Hydrodesulfurization activity measurements

Organic sulphur- and nitrogen-compounds in motor fuels are a source for acid rain and harmful to the environment. Moreover, they are poisonous to the auto exhaust catalysts. To meet new developments in EU regulations on the S-concentration, a commonly applied one-step hydrodesulfurization (HDS), using conventional catalysts, e.g. C0-M0/7-AI2O3, is insufficient. A second HDS step, viz. a deep HDS step, can be more economical to reduce the S-content to the currently allowed European level of 350 ppm. This level will be reduced further to 50 ppm in 2005 [1]. In the first HDS step, often the heavy organic sulfur-containing polyaromatics survived, such as dibenzothiophene (DBT) and (4-, and/or 6-) alkylated DBTs [2,3]. They are the most refractory. In crude oils, there are also aromatic N-compounds, which suppress the performance of the HDS catalysts. Hence, a model feed for representative HDS-activity measurements should contain characteristic S- and N- compounds for practical relevance. 

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

Activity Tests with Model Compounds. Activity tests with model compounds were also carried out for the fresh, regenerated, and aged catalysts in a fixed bed reactor under a vapor phase condition at 5.0 MPa. 3 cm of crushed catalyst (0.35 - 0.5mm) was diluted with 9 cm of inactive alumina particles. Catalyst activities, such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrogenation (HG), were measured, feeding a mixture of 1 wt% carbon dioxide, lwt% dibenzothiophene, 1 wt% indole, and 1 wt% naphthalene in n-heptane. The catalysts were presulfided with a 5% H2S/H2 mixture at 400 °C for two hours and aged with a liquid feed at a reaction condition for 24 hours. Tests for HDS and HDN reactions were conducted at 275 °C, while those for a HG reaction were done at 325 °C. Condensed liquid products were analyzed with gas chromatography. Since all the reactions took place with the crashed catalysts in the vapor phase, we assumed that effectiveness factors were unity (9). 

It is interesting to note that Reddy and Manohar could show that M0S2 did also spread on the surface of AI2O3 supports. The resulting material developed a thiophene hydrodesulfurization activity which was identical to that measured on a conventionally impregnated and sulfided catalyst. 

The activity and selectivity of 12.5% M0/AI2O3 nitrided at various temperatures for the hydrodesulfurization (HDS) of dibenzothiophene and the effect of re-treatment of NH3 on dibenzothiophene HDS were studied. The nitrided catalyst was significantly more active toward the scission of the C-S bond from dibenzothiophene with hydrogenation of dibenzothiophene. The sulfur species accumulated on the surface of the nitrided M0/AI2O3 catalysts by replacement of nitrogen species after reaction was analyzed by XPS measurement. The formation of molybdenum sulfide during the HDS dibenzothiophene led to a decrease in the activity of the nitrided catalyst, which approached that of the sulfided catalyst. 

The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts were measured in a high pressure reactor. The experimental conditions for the activity tests were a feedstock of thiophene (15000 ppm), cyclohexane (90%) and cyclohexene (10%), flow rate 0.353 ml/min, total pressure = 26 Kg/cm and LHSV=52 l/h. Additional experiences with toluene (90%) and cyclohexene (10%) were also carried out. The operative conditions for the hydrotreating test were selected according to the recent experience about the C0M06 [3]. The products were analysed by gas chromatography by means of a Varian Start 3400 gas chromatograph, with FID detector. 

The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts have been measured in a high pressure reactor, the operative conditions having been selected according to the recent experience about the CoMoe and NiMoe based systems because of the common structural and physical-chemical properties [3, 4]. Table 3 shows chemical data and conversion obtained for selected catalysts based on RhMoe. In addition, the data for CoMoe Anderson, CoMo commercial and Rh commercial 7-AI2O3 supported catalysts are included for comparative purposes. 

Several Pd-Pt (Pd Pt=4 l) catalysts for deep hydrodesulfurization (HDS) were prepared using a commercial amorphous silica-alumina (ASA), a novel mesoporous silica (MPS) and a novel mesoporous silica-alumina (MPSA) as carriers with dispersions of > 50%. They were characterized by various techniques and their performance in deep HDS of 4-ethyl, 6-methyl-dibenzothiophene measured at 633 K and 6.0 MPa in the presence of carbazole and H2S. The Pd-Pt catalysts showed good activity as compared to typical conventional HDS catalysts. Their activity decreased with a decreasing Al content in the support (thus support acidity) ASA > MPAS > MPS. Enhanced support acidity seems to favor the stabilization of the dispersed metal particles against sintering. 

Koschier (1999) reviewed the potential of dermal exposure to kerosene to cause adverse health effects. The author described a study in which rats were exposed to hydrodesulfurized kerosene dermally at 0,165, 330, and 495 mg/kg for 5 days/wk for 13 wk. The rats were assessed with a FOB, and motor activity, startle response, and histologic characteristics were measured. All groups were examined after the 13-wk exposure, and the control and high-dose groups were also examined after a 4-wk recovery period. No significant differences were observed in any of the measures in any of the exposure groups. 

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