Coke burning temperature dependency

Weisz (2) carried experiments on silica- alumina beads (many times the size of an average FCC particle). Heobservedthattheintrinsic cokebuming rate was independent of the coke composition and the catalyst characteristics but dependent on initial coke level and the diffusivity. Weisz (3) inanotherstudy, found that the CO /CO ratio during intrinsic coke burning is only a function of temperature. He also observed that this ratio i s affected by the presence oftrace metals like iron and nickel etc. Even though this study was elaborate, it was limited to only silica-alumina catalysts in the form of beads. 

The sulfate (Equation 6) and sulfite (Equation 5) can be reduced by numerous substances. Fluidized coke is particularly good for this application because it is relatively inexpensive and has a low ash content. Depending on the sulfate to carbonate ratio, about 1.5-2 lbs of coke are consumed per pound of sulfur reduced. This includes coke consumed in the reaction and coke burned to cover heat of reaction, heat required to increase salt temperature to reducing temperature, and heat losses from the system. The reaction rate increased by about 2 to 3 for each 50 °C temperature rise reduction times of 170, 30,15, and 4 min were observed at 700, 800, 875, and 950°C, respectively. Therefore, the reduction reaction is carried out at about 850°C or above. 

The coke deposited on the metal particles, or in the proximity, burns at lower temperature than the coke deposited on the acid sites. This is due to the catalytic effect of the metal (platinum) for the coke combustion reaction. Therefore, depending upon catalyst and reaction conditions, the TPO profile displays two peaks. The low temperature peak, between 250 and 400°C approximately, corresponds to the coke deposited on the metallic phase, and the coke burning above this temperature corresponds to the coke on the acid support (101). Figure 12 shows TPO profiles, that corresponds to unsulfided Pt and Pt-Re catalysts (102). The first peak is evident in both cases. However, the definition of this peak decreases when the total amount of coke increases (102,103). The regeneration of these catalysts at low temperature, for example 350°C, eliminates the small amount of coke that is deposited on the metal, and allows the recovery of the metal activity. 

The high temperature necessary for pyrolysis is obtained by burning fuel in excess air in a combustion chamber. Natural gas is still the fuel of choice, but other gases, e.g., coke oven gases or vaporized liquid gas, are occasionally used. Various oils including the feedstock are occasionally be used as fuel for economic reasons. Special burners, depending on the type of fuel, are used to obtain fast and complete combustion. 

The bed temperature in the burner is of the order of 590 to 650°C (1095 to 1200°F), and any excess coke that is not removed as part of the burning is periodically removed from the burner. The coke yield from the process may be as little as 1.2 or as high as 1.7 times the carbon residue of the feedstock. As with delayed coking, the fluid coking process is capable of producing liquid products with substantially lower sulfur contents than the feedstock (Table 7-14), but part of the sulfur in the feedstock is concentrated in the coke. There is elimination of sulfur into the gaseous products but uses for the coke depend very much upon the amount of feedstock sulfur. 

Particle Temperature Overshoot. The temperature of the burning char particles will run hotter than that of the bed by amounts that depend upon particle size, reactivity, bed temperature. It is determined in part by the heat released at the particle surface due to reaction and in part to the additional heat released by carbon monoxide oxidation near the particle surface (54-58). Measurements for 1.8 to 3.2 millimeter size coke particles burning in a fluidized band of sand at 1173 K increased from the bed temperature at low oxygen concentrations to values 150 to 200 K above the bed temperature for oxygen concentrations approaching that of air (72). Estimation of this temperature rise is important for purposes of evaluating the NO/C reaction and also for prediction of the burnout times of fines. 

Figure 1 shows the DTA curve obtained when a sample of diesel soot is burned in the absence of a catalyst. With increasing the temperature, four exothermic peaks appear. These peaks can be attributed to the combustion of different types of hydrocarbons constituting the soot (11). Indeed, it is known that a real soot consists of a volatile fraction, which is more active than a carbonaceous solid fraction. The composition of the volatile fraction can also vary depending on the quality of fuel and the engine s mode of operation. In some cases, aromatic, oxygenated and paraffinic compounds can be present, as well as residual coke of the lubricant (12). 

The process is cyclic. The feedstock and C recycle are preheated to 600 C and sent to the catalyst bed, forming butadiene, butenes, a number of gaseous by-products and coke. After a reaction period of 5 Id 10 min. depending on the number of reactors in the unit, the temperature drops by-15 to 20°C. Regeneration is then carried out, lasting 5 to 10 min. The reactor is first purged with steam, and air at 600 C is then introduced to burn the carbon deposits formed. The heat liberated raises the temperature of the... 

Isobutane Alkylation. The deactivation of solid acid catalysts due to coke deposition is the cause of not having as yet, a commercially available process for isobutane alkylation with C4 olefins, using solid acid catalysts. The coke on these catalysts have been characterized with TPO analyses . The TPO profiles on zeolites used in this reaction, displayed two well defined burning zones. One peak below 300°C, and the other at high temperatures. The relative size of these peaks depends on the zeolite and the reaction temperature. In the case of the mordenite, the first peak was the most important, and in the case of the Y-zeolite, at 50°C or... 

Fig. 61. Dependence of minimum rate of advance of burning front on thickness of bed and on oil concentration in the bed at reservoir temperature of 260°C. Fuel/coke concentration, kg/m3 1-32, 2-24, 3-20, 4-19.2, 5-18.4.

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