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 otigomeric constituents. The DGE process was carried out in a semibatch mode using the solvent toluene (T-c = 318.6 degreesC, P, = 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 320degreesC and the column temperature increasing linearly up to the reflux condenser temperature of 360degreesC. 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 (MW, = 299, PDI = 1.10), a dimer-rich extract with a purity approaching 90% (MW, = 52 1, PDI = 1.07) was obtained, with the remaining residue (MW, = 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. (C) 2004 Elsevier B.V All rights reserved.