Characterization of Heavy Crude Oils, Their Fractions, and

7 Dec 2011 - Chevron Energy Technology Company, 100 Chevron Way, Richmond, California ... industry moves to more offshore production and into deep...
6 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/EF

Characterization of Heavy Crude Oils, Their Fractions, and Hydrovisbroken Products by the Asphaltene Solubility Fraction Method Cesar Ovalles,* Estrella Rogel, Michael Moir, Lori Thomas, and Ajit Pradhan Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States ABSTRACT: An improved method for the separation of asphaltene solubility fractions is presented and has been proven useful for the characterization of heavy crude oils and their fractions. Mixtures of heptane, dichloromethane, and methanol are used to obtain four different and well-defined asphaltene fractions with increasing solubility parameter. A good correlation (0.95) is found between the solubility fraction method and the gravimetric asphaltenes for virgin materials. For processed samples, the correlation depends upon the type of conversion process chosen [fluidized catalytic cracking, thermal cracking, or hydroprocessing]. The characterization of asphaltenes by the asphaltene solubility fraction method for a heavy oil feed and its visbroken products indicated that the low solubility parameter asphaltenes are processed first (“easy-to-react”) and then the higher solubility parameter counterparts (“hard-to-process”). Preparative separations and characterization of Mexican vacuum residue asphaltenes and a thermally cracked residue were carried out using an automatic solvent extractor (ASE) apparatus and the same set of solvents as the solubility fraction method. The results indicated that the H/C ratio of the extracted asphaltene fractions decreased and the aromaticity increased with the solubility parameter of the solvent. However, small differences in the distributions of asphaltene fractions were observed and were attributed to the larger precipitant/sample ratio used in the asphaltene solubility fraction method (>50:1) compared to the ASE preparative separation (20:1).

■

INTRODUCTION Asphaltenes are considered to be the “bad actors” when heavy crude oils are transported or upgraded.1 Unwanted asphaltene precipitation is a serious concern to the petroleum industry because, in addition to the choking of the pipelines, asphaltenes plug up well bores and can decrease or stop oil production. These problems are expected to become significant as the industry moves to more offshore production and into deep water, where prevention and remediation costs rise dramatically.2 On the other hand, in downstream applications, asphaltenes are believed to be the source of coke during thermal upgrading processes, reducing and limiting the yield of residue conversion. Also, in catalytic upgrading processes, they contribute to catalyst poisoning by coke and metal deposition.1,2 Asphaltenes can also cause fouling in heat exchangers and other devices in refineries.3 It is well-known that asphaltenes are the heaviest and most polar molecules of the crude oil. These compounds are not classified by the structure but are defined by a solubility class, that is, compounds that are soluble in aromatic solvents, such as toluene, and insoluble in n-alkanes, such as n-heptane.1 The characterization of asphaltenes by separating them into solubility fractions has been carried out by several authors. Pioneer work was reported by Boduszynski et al.4 in 1982, who developed a gravimetric sequential elution fractionation (SEF) method for separation of “non-distillable” petroleum residua into a set of well-defined fractions for subsequent comparison and analysis. An on-column separation methodology was reported4 using heptane, toluene, and pyridine fractions to extract coal liquids deposited onto an inert column packing. Later, Boduszynski applied a similar SEF technique to separate petroleum residua into different solubility fractions.5 © 2011 American Chemical Society

Other fractionation procedures have also been reported in the literature6−14 that differ in the type of solvents used for precipitation and redissolution, temperature, contact time, and sample/solvent ratio. However, several general observations can be made. For example, Andersen et al.6 separated and characterized subfractions of Boscan asphaltenes by precipitation and extraction using n-heptane/toluene mixtures at different temperatures and contact times. The characterization revealed that the asphaltenes obtained by precipitation have a higher molecular weight [by vapor pressure osmometry (VPO)] than those obtained by extraction. They found that the H/C molar ratio decreased and the aromaticity and N/C ratio increased with an increased polarity of the asphaltene fraction.6 Suhara and co-workers7 reported the separation of Middle East vacuum residue (VR) asphaltenes into two fractions using ultracentrifugation and different mixtures of toluene−heptane. Consistent with the findings by Anderson et al.,6 those authors found that the most polar fraction has the lowest H/C molar ratio and highest aromaticity and molecular weight (by VPO). They also separated asphaltene fractions from hydroprocessed samples and found that the most polar fractions showed a decrease in the H/C molar ratio and an increase in aromaticity and molecular weight (by VPO) as the hydroprocessing temperature increases. Also, using the Heithaus stability method, decreases in pa (measure of the peptizability of asphaltene), po (indicates the peptizing power of maltene), and P (overall state of peptization of the system) were also observed Received: October 2, 2011 Revised: November 28, 2011 Published: December 7, 2011 549

dx.doi.org/10.1021/ef201499f | Energy Fuels 2012, 26, 549−556

Energy & Fuels

Article

lacks a gradual change in the strength and type of interactions of the solvent. The second issue is that the percentages of areas for the toluene-soluble asphaltenes are much greater than those obtained with cyclohexane and dichloromethane.14,15 Therefore, changes in the content of the low- and high-polarity asphaltene solubility fractions will have high associated errors and will be hard to determine with precision. Finally, it was found17 that a significant amount of material is retained in the column after the eluting with CH2Cl2 and that the presence of these compounds cannot be detected by visual inspection.14,15 With these ideas in mind, a modification of the methodology by Schabron for the separation of asphaltene solubility fractions is presented that decreases the retention of material in the column and produces better defined fractions. It is expected that these changes would make the methodology more robust and repeatable. This new methodology uses mixtures of heptane, dichloromethane, and methanol in an attempt to make the changes in solvent power quantifiable and gradual. This new method has been proven useful for the characterization of heavy crude oils and their fractions and correlates well with the gravimetric method American Society for Testing and Materials (ASTM) D6560.16 Also, preparative separations and characterization of the asphaltene fractions are presented for Mexican VR asphaltenes and from a thermally cracked residue from Canadian crude oil.

for processed samples as the temperature of the reactor increased.7 Strausz and et al.8 carried out the separation of Athabasca asphaltenes into five fractions using gel permeation chromatography (GPC) fractionation. The results showed that the percentage of aromaticity declined (from 48 to 35%) with an increase of the molecular weight of the sample (from 1200 to 17 000 uma). No differences were found in the nitrogen, oxygen, and sulfur contents of the isolated fractions.8 Acevedo and co-workers9 reported the fractionation of asphaltenes by complex formation with p-nitrophenol. The latter compound is known to produce charge-transfer complexes with aromatic compounds, and it was suggested that the same type of compounds are formed with asphaltenes, leading to precipitation. Using this procedure, two asphaltene fractions were isolated of very different solubility.9 The results showed a lower molecular weight (by VPO) but a higher H/C molar ratio for the more soluble fraction compared to the less soluble counterpart. The results were used to propose a model for colloidal solution of asphaltenes in aromatic solvents.9 Fogler and co-workers10−12 fractionated asphaltenes from Mobil crude oil into components of different polarities using binary mixtures of dichloromethane and n-pentane. In their procedure, the asphaltene fractions that precipitated first are the more polar fractions, followed by precipitation of less polar fractions upon the addition of more pentane.10−12 Consistent with other authors,6,7 the most polar fraction (solubles in 30:70 CH2Cl2/pentane) has the lowest H/C ratio and highest content of heteroatoms and metals (V, Ni, and Fe) compared to the less polar counterparts.10−12 Also, they found that the subfraction of asphaltenes with the largest average size and the highest aromaticity also contained the asphaltenes that have the strongest tendency to flocculate.12 Kilpatrick et al.13 carried out preparative separation of asphaltenes into 20−30 discrete fractions from three different crude sources by sequential precipitation in mixtures of nheptane and toluene. For two of the crudes, H/C, N/C, and S/ C ratios appeared to obey a Gaussian distribution.13 Smallangle neutron scattering (SANS) experiments suggested that aggregate size generally decreases with the decreasing solubility parameter of the asphaltene fractions. That is, the more polar asphaltenes have a higher tendency to aggregate than the less polar counterparts.13 In all of the studies discussed previously, extensive and timeconsuming experimental methodologies were made to separate and quantify the asphaltene solubility fractions. Recently, Schabron et al.14,15 developed a faster and simpler separation technique that allows for the separation of asphaltenes into solubility fractions using on-column precipitation and redissolution of asphaltenes. In this technique, also called the “four solvent method”, a sample of crude oil or petroleum residue is injected onto a stainless-steel column packed with ground polytetrafluoroethylene (PTFE) using n-heptane as carrier solvent.14,15 Once the asphaltenes are precipitated on the column and after the maltenes were eluted, the solvent is first switched to cyclohexane, followed by toluene, and finally, CH2Cl2.14,15 In particular, Schabron et al.14,15 were interested in correlating the cyclohexane fraction with the coke tendency of the materials; therefore, this solvent was their first choice. The first issue in the method by Schabron et al. is that the last three solvents used (cyclohexane, toluene, and dichloromethane) have very different dispersive, polar, and hydrogenbond-forming interactions, and therefore, this solvent sequence

■

EXPERIMENTAL SECTION

Materials. All solvents used were high-performance liquid chromatography (HPLC)-grade, unless otherwise noted. A wide range of petroleum samples was tested, including virgin light, medium, and heavy crude oils and some of their VRs, visbroken residues, hydrotreated and fluidized catalytic cracking (FCC) products, and other hydro- and thermal-treated materials. Gravimetric asphaltene determinations were carried out using a modification of the ASTM test method D6560,16 as described elsewhere.17 This method consists of mixing the sample with heptane and heated under reflux. The precipitated asphaltenes, waxy substances, and inorganic material are collected on filter paper, and the waxy substances are removed by washing with hot heptane in an extractor. Using this method, sufficient asphaltenes were isolated and characterized by elemental analysis and nuclear magnetic resonance (NMR). Separation of Asphaltenes in Solubility Fractions. In a typical analysis, the whole feedstock is dissolved in dichloromethane (0.1 g in 10 mL) and injected (40 μL) into a 10 mm inner diameter × 250 mm stainless-steel column packed with poly(tetrafluoroethylene) (PTFE) using a heptane mobile phase. Maltenes (heptane solubles) elute from the column as the first peak (see Figure 1). The mobile phase is then

Figure 1. Typical liquid chromatography (LC) trace of the asphaltene separation using the solubility fraction method. 550

dx.doi.org/10.1021/ef201499f | Energy Fuels 2012, 26, 549−556

Energy & Fuels

Article

Table 1. Characterization Data for the Preparative Separation of Mexican VR Asphaltenesa 100% C7 preparative fractionb

15:85 CH2Cl2/C7 preparative fractionc

30:70 CH2Cl2/C7 preparative fractiond

100% CH2Cl2 preparative fractione

10% MeOH/CH2Cl2 preparative fractionf

2.04

0.16 7.9

0.18 8.9

0.24 11.8

1.4 69.0

0.05 2.5

99.5

80.84 7.66 1.47 7.34 1.14 0.64

83.26 9.83 0.30 4.87 1.42 0.37

77.30 7.70 1.30 7.10 1.20 0.57

81.10 7.50 1.50 7.20 1.11 0.65

79.90 7.00 1.60 7.30 1.05 0.64

nd nd nd nd nd nd

96 93 96 93

Mexican VR asphaltenesa mass (g)h yield fraction (%)i C (%)j H (%)k N (%)l S (%)m H/C molarn aromaticity (fa)o

mass balance (%)g

93

a

Mexican VR asphaltenes separated using ASTM D6560.16 bPreparative fraction extracted with heptane (maltenes). cPreparative asphaltene fraction extracted with 15:85 CH2Cl2/C7. dPreparative asphaltene fraction extracted with 30:70 CH2Cl2/C7. ePreparative asphaltene fraction extracted with 100% CH2Cl2. fPreparative asphaltene fraction extracted with 90:10 CH2Cl2/MeOH. gMass balance with respect to Maya VR asphaltenes. hMass of feed used and masses of product obtained after extraction with the ASE. iPercentage of yield of each asphaltene fraction. jPercentage of weight of carbon by elemental analysis. kPercentage of weight of hydrogen by elemental analysis. lPercentage of weight of nitrogen by elemental analysis. m Percentage of weight of sulfur by elemental analysis. nHydrogen/carbon atomic ratio. oAromaticity measured by 13C NMR.

Table 2. Characterization Data for the Preparative Separation from a Thermally Cracked Residuea 100% C7 preparative fractionb

15:85 CH2Cl2/C7 preparative fractionc

30:70 CH2Cl2/C7 preparative fractiond

100% CH2Cl2 preparative fractione

10% MeOH/ CH2Cl2 preparative fractionf

10% MeOH/CH2Cl2 at 100 °C preparative fractiong

mass balance (%)h

4.60

2.85 65.1

0.21 4.8

0.23 5.3

0.85 19.4

0.24 5.5

0.32 7.3

102

76.73 7.04 1.05 5.83 1.10 494 198 0.57 4.8 11.0 89.0

83.72 10.22 0.80 4.45 1.47 69 46 0.35 5.5 7.1 92.9

80.30 7.70 1.30 6.80 1.15 605 250 0.72 3.8 13.4 86.6

81.40 7.10 1.40 5.80 1.05 893 286 0.69 3.5 12.6 87.4

80.40 6.30 1.60 6.00 0.94 1195 477 0.77 4.5 13.3 86.7

80.30 5.70 1.80 5.80 0.85 1344 554 0.75 4.4 14.2 85.8

80.56 5.49 1.71 6.09 0.82 1398 579 0.75 4.4 14.2 85.8

110 126 108 89

cracked residuea mass (g)i yield fraction (%)j C (%)k H (%)l N (%)m S (%)n H/C molaro V (ppm)p Ni (ppm)q aromaticityr chain lengths Haromatict Haliphaticu

102 107 90 107 90 104

a Thermally cracked residue from Canadian crude oil. bPreparative fraction extracted with heptane (maltenes). cPreparative asphaltene fraction extracted with 15:85 CH2Cl2/C7. dPreparative asphaltene fraction extracted with 30:70 CH2Cl2/C7. ePreparative asphaltene fraction extracted with 100% CH2Cl2. fPreparative asphaltene fraction extracted with 90:10 CH2Cl2/MeOH. gPreparative asphaltene fraction extracted with 90:10 CH2Cl2/ MeOH at 100 °C. hMass balance with respect to cracked residue. iMass of feed used and masses of product obtained after extraction with the ASE. j Percentage of yield of each asphaltene fraction. kPercentage of weight of carbon by elemental analysis. lPercentage of weight of hydrogen by elemental analysis. mPercentage of weight of nitrogen by elemental analysis. nPercentage of weight of sulfur by elemental analysis. oHydrogen/ carbon atomic ratio. pVanadium content in wppm. qNickel content in wppm. rAromaticity measured by 13C NMR. sAverage alkyl side-chain length as determined by 1H NMR. tTotal aromatic hydrogen as determined by 1H NMR. uTotal aliphatic hydrogen as determined by 1H NMR

switched in successive steps to a blend of solvents of increasing solubility parameter: 15% dichloromethane/85% n-heptane, 30% dichloromethane/70% n-heptane, 100% dichloromethane, and 10% methanol/90% dichloromethane, to separate the asphaltenes into four different solubility fractions. The eluted fractions are quantified using an evaporative light scanning detector (ELSD), as described elsewhere.17 Preparative Separations. The preparative separations of the solubility fractions were carried out using Mexican VR asphaltenes and a thermally cracked residue from Canadian crude oil (percent conversion of ∼35%). The feed and fraction characteristics are listed in Tables 1 and 2, respectively. For the preparative separation, an accelerated solvent extractor (ASE) Dionex 300 apparatus was used. A total of 2 g of Mexican VR asphaltenes obtained by modified ASTM D656016 was dissolved in the minimum amount of dichloromethane. This solution was stirred with 45 g of PTFE (40−60 mesh) at room temperature for about 1 h. The solvent was removed by heating at 60 °C under nitrogen overnight.

The PTFE-supported asphaltenes were placed into a 100 mL stainlesssteel cell and extracted with heptane at room temperature with 60 min of soaking time. After flushing the cell with nitrogen and removing the solvent, the maltene fraction (heptane solubles) was collected and weighted. The asphaltene-containing cell was sequentially extracted with 15:85 CH2Cl2/C7, 30:70 CH2Cl2/C7, and 100% CH2Cl2 at room temperature for 60 min during each step. Then, the cell was extracted another 3 times but with 90:10 CH2Cl2/MeOH at room temperature for 60 min during each cycle. Finally, the cell was heated to 100 °C and extracted 3 more times with 90:10 CH2Cl2/MeOH for 60 min during each cycle. A very small amount of material (50:1) compared to the preparative separation using ASE (20:1). In addition, a thermally cracked residue was also preparatively separated using the same methodology as before, and the results are shown in Table 2. The elemental analysis and the NMR data showed mass balances between 90 and 110% (see Table 2) for the maltenes and all asphaltene fractions. As in the previous case, these results indicate the validity and consistency of the data within the experimental error of the techniques. Only the balance in hydrogen is out (126%) of the 100 ± 10 range, which could be attributed to the presence of condensed water in the sample. In this case, the largest fraction extracted from the thermally cracked residue was the heptane solubles (65 wt %), whereas the largest asphaltene fraction was the fraction soluble in 100% CH2Cl2. As mentioned in the Experimental Section, after heating the cell at 100 °C and extracting 3 more times with 90:10 CH2Cl2/CH3OH, an additional asphaltene fraction was obtained, as shown in Table 2. Consistent with the previous case and the literature reports,6,7,10,11 the H/C atomic ratios of the asphaltene fractions isolated from thermally cracked residue were found to decrease (from 1.15 to 0.82), whereas aromaticity and percentage of aromatic hydrogen increased slightly with the solubility parameter of the solvent (Figure 9 and Table 2).

is not only characteristic of thermal processes but could also be extended to catalytic hydroprocessing as well. Therefore, the asphaltene solubility fraction method could be useful to monitor changes during upgrading processes of crude oil, heavy fractions, or residua. Preparative Separation. To obtain compositional information on the asphaltene solubility fractions, Mexican VR asphaltenes were supported in PTFE and extracted with the same set of solvents used for the asphaltene solubility fraction method. The results on the preparative separation and characterization for maltenes and asphaltenes are shown in Table 1. First at all, the elemental analysis and the aromaticity determined by NMR showed mass balances between 93 and 100% (see Table 1) for all extracted fractions. These results indicate the validity and consistency of the data within the experimental error of the techniques. As shown in Table 1, a very small amount of n-heptane solubles was obtained (8%) and the most abundant fraction is the fraction extracted using 100% CH2Cl2 (69 wt %). These results are different from those obtained analyzing the original Mexican VR asphaltenes by the asphaltene solubility fraction method. As seen in Figure 8, the n-heptane fraction for Mexican VR asphaltenes represents only 36% of the total area. This

Figure 8. Percentages of areas obtained by the solubility fraction method for Mexican VR asphaltenes, maltenes, and preparatively separated fractions (see Table 1 for characterization data).

difference could be attributed to the larger precipitant/sample ratio used in the former method (>50:1) compared to the preparative separation using ASE (20:1). In other words, when a high concentration of n-heptane is used, parts of the lowpolarity asphaltenes that precipitate during the preparative separation become soluble and it is determined as maltenes (nheptane solubles) by the asphaltene solubility fraction method. Also, it is possible that the n-heptane solubles have a different response factor in the ELSD. Consistent with the observations reported in the literature,6,7,10,11 the H/C ratio of the asphaltene fractions extracted decreased and the aromaticity increased with the solubility parameter of the solvent (Table 1). In effect, the H/C molar ratio decreases from 1.20 to 1.05 and the aromaticity increases from 0.57 to 0.64 in going from 15:85 CH2Cl2/C7 to 100% CH2Cl2. Similar results were reported by Fogler and coworkers10 using binary mixtures of dichloromethane and npentane. These authors found that the fraction solubles in 30:70 CH2Cl2/pentane had the lower H/C ratio and higher

Figure 9. H/C molar ratio and aromaticity for the asphaltene fractions extracted from a thermally cracked residue.

The metal contents for the asphaltene fractions extracted from thermally cracked residue can be seen in Figure 10. As shown, the greatest concentrations (Table 2) of vanadium and nickel are found in the most polar asphaltene fractions (100% CH2Cl2, 90:10 CH2Cl2/CH3OH, and 90:10 CH2Cl2/CH3OH at 100 °C). These results are consistent with those reported by Fogler and co-workers,10 as mentioned before. As in the case of Mexican VR asphaltenes, differences are observed between the thermally cracked residue preparative separation and the result by the asphaltene solubility fraction 554

dx.doi.org/10.1021/ef201499f | Energy Fuels 2012, 26, 549−556

Energy & Fuels

Article

For virgin samples, good correlations were found between the gravimetric asphaltenes and those determined by the solubility fraction method. For processed samples, this correlation depends upon the type of process chosen (FCC, thermal cracking, and hydroprocessing). (3) The characterization of asphaltenes by the asphaltene solubility fraction method for a heavy oil feed and its visbroken products indicated that the low solubility parameter asphaltenes are processed first (“easy-toreact”) and then the higher solubility parameter counterparts (“hard-to-process”). (4) Preparative separation of Mexican VR asphaltenes and a thermally cracked residue using the same set of solvents yielded asphaltene fractions in which the H/C ratio decreased and the aromaticity increased with the solubility parameter of the solvent. (5) For a thermally cracked residue, the greatest concentrations of vanadium and nickel were found in the most polar asphaltene fractions (100% CH2Cl2, 90:10 CH2Cl2/CH3OH, and 90:10 CH2Cl2/CH3OH at 100 °C). (6) Differences are found between the asphaltene solubility fraction method and the preparative separation, which were attributed to the larger precipitant/sample ratio in the first method compared to the second method.

Figure 10. Metal yields (wt %) determined for the preparatively extracted asphaltene fractions from a thermally cracked residue.

method. Using the first methodology, 65 wt % n-heptane solubles (Table 2) was found, whereas using the second methodology, 45% of the total area was determined (Figure

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: 510-242-2991. E-mail: [email protected].

■

ACKNOWLEDGMENTS We thank the Chevron Energy Technology Company and, in particular, the Measurement and Chemistry Focus Area for providing funding and the permission to publish this paper. Many thanks to Bin Zhang for her technical assistance.

■

REFERENCES

(1) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999 and references therein. (2) Speight, J. G. Oil Gas Sci. Technol. 2004, 59, 467 andreferences therein. (3) Lemke, H. K. Proceedings of the American Institute of Chemical Engineers (AIChE) National Spring Meeting; Houston, TX, March 14− 18, 1999; Paper 57a. (4) Boduszynski, M. M.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1982, 54, 372. (5) Boduszynski, M. M. Energy Fuels 1987, 1, 2. (6) Andersen, S. I.; Keul, A.; Stenby, E. Fuel Sci. Technol. Int. 1997, 15, 611. (7) Tojima, M.; Suhara, S.; Imamura, M.; Furuta, A. Catal. Today 1998, 43, 347. (8) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809. (9) Gutierrez, L. B.; Ranaudo, M. A.; Mendez, B.; Acevedo, S. Energy Fuels 2001, 15, 624. (10) Nalwaya, V.; Tantayakom, V.; Piumsomboon, P.; Fogler, S. Ind. Eng. Chem. Res. 1999, 38, 964. (11) Ostlund, J.-A.; Nyden, M.; Fogler, H. S.; Holmberg, K. Colloids Surf., A 2004, 234, 95. (12) Ostlund, J.-A.; Wattana, P.; Nyden, M.; Fogler, H. S. J. Colloid Interface Sci. 2004, 271, 372. (13) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Energy Fuels 2006, 20, 705. (14) Schabron, J. F.; Rovani, J. F. Fuel 2008, 87, 165. (15) Schabron, J. F.; Rovani, J. F. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 2010, 55, 22. (16) American Society for Testing and Materials (ASTM). ASTM D6560, Standard Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products; ASTM International: West Conshohocken, PA, 2005.

Figure 11. Percentages of areas obtained by the solubility fraction method for a thermally cracked residue, maltenes, and preparative separated fractions (see Table 2 for characterization data).

11). Furthermore, the first two asphaltene fractions extracted from the cracked residue (15:85 CH2Cl2/C7 and 30:70 CH2Cl2/C7) are primarily composed (97−99%) of n-heptane solubles (Figure 11). In addition, the other three asphaltene fractions (100% CH2Cl2, 90:10 CH2Cl2/CH3OH, and 90:10 CH2Cl2/CH3OH at 100 °C) are mainly 100% CH2Cl2-soluble asphaltenes with small contents of 90:10 CH2Cl2/CH3OH (Figure 11). As mentioned before, these differences could be attributed to the larger precipitant/sample ratio used in the asphaltene solubility fraction method (>50:1) compared to the preparative separation (20:1).

■

CONCLUSION (1) An improved analytical method for the separation and quantification of asphaltenes by solubility fractions was developed. The asphaltene fractions were separated according to their solubility parameters from 15% dichloromethane/85% n-heptane, 30% dichloromethane/70% n-heptane, 100% dichloromethane to 10% methanol/90% dichloromethane. (2) 555

dx.doi.org/10.1021/ef201499f | Energy Fuels 2012, 26, 549−556

Energy & Fuels

Article

(17) Rogel, E.; Ovalles, C.; Moir, M. Energy Fuels 2009, 23, 4515. (18) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1983. (19) Michael, G.; Al-Siri, M.; Khan, Z. H.; Ali, F. A. Energy Fuels 2005, 19, 1598. (20) Ancheyta, J.; Centeno, G.; Trejo, F.; Speight, J. G. Catal. Today 2005, 109, 162. (21) Robert, E. C.; Merdrignac, I.; Rebours, B.; Harle, V.; Kressmann, S.; Colyar, J. Pet. Sci. Technol. 2003, 21, 615. (22) Storm, D. A.; Decanio, S. J.; Edwards, J. C.; Sheu, E. Y. Pet. Sci. Technol. 1997, 15, 77. (23) Speight, J. G. Catal. Today 2004, 98, 55.

556

dx.doi.org/10.1021/ef201499f | Energy Fuels 2012, 26, 549−556