Coke (often called petroleum coke or pet coke) is a carbonaceous solid produced in the oil refining process. Coking reactions follow free-radical mechanisms in which the breaking and formation of chemical bonds occur simultaneously. The most distinctive reactions are the dealkylation of aromatic compounds and the dehydrogenation of saturated compounds. Aromatization, dehydrogenative cyclization, and polymerization are also prevalent reactions.
The feedstock for coking processes is generally low economic value crude oil residue streams, such as vacuum residue or atmospheric residue. Although liquid product from the coking process needs further treatment steps, the simplicity of the coking process has attracted interest. Two major grades of coke are available in the market based on their quality: fuel grade coke, which contains a lot of impurities, such as sulfur and metals, and is often used in combustion applications to generate heat; and anode grade coke, which has fewer impurities than fuel-grade coke, and thus, is used in more applications, such as in the aluminum industry. Typical compositional properties of anode grade coke are shown in Table 1 as follows.
TABLE 1Component AmountSulfur <3.0 weight (wt) % Volatile matter <11.5 wt % Vanadium <250 wt parts permillion (ppm) Nickel <150 wt ppm Iron <200 wt ppm Calcium <100 wt ppm Sodium <100 wt ppm Silicon <150 wt ppm
For suitability in industrial applications, anode grade coke requires low metal content, low sulfur content, and high aromatic content. For these reasons, pre-treatment steps in upstream of delayed coker units and other coke production processes may be required. The pre-treatment methods can be classified into two groups: solvent extraction, and hydroprocessing.
In solvent extraction processes, for example, the ROSE® (Residuum Oil Supercritical Extraction) solvent extraction process developed by Kellogg Brown & Root, Inc a Solvent Deasphalting (SDA) process is utilized that separates a resin fraction from asphalthene. The resulting resin is directed to a delayed coker for generating high-end coke such as anode grade coke. While generating feedstock suitable for the production of high-end coke, the ROSE process demonstrates poor conversion of the vacuum residue and thereby is not an economically feasible anode coke producing process. Specifically, to reduce the metal content of the vacuum residue of Arabian heavy crude oil from 250 weight parts per million (wt ppm) to 9 wt ppm by the ROSE process, over 50% of vacuum residue is rejected as asphaltene pitch. Consequently, while the ROSE process can generate feedstock suitable for anode coke; the ROSE process is ineffective at producing upgraded petroleum, because it rejects so much of the vacuum residue and lacks chemical conversion function.