Separation of compounds from gasifier tars by extraction with supercritical carbon dioxide
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Coal gasification tars were extracted with supercritical carbon dioxide at different temperatures and pressures, and the properties of the extraction residues (pitches) were determined by elemental analysis, size-exclusion chromatography and simulated distillation. Extract fractions were analysed by supercritical fluid chromatography. The pitches generally had softening points near the temperature of extraction. Raising the extraction temperature at constant pressure reduces the solvent density, and hence reduces the solubility of tar constituents. Extraction is optimized by extracting at low flow rates, initially at low temperature (high density) to remove low molecular mass compounds, and then increasing the temperature to ensure good contact with molten pitch and increasing the pressure to increase the density. Contact with powdered pitch is preferable. In agreement with calculated solubilities, polycyclic aromatic hydrocarbons, especially naphthalene, fluorene and phenanthrene, are the principal constituents of the extracts. More polar compounds remain in the pitch.