NMR investigations on products from thermal decomposition of Kuwaiti vacuum residues

Abstract

Three vacuum residues (VR) obtained from Ratawi Burgan (RB), Lower Fars (LF) and Eocene (EOC) crude oils were thermally cracked at three different temperatures (T: 400°C, 415°C, 430°C) and reaction times (t: 30, 50, 60min). The chemical compositions of the non-gaseous products (i.e. cracked oil and pitch) were investigated in terms of changes in operating conditions and nature of feedstock. The findings show that under severe operating conditions, the type of feed affects the VR’s thermal stability (RB–VR<LF–VR≈EOC–VR). The sulfur content of the cracked oils (CO) varies between 3.6 and 4.4wt%, depending on the feedstock (RB–CO<EOC–CO<LF–CO). The pitch contains between 7.1wt% and 9.6wt% sulfur. SARA-separation of the VRs shows that the VRs consist mainly of aromatic hydrocarbons (95%). 1H and 13C NMR were used to obtain quantitative data (average structural parameters) about the structural composition of the VRs and the non-gaseous products. The VRs are mainly made of alkyl aromatics with more than three cata-condensed aromatic rings that decompose under heat into saturated hydrocarbons and aromatics with less aliphatic carbon. Under the chosen reaction conditions, the effect of feedstock and cracking severity on the chemical composition of the cracked oil is only minor. Throughout all experiments, the cracked oil contains three times more aliphatic carbon (≈62%) than aromatic carbon (≈21%). The aromatic moiety is mono- or di-aromatic. The pitch, on the other hand, shows a steady increase in aromatic carbon with increasing reaction severity (41–71wt%). Under the most severe cracking conditions, about 30% of the aromatic carbon is found in bridge head position of peri-condensed polyaromatic hydrocarbons (PAHs) (PAHRB<PAHLF≈PAHEOC).