Aromatic saturation of distillates: An overview

The saturation of aromatic compounds in distillate fractions and in particular in diesel fuel has received considerable attention in recent years. A high aromatic content is associated with poor fuel quality, giving a low cetane number in diesel fuel and a high smoke point in jet fuel. There is also evidence that particulate emissions in diesel exhaust gases correlate with the aromatic content of the fuel. New legislation has been introduced to limit aromatics in diesel fuel and this has led to new catalyst and process developments for aromatic saturation. This paper gives an overview of these developments. The types of aromatic compounds found in distillate streams are described, and the kinetics of both single (model) compounds and groups of compounds as found in industrial feedstocks are discussed. Both supported metal sulfide and supported noble metal catalysts an used industrially and the paper outlines the role of the active species in these catalysts and compares reaction conditions used for each. The tolerance of different catalyst systems towards sulfur and nitrogen in the feed is dealt with in some detail. Commercial processes employ either single- or dual-stage catalyst systems depending on the nature of the aromatic saturation catalyst. The paper considers the merits of different process configurations. The paper concludes with a brief survey of possible future applications for distillate aromatic saturation catalysts.