Evaluation of Polycyclic Aromatic Hydrocarbon Removal from Hydrocracking Recycle Streams

Abstract

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 n-alkanes 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 be 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.

Applications of High Performance Liquid Chromatography in the Petroleomic Analysis of Crude Oil: A Mini-Review

“…Quantification was successful, but sulfur-containing species were found to interfere with measurement, limiting the method’s applicability to low-sulfur streams. Additional work by the authors of that study applied this methodology to evaluate the effectiveness of PAH removal from hydrocracker recycle streams …”

“…49 HPLC methods for PAH analysis have also been developed for use in monitoring industrial processes. 52,53 Nonaqueous reversed phase HPLC with diode array UV detection was used to measure larger PAH molecules, up to coronene (C24) in size, which are of concern in hydrocracker units. 52 Eight specific PAHs were targeted, and the chosen stationary phase and conditions were optimized based on the FT-ICR MS and UV-vis spectroscopic data of the HPLC-separated fractions.…”

“…Additional work by the authors of that study applied this methodology to evaluate the effectiveness of PAH removal from hydrocracker recycle streams. 53 A two-part paper takes an interesting approach to the grouptype characterization of oils, by developing a novel system to automate the separation and analysis of samples' saturates, aromatics, resins and asphaltenes fractions, 28 and the analysis of normal paraffins, iso-paraffins and naphthenes groups, which are traditionally done by GC. 1,54,55 The automated system shows reduced variability and improved stability when compared to the traditional alternative tests, and HPLC columns are used in place of open columns that offer less efficient separations.…”

Direct analysis at temporal and molecular level of deactivating coke species formed on zeolite catalysts with diverse pore topologies

“…21 In-situ techniques, from spectroscopy 22,23 to microscopic-tomographic 24,25 techniques, have contributed substantially to the understanding of the coke formation mechanisms. To date, in-situ techniques cannot resolve the molecular-level characterization, and ex-situ techniques, such as disaggregation and extraction 26 or extraction alone, 27,28 are widely used instead. These ex-situ methods enable liquid separation from the solid phase, classifying coke into soluble and insoluble fractions.…”

Practical Online Characterization of the Properties of Hydrocracking Bottom Oil via Near-Infrared Spectroscopy

“…HBO is obtained from the hydrocracking reaction unit (HCR), which is an essential process that converts heavy oil fractions with catalysts and excess hydrogen into a variety of valuable products [5]. Recently, the cost of feedstock accounts for over 90% of the total expense of ethylene production.…”