Wet Air Oxidation of Oil Shales: Kerogen Dissolution and Dicarboxylic Acid Formation

Abstract

Until now, the oil shale kukersite has been used mainly for energy and oil production. To broaden the possible applications of oil shales, the wet air oxidation of kukersite (an organic-rich sedimentary rock from Estonia) was studied. Kukersite was oxidized with an oxygen-rich gas in water at temperatures up to 200 °C and pressures up to 60 bar. The efficiency of this batch process was evaluated from organic matter conversion, from the amount of solubilized organics obtained, and from the rate of dicarboxylic acid (DCA) formation. The effect of several reaction parameters—pressure, temperature, time, acid/base additives, substrate concentration, the origin of a substrate and its organic matter content, and so forth—was measured. A conversion of 91% in total organic carbon was achieved at 175 °C with 40 bar of the 1:1 oxygen/nitrogen mixture in 3 h without the presence of any additives. Under basic conditions, high yields (up to 50%) of dissolved organic matter were obtained with 8% of DCA; the best results are obtained with K 2 CO 3 and KOH. The highest DCA outcome (12%) within the 3 h reaction time was obtained in the presence of acetic acid. It was found that temperatures higher than 185 °C, pressures over 30 bar of pO 2 , and long reaction times in the acidic media caused a considerable decrease in the DCA outcome. It was also found that the same process can be applied to shales of different origins, although with lower DCA yields.

Aspects of kerogen oxidative dissolution in subcritical water using oxygen from air

“…The studied samples, K-70 and K-45, had particle sizes < 45 μm and < 90 μm, respectively. The elemental analysis results for these oil shale samples are given in Table 1 and additional information on their mineral contents can be found in the research by Kaldas et al [24]. For WAO experiments the same protocol was followed as previously [24].…”

“…The elemental analysis results for these oil shale samples are given in Table 1 and additional information on their mineral contents can be found in the research by Kaldas et al [24]. For WAO experiments the same protocol was followed as previously [24]. Large scale experiments were carried out in a 500 mL stainless steel pressure reactor (4575A, Parr Instrument Company, Moline, Il, USA) having similar properties as the 100 ml reactor used earlier.…”

“…Kaldas et al [24] have reported that oil shale could be dissolved via oxidative degradation using a wet air oxidation (WAO) process. The main advantage of the WAO method compared to conventional petrochemical processes is a shorter route from organic sediments to chemicals, lower temperatures, use of water as a solvent and air as an oxidant.…”

“…As the WAO process is widely used in wastewater treatment, technical solutions are readily available [25][26][27]. It has been demonstrated that by use of the WAO process at 175 °C for 3 h the yield of water soluble chemical compounds can be up to 50% of the initial kerogen content [24]. Moreover, 20-30% of these soluble compounds represent aliphatic dicarboxylic acids.…”

“…In the present work, the main focus was on determining the distribution of organic carbon between solid, liquid and gaseous phases throughout the reaction period rather than on the yield of DCA. Kukersite was subjected to the WAO process in wider pressure and substrate concentration ranges than in the work by Kaldas et al [24], namely 0.5-4 MPa and 2.5-40 g/L, and at temperatures of 155-185 °C in 500 ml batch reactors. The aim was to establish the optimal conditions for obtaining a maximum yield of organic carbon in liquid phase by avoiding its over-oxidation to CO 2 .…”