Metal Hydroxide-Catalyzed Heavy Oil Upgrading in Supercritical Water: Deuterium Tracing Study

Abstract

Conversion of heavy oil is urgent as most of the explored fossil oil reserves and renewable raw biodiesel can be categorized as heavy oil. Supercritical water upgrading (SCWU) is a green and promising technology for heavy oil conversion, but the hydrogen-donating capacity of pure water is inadequate. This study adopts transition-metal hydroxide (M−OH) to enhance the in situ hydrogen-donating capacity of water for the first time. Both M− OH and the corresponding metal oxide nanoparticle (MO n ) of one post-transition metal (Al) and five transition metals (Cr, Cu, Fe, Ni, Zn) are tested. The SCWU conditions are selected as 425 °C, 60 min, water-to-oil ratio of 4:1, catalyst-to-oil ratio of 1:10, and water density of 308 kg m −3 . Deuterium oxide (D 2 O) instead of H 2 O is used as the reaction medium for a deuterium tracing study. Results indicate that the different metal-based MO n nanoparticles display various effects on heavy hydrocarbon cracking, gasification, coke suppression, and D addition reactions. The overall performance is found to be on the order of NiO > Cr 2 O 3 > Fe 2 O 3 > CuO > Al 2 O 3 > ZnO. Compared to MO n , M−OH can generally improve the oil yield and H/C ratio and reduce the coke yield simultaneously. The mechanism analysis suggests that the M−OH decomposition behavior under hydrothermal conditions is the primary cause by which MO n and lattice H 2 O are generated in the oil-rich phase. The lattice H 2 O can not only take part in SCWU reactions but also suppress coke formation. In conclusion, M−OH is a promising catalyst as it combines the good dispersibility and catalytic activity of MO n nanoparticles and the low cost of water-soluble inorganic metal salts (M-IAc) but does not result in any corrosive species, such as inorganic acid from M-IAc.

Safety Management and Accident-Control Strategy for a Commercial-Scale Plant for Supercritical Water Oxidation of Sludge

“…For the normal shutdown process, the feedstock of the system is changed from sludge to water gradually, by adding water into the sludge tank (1). The temperature of the reactor ( 7) is gradually decreased with a decreasing rate of 100 • C per hour, which is achieved by interlocking the temperature signal from the reactor (7) and the power of the electrical heater (5). According to our estimation, it takes about 6 to 8 h for the system to shut down by the normal procedure.…”

“…(1) Overpressure accident: if overpressure occurs in the electrical heater (5), the first step is to change the location of the heating module where salt precipitation forms, as introduced in Section 3.2. If the overpressure occurs in the heat exchanger (4) or the reactor (7), the fast shutdown procedure should be carried out directly.…”

“…Supercritical water oxidation is a powerful organic-waste treatment method that can completely decompose the organic materials without a pre-drying procedure and secondary pollution [5][6][7]. Compared with other advanced oxidation technologies, there are several advantages to SCWO.…”