Clean diesel

hydrotreating Design considerations for clean diesel hydrotreating. Critical issues are discussed when designing a hydrotreating facility to produce diesel fuel with very low levels of total sulphur For over three decades, refiners worldwide have been implementing various projects in their facilities to accommodate a variety of regulations to improve the quality of transportation fuels in order to reduce vehicle emissions. One of the key areas of interest has been the reduction of sulphur in diesel fuel to very low levels. For example, since mid-2006, the maximum sulphur content of on-road diesel fuel in the US has been limited to 15 wppm. Similar regulations are in place or are in various phases of implementation in many countries. Design considerations Middle distillates contain various types of sulphur species, including mercaptans, sulphides, thiophenes and aromatic sulphur compounds. Sterically hindered dibenzothiophenes are a group of aromatic sulphur compounds that are among the most difficult to remove when hydrotreating to very low sulphur levels. This is particularly true for diesel fuels that contain significant quantities of cracked stocks, such as FCC light cycle oil (LCO), which contains a large concentration of aromatic sulphur compounds. The effective removal of these species requires tailored catalysts and process conditions, as well as consideration of other factors such as feed nitrogen content and aromatics equilibrium. There are numerous issues to be addressed in the design of a hydrotreater, including: — Feed characteristics and variability — Other product quality requirements, especially cetane index — Catalysts selection — Optimisation of reactor process variables — Equipment design requirements — Reliability — Minimising product contamination — Handling of off-spec diesel product. All of these factors should be carefully considered during the front-end process design. Process flow Figure 1 shows a simplified process flow diagram for a diesel hydrotreater. Fresh feed from the surge drum is heated with stripper bottoms, then mixed wit