Influence of chemical reaction rate, diffusion and pore structure on the regeneration of a coked Al 2O 3-catalyst

Abstract

As a contribution to a better basic understanding and an accurate modelling of the regeneration of coked catalyst particles, the following investigations were done with pure Al 2O 3 as a model catalyst: (1) determination of the intrinsic and effective kinetics of coke burn-off; (2) characterisation of the catalyst’s morphology with respect to the influence of the carbon load on the surface area, porosity, pore diameter, and tortuosity; (3) measurement of radial coke profiles within partly regenerated particles; (4) numerical simulation of the regeneration within coked particles and comparison with experimental data of radial coke profiles and the time needed for a certain degree of burn-off. For the modelling of coke burn-off within single particles, the chemical reaction rate, pore diffusion, radial gradients of the O 2- and the carbon-concentration, and the influence of the carbon load on the porosity and tortuosity have to be considered. Only the resistances of external heat and mass transfer and of intraparticle heat conduction can be neglected, at least for particle sizes and temperatures of technical relevance for fixed beds (<5 mm, <700 °C). The measured and numerically simulated data according to this model presented in detail are in good agreement. The results show that temperatures above about 400 °C are needed to achieve regeneration within an acceptable timeframe. On the other hand, a temperature of more than about 500 °C will not anymore accelerate the burn-off, at least in case of fixed bed reactors with particles in the region of mm. This effect can be attributed to the increasing strength of pore diffusion resistance, and eventually the complete resistance is confined to the outer carbon-free shell.