748
Ind. Eng. Chem. Res. 1996,34,748-754
Hydrodesulfurization Reactivities of Narrow-Cut Fractions in a Gas Oil Xiaoliang Ma, Kinya Sakanishi, Takaaki Isoda, and Isao Mochida* Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan
A gas oil (bp 227-377 "C)was fractionated by the distillation at intervals of 20 "C into five fractions. Each fraction was desulfurized over the NiMo and CoMo catalysts, respectively, in an autoclave reactor at 360 "C under a total pressure of 2.9 MPa to examine its hydrodesulfurization (HDS) reactivities. The fractions exhibited the different reactivities which were expressed by the pseudo-second-order kinetics, giving the decreasing rate constants of 0.636, with a n increase of their boiling point ranges. 0.270, 0.118, 0.039, and 0.023 [min-wt %]-l Composition analyses of the fractions indicated that alkyldibenzothiophenes, polyaromatic hydrocarbons, and nitrogen compounds in the heavier fractions restricted their reactivities. Steric hindrance of methyl groups as well as the competitive retardation of latter compounds are suggested. The NiMo catalyst showed higher activity for HDS of the heavier fractions than the CoMo through its higher hydrogenation activity, while the latter was slightly superior to the former in HDS of the lighter fractions. The polyaromatic species existing dominantly in the heaviest fraction (>340 "C) are responsible for the fluorescent color in the oils. On the basis of such understanding, a deep HDS process which combined a single-stage HDS for lighter fractions and a multistage HDS for the heavier fractions was proposed and examined. Such a process achieved the sulfur level below 0.05 wt % with the color removal and its superiority over the conventional HDS process was discussed. Introduction Hydrodesulfurization (HDS) of gas oil has been one of the major refining processes. Recently, the demand for low-sulfur diesel fuel below 0.05 wt % of sulfur is increasing to minimize the emission of sulfur oxides from diesel engines. On the other hand, heavier feed which contains more unreactive sulfur compounds has to be refined t o increase the supply for diesel fuel. In addition, both the deep desulfurization by increasing reaction temperature and the adoption of the heavier feed always induce the fluorescent color to deteriorate the quality of product oil (Takatsuka et al., 19921, which is regarded as the most sensitive visual index of quality concerned by the end-user. Consequently, it has been recognized that it is difficult to meet such requirements by the conventional HDS process operated under moderate conditions (3-5 MPa, 350-360 "C). Although more severe reaction conditions andlor catalysts of higher activity are some ways to solve the problems, some new processes have been researched rather extensively to achieve deep HDS under moderate conditions. For example, two- (Haun et al., 1992a; Sakanishi et al. 1991, 1992) or three-stage HDS (Haun et al., 199213; Sakanishi et al., 1993; Ma et al., 1994b) as well as the SynSat process with both concurrent and countercurrent liquidlvapor flows in two separate catalyst beds (Suchanek et al., 1992) have been proposed. Sugimoto et al. (1992)proposed another HDS where two fractions of a gas oil were separately desulfurized. In our previous study, more than 60 kinds of sulfur compounds were found existing in a gas oil, all exhibiting very different HDS reactivities (Ma et al., 1994b). Some alkyldibenzothiophenes with alkyl groups at the 4 andfor 6 positions, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), showed very low reactivity about 2 orders of magnitude less than that of benzothiophene. In general, the lighter fraction containing reactive sulfur compounds can be easily desulfurized within the first few minutes of the reaction. Such fractions act as diluents for the less-reactive heavier
fractions in conventional HDS processes, reducing the reactor efficiency. These results indicated that when gas oil is separated into fractions which have more homogeneous reactivities, the HDS should be more effective, since each fraction can be more effectively desulfurized under conditions suited to each. Hence, the respective HDS of the fractions appears more reasonable to achieve deep desulfurization, although the separation step and two or more reactors, or two reaction zones at least are necessary. The reactivities of the respective fraction are most concerned in such a case to be quantified. In the present study, gas oil was first fractionated by distillation into five fractions with different boiling range to quantify the reactivity of each fraction for desulfurization at 360 "C. The catalytic activities of commercial NiMo and CoMo catalysts for HDS of respective fractions were compared. The fluorescent color of various fractions and their desulfurized products was also examined. On the basis of the results, a HDS process which combined a single stage for the lighter fraction with a three stage for the heavier fractions (being termed as separate HDS of two fractions) was proposed and performed. The superiority of such HDS process over the conventional HDS process is attempted to clarify. In this study, an autoclave was used to observe the HDS reactivity of the fractions derived from a gas oil. The batch reactor including heating and cooling by using a fresh catalyst is often complained unsuitable for kinetic study. However, rather rapid heating and cooling relative to the reaction period and rather large activation energy of HDS moderate the problem of difficulty to define the exact reaction time. The catalytic activity of the presulfided catalyst has been recognized as being unstable in the early stage of the reaction, leading to a difficulty of estimating the stationary activity in the autoclave reactor, which corresponds to that observed by the practical reactor. Nevertheless, the relative reactivities of sulfur species in the fuel oil
0888-5885l95/2634-0748$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 749 Table 1. Distribution and Composition of the Fractions Derived from Gas Oil B-1 P-2 F-3 boiling range ("C) density (15 "C) (g/mL) kinematic viscosity (30 "C, cSt) fraction distribution ( w t %) sulfur content (wt %) sulfur distribution (wt %) nitrogen content (wt ppm) group composition (HPLC) (wt %) saturates monoaromatics diaromatics polyaromatics re1 fluorescent intensity (RFI)
coo
NiO Si02
so4
physical properties surface area (m2/g) pore vol (mug) shape av diam (mm) av length (mm)
F-5
T-F
~280 0.8409 2.721 29.0 0.544 15.7 11
280-300 0.8512 4.457 16.6 0.819 13.5 34
300-320 0.8545 5.998 15.8 0.925 14.5 62
320-340 0.8675 8.257 13.1 1.262 16.3 126
> 340 0.8843 17.010 25.4 1.582 40.1 380
227-377 0.8589 5.655
74.7 15.8 9.4 0.1 0.0
70.3 16.6 12.6 0.5 1.0
66.4 18.2 14.2 1.3 9.5
57.5 18.2 21.8 2.5 43.8
44.7 19.4 30.2 5.6 260.0
63.1 18.1 16.9 1.9 72.3
Table 2. Chemical Composition and Physical Properties of Catalysts CoMo NiMo chemical composition (wt MOO3
F-4
14.9 4.4
14.9
0.95 0.50
3.1 4.8 0.65
268 0.53 four leaves 1.4 x 1.2 2.8
273 0.52 four leaves 1.3 x 1.1 3.5
Remainder Al203.
are obtained with reasonable accuracy by the autoclave which is easily operative in the laboratories, providing sufficient data to propose a novel scheme of efficient HDS, although the catalyst evaluation will not be performed satisfactorily for commercial examination.
Experimental Section The gas oil feed (T-F) used in the present study was obtained from a Middle East crude (Arabian-Light)with sulfur content of 1.007 wt % and boiling range of 227377 "C. Its properties and composition are summarized in Table 1. T-F was separated by atmospheric distillation at intervals of 20 "C into 5 fractions with boiling range of '280, 280-300, 300-320, 320-340, and '340 "C, respectively, which were denoted as F-1, F-2, F-3, F-4, and F-5, respectively. T-F and its fractions were provided by Research Institute of Petroleum, Japan Energy Co. The catalysts used in the present study were commercial CoMo/AlzOs (KF 742) and NiMo/AlnOs (KF 842) supplied by Nippon Ketjen Co. Their properties and composition are listed in Table 2. After the catalysts were presulfided with a 5% H2S/H2 flow under atmospheric pressure a t 360 "C for 6 h at a heating rate of 120 "CAI and cooled to room temperature under the same atmosphere, they were immediately used for the reactions. T-F and each fraction were desulfurized, respectively, over the NiMo and the CoMo catalysts at 360 "C, 2.9 MPa in a 50 mL magnetically stirred (1000 rpm) batchautoclave with a catalyst-to-oil weight ratio of 0.10. The heating rate was ca. 25 "C/min and the cooling rate was ca. 30 "C/min. The reaction time was counted from the moment when the temperature in the reactor reached the prescribed level. The hydrogen gas-to-oil ratios were 110 vol/vol. The total reaction pressure was controlled
1.007 143
a t the designed pressure throughout the reaction by adding gaseous hydrogen into the autoclave to compensate for its consumption in order to ensure the approximately constant hydrogen partial pressure throughout the reaction. In the separate HDS of lighter (340 "C), which only occupies 25 wt % of the gas oil. The lighter fractions (e300 "C) contain primarily alkyl BTs, while the heavier fractions (2300 "C) contain primarily alkyl DBTs. The fractions exhibit different HDS reactivities, which decrease sharply with their rising boiling ranges. Their HDS reactions can be expressed by the pseudo-secondorder kinetics, giving the rate constants of 0.636,0.270, 0.118, 0.039, and 0.023 [minwt for F-1 (340 "C), respectively. The heavier fractions are less reactive since they contain more alkyl DBTs of lower reactivity, more polyaromatic hydrocarbons and nitrogen compounds which are inhibitors against HDS. The NiMo catalyst exhibits higher catalytic activity than the CoMo for HDS of such heavier fractions, since unreactive sulfur compounds existing in the heavier fraction require the hydrogenation of neighboring aromatic ring to the thiophene ring prior to their desulfurization. The F-5 fraction and its desulfurized product exhibit the strongest fluorescent color among all fractions, while basically no fluorescent color is observed in the F-1, F-2, and F-3 fractions as well as in their desulfurized products. The polyaromatic species in the heaviest fraction are responsible for the fluorescent color of the oils. The separate HDS of two fractions with a single-stage for the lighter fraction and three-stages for the heavier fraction allows the fractions to be desulfurized more effectively under the conditions suited to their composition and reactivity. Hence, it has high superiority over
754 Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995
the conventional single-stage process in both the deep HDS and the color removal.
Miki, Y.; Sugimoto, Y.; Yamadaya, S. Desulfurization of ThiopheneType Compounds and Addition Reactions of the Products (Part 1): The Relation between Skeletal Structure and Reactivity.
Literature Cited
Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C. Hydrodesulfixization of Polycyclic Aromatics Catalyzed by Sulfided CoO-MoO3/y-Al2O3: The Relative Reactivities. J . Catal. 1979,
Bhinde, M. V. Quinoline Hydrodenitrogenation Kinetics and Reaction Inhibition. Ph.D. Dissertation, University of Delaware, Newark, 1979. Davies, I. B.; Bartle, K. D. On-Line Fractionation and Identification of Diesel Fuel: Polycyclic Aromatic Compounds by TwoDimensional Microbore High-Performance Liquid Chromatography/Capillary Gas Chromatography. Anal. Chem. 1988,60,
204. Galiasso, R.; Garcia, W.; Rammirez De Agudelo, M. M.; Andreu, P. Hydrotreatment of Cracked Light Gas Oil. Catal. Rev. Sci.
Eng. 1984,26,445. Haun, Edward C.; Thompson, Gregory J.; Gorawara, Jayant K.; Sullivan, Dana K. Process for Two-stage Hydrodesulfurization and Hydrogenation of Hydrocarbon Distillates. U S . Patent
5,114,562,1992a. Haun, Edward C.; Thompson, Gregory J.; Gorawara, Jayant K. Multi-Stage Hydrodesulfurization and Hydrogenation Process for Distillate Hydrocarbons. US.Patent 5,110,444,1992b. Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. IC;de Beer, V. H. J.; Gates, B. C.; Kwart, H. Hydrodesulfurization of Methylsubstituted Dibenzothiophenes Catalyzed by Sulfided CO-MO/y-&03. J . Catal. 1980,61,523. Isoda, T.; Ma, X.; Mochida, I. Reactivity of Refractory Sulfur Compounds in Diesel Fuel (Part 1)Desulfurization Reactivity of Alkyldibenzothiophenes in Decalin. J . Jpn. Pet. Znst. 1994a,
37,368. Isoda, T.; Ma, X.; Mochida, I. Reactivity of Refractory Sulfur Compounds in Diesel Fuel (Part 2) Inhibition of Hydrodesulfurization Reaction of 4,6-Dimethydibenzothiopheneby Aromatic Compounds. J . Jpn. Pet. Znst. 199413,37,506. Kilanowski, D. R.; Teeuwen, H.; de Beer, V. H. J.; Gates, B. C.; Schuit, G. C. A.; Kwart, H. Hydrodesulfurization of Thiophene, Benzothiophene, Dibenzothiophene, and Related Compounds Catalyzed by Sulfided C00-M003/y-&03: Low-pressure Reactivity Studies. J. Catal. 1978,55,129. La Vopa, V.; Satterfield, C. N. Poisoning of Thiophene Hydrodesulfurization by Nitrogen Compounds. J . Catal. 1988,110,
375. Lo, H. S.Kinetic Modeling of Hydrotreating. Ph.D. Dissertation, University of Delaware, Newark, 1981. Ma, X.; Sakanishi, K.; Mochida, I. Hydrodesulfurization Reactivities of Various Sulfur Compounds in Diesel Fuel. Znd. End.
J . Jpn. Pet. Znst. 1992,35,332.
57,509. Nagai, M.; Kabe, T. Selectivity of Molybdenum Catalyst in Hydrodesulfurization, Hydrodenitrogenation, and Hydrodeoxygenation: Effect of Additives on Dibenzothiophene Hydrodesulfurization. J . Catal. 1983,81, 440. Nagai, M.; Sato, T.; Aiba, A. Poisoning Effect of Nitrogen Compounds on Dibenzothiophene Hydrodesulfurization on Sulfided NiMo/Al203 Catalyst and Relation to Gas-Phase Basicity. J . Catal. 1986,97,52. Sakanishi, K.; Ando, M.; Abe, S.; Mochida, I. Extensive Desulfurization of Diesel Fuel through Catalytic Two-Stage Hydrotreatment. J . Jpn. Pet. Znst. 1991,34,553. Sakanishi, K.; Ando, M.; Mochida, I. Extensive Desulfurization of Diesel Fuel tbough Catalytic Two-Stage Hydrotreatment (Part 2) Influence of Reaction Pressure on Desulfurization and Reactivities of Refractory Sulfur Compounds. J . Jpn. Pet. Znst.
1992,35,403. Sakanishi, K.; Ma, X.; Mochida, I. Three-stage Deep Hydrodesulfurization of Diesel Fuel under 30 kg/cm2 Hz Pressure without Color Development. J. Jpn. Pet. Znst. 1993,36,145. Satterfield, C. N.; Modell, M.; Mayer, J. F. Interactions between Catalytic Hydrodesulfurization of Thiophene and Hydrodenitrogenation of F’yridine. AICHE J. 1975,21,1100. Satterfield, C. N.;Modell, M.; Wilkens, J . A. Simultaneous Catalytic Hydrodenitrogenation of Pyridine and Hydrodesulfurization of Thiophene. Znd. Eng. Chem. Process Des. Dev. 1980,
19,154. Stanislaus, A.; Cooper, B. H. Aromatic Hydrogenation Catalysis: A Review. Catal. Rev. Sci. Eng. 1994,36,75. Suchanek, A.; Hamilton, G. Update on Diesel Deep Desulfurizatiodkomatics Saturation by SynSat Process. 1992 NPRA Annual Meeting, New Orleans, Louisiana, 1992. Sugimoto, A.; Kondou, S.; Niizuma, M.; Sekido, F. HDS of Gas Oil.Jpn. Kokai Tokkyo Koho. JP 0446,92,46993,1992. Takatsuka, T.; Wada, Y,; Suzuki, H.; Komatsu, S.; Morimura, Y. Deep Desulfurization of Diesel Fuel and Its Color Degradation.
J . Jpn. Pet. Znst. 1992,35,179. Received for review June 16,1994 Accepted December 5, 1994 @
Chem. Res. 1994a,33,218. Ma, X.;Sakanishi, K.; Mochida, I. Three-stage Deep Hydrodesulfurization and decolorization of Diesel Fuel with CoMo and NiMo Catalysts at Relatively Low Pressure. Fuel 1994b,73,
1667.
IE9400863 @
Abstract published in Advance ACS Abstracts, February
15,1995.
