Question
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What catalyst additive technologies are available to enhance propylene production in our FCC unit?
Jan-2022
Answers
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Bani Cipriano, W. R. Grace & Co, Bani.Cipriano@Grace.com
Increasing the yield of propylene from the FCC unit can create significant value for refiners. To raise the propylene yield from the FCC unit, refiners can inject additives containing ZSM-5 zeolite. These additives have physical properties (particle size distribution, ABD, attrition resistance) that match those of the base catalyst, yet, on account of their molecular structure, catalyse different reactions in the FCC unit vs Y-zeolite catalyst. ZSM-5 additives are available with varying levels of ZSM-5 zeolite content and therefore cracking activity. While it is common practice to compare additives from different suppliers based on ZSM-5 zeolite content, this does not always reflect true performance in the FCC unit, as the activity of ZSM-5 additive is highly dependent on the chemistry used to stabilise the zeolite.
Aside from zeolite Y, ZSM-5 is the only zeolite that has had commercial significance in FCC. In contrast to zeolite Y, which has 12-membered ring openings, ZSM-5 is a medium-pore zeolite consisting of 10-membered ring openings. The pore dimension of ZSM-5 is 5.4-5.6 Ã…, which is considerably smaller than the 7.4 Ã… openings of zeolite Y. Due to its smaller pore size, ZSM-5 exhibits size-exclusion shape selectivity, which allows it to crack only gasoline range molecules into valuable LPG olefins, namely propylene and butylene. Moreover, unlike zeolite Y, which has a large cavity (supercage) at the intersection of its pore openings, the intersections of the ZSM-5 pore channels do not form a large cavity. Thus, ZSM-5 exhibits transition-state shape selectivity due to its inability to accommodate the large transition state complexes associated with bimolecular reactions. Due to these restrictions, ZSM-5 has very low hydrogen transfer activity.
Using a ZSM-5 zeolite additive in the FCC unit therefore increases the yield of propylene at the expense of gasoline (butylene also increases, and a minor amount of ethylene is produced, although propylene is the predominant specie). Fundamental studies on cracking of model compounds1 reveal that as the gasoline olefin species become smaller (lower carbon number 6 and 5), these are more difficult to crack vs larger gasoline olefins (7 to 9 carbon), yet these smaller olefins when cracked yield higher selectivity towards propylene. Therefore, the key to reaching increasingly higher yields of propylene is to increase ZSM-5 cracking activity, either by increasing the additive level in the FCC unit inventory or increasing the cracking activity of the ZSM-5 additive. It is worth mentioning that use of ZSM-5 additives results in an increase in the octane of the FCC gasoline as well, delivering additional value to octane short refineries.
The traditional route to increasing the cracking activity of ZSM-5 zeolite additive is focused on increasing the content of ZSM-5 zeolite in the additive. However, as mentioned above, the chemistry and methods employed in the manufacturing of the ZSM-5 additive play a critical role in the stabilisation of the zeolite. As a result, two additives may have similar crystal content, yet their performance and effective zeolite activity is very different in the FCC unit. Next generation ZSM-5 additives exploit this concept and through novel chemistry seek to boost the activity of the zeolite itself, delivering step out cracking activity. This is the concept behind Zavanti technology, the newest generation of ZSM-5 additive available from W. R. Grace & Co. and intended for maximum propylene applications.
References
1 For more information about fundamental studies of ZSM-5 cracking of model compounds, the reader is referred to Buchanan, et. al. Journal of Catalysis. Vol 158, 279, 1996.
2 For more information about ZAVANTI™ technology, the newest generation of ZSM-5 additives, the reader is referred to Serban, et. al. PTQ Q3 2021, 43.Jan-2022
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