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Energy & Fuels 2003, 17, 1372-1381
Product Selectivity during Hydrotreating and Mild Hydrocracking of Bitumen-Derived Gas Oil Christian Botchwey,† Ajay K. Dalai,*,† and John Adjaye‡ Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK, S7N 5C9, Canada, and Syncrude Edmonton Research Centre, Edmonton, AB, T6N 1H4, Canada Received September 24, 2002. Revised Manuscript Received July 22, 2003
The hydrotreating (HT) and mild hydrocracking (MHC) of gas oil derived from Athabasca bitumen have been performed in a micro trickle-bed reactor, using a commercial NiMo/Al2O3 catalyst. The operating conditions were varied as follows: temperature range, 340-420 °C; reactor pressure, 6.5-11.0 MPa; liquid hourly space velocity (LHSV) range, 0.5-2.0 h-1; and hydrogen/ gas oil ratio, 600 mL/mL. The removal values of sulfur and total nitrogen, basic nitrogen, and non-basic nitrogen obtained under optimum conditions were 99, 92, 99, and 88 wt %, respectively. The highest selectivities for sulfur and basic nitrogen removal occurred at the lowest temperature and pressure and the highest LHSV values (i.e., 340 °C, 6.5 MPa, and 2 h-1, respectively), whereas those for total and non-basic nitrogen removal occurred at the highest temperature and pressure and the lowest LHSV values (i.e., 420 °C, 11 MPa, and 0.5 h-1, respectively). High levels of aromatic saturation were also observed (22.7 wt % aromatics at 400 °C and 11 MPa). The overall boiling point of each product fraction decreased over the entire temperature range, because of (i) the conversion of sulfur and nitrogen heteroatoms at lower temperatures (e380 °C) and (ii) MHC at higher temperatures (>380 °C). The yield and selectivities of gasoline, kerosene, and light gas oil (LGO) increased as the operating severity increased. The highest yield of gasoline (10 wt %), kerosene (12 wt %), and LGO (19 wt %) were obtained at the highest severity of 420 °C. Vacuum gas oil (VGO) was the main fraction of the gas oil feed that apparently underwent conversion. No apparent significant change was observed in the net content of the heavy gas oil (HGO) fraction under all operating conditions. A reaction pathway is postulated for the conversion of the gas oil to products via heteroatom removal, saturation of aromatics, and hydrocracking.
1. Introduction Recently, there has been progressive interest in the production of synthetic fuel from oil sands,1 partly because of the gradual decline in the availability of conventional petroleum fuel.2 With the ever-increasing demand for transportation fuel, supplementing conventional crude with synthetic crude has become imperative. However, synthetic crudes are of much lower quality, compared to conventional crude oil, because of their high sulfur and nitrogen contents. Also, their nature is very aromatic, which results in low-quality products, especially where the use of diesel fuel is desired. To use them as transportation fuel with the required standard specifications, they require a relatively intensive upgrading, in the form of hydrotreating (HT) and/or hydroconversion.3 Earlier studies on the HT of synthetic crude oil have targeted the removal of heteroatoms, namely, sulfur and * Author to whom correspondence should be addressed. E-mail:
[email protected]. † University of Saskatchewan. ‡ Syncrude Edmonton Research Centre. (1) Bej, S. K.; Dalai, A. K.; Adjaye, J. Pet. Sci. Technol. 2002, 20 (7&8), 895. (2) Tian, K. P.; Mohamed, A. R.; Bhatia, S. Fuel 1998, 77, 1221. (3) Nagai, M.; Masunaga, T.; Hana-oka, N. Energy Fuels 1988, 2, 645.
nitrogen, which are responsible for deactivating acidic catalysts in downstream secondary processes, and the saturation of aromatics, to meet the stringent emission standards.4-10 For instance, Sambi et al.4 studied the HT of heavy gas oil (HGO) over a CoMo/Al2O3 catalyst within the temperature range of 300-450 °C and pressures of 4.1-12.4 MPa. They obtained conversions of up to 92% sulfur and 72% nitrogen. They also observed mild hydrocracking (MHC) at temperatures of >400 °C. Diaz-Real et al.8 also studied the HT of Athabasca bitumen-derived gas oil over Ni-Mo, NiW, and Co-Mo catalysts in a trickle-bed reactor at 623698 K (350-425 °C), a liquid hourly space velocity (LHSV) of 1-4 h-1, and a pressure of 6.9 MPa. They reported 99% sulfur and 89% nitrogen removal at 425 °C. Similarly, Bej et al.6 studied the hydrodenitrogenation (HDN) of HGO over a commercial Ni-Mo catalyst (4) Sambi, I. S.; Khulbe, K. C.; Mann, R. S. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 575. (5) Yui, S. M. Chem. Ind. (Dekker) 1994, 58, 235. (6) Bej, S. K.; Dalai, A. K.; Adjaye, J. Energy Fuels 2001, 15, 375. (7) Yui, S. M.; Ng, S. H. Energy Fuels 1995, 9, 665. (8) Diaz-Real, R. A.; Mann, R. S.; Sambi, I. S. Ind. Eng. Chem. Res. 1993, 32, 1354. (9) Mann, R. S.; Sambi, I. S.; Khulbe, K. C. Ind. Eng. Chem. Res. 1987, 6, 410. (10) Mann, R. S.; Sambi, I. S.; Khulbe, K. C. Ind. Eng. Chem. Res. 1988, 27, 1788.
10.1021/ef020214x CCC: $25.00 © 2003 American Chemical Society Published on Web 08/20/2003
Hydrotreating of Bitumen-Derived Gas Oil
Energy & Fuels, Vol. 17, No. 5, 2003 1373
Figure 1. Schematic diagram of experimental setup. Legend is as follows: PG, pressure gauge; TC, temperature controller; BPR, backpressure regulator; F, furnace; H2-C, hydrogen cylinder; H-C, helium cylinder; N-C, nitrogen cylinder; FT, feed tank; R, reactor; W-S, water scrubber; H2S-S, hydrogen sulfide scrubber; HPS, high-pressure separator; NV, needle valve; CV, check valve; B, balance; PST, product storage tank; and PCT, product collection tank.
in a trickle-bed reactor; in their study, they compared the effect of operating conditions on basic nitrogen (BN) and non-basic nitrogen (NBN) compounds in the oil. They observed that the rate of removal of BN was faster than that of NBN. As gas oil production from the oil sands becomes increasingly important, there is the need not only to remove sulfur and nitrogen but also to determine the conditions that optimize the selectivity and yield for products such as naphtha, diesel, kerosene, and light cycle oil (LCO). In addition, the quality of these products becomes very important. This requires both HT and MHC tests that cover a wide range of conditions. Information in the literature is scarce and scattered in the area of conversion of heavy fuels to produce desirable products such as gasoline, diesel fuel, and jet fuels.2,11-14 Yui15 studied the MHC of bitumen-derived coker gas oil with the conversion of heavy materials in the feed into lighter materials. Similarly, Yang et al.16 studied the MHC of synthetic crude gas oil at temperatures of 260-380 °C. They asserted that MHC has advantages over conventional hydrocracking in minimizing the production of undesirable light hydrocarbons and reducing hydrogen consumption. They also reported that undesirable secondary products can be avoided by avoiding overcracking at higher reaction temperatures. Ng and Rahimi17 studied the cracking of Canadian nonconventional feedstock. They observed a maximum conversion of 400 °C. (3) The yield of lighter products such as gasoline (10 wt %), kerosene (12 wt %), and light gas oil (19 wt %) and their selectivities were highest at the highest severity of 420 °C. However, the yield of light cycle oil increased at a faster rate than naphtha yield at the highest severity. EF020214X
