Control of mesophase pitch properties by supercritical fluid extraction

Supercritical fluid (SCF) extraction has been used to fractionate an isotropic petroleum pitch and produce a number of pitch fractions, many of which contain 100% mesophase. Toluene was used as the supercritical solvent, and experiments were carried out in a region of`liquid-liquid equilibrium that exists for mixtures of pitch and toluene above the critical pressure of toluene. A central composite design was used to investigate the effects of the operating variables of temperature, solubility parameter and solvent-to-pitch (SIP) ratio on the yield and softening point of the produced mesophase pitch fractions. Temperatures, pressures and SIP ratios from 310 to 360 degrees C, 45 to 155 bar and 2.5 to 4.0, respectively, were investigated. Using the developed statistical models, the effect of operating variables on softening point and mesophase yield can be quantitatively predicted. Chemical analyses of the mesophases by elemental analysis and DRIFT indicate that the extraction process can also be used to alter the chemical composition (e.g. the degree of alkyl substitution) of the produced mesophases. The flexibility of SCF extraction for processing pitches is illustrated by the fact that one can change the operating temperature and pressure and still maintain a constant mesophase yield or softening point. (C) 1998 Elsevier Science Ltd. All rights reserved.