Extraction of Taiheiyo coal with supercritical water–phenol mixtures

Taiheiyo Japanese sub-bituminous coal was extracted with supercritical water (SCW) and phenol mixtures at 673 K and at over fluid densities ranging from 0 to 0.5 g/cm3. The extraction yield with SCW was 0.55–0.60, but increased with increasing the ratio of phenol to water, showing a maximum of 0.7 at water–phenol ratio of 4.5:0.5 and then decreased to 0.50–0.55 for pure phenol. in products for the SCW–phenol extraction were bisphenol alkyl compounds, while these compounds could not be detected when SCW was used as the solvent. For elucidating the mechanism of SCW–phenol extractions, reactions between phenol and model compounds of hydrolysis products (formaldehyde, acetone, propionic acid, and 2-propanol) were conducted. In SCW, formaldehyde reacted with phenol to produce polymers, while neither acetone nor propionic acid reacted with phenol. The 2-propanol dehydrated to form propene, which reacted with phenol to form 2-isopropylphenol. The reaction rate increased with increasing water density. In SCW–phenol extraction of coal, phenol seems to inhibit reactions that lead to hydrolysis products or those that might cross-link to form the macromolecules. Phenol can be used with SCW to reduce retrograde reactions in residual coals.