Article pubs.acs.org/EF
Extracting Coal Liquids from Direct Coal Liquefaction Residue Using Subcritical Water Xingjia Jiang,†,‡,§ Hong Cui,∥ Muxin Liu,†,‡,§ Qiang Guo,§ Jian Xu,§ Jianli Yang,*,†,§ Yong Yang,*,†,§ and Yong-Wang Li†,§ †
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P.R. China University of Chinese Academy of Sciences, Beijing 100049, P.R. China § National Energy Center for Coal to Liquids, Synfuels China Co., Ltd., Beijing 101400, P.R. China ∥ Hawaii Natural Energy Institute, University of Hawaii at Manoa, Hawaii, Hawaii 96822, United States ‡
S Supporting Information *
ABSTRACT: 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 n-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 n-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 n-hexane or methanol extraction when the (SBCW3/DCLR)mass is high enough. In comparison with n-hexane-extractable, SBCW-extractable contains more high-molecular-weight and heteroatomcontaining components. The group composition balances of several SBCW extractions reveal that SBCW-extractable is mainly from the n-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-molecularweight and low-solubility/emulsibility components are extracted from DCLR with the increase of extraction yield. Similar phenomena are found when n-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. DCL process solvent.9,10 Hence, effective separation of coal liquids from DCLR is desirable. Coal liquids in DCLR are very complex mixtures, containing high-boiling-temperature alkanes, polycyclic aromatics, and heterocyclic compounds.10 Some traditional organic solvents,6,9,14,15 such as methanol, ethanol, n-hexane, toluene, benzene, and tetrahydrofuran (THF), as well as ionic liquids,16,17 have been studied for extracting coal liquids from DCLR. However, the toxicity, volatility, and high cost of solvent, as well as the complexity of separating extract from solvent, may limit their application in industry. Theoretically, water around the critical state may have properties equivalent to those of organic solvents to some extent, depending on water conditions. It is known that the properties of subcritical water (SBCW, liquid water with a temperature >100 °C and
