Catalyst regeneration coke burning, kinetics

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

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. 

Modeling of the Regeneration of a Coked Fixed Bed Catalyst based on Kinetic Studies of Coke Burn-Off... 

Intrinsic Kinetics of Coke Burn-Off The intrinsic kinetics of deactivation and regeneration of a reforming catalyst have been studied by Kern et al. (Kern, 2003 Kem and Jess, 2005 Ren et al., 2002). The main results are ... 

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). 

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