Anode-grade coke from traditional crudes
A combination of solvent deasphalting and delayed coking is an option to minimise fuel oil production and produce anode-grade coke
Mitra Motaghi, Kanu Shree and Sujatha Krishnamurthy
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In an era of economic and political uncertainties, refinery margins will continue to be dictated by processing heavier, sourer crudes. The dramatic increase in residuum content from 10% in light sweet crudes to 50% in extra-heavy crudes poses interesting challenges, while presenting some unique opportunities. This is especially true when it comes to producing high-value products from low-value, bottom-of-the-barrel streams.
According to conventional wisdom, the residuum is either removed as fuel oil or asphalt, or subjected to thermal conversion processes for upgrading. Traditional resid upgrading methods include resid fluidised catalytic cracking (RFCC), visbreaking (VB) and delayed coking (DC).
RFCC is a widely used carbon rejection technology to convert high-boiling, high-molecular-weight hydrocarbon fractions to more valuable gasoline, olefinic gases and other products. However, due to the nature of the process, it is limited to processing lighter, low-metals, low-sulphur residues.
Visbreakers are essentially a means of improving the viscosity of the residuum so as to minimise the addition of valuable distillate boiling-range cutter stock to meet fuel oil specifications. As world economics seems to be influenced by the use of natural gas, the production of fuel oil has a negative effect on refinery product slate and economics. This situation is expected only to worsen as refiners face regulatory pressures ranging from new maritime bunker fuel specifications to carbon dioxide cap and trade and carbon footprint limitations. This leaves refineries with the challenge to minimise fuel oil production.
Carbon rejection choice
Residues from heavy crude oils contain high concentrations of sulphur, complex hydrocarbons and heavy metals such as nickel and vanadium. Due to the nature of these residues, delayed coking technology is the most commonly used carbon rejection technology. In addition, it enables the refiner to significantly reduce production of low-value fuel oil. Coking is a thermal cracking process in which, typically, a low-value residual oil, such as atmospheric or vacuum residue (VR), is converted into valuable distillate products and off-gas, leaving behind low-value fuel-grade coke. High-sulphur petroleum coke prices are distressed and, as is evident in Canada, coke is just being piled up in large quantities with no real economic outlet.
On the other hand, anode-grade coke is in high demand in the electrode industry. The world market for anode-grade coke is projected to be approximately 17–20 million tpa. The high price differential between the two grades, coupled with increasing demand for anode-grade coke, creates an unprecedented need to find an alternate path to improve the economics of coke production while maintaining higher refinery margins.
Production of anode-grade coke is greatly influenced by the sulphur and metal content of the feed or, for all practical purposes, the VR. The volume and quality of the residue is essentially determined by the quality of the vacuum gas oil fraction and the ability to process this fraction through conventional hydroprocessing or catalytic cracking conversion units. In most cases, the limiting factor is the metals content or the Conradson carbon residue (CCR) in the gas oil.
The residue volume and quality is by balance a reject defined by gas oil quality. Furthermore, not much attention has been paid to improving the quality of the residue prior to coking, primarily because of issues associated with the methods used to improve the residue quality.
One approach to reduce the metals and sulphur content of the residuum is hydrotreating. While hydrotreating addresses the sulphur and metals content of the feed, it is an expensive proposition incurring high capital investment due to high operating pressures and high hydrogen consumption with poor catalyst cycle length. In addition, hydrotreating increases the level of saturates in the residuum, which may make it unsuitable for anode coke production because other physical requirements, such as volatile carbonaceous material content, bulk density and grindability, may no longer be met. So, in reality, hydrotreating is not an economic option for residuum upgrading for anode coke production, and is therefore not widely practised.
The solution to obtaining anode-grade coke from traditional crudes, therefore, lies in alternative low-sulphur, low-metals content feed options to the coker unit. The options become obvious when analysing the residuum at the molecular level, where it is clear that the undesirable impurities in the coke are essentially asphaltenic in nature and can be separated by solubility-driven processes.
The solution involves the use of a proven solubility-based physical separation process — solvent deasphalting — in which a paraffinic solvent preferentially extracts paraffinic and resinic molecules, leaving behind asphaltenic products. While solvent deasphalting is primarily an aromatics rejection process, it is also a metals and CCR rejection process. The aromatic molecules that are rejected contain the majority of the metals and CCR, thereby producing a deasphalted oil (DAO) that can be processed in downstream units directly or after the removal of resins.
While DAO has been traditionally hydrotreated and/or catalytically cracked owing to its higher value molecules, the resin that is produced has so far been used only for production of fuel oil or road asphalt. The resin product is a relatively low-metal, low-sulphur residuum that is high in asphaltene-free CCR. Due to these characteristics, resin is very good for producing higher quality coke, and an excellent feedstock for the production of anode-grade coke.
Inherent in the solvent deasphalting process is the ability to draw out the resinic molecules and to adjust the volume and quality of the resin. The operating conditions of the asphaltene separator can be adjusted to lift the resinic molecules in the DAO. The resinic molecules are then recovered from the DAO by partially expanding the solvent under supercritical conditions. This arrangement provides the flexibility to balance the streams to downstream processing needs, while consistently meeting the required DAO quality and exercising other disposition options for the intermediate resin streams.
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