Desulfurization petroleum coke

To date, all of the research effort has not produced a commercial process to desulfurize petroleum coke for use in aluminum cell anodes. However, there is a new and unique process that promises to succeed where so many previous attempts have failed. This new process has been patented (14) by Diamond West Energy Corporation, and the research work, conducted at The Cooper Union in New York City, has been jointly funded by Kennedy Van Saun Corporation and Diamond West. 

Desulco is a granular graphitic carbon made by desulfurizing petroleum coke at temperatures approaching 2500C. Desulco meets a sulfur specification of 0,03 percent sulfur maximum as required for use of the product as a recarburiser for production of ductile iron,... 

This paper briefly reviews the technology of petroleum coke desulfurization and the effects of sulfur removal processes on coke physical properties. 

That leaves us with the challenge to desulfurize the coke itself. There has long been an incentive to remove sulfur from coke since low sulfur coke has always commanded a premium price over its higher sulfur counterpart. There have been dozens of patents filed on petroleum coke desulfurization technology (1-18). Although far from exhaustive, the list of 18 patents cited are those most pertinent to this discussion. 

Laboratory tests on two-pound sampl 5 of green petroleum cokes with sulfur contents of 4 wt% to 6 wt% have successfully reduced the sulfur levels to as low as 0.7 wt%. The degree of desulfurization can readily be controlled by adjusting the temperature (nominally between 2500 and 2800°F), the holding time in the reactor (between 5 and 90 minutes), and the reactant concentrations and flow rates. 

El-Kaddah, N. Ezz, S. Y., "Thermal Desulfurization of Ultra-High Sulfur Petroleum Coke," Fuel, 1973, 52(2), 128. 

Hussein, M. H. El-Tawil, S. Z. Rabah, M. A., "Desulfurization of High-Sulfur Egyptian Petroleum Coke," J. Institute Fuel, September 1976, 49, 139. 

Kalinowsky, B. Wlodarski, R., "The Mechanism of Thermal Desulfurization of Petroleum Cokes with High Sulfur Content," Przemysl Chemiczny, 1968, 47, 351. 

Review of Process Alternatives, Superior Graphite began in 1968 to investigate possible alternatives for desulfurization of petroleum cokes. The methods considered included various chemical treatments and direct thermal purification processes. The chemical treatment methods included hydrodesulfurization and reactions with various alkali metal compounds. Fine grinding of the coke appeared to be required and reaction conditions generally involved high pressure. 

The chemical methods reported at the time therefore appeared complex and unlikely to achieve at a reasonable cost the low levels of product sulfur required. Alternatively, thermal desulfurization at about 2500C was known to yield a product meeting sulfur specifications as made evident by the conventional process of graphitizing shaped mixtures of petroleum coke and coal tar pitch. In addition to simplicity, thermal purification offered a significant advantage in not requiring fine grinding of the coke. 

A review of available data on thermal desulfurization of various petroleum cokes indicated a generalized desulfurization relationship as shown in Figure 1. To achieve nearly complete desulfurization it was recognized that temperatures in excess of 2400C would be required. 

Figure 1. Desulfurization of Petroleum Coke by Thermal Treatment.

Figure 2. Desulfurization of Calcined Petroleum Coke as a Function of Time at Temperature.

One of the reasons that Powder River Basin (Wyoming) coals are widely used for pulverized coal combustion power plants (despite the relatively low heating value) is the low sulfur content. With the increased restrictions on sulfur dioxide emission regulations, coal combustion power plants looking to avoid expensive and efficiency-reducing flue gas desulfurization retrofits have switched to low-sulfur Powder River Basin coal. There is also the possibility that western coals can be combined with petroleum coke in order to increase the heating value and decrease the moisture content of the gasification feedstock. 

FIGURE 6.9 Flexicoking is a commercial process for refining petroleum that has been applied to heavy oil and tar sand fractions. The process employs circulating fluidized beds and operates at moderate temperatures and pressures. The reactor produces liquid fuels and excess coke. The latter is allowed to react with a gas-air mixture in the gasifier fluidized bed to provide a low-value heating gas that can be desulfurized and used as a plant fuel. Courtesy, Exxon Research and Engineering Company. 

Sulfur removal, as practiced in various refineries, can take several forms for instance, concentration in refinery products such as coke (Table 7-1), hydrode-sulfurization, or chemical removal (acid treating and caustic treating, i.e., sweetening or finishing processes). Nevertheless, the desulfurization of petroleum is almost universally accomplished by the catalytic reaction of hydrogen with the sulfur-containing constituents of petroleum to produce hydrogen sulfide that can be readily separated from the liquid products (Chapter 9). 

The petroleum feedstocks that contain sulfur as an impurity are handled under ambient conditions in carbon-steel tanks and pipelines where the corrosion attack by sulfur is less severe. In the desulfurization step, vaporized feedstock is processed at 400°C in the presence of hydrogen sulfide (H2S) and carbonyl sulfide (COS) - both of which are highly corrosive. Stainless Steels (SS) 304, 316 or 321 (for the fired heater) are used as the material of construction for various pieces of process equipment in this section of the plant. Equipment failures occur because of external corrosion and thinning of fired-heater coils and interior deposition of carbon from coking which leads to overheating. Fuel-gas lines, that contain hydrocarbon vapors and H2S, should be constructed of SS 304 and heat traced to avoid condensation88. 

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