatile matter and char), devolatilization kinetics, and char oxidation kinetics. In the management of emissions formation (e.g., NO emissions), manipulation of specific mechanisms becomes important. Fuel particle size, heating rate, and combustor temperature influence the proportional distribution between volatile matter and char. The chemical structure of the fuel—various coals, coal waste, petroleum coke, wood waste. 

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