Extraction of coal tar pitch using a mixture of compressed CO₂ and toluene

The removal of low-molecular-weight components from commercial pitches is a critical step in the production of suitable precursors for isotropic carbon fiber formation. A unique extraction process employing a one-phase binary mixture of carbon dioxide and toluene has been developed to remove low-molecular-weight components from coal tar pitch, with an ultimate goal of increasing the softening point of the pitch. The mass fraction of coal tar pitch extracted was determined as a function of extraction temperature (25-75 °C), pressure (8.7-14.9 MPa), and extractive solvent composition (40-70 wt % toluene) using a factorial experimental design. As much as 44 wt % of the coal tar pitch was removed at these extraction conditions, and softening points greater than 250 °C were achieved. The separation is controlled by the temperature and composition of the extractant solvent. This compressed gas/organic solvent extraction process removes low-molecular-weight material from the coal tar pitch using significantly milder solvents and reduced temperatures relative to traditional extractive processes.