Aromatics Saturation of Diesel via Two-Stage Hydrotreating

Published September 2001

The focus of this review is a two-stage hydrotreating process for the reduction of aromatics in diesel fuel to a level below 10 wt%, as currently mandated by California Air Resources Board (CARB) regulations. New noble-metal catalysts have recently been commercialized to achieve such levels of dearomatization but require a very low sulfur reaction environment - as low as 10 ppmw. A two-stage reaction design configuration is therefore required, with the first stage primarily applied to hydrodesulfurization (HDS) while the noble-metal catalyzed second stage is primarily applied to hydrodearomatization (HDA). In addition to the dearomatization, another apparent advantage of this design approach is to achieve "ultra-low" levels of sulfur removal from diesel fuel - potentially well below 5 ppmw. Such low levels of sulfur removal would not only exceed recently announced U.S. Environmental Protection Agency (EPA) sulfur regulations for 2006, but would also meet even more stringent diesel fuel sulfur limitations currently anticipated in other major jurisdictions around the world.

The Cetane Index of the diesel product is expected to improve by 8 to 10 points, and perhaps more for highly paraffinic gas oil feedstock cuts. The total hydrogen consumption is in the range of 700-900 scf/b of feed, depending on sulfur, aromatics, and olefins content of the feedstock. The technology is licensed by Haldor Topsoe and has been recently demonstrated by San Joaquin Refining in Bakersfield California, where a 3,000 bpsd aromatic saturation unit is in operation.