Energy Fuels 2010, 24, 1495–1501 Published on Web 01/08/2010
: DOI:10.1021/ef9012104
Assessment of Selected Apparent Kinetic Parameters of the HDM and HDS Reactions of Two Kuwaiti Residual Oils, Using Two Types of Commercial ARDS Catalysts Dawoud Bahzad,* Jamal Al-Fadhli, Ayyad Al-Dhafeeri, and Ali Abdal Petroleum Refining Department, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Received October 27, 2009. Revised Manuscript Received December 13, 2009
Selected apparent kinetic parameters such as rate constants and activation energies of the hydrodemetallation (HDM) and hydrodesulfurization (HDS) reactions were assessed for two Kuwaiti Residual oils using two types of commercial atmospheric residual desulfurization (ARDS) catalysts. The feeds investigated in the study were 360 °Cþ cuts of Kuwait Export crude (KEC) and Kuwait Ratawi/Burgan crude (KHC). The catalysts used in the study were two different catalysts of a complete ARDS catalyst system (Cat-B) consisting of five different catalysts. Cat-BA is an HDM catalyst used in the front reactor of an industrial ARDS process to remove possible contaminants present in the feed, such as metals, sulfur, nitrogen, and to reduce the feed asphaltene content to lowest levels possible. Cat-BC is an HDS catalyst, usually employed in the middle reactors, mainly to reduce the feed sulfur content to the desired levels. Testing was performed using a bench scale unit that was equipped with three fixed-bed reactors working under trickle flow mode. The findings showed that KEC-AR is generally more reactive than KHC-AR, in terms of metals and sulfur removal over both catalysts. However, KHC-AR showed higher reactivity for sulfur removal over CatBA, although KHC-AR contains higher levels of sulfur and asphaltenes, in comparison to KEC-AR.
feeds are upgraded in the presence of hydrogen and several catalysts of different types grouped as one system.9-11 A typical ARDS catalyst system is usually a combination of at least three different types of hydrotreating catalysts, namely, hydrodemetallation (HDM), HDS, and hydrodenitrogenation (HDN), with a considerable hydrocracking function.12 Every individual catalyst of an ARDS catalyst system is employed to achieve a main task. HDM catalysts are employed to achieve metal removal and asphaltene reduction, as well to achieve mild HDS. HDS catalysts are employed to lower the feed sulfur content to the desired levels, whereas HDN catalysts are employed to reduce the feed nitrogen content to the lowest levels possible and to lower the feed boiling range through hydrocracking.4,13-15 However, grouping different types of hydrotreating catalysts to work connectively as one system is not as easy as it sounds, because several factors are involved. For example, the physicochemical properties of the front catalyst(s) must be suited to handle a raw feed (a fresh residue) with all its bad processing features. Similarly, the physicochemical properties of middle catalyst(s) must be featured to treat a feed already demetallized and partially desulfurized and deasphalted over the front catalyst(s) bed. For the end catalyst(s), they must be highly active for nitrogen removal, cracking, and hydrogen addition reactions.16,17 Hence, the individual elements of an ARDS catalyst system must be complementary to each other; otherwise, the
1. Introduction Upgrading heavy oils to light petroleum products currently is an unavoidable practice, because of the growing demand on light and midheavy petroleum fractions.1-3 The current petroleum refining technology provides various options for upgrading heavy oils: either by carbon rejection or by hydrogen addition. Among the available processes, atmospheric residual desulfurization (ARDS) is a significant hydrogen-addition upgrading process, which is used mainly to upgrade low-value petroleum feeds to high-quality products of high commercial value and wider usability.4,5 Unlike simple hydrodesulfurization (HDS) processes, ARDS is a multitask process, where heavy petroleum feeds are hydrotreated and converted to light products that have lower boiling range, in comparison to the original feed.6-8 In an industrial ARDS process, low-value *Author to whom correspondence should be addressed. Fax: (þ965)23980445. E-mail:
[email protected]. (1) Ancheyta, J.; Speight, J. G. Hydroprocessing of Heavy Oils and Residua; Taylor and Francis Group/CRC Press: Boca Raton, FL, 2007. (2) Absi Halabi, M.; Stanislaus, A. Oil Arab Coop. 2005, 31 (112), 25–46. (3) Ancheyta, J.; Rana, M. S.; Furimsky, E. Catal. Today 2005, 109, 3–15. (4) Selah, T. M.; Ismail, H.; Corbett, J. E.; Bali, R. S. Stud. Surf. Sci. Catal. 1989, 53, 175–188. (5) Al-Nasser, A.; Chaudhuri, S. R.; Bhattacharya, S. Stud. Surf. Sci. Catal. 1995, 100, 171–180. (6) Abbas, A. K.; Chaudhuri, S. R.; Bhattacharya, S. Proceedings of the 8th Annual Saudi-Japanese Symposium, November 29-30, 1998; pp 175-189. (7) Al-Hajji, N.; Bhattacharya, S. Proceedings of the Kuwait/Japan 4th Joint Symposium on Catalysts in Petroleum Refining: Hydrotreating and Hydrocracking of Heavy Ends, Kuwait, 2001; pp 13-22. (8) Furimsky, E. Stud. Surf. Sci. Catal. 2007, 169. (9) Gary, J. H.; Handwerk, G. E. Petroleum Refining Technology and Economics, 3rd Edition; Marcel Dekker: New York, 1994. (10) Furimsky, E. Appl. Catal. A 1998, 171, 177–206. (11) Kam, E. K.; Al-Shamali, M.; Qabazard, H. Energy Fuels 2005, 19, 753–764. r 2010 American Chemical Society
(12) Kressmann, S.; Morel, F.; Harle, V.; Kasztelan, S. Catal. Today 1998, 43, 203–215. (13) Speight, J. G. Catal. Today 2004, 98, 55–60. (14) Leyva, C.; Rana, M. S.; Trejo, F.; Ancheyta, A. Ind. Eng. Chem. Res. 2007, 46, 7448–7466. (15) Nunez, M.; Villamizar, M. Appl. Catal. A 2003, 252, 51–56. (16) Qabazard, H.; Adarme, R.; Crynes, B. L. Stud. Surf. Sci. Catal. 1989, 53, 61–75. (17) Papayannakos, N.; Marangozis, J. Chem. Eng. Sci. 1984, 39 (6), 1051–1061.
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catalyst system would fail to achieve the desired operating objective. Generally, ARDS catalysts are optimized for a range of feeds (residues) having similar bulk properties. Unfortunately, however, not all feeds of similar bulk properties would behave exactly similarly over the same catalysts. This phenomenon is regarded mainly to differences in the physicochemical characteristics and reactivity of the compositional elements (individual molecules) that comprise residues. In reality, it is likely to happen that two residues obtained from different crudes would possess same exact bulk properties, but it is unlikely to happen that both residues would possess the same exact chemical composition, qualitatively and quantitatively. In the petroleum refining industry, residual oils are ranked according to their bulk properties, such as viscosity, density, heteroatoms content, and asphaltene and Conradson carbon contents. However, such information is not sufficient to select and optimize ARDS catalysts suitably.11,18 In an industrial ARDS process, individual feeds respond differently, each according to the physicochemical properties of the individual compositional constituents that comprise it.8 For example, sulfur that is present in residual oils exists in ratios among diverse chemical environments, which vary significantly from one to another, in terms of reactivity. During hydrotreatment of a residue over a solid catalyst, individual sulfur-bearing molecules react with different mechanisms and rates, some of which are considerably responsive to sulfur removal, such as the saturates, which obey a direct sulfur removal mechanism, whereas aromatics, resins, and asphaltenes are comparatively less reactive, obeying complicated sulfur removal mechanisms that involve several parallel and consecutive reaction steps. Moreover, sulfur-bearing molecules of the same class (i.e., aromatics) exist in residual oils in diversity, in terms of type, such as thiophene, benzothiophene, and dibenzothiophene and its alkyl derivatives.19,20 Although these sulfur-bearing molecules are all aromatics, they obey different HDS pathways and, thus, they react with different rates, each according to its unique physicochemical properties. Hence, the reactivity of a resid feed for a particular hydrotreating reaction is greatly dependent on the quality and quantity of the individual molecules of concern. In actual practice, it is very difficult to identify the intrinsic reactivity of a resid feed by identifying the intrinsic reactivity of its individual compositional constituents, bceause of the difficulty in identifying the exact quality and quantity of every molecule.21,22 Nevertheless, the reactivity of residual oils still can be identified through their apparent behavior.23,24 This approach is widely practiced for ARDS catalyst selection and optimization, because of its simplicity and the wide range of useful information that can easily be provided.5,25 This work was conducted to investigate the apparent behavior of two Kuwaiti atmospheric residual oils for feeds to a commercial ARDS process. The apparent behavior of the feeds involved in the study was investigated over two types of
Table 1. Key Properties of KEC-AR and KHC-AR Feedstocks property
KEC-AR
KHC-AR
TBP API gravity total sulfur kinematic viscosity @ 100 °C CCR asphaltene metals in residue vanadium nickel SARA fractions saturates aromatics resins asphaltene (preparative) metals in asphaltene vanadium nickel sulfur in SARA fractions saturates aromatics resins asphaltene (preparative)
360 °Cþ 12.89 4.60 wt % 65 cSt 12.3 wt % 4.9 wt %
360 °Cþ 9.58 5.75 wt % 187 cSt 16.8 wt % 9.9 wt %
69 μg/g 21 μg/g
73 μg/g 35 μg/g
27.91 wt % 60.64 wt % 6.35 wt % 5.3 wt %
20.35 wt % 62.92 wt % 6.88 wt % 9.85 wt %
777 μg/g 275 μg/g
514 μg/g 309 μg/g
0.4 wt % 6.3 wt % 5.4 wt % 6.7 wt %
0.5 wt % 6.0 wt % 6.4 wt % 8.6 wt %
Table 2. Key Characteristics of the Catalysts Used in this Study Value/Comment property catalyst type bulk density (g/mL) surface area (m2/g) pore volume (cm3/g) pore diameter (A˚) macropores mesopores micropores metal content
Catalyst CAT-BA
Catalyst CAT-BC
HDM 0.4-0.6 120-220 0.7-1.2
HDS 0.55-0.75 230-270 0.3-0.8
fair predominant few low
few predominant good high
commercial ARDS catalysts. The major purpose of the study was to collect data on selected apparent kinetic parameters such as rate constants and activation energies of the HDM and HDS reactions. 2. Experimental Work 2.1. Feedstock. The feedstocks used in the study were 360 °Cþ cuts of Kuwait Export Crude (KEC) and Kuwait Ratawi/ Burgan crude (KHC). KEC-AR, which is the atmospheric residue of KEC, was acquired from one of the petroleum refineries of Kuwait National Petroleum Company (KNPC). KHC-AR, which is the atmospheric residue of KHC, was prepared by PARC, Inc., in Pennsylvania. Table 1 lists the key properties of KEC-AR and KHC-AR. 2.2. Catalyst Properties. The catalysts used in this study were two types of catalysts of a complete commercial ARDS catalyst system. The catalysts were acquired from a catalyst manufacturer. The catalysts involved in the study were abbreviated as Cat-BA and Cat-BC. Cat-BA is an HDM catalyst used in the front reactor of an industrial ARDS process to achieve hydrodemetallation, hydrodesulfurization, and hydrodeasphalting of the feed to be processed. Cat-BC is an HDS catalyst, normally used in the middle reactors of an ARDS process to reduce the sulfur content of the product from the HDM catalyst reactor to lowest levels possible. Table 2 presents key information on Cat-BA and Cat-BC. 2.3. Pilot Plant. The bench-scale unit used in this study was a product of Xytel Technology Corporation, India. The unit consists of three parallel fixed-bed reactors working in the downflow mode with the following dimensions: a total volume of 74 mL and a total length of 58 cm. Every reactor is equipped
(18) Variant, M. L. Appl. Catal. 1983, 6, 137–158. (19) Callejas, M. A.; Martinez, M. T. Energy Fuels 1999, 13, 629–636. (20) Bhan, O. K.; George, S. E. Catal. Pet. Refin. Petrochem. Ind. 1995, 100, 135–145. (21) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607–631. (22) Ho, T. C. Stud. Surf. Sci. Catal. 1999, 127, 179–186. (23) Gray, M. R.; Ayasse, A. R.; Chan, E. W.; Veljkovic, M. Energy Fuels 1995, 9 (3), 500–506. (24) Hung, C.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19 (2), 250–257. (25) Alvarez, A.; Ancheyta, J. Appl. Catal. A 2008, 351, 148–158.
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Bahzad et al. Table 3. Operating Conditions Applied during Kinetic Evaluation Tests operating parameter
value
temperature liquid hourly space velocity, LHSV pressure hydrogen flow rate H2/oil ratio
370, 390, 410 °C 1.0 h-1 12000 kPa 10.3 N L/h 680 mL/mL
Kuwait Institute for Scientific Research. After adapting the catalyst for kinetic evaluation, kinetic parameters of the HDM and HDS reactions were evaluated at three temperature levels, specifically, 370, 390, and 410 °C. During testing, product samples were collected and analyzed for certain properties. Kinetic evaluation test runs were performed under the conditions listed in Table 3. 2.4.4. Product Analysis. The sulfur content of the feed and product liquid samples was determined using an Oxford 3500 sulfur analyzer, according to ASTM Testing Method D3120. Metals (vanadium and nickel) were analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Varian model), according to ASTM Testing Method D5184. Figure 1. Reactor loading and heating shell setup.
3. Data Processing and Results
with independent liquid feeding, gas feeding, and product receiving facilities. The reactors are contained in one common heating shell. The heating shell is divided to three zones aligned top to bottom as follows: preheat zone, catalyst heat zone, and postheat zone. Each of the reactors is equipped with three j-type thermocouples that are contained in thermowells aligned axially along the internal centerline of reactors tubes. During testing, the reactor temperature was controlled through several skin thermocouples that were attached to the outer skins of the reactors. In each test run, the three reactors were filled with one catalyst and operated under identical operating conditions to assess the repeatability of test results. 2.4. Pilot Plant Test Procedure. The test procedures for evaluating the kinetic parameters for both catalysts using KEC-AR as feedstocks are detailed as follows: 2.4.1. Catalyst Loading. Fifteen milliliters (15 mL) of a catalyst mixed with an equal amount of medium-grade carborundum (mesh 20) was placed in the middle of the reactor, as shown in Figure 1. The upper and lower zones of the reactor were filled with medium-grade carborundum to reduce channeling effects and enhance even oil distribution over the reactor cross section. 2.4.2. Test Procedure and Operating Conditions. After loading the catalyst into the reactors, the reactors were assembled and attached to the testing unit, each in its corresponding position. First, the unit was tested for leakage, using nitrogen gas under a pressure equal to 20 bar above the targeted operating pressure. Nitrogen then was purged and replaced with hydrogen. Leak testing was applied again, using hydrogen at 135 bar. When the unit confirmed safe operation, the hydrogen pressure was reduced to 120 bar and the hydrogen flow rate was maintained at 13 L/h. The reactor temperature was increased from ambient temperature to 150 °C by ramping at a rate of 20 °C/h. At 150 °C, the presulfiding feed, which consisted of 1% CS2 and 99% straight run gas oil (SRGO) (on a volume basis) was pumped into the reactors at a rate of 30 mL/h. This condition was maintained constant until liquid product started to collect in the gas/ liquid separators. The reactor temperature then was increased to 230 °C. At 230 °C, the operating conditions were maintained constant for 4 h and then, the reactor temperature was increased to 300 °C. At 300 °C, operating conditions were fixed constant for 6 h, and then the sulfiding feed was replaced with real residue and the hydrogen flow rate was adjusted to 10.3 L/h. 2.4.2. Kinetic Evaluation Test Run. After catalyst presulfiding, the catalyst was stabilized using a technique developed at the
The hydrotreating reactions of Kuwaiti residual oils obey second-order kinetics.26,27 Rate constants of the HDM and HDS reactions were determined using the following secondorder kinetic expression: ! LHSV 1 1 ðfor n ¼ 6 1Þ ð1Þ k ¼ n -1 Mp n -1 Mf n -1 where k is the reaction apparent rate constant, LHSV the liquid hourly space velocity (on a volume basis), n the reaction kinetic order, Mp the concentration of material in the product, and Mf the concentration of material in the feed. Magnitudes of the activation energy of the investigated reactions were determined using the linear form of the standard Arrhenius equation: Ea ð2Þ ln k ¼ ln A RT where A is the frequency factor, Ea the activation energy, R the gas constant, and T the absolute temperature. Table 4 lists values of the apparent rate constant of the HDM and HDS reactions of both feedstocks investigated in the study, and Table 5 lists values for the conversion and the activation energies of the mentioned reactions of the same feeds. 4. Discussion The commercial ARDS catalysts used in this study were Cat-BA, which was composed of molybdenum deposited on gamma alumina (Mo/γ-Al2O3), and Cat-BC, which was composed of nickel molybdenum deposited on gamma alumina (NiMo/γ-Al2O3). The main characteristics of both Cat-BA and Cat-BC are listed in Table 2. 4.1. Hydrodemetallation Reaction. The results showed that vanadium removal is relatively higher than nickel removal for both feeds over both catalysts, despite the differences in the feed properties and the catalyst characteristics. This phenomenon can be regarded as being mainly due to the (26) Ho, T. C.; Aris, R. AIChE J. 1987, 33, 1050–1051. (27) Bahzad, D.; Al-Fadhli, J. Proceedings of the Japan/Kuwait Joint Symposium, January 22-23, 2008, Kuwait; 2008; pp 178-196.
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Table 4. Apparent Rate Constants of HDM and HDS Reactionsa Apparent Rate Constants KEC-AR temperature (°C)
HDS
HDV
KHC-AR HDNi
HDS
HDV
HDNi
0.059 0.115 0.222
0.008 0.017 0.037
0.007 0.013 0.024
0.222 0.381 0.702
0.009 0.014 0.022
0.009 0.015 0.024
Catalyst CAT-BA 370 390 410
0.050 0.104 0.216
0.014 0.023 0.054
0.015 0.025 0.048
Catalyst CAT-BC 370 390 410
0.326 0.597 1.318
0.014 0.022 0.040
0.017 0.026 0.054
a Abbreviations: HDM, hydrodemetallization; HDS, hydrodesulfurization; HDV, hydrodevanadization; HDNi, hydrodenickelization.
Figure 2. Effect of catalyst type on HDV reactions using KEC-AR and KHC-AR.
polarity, which makes nickel more associated and hindered.32-34 Concerning the effect of catalyst type on hydrodevanadization (HDV) reactions, it was noticed that HDV removal was higher over BA, which had a larger average pore diameter. Figure 2 shows the effect of catalyst type on HDV removal. For KEC-AR, the extent of vanadium removal was almost the same over both catalysts in the temperature range of 370-390 °C. At 410 °C, on the other hand, KEC-AR exhibited higher reactivity over BA, although the catalytic strength of BC (NiMo) is higher than that of BA (Mo), in terms of metal loading, surface area, and hydrogenation function. Based on the results of Hauser et al.,35 most of the vanadium present in KEC-AR is accommodated in the asphaltene fraction with a ratio reaching ∼60% of the total vanadium content. Thus, reaching high conversion levels of vanadium removal imposes deep removal of the asphaltenic vanadium.36 However, for the asphaltenic vanadium to be removed effectively, the asphaltene itself must unrestrictedly reach the catalyst interior surfaces where most of the active sites are located.37,38 Unlike BC, BA pores are relatively wide and passable for the large molecules, such as the resins and asphaltene. This suggests that BA interior surfaces were virtually accessible for the asphaltenic vanadium due to BA large pores, and, hence, the enhanced vanadium removal. As reflected in Figure 2, vanadium removal from KHC-AR seems to be greatly affected by the pores (size) of the catalyst rather than the catalytic strength of the catalyst. Furthermore, with regard to the effect of catalyst pores, the ratio of metal loading to support of the catalysts could have another effect on vanadium removal reactions of both feeds. A typical ARDS catalyst is usually composed of metals deposited on an acidic support. The metallic moiety is responsible for the hydrogenation reactions, while the acidic moiety (support) is responsible for the cracking reactions. For an HDM catalyst such as BA, the acidic moiety must be
Table 5. Conversion and Magnitude of the Activation Energy of the HDM and HDS Reactions KEC-AR HDS
HDV
KHC-AR HDNi
HDS
HDV
HDNi
25.43 39.71 56.12 28.9
35.62 55.23 73.06 34.7
20.66 31.79 45.16 25.1
56.09 68.68 80.14 25.1
39.46 49.65 61.45 18.1
23.97 34.09 45.94 21.6
Catalyst CAT-BA conversion 370 °C 390 °C 410 °C Ea (kcal/mol)
18.56 32.40 49.89 32.2
48.23 61.07 78.89 30.2
24.32 34.37 50.30 25.0
Catalyst CAT-BC conversion 370 °C 390 °C 410 °C Ea (kcal/mol)
60.00 73.31 85.84 30.4
48.50 60.72 73.36 23.4
25.96 35.58 53.05 25.5
chemistry of the vanadium- and nickel-containing molecules. Vanadium and nickel present in residual oils exist in two distinct chemical environments, namely, porphyrins and nonporphyrins. Porphyrins exist as isolated molecules or linked to large asphaltenic groups, whereas nonporphyrins refer to the metals present within the asphaltene skeleton, but in an undefined manner.19,20,24,28,29 Unlike nickel porphyrins, vanadium porphyrins contain an O atom that is linked to the V atom through a double bond. The O atom protrudes over the porphyrin plane, which provides an enhanced site for adsorption over the catalyst surface and, thus, makes vanadium removal relatively easier, in comparison to nickel removal.30,31 For the nonporphyrins, on the other hand, vanadium tends to concentrate on the outer edges of the asphaltene skeleton, while nickel tends to exist in the centers of the asphaltene skeleton in locations of relatively high (28) Bonne, R. L. C.; Van Steenderen, P.; Van Langeveld, A. D.; Moulijn, J. A. Ind. Eng. Chem. Res. 1995, 34 (11), 3801–3807. (29) Ali, M. F.; Perzanowski, H.; Bukhari, A.; Al-Haji, A. A. Energy Fuels 1993, 7 (2), 179–184. (30) Choudhary, T. V.; Parrott, S.; Johnson, B. Catal. Commun. 2008, 9, 1853–1857. (31) Marafi, A.; Al-Bazzaz, H.; Al-Marri, M.; Maruyama, F.; Absi-Halabi, M.; Stanislaus, A. Energy Fuels 2003, 13, 1191–1197. (32) Leyva, C.; Rana, M. S.; Trejo, F.; Ancheyta, J. Catal. Today 2009, 141, 168–175. (33) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal, A. Energy Fuels 2000, 14 (1), 25–30.
(34) Fish, R. H.; Komlenic, J. J.; Wines, B. K. Anal. Chem. 1984, 56 (13), 2452–2460. (35) Hauser, A.; Bahzad, D.; Stanislaus, A.; Behbahani, M. Energy Fuels 2008, 22, 449–454. (36) Koinuma, Y.; Kushiyama, S.; Aizawa, R.; Kobayashi, S.; Uemasu, I.; Mizuno, K. ACS Div. Pet. Chem., Prepr. 1997, 42 (2), 331–335. (37) Li, C.; Chen, Y. W.; Tsai, M. C. Ind. Eng. Chem. Res. 1995, 34 (3), 898–905. (38) Tsai, M. C.; Chen, Y. W.; Li, C. Ind. Eng. Chem. Res. 1993, 32 (8), 1603–1609.
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reasonably effective to crack the large feed molecules, such as the resins and asphaltenes, to smaller fragments that have higher intrinsic reactivity, whereas, for an HDS catalyst such as BC, the metallic moiety must be efficient with regard to heteroatom removal and hydrogen-addition reactions.3,8,10,39 Accordingly, the enhanced activity exhibited by BA for vanadium removal is most likely a result of a combination of factors, including effectively passable pores for large molecules and efficient acidic functionality.32,40 Concerning reactivity of the two feeds, it is obvious that KEC-AR is more reactive than KHC-AR over both catalysts, although the vanadium content in both feeds is practically the same. This variation is most probably due to differences in the reactivity of the individual vanadium species of both feeds. Vanadium present in residual oils can possibly occur in three distinct chemical environments, namely, aromatics, resins, and asphaltenes.41,42 For KECAR and KHC-AR, most of the vanadium is concentrated in the asphaltene fraction and the remaining amount may be distributed in ratios between the aromatics and resins fractions. As shown in Table 1, the asphaltene fraction of KECAR accommodates ∼60% of the total vanadium content of this feed, while asphaltenic vanadium in KHC-AR represents ∼70% of the total vanadium content.35 Thus, it is apparent that the extent of vanadium removal of both feeds is dependent primarily on the reactivity of the asphaltenic vanadium species. Although the amount of vanadium accommodated in the asphaltene fraction of KEC-AR is less than the corresponding amount of KHC-AR, the vanadium in the asphaltene fraction of KEC-AR is comparatively 1.5-fold denser, as a 1 g of asphaltene of KEC-AR contains 777 μg of vanadium, compared to 514 μg of vanadium per gram of asphaltene for KHC-AR. This suggests that the asphaltenic vanadium of KEC-AR is comparatively more reactive than the corresponding asphaltenic vanadium of KHC-AR.31,35,43,44 The HDV activation energy of KEC-AR and KHC-AR over BA and BC is presented in Figure 3. As illustrated in the figure, the activation energy of both feeds clearly was influenced by the type of catalyst used. Both feeds exhibited higher activation energy over BA, although BA catalytic strength is relatively low, in comparison with BC. The activation energy of hydrotreating reactions of a residual oil undergoing catalytic hydrotreatment can be influenced by several factors, including catalyst type, feedstock reactivity, reactor configuration, and operating conditions.45 For BA, the pores are relatively wide and passable for a wide range of vanadium species, including the large ones, such as the resins and asphaltenes, whereas BC pores are relatively small and impassable for the large vanadium species. This suggests that HDV activation energy recorded over BA represents the reactivity of a wide spectrum of vanadium species of varied
Figure 3. Arrhenius plot of the HDV activation energy of KEC-AR and KHC-AR over BA and BC.
Figure 4. Effect of catalyst type on HDNi, hydrodenickelization reactions using KEC-AR and KHC-AR.
reactivities, whereas the activation energy recorded over BC represents the reactivity of a narrow spectrum of vanadium species, most likely representing the reactivity of the small vanadium species. Unlike vanadium removal, nickel removal reactions did not show appreciable dependence on the catalyst porosity, because both feeds were slightly more reactive over BC, although BC pores are relatively small, in comparison to BA pores, as shown in Figure 4. Regardless of the diffusion effect, this behavior is not clearly understood, because vanadium removal reactions were not affected in the same manner, considering that most of the nickel present in the feeds is accommodated in the asphaltene fractions in the feeds, with ratios reaching 70% for KEC-AR and ∼87% for KHC-AR. Nevertheless, BC seemed quite selective for nickel species, which may be attributed to the chemical composition of BC, most probably to BC hydrogenation function that is enhanced by the presence of nickel.3,29,46 On the other hand, KEC-AR exhibited higher reactivity over both catalysts, in comparison with KHC-AR. This behavior can be attributed mainly to the asphaltene content of both feeds, because residues with low asphaltene content are normally more reactive than residues with high asphaltene content.8,31,43 For the activation energy, it was found that neither the type of catalyst nor the chemical composition of the feeds had
(39) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis: Science and Technology, Vol. 11; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, Germany, 1996. (40) Breysse, M.; Afanasiev, P.; Geantet, C.; Variant, M. Catal. Today 2003, 86, 5–16. (41) Savage, P. E. Chem. Eng. Sci. 1990, 45 (4), 859–873. (42) Savage, P. E.; Klein, M. T.; Kukes, D. G. Ind. Eng. Chem. Process Des. Dev. 1985, 24 (4), 1169-1174. (43) Al-Mutairi, A.; Bahzad, D.; Absi Halabi, M. Catal. Today 2007, 125, 203–210. (44) Callejas, M. A.; Martı´ nez, M. T. Energy Fuels 2000, 14 (6), 1304– 1308. (45) Kam, E. T.; Al-Bazzaz, H.; Al-Fadhli, J. Ind. Eng. Chem. Res. 2008, 47 (22), 8594–8601.
(46) Rana, M. S.; Ancheyta, J.; Rayo, P.; Maity, S. K. Fuel 2007, 86, 263–1269.
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Figure 7. Arrhenius plot of the HDS activation energy of KEC-AR and KHC-AR over BA and BC.
Figure 5. Arrhenius plot of the HDNi activation energy of KEC-AR and KHC-AR over BA and BC.
asphaltenes is removed through a complicated reaction mechanism that consists of several consecutive and parallel steps, including dehydrogenation, cracking, and hydrogenation reactions, followed by sulfur removal.47,48 Unfortunately, ∼98% of the total sulfur in each KEC-AR and KHC-AR is contained in the aromatics, resins, and asphaltenes (as shown in Table 1). For these types of sulfur molecules to be removed effectively over a contact catalyst, the catalyst must possess a strong hydrogenation function to promote hydrogenation reactions associated with hydrodesulfurization reactions. Numerous studies have been conducted on the hydrodesulfurization of residual oils using different types of solid contact catalysts of varied hydrogenation strength.8,10,39 With the exception of those studies, which reported a diffusion effect, the findings of the other many studies provided solid evidence confirming that the higher the catalyst hydrogenation strength (function), the more effective the catalyst for hydrodesulfurizing residual oils. Thus, it is strongly accepted that the relatively higher reactivity exhibited by both feeds over BC was primarily due to the hydrogenation strength of BC, which is enhanced by the presence of nickel. Interestingly, sulfur removal of both feeds appeared unaffected by BC pores, considering that BC pores are relatively small and resistive. Unlike vanadium, which is concentrated in the asphaltene fractions of the examined feeds, sulfur that is contained in the resins and asphaltene fractions accounts for ∼15% of the total sulfur in KEC-AR and ∼25% of the total sulfur in KHC-AR. Assuming that BC pores are impassable for the resins and asphaltene molecules, the pores are yet virtually passable for the saturate- and aromatic-containing compounds of both feeds, taking into consideration that such types of compounds are characteristically small and can unrestrictedly diffuse throughout small pores as wide as e80 A˚.10,37,38 With regard to activation energy, it was noticed that the activation energy of KEC-AR is higher than the activation energy of KHC-AR over both catalysts, as illustrated in Figure 7. This suggests that sulfur species present in KECAR are relatively more responsive to increases in temperature, compared to the corresponding sulfur species present in KHCAR. The reactivity of individual sulfur compounds present in real heavy oils has been reported well by Choudhary et al.49
Figure 6. Effect of catalyst type on HDS reactions of KEC-AR and KHC-AR.
appreciable influence on the magnitude of the HDNi activation energy, because both feeds exhibited almost the same value over both catalysts, as shown in Figure 5. 4.2. Hydrodesulfurization Reaction. For hydrodesulfurization, the results showed that both feeds are comparatively more reactive over BC, as presented in Figure 6. This behavior can be attributed mainly to the physical and chemical properties of BC. BC is characterized by a large surface area and relatively high metal loading, compared to BA. For a solid contact catalyst, assuming a negligible diffusion effect, the larger the catalyst surface area, the higher is the number of molecules that can react simultaneously and, consequently, the higher the reaction rate, in terms of the number of molecules reacted per unit surface area per unit time.22,26 Also, BC contains nickel, which is a promoting element for the hydrogenation reactions. Sulfur that is present in residual oils occurs in diverse chemical environments of varied physicochemical characteristics and reactivity.35,45,47 In residual oils, sulfur exists in ratios among four distinct chemical environments, namely, saturates, aromatics, resins, and asphaltenes. Sulfur that is contained in saturates is removed through the direct elimination of the S atom (hydrogenolysis) by the cleavage of the S-H bond, whereas sulfur contained in the aromatics, resins, and
(48) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30 (9), 2021-2058. (49) Choudhary, T. V. Ind. Eng. Chem. Res. 2007, 46, 8363–8370.
(47) Chang, J.; Liu, J.; Li, D. Catal. Today 1998, 43, 233–239.
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Energy Fuels 2010, 24, 1495–1501
: DOI:10.1021/ef9012104
Bahzad et al.
On the other hand, it was observed that neither the chemical composition of the catalysts nor their porosities had appreciable influence on the activation energy of KEC-AR. For KHCAR, the activation energy was influenced by the catalyst porosities, because the reported value over BC is lower by 5 kcal/mol, compared to the reported value over BA. This variation could be due to diffusion limitation, as BC pores are relatively small and restrictive.
smaller pores, in comparison to Cat-BA pores. With regard to sulfur removal, we found that KEC-AR was generally more reactive than KHC-AR over both catalysts involved in the study. On the other hand, however, we noticed that KHC-AR is slightly more reactive than KEC-AR at 370 °C, using Cat-BA. For the activation energies of the reactions investigated in the study, we found that HDS and HDNi activation energies of both feeds were not affected appreciably by catalyst type. For the HDV, we noticed that the activation energy was considerably higher over Cat-BA, which had large pores in comparison with Cat-BC.
Conclusions Selected apparent kinetic parameters of the hydrodesulfurization (HDS) and hydrodemetallation (HDM) reactions of two Kuwaiti residual oils were determined over two types of commercial ARDS catalysts. The results showed that KECAR is generally more reactive than KHC-AR, in terms of hydrodemetallation and hydrodesulfurization over both catalysts involved in the study. On the other hand, we found that vanadium removal from KEC-AR and KHC-AR correlates proportionally with the porosity of the catalysts used, because both feeds exhibited higher reactivity overt Cat-BA, which had large pores. Unlike vanadium removal, nickel removal from both feeds was not affected by catalyst porosity, because both feeds were slightly more reactive over Cat-BC, which had
Acknowledgment. The authors would like to acknowledge the financial support of the Kuwait Institute for Scientific Research (KISR), and the Japan Cooperation Center, Petroleum (JCCP), a Japanese organization supported by Japan Ministry of Economics, Trade and Industry (METI). The authors also give their special thanks to the management of Japan Energy Corporation (JEC) for their support and technical assistance in pursuing this project. The author also acknowledges Kuwait National Petroleum Company for their support and cooperation. Special thank to Kuwait Oil Company for providing us with heavier feedstocks. The authors would also like to thank the National Scientific and Technical Information Center (NSTIC).
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