Energy Fuels 2010, 24, 5008–5011 Published on Web 08/16/2010
: DOI:10.1021/ef1005385
Influence of Asphaltene on the Residue Hydrotreating Reaction Yu-dong Sun,*,†,‡ Chao-he Yang,‡ Hui Zhao,‡ Hong-hong Shan,‡ and Ben-xian Shen† †
East China University of Science and Technology, Shanghai, 200237, and ‡State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong, 266555 Received May 2, 2010. Revised Manuscript Received July 23, 2010
The influence of asphaltene contents on residue hydrotreating was studied in an autoclave. The feedstocks were Tahe deasphalted oil (DAO) mixed with different contents of asphaltene. The changes of asphaltene structure pre- and postreaction were analyzed. The results showed that the conversion of residue was improved by 11% age points when the asphaltene contents increased from 2% to 12%; the yield of gas, coke, gasoline and diesel increased to different degrees, but as the yield of vacuum gas oil (VGO) and >500 °C residue decreased, the removal rate of sulfur and nitrogen first increased and then decreased. The properties of asphaltene were changed after hydroprocessing. It was found that the molecular weight and hydrogen/carbon ratio of asphaltene decreased, the sulfur content decreased but the nitrogen content increased, and the aromaticity and the condensation degree increased. The sulfur and nitrogen contents in the hydrotreated residue increased with asphaltene content as the feedstock increased.
1. Introduction
Table 1. Properties of Tahe AR Asphaltene
Asphaltenes are the most complex and the biggest molecules in petroleum. They consist of condensed polyaromatic nuclei carrying alkyl groups and alicyclic systems and also contain various heteroatoms. Asphaltenes can lead to negative impacts on residue hydroprocessing.1-3 Because the unit structure, chemical structure, aggregate states of the asphaltene units, and combination states of the heteroatoms in the asphaltene are complex, there is a lack of enough deep understanding of asphaltene in petroleum processing. During hydroprocessing, asphaltenes undergo a multitude of reactions involving both cracking and hydrogenation, which change their structure.2,4-6 In this paper, the conversion characteristics of Tahe deasphalted oil (DAO) with different asphaltene contents were studied, and the structural changes of asphaltene pre- and posthydrotreating were also analyzed.
value
analysis items
value
molecular weight, M carbon, wt % hydrogen, wt % H/C, mol/mol sulfur, wt % nitrogen, wt % aromaticity factor, ( fa)
7122 84.52 7.46 1.05 4.85 1.36 0.52
σ HAU/CA total rings, RT aromatic rings, RA naphthenic rings, RN total carbon numbers, CT aromatic carbon numbers, CA
0.15 0.44 107.65 86.28 21.37 501.63 262.83
in Table 1. The properties of the mixed feedstocks are shown in Table 2. 2.2. Analysis. (1) The molecular weight was measured by vapor pressure osmometry (VPO) with Knauer equipment. The operating conditions were as follows: toluene was the solvent and a temperature of 80 °C. (2) The SARA composition was separated by “The method for contents determinate of petroleum asphalt”, which meets the standard of SH/T 0509-92 (1998). (3) The carbon, hydrogen, sulfur, and nitrogen contents of the asphaltenes were determined with a Vario EL β CNHS/O elemental analyzer. (4) The structural parameters of asphaltene were calculated by the improved Brown-Ladner (B-L) method according to the data of proton nuclear magnetic resonance (1H NMR). 1H NMR data were analyzed by an AV500 NMR spectrometer as follows: CDCl3 was the solvent, resonance frequency SF = 500.13 MHz, sampling interval D1 = 2 s, sampling time AQ = 1.6 s, and 90° pulse power P1 = 13.50 μs. 2.3. Experiment. The ratio of catalyst to feedstock was 1:10. Then the mixture was injected into an autoclave to react. The run conditions for the hydrotreating experiment were temperature of 400 °C, initial hydrogen pressure of 8.0 MPa, reaction time of 2 h. At the end of each run, a little of lthe iquid product was analyzed by simulated distillation in an HP5880A gas chromatograph (meeting the ASTM D5307 standard) after centrifugal separation to remove the catalyst. The reaction mixture (liquid product and catalyst surplus in the autoclave) was diluted by analytically pure toluene and then extracted in a Soxhlet extractor with toluene. After extraction, the washed and dried spent catalyst samples were analyzed for carbon content. Sulfur, nitrogen, SARA composition, and so on in the >350 °C residue were analyzed after distillation. The structural parameters of asphaltene in the residue were measured by 1H NMR.
2. Experimental Section 2.1. Feedstocks. The feedstocks were THAR deasphalted oil (DAO) mixed with a different content THAR asphaltene (precipitated by solvent extraction, n-heptane was used as the solvent). The properties of THAR asphaltene are shown *To whom correspondence should be addressed. Telephone: þ86532-86984702. Fax: þ86-532-86981787. E-mail:
[email protected]. (1) Gawel, I.; Bociarska, D.; Biskupski, P. Effect of asphaltenes on hydroprocessing of heavy oils and residua. Appl. Catal., A: General 2005, 295, 89–94. (2) Ancheyta, J.; Centeno, G.; Trejo., F; Marroquın, G. Changes in Asphaltene Properties during Hydrotreating of Heavy Crudes. Energy Fuels 2003, 17, 1233–1238. (3) Li, D.-d. Process. Eng. Residue Hydrogenation 2004, 12, 1133– 1175. (4) Bartholdy, J.; Andersen, S. I. Changes in Asphaltene Stability during Hydrotreating. Energy Fuels 2000, 14 (1), 52–55. (5) Buch, L.; Groenzin, H.; Buenrostro-Gonzalez, E.; et al. Molecular size of asphaltene fractions obtained from residuum hydrotreatment. Fuel 2003, 82 (9), 1075–1084. (6) Seki, H.; Kumata, F. Structural Change of Petroleum Asphaltenes and Resins by Hydrodemetallization. Energy Fuels 2000, 14 (5), 980– 985. r 2010 American Chemical Society
analysis items
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pubs.acs.org/EF
Energy Fuels 2010, 24, 5008–5011
: DOI:10.1021/ef1005385
Sun et al.
Table 2. Properties of Residue with Different Asphaltene Content properties
2% asphaltene
4% asphaltene
7% asphaltene
10% asphaltene
12% asphaltene
molecular weight, M relative density, (d20 4 ) carbon, wt % hydrogen, wt % H/C, mol/mol sulfur, wt % nitrogen, wt %
737 1.0057 85.68 10.71 1.49 2.73 0.47
741 1.0109 85.61 10.59 1.47 2.81 0.46
757 1.0139 85.66 10.52 1.46 2.88 0.49
773 1.0216 85.62 10.34 1.44 2.89 0.52
776 1.0247 85.72 10.27 1.43 2.93 0.53
saturate, wt % aromatic, wt % resin, wt % asphaltene, wt %
28.54 39.53 29.31 2.62
Four Subfractions 27.83 37.78 30.60 3.80
27.10 36.59 29.35 6.97
25.58 34.27 29.54 10.61
25.10 34.04 28.46 12.39
aromaticity factor, fa condensation index, CI total rings, RT aromatic rings, RA naphthenic rings, RN characterization index, KH
0.31 0.20 6.29 3.59 2.69 6.55
Structure Parameters 0.32 0.20 6.39 3.77 2.62 6.45
0.33 0.21 6.58 3.97 2.61 6.36
0.35 0.21 6.84 4.33 2.51 6.19
0.36 0.21 6.97 4.47 2.50 6.12
Table 3. Hydrotreating Results of Residue with Different Asphaltene Content P = 8.0 MPa, t = 2 h, T = 400 °C
reaction conditions asphaltene contents, wt % conversion, wt % sulfur removal, wt % nitrogen removal, wt % coke gas 500 °C
increasing in the feedstock because more asphaltene transformed into gasoline and diesel. Asphaltenes are the largest molecule in the residue. The particle size of asphaltene is bigger than the pore size of the hydrotreating catalyst, so there is a diffusion limitation when asphaltene enters the pore of the catalyst. Asphaltene is inclined to precipitate on the catalyst surface to form coke and block the pore mouth. The coke deposits on the catalyst and covers the acidic sites of the catalyst to reduce the activity of the catalyst. Asphaltene is the precursor of coke8 and has a high tendency to form coke in hydrotreating. The relationship between coke yields and asphaltene contents in the feedstocks is shown in Figure 2. The coke yield increased with the asphaltene content increasing apparently. The residue (>350 °C) was obtained by distilling the liquid product after the hydrotreating reaction. The properties of the hytrotreated residues are shown in Table 4. With comparison with the feedstocks, the changes of the hydrotreated residue are as following: molecular weight, H/C, sulfur content, aromatic, and resin content decreased; density, nitrogen content, saturate content, and asphaltene content increased. In the hydrotreating processing, different reactions occurred for different components. Aromatics can transform to saturate by hydrogenation saturation. Part of the resin cracks to form light hydrocarbons, and a small part of the resin converts to asphaltene by a condensation reaction and dealkylation, thus the asphaltene content in the residue should increase. On the other hand, part of the asphaltene could transform to light components by dealkylation and/or breaking weak bonds in the molecule. The hydrocracking of asphaltene plays a main role when the asphaltene content in the feedstock is high. It can be seen from Table 2-4 that the value of the asphaltene in the hydrotreated residue plus the coke yield to asphaltene in the feedstock reduced with asphaltene content in the feedstock increasing, and the value is less than 1 when the asphaltene content is more than 10% in the feedstock. It is shown that asphaltene is easier to convert into a light component when the asphaltene content is high in the feedstock.
2.62 25.55 60.12 15.92
3.80 28.89 61.09 20.19
6.97 32.80 63.14 25.33
10.61 35.83 62.84 23.38
12.39 36.96 59.77 21.50
Material Balance, wt % 1.25 1.69 1.88 1.52 1.56 1.59 6.68 8.78 11.89 16.10 16.86 17.44 52.30 53.26 52.76 22.15 17.85 14.44
1.98 1.64 12.64 19.57 50.68 13.49
2.06 1.68 13.30 19.92 49.83 13.21
3. Results and Discussion Table 3 presents the hydrotreating results of rthe esidue with different asphaltene content. 3.1. Products Distribution. The relationships between asphaltene content and conversions and product distributions are shown in Figure 1. The residue conversion, light oil (gasoline þ diesel) yield, gas yield, and coke yield increased with asphaltene content increasing, but the VGO yield and >500 °C residue yield decreased. The curves in Figure 1 show better regularities with an increase of the asphaltene content. Asphaltene has two main reaction directions in hydrotreating.7 One is the depolymerization and hydrocracking of asphaltene micelles, which can make asphaltene turn into light hydrocarbons such as gas, gasoline, and diesel. It plays an important role in desulfurization, denitrogenation, demetalization, and converting heavy oil to light products. The other is a condensation reaction to generate insoluble toluene. The latter can form coke, which precipitates on the catalyst surface to block the pore mouth, leading to the deactivation of the catalyst and shortening the run time of the unit. Asphaltenes are the most polar and high-molecular weight compounds in the residue. At high temperatures, asphaltenes can decompose to small molecular substances by rupturing the unstable bonds (e.g., bonds with heteroatoms). Therefore, the liquid yields increased with the asphaltene content
(8) Huannian, J.; Wenan, D.; Guohe, Q. The thermal reaction behavior of resin and asphaltene over dispersed catalyst in the presence of hydrogen. Pet. Process. Petrochem. 2006, 37 (11), 11–14.
(7) Li, J.; Luan, X.-d. Relation of residue hydro-upgrading between suspended bed and fixed bed. Contemp. Chem. Ind. 2007, 36 (5), 447–450.
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Energy Fuels 2010, 24, 5008–5011
: DOI:10.1021/ef1005385
Sun et al.
Figure 1. Influence of asphaltene content on product distribution and residue conversion. Table 4. Properties of >350°C Hydrotreated Residues properties
2% asphaltene
4% asphaltene
molecular weight, M relative density, (d20 4 ) carbon, wt % hydrogen, wt % H/C, mol/mol sulfur, wt % nitrogen, wt %
588 1.0109 86.75 10.59 1.46 1.46 0.53
609 1.0148 86.83 10.50 1.44 1.64 0.53
saturate, wt % aromatic, wt % resin, wt % asphaltene, wt % asphaltene in residue to feedstock, wt %
37.80 34.79 20.79 6.92 5.15
Four Subfractions 34.96 35.64 22.42 6.98 4.96
7% asphaltene
10% asphaltene
12% asphaltene
622 1.0178 86.81 10.43 1.43 1.58 0.53
607 1.0298 86.80 9.93 1.40 1.76 0.67
611 1.0285 86.52 10.18 1.36 1.87 0.66
35.78 33.17 21.45 9.59 6.44
32.16 32.25 21.63 13.97 8.96
33.40 31.91 21.58 13.11 8.26
Figure 2. Coke yield change curve with asphaltene content.
Figure 3. S, N removal rate changes versus asphaltene content.
Most of the nitrides exist in asphaltene in the stable forms of pyrrole and pyridine. Denitrogenations are more difficult under the hydrotreating operation conditions, and most nitrogen should be retained and accumulate in the products. Therefore, the nitrogen contents in the hydrotreated residue increased with the asphaltene content increasing. 3.2. Influence of Asphaltene on Desulfurization and Denitrogenation. The plots of sulfur and nitrogen removal rates versus the asphaltene content are shown in Figure 3. At first, the sulfur and nitrogen removal increased and then decreased with the asphaltene content increasing. The variation range of the nitrogen removal rate was more obvious than sulfur. The nitrogen removal rate reaches the maximum when the asphaltene content is about 7%. Asphaltene with a higher sulfur content was easier to convert than those with lower sulfur contents. The reason is that the bond energy of C-S is weaker than that of C-C.1 The sulfur mainly exists in asphaltene between aromatic
nuclei in the form of a sulfur bridge bond. The weaker C-S bonds have more opportunities to break when the sulfur content is higher in asphaltene. When asphaltene turns into small molecules by C-S bond breaking, it is easier for the intermediate products to enter into the pore channel of the catalysts to crack further. Therefore, the nitrogen in the aromatic nucleus can easily enter the pore and absorb on the active points of the catalyst to react with hydrogen. Nitrogen removal increased with the asphaltene content increasing. However, when the asphaltene content is beyond a certain value, the large molecule content is high enough and the steric hindrance effect become obvious, so it is dfficult for asphaltene to enter the pore of catalyst and absorb on the active sites to crack. It is becoming more and more difficult to remove sulfur and nitrogen (especially nitrogen) in the residue. At the same time, the molecular structure of nitrides becomes complicated and the steric hindrance effect is enhanced with the asphaltene increasing; aromatic heterocyclic nitrides increase with the 5010
Energy Fuels 2010, 24, 5008–5011
: DOI:10.1021/ef1005385
Sun et al.
Table 5. Structure Composition of Asphaltene after Hydrotreating properties
2% asphaltene
4% asphaltene
7% asphaltene
10% asphaltene
12% asphaltene
molecular weight, M carbon, wt % hydrogen, wt % H/C, mol/mol sulfur, wt % nitrogen, wt % aromaticity factor, fa total carbon numbers, CT total hydrogen numbers, HT aromatic carbon numbers, CA saturated carbon numbers, CS σ HAU/CA total rings, RT aromatic rings, RA naphthenic rings, RN
3353 84.82 6.24 0.88 2.96 1.47 0.63 237.00 207.59 149.20 87.80 0.39 0.35 59.61 48.40 11.21
3912 84.40 6.09 0.86 3.36 1.49 0.65 275.14 236.41 178.89 96.25 0.37 0.36 68.49 58.30 10.19
5351 84.99 6.10 0.86 3.24 1.52 0.65 378.98 323.85 244.97 134.02 0.35 0.35 95.57 80.32 15.25
5793 86.32 5.80 0.80 3.45 1.62 0.68 416.71 333.36 282.23 134.48 0.33 0.34 109.91 92.74 17.17
6149 85.77 5.72 0.79 4.44 1.64 0.68 439.50 348.97 298.56 140.94 0.35 0.35 116.74 98.19 18.55
feedstock becoming heavy. All of these have caused sulfur and nitrogen removal to become more and more difficult. 3.3. Asphaltene Structure Changes Pre- and Posthydrotreating. The asphaltene structure could change because asphaltene can decompose and condense in residue hydrotreating. Temperature plays a key role in residue hydrotreating. At lower hydrotreating temperatures, the hydrogenation of the polycondensed aromatic rings was observed.2,4,5 Hydrogenation can lead to part of the asphaltenes converting into resins.5 However, when the temperature was higher than 380 °C, cracking reactions become dominant. Table 5 gives the structural parameters of asphaltene after reaction, which is analyzed by 1H NMR at the reaction temperature of 400 °C in our study. The asphaltene in the residue after the reaction is not the original asphaltene in the petroleum. It is a mixture of unreacted original asphaltene after the reaction and secondary asphaltene created by the resin. With the comparison of the data in Tables 1 and 5 that were pre- and post hydrotreating, respectively, we obtain the following results: (1) The molecular weight and H/C of asphaltene decreased after hydrotreating. (2) The sulfur content decreased but nitrogen increased in asphaltene after the reaction. The contents of sulfur and nitrogen increased with asphaltene increasing. (3) The aromatic-carbon ratio and total rings increased, but σ and HAU/CA decreased. It is shown that the condensation degrees of asphaltane increased after the reaction. Resin can convert to asphaltene by dealkylation and the condensation reaction in hydrotreating. At the same time, asphaltene will convert into low molecular weight and high polar products by the rupture of the bridge bond and the dealkylation reaction. Therefore, the contents and average molecular weights of asphaltene after hydrotreating were reduced. The H/C of asphaltenes decreased and the aromaticity ( fa) and condensation degree increased after hydrotreating. The reason9 is that part of the chain structures (high H/C) in asphaltene became light oils by chain scission and part of the naphthenes became aromatics by dehydrogenation. After hydroprocessing, the aromatic carbon numbers in asphaltene increased stably and the peripheric substitution rate, the numbers and length of the alkyl side chains of the aromatic rings, decreased. Ultimately, another part of the asphaltene turned into coke by condensation and was deposited on the
catalyst with the processing of the reaction. When compared with the original asphaltene, the asphaltene after hydrotreating has a higher aromaticity and condensation degree, lower alkyl side chain numbers, and shorter alkyl side chains. The sulfur content was reduced, and the nitrogen was enhanced in asphaltene after hydrotreating compared with those in the original asphaltene. The sulfur existing in asphaltene is mainly in the forms of thioether and thiophene sulfur. Because the sulfur-carbon bond is weaker than those corresponding carbon-carbon bonds, asphaltenes with high sulfur content are easier to convert than those with lower sulfur content. The nitrogen content of asphaltenes does not decrease and sometimes even increases under the hydrotreating conditions.1 It has been reported that almost all nitrogen in asphaltenes is present in the aromatic rings located inside the asphaltene molecule,10 and the hydrogenation of these compounds is thermodynamically unfavorable. The removal of nitrogen is adverse at high temperature, and most of the nitrogen should condense in asphaltene, so the content of nitrogen in asphaltene increased. 4. Conclusions With the increase of asphaltene in the feedstock, conversion, gas yield, gasoline yield, diesel yield, and coke yield of the residue hydrotreating were increased but the VGO and the >500 °C residue yields were decreased. The molecular weight, H/C, sulfur content, aromatics, and resin of the >350 °C residue after hydrotreating were decreased. The density, nitrogen content, and saturate of the >350 °C residue were increased compared with the feedstock. The asphaltene yield in the product was higher and then lower than the feedstock with the asphaltene contents increasing in the feedstock. The removals of sulfur and nitrogen first increased and then decreased with the asphaltene content increasing in the feedstock. After hydrotreating, the molecular weight, H/C, and sulfur content of asphaltene reduced; the aromaticity, condensation degree, and nitrogen content increased. The sulfur and nitrogen contents of the asphaltene after hydrotreating were increased with the asphaltene content increasing in the feedstock. Acknowledgment. We thank the financial support by the State Key Basic Research Program of National Science and Technology Ministry of PRC (Grant No. 2006CB202505).
(9) Ancheyta, J.; Centeno, G.; Trejo, F.; Speight, J. G. Asphaltene characterization as function of time on-stream during hydroprocessing of Maya crude. Catal. Today 2005, 109, 162–166.
(10) Mitra-Kirtley, S.; Mullins, O. C.; Elp, J. V.; et al. Determination of the nitrogen chemical structure in petroleum using XANES spectroscopy. J. Am. Chem. Soc. 1993, 115 (1), 252–258.
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