Control of molecular weight distribution of petroleum pitches via multistage supercritical extraction

Packed-column supercritical extraction (SCE), followed by low-pressure gas stripping, was used to produce a dimer-rich pitch fraction from an oligomeric petroleum pitch, Marathon M-50, of broad molecular weight distribution (MWD). Both solvent-to-pitch ratios (S/P) >5 and a positive retrograde temperature gradient of 380–330 °C at 70 bar were found to reduce significantly the amount of trimer+ oligomers in the overhead product from the SCE column. This monomer- and dimer-rich overhead was subsequently stripped of monomer with gaseous toluene in a second packed column at 380 °C and 1.5 bar to obtain an 80+ mol% dimer product with an overall yield, based on the original feed, of 30%. To our knowledge, this is the first reported fractionation of a dimer-rich cut from a petroleum pitch with a demonstrably low level of both lower and higher mol wt impurities.

An oligomeric petroleum pitch was fractionated by means of a packed column operating under supercritical toluene conditions, followed by a low-pressure gas stripper. The effect of solvent-to-pitch ratio and temperature profile at supercritical operating conditions are shown to play an important role in controlling the extent of extraction. Dimer-rich pitches with purities up to 81 mol% have been produced in this continuous process at yields of up to 30% of the feed pitch.Figure optionsDownload as PowerPoint slide