Sulfur hydroprocessing units

Sulfur is removed from a number of refinery process off-gas streams (sour gas) to meet the sulfur oxide emissions limits of the Clean Air Act and to recover salable elemental sulfur. Process off-gas streams, or sour gas, from the coker, catalytic cracking unit, hydrotreating units, and hydroprocessing units can contain high concentrations of hydrogen sulfide mixed with light refinery fuel gases. 

Liquid products contain sulfur and nitrogen and must be hydroprocessed to improve quality. Separate hydroprocessing units for upgrading the naphtha, kerosene, and gas oil fractions can be used to optimize the overall process. Refined gas oil or diesel fuel is aromatic in character and contains more cycloparaffins than conventional crude oil. The resulting fuel is low in cetane number, high in density, and typically has very good low-temperature handling properties. 

Based on elemental analyses and microprobe tracing (Dautzenberg et al., 1978), metal deposits appear to be present in sulfide forms and not as adsorbed porphyrin-type compounds or as metals in the elemental or metallic state. Takatsuka et al. (1979) and Rankel and Rollmann (1983) have reported direct linear correlations of the spent catalyst sulfur content with the deposited metal content. The sulfide forms of nickel and vanadium are consistent with expectations based on thermodynamics for the conditions typically encountered in residuum hydroprocessing units (600-800°F, 1000-2200 psig, H2/H2S environment). 

For hydroprocessing units, product specifications are set to meet plantwide objectives. For example, the naphtha that goes to catalytic reforming and isomerization units must be (essentially) sulfur free. Before it can be sold as... 

Desulfurization of FCC feedstocks reduces the sulfur content of FCC products and SOX emissions. In the United States, road diesel sulfur can be 500 ppm (0.05 wt%). In some European countries, for example in Sweden, the sulfur of road diesel is 50 ppm or less. In California, the gasoline sulfur is required to be less than 40 ppm. The EPA s complex model uses sulfur as a controlling parameter to reduce toxic emissions. With hydroprocessed FCC feeds, about 5% of feed sulfur is in the FCC gasoline. For non-hydroprocessed feeds, the FCC gasoline sulfur is typically 10% of the feed sulfur. 

Transition metal sulfide units occur in minerals in nature and play an important role in the catalytic activity of enzymes such as hydrogenase and nitrogenase. Industrial processes use transition metal sulfides in hydroprocessing catalysis. Both the metal and the sulfur sites in these compounds can undergo redox reactions which are an important part of their reactivity. Thus, the electronic situation of the ReS4 anion and related complexes is of considerable interest and has been evaluated applying quantum chemical methods. 

Short contact time SRC can be upgraded via catalytic hydroprocessing into low sulfur boiler fuels. However, the solid SCT SRC feedstock requires solvent dilution to reduce its viscosity. Furthermore, even for a 50 wt % W. Kentucky SCR SRC blend, all pilot unit lines and valves have to be heat-traced above 350°F in order to achieve smooth mechanical operations. 

As in the traditional acid extraction process, the feedstock is generally dewaxed solvent refined base stock, since levels of the aromatics, polynuclear aromatics, and nitrogen and sulfur compounds are already reduced relative to a straight-run gas oil. This facilitates hydroprocessing by lightening the load on the catalysts and extending their lives. Equally important is that this is an already dewaxed feed, so the white oil producer does not have to bear the capital costs of crude fractionation and dewax units. 

Reactors, Catalyst Beds and Quench Zones. As shown in Table 4, most hydroprocessing reactions are exothermic. The heat released in naphtha and kerosene hydrotreaters is relatively low, so units designed for these feeds may use just one reactor that contains a single catalyst bed. However, for heavier feeds and/or feeds that contain large amounts of sulfur, aromatics or... 

---