Slow coke burning

A. Development of an Explicit Solution for the Kiln Equation for Slow Coke Burning... 

In treating the simultaneous burning of both fast and slow coke, it is convenient for computation purposes to reformulate the slow-coke kinetics in terms of a new effectiveness factor t)t. The total rate of slow coke burning dyjdt)j of Eq. (12) is set equal to r)j(dyjdt)i ... 

Fig. 19. Effect of fast coke on slow coke burning. All cases 1.0 wt. % original slow coke. Slow coke profiles are given for the top (a) and bottom (b) sections.

Thus, for the monolayer region of coke concentration, the intrinsic rate of burning of slow coke is given by... 

A third problem was that of fast coke. This coke also exhibited first-order intrinsic burning, but with a rate constant 17 times that of slow coke at 950°F (783 K). Again a fortunate simplification was found so that the kiln equations would not have to be solved for two kinds of coke burning... 

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.

At the same time as the basic work was being done on the kinetics and difFusivity effects in coke burning, the kinetics of the processes that determined the CO/CO2 ratio from slow coke was investigated by Weisz (1966). Studies were made of the cumulative CO2/CO ratios for individual, whole, spherical catalyst beads. The results, shown in Fig. 28, scattered very badly. 

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. 

The calculated influence of pore diffusion on the coke bum-off (Fig. 4) is also reflected by the measured carbon distribution over the particle cross section for catalyst samples, which were regenerated at different temperatures up to a defined bum-off degree of about 55%. Fig. 5 shows that a pronounced gradient of the carbon load over the particle cross section for a bum-off temperature of 530°C is developed. For a temperature of430°C no gradient is determined. The rate of the oxygen conversion is then so slow, that the diffusion in the pores has no influence and the coke is uniformly burned within the particle according to the (intrinsic) rate of the chemical reaction. Both results are consistent with the kinetic measurements und calculations (see Fig. 4). 

Coal- and coke-fired, semidry process kiln, 1850 tons/ day. High maximum temperature, long burning time, slow heating rate, quick to moderately quick cooling,... 

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