Evaluation of Polycyclic Aromatic Hydrocarbon Removal from Hydrocracking Recycle Streams

Large polycyclic aromatic hydrocarbon (PAH) compounds with 7 or more rings, such as coronene, can be problematic in petroleum refining processes, where they build up over time and eventually enhance coke formation. In this work, large PAH molecules were characterized in a recycling stream returning the distillation bottoms of a commercial hydrocracking reactor back into the unit. High-performance liquid chromatography and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) methods were in good agreement with PAH compounds extracted with DMSO, which provided a selective way to determine PAHs with up to 10 aromatic rings. However, after Soxhlet extraction with pentane, PAH compounds with up to 14 aromatic rings were tentatively identified using FT-ICR MS, extending the naphthalene zigzag pattern. With the characterization method established, solvent extraction, solvent precipitation, and adsorption processes were tested to remove the problematic, large PAH compounds from the recycle stream. For each process, including different parameters, the PAH-rich and cleaned streams were analyzed. Adsorption on clay and adsorption on active carbon were the most effective processes in terms of minimizing the rejected fraction and for removing both types of aromatic compounds: unsubstituted (hard) and heavily alkylated (soft) PAHs. Alternatively, solvent precipitation with different nalkanes offered a choice to increase the selectivity for PAH through longer alkane solvents, but showed only a modest removal with PAH remaining in the cleaned streams. Soxhlet extraction was found to he superior in comparison to other solvent extraction processes, yielding the PAH with the highest double bond equivalent values, even though it remains unpractical for real-life hydrocracking operation cleanup. Consequently, the detailed characterization by FT-ICR MS suggests that adsorptive removal of PAH is the most effective and practical cleanup process for the studied hydrocracker recycle stream.