Production of mesophase pitch by supercritical fluid extraction: A study of the region of liquid-liquid equilibrium

Supercritical fluid (SCF) extraction is investigated for the production of mesophase pitch, the liquid-crystalline precursor for the manufacture of carbon fibers. The mesophase pitch is produced by fractionation of an isotropic petroleum pitch with supercritical toluene in a region of liquid-liquid equilibrium that exists at approximately 340{dollar}spcirc{dollar}C and 40 bar. A continuous-flow apparatus was used for measuring the liquid-liquid equilibria, and for producing significant quantitities of mesophase pitch fractions.; An experimental program was conducted to investigate the effect of operating conditions on the properties and yield of the mesophase pitch. The experiments were organized according to a central composite design in which temperature, solvent solubility parameter, and solvent-to-pitch (S/P) ratio were chosen as independent variables. A rich variety of fractions with significantly different softening points, viscosities, and mesophase contents were produced. Several fractions that are 100% mesophase each exhibited distinctly different properties. Thus, SCF extraction can be used to manipulate the mesophase pitch properties to produce a diversity of fractions from which carbon fibers and carbon matrices could be made.; The variation of the yield of mesophase pitch with the S/P ratio indicates that supercritical toluene is an antisolvent for petroleum pitch. These results were interpreted by using ternary phase diagrams in which softening point and toluene content of the phases are used as coordinates. Additional experiments at different S/P ratios demonstrated that the ternary phase diagrams are useful to understand the main features of the SCF extraction process and to estimate product yields and properties for conditions for which no measurements are available.; A thermodynamic model that uses an equation of state based on the Statistical Associating Fluid Theory was used to correlate the experimental phase equilibria. The feed pitch was represented by 21 aromatic pseudocomponents that were chosen by applying a Gaussian quadrature method to the molecular weight distribution of the feed pitch, which was determined by gel permeation chromatography. By using three adjustable parameters, the model correctly predicts the response of phase compositions and mesophase pitch molecular weights to changes in the operating conditions.