Regeneration of coked catalysts via ozonation: Experimental study of diffusion–reaction mechanisms at pellet and reactor scales

Abstract

This work focuses on regeneration of coked catalysts using ozonation as an alternative process to typical coke combustion with oxygen. Different samples of coked ZSM-5 zeolites obtained during catalytic pyrolysis of polyethylene at 450 °C were exposed to an ozone enriched stream in a fixed-bed reactor. The influence of main operating parameters (time of exposure, temperature, O3 inlet concentration and gas volumetric flowrate) were studied by evaluating coke removal efficiency and by observing regeneration behavior both at pellet and reactor scales. These experimental results led to a better insight of the diffusion–reaction phenomena occurring during the process. Diffusion limitations were illustrated thanks to an innovative method, the analytical acquisition of carbon radial profiles on catalyst pellets, while the apprehension and quantification of competing reactivity (thermal, catalytic and oxidation) gave a better understanding of the ozonation process. Time of exposure and temperature were found to significantly affect carbon removal efficiency, while inlet ozone concentration and flowrate influence were respectively moderate and null in the investigated range. Similarly to combustion, coke ozonation is driven by a balance between diffusion and reactivity, leading to the appearance of a mix of known regeneration regimes depending on the operating conditions: homogeneous and shrinking-core.