This paper presents an overview of the critical factors governing the production of ultra low sulfur diesel (ULSD) i.e. diesel fuel with less than 50 ppm sulfur. To produce ULSD it is necessary to remove the most refractive sulfur compounds, which are certain alkyl-substituted dibenzothiophenes. Alkyl-substituted dibenzothiophenes are desulfurized via one of two routes: by direct extraction of the sulfur atom, or by hydrogenation of one of the aromatic rings followed by sulfur extraction. Factors affecting the relative rates of reaction for the two routes are discussed, in particular the inhibiting effect of certain nitrogen containing components of diesel oils on the hydrogenation route. CoMo catalysts are generally more active for the direct desulfurization route, whereas NiMoP catalysts show relatively higher activity for the hydrogenation route. The consequences for ULSD are demonstrated through a number of cases studies, which serve to illustrate the effect of catalyst choice on required catalyst volume, hydrogen consumption and product properties. The case studies are used to discuss the merits of revamps versus grassroots units. Many diesel hydrotreaters suffer from poor liquid distribution resulting in poor catalyst efficiency.
Examples are given from industrial operation of how the application of state-of-the-art reactor internals improves reactor performance.
Diesel fuel specifications are being tightened throughout the world as part of efforts to improve air quality. At the same time, the demand for diesel is increasing necessitating use of lower quality feedstocks. The combination of these factors places a heavy burden on the refiner's hydroprocessing capabilities. New hydrotreating capacity and revamp of existing facilities are needed to meet the future diesel specifications. The present emphasis is on the reduction of sulfur, but future requirements may include improvement of cetane number, reduction in polyaromatic content and reduction in density.
For the production of ultra low sulfur diesel (ULSD), the refiner has to decide whether to revamp an existing hydrotreater or to build a new, grassroots unit. A revamp is less costly but will often be less flexible with respect to changes in feedstock and in required product properties. Many factors need to be considered in choosing the most cost-effective solution, but in all cases it is essential to have a thorough understanding of the kinetics for removal of the most refractive sulfur compounds.
The kinetics of deep desulfurization is governed by the extent to which desulfurization (HDS) occurs by direct sulfur extraction, or by hydrogenation of the sulfur-containing molecule. The direct route is primarily inhibited by hydrogen sulfide, and the hydrogenation route by specific nitrogencontaining compounds. CoMo and NiMoP catalysts exhibit di