Isolating polycyclic aromatic hydrocarbon (PAH) oligomers via continuous, two-column supercritical extraction

A commercially produced petroleum pitch (M-50) consisting of PAH oligomers was fractionated via supercritical (SC)/dense-gas extraction, using two packed columns operating in series on a continuous basis. In the 1st column, toluene was used as the SC solvent, and a monomer- and dimer-rich cut was obtained as the top product. In the 2nd column, supercritical pentane was used to obtain 96+% pure monomer (as top product) and 96+% pure dimer (as bottom product) streams. All of the monomer and almost 90% of the dimer present in the original feed pitch were recovered in these two product streams. High (>10) solvent-to-feed ratios, positive column temperature gradients, and rectifying/stripping (vs. stripping only) operation were all found to significantly improve selectivities for a given oligomer. The above two-column setup was also used to isolate a trimer-rich cut by using SC toluene as the solvent for both columns.