Fractionation of pitches by molecular weight using continuous and semibatch dense-gas extraction

Abstract

A countercurrent, multistage, dense-gas extraction technique with reflux was investigated for the fractionation of carbonaceous pitches. Two modes of operation were investigated: continuous-stripping and semibatch operation. For example, continuous stripping with dense-gas toluene in the supercritical state, a positive column temperature gradient from 330 to 380 °C, and a pressure of 49 bar was used to strip the monomer and dimer species from an A-240 petroleum pitch feed, yielding a high molecular weight (mol wt) bottoms product rich in trimer and higher oligomers. Afterwards, semibatch operation was used with supercritical, dense-gas toluene, a temperature gradient of 330 to 380 °C, and pressures from 84 to 111 bar to fractionate the above bottoms product, yielding a trimer-rich overhead (average mol wt ( M w) = 800) and a tetramer and higher residue with M w ∼ 1000. Considering the two operations as a unit, a combined selectivity factor of ∼350 was obtained. Not only is this at least an order of magnitude better than what can in principle be accomplished by conventional, single-stage solvent extraction, but such extraction is inapplicable to our system because of the insolubility of the pitch fractions of interest in typical liquid solvents. Matrix-assisted, laser desorption/ionization time-of-flight mass spectrometry (MALDI) was used to verify that separation was indeed occurring by mol wt and to study the relationship between the M w, softening point, and C/H ratio of the fractions produced.