Energy & Fuels 2006, 20, 2475-2477
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Effect of n-Pentane and n-Heptane Insolubles on the Pyrolysis of Vacuum Residue Budeebazar Avid,*,† Shinya Sato,‡ Toshimasa Takanohashi,‡ and Ikuo Saito‡ Institute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar-51, Mongolia, and Institute of Energy Utilization, National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ReceiVed September 5, 2005. ReVised Manuscript ReceiVed April 6, 2006
Tests have been performed using a vacuum residue from the Middle East and its subfractions, pentane solubles (C5S), pentane insoluble-hepane solubles (C5IC7S), and heptane insolubles (C7I), as feedstock in different solvents, to evaluate their ability upon coke formation. The feedstock including those three subfractions produced a negligible amount of coke (1-1.2 wt %) and 14.7-16.7 wt % asphaltene; the latter was formed more in the feedstock (9.4 wt %), at 420 °C. There was no significant effect on coke and asphaltene formations in the products depending upon the solvent used in the tests. Heptane-soluble subfraction (C7S) formed 9.213.6 wt % asphaltenes but no coke at 420 °C using 1-methylnaphthalene and decahydronaphthalene, while the asphaltene contents were only 2.4-6.0 wt % in the case of C5S. The properties and average molecular structural parameters indicated that C5S included only 5.1 aromatic rings per average fused ring system, while C5IC7S and C7I contained 8.3 and 10.1 aromatic rings, respectively, which showed that C5IC7S was much closer to C7I than C5I. The results suggest that removing C5IC7S from the subfractions is very effective in suppressing coking trouble during upgrading.
Introduction Our increasing reliance on liquid fuels, coupled with the decrease in oil reserves, has provided the impetus for research into how alternative fuel sources, such as bitumen and petroleum, might be utilized. This can be achieved by primary upgrading of bitumen and petroleum residues to distillate products, where the objective is to maximize the yield of cracked products. It is well-known that heavy oil and residues can contain significant amounts of asphaltene. Asphaltenes are defined as that portion of crude oil that is insoluble in n-alkanes, such as n-heptane or n-pentane, but soluble in benzene or toluene, and they are responsible for coke deposition and catalyst deactivation.1-4 One of possible ways to avoid these problems is to remove the asphaltene fractions from the feedstock. However, the amount and composition of the asphaltene fractions are important for both production and processing considerations. Although there are many papers about asphaltene where both n-heptane5-10 and n-pentane11-16 have been used to precipitate the asphaltenes, * To whom correspondence should be addressed. Fax: +81-29-8618432. E-mail:
[email protected]. † Mongolian Academy of Sciences. ‡ National Institute of Advanced Industrial Science and Technology. (1) Pfeiffer, J. PH. The Properties of Asphaltic Bitumen; Elsevier: The Netherlands, 1950; p 289. (2) McLean, J. D.; Spiecker, P. M.; Sullivan, A. P.; Kilpatrick, P. K. In Structures and Dymamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum: New York, 1998; pp 377-422. (3) Deo, M. D.; Hwang, J.; Hanson, F. V. Fuel Process. Technol. 1993, 34, 217-228. (4) Sheu, E. Y. Energy Fuel 2002, 16, 74-82. (5) Avid, B.; Sato, S.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 283-284. (6) Rahmani, S.; McCaffrey, W.; Elliott, J. A. W.; Gray, M. R. Ind. Eng. Chem. Res. 2003, 42, 4101-4108. (7) Miller, J. T.; Fisher, R. B.; Thiyagaranjan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290-1298. (8) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2001, 15, 429-437. (9) Ostlund, J. A.; Wattana, P.; Nyden, M.; Fogler, H. S. J. Colloid Interface Sci. 2004, 271, 372-380.
there is much less emphasis on the comparison of these asphaltenes with respect to the properties and coke formation during pyrolysis. Several works have been published on colloidal behavior of the asphaltenes, where surface-tension measurements have been used in solutions formed by any of these two types of asphaltenes, pentane insolubles (C5I) or heptane insolubles (C7I).17-18 Kilpatrick noted that n-pentane would generally precipitate more heavy-end components with a wider variety of molecular weights and polarities than n-heptane.2 Andersen and Speight provided a detailed discussion about petroleum subfractions, and they noted that the fraction that is soluble in n-heptane but insoluble in n-pentane is a high-molecular-weight resin.19 The object of this work is to examine the properties of subfractions of the petroleum feedstock precipitated by n-pentane and n-heptane using elemental analysis, nuclear magnetic resonance (NMR), and gel-permeation chromatography (GPC) and to evaluate the ability of these subfractions upon coke formation. Experimental Section The feedstock was prepared from Middle East vacuum residue by removing neutral fractions using liquid propane.3 The feedstock (10) Andersen, S. I.; Keul, A.; Stenby, E. Petrol. Sci. Technol. 1997, 15, 611-645. (11) Yang, M. G.; Eser, S. Prepr.-Am. Chem. Soc., DiV. Fuel Chem. 1999, 44, 768-778. (12) Dehkissia, S.; Larachi, F.; Chornet, E. Fuel 2004, 83, 1323-1331. (13) Rahimi, P. M.; Gentzis, T. Fuel Process. Technol. 2003, 80, 6979. (14) Storm, D. A.; Decanio, S. J.; Edwards, J. C.; Sheu, E. Y. Petrol. Sci. Technol. 1997, 15, 77-102. (15) Chakma, A. Fuel Process. Technol. 1993, 36, 147-153. (16) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287-296. (17) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299-302. (18) Ramos, A. C. D.; Haraguchi, L.; Notrispe, R.; Loh, W.; Mohamed, R. S. J. Petrol. Sci. Eng. 2001, 32, 201-216. (19) Anderson, S. I.; Speight, J. G. Petrol. Sci. Technol. 2001, 19, 1-34.
10.1021/ef050287r CCC: $33.50 © 2006 American Chemical Society Published on Web 10/28/2006
2476 Energy & Fuels, Vol. 20, No. 6, 2006
AVid et al. Table 1. Results of the Analysis for the Feedstock and Its Subfractions sample name recovery (wt %) elemental analysis (wt %) C H N S O (diff.) H/C 1H NMRa Ha HR Hβ Hγ fa Mn (GPC)
Figure 1. Scheme for the separation of the petroleum feedstock.
sample was further separated into four subfractions, pentane solubles (C5S), heptane solubles (C7S, including C5S), pentane insolubleheptane solubles (C5IC7S), and heptane insolubles (C7I), using n-heptane and n-pentane according to the scheme shown in Figure 1. Each fraction was characterized by elemental analysis, GPC, NMR, and a coking test. NMR analyses were performed using a JEOL model Lambda 500 spectrometer. Samples were prepared for 1H NMR by mixing 10 mg of sample with 700 µL of deuteriochloroform (CDCl3). Samples for 13C NMR were preparedby dissolving ∼100 mg of sample in 700 µL of CDCl3. Two types of pulse sequences were used with the 13C NMR analysis. One was an inverse-gated decoupling system (NNE, pulse width of 4.85 µs, acquisition time of 0.967 s, and pulse delay of 7 s). The resulting spectrum was the product of 9000 scans. The other pulse sequence was a DEPT pulse sequence, in which the signals of quaternary carbons were suppressed to quantify protonated aromatic carbons. DEPT spectra were collected at flip angles of 135° and 45° and a pulse delay of 3 s. The other conditions were the same as those for NNE. The average molecular weight of the asphaltene subfractions was determined using a GPC system (JASCO) fitted with a KF403HQ Shodex column (exclusive limit of 70 000) and an evaporative laser scattering detector (ELSD). Chloroform was the elution solvent, and polystyrene was used as the calibration standard for the molecular weight. The coking test was undertaken in a batch autoclave using a 18 mL quartz tube. The tube, in which about 0.5 g of sample and 2 mL of solvent were loaded, was placed in a 50 mL autoclave and pressurized with nitrogen to 1 MPa. Decalin, 1-methylnaphthalene (1-MN), and quinoline were used as solvents. The autoclave was then heated in an oven at 300-420 °C for 1 h while being agitated at a rate of 30 rpm and cooled to room temperature. Then, the reactor was vented, and the liquid product was washed with 40 parts of toluene and kept overnight. The toluene-insoluble product (coke) was separated using a filter and then dried in a vacuum at 60 °C overnight. Toluene was removed from the filtrate using a rotary evaporator at 60 °C. The residue was kept overnight after adding 40 times n-heptane, and then the insoluble material was filtered and dried to give the asphaltene yield. The remaining fraction was recovered from the solution by removing n-heptane and dried to give the oil yield. The procedure of structural analysis is described elsewhere.5
Results and Discussion Properties. The elemental analysis of the subfractions (Table 1) shows that the H/C atomic ratio of fractions C7I and C5IC7S is similar (i.e., around 1.0) and that these fractions have more heteroatoms and less hydrogen content than the C5S fraction. The carbon aromaticity (fa) of the C5IC7S was close to that of the C7I fractions aromaticity, and it is much higher than the C5S aromaticity. Molecular weight (Mn) for C5IC7S and C7I fractions was in the range of 800-900 Da, while C5S has a molecular weight of around 1200 Da. The solubilities of maltene
feedstock
C5S
C5IC7S
C7I
87.4
3.2
9.4
84.5 10.4 0.4 3.8 0.9 1.48
85.1 10.1 0.4 3.6 0.8 1.41
84.6 7.8 0.8 5.1 1.7 1.10
84.6 6.8 1.0 5.9 1.7 0.96
5.8 12.5 60.7 21.0 0.32
6.3 12.3 61.0 20.4 0.30 1219
9.3 18.9 52.1 19.7 0.50 971
10.4 15.2 48.5 25.9 0.51 860
100
a Ha, aromatic hydrogen; HR, hydrogen-attached carbon at R to the aromatic ring; Hβ, other hydrogen; Hγ, hydrogen at terminal methyl group.
Table 2. Results of the Coking Test on Feedstock and Its Subfractions distribution of the coke formation (wt %) sample
petroleum feedstock
C7S
C5S
temperature (°C) 300 350 400 420 300 350 400 420 300 350 400 420 300 350 400 420 300 350 400 420 300 350 400 420 300 350 400 420
solvent 1-MN
decalin
quinoline
1-MN
decalin
1-MN
decalin
coke
asphaltene
maltene or oil, by diff.
0 0 0 2.1 0 0 0 1.5 0 0 0 1.0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
7.1 7.6 12.9 15.5 8.1 7.4 12.4 14.7 6.8 7.1 13.2 16.7 1.1 2.4 8.2 13.6 0 0 7.2 9.2 0 0 2.1 6.0 0 0 2.0 2.4
92.9 92.4 87.1 82.4 91.9 92.6 87.6 83.8 93.2 92.9 86.8 82.3 98.9 97.6 91.8 86.4 100 100 92.8 90.8 100 100 97.9 94 100 100 98 97.6
and asphaltene decrease with an increase in their molecular weight, aromaticity, and polarity. It is considered that C5IC7S and C7I were quite aromatic and more polar than C5S because of the high nitrogen content. Therefore, C5S with a higher Mn and lower aromaticity and polarity may be more soluble than C5IC7S and C7I with a lower Mn and higher aromaticity and polarity. The fraction C7I (insoluble in n-heptane) is asphaltene, but the chemistry of the C5IC7S fraction is not so clear; is it the asphaltene, resin, or maltene? Coking Tests. The feedstock and C7S and C5S subfractions were studied by coking tests to evaluate their coke formation abilities. Table 2 shows that the content of asphaltene (C7I) in the coking test of feedstock was increased from 7 to about 16 wt % depending upon the reaction temperature regardless of the kind of solvent. In most cases, the experimental reproducibility was satisfactory. In the case of the C7S fraction, the amount of asphaltene produced at 420 °C was around 9.213.6 wt %; i.e., the asphaltene yield decreased by 3-4 wt % from the correlating yield for the feedstock, and even 9.4 wt % C7 asphaltene (C7I) had been removed from the feedstock. The amount of asphaltene yielded at 420 °C from the C7S fraction
Pyrolysis of Vacuum Residue
Energy & Fuels, Vol. 20, No. 6, 2006 2477
Table 3. Structural Parameters per Molecule for Feedstock and Its Subfractions
Table 4. Structural Parameters per Unit for Feedstock and Its Subfractions
sample name
C5S
C5IC7S
C7I
sample name
C5S
C5IC7S
C7I
number of total carbons number of total hydrogens number of aromatic carbons number of naphthenic carbons number of paraffinic carbons number of fused ring units number of total rings number of aromatic rings number of paraffinic chains average chain length
88.1 123.4 27.9 18.1 42.1 1.4 13.5 7.2 7.6 5.5
70.5 77.4 36.1 10.9 23.5 1.3 14.8 10.8 3.5 6.7
62.8 60.5 33.3 16.7 12.8 1.0 17.0 10.1 6.0 2.1
unit foluma weight number of total carbons number of total hydrogens number of aromatic carbons number of naphthenic carbons number of paraffinic carbons number of total rings number of aromatic rings number of paraffinic chains average chain length
843 62.9 88.1 19.9 12.9 30.1 9.6 5.1 5.4 5.5
710 54.2 59.5 27.8 8.4 18.1 11.4 8.3 2.7 6.7
814 62.8 60.5 33.3 16.7 12.8 17.0 10.1 6.0 2.1
was much higher itself (3.2%), which means that during thermal reactions some parts of C5IC7S and C5S fractions were converted into asphaltene. However, after the C5IC7S fraction was removed from C7S, the asphaltene yield was decreased to 6 wt % at 420 °C in the case of 1-MN and 2.4 wt % in the case of decalin as the solvent. It is likely that decalin is more effective than 1-MN for fractions C7S and C5S; however, there was no solvent effect on feedstock conversion. It is likely that the C5IC7S has an effect on the conversion of maltene into asphaltene, because after removing C5IC7S, the conversion of maltene into asphaltene was much lower. Takanohashi et al. estimated the behavior of asphaltene aggregates using molecular dynamics and reported that some asphaltene aggregates still exist at 673 K.20 It is plausible that the aggregation of C7I was so strong that the solvent used here could not dissolve such aggregates. That is the reason that no solvent effect was observed. In the case of C5IC7S and C5S, it is plausible that their aggregations were weaker than C7I and the solvent could surround a molecule to prevent the formation of new aggregates. At the reaction temperatures of 300 and 350 °C, there are no asphaltenes, while there are just around 2 wt % at 400 °C. C7S and C5S produced no coke at any reaction temperatures investigated, while the feedstock produced about 1-2 wt % coke in each solvents. Even if there was a small amount or no coke, the produced asphaltene yield was high, which causes many problems such as producing coke or deactivation of the catalyst. The amount of C5IC7S is not high, but removing the C5IC7S fraction is very effective from the point of conversion of petroleum subfractions; i.e., to suppress the coking or catalysts trouble during the upgrading of vacuum residue, it is better to separate asphaltene fractions using n-pentane. Therefore, it was a reason that the fraction C5IC7S was separated from C7S. Structural Analysis. A spreadsheet program that was based on the Brown-Ladner method developed by Sato5,21 used results of NMR, GPC, and elemental analysis to calculate the structural parameters of the subfractions. The structural parameters per molecule and per unit are shown in Tables 3 and 4, respectively. Here, unit means the structure including only one fused ring system. C5IC7S consisted of 1.3 fused ring systems in a molecule. It is sufficient to understand that an average molecule (20) Takanohashi, T.; Sato, S.; Tanaka, R. Petrol. Sci. Technol. 2004, 22, 901-914. (21) Sato, S.; Gakkaishi, S. 1997, 40, 46-51.
of C5SC7I consists of 30% dimers bridged by an alkyl chain and 70% monomers. From a standpoint of coking, dimer structures are not important because the bridged chain is easily decomposed during thermal decomposition. As shown in Table 4, C5S contains 5.1 aromatic rings and 5.4 paraffinic chains with carbons. The unit structure has relatively few aromatic rings and many long alkyl chains. C5IC7S has 8.3 aromatic rings and 2.7 side chains with 6.7 carbons. It is less aromatic than C7I, and the chain length was similar to C5S. C7I contain 10.1 aromatic rings and 6.0 parraffinic chains with 2.1 carbons. It is more aromatic, with many short alkyl chains. The results from the coking tests suggested such structural differences. The aggregates of C7I existed originally because of their strong aggregation power. C5IC7S was originally dispersed by C5S but forms new stronger aggregates, and it was observed as C7I. This means that C5IC7S can become a core of aggregates classified as C7I. In the case of C5S, a part of the components are more aromatic than the average and may behave similar to C5IC7S but the amount is small. On the basis of the properties obtained for the C5IC7S fraction, such as the solubility in n-heptane, and the important role upon conversion of C7S into asphaltene, C5IC7S could be the most polar and aromatic part of the resin that causes the coke formation or catalyst deactivation and it also has an effect on maltene (C5S) conversion into asphaltene. Conclusions The properties of the C5IC7S (soluble in n-heptane but insoluble in n-pentane) fraction were much closer to C7I (nheptane asphaltene) than C5S (n-pentane maltene). The results of the coke formation suggest that removing the C5IC7S fraction from the subfractions is a very effective method to suppress coking trouble during upgrading. Most researchers use n-heptane for the precipitation of asphaltene; however, more favorable results with respect to the suppression of coking are obtained when the asphaltene is removed using n-pentane. Acknowledgment. B.A. acknowledges the JSPS fellowship. The authors extend their gratitude to Dr. Y. Sugimoto, M. Kouzu, and Mr. H. Ozawa (AIST) for assistance with the apparatus used in the coking tests. EF050287R
