The products of petroleum refining must meet tight specifications, including limits on sulfur, nitrogen, olefins, aromatics, and other contaminants. Hydrotreating removes these contaminants from distilled crude oil fractions and intermediate process streams. Hydrocracking converts heavy oil fractions into lighter, more valuable products. Hydrotreating and hydrocracking processes share many common features, so they often are discussed together as “hydroprocessing.” Most hydroprocessing units employ specialized catalysts. As the name implies, they all consume hydrogen. Important chemical reactions include hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation, and the saturation of olefins and aromatics. Most catalysts are comprised of active metals and promoters on solid supports. For hydrotreating, the active component is molybdenum disulfide (MoS2). MoS2 is also used in some hydrocracking catalysts. Other hydrocracking catalysts employ either tungsten disulfide (WS2), palladium, or platinum. Promoters for MoS2 and WS2 catalysts include nickel and/or cobalt. The main support for hydrotreating catalysts is g-alumina. Hydrocracking requires highly acidic supports such as amorphous silica/alumina or synthetic zeolites. Catalysts can be prepared in various ways. In some cases, the support is manufactured first and the metals are added afterwards. In other cases, the components are co-mulled prior to shaping, drying and calcination. Hydroprocessing configurations can include fixed-bed reactors, ebullating bed reactors, or slurry-phase reactors, each of which requires a different kind of catalysts or (in the case of slurry-phase processing) a non-catalytic additive. Catalyst deactivation plays a major role in the economics of hydroprocessing.
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