2438
Energy & Fuels 2005, 19, 2438-2444
Specific Nitrogen Boiling Point Profiles of Vacuum Gasoils Nade`ge Revellin, Hugues Dulot,* Clementina Lo´pez-Garcı´a, and Franck Baco Institut Franc¸ ais du Pe´ trole IFP Lyon, P.O. Box 3, 69390 Vernaison, France
Jacques Jose Laboratoire des Sciences Analytiques, Universite´ Claude Bernard Lyon I, 43 boulevard du 11 Novembre 1918, 69100 Villeurbanne, France Received April 29, 2005. Revised Manuscript Received September 2, 2005
Vacuum gasoils are heavy petroleum cuts that are hydrotreated prior to being transformed into valuable fuels by hydrocracking. Hydrotreatment is a refining process carried out to reduce the content of heteroatomic molecules in petroleum cuts to meet the stringent fuel specifications and to prevent catalyst poisoning in downstream conversion processes. Kinetic modeling of hydrotreatment reactions requires information about the distribution of nitrogen compounds in vacuum gasoils and in their hydrotreated effluents. Analysis of heavy distillates is a challenge since nitrogen molecules are present in small amounts in the hydrocarbon matrix. Moreover, a large number of isomers are distributed over a large boiling point range (350-650 °C). This paper presents a GC-NCD method developed to determine the nitrogen distribution in vacuum gasoils. To validate the GC-NCD results, vacuum gasoils, narrow cuts as well as hydrotreated vacuum gasoils, have been analyzed. Quantitative data were compared to the total nitrogen content determined by organic elementary analysis, and an excellent agreement between both methods was found.
1. Introduction Hydrocracking is performed to upgrade heavy petroleum cuts such as vacuum gasoils (VGOs) mainly into gasoline and diesel. Even at low concentrations, organic nitrogen compounds are known to be catalyst poisons for the hydrocracking process. Therefore, a hydrotreatment stage is carried out before hydrocracking to reduce the content of impurities (sulfur and nitrogen) in the vacuum gasoils. This process requires severe conditions such as high temperatures and high hydrogen partial pressures, in the presence of a NiMo/Al2O3 type catalyst. Basic nitrogen compounds are strongly adsorbed on the catalyst surface,1 inducing competition and inhibition between hydrotreatment reactions: hydrodearomatization, hydrodesulfurization, and hydrodenitrogenation.2-4 According to the literature,5-10 nitrogen compounds in * Corresponding author. Tel: + 33 (0)4 78 02 20 20. Fax: +33 (0)4 78 02 20 15. E-mail:
[email protected]. (1) Koltai, T.; Macaud, M.; Guevara, A.; Schulz, E.; Lemaire, M.; Bacaud, R.; Vrinat, M. Appl. Cat. A 2002, 231, 253. (2) Magne-Drisch, J. Cine´tique des re´actions d′hydrotraitement de distillats par de´composition en familles et par coupes e´troites. Ph.D. Thesis, Universite´ Pierre et Marie Curie, Paris VI, France, 1995. (3) Bonnardot, J. Mode´lisation cine´tique des re´actions d’hydrotraitement par regroupement en familles chimiques. Ph.D. Thesis, Universite´ Claude Bernard Lyon I, France, 1998. (4) Lo´pez-Garcı´a, C. Analyse de la re´activite´ des compose´s soufre´s dans les coupes pe´trolie`res: cine´tique et mode´lisation de l’hydrotraitement. Ph.D. Thesis, Universite´ Claude Bernard Lyon I, France, 2000. (5) Bej, S. K.; Dalai, A. K.; Adjaye, J. Energy Fuels 2001, 15, 377. (6) Ferdous, D.; Dalai, A. K.; Adjaye, J. Energy Fuels 2003, 17, 164. (7) Ho, T. C. Catal. Rev. 1988, 30, 117. (8) Shin, S.; Sakanishi, K.; Mochida, I. Energy Fuels 2000, 14, 539.
vacuum gasoils have different reactivities: nonbasic nitrogen species such as carbazole are known to be more difficult to eliminate than the basic acridine-type species (Figure 1). Our work aims at providing the basis for the development of a kinetic model of vacuum gasoil hydrotreatment reactions, with particular attention given to hydrodenitrogenation modeling. Quantitative information about nitrogen distribution in vacuum gasoil cuts is needed for this purpose as well as for further understanding of the hydrodenitrogenation process. Usually, the nitrogen data available for vacuum gasoils are the total and basic nitrogen analyses, which describe only global properties. To analyze nitrogen distributions, a chromatographic separation followed by a selective detector is required. Several specific nitrogen detectors are commercially available. GC-MS and GCAED are frequently used for nitrogen detection,8-19 but they face sensitivity and selectivity difficulties in the (9) Wiwel, P.; Knudsen, K.; Zeuthen, P. Ind. Eng. Chem. Res. 2000, 39, 533. (10) Zeuthen, P.; Knudsen, K. G.; Whitehurst, D. D. Catal. Today 2001, 65, 30. (11) Dzidic, I.; Balicki, M.; Hart, H. Fuel 1988, 67, 1155. (12) Quimby, B.; Grudowski, D.; Giarrocco, V. J. Chromatogr. Sci. 1998, 36, 435. (13) Qian, K.; Rodgers, R.; Hendrickson, C.; Emmett, M.; Marshall, A. Energy Fuels 2001, 15, 492. (14) Baco, F. Caracte´risation des distillats pe´trolieres par couplage chromatographie en phase gazeuse et de´tection par e´mission atomique. Ph.D. Thesis, Universite´ Claude Bernard Lyon I, France, 1997. (15) Baco, F.; Quignard, A.; Szymanski, R. Oil Gas Sci. Technol. 1999, 54, 473. (16) Laredo, G.; Leyva, S.; Alvarez, R.; Mares, M.; Castillo, J.; Cano, J. Fuel 2002, 81, 1341.
10.1021/ef050127f CCC: $30.25 © 2005 American Chemical Society Published on Web 10/14/2005
Nitrogen Boiling Point Profiles of Vacuum Gasoils
Energy & Fuels, Vol. 19, No. 6, 2005 2439
complex hydrocarbon matrix. As far as we know, no applications of GC-NCD for such heavy petroleum cuts are reported in the literature. Thus the challenge of this work was to determine whether the GC-NCD technology could be successfully applied to the analysis of VGOs, to determine if the NCD detection was nitrogen equimolar, sensitive, and selective in the presence of highboiling-point complex petroleum matrixes, and which chemical standards should be used for quantitative nitrogen measurements. This paper describes the results of the evaluation of the GC-NCD performances applied to the analysis of model molecules, VGOs feedstocks, narrow cuts, and several hydrotreated effluents. Figure 1. Examples of nonbasic and basic nitrogen molecules.
analysis of heavy petroleum cuts. The major drawbacks of the nitrogen-phosphorus detector (NPD) are the response dependence on the structure of the nitrogen compounds and the strong carbon matrix interference with the nitrogen detection.20-22 In recent years, the nitrogen chemiluminescence detector (NCD) has become one of the most powerful analytical tools for nitrogen detection. The first step of the chemiluminescence detection is the oxidation of the sample in the presence of oxygen at high temperature (eq 1):
R-N + O2 f NO + other oxides
(1)
The products resulting from the oxidation step are then transferred from the furnace to a chemiluminescence reaction cell where they react with ozone to form excited nitrogen dioxide. The radiation emitted by the excited nitrogen dioxide (eq 2),
NO + O3 f NO2* f NO2 + photon
(2)
is detected by a photomultiplier tube between 600 and 900 nm. Characterization and quantification of nitrogen compounds in gasoline and diesel fuels by GC-NCD have already been reported in the literature,23-29 demonstrating that the nitrogen chemiluminescence detector has high sensitivity and nitrogen selectivity. However, the analysis of nitrogen profiles of vacuum gasoils is particularly difficult for several reasons. The nitrogen compounds are present as hundreds of isomers distributed over a high-boiling-point range (350-650 °C) and in very low concentrations (
