Activation energy coke formation

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

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. 

It is conceivable, however, that deactivation of Tung s more active catalysts at lower temperatures is caused by coke formation which has accumulated on the more active catalyst at lower temperatures. The lack of return of activity upon regeneration by air remains to be explained. The jump of framework oxygen as the rate-controlling step in catalysis is not likely in view of the large apparent activation energy of dehydroxylation reported by Venuto (25), > 70-100 kcal/mole, a substantial... 

Grwin and Luss [ref- 37 simulated an adiabatic reactor for a first order irreversible reaction with an activation energy exceeding that for the coke formation and an exponential deactivation function in terms of the coke content The results of the... 

The most significant rate parameters are those calculated for Pi, P2 and P4. For all coked substrates, the corresponding A-factors and activation energies for these peaks are a similar magnitude. Any differences in A-factors between equivalent peaks for different substrates can be associated with differences in the amount of carbon per peak. Hence reflecting the propensity for coke formation. Similar activation energies for corresponding P2 and P4 indicate the oxidation reactivity of the final "hard coke is independent of the initial hydrocarbon source of the coke. 

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. 

Under these circumstances, Japan Energy Corporation has been conducted extensive research on the development of a new aromatization catalyst that exhibits high activity and excellent inhibition of coke formation. Based on this fundamental research, an LNA demonstration plant with a capacity of 2,250 BPSD has been operated in 1994. This paper describes the features of the LNA process and its performance. 

A comparison between the activation energies for coke formation from light aromatics (52-58 kJ/mol) and from asphaltenes and resins (34— 47 kJ/mol) shows that the reaction velocity of coke formation from light aromatics grows faster with increasing temperature than for coke formation from asphaltenes or resins. 

The formation of coke by pyrolysis increases strongly with temperatures as indicated in Fig. 4 which shows results from TGA-measurements on cracking of ethylene (10). With no catalyst, the activation energy was estimated to 458 kJ/mol. In presence of an alkali promoted support, (Zr02, 0.5% K), the coking was retarded probably by the promotion of alkali of the reaction ... 

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