Coke burning temperature

The chemical processes occurring inside the blast furnace can be stated to start basically from the hot air coming into contact with the white-hot coke. The coke burns to form carbon dioxide. This reaction generates a very large quantity of heat, and it is this heat which maintains the high temperature necessary for the reduction process. As the gas is... 

Parameters in the model are listed in Table I. The flow, structural, and boundary conditions are known quantities. The frequency factor and activation energy for coke burning were the values determined by Weisz and Goodwin (1966) from the experiments discussed earlier, and the catalyst diffusivity D was measured directly in the laboratory. The value of a was determined from direct observations of the CO/CO2 ratio in each zone of the operating kiln. The remaining parameters are known quantities. Thus, there are no adjustable parameters available to tune the fitting of predicted values to observed data, for the fraction of coke remaining and for the vertical temperature versus distance from the top of the kiln. 

Figure 19 shows the slow coke profiles for the top and bottom zones obtained when 0.0, 0.1, and 0,2% fast coke is present. When no fast coke is present, the catalyst does not bum clean ( 0.2% residue carbon at kiln exit). This is consistent with the rapid rise in bum-off distance at 875 F (469°C) seen in Fig. 17. However, the presence of 0.1% fast coke gives an essentially clean catalyst, using only 0.6 of the bottom zone. Only a quarter of the bottom zone is needed when 0.2% fast coke is present. This improvement in bum-oflf distance is caused by the temperature boost obtained from the rapidly burning fast coke. This temperature boost is shown by the temperature curves in Fig. 20b. The temperature of the catalyst at the top of the upper bed has increased from 875°F (741 K) to 900°F (755 K). This is sufficient to make a large improvement in bum-off distance as shown by Fig. 17.

Fig. 20. Effect of fast coke on slow coke burning (1% slow coke). The profiles shown are for the top section (a) oxygen profile (b) temperature profile.

The introduction of the kinetics of the formation of CO and CO2 into the model modifies the temperature equation (31) and the oxygen equation (33). The slow and fast coke burning equations are unchanged except for a change in the effectiveness factor tjt of Eq. (29) to tjtc given by Eq. (73). A new equation for the conversion of CO is introduced. 

Catalyst stability with time on stream is an important characteristic. Acidic catalysts can be deactivated by basic poisons such as nitrogen. Carbonaceous species can build up on both metal and acid sites. These are the two prevalent mechanisms for catalyst deactivation. Other ways that a catalyst can be damaged, such as a temperature excursion, may be more likely to occur during the initial start up or during coke burning regenerations. Regeneration is discussed in the next section. 

The impact of temperature on the rate of combustion is exponential. The rate increases by a factor of 2.4 going from 1200 to 1300°F. However, the rate increases by factor of 7.2 going from 1200 to 1400°F. The impact of carbon concentration on catalyst is also nonlinear. The relative amount of residence time required to decrease carbon concentration by 0.1% increases by a factor of 10 from an initial concentration of 1.0-0.15 wt%. The impact of oxygen partial pressure is linear. The unit feed rate will also inflnence coke burning kinetics. As feed is increased, the coke production will increase requiring more air for combustion. Since the bed level is constant, the air residence time in the bed will decrease causing the O2 concentration in the dilute phase to increase. This will lead to afterbum, which is defined as the combustion of CO to CO2 in the dilute phase or in the cyclones of the regenerator. 

Coke burning during regeneration. Co F-T catalysts deactivate due to coke formation blocking the active sites. This coke can be burned off by an oxidative treatment. The addition of promoter elements may decrease the temperature of this oxidative treatment, preventing the possible clustering of supported cobalt particles. 

Water gas is often called blue gas because of the color of the flame when it is burned. It is produced by the reaction of steam on incandescent coal or coke at temperatures above 1000°C. 

A reliable simulation is important, for example, in planning coke burning from catalytic reformers, where excessively high temperatures can cause sintering of metal crystallites. 

Clearly, burning should be performed at a temperature high enough to give a sharp bum front, but alone this is still not sufficient to reduce the coke to an acceptable level in a short time, for two reasons. First, the rate of coke burning decreases markedly when the center of the burn front passes out of the bed and, second, coke consists of a mixture, some of whose components bum much more slowly than the others. Consequently, it is common to switch to more severe conditions for a period of secondary burn following the primary bum. 

HC ratio of 2,5-3,2, Aged catalyst was regenerated using normal regeneration procedure of coke burning, oxychlorination and calcination under controlled conditions of temperature and air supply with nitrogen flow. Test in next cycle was performed to see its regenerability. 

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. 

An advantage cited for the multizone TCC regenerator is that high temperatures can be used in the bottom zones without fear of the deactivating influences of steam or sulfur compounds, because hydrogen and sulfur in the coke burn more readily than carbon and are largely removed in the upper zones (241). 

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