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Three vacuum residual oils (VR) derived from Ratawi Burgan (RB), Lower Fars (LF), and Eocene (EOC) crude oils were subjected to thermal cracking in a pilot plant, which simulates the Eureka process, to produce cracked distillate petroleum products and residual pitch. The cracking reaction was performed at 430 °C for 50 min. The chemical composition of the produced cracked petroleum products and byproduct pitch was studied to determine its relationship to the variations in the properties of the feedstock. Saturates, aromatics, resins and asphaltenes (SARA) analysis of the vacuum residues, cracked oils, and pitch show that the residues and pitch consist mainly of aromatic hydrocarbons (VR: 94 wt %; pitch: 99 wt %), while the oils themselves contain about 42 wt % saturated hydrocarbons (oil RB : 46 wt %; oil EOC : 44 wt %; oil LF : 36 wt %). 1 H and 13 C NMR revealed that the VRs consist predominantly of alkyl aromatics with di-, tri-(aromatics, resins), and polyaromatic rings (asphaltenes) that thermally decompose splitting the molecules into saturated lower molecular weight hydrocarbons and aromatics having lower aliphatic carbon attached to it. Regardless of the feed, all oils contain more aliphatic (∼62 wt %) than aromatic carbon (∼21 wt %). The cata-condensed aromatic moiety in the oil is triaromatic. The effect of feedstock on the chemical composition of the oil and pitch is most prominent for the aromatic and asphaltenic fractions.

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Two asphaltenes obtained from a residual oil before and after hydrotreatment were subjected to preparative GPC. Selected GPC fractions were analyzed by NMR and XRD to derive structural parameters to investigate whether there is a relationship between the molecular size of the asphaltene fraction and its structural features. Under the GPC experimental conditions (∼5 wt % THF solution), some eluting species are aggregates rather than monomeric molecules. Nevertheless, a correlation between the molecular size of the GPC fraction and its structure was found. The higher the molecular weight (MW), the more aromatic and condensed is the fraction. Moreover, the asphaltene fractions with the highest MW of both residues, before and after hydrotreating, show very similar structural features. This fact suggests that during hydrotreatment the low MW asphaltenes are converted to distillates, whereas the higher MW fractions (>6000 Da) stay unaltered and can cause severe coking in the downstream processes because of the highly refractory asphaltenes, which are still present. Structure presentations derived from NMR and XRD data demonstrate that the higher MW fractions (≈5250 Da) of both residues consist of clusters of 4-5 aromatic rings, which are interlinked by aliphatic chains of 9-11 carbon atoms.

The main objective of this work is to determine the effect of thermal cracking on asphaltene molecular structure by using nuclear magnetic resonance (NMR). The NMR is used to identify and quantify the structural parameters and the functional groups, which subsequently helped in deriving a hypothetical average structure for the asphaltene molecules. The NMR analysis revealed a number of valuable findings and developed a better understanding about the nature of structural changes that asphaltene molecules undergo during the thermal cracking of vacuum residues (VRs). For instance, the polyaromatic structure of asphaltene in the parent VRs consists of peri-condensed cores interlinked through aliphatic bridges. This peri-condensed polyaromatic core converts to cata-condensed polyaromatic as the reaction severity increases. The NMR analysis confirmed that the observed reduction in the average molecular weight of asphaltene molecules, as reaction severity increases, is mainly due to cracking of the saturated parts of the asphaltenes molecules, loss of sulfur and nitrogen located in the saturated parts, and shrinking in the aromatic core. Another observation is the growth in the percentages of aromatic carbon and aromatic carbon bearing hydrogen with the increase in cracking severity.