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/cm(3). 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. The main 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. (C) 2002 Elsevier Science Ltd. All rights reserved.