Dense-gas fractionation of mixtures of petroleum macromolecules

Dense-gas extraction (DGE) was used to fractionate a petroleum pitch feedstock of broad molecular weight distribution (MWD) into selected oligomeric constituents. The DGE process was carried out in a semibatch mode using the solvent toluene (Tc = 318.6 °C, Pc = 41.1 bar) and a pitch charge of 10-15 g. A packed column with reflux produced the equivalent of several equilibrium stages. The apparatus was operated at near and supercritical temperatures, with the stillpot at 320 °C and the column temperature increasing linearly up to the reflux condenser temperature of 360 °C. Operating pressures were determined by the conditions required to extract a given oligomer and varied from 44 to 84 bar. Using a monomer-rich petroleum pitch feed (MWn = 299, PDI = 1.10), a dimer-rich extract with a purity approaching 90% (MWn = 521, PDI = 1.07) was obtained, with the remaining residue (MWn = 883, PDI = 1.24) consisting of 95% trimer and higher oligomers. MALDI-TOF mass spectrometry was used to determine the MWD of the feed pitch and pitch fractions, with a unique matrix being used to enhance the response of higher MW species. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels.