Extracting Coal Liquids from Direct Coal Liquefaction Residue Using Subcritical Water

Efficient recovery of coal liquids from direct coal liquefaction residue (DCLR) is beneficial for improving the economics of the direct coal liquefaction process. An attempt was made to evaluate the possibility of extracting coal liquids from DCLR using subcritical water (SBCW). The properties of water are compared with those of typical organic solvents. With regard to the ability of dissolving/emulsifying organic components, SBCW compares favorably with some typical organic solvents under certain conditions. This is evidenced by the fact that the SBCW3 (320 °C/11.7 MPa) extraction yield is similar to the <i>n</i>-hexane extraction yield, although the SBCW1 (250 °C/5.2 MPa) and SBCW2 (300 °C/8.9–11.6 MPa) extraction yields are lower than the <i>n</i>-hexane extraction yields under comparable conditions. The recovery rate of coal liquids from DCLR by SBCW3 extraction can be higher than the maximum recovery rate by <i>n</i>-hexane or methanol extraction when the (SBCW3/DCLR)_mass is high enough. In comparison with <i>n</i>-hexane-extractable, SBCW-extractable contains more high-molecular-weight and heteroatom-containing components. The group composition balances of several SBCW extractions reveal that SBCW-extractable is mainly from the <i>n</i>-hexane-extractable fraction of the parent DCLR, with a small amount of components from the asphaltene-type materials. The solvent utilization index decreases with the increase of extraction yield, indicating that the overall solubility/emulsibility of coal liquids in SBCW3 decreases as the extraction proceeds. This implies that more and more high-molecular-weight and low-solubility/emulsibility components are extracted from DCLR with the increase of extraction yield. Similar phenomena are found when <i>n</i>-hexane and methanol are used as the extraction solvents. It is also found that the SBCW3 extraction yield can be higher than the 320 °C-pyrolysis extraction yield when the (SBCW3/DCLR)_mass is high enough