Coke oxidation reaction

Whatever the operating conditions, in the presence or absence of NH3, the CO yield increased with the reaction temperature and a 100% yield was obtained at 400°C (Fig. 5.13b). However, at 300°C, while 1-MN was fully transformed in the presence of NH3, only 10% carbon dioxide was produced, mainly because of the formation of oxygenated compounds retained in the zeolite pores ( coke )7 which was favoured by NH3 as revealed by elemental analysis showing a larger amount of carbon after the reaction. As for the coke oxidation reaction, it was shown that 1-MN oxidation into carbon dioxide required strong Brpnsted acid sites. In our case, the basic character of NH3 favours its adsorption at low temperatures on strong acid sites, which are able to transform 1-MN into carbon dioxide. Indeed, at 300°C the carbon dioxide yield was close to 20% in the absence of NH3 as opposed to 10% in the presence of NH3. When the reaction was carried out with NH3, only the weaker sites were able to work, but these sites seemed to be active only in the conversion of 1-MN into intermediate oxygenated compounds that remained adsorbed on the solid surface. 

Using a "home made" aneroid calorimeter, we have measured rates of production of heat and thence rates of oxidation of Athabasca bitumen under nearly isothermal conditions in the temperature range 155-320°C. Results of these kinetic measurements, supported by chemical analyses, mass balances, and fuel-energy relationships, indicate that there are two principal classes of oxidation reactions in the specified temperature region. At temperatures much lc er than 285°C, the principal reactions of oxygen with Athabasca bitumen lead to deposition of "fuel" or coke. At temperatures much higher than 285°C, the principal oxidation reactions lead to formation of carbon oxides and water. We have fitted an overall mathematical model (related to the factorial design of the experiments) to the kinetic results, and have also developed a "two reaction chemical model". 

We propose that the complicated dry oxidation of bitumen can be represented as the sum of contributions from two classes of oxidation reaction. One class of reactions is the partial oxidation that leads to deposition of coke and formation of "oxygenated bitumen", with very little production of carbon oxides and water. This class of reactions is concisely summarized by... 

At 316C inlet temperature, reaction rate controls, and the coke oxidizes slowly everywhere in the bed. There is no dis-cernable temperature wave and a significant concentration of oxygen leaves the bed throughout the bum. 

The oxidation of coke molecules begins by their hydrogen atoms with formation of oxygenated compounds which can undergo various reactions decarbonylation, decarboxylation, condensation. The greater the density of the acid sites the faster the oxidation of coke. Radical cations formed through reaction of molecular oxygen with coke molecules adsorbed on protonic sites would be intermediates in coke oxidation on acid zeolites. 

Coke, deposited on a catalyst, may be removed by one of several reactions oxidation, reaction with water to form carbon monoxide and hydrogen methanation, and the Boudouart reactions. 

The oxidation reactions of carbon and sulfur on hydroprocessing catalysts seem to be kinetically controlled by oxygen diffusion inside the catalyst porosity. Figure 3 shows the carbon and sulfur removal for Cat C which contains a very high amount of nickel and molybdenum, and an appreciable load of carbon. It is clear that the sulfur elimination occurs at higher temperatures than for the other catalysts and is simultaneous to carbon combustion. A tentative explanation of this phenomenon would be that the diffusion of oxygen in the microporosity is limited by coke deposit which needs to be at least partly removed to allow complete sulfur oxidation. 

Two aspects of these results will be discussed here, namely the low platinum dispersion and the presence of significant amounts of coke on alumina and zeolite catalysts after testing. The latter finding is prohahly due to a combination of test tenq>eratures below 2S0°C and the propensity for toluene to form coke. In any event, coke formation did not seem to inhibit the oxidation reaction to any significant extent. The low platinum diversions, confirmed by XRD measurements, indicate that the active reside outside the zeolite micropores on... 

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. 

As a rule, present-day catalysts are based on iron oxide Fe203, although, at the outset, the pronounced tendency of this compound to lead to elemental iron, which favors dealkylation and coke formation reactions, caused it to be discarded in favor of other systems such as ternary oxide mixes, such as ZnO, A1203 and CuO (IG Farbenl, or more recently the attempt to employ a combination of V2Os and A1,03, first alone, and then in the presence of alkali metals (Dow, CC1 Catalysts and Chemicals Inc a United Catalysts subsidiary). Gradually, the good activity of iron oxide was exploited by... 

Syngas can be obtained as equilibrium product of reaction (1). The main requirement for a syngas catalyst is therefore to activate methane and oxygen under conditions where the equilibrium composition is favorable, and where the catalyst is not active for total oxidation (reaction 2) or coke formation. 

In contrast to hydrogenation and oxidation reactions, much less is known about the ability of materials to effect the catalysis of hydrogasification reactions. Alkali carbonates, 1-10 wt % catalyze the hydrogasification of coals and cokes at 800°-900°C (6). The suggested mechanism is that adsorption of the alkalies by carbon prevents graphitization of the surface. Zinc and tin halides are effective hydrogasification catalysts. There is, however, little kinetic information on any of the catalyzed hydrogasification reactions. 

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... 

The deactivation of Mg-Al mixed oxides during the gas-phase self-condensation of acetone was studied. Main aldol condensation reaction and secondary coke-forming reactions take place on both basic and acidic sites. Although deactivation is caused by carbon deposition, it was found that coke formed on basic sites rather than on acidic sites is responsible for the activity lost. 

From the slope of the plots of Fig. 6, the initial deactivation rate was calculated as do = [- da/dt ]h). In Fig. 7, the do values obtained for all the samples are represented in open squares as a fimction of the carbon content measured after the catalytic runs (Table 2) clearly, it does not exist any correlation between do and the amount of coke. Initial deactivation is lower on AI2O3 (do = 0.14 h" ) than on MgO (do = 0.53 h ) in spite that alumina forms more coke during reaction and that the coke is more difficult to oxidize as compared to MgO (Table 2 and Fig. 4). These results show that neither the coke amount nor its polymerization degree account for the catalyst deactivation order observed in Fig. 6. A better explanation is obtained by considering the nature of the surface sites that are responsible for the formation of coke precursors on pure AI2O3 or MgO. Alumina contains Brbnsted (OH groups) and Lewis (metal... 

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