Canadian crudes: A comparative study of SARA fractions from a modified HPLC separation technique

In recent years, a worldwide reduction in economically recoverable conventional petroleum reserves has led to an increase in exploration and production activity in heavier crude oils. In Canada, up-graders have been required to deal with more accessible, but difficult to process, heavy oils and bitumen from oil sands. In order to optimize plant operating conditions and assess their impact on the environment, a thorough knowledge of the molecular structure and behaviour of the source petroleum is needed. The problems associated with hydro-processing of fractions rich in nitrogen is of particular concern. The approach applied here involves separation of a series of diversified Canadian crude oils (oil sands bitumens plus heavy and conventional oils) into asphaltenes and maltenes, followed by further fractionation of the maltene components by High Performance Liquid Chromatography (HPLC). This approach differs from conventional SARA (saturates, aromatics, resins, asphaltenes) separation in that multiple fractions are easily separated on the basis of polarity differences thereby providing more detailed information on component class distribution. The separated fractions are subjected to characterization by various analytical methods, including: gel permeation chromatography (GPC) for number average molecular weight determination, molecular parameter calculation using CHNS analyses in combination with H-1 and C-13 NMR spectroscopy and group analysis by peak deconvolution of X-ray photo-electron spectra (XPS). The bitumens comprise less saturates but more resins and asphaltenes than any of the other heavy oils tested. Conversely, the conventional crude is associated with the highest saturates content and the least amount of resins and asphaltenes. Yields of aromatic fractions from different sources all fall within a relatively narrow range. It is noteworthy that the SARA fractions from each oil produced relatively similar bulk property values. All of the resin fractions contained more than 40% of the total nitrogen, i.e., greater than the amounts contributed by the corresponding asphaltene fractions. For the resin sub-fractions relatively minor differences between molecular weights, atomic H/C ratios and aromaticity (C-ar) were observed. The substantial difference in the HPLC elution behaviour for these subfractions appears to be attributable to the asymmetric distribution of polar nitrogen compounds for material collected at longer elution times. This observation may allow selective removal of intractable nitrogen compounds, possibly leading to cost savings through improved catalyst utilization in a modified upgrading process.