Extraction of Huadian oil shale with water in sub- and supercritical states

Water extraction experiments were carried out in a semi-continuous operation apparatus with the substrate being Huadian oil shale from Jilin Province of China. Both non-isothermal and isothermal extraction techniques were employed. The effects of temperature and pressure were investigated in terms of degree of conversion, extract as well as gas yield and formation rate, and the compositions of extract and gas. For non-isothermal extraction, temperature varied from ambient value to 500 °C and pressure stayed either at 15 or 30 MPa. Isothermal extraction experiments were carried out at four levels of temperature from subcritical to supercritical conditions and two supercritical pressures of 25 and 30 MPa. The results indicated that the extract was obtained mainly in the temperature range between 300 °C and 500 °C and gas formation was only observed at temperatures higher than 350 °C. A maximum formation rate exists for extract and gas at about 390 °C. The major extract component is asphaltene and the major gas formed is CO2for non-isothermal extraction. The large increase in the formation rate of H2and CH4at high temperature results in a secondary increase of gas formation rate starting at 460 °C. The isothermal extractions show that a conversion plateau exists when temperature is over 400 °C where a maximum extract yield was also found. The experimental findings were explained well in terms of the variation of hydrocarbon solubility in water and the degree of pyrolysis of oil shale. Water extraction shows a higher conversion, slightly lower extract yield, more oil content than toluene extraction. The isothermal results re-establish that non-isothermal extraction technique is a useful tool in searching the optimal extraction conditions.