Supercritical fluid extraction of the coal liquids was carried out using single solvent such as benzene, toluene or p-xylene and an equimolecular mixed solvent of benzene and toluene. The correlations between the extraction yield and extraction conditions, e. g. temperature and pressure, were investigated.
The increase in the extraction temperature resulted in an increase in the extraction yield, and so did the increase in the density of the solvent. The extraction pressure was an indirect factor which influenced the extraction yield through changing the density of the solvent. The following empirical equation concerning the extraction yield (x) was obtained as functions of reduced temperature (T/TC) and reduced density (d/dc) for each single solvent.
x=A (d/dc) +B (T/TC) +C...(1) where, A, B and C are empirical constants for each solvent. The extraction yields calculated from this equation were in good agreement with the experimental values.
The extraction yield of the coal liquids using mixed solvent could be calculated from the equation which was composed of the equations (1) for each single solvent.