Hydrotreating Process



Generally speaking, the hydrotreating process removes objectionable materials from petroleum distillates by selectively reacting these materials with hydrogen in a catalyst bed at elevated temperature. These objectionable materials include sulfur, nitrogen, olefins, and aromatics. Lighter materials such as naphtha are generally treated for subsequent processing in catalytic reforming units, and the heavier distillates, ranging from jet fuel to heavy vacuum gas oils, are treated to meet strict product-quality specifications or for use as feedstocks elsewhere in the refinery. Many of the product-quality specifications are driven by environmental regulations that are becoming more stringent each year. This push toward more environmentally friendly products is resulting in the addition of hydroprocessing units in refineries throughout the world.

Hydrotreating is one of the most mature technologies found in the refinery, rivaling the history and longevity of the thermal process. In 1952, UOP and Union Oil Co. of California began licensing hydrotreating under the name of the Unifining process. The partnerships and the development of this technology have gone through a series of changes over the years, and in 1995 the acquisition of the Unocal Process Technology and Licensing group by UOP resulted in the merger of two premier hydroprocessing companies and the combination of their expertise under the UOP Unionfining banner.


The actual flow scheme of the UOP Unionfining process varies, depending on the application. Figure above provides a generic look at the flow scheme of a UOP Unionfining unit. The feed is exchanged with the reactor effluent, mixed with recycle hydrogen, and then heated to reaction temperature in a fired heater. The combined feed then flows through the reactor, which contains the catalyst that will accelerate the reaction. The reactor effluent is cooled in exchange with the feed and then in a series of coolers before being separated in a vapor-liquid separator. The vapor portion is recompressed, combined with fresh hydrogen, and returned to the reactor feed. The liquid portion is fed to a fractionator, where it is stripped of light ends, H2S, and NH3.

The chemistry behind the hydrotreating process can be divided into a number of reaction categories: (hydro)desulfurization, (hydro)denitrification, saturation of olefins, and saturation of aromatics. For each of these reactions, hydrogen is used to improve the quality of the petroleum fraction.

Desulfurization is by far the most common of the hydrotreating reactions. Sulfur-containing hydrocarbons come in a number of forms, and the ability to remove sulfur from the different types of hydrocarbons varies from one type to the next. The degree to which sulfur can be removed from the hydrocarbon varies from near-complete desulfurization for light straight-run naphthas to 50 to 70 percent for heavier residual materials. The desulfurization reaction results in the production of hydrogen sulfide (H2S) in the reactor section of the plant. To complete the desulfurization reaction, the H2S must be removed in downstream fractionation.

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