Coke burning rate

The coke burning rate is a function of temperatnre, oxygen partial pressure, carbon content of catalyst, and residence time. A typical relationship between these variables is shown in Figure 15.2. 

Here K is the intrinsic coke burning rate constant. 

Figure 7.26 Observed coke burning rates for a silica/alumina cracking catalyst with an initial coke content of 3.4 wt% big beads and fine powder. [From P.B. Weisz and R.D. Goodwin, Jr., J. Catal., 2, 397, with the permission of Academic Press, Inc., New York, NY, (1963).]...

Figure 5.16P Coke burning rates of silica-alumina cracking catalyst. Initial carbon content is 3.4 wt%. (Weisz and Goodwin 1966. Reprinted with permission from Journal of Catalysis. Copyright by Academic Press.)...

The rate of coke burning for coke deposited on a zeolite-containing catalyst has been reported to be first order with respect both to coke concentration and oxygen partial pressure (23) ... 

The hydrocarbon feed rate to the reactor also affects the burning kinetics in the regenerator. Increasing the reactor feed rate increases the coke production rate, which in turn requires that the air rate to the regenerator increase. Because the regenerator bed level is generally held constant, the air residence time in the dense phase decreases. This decrease increases the O2 content in the dilute phase and increases afterbum (Fig. 5). 

The hydrogen contained in coke bums at a higher rate than carbon. Hydrogen-burning rates are four to five times greater than carbon-burning rates. 

The rate of oxygen utilization is related to the intrinsic rate of carbon burning by the ratio of CO to CO2 produced in the burning reaction. This relation can be expressed in terms of a constant a that is defined as the moles of coke burned per mole of O2 consumed, and varies between 1 and 2. The relation between the two rates is given by... 

A variety of material could be used as the basis for cracking catalyst, including synthetic silica-alumina, natural clay, or silica-magnesia. If these materials did not contain significant amounts of metals such as chromium or platinum that catalyzed the burning of carbon, the burning rate of the coke is independent of the base as shown in Fig. 7. 

Fig. 6. Burning rate constant is independent of origin of coke. Five different modes of coking are represented by different symbols. From Weisz and Goodwin (1963).

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... 

R/Q)P = T Increase of burning rate in relation to coke consumption... 

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 ... 

Yields and Feed Rate at Maximum Recycle Subject to Coke Burn Limit Yields Are on Fresh Feed Basis... 

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. 

In our simple coke burning model, the gaseous reactant (oxygen) is assumed to be present uniformly throughout the catalyst particle at a constant concentration without diffusional resistance. The oxygen reacts with the solid reactant B, consisting of coke deposited on the catalyst according to the rate equation... 

The rate of coke burning is given in equation (7.28) and the value of the preexponential factor and activation energy are... 

Calculate the rate of coke burning from equation (7.41). 

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