ARTICLE pubs.acs.org/EF
Asphaltene Chemical Characterization as a Function of Solubility: Effects on Stability and Aggregation Estrella Rogel,* Cesar Ovalles, and Michael Moir Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States ABSTRACT: Rather recently, a new technique called the “asphaltene solubility profile” was developed to analyze the stability of asphaltenes. In this technique, a distribution of the asphaltenes in terms of their respective solubilities is provided, allowing for a more comprehensive view of how the different molecules comprising asphaltenes behave and interact. In the present work, maltenes and asphaltenes are preparatively separated according to the classification established by the asphaltene solubility profile analysis, i.e., “transitional material” or easy-to-dissolve asphaltenes (EDA) and difficult-to-dissolve asphaltenes (DDA). The results obtained for blends of DDA and EDA show that the latter fraction has a peptizing effect on the former. EDA help in the solubilization of DDA, as shown by the asphaltene solubility profile of the blends, and also decreases aggregation, as shown by the size-exclusion chromatography experiments. These findings support the following: (a) the idea that this so-called “transitional fraction or EDA” plays a key role in the stabilization of asphaltenes similar to the fraction known as “resins” in the literature, (b) the use of solubility profile analysis as a valid test to evaluate asphaltene stability, and (c) the model of crude oils as a continuum of molecules in which their properties vary gradually.
’ INTRODUCTION Understanding the solubility of asphaltenes in petroleumrelated materials is of central importance in preventing or minimizing asphaltene precipitation during production and refining operations. However, this is not an easy task. Asphaltene is a broad definition that includes more than 105 different molecules according to one estimate, and its solubility depends upon the complex interaction with other species present in the crude oil.1 Rather recently,2,3 a new technique was developed to analyze the solubility of asphaltenes with particular emphasis in understanding how the distribution of asphaltenes with different solubilities in the media affects the stability of the material. Generally speaking, evaluation of asphaltene solubility using traditional methods yields a value that is determined by the least soluble asphaltene that can be detected.4 In the present technique, a distribution of the asphaltenes in terms of their respective solubilities is provided, allowing for a more comprehensive view of how the different molecules comprising asphaltenes behave and interact. The solubility profile method has been successfully used to evaluate stability of virgin and processed materials,2 and it has shown great potential to follow asphaltene conversion during different processes.3,5 In a typical solubility profile of asphaltenes (see Figure 1), two main regions can be identified, corresponding to the so-called “easy-to-dissolve” asphaltenes (EDA) and “difficult-to-dissolve” asphaltenes (DDA), and it has been postulated that the presence of EDA is a key factor in keeping the rest of the asphaltenes in solution.2 In fact, it has been established6 that the capacity of a material to act as a solvent for a substance can be estimated by a comparison of their solubility parameters. The closer their values, the more likely they are to be miscible. Two substances with solubility parameters different enough so that they are immiscible in each other can be mutually solubilized by the addition of a third component whose solubility parameter lies between the r 2011 American Chemical Society
Figure 1. Example of a typical solubility profile for a virgin material and the deconvolution of the signal in EDA and DDA peaks.
two previous substances. An experimental proof about how this concept works for crude oils and related materials has been around for some time because it was found that asphaltenes from a saturates, aromatics, resins, and asphaltenes (SARA) analysis cannot be redissolved unless resins are present.7 Theoretical calculations8 as well as asphaltene separations9 have shown the
Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 15, 2011 Revised: November 7, 2011 Published: November 12, 2011 2655
dx.doi.org/10.1021/ef2013979 | Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
ARTICLE
importance of fractions, such as resins, that can act as intermediate material between maltenes and asphaltenes. The main goal of this work is to obtain a deeper understanding on how interactions between different solubility fractions affect the phase behavior of asphaltenes. In this case, the main idea is to determine if the EDA can act as peptizing agents for the DDA, so that the original hypothesis about EDA being a key factor in keeping asphaltenes in solution can be tested. To this end, preparative separations of maltenes, EDA, and DDA were carried out, and the fractions obtained were studied using a variety of techniques, including solubility profile analysis and size-exclusion chromatography (SEC).
’ EXPERIMENTAL SECTION Materials. A total of 20 crude oils from different origins were studied to evaluate asphaltene content using the solubility profile. Three different samples were fractionated in maltenes, EDA, and DDA: two vacuum residues (A and B) obtained from heavy crude oils and a visbroken residue (C) obtained from a commercial visbreaking unit. High-performance liquid chromatography (HPLC)-grade solvents methylene chloride, methanol, n-heptane, and tetrahydrofuran (THF) were purchased from Fisher Scientific and used without further purification. Asphaltene Determination by the Gravimetric Method. The asphaltene content was determined using a modification of the American Society for Testing and Materials (ASTM) D6560 test.10 In this modified version, a 1:20 sample/n-heptane ratio is used, and this solution is filtered at 80 °C. The precipitated material is washed using hot heptane prior to drying and weighing. The last traces of precipitate are removed from the digestion beaker using chloroform and then recovered.
Solubility Profile of Asphaltenes by the On-Column Method. Solutions of the samples in methylene chloride were prepared (1 wt % for residues and variable concentration for fractions). The solutions are injected in a column packed with an inert material using n-heptane as the mobile phase. This solvent induces the precipitation of asphaltenes and, as a consequence, their retention in the column. The first eluted fraction from the column is the maltenes, which is soluble in n-heptane. After all of this fraction has eluted, the mobile phase is changed gradually from pure n-heptane to 90:10 methylene chloride/methanol and then to 100% methanol. Asphaltenes are quantified using an evaporative light scattering detector (ELSD). The HPLC system consisted of a HP Series 1100 chromatograph and an Alltech ELSD 2000 detector. A detailed account of the technique is presented elsewhere.2,3 This procedure redissolves the asphaltenes gradually from the EDA (low-solubility parameter) to the DDA (high-solubility parameter). On the basis of the ELSD, a curve is generated that is related to the solubility properties of the asphaltenes and can be quantified to reflect the tendency of the sample toward asphaltene precipitation. In fact, it has been shown that the characteristic parameter of the curve, also known as ΔPS, is correlated to the p value of the sample.2 Accelerated Solvent Extraction of Samples. For the preparative separation of the fractions, an accelerated solvent extractor (ASE) Dionex 300 was used. A total of 4 g of sample was dissolved in 50 mL of methylene chloride. This solution was stirred with 45 g of polytetrafluoroethylene (PTFE) (4060 mesh) at room temperature for about 1 h. The solvent was removed by heating at 60 °C under nitrogen overnight. The PTFE-supported sample was placed into a 100 mL stainless-steel 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. The first asphaltene fraction (EDA) was extracted 3 times with heptane at 80 °C for 10 min. The second asphaltene fraction
Figure 2. Comparison between the asphaltene solubility profiles of gravimetric and on-column precipitated asphaltenes from a virgin crude oil (normalized responses for comparison purposes). (DDA) was extracted with 10:90 methanol/dichloromethane at room temperature. The fractions were dried under nitrogen, weighed, and analyzed using asphaltene solubility profile analysis, SEC, and thermogravimetric analysis (TGA). Elemental analysis of the samples was carried out using a Carlo-Elba analyzer (C, N, and H), and sulfur content was determined using ASTM D1552.11 SEC. The SEC studies were performed using a 30 0.10 cm Mixed E column with THF as the eluent during the first 20 min. After that, the solvent is switched to 90:10 methylene chloride/methanol. The flow rate used was 1.0 mL/min. The temperature was kept constant at 25 °C. Sample solutions in THF were prepared, dissolving around 0.01 g of sample in 10 mL of solvent. The injection volume was 40 μL, and all of the samples were run by duplicates. An evaporative light scattering detector (ELSD) was used (Alltech 2000). TGA. The analyses were carried out using a TGA apparatus. Approximately 310 mg of sample was used in each run. The samples were heated from 30 to 900 °C with a heating rate of 10 °C/min under a nitrogen atmosphere.
’ RESULTS AND DISCUSSION Correlation between On-Column Asphaltenes and Gravimetric Asphaltenes. A qualitative comparison between on-
column asphaltenes and gravimetric asphaltenes indicates that both materials have different solubility properties. An example of such a comparison is shown in Figure 2. As noticed, being the case for all of the virgin materials studied, the on-column precipitated asphaltenes contain a significant proportion of easily soluble compounds (EDA) that elute at the beginning with the lower solvent power blends. This can be mainly attributed to the different extraction temperatures used in both procedures. The higher temperature used during the traditional method (80 °C) contributes in the dissolution of the more soluble species, leaving behind only the DDA that correspond to the gravimetric asphaltenes obtained using the modified ASTM D6560. For the on-column asphaltenes, the obtained trace can be separated in two peaks that correspond to two different components: relatively easy to dissolve asphaltenes (EDA) and highly insoluble material (DDA). As mentioned, EDA should contain compounds that help to keep the hard-to-dissolve asphaltenes in solution, providing the transition in terms of solubility parameter 2656
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
ARTICLE
from the maltenes to DDA. They precipitate in heptane at room temperature, but they are soluble at high temperature and have also been called “binding resins”.12 Without this transitional material, the DDA would be immiscible in the maltenes. In Figure 2, it can be noticed that the eluted time for the gravimetric asphaltenes roughly corresponds to the shoulder and/ or peak in the on-column asphaltenes at 1518 min. The relative areas of these two peaks in the on-column asphaltene solubility profile can be calculated using a peak-fitting numerical subroutine. The log-normal function was used to fit the two peaks. Figure 1 shows an example of the peak fitting for a crude oil sample. On the basis of these calculations, a good correlation (r2 = 0.89) was found between the second peak area and the asphaltene content of the sample determined by the gravimetric technique. This indicates that it is possible to estimate the amount of asphaltenes determined by the standard gravimetric technique using the area of the second peak of the solubility profile. In the next section, a calibration procedure is used to quantify the amount of the hard-to-dissolve asphaltenes, which correspond to the asphaltenes obtained by the gravimetric standard technique. Even though the solubility profile method was not designed to determine asphaltene content, it can be used for this purpose. In particular, the main interest is to determine the amount of DDA and compare the results to the gravimetric values. A calibration procedure was developed on the basis of the following equation that relates the measured peak area (A) to the sample mass (M) when ELS detectors are used:13 A ¼ CM
B
Table 1. Content and Elemental Analyses of Vacuum and Visbroken Residue Fractions content sample
fraction maltenes
ð1Þ
where C and B are coefficients depending upon droplet size, concentration, solvent, nature of the solute, and some other variables, including the design of the detector.1416 Several asphaltene solutions are prepared using gravimetric asphaltenes from a heavy crude oil, and their asphaltene solubility profiles were determined. The areas of the second peaks in the solubility profile for these solutions are related to the masses M of gravimetric asphaltenes according to log M ¼ 0:5336 log A 6:097
Figure 3. Comparison between asphaltene contents measured using asphaltene solubility profile and measured using the traditional gravimetric technique.
vacuum residue A
EDA
N
S
76
86.2
10.7
1.2
nda
3
95.7
10
1.5
nd
94.6 82.0
8.4 10.3
1.9 1.0
nd 4.4
EDA
7
82.8
9.0
1.3
nd
DDA
28
81.7
7.6
1.6
6.9
maltenes
85
84.8
10.7
1.0
0.8
3
86.5
8.7
1.1
nd
11
88.9
6.7
1.3
1.2
visbroken residue C EDA DDA a
H
21 75
DDA maltenes vacuum residue B
C
(wt %) (wt %) (wt %) (wt %) (wt %)
nd = not enough was obtained to carry out the test.
ð2Þ
where M and A are the mass and area, respectively. On the basis of eq 2, the contents of DDA in several samples were calculated and their values were correlated to the content of asphaltenes directly determined by the gravimetric method. As seen in Figure 3, there is a good correlation (r2 = 0.88) between both sets of values. Therefore, it can be concluded that the second peak corresponds to the gravimetric C7 asphaltenes. Preparative Separation of Fractions. On the basis of the findings of the previous sections, a procedure for the separation of maltenes, first-peak EDA, and shoulder DDA was devised as described in the Experimental Section. Basically, maltenes are separated as the portion soluble in heptane at room temperature, and the first asphaltene peak (EDA) is separated as the portion insoluble in heptane at room temperature but soluble in heptane at 80 °C. The last asphaltene portion (DDA) is insoluble in heptane at 80 °C but soluble in the methylene chloride/ methanol blend that is used at the end of the separation scheme. The results on the preparative separation of the samples by accelerated solvent extraction are shown in Table 1. Mass balances for the extractions varied between 92 and 98%, and the extractions were run in duplicate in all of the cases. Values reported in Table 1 reflect the average between two measurements.
Figure 4. Solubility profiles of the samples studied.
In terms of fraction composition, it is noticeable that all samples show a remarkably small content of EDA in comparison to what is observed from the solubility profiles shown in Figure 4 for the samples. Three main factors can be responsible for this 2657
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
ARTICLE
Figure 5. H/C and N/C for the fractions from different samples. Figure 7. Comparison of the TGA first-derivative profiles (DTG) for fractions extracted from vacuum residue B.
Figure 6. Comparison of the TGA first-derivative profiles (DTG) for fractions extracted from vacuum residue A.
difference: lower efficiency of the preparative separation scheme, effect of the solvent/sample ratio, and unequal response of the ELSD to different materials/solvent blends. In the next sections, these factors will be discussed thoroughly. As expected, the elemental analysis indicates a decrease in the H/C ratio from the maltene fraction to the DDA fraction, as seen in Figure 5. It was also found that the N content increased, which supports the idea of more polar and more hydrogen-deficient species as the fraction becomes less soluble. Similar results were found during a preparative separation using a similar method but a different separation scheme.17 Thermogravimetric Study of the Fractions. Comparisons of the TGA first-derivative profiles (DTG) of the fractions for all of the samples are shown in Figures 68. As seen, fractions exhibit similar behaviors. As expected, weight losses start at higher temperatures (from 250 to 350 °C) in the order DDA > EDA > maltenes. For all of the fractions, the maximum weight loss is seen at 450 °C. In terms of the coke yields (defined as the residue at 900 °C) calculated using the TGA analysis, it has been found that they correlated well with the elemental composition (H/C molar
Figure 8. Comparison of the TGA first-derivative profiles (DTG) for fractions extracted from visbroken residue C.
ratio) of the fractions, as shown in Figure 9. The DDA fraction from the visbroken residue C produced the largest amount of coke (67 wt %), followed by the DDA fractions from vacuum residues A and B (around 5055 wt %). EDA coke yields are around 35 wt %, while maltenes produced 1015 wt % coke. Asphaltene Solubility Profile of the Fractions. The fractions were studied using the asphaltene solubility profile technique. They were injected under the same conditions using sample solutions of 1000 ppm in methylene chloride. For maltenes, a smaller amount of material was used (200 ppm) because the signal was out of scale when larger concentrations were employed. In general, it was observed that the first fraction or maltenes come out in the right region of the chromatogram, also showing a small peak in the area of the EDA fraction. This behavior can be seen in Figure 10 for the maltenes extracted from vacuum residue A. For the second fraction or EDA, a partition of the material was observed between maltenes and the expected second region 2658
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
ARTICLE
Figure 9. Coke formation as a function of the H/C ratio of the fractions. Figure 11. Asphaltene solubility profile for the EDA extracted from vacuum residue A.
Figure 10. Asphaltene solubility profile for the maltenes extracted from vacuum residue A.
(around 13 min), indicating that the preparative procedure was unable to achieve a complete separation of the second fraction, EDA. Figure 11 shows an example for the EDA extracted from vacuum residue A. As seen, for this fraction, a large maltene peak is obtained that can be attributed to an inefficient separation and also to the higher response of the maltenes in comparison to the other fractions. For the DDA fraction, most of the material comes out in the expected region (1518 min), with relatively small peaks in the first (maltenes) and second fractions, as seen in the example in Figure 12. In summary, the preparative extractions were relatively successful in separating maltenes and DDA, but they could not completely separate the EDA fraction with the same efficiency. It can be argued that the asphaltenes might have an effect on the distribution of the transitional material by interacting strongly with these species, and therefore, these species co-precipitated with asphaltenes during the solubility profile test and eluted together. However, during the preparative extraction, repeat washes with heptane eliminated most of this fraction, and when they were injected alone, some of the material eluted as maltenes. Similar solubility changes have been reported in the past for asphaltenes and resins after extraction.9
Figure 12. Asphaltene solubility profile for the DDA extracted from the visbroken residue C.
Effect of EDA on the Solubility of DDA. Another important aspect related to the characterization of the fractions was to study the effect that the addition of the second fraction (EDA) could have on the solubilization of the third fraction (DDA). To this end, several blends of the fractions from vacuum residue A were evaluated using the asphaltene solubility profile analysis. Figure 13 shows a sequence where separated fractions and blends are compared. For all of the cases, the amount of DDA was the same, while the amount of EDA was changed to reflect different EDA/DDA ratios. The solubility profiles of the blends are compared in the plots to the solubility profile of the components at the same composition as present in the blend. The comparison of panels ad of Figure 13 indicates that there are strong interactions between both fractions. Specifically, the second peak or shoulder corresponding to the DDA moves to the left as the amount of EDA in the blend increases. This seems to indicate that the DDA become more soluble because they eluted earlier at a lower solubility parameter than if they are alone. The calculation of the ΔPS of the actual blends shows a decrease in this stability parameter as the EDA/DDA ratio increases, as seen 2659
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
ARTICLE
Figure 13. Asphaltene solubility profiles of different blends of EDA and DDA at different EDA/DDA weight ratios: (a) 0.2, (b) 0.6, (c) 1.0, and (d) 2.0. Comparison to solutions of these fractions alone.
in Figure 14. A lower value of ΔPS indicates a more stable material; therefore, the stability of the blend increases as the relative amount of EDA increases.2,3,5 In another approach to study stability in these samples, theoretical ΔPS values were calculated on the basis of the signal addition of the two asphaltene fractions and compared to the experimental values shown in Figure 15. This comparison indicates that theoretical ΔPS values are larger than the experimental values probably because of the existence of a solubilizing effect of EDA on DDA. This effect decreases the ΔPS of the blends in correspondence with a smaller tendency to asphaltene precipitation when the second fraction (EDA) is present. This result is not new. It has been shown previously that resin materials have a solubilizing effect on asphaltenes;4,7,9,18 however, to our knowledge, this is the first time in which this effect is linked to the solubility continuum of petroleum-related materials. Also, this result explains the reason why the solubility profile analysis can be used as an effective tool to evaluate stability in petroleum-related materials. SEC of the Fractions. The different fractions were also studied in terms of size distribution. As an example, Figure 16 shows the combined chromatographs of the vacuum residue A. As expected, the maltenes show apparent molecular sizes slightly smaller than EDA, whereas the latter show apparent molecular sizes slightly smaller than DDA. These results are in agreement
Figure 14. Stability parameter ΔPS of the blends as a function of the EDA/DDA ratio.
with previous findings that showed similar tendencies for maltenes, resins, and asphaltenes. In another set of experiments, a series of blends of the two last asphaltene fractions (EDA and DDA) were compared, as shown in Figure 17, for vacuum residue B. The comparison indicates 2660
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
Figure 15. Stability parameter ΔPS of the blends as a function of the EDA/DDA ratio.
ARTICLE
Figure 17. Comparison of the SEC chromatograms of fractions and blends of fractions at different EDA/DDA ratios (vacuum residue B).
analysis.23 In this work, it was found that the least soluble resins tend to aggregate with asphaltenes to enhance their stability in mixtures of heptanes and toluene.23
Figure 16. Comparison of the SEC chromatograms for the different fractions extracted from vacuum residue A.
that, as more EDA are added to DDA, the distribution shows a shift to the right, indicating lower sizes. This can indicate deaggregation of DDA induced by the presence of EDA, and the effect seems to be larger as more of this fraction is added. A similar behavior was found for the other two vacuum and visbroken residue samples. The deaggregation of asphaltenes in the presence of resins has been reported previously.1921 According to small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements, the size of the aggregates seems to decrease when resins are added. According to the SEC results presented in this work, the transitional material or EDA seem to have the same effect as the resins extracted using other methods. It is important to mention that this transitional material with peptizing activity is very close to asphaltenes in terms of solubility. In fact, recently, a thermodynamic model22 of the mixed aggregation of asphaltenes and resins revealed that the effectiveness of different resins to split asphaltene aggregates is closely related to their solubility. Highly soluble resins do not become incorporated into the aggregates and, therefore, cannot act as deagglomerating agents. This behavior has also been confirmed using impedance
’ CONCLUSION (1) Accelerated solvent extraction was used to separate the asphaltenes according to the classification established by the asphaltene solubility profile analysis: EDA and DDA. The separation was successful in isolating the DDA and maltene fractions. For EDA, a partition of the material was observed between maltenes and the expected EDA, indicating that the proposed procedure was unable to reach a complete separation of this fraction. (2) It was shown that EDA have a peptizing effect on DDA. This fraction helps in the solubilization of DDA, as shown by the asphaltene solubility profiles of the blends. It also decreases aggregation, as shown by the SEC experiments. These findings support the following: (a) the idea that this so-called “transitional fraction or EDA” plays a key role in the stabilization of asphaltenes similar to the fraction known as “resins” in the literature, (b) the use of solubility profile analysis as a valid test to evaluate asphaltene stability, and (c) the model of crude oils as a continuum of molecules in which their properties vary gradually. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: (510) 242-1725. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank Chevron ETC for financial support and permission to publish this work. The technical assistance of Lori Thomas, Richard Dutta, and Bhavani Shetty as well as enlightening conversations with L. Carbognani about size-exclusion chromatography are greatly appreciated. ’ REFERENCES (1) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (2) Rogel, E.; Ovalles, C.; Moir, M. Energy Fuels 2010, 24, 4369. 2661
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
Energy & Fuels
ARTICLE
(3) Rogel, E.; Ovalles, C.; Carbognani, L.; Lopez-Linares, F.; Fathi, M. M.; Pereira-Almao, P. Prepr. Pap.Am. Chem. Soc., Div. Pet. Chem. 2011, 56, 3. (4) Rogel, E.; Leon, O.; Contreras, E.; Carbognani, L.; Torres, G.; Espidel, J.; Zambrano, A. Energy Fuels 2003, 17, 1583. (5) Lopez-Linares, F.; Carbognani, L.; Pereira-Almao, P.; Rogel, E.; Ovalles, C.; Pradham, A.; Zintmaster, J. Prepr. Pap.Am. Chem. Soc., Div. Pet. Chem. 2011, 56, 20. (6) Hildebrand, J. H.; Scott, R. L. Regular Solutions; Prentice-Hall: Englewood Cliffs, NJ, 1962. (7) Koots, J. A.; Speight, J. G. Fuel 1975, 54, 179. (8) Rogel, E. Energy Fuels 2008, 22, 3922. (9) Selucky, M. L.; Kim, S. S.; Skinner, F.; Strausz, O. P. Adv. Chem. Ser. 1979, 195, 83. (10) 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. (11) American Society for Testing and Materials (ASTM). ASTM D1552, Standard Test Method for Sulfur in Petroleum Products (HighTemperature Method); ASTM International: West Conshohocken, PA, 2008. (12) Graham, B. F.; May, E, F.; Trengove, R. D. Energy Fuels 2008, 22, 1093. (13) Mengerink, Y.; de Man, H. C. J.; van der Wal, S. J. Liq. Chromatogr. 1991, 552, 593. (14) van der Meeren, P.; Vanderdeelen, J.; Baert, L. Anal. Chem. 1992, 64, 1058. (15) Righezza, M.; Guiochon, G. J. Liq. Chromatogr. 1988, 11, 1967. (16) Luzio, G. A. J. Liq. Chromatogr. Relat. Technol. 2006, 29, 185. (17) Ovalles, C.; Rogel, E.; Moir, M.; Thomas, L.; Pradhan, A. Prepr. Pap.Am. Chem. Soc., Div. Pet. Chem. 2011, 56, 8. (18) Leon, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18, 5106. (19) Barre, L.; Espinat, D.; Rosenberg, E.; Scarsella, M. Rev. Inst. Fr. Pet. 1997, 52, 161. (20) Bardon, Ch.; Barre, L.; Espinat, D.; Guille, V.; Li, M. H.; Lambard, J.; Ravey, J. C.; Rosenberg, E.; Zemb, T. Fuel Sci. Technol. Int. 1996, 14, 203. (21) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Pet. Sci. Technol. 2003, 21, 461. (22) Rogel, E. Energy Fuels 2008, 22, 3929. (23) Sedghi, M.; Goual, L. Energy Fuels 2010, 24, 2275.
2662
dx.doi.org/10.1021/ef2013979 |Energy Fuels 2012, 26, 2655–2662
