More Benign and Effective SCF Solvent System for Producing Mesophase Pitch

摘要

The goal of this study was to investigate the use of more benign solvents, in particular both toluene-water mixtures and neat water, for producing mesophase pitches. Extraction operating conditions of 340 degrees C and 200 bar were selected such that toluene and water would form a homogeneous, dense supercritical phase. An overall solvent-to-pitch (S/P) ratio of 3.0 was selected for this work, with the solvent compositions being varied from 0 to 100 mol % water. The results of our exploratory work can be summarized as follows. Hot, compressed near critical water can be used to dissolve pitches, but only the lower molecular weight fractions of the pitch are readily dissolved. At moderate S/P ratios (e.g., 3:1), the water functions primarily as an antisolvent, as most of the feed pitch simply precipitates out as a bottom phase that does not differ significantly from the feed pitch in composition or molecular weight. To effectively fractionate the feed pitch with neat aqueous solvents, high S/P ratios (e.g., 10:1) would probably be required. On the other hand, the addition of small amounts of water (e.g., 5-10 wt %) to toluene solvent can be used to precipitate pitches with relatively high melting points that contain mesophase.