Control of the Molecular Weight Distribution of Petroleum Pitches via Dense-Gas Extraction

摘要

Dense-gas extraction(DGE)was used to fractionate an isotropic petroleum pitch(number-average molecular weight M_n=516)into oligomeric cuts.A counter-current-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 deg 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 deg C.For isother mal operation,the molecular weight distribution of the bottom products con sisted primarily of dimer(M_n=508)and trimer(M_n=759)species,with pres sure 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(M_n=997).