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Significance Statement
Increasing refining capacity and expanding the processible range of heavy crude oils are both important for the efficient use of oil resources. In modern petroleum refineries, the cracking of high molecular weight components is performed with the fluid catalytic cracking (FCC) process. However, the current fluid catalytic cracking process has been considered not to be able to directly refine polycyclic aromatic hydrocarbons (PAHs), which are one of the main components of the heavy crude oils, because of difficulties in activation of aromatic rings in an atmosphere without hydrogen. Therefore, for the efficient utilization of the heavy crude oils, it has been considered necessary to hydrogenate the PAHs with pressurized hydrogen atmosphere so as to convert them into naphthenes before they are supplied to the fluid catalytic cracking process. Our research objective in this study is to achieve direct conversion of the PAHs into monocyclic aromatic hydrocarbons (MAHs) in the fluid catalytic cracking process. For that purpose, we focused on a hydrogen transfer reaction that proceeds in the fluid catalytic cracking process. Hydrogen transfer reaction is a bimolecular reaction in which dehydrogenation of one molecule (hydrogen donor) and hydrogenation of the other (hydrogen acceptor) proceed simultaneously. Actually, this reaction has been suppressed by the recent design of the fluid catalytic cracking catalysts because it converts olefins into paraffins, which results in a lower octane number of the produced gasoline. On the other hand, we expected the hydrogen transfer reaction to contribute to the hydrogenation and decomposition of the PAHs in the fluid catalytic cracking process. In this study, we investigated the catalytic cracking of the PAHs using an fluid catalytic cracking catalyst with rare-earth ion exchanged USY zeolite, which is known to exhibit high hydrogen transfer activity. Reaction products analysis revealed that the yield of MAHs produced from the cracking of the 3-ring PAHs/n-hexadecane mixture was higher than that from the cracking of n-hexadecane alone. This result suggests that the 3-ring PAHs were highly reactive on the fluid catalytic cracking catalyst and they were converted into MAHs. Further investigation revealed that the conversion of the 3-ring PAHs were initiated by the hydrogen transfer reaction between the 3-ring PAHs (hydrogen acceptor) and n-hexadecane derivatives (hydrogen donor). Then, the saturated ring formed by the hydrogen transfer reaction was cracked and produced MAHs. From the results shown in this paper, we concluded that the hydrogen transfer reaction can sufficiently contribute to hydrogenate the aromatic rings of the 3-ring PAHs in the fluid catalytic cracking process. This result suggests the possibility for the direct conversion of the PAHs into MAHs in the fluid catalytic cracking process without hydrogen atmosphere, which may be achieved by controlling the hydrogen transfer activity of the fluid catalytic cracking catalysts.
Journal Reference
Fuel, Volume 161, 1 December 2015, Pages 207-214.
Iori Shimada, Kouhei Takizawa, Hiroshi Fukunaga, Nobuhide Takahashi, Toru Takatsuka
Materials and Chemical Engineering Course, Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan
Abstract
With the aim of enhancing oil refining processes based on fluid catalytic cracking (FCC), the catalytic cracking of polycyclic aromatic hydrocarbons (PAHs) was investigated using an fluid catalytic cracking catalyst consisting of a rare earth ion exchanged USY zeolite. In these trials, model PAHs were dissolved in n-hexadecane and were fed into a fixed bed microactivity test reactor operating at 516 °C. Reaction product analysis indicated very little cracking of the 2-ring PAH over the fluid catalytic cracking catalyst, while in contrast the 3-ring PAH was highly reactive, and was rapidly converted into monocyclic aromatic hydrocarbons, 2-ring PAHs and coke. Tests using fluid catalytic cracking catalysts with different rare earth loadings revealed that the loading amount has little effect on the conversion of the 3-ring PAH. In addition, catalysts containing USY zeolites with comparable unit cell sizes, and thus having comparable hydrogen transfer activities, exhibited similar catalytic activities for 3-ring PAH conversion, even though they contained different amounts of the rare earth metal oxide. This result suggests that the hydrogen transfer reaction plays an important role in 3-ring PAH conversion and that the main effect of rare earth loading is to maintain the hydrogen transfer activity of the catalyst by stabilizing the USY zeolite against steam deactivation. In summary, this study successfully demonstrated a potential fluid catalytic cracking process for converting PAHs into useful light fractions without the necessity of employing a pressurized hydrogen atmosphere.
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