Energy & Fuels 2005, 19, 2455-2459
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Tracing Sodium Naphthenate in Asphaltenes Precipitated from Athabasca Bitumen Xiaoli Yang† and Jan Czarnecki* Syncrude Canada Ltd., Edmonton Research Centre, 9421 17th Avenue, Edmonton, Alberta T6N 1H4, Canada Received May 16, 2005. Revised Manuscript Received September 8, 2005
Athabasca bitumen contains 1-2 wt % naphthenic acids, ∼18 wt % asphaltenes, and ∼40 wt % resins, all of which can stabilize bitumen-based water-in-oil (W/O) emulsions. The asphaltenes fraction is generally considered as the most important fraction in stabilization of W/O emulsions. It is also known that, once the asphaltenes are removed from a crude oil, the oil ability to form stable W/O emulsions is decreased. In conventional bitumen extraction, sodium hydroxide is used as a process aid. Therefore, naphthenic acids in bitumen can be converted to their sodium salts. Commercial sodium naphthenate (SN) is known to stabilize both oil-in-water and water-in-oil emulsions. The questions are whether SN precipitates with the asphaltenes and, if it does, whether it is SN that is responsible for emulsion stabilization rather than asphaltenes. Asphaltenes precipitation experiments were conducted with and without the addition of SN. It was found that SN does not co-precipitate with asphaltenes and likely reports into the maltenes fraction. Emulsion stabilization ability of SN was lower than that of asphaltenes. Our results indicate that the leading role in the stabilization of W/O emulsions is likely played by a not yet identified subfraction of asphaltenes. It is unlikely that asphaltenes as a whole or this subfraction contains SN.
Introduction Athabasca bitumen is a heterogeneous mixture containing thousands of different chemical species. There is about ∼18 wt % asphaltenes (pentane as precipitation solvent) and 1-2 wt % naphthenic acids in Athabasca bitumen.1-4 Naphthenic acids can be easily converted to their sodium salts, sodium naphthenates (SN), during bitumen extraction, which is routinely performed under alkaline conditions. Breaking bitumen emulsion is a challenge in the Athabasca oil sand industry due to the abundant presence of these natural surfactants. It is widely accepted that asphaltenes play a very important role in stabilizing water in crude oil emulsions.5-11 * To whom correspondence should be addressed. Tel: (780)970-6825. Fax: (780)970-6805. E-mail:
[email protected]. † Current address: Champion Technologies, 2300 Premier Way, Sherwood Park, Alberta T8H 2L2, Canada. (1) Strausz, O. P. AOSTRA/University Reports for Industry Agreement No. 30, Final Report, 1979. (2) Cyr, T. D.; Strausz, O. P. J. Chem. Soc. Chem. Commun. 1983, 1028-1083. (3) Cyr, T. D.; Strausz, O. P. Org. Geochem. 1984, 7 (2), 127-140. (4) Nowlan, V. Internal Syncrude report. (5) Siffert, B.; Bourgeois, C.; Papirer, E. Fuel 1984, 63, 834. (6) Papirer, E.; Bourgeois, C.; Siffert, B.; Balard, H. Fuel 1982, 61, 732. (7) Acevedo, S.; Escobar, G.; Gutierrez, L.; Rivas, H. Fuel 1992, 71, 619. (8) Kilpatrick, P. K.; Spiecker, P. M. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, 2001; Chapter 30. (9) Gu, G.; Xu, Z.; Nandakumar, K.; Masliyah, J. H. Fuel 2002, 81 (14), 1859. (10) Sjo¨blom, J.; Aske, N.; Auflem, I. H.; Brandal, Q.; Havre, T. E.; Sæther, Q.; Westvik, A.; Johnsen, E. E.; Kallevik, H. Adv. Colloid Interface Sci. 2002, 100-102, 399-473. (11) Yang, X. L.; Hamza, H.; Czarnecki, J. Energy Fuel 2004, 18 (3), 770-777.
SNs are surfactants also known to be able to stabilize water-in-oil (W/O) emulsions. It was reported that water-soluble commercial SN in water-toluene (or heptane-toluene mixtures) systems can form liquid crystals over a wide concentration range and stabilize W/O emulsions at room temperature.12,13 Friberg and other researchers showed that the liquid crystal formation dramatically increases emulsion stability.5,14,15 We could not find any data on liquid crystal formation by SN at higher temperatures (i.e., where most of the experiments reported here were conducted). Asphaltenes are a solubility class and contain a wide variety of chemical species and some of the most polar oil fractions. Some researches have reported that only a small subfraction of the total asphaltene class is responsible for water-in-crude oil emulsion stabilization.11,16 We separated six subfractions from Athabasca asphaltenes and found that their emulsion stabilization abilities were different.11 A similar observation was addressed in the Spiecker work.16 He fractionated B6 asphaltenes (from California crude) into more-soluble and less-soluble fractions and found that the less-soluble asphaltenes fractions play a more important role in emulsion stability than the more-soluble ones. (12) Szabo, G. H.; Czarnecki, J.; Masliyah, J. J. Colloid Interface Sci. 2001, 236, 233-241. (13) Szabo, G. H.; Czarnecki, J.; Masliyah, J. J. Colloid Interface Sci. 2002, 253, 427-434. (14) Friberg, S.; Mendell, L.; Larsson, M. J. Colloid Interface Sci. 1969, 29 (1), 155∼156. (15) Friberg, S. E.; Solans, C. Langmuir 1986, 2 (2), 121-126. (16) Spiecker, P. M. The Impact of Asphaltenes Chemistry and Solvation on Emulsion and Interfacial Film Formation. Ph.D. Thesis, North Carolina State University, Raleigh, NC, 2001.
10.1021/ef058017w CCC: $30.25 © 2005 American Chemical Society Published on Web 10/06/2005
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Therefore, a question arises as to whether a simple chemical that precipitates with Athabasca asphaltenes is responsible for W/O emulsion stabilization, thus leading to attribution of the stabilization role to the whole asphaltenes fraction, and if so, whether the chemical is SN. There is little reported on the fate of SN when asphaltenes are precipitated out of the oil. In this work, SN was added to the oil before asphaltenes precipitation so it could be traced more easily. PASFTIR was used to identify the chemical composition of the precipitates and maltenes to determine the likely destination of SN. At the same time, the stabilities of water-in-SN model oil emulsions and water-in-asphaltenes model oil emulsions were compared. Experimental Details Materials. Coker-feed bitumen and commercial naphtha supplied by Syncrude Canada Ltd. were used in experiments. SN, which is a white-yellow crystalline material, was supplied by Eastman Kodak and used without further purification. The main components of this chemical are molecules with 15-17 carbon number and one or two cyclic rings as analyzed with electrospray ionization mass spectrometry.17 Toluene and n-heptane were HPLC grade, supplied by Fisher Scientific. A solution of 25 mM NaCl and 15 mM NaHCO3 in deionized (DI) water was used in all emulsion experiments to emulate the pH and ionic strength of Syncrude’s plant process water. Asphaltenes Precipitation. Bitumen was initially diluted with naphtha at the naphtha to bitumen (N/B) ratio of 0.35 and centrifuged at approximately 27000g for 30 min to remove fine solids. The amount of naphtha used to facilitate solids removal is so small, as compared with the amount of heptane used later for asphaltene precipitation, that it is not expected to alter the properties of the asphaltenes obtained as compared with those obtained by direct precipitation without prior dilution with naphtha.11 Asphaltenes were then precipitated from the supernatant using one of the following three methods: (1) n-Heptane was added to above diluted bitumen to precipitate asphaltenes. The n-heptane to bitumen ratio (wt/wt) was 40:1. (2) A known amount of SN was first dissolved in a known amount of heptane, and the SN- heptane solution was then used to precipitate asphaltenes. The final bitumen dilution ratio with heptane was 40:1, and the concentration of SN based on bitumen was 1%. (3) The same conditions as in method 2, but 10% (v/v) DI water was added to the mixture of diluted bitumen and SN heptane solution. Known amounts of diluted bitumen and heptane (with or without SN and water) were weighed into a 100-mL centrifuge tube. Then the mixture was shaken for 24 h on a plate shaker. The mixture was then centrifuged at 1300g for 30 min. The supernatant was syringed out carefully to separate it from the precipitated material (and water). The precipitated material (and water) was transferred to a Buˆchner funnel with 1.5-µm Whatman 934-AH glass microfiber filter paper, filtered, and washed with 4× n-heptane based on bitumen (wt/wt). Toluene was then used to re-dissolve the precipitated material (asphaltenes). n-Heptane or toluene was removed from the maltenes or asphaltenes samples by rotary evaporation. Maltenes and asphaltenes were further dried in a helium-flushed vacuum oven at 60 °C for 48 h and then stored in a desiccator. PAS-FTIR Measurement. The spectra of all isolated samples were obtained using a PAS-FTIR spectroscope (Thermo-Nicolet Magna 560) equipped with a MTECH model 300 photoacoustic cell (Nicolet Instrument Corporation, Madison, WI). The instrument was operated using the following (17) Wu, X. Energy Fuels 2003, 17, 179-190.
Figure 1. Procedure for preparing asphaltenes samples for PAS-FTIR measurements. parameters: aperture of 150, interferometer mirror velocity of 0.1582 cm/s, 256 scans per run, resolution of 4 cm-1, and a data spacing of 0.964 cm-1. The samples for photoacoustic spectroscopy underwent two-stage purging with helium to remove CO2. The total purging time was approximately 30-60 min. Stability of Water-in-Asphaltenes or SN Model Oil Emulsions. Two series of model oils were prepared for emulsion stability experiments. One series of model oils was 2 wt % asphaltenes solutions in toluene or in a 70/30 mixture of toluene/heptane (referred to as T/H). The other series of model oils was 2 wt % SN solutions in toluene, n-heptane, naphtha, or 70/30 T/H. Water-in-model oil emulsions were prepared at a water to oil ratio of 1:9, which is close to the water content in commercial bitumen extraction froth. A known amount of the model oil and the water were separately weighed into 10-mL centrifuge tubes with 0.1-mL graduations and preheated in a water bath at 60 °C for 1 h. Then the water was poured into the tubes that contained the model oil. The water and the model oil were agitated for 2 min on a Wortex-2 Genie shaker (Scientific Industries Inc., Bohemia, NY) at maximum rotational speed. The test tubes were then placed back in the water bath. Emulsion stability was determined by evaluating the amount of water resolved as a function of time at 60 °C.
Results and Discussion Detecting SN in Asphaltenes and Maltenes. As mention above, three methods were employed to precipitate asphaltenes from bitumen. The first is the conventional method for asphaltenes precipitation. In the second method, SN solution in heptane was added to bitumen to precipitate asphaltenes and to trace the fate of SN in asphaltenes precipitation. Because SN is water-soluble, in the third method DI water was added to detect whether SN dissolves in the added water, remains in the oil phase, or precipitates with the asphaltenes. To easily detect whether SN tended to end up in maltenes or in asphaltenes, maltenes and asphaltenes samples isolated by the first two methods were mixed with water at 60 °C for 8 h. Then the water solution was separated from the maltenes or asphaltenes and dried. The detailed procedures are shown in Figures 1 and 2. It was found that no water-soluble materials were washed out from the asphaltenes samples; conversely, remnant materials were extracted from the maltenes samples. In method 3, maltenes, asphaltenes, and
Sodium Naphthenate in Ashaltenes
Energy & Fuels, Vol. 19, No. 6, 2005 2457
Figure 2. Procedure for preparing maltenes samples for PAS-FTIR measurements.
Figure 4. PAS-FTIR spectra of M (a) and M-w (b).
Table 1. Symbols for Maltenes, Asphaltenes and Remnant Materials Extracted from Maltenes method
maltenes
asphaltenes
remnant materials extracted from maltenes
1 2 3
M M-s M-sw
A A-s A-sw
M-w M-s-w W-swa
a
Directly separated from water phase.
Table 2. Elemental Analysis of Asphaltenes from Three Isolation Methods samples
C
H
N
H/C
A A-s A-sw
83.2 82.1 82.8
8.7 8.8 8.7
1.1 1.0 0.8
1.25 1.29 1.26
remnant materials dried from water solution were obtained. Table 1 lists the symbols for maltenes, asphaltenes, and remnant materials isolated by the three methods. Table 2 lists the results of C, H, and N analysis of three asphaltene samples. The yields of asphaltenes obtained by the three methods were about 16 wt % based on bitumen. Figure 3 shows the PAS-FTIR spectra of the three asphaltene samples. There are no noticeable differences among their spectra. Their yields, the elemental analyses, and PAS-FTIR spectra are practically the same for the three asphaltene samples. No SN was detected in any of these samples, and no water-soluble materials were washed out from either A or A-sn (see Figure 1).
Figure 3. PAS-FTIR spectra of asphaltenes obtained: (a) A; (b) A-s; (c) A-sw.
Figure 5. PAS-FTIR spectrum of M-s-w.
Figure 6. PAS-FTIR spectrum of pure commercial SN.
Although the FTIR spectra of M, M-s, and M-sw show no noticeable differences (Figure 4 (a)), the PAS-FTIR spectra of remnant materials extracted from both M and M-s by water were significantly different. The PAS-FTIR spectrum of remnant material M-w (Figure 4 (b)) is similar to that of its source M (Figure 4 (a)), which was part of the maltenes fraction emulsified with water that remained in the water. The spectrum of remnant material from M-s-w (Figure 5) has a strong peak clearly appearing at 1575 cm-1 and a shoulder peak at 1415 cm-1. Comparison with the spectrum of pure SN (Figure 6) clearly shows that water-extracted residue from M-s is dominated by SN and a small amount of M-s. No SN was detected in either A-sw or M-sw; however, SN was identifiably detected in W-sw and its PAS-FTIR spectrum is similar to that of M-s-w shown in Figure 7. In other words, SN preferentially dissolves in water and is unlikely to remain in the oil
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Figure 7. PAS-FTIR spectrum of W-sw. Table 3. Naphthenic Acids in Athabasca Bitumen samples
formula
H/C
weight
ref
C21 tricyclic terpenoid acid C24 tricyclic terpenoid acid commercial SN
C21H34O2
1.71
326
2-4
C24H40O2
1.67
362
2-4
C15-C26
1.83-1.64
236-386
4, 18
Figure 9. Emulsion stabilization capabilities of SN and asphaltenes (T, toluene; N, naphtha; Asph, asphaltenes; SN, sodium naphthenates; H, heptane; TH 73-70%, toluene with 30% heptane).
phase or to precipitate with asphaltenes when water is present. According to some research results for naphthenic acids in Athabasca bitumen,2-4 90% of the naphthenic acids in bitumen are C21-C24 tricyclic terpenoid acids. Their formulas, molecular weights, and H/Cs, are listed in Table 3. The structures of C21 and C24 naphthenic acids are shown in Figure 8. The molecular weights of
Figure 10. Influence of temperature on the stability of waterin-SN model oil emulsions (N, naphtha; SN, sodium naphthenates; Asph, asphaltenes; TH 73-70%, toluene with 30% heptane). Figure 8. Structures of naphthenic acids in Athabasca bitumen.
most of Athabasca SN are around 300-400,2-4 and they should therefore be partially water-soluble.18 From its H/C, naphthenic acid is an aliphatic material and likely dissolves in alkane solvent. In this study, it was determined that 2% SN can totally dissolve in either heptane or toluene. In method 2, assuming there is 1% SN in Athabasca bitumen, the concentration of SN in heptane will decrease to 0.025% because a heptane to bitumen ratio of 40 is used. At this low concentration SN dissolved readily in the maltenes-heptane solution. Method 3 simulates the hot water extraction process. It has been shown that, when water appears in a bitumen system, water-soluble SN is lost with the water and is unlikely to co-precipitate with asphaltenes. In the commercial process, significant amounts of SN can be detected in tailings pond samples.19-21 Stabilities of Water-in-SN Model Oil Emulsions and Water-in-Asphaltenes Model Oil Emulsions. Figure 9 shows the results of experiments comparing water-in-asphaltenes model oil emulsion stability and water-in-SN model oil emulsion stability at 60 °C. The figure shows the amount of water forming a separate
phase at the bottom of a tube, as a percentage of the total water present. Thus, more resolved water indicates lower emulsion stability. It can be seen that the stability of emulsions stabilized by SN either with toluene, heptane, or naphtha as solvent are not significantly different, although the emulsions stabilized by SN in naphtha or in heptane are slightly more stable than those stabilized by SN in toluene or T/H mixtures. However, all emulsions stabilized by SN are less stable than those stabilized by asphaltenes, regardless of the kind of oil phase. After 6 h, almost all water separates from SN stabilized emulsions. Conversely, after extended times, emulsions formed by asphaltenes model oils are still very stable. When toluene is used as solvent, the capability of asphaltenes to stabilize emulsions is obviously reduced as compared to when the 70/30 T/H mixture is used as solvent. The aromaticity of solvent significantly affects the capability of asphaltenes to stabilize emulsions.8,22,23 (18) Wu, M. Q. Study on Colloid Chemistry for the Caustic Treatment of Oil Fractions. Ph.D. Thesis, Research Institute of Petroleum Processing, Beijing, 1997. (19) Smith, R. G.; Tychkowsky, M. Internal Syncrude report. (20) Srinivasan, N. S. Internal Syncrude report. (21) Yeung, A.; Maslanko, R. Internal Syncrude report.
Sodium Naphthenate in Ashaltenes
Energy & Fuels, Vol. 19, No. 6, 2005 2459 Table 4. Properties of Asphaltenes
source
H/C
Athabasca Athabasca Athabasca typical asphaltenes
1.17 1.2 1.18 1.198
formula
Mt
method
ref
C420H476N6S14O4V C84H100N2O3S2,
1800-4800 5900 6191 1250
VPO GPC model based on elemental analysis, functional groups of spectra analysis
32, 33 1 33 8
Figure 10 shows that the stability of water-in-SN model oil emulsion at room temperature is obviously greater than that at 60 °C, even though it is still lower than the stability of water-in-asphaltenes model oil emulsion. This result implies that emulsions stabilized by SN are less stable than emulsions stabilized by asphaltenes and that their stability is sensitive to variations in temperature. The different capabilities of SN and asphaltenes to stabilize emulsions can be explained based on their different structures. Asphaltenes are polyaromatic hydrocarbons with heteroatom functional groups including some acids and bases.1,9,24,25 Although there are many different views about asphaltenes structures and molecular weights,26-33 some common features of asphaltenes seem to suggest that the aromatic carbon content of asphaltenes falls typically between 40 and 60 wt %, with a corresponding H/C of 1.1 ( 0.1 (see Table 4).1,8,32,33 Figure 11 shows average molecular structures
gregate composed of two to three “monomer” molecules rather than a single asphaltene molecule.30,31 Compared to the structure of asphaltenes, naphthenic acids (Figure 8) in bitumen are relatively simple chemicals and have carboxylic functional groups. The major difference is in the structures of their hydrophobic parts. The high percentage of aromatic carbon (low H/C aromatic ratio) is the major characteristic of asphaltenes structure. Besides making asphaltene insoluble in alkanes, the higher aromaticity of asphaltenes also enables them to associate at the water/oil interface and to form rigid films that are thought to stabilize emulsions.8,16,17,34 As a result of their aromaticity, their emulsion stabilizing capability is sensitive to variations in the aromaticity of the oil phase.8,22,23 There is less aromatic carbon in naphthenic acids (and SN), with a corresponding higher H/C ratio of 1.6-1.7. Because of their aliphatic character, naphthenic acids are easily soluble in alkanes, and at least 2% SN can be dissolved in toluene or heptane. The capability of SN to stabilize emulsions is less affected by variations in the aromaticity of the oil phase. In the emulsion stabilization testing, it was noted that there is a synergy effect between asphaltenes and SNs. When 1% asphaltene and 1% SN were both present in the toluene or Heptol solution, they formed emulsions even more stable than when only asphaltenes were present (see Figure 9). It might be one of the reasons why Athabasca bitumen can form very stable W/O emulsions. Conclusions
Figure 11. Model “average” structure of asphaltene molecules.8
of “typical” asphaltenes.8 It should be noted, however, that there are compelling arguments that the structure shown in Figure 11 represents composition of an ag(22) Fordedal, H.; Midttum, Q.; Sjo¨blom, J.; Kvalheim, O. M.; Schildberg, Y.; Volle, J. J. Colloid Interface Sci. 1996, 182, 117-125. (23) McLean, J. D.; Kilpatrick, P. K. The Role of Petroleum Asphaltenes in the Stabilization of Water-in-Oil Emulsions. In Structures and Dynamics of Asphaltenes, Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; pp 377-422. (24) Moschopedis, S. E.; Speight, J. G. Fuel 1976, 55 (7), 184. (25) Ignasiak, T.; Strausz, O. P.; Montgomery, D. S. Fuel 1977, 56 (10), 359. (26) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39 (14), 1847. (27) Yen, T. F. Energy Sources 1974, 1 (4), 447-463. (28) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227. (29) Strauzs, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355. (30) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972-978. (31) Groenzin, H.; Mullins, O. C. Energy Fuels 2001, 14, 677-684.
Experiments were carried out to trace the fate of SN in asphaltenes precipitation. PAS-FTIR analysis of asphaltenes and maltenes samples show that SN does not tend to co-precipitate with asphaltenes but remains in the maltenes fraction. The capability of SN to stabilize emulsion is lower than that of asphaltenes and is sensitive to temperature. This investigation of asphaltenes precipitation and emulsion stability shows that asphaltenes can form a very stable emulsion and that their emulsion stabilization capability is less affected by temperature. These results indicate that the capability of asphaltenes to stabilize water-in-crude emulsions cannot be simply attributed to SN coprecipitating with asphaltenes. Acknowledgment. The authors thank the NSERC for financial support to X.Y. We also thank Mr. D. Rioux from NCUT, CANMET for elemental analysis. EF058017W (32) Speight, J. G. The Structure of Petroleum AsphaltenessCurrent Concepts; Information Series 81; Alberta Research Council: 1978. (33) Yarranton, H. W. Asphaltenes Solubility and Asphaltenes Stabilized Water-in-Oil Emulsions. Ph.D. Thesis, University of Alberta, Edmonton, 1997. (34) Yang, X. L.; Lu, W. Z.; Sjo¨blom, J. Film Properties of Asphaltenes and Resins. In Encyclopedic Handbook of Emulsion Technology; Sjo¨blom, J., Ed.; Marcel Dekker: New York, 2000; Chapter 23.
