Energy & Fuels 2003, 17, 1233-1238
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Changes in Asphaltene Properties during Hydrotreating of Heavy Crudes J. Ancheyta,*,† G. Centeno,†,‡ F. Trejo,†,§ and G. Marroquı´n† Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Col. San Bartolo Atepehuacan, Me´ xico D.F. 07730, Instituto Tecnolo´ gico de Ciudad Madero, Juventino Rosas y Jesu´ s Urueta, Col. Los Mangos, Cd. Madero, Tamps., 89440 Me´ xico, and Facultad de Quı´mica, UNAM, Ciudad Universitaria, Me´ xico D.F 04510 Received January 22, 2003
Precipitation with n-heptane and characterization of asphaltenes were carried out for products obtained during hydrotreating of Maya heavy crude oil. Hydrotreating experiments were conducted in a pilot plant at constant pressure, hydrogen-to-oil ratio, and space-velocity. Reaction temperature was varied in the range of 380-440 °C. Maya crude and hydrotreated products were characterized by API gravity and sulfur, nitrogen, nickel, vanadium, and asphaltenes contents. Elemental analysis, metals content, VPO apparent molecular weight, and NMR measurements were performed in the asphaltenes fraction. The effects of reaction temperature on asphaltenes properties during hydrotreating are discussed in terms of changes in heteroatoms contents and structural parameters.
1. Introduction Asphaltenes are the most complex molecules present in petroleum and consist of condensed polynuclear aromatics carrying alkyl, cycloalkyl, and heteratom constituents. In general, asphaltenes have higher aromaticity (low H/C molar ratio), heteroatoms content (S, N, O), metal contents (mainly V and Ni), and apparent molecular weight as compared with lighter petroleum fractions, and thereby they are commonly presumed to represent the most refractory and difficult portion of the feedstock to process.1 On the other hand, due to the increasing production of heavier crude oils, refiners face drastic changes in petroleum feed properties, which will affect all refinery conversion processes. Asphaltenes content in this new refinery feed will also be also high. Consequently, as more heavy oil is refined, its processing becomes increasingly difficult. In the particular case of hydrotreating (HDT) of heavy oil, the following problems can be caused by asphaltenes:2-4 •They affect the overall rate of HDT reactions. •Asphaltenes are precipitated on the catalyst surface and block the pore mouth. •They act as coke precursors which in turn lead to deactivation of the HDT catalysts. •Asphaltenes may limit the maximum level of conversion due to sludge formation. * Corresponding author. Fax: (+55) 3003-8429. E-mail: jancheyt@ imp.mx. † Instituto Mexicano del Petro ´ leo. ‡ Instituto Tecnolo ´ gico de Ciudad Madero. § Facultad de Quı´mica, UNAM. (1) Shirokoff, J. W.; Siddiqui, M. N.; Ali, M. F. Energy Fuels 1997, 11, 561-565. (2) Callejas, M. A.; Martı´nez, M. T. Energy Fuels 2000, 14, 13041308. (3) Calemma, V.; Rausa, R.; D’Antona, P.; Montanari, L. Energy Fuels 1998, 12, 422-428. (4) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquı´n, G.; Garcı´a, J. A.; Tenorio, E.; Torres, A. Energy Fuels 2002, 16, 1121-1127.
The main problem caused by asphaltenes is without any doubt catalyst deactivation, which affects catalyst life and, hence, operation and economics of the HDT process. To understand the mechanisms of catalyst deactivation, it is then mandatory to examine the change of asphaltene structure and composition. A deeper knowledge of their properties can be a valuable aid in achieving a better comprehension of their behavior during HDT of the heavy oils.3,5 There are a few reports in the literature related to this type of investigations. Seki and Kumata5 have reported that asphaltene quality rather than quantity plays a key role in coke deactivation, and that asphaltene aromaticity is closely related to the catalyst deactivation. Bartholdy et al.6 concluded that around 370 °C, the overall apparent reaction mechanism changes from being hydrogenation-dominated to hydrocrackingdominated, and when 380 °C temperature is reached, the asphaltenes chemistry of hydrotreated products changes. Merdrignac et al.7 observed a general decrease of high molecular weight compounds of asphaltenes as the severity of the conversion increased. Their observations were supported on size exclusion chromatography (SEC) analysis, and they drew some hypotheses for explaining the shift of the molecular weight distributions toward lower masses: •Higher mass molecules can be preferentially converted. •The change in the low/high molecular weight peak proportion may also indicate that structural changes of asphaltenes occur on conversion. (5) Seki, H.; Kumata, F. Energy Fuels 2000, 14, 980-985. (6) Bartholdy, J.; Andersen, S. I. Energy Fuels 2000, 14, 52-55. (7) Merdrignac, I.; Truchy, C.; Robert, E.; Desmazieres, B.; Guibard, I.; Haylle, F. X.; Kressmann, S. In 2002 International Conference on Heavy Organic Depositions; Lira-Galeana, C., Ed.; Jalisco, Mexico, 2002.
10.1021/ef030023+ CCC: $25.00 © 2003 American Chemical Society Published on Web 07/31/2003
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•Dealkylation of side alkyl chains during the hydrogenation process may lead to a decrease in the apparent masses detected by SEC. More recently, Tanaka et al.8 examined the size and shape of petroleum asphaltene aggregates in different solvents in situ with small-angle neutron scattering (SANS) and found that Maya asphaltene aggregate was larger than Khafji and Iranian Light ones. They also concluded that the fractal network may be related to the high coking tendency of Maya asphaltene. Similar results were reported by Liu et al.9 All these hydrotreating studies have been conducted by using petroleum residua with low asphaltenes and metals contents (asphaltenes ≈ 4 wt %, Ni + V ≈ 80114 wppm), and providing that the trend in worldwide crude oil supply indicates a growing volume of heavy crude oils, it is then important to study the modifications in asphaltene quality in the HDT of these crudes, such as Maya heavy crude oil, which contain high amounts of metals and asphaltenes. Additionally, catalysts for HDT of heavy oils are different from those used for lighter fractions, since the former are operated comparatively at higher reaction severity than the latter. The main problem when designing catalysts for hydrotreating of heavy crude oils is the presence of asphaltenes which deactivate the catalyst at a faster rate. Therefore, to develop suitable catalysts for heavy crude oil hydrotreating, studies of asphaltenes characterization are very important.10 In this work we precipitated and characterized asphaltenes obtained from Maya heavy crude oil and from various hydrotreated products in order to study the changes of asphaltene properties during catalytic hydrotreating. 2. Experimental Section 2.1. Hydrotreating Experiments. Catalytic hydrotreating of Maya crude oil was carried out in a pilot plant described elsewhere.11 The heart of the pilot plant is an isothermal fixedbed reactor. The reactor temperature is maintained at the desired level by using a three-zone electric furnace, which provides an isothermal temperature along the active reactor section. Catalytic bed temperature is measured during the experiments by three thermocouples located in a thermowell mounted at the center of the reactor. The temperature profiles are measured at the middle of each experiment by a movable axial thermocouple located inside the reactor. The greatest deviation from the desired temperature values was about 4-5 °C. The catalyst employed for hydrotreating reactions was an HPC series commercial NiMo/Al2O3 sample (175 m2/g specific surface area, 0.56 cm3/g pore volume, and 127 Å mean pore diameter). The catalyst is loaded to the reactor in the oxide form and it is in-situ sulfided during 18 h at the following conditions: pressure of 54 kg/cm2, H2/oil ratio of 2000 ft3/bbl, temperature of 230 °C, and LHSV of 3.2 h-1. The catalyst was sulfided with a desulfurized naphtha contaminated with 0.6 wt % carbon disulfide. When sulfiding is completed, the feedstock is introduced, and naphtha flow is stopped without stopping the hydrogen
Ancheyta et al. flow, and the reactor temperature and other conditions are adjusted to the desired start-of-run conditions. The HDT experiments were conducted at the following constant operating conditions: pressure of 70 kg/cm2, H2-tooil ratio of 5000 ft3/bbl, and liquid hourly space-velocity of 0.5 h-1. Reaction temperature was varied in the range of 380440 °C. Product samples were collected at 4-8 h intervals, after allowing a 2 h stabilization period under each set of conditions. Mass balances for each run were in the range 100 ( 3%. Sulfur, nickel, and vanadium contents of Maya crude and hydrotreated products were measured by combustion using a LECO SC-444 analyzer and by atomic absorption in a PerkinElmer 5000, respectively. Nitrogen content was measured by chemiluminescence. 2.2. Precipitation and Characterization of Asphaltenes. Asphaltenes were extracted from feed and hydrotreated products according to the method described in ASTM D-3279, which uses n-heptane for solvent extraction. Sulfur and metals (Ni and V) contents of asphaltenes were determined by the aforementioned techniques. Additionally, elemental composition, apparent molecular weight, and liquid state 1H and 13C NMR of asphaltenes were determined by an Elementar VARIO EL model analyzer, by vapor pressure osmometry (VPO) in a Corona Wescan 232A equipment and in a JEOL Eclipse 300 spectrometer operating at 1H resonance frequency of 300 MHz and 13C resonance frequency of 75 MHz, respectively. 1H NMR spectra were obtained as deuterated chloroform (CDCl3) solution with a flip angle of 75°, tube diameter of 5 mm, and spectral width of 220 ppm, while 13C NMR spectra were evaluated by applying an inverse-gated decoupling technique to suppress theNOE effect. Chromium acetylacetonate (Cr(acac)3 in the final solution) was added to ensure complete nuclear magnetic moment relaxation between pulses. In addition, tetramethylsilane (TMS) was employed as an internal reference. These conditions are necessary to have quantitative 13C NMR signals. Operating conditions were as follows: flip angle of 75°, tube diameter of 5 mm, CDCl3 solvent, and spectral width of 220 ppm. The measurements were performed for 30000 scans in a gated, proton decoupled mode. For 1H NMR, the spectrum was divided into three regions (0.5-2 ppm: β + γ hydrogen-toaromatic ring, 2-4 ppm: R hydrogen-to-aromatic ring, and 6-9 ppm: aromatic hydrogen) and the 13C NMR spectrum has been divided only into two different integration domains (1060 ppm: aliphatic carbon, and 110-160 ppm: aromatic carbon). The main molecular parameters from NMR spectra are the aromaticity factor (fa), average number of carbons per alkyl side chain (n), percent of substitution of aromatic rings (As), and the aromatic ring number (Ra), which are evaluated as follows:5,12
fa )
(1)
Cal Csub
(2)
n) As ) 100
substituted aromatic carbon (percent percent nonbridge aromatic carbon ) Ra )
(8) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P. Energy Fuels 2003, 17, 127-134. (9) Liu, Y. C.; Sheu, E. Y.; Chen, S. H.; Storm, D. A. Fuel 1995, 74, 1352-1356. (10) Ancheyta, J.; Maity, S. K.; Betancourt, G.; Centeno, G.; Rayo, P.; Go´mez, M. T. Appl. Catal. A 2001, 216, 195-208. (11) Ancheyta, J.; Betancourt, G.; Marroquı´n, G.; Pe´rez, A.; Maity, S. K.; Cortez, M. T.; Del Rı´o, R. Energy Fuels 2001, 15, 120-127.
Car Car + Cal
Car - Cp -1 2
(3) (4)
where Car are the total aromatic carbons, Cal the total aliphatic carbons, Csub the alkyl-substituted aromatic carbons, and Cp (12) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. O. Energy Fuels 1995, 9, 225-230.
Asphaltene Properties during Hydrotreating of Heavy Crudes
Energy & Fuels, Vol. 17, No. 5, 2003 1235
Table 1. Properties of Maya, Isthmus, and Hydrotreated Products hydrotreated products API gravity sulfur, wt % nitrogen, wppm asphaltenes in nC7, wt % vanadium, wppm nickel, wppm HDS, % HDN, % HDAsp, % HDV, % HDNi, %
Maya
380 °C
400 °C
420 °C
440 °C
Isthmus
20.9 3.4 3700 12.4 298.9 54.8
23.5 2.5 3350 8.7 201.0 49.3 26.5 9.5 29.8 32.8 10.0
24.1 2.3 3000 7.9 180.0 44.8 32.4 18.9 36.3 39.8 18.2
27.9 2.0 2870 6.5 144.0 36.9 41.2 22.4 47.6 51.8 32.7
29.7 1.5 2813 4.9 91.0 29.4 55.9 24.0 60.5 69.6 46.4
33.2 1.8 1446 3.1 79.3 20.4
the peripheral aromatic carbons (Cp ) Cus + Csub, where Cus are the unsubstituted aromatic carbons).
3. Results and Discussion 3.1. HDT Experiments. Sulfur, nickel, vanadium, nitrogen, and asphaltenes contents were substantially reduced when Maya crude oil was hydrotreated. At the most severe reaction temperature (440 °C), the sulfur content in product was lower and asphaltenes and metal contents were slightly higher than those reported in the Isthmus crude oil (reference light crude), as can be observed in Table 1. Conversions of all these contaminants, including asphaltenes, substantially increased (more than twice) when reaction temperature was increased from 380 to 440 °C. It is important to note that the change in conversions is not proportional to every change in temperature of 20 °C. In other words, if temperature is increased from 380 to 400 °C, conversions of all contaminants increase at different rates than at other increments of temperature, as can be observed in Figure 1. It is also seen from this figure that the increase in temperature from 420 to 440 °C yields higher increments in conversion than other increases of temperature except for hydrodenitrogenation which showed an inverse behavior. It indicates that hydrocracking of asphaltenes is higher at elevated temperature, and those heteroatomcontaining compounds located at the external part of the asphaltene molecule are released and hence they can be easily removed. This also implies that asphalt-
Figure 1. Variation of conversions as function of increase of reaction temperature. (Patterned box) 380-400 °C, (9) 400420 °C, (0) 420-440 °C.
Figure 2. Conversion of contaminants: (b) HDS, (O) HDNi, (9) HDN, (0) HDV.
enes composition will change as the reaction temperature is increased. It is also observed from Table 1 that V removal (HDV) is much higher than Ni removal (HDNi). This behavior has been reported previously. One reason is that the oxygen atom perpendicularly linked to V porphyrin structures forms a strong bond with the catalyst surface. This oxygen link is not present in Ni porphyrins.13-15 HDV, HDNi, and HDS follow an approximately linear correlation with HDAsp as the reaction temperature is increased (Figure 2), which is in complete agreement with reports of the literature.5 Heavy fractions of Maya crude oil were converted to lighter products, which can be observed as an increase in API gravity of the hydrotreated products, from 20.9 up to 29.7° (Table 1). The highest value of API gravity was observed at the highest reaction temperature (440 °C). It means that heavy fraction of Maya crude oil was hydrocracked, and lighter products were produced. 3.2. Characterization of Asphaltenes. The elemental analyses (C, H, O, N, S), and metal contents (Ni and V) of asphaltenes are shown in Table 2. It is clearly seen that the major constituents in order of decreasing abundance are C, H, S, N, O, V, and Ni. From values of this table and referring to a proposed asphaltene (13) Kobayashi, S.; Kushiyama, S.; Aizawa, R.; Koinuma, Y.; Keiichi, I.; Shimizu, Y.; Egi, K. Ind. Eng. Chem. Res. 1987, 26, 2245. (14) Chen, Y. W.; Hsu, W. C. Ind. Eng. Chem. Res. 1997, 36, 25262532. (15) Ancheyta, J.; Betancourt, G.; Marroquı´n, G.; Centeno, G.; Castan˜eda, L. C.; Alonso, F.; Mun˜oz, J. A.; Go´mez, M. T.; Rayo, P. Appl. Catal. A 2002, 233, 159-170.
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Ancheyta et al.
Figure 3. Structure of asphaltenes proposed by LePage et al.16
structure shown in Figure 3,16 which was represented as an open two-dimensional structure of naphthenic, aromatic linkage by aliphatic chains, sulfur bridges, and containing nickel and vanadium in porphyrin moieties, the following behavior is observed: •The H/C molar ratio of asphaltenes decreases as the temperature is increased which means that the smaller molecules formed must be more aromatic. This was also confirmed with the aromaticity factor. •Nitrogen and metal contents in asphaltenes increased as reaction temperature also increased. It has been reported in the literature that all nitrogen in
asphaltenes is aromatic,17 and these compounds are commonly located inside the asphaltene molecule (Figure 3), and because of this, they remain almost unchanged after hydrotreating, and hence they are concentrated in asphaltenes. •Sulfur content decreases as temperature is increased. During HDT reactions a scission of side chains, where asphaltenic sulfur is sometimes located in the form of sulfur bridges (Figure 3), and cracking of naphthenes are observed. It implies that sulfur is separated from the asphaltene molecule, and its removal is easier compared with nitrogen and metals, which remain in the unaffected aromatic structure. •Compared with a lighter crude oil (Isthmus) asphaltenes from hydrotreated products are more aromatic with more nitrogen, sulfur, and metals contents. 13C NMR spectra of asphaltenes from Maya, Isthmus, and hydrotreated products are presented in Figure 4. The differences in aliphatic (Cal: 10-60 ppm) and aromatic (Car: 110-160 ppm) carbons are clearly distinguished. Structural properties (fa, n, As, Ra) calculated with eqs 1-4 by using NMR data, elemental analyses, and apparent molecular weights as determined by VPO of asphaltenes are also presented in Table 2. Some of these properties are shown graphically in Figure 5 as a function of hydrotreating temperature. The following changes and behavior in values of these parameters are observed: •Apparent MW was reduced as reaction temperature was increased which implies a decrease in the size of asphaltene molecule. As a consequence of this reduction, other molecular parameters are also different than asphaltenes from Maya crude.
(16) LePage, J. F.; Morel, F.; Trassard, A. M.; Bousquet, J. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1987, 23 (2), 470.
(17) Mitra-Kirtley, S.; Mullins, O. C.; Elp, J. V.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252-258.
Table 2. Properties of Asphaltenes in nC7 hydrotreated products Maya
380 °C
400 °C
420 °C
440 °C
Isthmus
84.59 6.42 0.66 1.58 6.36
84.29 8.25 0.60 1.04 5.56
C H O N S
82.54 8.46 0.68 1.11 7.10
Elemental Analysis, wt % 83.18 83.43 83.24 7.95 7.70 7.61 0.74 0.75 0.71 1.13 1.27 1.46 6.95 6.78 6.47
H/C O/C N/C S/C
1.230 0.006 0.012 0.032
1.147 0.007 0.012 0.031
Atomic Ratios 1.108 1.097 0.007 0.006 0.013 0.015 0.030 0.029
0.911 0.006 0.016 0.028
1.170 0.005 0.011 0.025
Ni V
320 1509
302 1609
Metals, wppm 362 378 1922 1893
474 1961
180 746
MW fa n Ra As
5190 0.52 6.80 62.0 38.9
Structural Properties 2288 1861 1513 0.53 0.55 0.58 6.21 5.09 5.05 24.5 20.7 17.3 36.9 40.1 35.6
1448 0.73 4.70 21.7 19.5
3375 0.57 5.0 45.0 41.0
Asphaltene Properties during Hydrotreating of Heavy Crudes
Energy & Fuels, Vol. 17, No. 5, 2003 1237
Figure 4. NMR spectra of asphaltenes: (A) Maya, (B) 380 °C, (C) 400 °C, (D) 420 °C, (E) 440 °C, (F) Isthmus.
Figure 5. Change of apparent MW as assessed by vaporphase osmometry (VPO), aromaticity factor (fa), and average number of carbons per alkyl side chain (n) with reaction temperature.
•In general, fa increased, and n, As, and Ra decreased. •The aromaticity factor remained almost at the same value as the Maya crude asphaltenes up to 420 °C temperature; however, at higher temperature (440 °C),
it changed drastically from 0.52 in the feed to 0.73 in the product. •It should be mentioned that temperature was increased during time-on-stream to compensate for catalyst deactivation which means that samples represent increasing reaction temperatures of catalyst deactivation. Then, the increase in aromaticity factor can also be correlated to the catalyst deactivation. •Ra in asphaltenes from hydrotreated products was reduced by more than 50% compared with asphaltenes from Maya crude. As also exhibited a slight decrease up to 420 °C; however, at 440 °C it drastically reduced. This behavior suggests that hydrogenated polycondensed aromatics were converted to other fractions and that those remaining unchanged are still in the asphaltene fraction. This agrees with literature reports,5,6 but the important change in asphaltene properties was observed at lower HDT temperature. •These changes in structural properties of asphaltenes may confirm that nitrogen and metals concentrate in asphaltenes as the temperature is increased, since they are located inside the asphaltene structure. It also explains the decrease in the average number of carbons per alkyl side chain from 6.8 up to 4.7. •Aromaticity factors of asphaltenes from hydrotreated products were almost similar than Isthmus crude up to 420 °C temperature. At 440 °C, fa was higher in the hydrotreated product. •The number of carbons per alkyl side chain at temperatures in the range of 400-440 °C was more or less the same as that for Isthmus crude oil (around 5.0).
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Ra and As were considerably lower in hydrotreated products. It is seen from the above observations that temperature has an important influence in asphaltenes properties during HDT of heavy oils. At temperatures lower than 420 °C, only dealkylation of side alkyl chains was observed and at higher temperature, hydrocracking of the asphaltene molecule is prominent. The hydrotreating temperature where asphaltenes structure change depends on the other reaction conditions and properties of the HDT feed. 4. Conclusion On the basis of our characterization results, we can conclude that asphaltenes precipitated from Maya crude oil exhibit different changes in composition during catalytic hydrotreating, depending on the severity of the reaction. The temperature of 440 °C was found to be
Ancheyta et al.
the value where structural properties of asphaltenes obtained from hydrotreating of Maya crude change drastically. While nitrogen and metals content increase, sulfur decreases as the reaction temperature is increased. This different behavior was attributed to the localization of each heteroatom in the asphaltene molecule. Asphaltenes aromaticity, measured as changes of fa and H/C molar ratio, increased as the reaction temperature was also increased. Hydrocracking of asphaltenes from Maya crude was deep enough to obtain lighter asphaltenes in hydrotreated products compared with Isthmus crude oil. Acknowledgment. The authors thank Instituto Mexicano del Petro´leo for its financial support. F. Trejo also thanks CONACyT for financial support. EF030023+
