(48f) Supercritical Water Treatment of Fractions of Crude Oil: Quantification of the Products

The Arabian Heavy (AH) crude oil was fractionated into distillate (≤C₂₀) fraction and bottoms fraction. The distillate and the bottoms fraction of crude oil were treated with supercritical water (SCW) in a batch reactor setup. The gaseous and liquid phase products were analyzed using gas chromatography (GC-MS, GCxGC, FID, SCD). The change in structural composition of maltenes and asphaltenes in the bottoms fraction before and after supercritical water treatment was investigated using proton NMR, FTIR and Raman spectroscopy.

 The distillate fraction primarily consisted of n-alkanes (≤C₂₀) plus a minor fraction of alkyl aromatics and sulfur compounds. SCW treatment of the distillate fraction resulted in cracking of higher carbon number alkanes to lower carbon number alkanes, and production of gaseous species such as methane, ethane and propane.

 SCW treatment of the bottoms fraction caused much more dramatic changes. Much of the heavy components in the crude oil cracked to form compounds in the gasoline and diesel range which are observed by GCxGC. The higher carbon number aliphatics (≥ C₂₀) and long chain (≥ C₉) alkyl aromatic compounds in the bottoms fractions underwent cracking and produced C₆-C₁₈ alkanes and C₃-C₈ alkylbenzenes. Supercritical water treatment of the bottoms fraction cracked the heavy sulfur compounds, and resulted in increased recovery of lighter sulfur compounds such as alkylated benzothiophene and dibenzothiophenes. The SCW-treated bottoms also showed increased recovery of polycyclic aromatic hydrocarbons such as naphthalene. Despite being carried out at temperatures similar to those used in conventional thermal coking processes, the SCW process did not form significant amounts of coke.

 The maltenes and asphaltenes were separated from the bottoms fraction and the SCW-treated bottoms fraction. The proton NMR of the SCW-treated maltenes fraction showed a decrease in the protons corresponding to aliphatic CH₂ and CH₃ groups. There was also an increase in protons in single ring and polycyclic aromatic hydrocarbons. The proton NMR results support the observations made by the GCxGC measurements.

 The proton NMR, FTIR and Raman spectroscopy on untreated and SCW-treated asphaltenes showed changes in the structural composition of asphaltenes post-SCW treatment. The proton NMR experiments showed a decrease in the protons which correspond to aliphatic CH₂ and CH₃ groups. The decrease in aliphatic CH₂ and CH₃ functional groups could be due to scission of the long alkyl chains attached to the aromatic core structure of the asphaltenes. The proton NMR results also showed 49 % increase in the mono-aromatic protons and 1 % decrease in di-aromatic and poly-aromatic protons, which suggests formation of new aromatic rings and/or perhaps conversion of maltenes containing single aromatic rings into asphaltenes (e.g. by the cleavage of alkyl tails, affecting the solubility).

 FT-IR spectroscopy showed similar results to proton NMR, wherein an increase in aromatic and heteroatom content (O-H/N-H) was observed in the SCW-treated asphaltenes. Raman spectroscopy also showed evidence of chemical rearrangement and higher aromatic content in the SCW-treated asphaltenes when compared to asphaltenes from untreated bottoms fraction.

 Overall, the SCW treatment of the crude oil fractions resulted in cracking of heavier hydrocarbon components into more valuable lighter molecules, which could be used in transportation fuel or as chemicals.  

 Acknowledgement:  This research was supported through a Research Agreement with Saudi Aramco, a Founding Member of the MIT Energy Initiative.