Hydrotreatment Of Diesel Fraction Over A Commercial NiW/Al₂O₃ Catalyst

More and more strict environment legislation limit the content of sulfur in diesel oil all over the world, which attracts attention of both researchers and refiners. Hydrodesulfurization (HDS) is still the key to produce the high quality fuel with low sulfur content. Development of novel catalyst and new reactor systems for HDS plays an important role in fuel hydrotreatment. Studies on kinetics of HDS are helpful to understand the mechanism of HDS over various catalysts, improve the design of HDS reactor, optimize the HDS process, and predict the sulfur content in product. In this thesis the kinetics of HDS of dibenzothiophene (DBT), as the model compound for S-bearing organics, and the kinetics of hydrodenitrogenation (HDN) of quinoline, as the model compound for N-bearing organics, were studied over the commercial NiW/Al₂O₃ catalyst (RN-10) in a pressured trickle-bed reactor, respectively. The effect of reaction conditions, such as hydrogen partial pressure, reaction temperature, hydrogen/oil ratio and weight hourly space velocity on the catalytic behavior was investigated in detail. The influence of quinoline on HDS of DBT was also studied. HDS of various diesel oils, such as atmospheric distillation diesel, DCC, FCC and coking diesel oil were carried out under various kinds of reaction conditions. Different kinetic models were obtained on the basis of experimental data. The kinetics of HDS of DBT over the RN-10 catalyst was studied. The result showed that the hydrogen pressure and volume ratio of hydrogen/oil exerted little influence on the conversion of DBT at the high level of hydrogen pressures and volume ratios of hydrogen/oil. At low reaction temperatures, the conversion of DBT increased drastically with the increase of reaction temperature up to 330 ℃, while at high reaction temperatures it increased slowly. A kinetic model of HDS was established according to a second-order kinetic model at various reaction temperatures, and the parameters of the model were calculated. The correlation coefficient of the model was above 0.989. The apparent activation energy of the high reaction temperature region was less than that of the low temperature region, which was 13.4 and 121.4 kJ/mol, respectively. The kinetics of HDN of quinoline showed that the conversion of quinoline increased drastically with the increase of reaction temperature, and the hydrogen pressure also had a significant effect on HDN of quinoline. However, at the high hydrogen pressures and high volume ratios of hydrogen/oil, the conversion of quinoline was almost unaltered. Experimental data were fitted by a n-order (n<1) kinetic model at various reaction conditions, and kinetic parameters of the model were calculated. The apparent activation energy of 180.4 kJ/mol and the reaction order of 0.83 were achieved for this reaction. The experimental data were in good agreement with the model ones. In the presence of quinoline, which is considered as an inhibitor in HDS of DBT, the consumption kinetic model equation of DBT was studied at 0.5% (wt), 1%, and 1.5% concentration of quinoline, respectively. The result showed that quinoline strongly inhibited HDS of DBT. With the increase of concentration of quinoline, the inhibition became strong, but the decreasing rate of DBT conversion reduced when the concentration of quinoline was above 1%. In HDS of DBT, the hydrogenation (HYD) route was inhibited by quinoline more severely than the direct desulfurization (DDS) route. Experimental data were fitted by a pseudo first-order kinetic model with adsorption constant of quinoline, and kinetic parameters of the model were calculated. The experimental data were in good agreement with the model ones. The kinetics of HDS of the four diesel oils, i.e., atmospheric distillation diesel oil, DCC, FCC and coking diesel oil, were investigated under various reaction conditions in detail, such as reaction temperature, pressure, volume ratio of hydrogen/oil and weight hourly space velocity. Experimental data were fitted by a BP artificial neural network kinetic model, and the content of sulfur in product was predicted by BP artificial neural network under other reaction conditions. The model result indicates that the order of oil physical properties influencing HDS of diesel oil is as the following: density > 90% distillation range> content of nitrogen > content of sulfur >viscosity, and the order of reactions conditions influencing HDS of diesel oil is as the following: reaction temperature > weight hourly space velocity > volume ratio of hydrogen/oil > reaction pressure. This model can precisely predict the content of sulfur in diesel oils after HDS.