Fractionation of petroleum pitch by supercritical fluid extraction: Experimental phase behavior and analytical characterization.

摘要

This dissertation reports on the author's contributions to an ongoing research project in the Center for Advanced Engineering Fibers at Clemson University: the investigation of a supercritical fluid extraction process for the production of mesophase pitch. If this project is successful, the mesophase pitch will be used to produce carbon fibers with improved physical properties, including higher strength.; A continuous-flow apparatus which incorporates an equilibrium view cell was designed and constructed for two purposes: (1) to measure the phase behavior for mixtures of petroleum pitch and supercritical fluid solvents at temperatures and pressures to 673 K and 350 bar, respectively, and (2) to produce sufficient quantities of pitch fractions for subsequent analytical characterization and spinning into carbon fibers.; Vapor-liquid and liquid-liquid equilibrium phase compositions are reported for mixtures of supercritical toluene and two types of petroleum pitch: Ashland A-240 pitch and a heat-soaked pitch from Conoco, Inc. These measurements and analytical characterization by gel-permeation chromatography and vapor pressure osmometry indicate that supercritical fluid extraction is capable of fractionating petroleum pitch into a number of pitch fractions of different molecular weight. Although the vapor-liquid equilibrium region is effective for extracting the low-molecular weight species in pitch, the liquid-liquid equilibrium region is of more practical interest for the production of mesophase pitch. In this region, large amounts of the feed pitch can be extracted, and some of the produced fractions spontaneously form a high percentage of mesophase pitch.; Using a thermodynamic model developed expressly for petroleum pitch, the critical behavior of the toluene/Ashland A-240 pitch system has been calculated. Although results are only qualitative, they may be useful as a guide for future experimental work.; Finally, analytical characterization of the produced pitch fractions by gel-permeation chromatography, vapor pressure osmometry, elemental analysis, and proton and carbon-13 nuclear magnetic resonance spectroscopy are reported. The results of these analyses have been used to determine average molecular parameters and structures for each pitch sample.