Elsevier

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Volume 78, Issue 6, May 1999, Pages 645-651
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Extraction of Huadian oil shale with water in sub- and supercritical states

https://doi.org/10.1016/S0016-2361(98)00199-9Get rights and content

Abstract

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 CO2 for non-isothermal extraction. The large increase in the formation rate of H2 and CH4 at 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.

Introduction

Oil shale is fine-grained sedimentary rock that produces oil on distillation. It represents a vast potential energy resource in the world [1]. 400 billion tons of oil shale were found in China with 16 billion tons of oil stored in it. Huadian oil shale, located in Huadian county, Jilin province, has an oil reserve of 1.3 billion tons. It originates from algal flora and belongs to sapropelic nature. Its Fisher Assay oil yield is about 45 wt.%(daf). Various techniques are available for recovery of oil from oil shale based on the thermal decomposition (or retorting) of the organic matter. The main problems of these techniques are, however, of low carbon conversion and low oil yield. Supercritical fluid extraction (SFE) has also been tested as a new separation method to recover oils from oil shale. The carbon conversion and oil yield of SFE are found to be much higher than those of the conventional methods. Thus SFE is becoming a promising method for oil shale processing.

The fluids usually employed in SFE of coal and oil shale are hydrocarbon solvents such as toluene [2], [3], [4], [5], [6], [7]. A previous publication from our laboratory reported a conversion of 82.6 wt.% of kerogen in the Huadian oil shale using toluene in SFE with non-isothermal extraction [8]. Because of its interesting physical and physicochemical properties, supercritical water extraction has received considerable attention [9], [10], [11], [12], [13], [14], [15], [16], [17]. It is generally agreed that the conversion and oil yield for SFE of oil shale with water depend on the sources of the oil shale sample and also the extraction conditions. Experiments carried out recently in our laboratory have shown that SFE of the Huadian oil shale with water is amenable. This article reports the effects of the extraction conditions on the conversion and oil yield as well as the product compositions for SFE of the Huadian oil shale with water.

Section snippets

Experimental

The experimental facility is shown in Fig. 1. Extractions were carried out in a semi-continuous mode. About 75 g weighted sample of Huadian oil shale was filled into the fixed bed reactor that can be heated by an electrical oven. Analytical details of the oil shale are listed in Table 1. Water was then fed into the reactor continuously by a high-pressure metering pump. Both non-isothermal and isothermal fluid extraction techniques were tested in the present study. For non-isothermal experiments,

Results and discussion

The extraction of oil shale is believed to be a thermal pyrolysis process in the presence of solvent. Thus extraction temperature plays an important role in affecting the amount and composition of the extract. While non-isothermal extraction makes it possible to investigate the extraction characteristics at the whole temperature range, its results are expected to be different from those of isothermal experiments. Hence they are presented and analysed separately in this section.

Conclusions

The conversion (88.0 wt.%) from Huadian oil shale was obtained for non-isothermal water extraction at 30 MPa with the extract mainly formed in the temperature range between 300°C and 500°C. Of this conversion, the extract yield is 56.8 wt.%. Gas was noticed to form at about 350°C and its formation rate is very similar to that of extract until temperature of 460°C. Although the major gas formed is CO2, significant increases of H2 and CH4 were found at the high-temperature region which resulted in a

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