Supercritical fractionation of petroleum pitches: Experiment and prediction

Previous work at Clemson has shown that supercritical extraction (SCE) can be used to produce liquid crystalline (i.e., mesophase) pitches of excellent quality for use as starting materials for high-performance carbon products. However, to date the full potential of such extraction processes for manipulating the molecular composition of pitches for a desired end use has yet to be realized. The goals of this project were twofold: (1) to develop equations of state (EoSs) applicable to pitch-solvent systems and to the multicomponent pitch mixtures themselves; (2) to investigate the potential of stagewise SCE for producing fractions suitable for analytical characterization studies.; A thermodynamic model that incorporates the SAFT EoS, the MWD of the feed pitch, and mathematically generated pseudocomponents was developed for predicting phase compositions and extraction yields for the SCE process. Our model uses two binary parameters, which were determined by fitting a limited set of isothermal LLE data for pitch-solvent systems. SAFT was subsequently used with these parameters to successfully predict solvent-phase compositions at other extraction conditions.; As originally developed, SAFT cannot predict the existence of mesophase. Thus, in collaboration with Hurt and co-workers at Brown University, we created a modification of SAFT for these liquid crystalline pitches (i.e., SAFT-LC). Using only three binary parameters, SAFT-LC was successfully used to model both the SCE step, where solvent-pitch interactions dominate, and the mesophase formation step in the dried pitch product, where pitch-pitch interactions are significant.; A stagewise fractionation technique employing sequential, single-stage SCE was used to fractionate the heaviest portion of a petroleum pitch by operating at higher pressures in the LLE region. The pressure was sequentially reduced in each stage so as to precipitate out ∼5 wt % of the feed pitch in each stage. Five fractions comprising the heaviest 28.8 wt % of the feed pitch were obtained. Although analytical characterization (e.g., C/H ratio, softening point, and DRIFTS) indicated that the pitches were being separated according to molecular weight, more quantitative molecular information proved difficult to obtain, even after the heaviest three fractions were hydrogenated by Birch reduction so that GPC could be used.