Control of the molecular weight distribution of petroleum pitches via dense-gas extraction

Dense-gas extraction (DGE) was used to fractionate an isotropic petroleum pitch (number-average molecular weight M-n = 516) into oligomeric cuts. A countercurrent-flow packed column was used to effect the separation, with supercritical toluene being used as the dense-gas solvent and commercially available M-50 or A-240 pitch being used as the feed. Isothermal operation at 330, 350, and 380 degrees C was investigated, as well as operation with a linear positive temperature gradient (+Delta T), with the bottom of the column at 330 and the top at 380 degrees C. For isothermal operation, the molecular weight distribution of the bottom products consisted primarily of dimer (M-n = 508) and trimer (M-n = 759) species, with pressure changes of as little as 5 bar producing significant changes in their relative distribution, as observed by MALDI mass spectrometry. On the other hand, by operating with a +Delta T, we could produce a bottom product consisting primarily of trimers and tetramers (Mn = 997).