Energy & Fuels 2004, 18, 1001-1004
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Individual Hydrotreating of FCC Feed Components J. Ancheyta,* P. Morales, G. Betancourt, G. Centeno, G. Marroquı´n, and J. A. D. Mun˜oz Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Col. San Bartolo Atepehuacan, Me´ xico D.F. 07730 Received November 25, 2003
In this work, hydrotreating of heavy atmospheric gas oil, light vacuum gas oil, and heavy vacuum gas oil was carried out at pilot scale at the following operating conditions: reaction temperature of 340-380 °C, liquid hourly space velocity of 1.0-2.0 h-1, pressure of 54 kg/cm2, and a hydrogen-to-oil ratio of 2000 ft3/bbl. The hydrotreating of the three gas oils blended in a typical FCC feed volumetric ratio was also conducted at similar reaction conditions. From the experimental results it was confirmed that the mono-aromatics content exhibited an increase as the multi-ring compounds are saturated, which indicates that mono-aromatics are significantly more difficult to saturate. It was also observed that the quality of the final products obtained when the gas oils were hydrotreated separately is better than the typical FCC feed hydrotreating scheme. This behavior was attributed to the competition of the different reactions for active sites of the catalyst, which is higher when all the feeds are hydrotreated together due to the presence of sulfur and nitrogen in multi-ring aromatics type of structures in heavy feeds.
Introduction Catalytic hydrotreating (HDT) is one of the most mature technologies in petroleum refining industry. Generally speaking, the HDT process removes materials from petroleum distillates by selectively reacting these materials with hydrogen in a catalyst bed at elevated temperature. The HDT process is used to treat distillates for subsequent processing, to meet strict product-quality specifications, or to use as feedstocks elsewhere in the refinery. Hence a great variety of refinery streams are hydrotreated such as straight-run light and middle distillates (naphtha, kerosene, light gas oil), FCC feed (heavy gas oil, vacuum gas oils), atmospheric and vacuum residua, light cycle oil (LCO), FCC naphtha, and lube oils.1 In the case of FCC feed, which is mainly integrated by heavy atmospheric gas oil (HAGO), light vacuum gas oil (LVGO), and heavy vacuum gas oil (HVGO), its hydrotreating is more difficult than HDT of light and middle distillates because of the higher sulfur content and lower reactivity of sulfur compounds in heavy streams.2 It is well-known that FCC units are one of the major producers of gasoline in a refinery, and FCC gasoline is usually the major contributor to the sulfur levels in the gasoline pool (about 90%).3 LCO of FCC is also a very poor blending component for diesel because of its high sulfur content and low cetane number.4 For these * Corresponding author. Fax: (+55) 9175-8429. E-mail: jancheyt@ imp.mx. (1) Mellers, M. A. Handbook of Petroleum Refining Processes, 2nd ed. McGraw-Hill: New York, 1997. (2) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607-631. (3) Upson, L. L. Oil Gas J. 1997, Dec. 8, 47-51.
reasons, FCC feed pretreating is an excellent tool to meet required product quality. The benefits of hydrotreating the FCC feed have been discussed by various authors, and can be summarized as follows:3,5-7 • Cracking ability is improved by saturation of the poly-nuclear aromatics (PNA) compounds present in typical feeds. These PNA are converted into naphthenes, which are easily cracked into valuable products. • Coke selectivity is also decreased since PNA are the main coke precursors. • Since metals content is reduced from the feed, the dehydrogentation reactions promoted by nickel, which increase coke and gas production, are also decreased. Destruction of zeolite in the catalyst by vanadium is also diminished. • The reduction of Conradson carbon of the feed lowers the amount of coke on the spent catalyst going to the regenerator, which provides better catalyst selectivity with lower deactivation. • Nitrogen content in the feed is reduced and consequently its poisoning effect on the active sites of FCC zeolitic catalyst, which eventually reduces its activity and selectivity toward valuable products, is also decreased. • SOx and NOx emissions from the FCC regenerator are reduced. (4) Ancheyta, J.; Aguilar, E.; Salazar, D.; Betancourt, G.; Leiva, M. Appl. Catal. A 1999, 180, 195-205. (5) McLean, J. B.; Vance, P. W.; Peires, J. P. 1998 NPRA Annual Meeting, San Francisco, Paper AM-98-21, March 15-17, 1998. (6) Chung, H.; Kolbush, S.; de la Fuente, E.; Chistensen, P. PTQ 1997, 19-23. (7) Dahl, I. M.; Tangstad, E.; Mostad, H. B. Energy Fuels 1996, 10, 85-90.
10.1021/ef0301856 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/15/2004
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Table 1. Properties of HDT Feedstocks properties
HAGO
LVGO
HVGO
specific gravity at 20/4 °C refractive index at 20 °C sulfur, wt % total nitrogen, wppm basic nitrogen, wppm Ni + V, wppm total aromatics, wt % mono-, wt % di-, wt % tri-, wt % tetra-, wt % Conradson carbon, wt %
0.9010 1.5021 1.977 927 331 0.46 28.1 14.0 8.3 1.6 4.2 0.10
0.9291 1.5208 2.684 1332 447 0.52 37.2 13.7 11.0 4.1 8.4 0.10
0.9417 1.5295 2.761 1544 556 1.04 46.5 15.0 14.7 7.8 9.0 0.66
• HDT makes the products more environmentally friendly, since sulfur content in gasoline and LCO is decreased, which may avoid downstream treatment. • The HDT unit can be designed to reduce the sulfur level in the feed so that when the products from the FCC unit are blended with other refinery streams, the blend can meet the most stringent specifications. • Having the capability to remove metals from the FCC feed allows the refiners for processing heavier feeds and reducing catalyst consumption. • Higher value equilibrium FCC catalyst for sale or reuse in another FCC unit. Most of the experimental and commercial results reported in the literature about hydrotreating of FCC feed commonly employ the typical mixture of gas oils (HAGO, LVGO, HVGO) as raw material for experiments, and not too much work has been done on the study of the effect of hydrotreating on each of the streams of such a mixture. This segregation of streams is not new. The bestknown example commonly practiced commercially is the fractionation of straight-run naphtha after hydrodesulfurization, in which the main objective is the preparation of specific feeds for different processes, isomerization, and reforming. Since the composition of the feed is one of the most important factors affecting the yields and product quality in FCC units, in this work we report the individual processing of heavy atmospheric gas oil, light vacuum gas oil, and heavy vacuum gas oil in order to analyze their differences in hydrotreating reactivities. Experimental Section Hydrotreating Feedstocks. Three feeds, HAGO, LVGO, HVGO, were used in this study for hydrotreating experiments. The samples were recovered from atmospheric and vacuum distillation refinery units. The physical and chemical properties of the HDT feedstocks are presented in Table 1. ASTM D-1160 distillation curves are shown in Figure 1. Catalyst and Sulfiding. The hydrotreating catalyst used in the present study was a commercial NiMo/γ-Al2O3 sample. Its main properties are the following: 175 m2/g specific surface area, 0.56 mL/g pore volume, 127 Å mean pore diameter, 10.66 wt % Mo, 2.88 wt % Ni, and 2.5 mm diameter. The catalyst was in-situ activated by the following steps: (1) drying at atmospheric pressure and 120 °C with hydrogen flow during 2 h, (2) soaking with straight-run gas oil (SRGO) with 1.7 wt % sulfur at atmospheric pressure and 150 °C with hydrogen flow during 2 h, and (3) sulfiding with SRGO+1 wt % S (DMDS) at 260 °C (3 h) and 320 °C (8 h). The other
Figure 1. ASTM distillation curves of HDT feeds. sulfidation conditions were 28 kg/cm2 pressure, LHSV of 2.0 h-1, and 2000 ft3/bbl of H2/oil ratio. Pilot Plant Experiments. The hydrotreating studies were conducted under steady-state operation in a fixed-bed hydrotreating pilot plant. A detailed description of the HDT pilot plant, the isothermal reactor, and experimental procedure was presented elsewhere.8 Briefly, the isothermal reactor is designed as a tube with an inside diameter of 2.54 cm and a total length of 143 cm. The length of the reactor is subdivided into three sections. The first section was packed with inert particles and was used to heat up the mixture to the desired reaction temperature and to provide a uniform feedstock distribution. The following section contained the commercial catalyst mixed with the diluent. The exit section was also packed with inert particles. Between each section a wool glass plug was inserted in order to improve oil distribution. The inert used for diluting the catalyst was silicon carbide with an average particle size of 1.4 mm. The inert particle size is lower than that of the catalyst. The catalyst was diluted with an equal volume of inert in order to maintain a ratio of the volume of catalyst to that of diluent at a constant value of 1.0. The inert material was employed to minimize the problems when testing catalyst having commercially applied size and shape. Therefore, the hydrodynamics of the flowing fluids will be mainly dictated by the packing of small inert particles, whereas the catalytic conversion behavior is that of the catalyst in the actual size.9 In all HDT experiments pure hydrogen was used in a oncethrough mode, at the following operating conditions: reaction temperature: 340, 360, and 380 °C; liquid hourly space velocity (LHSV): 1.0 and 2.0 h-1; reaction pressure: 54 kg/cm2; and hydrogen-to-oil ratio: 2000 ft3/bbl. Product samples were collected at 4-8 h intervals after allowing a 2 h stabilization period.
Results and Discussion Feed Characterization. It is very clear that HVGO is the heaviest and HAGO the lightest streams among the three gas oils according to the API gravity data given in Table 1. Sulfur, nitrogen, metals, and aromatics contents are also high in HGVO. Conradson carbon residue (CCR) is the same in HAGO and LVGO, while it is very high in HVGO. Mono-aromatic contents are more or less the same in the three samples; however, (8) Marroquı´n, G.; Ancheyta, J. Appl. Catal. A 2001, 207, 407-420. (9) Ancheyta, J.; Marroquı´n, G.; Angeles, M. J.; Macı´as, M. J.; Pitault, I.; Forissier, M.; Morales, R. D. Energy Fuels 2002, 16, 10591067.
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Table 2. Properties of Hydrotreated Products from HAGO, LVGO, and HVGO (LHSV ) 1.0 h-1, H2/oil ratio ) 2000 ft3/bbl, pressure ) 54 kgf/cm2) HAGO HDT temperature, °C specific gravity at 20/4 °C total sulfur, wt % basic nitrogen, wppm total nitrogen, wppm metal content, wppm total aromatics, wt % mono-, wt % di-, wt % tri-, wt % tetra-, wt % Conradson carbon, wt %
340 0.8827 0.567 243 701 0.10 32.4 20.3 3.1 5.4 3.6 0.02
360 0.8805 0.365 191 637 0.09 32.0 20.3 3.0 5.2 3.5 0.02
LVGO 380 0.8761 0.206 93 443 0.08 31.4 20.6 3.0 4.7 3.1 0.01
340 0.9091 0.888 419 1146 0.15 39.8 18.6 7.0 9.8 4.4 0.05
360 0.9038 0.517 300 1027 0.08 37.7 18.3 6.8 8.5 4.1 0.03
HVGO 380 0.8987 0.289 188 769 0.07 37.5 18.0 7.4 8.2 3.9 0.01
340 0.9232 1.117 588 1490 0.40 43.0 23.8 7.7 7.6 3.9 0.35
360 0.9162 0.616 468 1283 0.26 38.0 23.1 5.5 6.2 3.2 0.26
380 0.9093 0.346 336 1081 0.08 37.4 23.8 5.3 5.7 2.6 0.17
Table 3. Properties of Hydrotreated Products from HAGO, LVGO, and HVGO (LHSV ) 2.0 h-1, H2/oil ratio ) 2000 ft3/bbl, pressure ) 54 kgf/cm2) HAGO HDT temperature, °C specific gravity at 20/4 °C total sulfur, wt % basic nitrogen, wppm total nitrogen, wppm metal content, wppm total aromatics, wt % mono-, wt % di-, wt % tri-, wt % tetra-, wt % Conradson carbon, wt %
340 0.8848 0.776 312 844 0.08 35.9 19.7 3.2 8.3 4.7 0.03
360 0.8827 0.467 256 742 0.07 34.8 19.9 3.2 7.4 4.3 0.02
LVGO 380 0.8796 0.299 185 612 0.06 34.1 20.2 3.1 6.7 4.1 0.02
340 0.9127 1.336 504 1263 0.53 40.6 18.3 8.2 9.8 4.3 0.08
di-, tri-, and tetra-aromatics contents are quite different, HVGO having the highest values. On the basis of distillation curves presented in Figure 1, it is also evident that HVGO has the heaviest and more complex compounds. HAGO is slightly lighter than LVGO. It is curious that HAGO and LVGO exhibit almost the same distillation curve after 20 vol %. The only difference is in the IBP-20 vol % range: 288-402 °C for LVGO, and 251-401 °C for HAGO. It means that the type of sulfur compounds present in these light fractions (IBP-20 vol %) is different in both feeds, HAGO having lighter ones, and hence the reactivity of these compounds will dictate the HDS reaction extent. Hydrotreating Experiments. Tables 2 and 3 show the effect of both reaction temperature and LHSV on sulfur, total nitrogen, basic nitrogen, CCR and metals removals, and hydrogenation of total, mono-, di-, tri-, and tetra-aromatics during hydrotreating of the three feedstocks. From these results the following behavior is in general terms observed when temperature is increased or LHSV is reduced: • Total sulfur, total nitrogen, basic nitrogen, CCR and metals removals increased. • Specific gravity of hydrotreated products diminished due to hydrocracking reactions. • Saturation of total aromatics is incremented, which indicates that the feeds are becoming less aromatic in nature. • Di-, tri-, and tetra-aromatics are reduced; however, mono-aromatics content is increased. The hydrogenation of poly-aromatics is carried out according to the following general reaction mechanism. According to this mechanism, aromatics saturation in the hydrotreating process begins with the partial saturation of multiple-ring aromatics. From our results, it is observed that the mono-aromatics content showed an increase as the multi-ring compounds are saturated,
360 0.9084 0.918 444 1126 0.20 40.0 18.4 7.9 9.6 4.1 0.05
HVGO 380 0.9035 0.474 329 1011 0.11 38.3 22.6 5.6 6.8 3.3 0.04
340 0.9249 1.663 660 1519 1.84 41.9 18.8 8.3 10.2 4.6 0.62
360 0.9191 1.247 618 1494 1.66 41.6 20.5 7.6 9.3 4.2 0.46
380 0.9111 0.816 527 1382 0.68 41.8 23.7 6.8 7.5 3.8 0.23
then it is confirmed that mono-aromatics are significantly more difficult to saturate, which agrees with experiments reported in the literature with model compounds that suggest that naphthalene and substituted naphthalenes are 1 order of magnitude more reactive than benzene and substituted benzenes.10 With respect to the type of feedstock, the following observations can also be made from Tables 2 and 3: • Removal of all heteroatoms showed the trend: HAGO > LVGO > HVGO, which indicated that HVGO has the more refractory and less reactive compounds. • On the contrary, total aromatic hydrogenation exhibited an inverse behavior: HVGO > LVGO > HAGO. This can be attributed to the relatively greater content of harder-to-hydrogenate aromatics in HVGO feed. HDT of Individual Feeds versus HDT of Combined Feed. For comparison purposes the three gas oils were blended in the following typical FCC feed mixture volumetric ratios: 38% HAGO, 18% LVGO, and 44% HVGO, and then the resulting blend was hydrotreated (10) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058.
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Table 4. Hydrotreating of FCC Feed (LHSV ) 2.0 h-1, H2/oil ratio ) 2000 ft3/bbl, pressure ) 54 kgf/cm2 Scheme A HDT temperature, °C specific gravity at 20/4 °C total sulfur, wt % basic nitrogen, wppm total nitrogen, wppm metal content (Ni + V), wppm total aromatics, wt % mono-, wt % di-, wt % tri-, wt % tetra-, wt % Conradson carbon, wt %
340 0.9083 1.339 513 1276 0.52 40.7 18.2 5.3 10.9 6.3 0.23
Figure 2. Process schemes for hydrotreating of FCC feed components.
at the same pressure and hydrogen-to-oil ratio than those used for each individual stream at only 2.0 h-1 LHSV and 340 and 360 °C reaction temperature (Scheme A in Figure 2). The quality of the hydrotreated products is shown in Table 4. Additionally, in this table is presented the theoretical quality of the product obtained by mixing each of the hydrotreated products of the three gas oils using the same volumetric proportions (Scheme B in Figure 2). It is observed that, in general, the quality of the final products is better in Scheme B. At the beginning one can think that product quality may be the same in both schemes, since all the feeds are hydrotreated at the same conditions; however, surprisingly it does not occur. The explanation for this behavior may be the following: when each feed is hydrotreated separately the competition of the different reactions for active sites of the catalyst is less since, for instance, in the case of HAGO, less complex heteroatoms molecules are present compared with the other two heavier feeds. The presence of sulfur and nitrogen in heavy molecules and multi-ring aromatics type of structures can also inhibit the hydrogenolysis reaction, especially of those lighter heteroatoms compounds. These competition of reactions for active sites and inhibiting effects
Scheme B 360 0.9027 0.929 457 1227 0.42 40.3 18.6 5.0 10.6 6.1 0.18
340 0.9075 1.275 503 1223 0.95 39.44 19.04 6.39 9.42 4.59 0.30
360 0.9033 0.898 452 1148 0.81 38.79 19.90 6.02 8.65 4.22 0.22
of some complex compounds are higher when all the feeds are hydrotreated together. It is also noted from Table 4 that total aromatics in the product are more or less the same in both schemes. However, aromatics distribution is quite different. In scheme B, hydrogenation of polyaromatics is higher than that of scheme A. On the contrary, mono- and diaromatics are higher in the products of scheme B, which is a consequence of the hydrogenation of the tri- and tetra-aromatics. If the products obtained from the two schemes are fed to catalytic cracking units we can anticipate that the lower content of PNA in the products obtained with scheme B will improve their cracking ability and reduce coke selectivity. In addition, lower sulfur contents in the liquid products are expected. Finally, it is important to highlight that the experiments in this study were carried out at 54 kg/cm2 pressure. This value of pressure is very low for hydrotreating of FCC feed, which is normally done at 70 kg/cm2 or higher. If reaction severity is increased conversion levels will also increase at a point in which both schemes analyzed here may give almost the same results. However, for those refineries having only low severity HDT plants the blending process proposed in our work is a good alternative to improve the quality of FCC feed. Conclusions As it was demonstrated here, the conventional scheme for hydrotreating the FCC feed components seems to have some disadvantages compared with the hydrotreating of individual streams; however, more experimental and process studies are needed in order to determine the best process configuration from both technical and economical points of view. One interesting result from this study, which needs to be highlighted, is that the complex molecules present in the heavy feed adversely affect removal of heteroatoms and hydrogenolysis of polynuclear aromatics when the streams are hydrotreated all together. From this investigation, attractive process configurations can be proposed for hydrotreating of FCC feed components, such as the use of different reactors, different type of catalysts, and different operating conditions for HDT of each of the streams in order to prepare higher quality FCC feeds. Acknowledgment. The authors thank Instituto Mexicano del Petro´leo for its financial support. EF0301856
