Group separation of crude oil by supercritical fluid chromatography using packed narrow bore columns, column switching, and backflushing

Determination of saturates, aromatics, and resins and asphaltenes in crude oil has been made using supercritical fluid chromatography (SFC) with a carbon dioxide mobile phase and flame ionization detection. The separation system consists of three columns. The first column separates resins and asphaltenes from saturates plus aromatics, and the other two columns separate the saturates from the aromatics. The saturates are eluted directly through the three columns, the aromatics are backflushed from the last two columns in the sequence, and the resins and asphaltenes are backflushed from the first column. The properties of three different systems — which respectively employ packed 0.25‐mm‐i.d. fused silica, 0.7‐mm‐i.d. glass‐lined steel, and 1‐mm‐i.d. steel columns — have been investigated. Column diameters of 0.5–0.7 mm are recommended to obtain the best flow rate compatibility with the detector. Good class separations were obtained on all three systems, but, due to practical considerations, the more rugged all‐steel columns were found to be more convenient. In order to improve column reproducibility and stability and increase column lifetime as compared to earlier SFC group separations, changes were made to the column packings. The current system consists of two cyano columns and one silver‐loaded cation exchanger. With standard mixtures and crude oils, close to 100% recoveries were obtained. The analysis time was approximately 15 min. Weight percents of aliphatics and aromatics were determined with response factors of 1.0 and those of the asphaltenes and resins with response factors of 0.7–0.8.