Inhibition of nitrogen compounds on the hydrodesulfurization of substituted dibenzothiophenes in light cycle oil

The influence of nitrogen compounds on the hydrodesulfurization (HDS) activities of a series of substituted dibenzothiophenes in light cycle oil (LCO) was studied over a NiMo/Al2O3 commercial catalyst. Three types of light cycle oil with nitrogen compounds of different concentrations and chemical natures were used as feed—an original fluid catalytic cracking light cycle oil (LCO), LCO with most of its basic nitrogen removed, and an ultra-low nitrogen LCO. Experiments were conducted in a fixed-bed microreactor at a total pressure of 70 atm, temperatures between 330 and 400 °C, and liquid hourly space velocities (LHSV) in the range of 1.0 to 3.5 h−1. The inhibition effects of nitrogen compounds on the HDS reactivity of the three sulfur groups—total sulfur, hard sulfur, easy sulfur—and 14 specific mono-, di- and tri-alkyl substituted dibenzothiophenes were investigated. The results showed that the HDS rate significantly increased using ultra-low nitrogen LCO. Pseudo first-order rate constants were estimated for the 14 mono-, di- and tri-alkyl substituted dibenzothiophenes. The HDS rates could be classified into three groups based on the position of the substituents. It was found that 4 and 6 substituted dibenzothiophenes had the lowest HDS rates. The HDS rate of the 14 substituted dibenzothiophenes were all increased when the ultra-low nitrogen feed was used. The improvement was greater for 4 and 6 substituted dibenzothiophenes than for those with one of the substituents at either the 4 or 6 positions. This finding indicates that the hydrogenation route is more strongly suppressed than hydrogenolysis route by nitrogen compounds since the hydrogenation route is believed to be the predominant reaction pathway for 4 and 6 alkyl-substituted dibenzothiophenes.