Fast coke burning

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. 

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... 

A major part of the fast coke is probably desorbed from the catalyst bed and burned in the gas phase. Even if none of the fast coke was desorbed, a calculation of the Thiele modulus tj for conditions in the plume burner and the top of the first zone of the kiln shows that rj is in the range 0.92-0.99. Thus, the fast coke can be assumed to bum without significant diffusion limitation. 

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 ... 

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. 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.

Fio. 21. Response to 20°F increase in temperature of catalyst when no fast coke is present = 0). Residual coke is present until 60 min, by which time the coke is burned away. 

Heat of reaction for combustion of coke to CO, Btu/lb dutff Heat of reaction for combustion of fast coke, Btu/lb AHx [axAHcoj + 2(1 - a,) AHJ/a k Rate constant for carbon burning, ft /(lb mole)(hr)... 

Van Deemter (1980) compared the kinetic constant fcc calculated from A and E obtained in the laboratory with the pseudo-kinetic constants k c from several commercial and pilot plants, and found that the values for k c are always lower than those corresponding to kc, as shown in Fig. 13. It appears, therefore, that the process of coke burning is principally controlled by gas interchange and mass transfer in the bubbling or turbulent bed. Fast... 

Fixed bed decoking involves time-dependent profiles of the oxygen concentration and the carbon load both within the particles (pore diffusion) and within the fixed bed (moving reaction zone). The reaction zone migrates through the reactor, which may lead to overheating of the catalyst, if the velocity of the zone is too fast. To model the coke burn-off process in the adiabatic fixed bed the so-called one-dimensional pseudo-homogeneous reactor model can be used. 

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. 

Photograph 7-81 Free-lime nest from coarse calcite particle in raw feed. Very low liquid phase. Coal- and coke-fired, dry-process kiln. Moderately high maximum temperature, long burning time, moderately fast heating rate, quick cooling rate. (S A6699)... 

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