Energy & Fuels 2007, 21, 3401–3405
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Studies on Hydrotreating of Diesel Streams from Different Kuwait Crudes for Ultralow Sulfur Diesel Production A. Marafi,* A. Al-Hendi, A. Al-Mutawa, and A. Stanislaus Petroleum Refining Department, Kuwait Institute for Scientific Research, P.O. Box 24885, 13109 Safat, Kuwait ReceiVed March 20, 2007. ReVised Manuscript ReceiVed August 10, 2007
One of the parameters that plays a key role in achieving deep desulfurization in gas oil hydrotreaters is the feedstock quality. This paper addresses the preliminary assessment of the deep hydrodesulfurization (HDS) of the diesel feed (straight run gas oil) produced from a new Kuwaiti heavy crude, namely Lower Fars (LF), compared to that from the conventional Kuwait Export Crude (KEC). The key properties of the two feeds with respect to the sulfur, nitrogen, and aromatic contents, distillation range, and distribution of different types of sulfur compounds were examined. Systematic experiments were conducted to compare the degree of desulfurization of both feeds at different temperatures. The gas oil feed from the heavy crude (GO-LF) that contained higher sulfur, nitrogen, and aromatics contents together with a higher concentration of sterically hindered alkyl dibenzothiophenes was more difficult to desulfurize than the gas oil feed (GO-KEC) derived from KEC. The desulfurized diesel fuel produced from the GO-LF was also poor in quality with a higher density, higher aromatics and polynuclear aromatics (PNA) contents, and a lower cetane index.
1. Introduction Deep desulfurization of diesel fuels has gained considerable importance in oil refineries in recent years due to environmental regulations which limit the sulfur content of diesel fuel to very low levels. In the USA, the sulfur content in diesel has to be reduced from 50 ppm down to less than 15 ppm starting this year.1,2 In Japan, the diesel sulfur content was reduced by regulation from 2000 ppm to 500 ppm in 1997 and is currently set at 10 ppm. In the European Union countries, the sulfur content of diesel fuel has already been reduced to 50 ppm and it will further reduced to 10 ppm in 2008. The sulfur level in most industrial countries is expected to be reduced to around 10 ppm within the next 10 years.3 In addition to ultralow-sulfur content, the new specifications proposed by worldwide fuel character are introducing limitations on the density, polyaromatic hydrocarbon content, cetane number, total aromatics content, and 95% ASTM distillation temperature of diesel.4 Although the use of clean diesel fuels with ultralow-sulfur levels in transportation vehicles is beneficial from an environmental point of view, it places a heavy burden on the refineries due to the following factors: (i) the quality of crude available to the refineries is declining and getting heavier with more sulfur and lower API values, (ii) the refineries have to use the low quality feeds to produce ultra-clean fuel, (iii) when hydrotreating * To whom correspondence should be addressed. Fax: (+965)3980445. E-mail:
[email protected]. (1) Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal. B: EnViron. 2003, 41, 207–238. (2) Absi-Halabi, M; Stanislaus, A. Latest developments in hydrotreating catalysts: Meeting forthcoming sulfur specifications for transportation fuels. Oil Arab Cooperat. 2005, 31, 25. (3) Breysse, M.; Mariadassou, G.; Pessayre, S.; Geanete, C.; Vrinat, M.; Perot, G.; Lemaire, M. Deep Desulfurization: Reactions, Catalysts and Technological Challenges. Catal. Today 2003, 84, 129. (4) Antalffy, L. P.; Haelsig, C. P.; West, G.; Danial, F. Global clean fuels and the Middle East. Pet. Technol. Q. 2002, (autumn), 73–85.
to lower and lower levels (e.g. < 50 ppm), it is necessary to remove sulfur from the compounds that are the most difficult to desulfurize, and (iv) deep desulfurization to ultralow-sulfur levels is difficult to achieve with conventional hydrotreating catalysts and conventional operating conditions. The major factors that are envisioned in controlling the production of the ultralow-sulfur diesel (ULSD) are the composition of the gas oil feedstock, catalyst type, process parameters, and chemistry of gas oil hydrotreating for deep desulfurization. Among these parameters, the feedstock quality plays a significant role in achieving deep desulfurization during gas oil hydrotreating. Industrial feedstocks used for diesel fuel production are usually different in their characteristics with regard to sulfur, nitrogen, and aromatic contents which may affect the degree of desulfurization during hydrotreating. Diesel fuel feedstocks mainly consist of middle distillates in the boiling range 220–360 °C with plus or minus some front or back ends. To produce ULSD cost effectively, it is important to understand the nature of the sulfur containing compounds found in the diesel feedstock and the chemistry involved in removing the sulfur. The distribution of sulfur compounds in a diesel feedstock depends on the crude oil type from which it was derived. Since their relative reaction rates can be vastly different, identifying the specific sulfur containing compounds that exist within the diesel feedstock is the first critical step in defining the appropriate operating severity to achieve the ULSD specification. Most refiners process a wide variety of crudes and routinely adjust the processing conditions of major processing units according to the changing feeds and overall product needs. Additionally, the types and volume of streams feeding into the diesel pool may change due to seasonal swing in the product demand and changes in upstream operations. These changes in upstream operations will cause the type and concentrations of sulfur and nitrogen compounds in the diesel feedstock to vary. In Kuwait, Kuwait National Petroleum Company (KNPC)
10.1021/ef070142f CCC: $37.00 2007 American Chemical Society Published on Web 09/28/2007
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refineries have operated their gas oil desulfurization (GOD) units for many years. The total gas oil hydrotreating capacity at KNPC is 190 000 bbl/d, and this is expected to increase in the future to fulfill the increase in the international demand for low-sulfur diesel. The sulfur content of the diesel fuel will be reduced from its current 450 ppm level to 45 or 10 ppm to conform to the export market requirements. The KNPC’s experience in the GOD process to produce ULSD was reported in some recent papers.5,6 KNPC is planning to process heavier crudes than the Kuwait Export Crude (KEC) in the future in the new refinery. Kuwaiti heavy crudes, namely Ratwai/Burgan, Eocene and Lower Fars (LF), and their blends are going to be refined by the year 2010. Information on various types of sulfur compounds present in the diesel (gas oil) fractions produced form different crudes and their hydrodesulfurization (HDS) reactivities under different operating conditions will be of a great value for the optimization and economics of the deep HDS process. Published data about the deep hydrotreating of gas oils from heavier Kuwait crude oils to produce ULSD is scarce. This paper addresses the preliminary assessment of the deep HDS of the diesel cut produced from a Kuwaiti heavy crude, namely Lower Fars (LF), compared to that from the conventional Kuwait Export Crude. 2. Experimental 2.1. Feedstock Preparation. Two different Kuwait oils, namely, Kuwait Export Crude and Lower Fars crude, were obtained from the Kuwait Oil Company (KOC). Straight run gas oil (SRGO) feedstocks (i.e. diesel cuts) boiling in the range of 240–370 °C were prepared by fractionation of each crude oil using a pilot plant distillation unit (Fisher, model 40992) available at the Pilot Plant Hall facility at the Kuwait Institute for Scientific Research (KISR). Both crude oils and their gas oils were subjected to detailed characterization in order to assess the quality of the feeds based on the bulk properties. The sulfur compounds distribution was also determined for both gas oil feeds. 2.2. Feed and Products Analysis. The total sulfur and nitrogen contents of feed and product samples were determined using an Antek 7000 sulfur analyzer equipped with an Sulfur Chemiluminescence Detector (SCD).7,8 Density was determined by the IP 190 method. Individual sulfur compounds distribution in different feeds was determined using a high-resolution gas chromatograph equipped with a 30 m × 0.32 mm × 0.04 µm SPB-I capillary column and a flame photometric detector (FPD). The total aromatics content and polynuclear aromatics (PNA) content of the feed and products were determined with a supercritical fluid chromatography (SFC) aromatic analyzer (Berger). 2.3. Hydrotreating Experiments. A high pressure fixed bed microreactor unit manufactured by Vinci Technologies, France, was used for testing and comparing the HDS of the two feeds. The microreactor unit had a cylindrical reactor with an inside diameter of 1 cm and a volume of 16 cm3. The reactor was equipped with a thermowell that extended along the center of the reactor. The (5) Al-Radwan, S.; Said, S.; Parthiban, J. V. The challenges of achieving deep desulfurization of gas oil at KNPC’s MAA refinery. Proceedings, Kuwait Japan 5th Joint Symposium on Catalysts in Petroleum Refining: Hydrotreating and Hydrocraking of HeaVy Ends, Kuwait, January 2003. (6) Said, H.; Al-Radhwanm S. Challenge of Achieving Ultra Low Sulfur Diesel Near to Zero Sulfur Diesel with Constraints of Unit Design and Operation. The 7th Japan-Kuwait Joint Symposium in Deep Desulfurization for Production of Clean Transportation Fuel, Kuwait, November 13–14, 2005, in press. (7) Qabazard, H.; Abu-Seedo, F.; Stanislaus, A.; Andari, M.; AbsiHalabi, M. Comparison between the performance of conventional and highmetal Co- Mo and Ni-Mo catalysts in deep desulfurization of Kuwait atmospheric gas oil. Fuel Sci. Technol. Int. 1995, 13, 1135. (8) Qabazard, H.; Stanislaus, A.; Al-Barood, A. Deep desulfurization of coker and straight run gas oils. OAPEC-IFP Joint Seminar, Paris, France, September 26–28, 2000.
Marafi et al. Table 1. Experimental Conditions Used for the Gas Oil Hydrotreating Test operating parameters temperature(°C) pressure (bar) LHSV (h-1) H2/oil (mL/mL)
values 335, 350, 365, and 380 40 1.3 200
Table 2. Properties of Kuwait Crude Oils feed properties
KEC
LF
density @ 15 °C (g/cc) API sulfur (% wt) nitrogen (ppm) kinematic viscosity @ 40 °C (cSt) CCR (% wt) asphaltanes (%wt) total metals (V + Ni) (ppm)
0.8782 30 2.6 1100 12.82 6.22 2.7 35
1.0732 14 5.3 4000 150.1 11.64 10.1 132
temperature in the reactor was measured at different levels using a four-point thermocouple. The feed was pumped from a stainless steel tank to the reactor using an HPLC pump (slow suction quick delivery). High-purity (99.999%) hydrogen was supplied from highpressure cylinders. The desired pressure of the reactor was controlled by pressure regulators and controllers. A commercial new generation CoMo/Al2O3 catalyst was used in the gas oil desulfurization experiments. For each test, 6 mL of crushed catalyst (12–18 mesh), diluted with 6 mL of carborundum (12–18 mesh), was loaded in the middle zone of the reactor. Carborundum was placed above and below the catalyst bed. Before starting a run, the reactor was purged with nitrogen (40 L/h) under atmospheric pressure for 10 min. Then, the unit pressure was set at 70 bar under the flow of nitrogen (40 L/h). After reaching 70 bar, the pressure was maintained for 30 min and checked for leaks. After confirming that there were no leaks, the system was depressurized and the catalyst was presulfided with a sulfiding feed containing 3 wt % Dimethyl disulfide (DMDS) in SRGO.9 After sulfiding, the feed was switched to the test feed and the operating conditions were adjusted to the test conditions required for the run. The operating conditions used for the hydrotreating tests are summarized in Table 1. Hydrotreated product samples were collected at each condition by an autosampler after stabilization for 24 h. H2S dissolved in the hydrotreated product oil was removed by online stripping with nitrogen gas before sample collection.
3. Results and Discussion 3.1. Quality of Crude Oils and Their Gas Oils. The sulfur content and other characteristics of the two crudes and their straight run gas oils (diesel cuts) are presented in Tables 2 and 3. The results clearly indicate that the LF crude is much heavier than the KEC in terms of density and viscosity. The concentrations of S, N, asphaltenes, Conradson Carbon Residue (CCR), and metals (V + Ni) in the LF crude are substantially higher than those of KEC. The characteristics of the gas oils derived from the two crudes also show significant differences (Table 3). The GO-LF has a higher density and contains higher concentrations of sulfur, nitrogen, and aromatics than the GOKEC. The sulfur compounds distribution in the two gas oils are compared in Figure 1. Compared to GO-KEC, the GO-LF contains higher concentrations of alkyl dibenzothiophenes (DBTs) with alkyl groups close to the sulfur atom (e.g., 4,6dimethyl DBT, 4,6-methyl ethyl DBT, etc.), that are less reactive (9) Marafi, A. Deep Desulfurization of KNPC MAA-Gas oil: Screening of potential Catalysts for the Production of Ultra Sulfur Diesel; Proposal 7923, KISR: Kuwait, 2005.
Ultralow S Diesel from Kuwait Crudes
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Table 3. Properties of SRGO Feeds feed properties
GO-KEC
GO-LF
density @ 15 °C (g/cc) sulfur (% wt) nitrogen (ppm) TBN mg/mg KOC total aromatics (% wt) momoaromatics (% wt) polyaromatics (% wt) kinematic viscosity @ 40 °C (cSt) cetane index
0.8508 1.52 50 0.179 30.45 15.09 14.92 4.32 50.2
0.8962 3.22 187 0.457 44.73 19.47 26.21 5.635 38.2
Distillation (°C) IBP 10 vol 30 vol 50 vol 70 vol 90 vol 95 vol
% % % % % %
220 245 264 305 334 370 370
236 261 288 314 340 366 375
and difficult to desulfurize. This might affect the deep desulfurization of the GO-LF feed. This point will be discussed in more detail later. 3.2. Hydrotreating Processability of GO-KEC and GO-LF. In this study, experiments were conducted to compare the degree of desulfurization of the two gas oils at different temperatures in the range 320–380 °C. The results presented in Figure 2 show that the degree of desulfurization of GO-KEC is remarkably higher than that of GO-LF at all temperatures. It is seen that deep HDS of GO-KEC to 50 ppm sulfur can be achieved at 365 °C, whereas, in the case of GO-LF, the sulfur level is not reduced below 1000 ppm even at temperatures as high as 380 °C. These results clearly indicate that GO-LF is more difficult to desulfurize than GO-KEC. These results could be explained on the basis of differences in the characteristics and quality of the two gas oils. The sulfur, nitrogen, and aromatics contents of both feeds are significantly different (Table 2). The nitrogen content of the GO-LF is nearly 4 times that of GO-KEC. The sulfur and aromatic contents of GO-LF are also substantially higher than those of GO-KEC. Furthermore, the concentration of the low reactive sulfur compounds such as 4,6-DMDBT and other sterically hindered alkyl DBTs with more than two alkyl carbon atoms are higher in the GO-LF than in the GO-KEC. A combination of all the above factors could have caused the low HDS reactivity of GOLF. To achieve a diesel product with less than 50 ppm sulfur, a much greater percentage of sulfur must also be removed from the group of difficult statically hindered alkyl DBT compounds. The reaction mechanisms and the factors influencing the extraction of these last remaining sulfur molecules from diesel feeds to achieve
