Isolating Petroleum Pitch Oligomers via Semi-continuous Supercritical Extraction

Dense-gas/supercritical extraction (DGE/SCE) was used for the fractionation of a representative petroleum pitch, M-50, into its oligomeric constituents using toluene as the extractive solvent. A small, pilot-scale column was operated in the semi-continuous/semi-batch mode under a linear positive temperature gradient of 380-330 degrees C from the top to the bottom of the column and over a pressure range of 15-75 bar. This DGE column was used to produce high-purity monomer, dimer, and trimer fractions of M-50 suitable for use as molecular standards for petroleum pitches and for polycyclic aromatic hydrocarbon (PAH) oligomers. During the experimental runs, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) was used to monitor in real time the progress of the separation (and oligomeric purity) by performing rapid analyses of the molecular-weight distributions (MWDs) of the overhead fractions being collected. These real-time analyses provided us with the ability to fine-tune in situ the operating conditions according to the separation desired. The separation of petroleum pitches and other heavy fossil fuels into narrow molecular-weight fractions by semi-continuous DGE has proven to be an invaluable first step in the isolation and structural characterization of the individual species present in these multi-component, poorly defined systems. Furthermore, these molecular standards are also suitable for quantitative analysis.