Coke formation during the refining of heavy oils has attracted extensive attention as a result of the effects on the liquid yield, catalyst deactivation, and operating period. Polycyclic aromatic hydrocarbons (PAHs) generally have the strongest tendencies to form coke during the refining processes, which are considered as coke precursors. In this work, a vacuum residue was treated by thermal conversion and deep hydroprocessing. The feedstock and products were characterized by Fourier transform ion cyclotron resonance mass spectrometry. The detailed distributions of aromatic hydrocarbons of the products behaved with clear boundaries, which were described in limit lines. The slopes of the limit lines differed greatly between the two kinds of products, indicating different mechanisms for the growth of PAHs. Thermal conversion and deep hydroprocessing of model compounds were also conducted. Thermal conversion products of phenanthrene and pyrene proved that only condensation reactions occurred at a temperature of 450 °C and the aromatic rings were not ruptured. Deep hydroprocessing of pyrene revealed that the aromatic ring structures were cracked and then the smaller aromatic substrates combined together to form highly condensed aromatic rings. As a conclusion, the different mechanisms of coke precursor formation resulted in the different slopes of limit lines for thermal conversion and deep hydroprocessing products.

Ion suppression or matrix effect has been widely concerned because of its influence on the characterization of electrospray ionization mass spectrometry. A clear understanding about the ion suppression effect of basic nitrogen compounds in heavy oils is helpful to obtain comprehensive information on the compositions. In this work, the ion suppression effects of basic nitrogen compounds in a processed vacuum gas oil (VGO) and the isolated basic nitrogen fraction were studied under different ionization conditions. The most abundant compounds in the VGO and the isolated basic nitrogen fractions were in the form of N 1 and N 2 species. The influences of the concentrations of the basic nitrogen fractions and formic acids were investigated, which were key factors of ion suppression. The ion suppression effect of the isolated basic nitrogen fraction was evaluated by comparing the nitrogen content obtained by mass spectrometry and elemental analysis. Deviations of less than 30% of the nitrogen content were defined as less suppression conditions, while deviations of more than 30% were defined as more suppression conditions. The low basic nitrogen fraction content and high formic acid content were helpful to reduce the ion suppression effect. It was also found that N 1 species with a low double bond equivalent (such as cyclic amines) were more inclined to ionize under the more suppression condition.

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Hydrodesulfurization (HDS) is a widely used technology to reduce the sulfur content of heavy oils, and the HDS reactivities of sulfur compounds are closely related to their structures. In this work, the tandem mass spectra of sulfur-containing model compounds were measured by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with collision-induced dissociation. The influences of naphthenic rings, size of aromatic rings, number of heteroatoms, and number of alkyl side chains to the fragmentation behaviors of sulfur-containing model compounds were studied. Finally, the structure differences of sulfur compounds in heavy oils before and after hydrogenation were compared by tandem mass spectra of FT-ICR MS. The most refractory sulfur compounds in the heavy oil were those with fully conjugated aromatic rings attached with fewer and longer side chains. These results would be helpful for the development of a HDS catalyst/process for heavy oils. For example, developing a new HDS catalyst with high alkyl side-chain cracking activity may be conducive to HDS of heavy oils.

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