Subcritical hydrothermal purification from oily sludge recovered from petroleum oily sludge by adding sodium hydroxide, sodium bicarbonate, and formic acid

Subcritical water upgrading has received a great deal of attention because of its high efficiency and environmental protection technology features, but its high temperature and high-pressure reaction conditions have limited its further development. This article investigates the hydrothermal upgrading recovered oil from oily sludge at 350°C and for a residence time of 60 min recovered oil. The effects of the type and concentration of various additives sodium hydroxide (NaOH), sodium bicarbonate (NaHCO₃) and formic acid (CH₃COOH) on the yield and quality of recovered oil were analyzed and discussed. The addition of 2 wt % NaOH increases the recovered oil yield to 78.6%, due to the demulsification effect of alkaline additives. Meanwhile, the CH₃COOH effectively increased the H/C ratio of the recovered oil to 1.69, which can be attributed to the hydrogenation of large molecules in the sludge with hydrogen generated from the added CH₃COOH. The effect characteristics and mechanisms of adding additives on the increase in the recovered oil yield and quality of oil-bearing sludge in a sub-critical hydrothermal environment were revealed. Subcritical water upgrading technology provides a new technological route for resource recovery of oily sludge with mild reaction conditions and high efficiency and simplicity.