Fractionation of petroleum pitch by supercritical fluid extraction: Experimental phase behavior and thermodynamic modeling.

摘要

Petroleum pitch is being considered as a raw material for the economical production of high-performance carbon fibers and composites. The raw pitch is generally treated to form an oriented mesophase which is used as the precursor for these materials. Existing treatment methods are not entirely successful in producing a mesophase with the desired composition and properties, so better treatment process is needed to improve the quality of the mesophase pitch.; This dissertation presents part of ongoing research that is investigating a supercritical fluid extraction process for producing a mesophase precursor pitch for carbon fibers. The process can be used to separate the parent pitch into several fractions having narrow molecular weight distributions. Hopefully, by selecting a particular fraction or blend of such fractions, one can produce a mesophase pitch of reproducible composition with improved chemical and physical properties. This could lead to significant improvements in the strength and quality of pitch-based carbon fibers and could expand markets for carbon composites.; A continuous-flow apparatus for measuring vapor-liquid and liquid-liquid phase equilibria is described that is capable of operation at pressures to 350 bar and temperatures to 675 K. A key feature of the apparatus is a nonvisual interface detection technique for operation with opaque phases. Phase equilibrium measurements are presented for mixtures of toluene with phenanthrene and with two petroleum pitches: Ashland A-240 pitch and a proprietary heat-soaked pitch obtained from Conoco. These measurements and analytical results for extracted fractions demonstrate that supercritical extraction can be used to separate pitch into fractions by molecular weight. High extraction yields and significant mesophase formation are observed in a region of liquid-liquid phase equilibrium, indicating this region is likely to be of practical interest for fractionating petroleum pitch.; Initial efforts toward the development of a thermodynamic model of the phase equilibria for these systems are also presented. The model uses generalized correlations for equation-of-state constants in terms of average molecular structure parameters and does reasonably well in fitting VLE data for the pitch/toluene system. The model should be useful for guiding future phase equilibrium measurements.