Several catalysts with increasing initial coke content were processed in the Microdowner unit in order to assess the activity and selectivity of coked catalyst. Tests were carried out at both constant operating conditions and changing catalyst-to-oil ratio to maintain conversion. It was found that coke on regenerated catalyst (CRC) has very little influence on the main product selectivity, while some effect on product quality was detected. CRC changed the hydrogen transfer to cracking relative rate as it lowered site density, resulting in a dramatic increase in the isobutene-to-isobutane ratio. Gasoline was more olefinic and less paraffinic, while sulfur content increased. LCO yield increased slightly, producing a distillate with better quality (fewer aromatics and more paraffins). When ZSM5 is used, it has been observed that propylene selectivity increases at constant conversion at a rate around 1 point of propylene per percent coke on catalyst. Apart from the cracking selectivity changes in the riser, the significant increase in catalyst circulation that is necessary to maintain conversion may have some beneficial effects for feed vaporization and reduced thermal cracking at the injection point.
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