(58c) Evolution of Coke in a Fluid Catalytic Cracking Process

Fluid catalytic cracking (FCC) is among the most important and widely used catalytic processes in petroleum refining to convert heavy petroleum fractions into gasoline, light olefins and cycle oils, typically using a zeolite-based catalyst. The contact time in the FCC reactor is only a few seconds in which the catalyst is covered by coke, and therefore, needs frequent regeneration. Recurrent swings between reaction and regeneration temperatures, coke deposition, high transport velocities through the reactor, deposition of metal contaminants, and steaming during regeneration can lead to severe structural defects and rapid catalyst deactivation.

Although the deactivation of FCC catalysts via coke deposition has been the subject of interest since the advent of the process, there is still a lack of knowledge on the contributions of the thermal, catalytic and metal-mediated mechanisms to the overall level of coke formation. With its short residence times, rapid deactivation process, catalyst age distribution in the equilibrium catalyst (E-Cat) and variations in the properties of feedstock, it is very difficult to scale down the commercial FCC process. In addition, there are limited studies the couple the analysis of E-Cat or zeolite catalysts with catalyst regeneration.

Transformation of an industrial zeolite-based fluid catalytic cracking (FCC) catalyst and its coke deposits during regeneration following FCC reactions are investigated. To obtain coked samples, the catalyst is used for the cracking of a representative refinery stream (a 1:1 weight basis mixture of light cycle oil and the heavy gas oil). The coked catalysts are regenerated on stream, between reaction cycles, in a regenerator maintained at 700 °C. Coked samples after 1, 22 and 45 reaction cycles (FCC-1, FCC-22 and FCC-45) are collected and analyzed for the present study. Our studies indicate that the higher acid site density of FCC-fresh, compared to the coked samples, resulted in a higher coke content in the first reaction cycle. The presence of coke drastically decreased the micropore area, micropore volume and total acidity of the coked catalysts. With successive reaction cycles, the total amount of coke decreases due to the loss in catalyst acidity and changes in the catalyst morphology. More aromatic coke is formed on FCC-22 and FCC-45, and the relative amount of surface carbon is significantly higher compared to FCC-1. Furthermore, severe catalyst agglomeration is observed as the catalyst is subjected to an increased number of reaction cycles. ²⁷Al MAS NMR spectroscopy demonstrates that the presence of coke induced quadrupolar shifts and severe distortion of the Al species in the coked samples. In-situ Raman spectroscopy shows that an increased number of reaction cycles and thermal ageing in an inert atmosphere alter the structural order of the graphitic species in the coke, making it difficult to oxidize. As a result, the combustion of thermally aged coked occurs at a higher temperature compared to un-aged coke. Most of the catalyst deactivation takes place within one reaction cycle and this loss contributes towards the overall deactivation of the catalyst. Overall, the present study provides an understanding of the evolution of coke species during reaction and regeneration in a FCC process. and gives insight towards improved process designs to minimize catalyst deactivation during the regeneration process.