Different Mechanisms of Coke Precursor Formation in Thermal Conversion and Deep Hydroprocessing of Vacuum Residue

Abstract

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.

Production of Non-Conventional Fuels by Catalytic Cracking of Scrap Tires Pyrolysis Oil

“…62 Furthermore, it is well established in the literature that coke deposition is affected by different properties of the catalyst: (i) porous structure, (ii) total acidity, and (iii) acidic strength of the acidic sites. 63 Hence, the higher coke yield has been obtained with CAT-3 that is about 0.7 and 1.8 wt % higher than the yield obtained for CAT-2 and CAT-1, respectively. Because coke deposition is boosted by the higher total acidity and the strength of the acidic sites, as well as by the highest capacity of the matrix to retain the heavy molecules found in the feedstock.…”

“…It is well-known that acidity is a critical property for the cracking of heavy fractions (LCO, HCO) into lighter ones (dry gas, LPG, naphtha), given the carbocationic reaction mechanism governing the chemical processes and their activity and selectivity . Furthermore, it is well established in the literature that coke deposition is affected by different properties of the catalyst: (i) porous structure, (ii) total acidity, and (iii) acidic strength of the acidic sites . Hence, the higher coke yield has been obtained with CAT-3 that is about 0.7 and 1.8 wt % higher than the yield obtained for CAT-2 and CAT-1, respectively.…”

Understanding bitumen partial upgrading through process modelling and simulation

“…As residue components are converted into lighter products, new asphaltenes are formed from polyaromatic moieties that progressively lose their alkyl chains, and/or undergo aromatization and condensation reactions; coke most likely arises from the successive condensation of these new asphaltenes. [9,[30][31][32] An excess of these hydrogen-deficient materials causes the thermally-cracked product to become unstable and incompatible for blending with other crudes in a refinery. [33,34] Refiners use various analytical measurements for liquid product stability, such as the peptization value (P-value), as guidance to adjust their thermal conversion limits.…”

Impact of Feedstock Properties on the Deactivation of a Vacuum Gas Oil Hydrocracking Catalyst

“…For example, aromatic feedstocks form more coke than aliphatic ones under similar processing conditions because polyaromatic structures are more easily formed from aromatics . Coking is more pronounced with condensed ring systems (e.g., naphthalene and anthracene) than with the corresponding linked systems (biphenyl and terphenyl). , Heterocycles produce more coke than hydrocarbon analogs . For alkyl-substituted aromatics, coke deposition increases with the length of the side chains rather than with the number of alkyl substitutions .…”

“…7 Coking is more pronounced with condensed ring systems (e.g., naphthalene and anthracene) than with the corresponding linked systems (biphenyl and terphenyl). 8,9 Heterocycles produce more coke than hydrocarbon analogs. 10 For alkyl-substituted aromatics, coke deposition increases with the length of the side chains rather than with the number of alkyl substitutions.…”