CONTROL OF MESOPHASE PITCH PROPERTIES BY SUPERCRITICAL-FLUID EXTRACTION

Supercritical fluid (SCF) extraction has been used to fractionate an i sotropic petroleum pitch and produce a number of pitch fractions, many of which contain 100% mesophase. Toluene was used as the supercritica l solvent, and experiments were carried out in a region of`liquid-liqu id 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, sol ubility parameter and solvent-to-pitch (SIP) ratio on the yield and so ftening 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 stat istical models, the effect of operating variables on softening point a nd mesophase yield can be quantitatively predicted. Chemical analyses of the mesophases by elemental analysis and DRIFT indicate that the ex traction 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 a nd still maintain a constant mesophase yield or softening point. (C) 1 998 Elsevier Science Ltd. All rights reserved.