Energy Fuels 2010, 24, 5081–5088 Published on Web 08/26/2010
: DOI:10.1021/ef100765u
Recovery of Bitumen from Utah Tar Sands Using Ionic Liquids Paul Painter,* Phillip Williams, and Aron Lupinsky Department of Materials Science and Engineering, Penn State University, University Park, Pennsylvania 16802 Received June 18, 2010. Revised Manuscript Received August 12, 2010
Hot or warm water processes are used to extract bitumen from Canadian oil or tar sands. The application of these methods to the processing of tar sand deposits found in the Western United States, notably Utah, has not been commercially successful, however, because of the consolidated nature of the deposits and the high viscosity of the bitumen. It is demonstrated here that a previously developed method employing ionic liquids (ILs) together with a nonpolar solvent such as toluene can effect a separation at ambient temperatures (∼25 °C), although with greater difficulty than Canadian oil sands. Essentially, a multiphase system consisting of a sand and clay slurry, an ionic liquid layer, and an organic layer containing the bitumen can be formed by simply mixing the components. More than 90% of the bitumen is released from the sand, but only in successive extractions. Water is not used in this stage of the separation, but relatively small amounts are used to separate entrained IL from the sand and clays.
used pretreatment with diluents such as kerosene was developed a few years ago, but the lack of water in the Western US, problems with fines that associate with the diluent, and emulsion build up in the recycle water limited development to the pilot plant scale (see refs 3 and 4 and citations therein). More recently, it has been shown that a water-based extraction process can be used,5 but this requires higher temperatures and higher mechanical energy levels than those used in processing Canadian oil sands. In recent work we have shown that certain ionic liquids (ILs) can be used to extract bitumen from both medium- and low-grade Canadian oil sands.6,7 Essentially, a tar or oil sand sample is simply mixed with an IL and a nonpolar hydrocarbon solvent at ordinary temperatures (∼25 °C). A multiphase system consisting of a sand and clay slurry, an ionic liquid layer, and an organic layer containing the bitumen is formed, and the components can be easily separated by decantation. Toluene was used in this initial work, and it served to both lower the viscosity of the bitumen phase and sharpen the boundaries between the phases, facilitating separation. Other nonpolar or relatively nonpolar solvents can be used, but toluene was convenient, and it is also well-known that it is an excellent solvent for bitumen, so that a comparison between IL-assisted extractions and those using toluene alone could be made. No water was required in this extraction step, but was subsequently used to wash the IL from the sand slurry (also at room temperature). Infrared spectroscopy was used to characterize the components, and within the detection limits of this technique it appeared that essentially all of the bitumen had been recovered from the oil sand. The separation was remarkably clean. The bitumen appeared to be free of minerals
Introduction A large proportion of the world’s known oil reserves are in the form of oil or tar sands, principally in Canada and Venezuela, although significant quantities can also be found in Eastern Utah in the US.1 Canadian oil sands are complex mixtures of sand, clays, water, and bitumen, a “heavy” or highly viscous oil. They are unconsolidated, and the bitumen is separated from the sand by a thin film of connate water. The tar sands found in Utah, on the other hand, are consolidated, and the bitumen is directly bound to the mineral particles, cementing them together in the form of a rock. The most commonly used process for the recovery of bitumen from Canadian oil sands involves the addition of hot or warm water and processing aids (historically NaOH) to form a slurry, which is then “conditioned” in processes involving shearing the oil sand particles so that bitumen is detached from the minerals, forming suspended droplets.2 Separation is then achieved by aeration to form an oil containing froth that can be skimmed off the surface. The froth also contains mineral fines and is subsequently diluted with a hydrocarbon solvent to lower the viscosity of the bitumen and facilitate removal of minerals and emulsified water droplets. Although small amounts of mineral fines apparently benefit bitumen recovery, in general, bitumen yield decreases with fines content, as a layer of these particles becomes attached to bitumen droplets forming a so-called slime coating that hinders froth formation. The presence of mineral fines can also lead to problems in subsequent processing. Low-grade oil sands usually have a high proportion of clay fines, making them very difficult to process economically. Utah tar sands are not only consolidated, but its bitumen has a much higher viscosity than bitumen extracted from Canadian oil sands. The application of water-based separation processes has therefore proved difficult. A process that
(3) Hupka, J.; Miller, J. D.; Drelich, J. Can. J. Chem. Eng. 2004, 82, 978–985. (4) Drelich, J. Miner. Metall. Process. 2008, 25, 1–12. (5) Mikula, R. J.; Munoz, V. A.; Omotoso, O. J. Can. Pet. Technol. 2007, 46, 50–56. (6) Painter, P; Williams, P.; Mannebach, E. Energy Fuels 2010, 24, 1094–1098. (7) Williams, P; Lupinsky, A.; Painter, P. C. Energy Fuels 2010, 24, 2172–2173.
*To whom correspondence should be addressed. E-mail: painter@ matse.psu.edu. (1) Oblad, A. G.; Bunger, W. B.; Hanson, F. V.; Miller, J. D.; Ritzma, H. R.; Seader, J. D. Ann. Rev. Energy 1987, 12, 283–356. (2) Masliyah, J.; Zhou, Z.; Xu, Z.; Czarnecki, J.; Hamza, H. Can. J. Chem. Eng. 2004, 82, 628–654. r 2010 American Chemical Society
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(even for the low-grade sample), and there was no detectable bitumen or residual IL on the sand (after two water washes at room temperature). Because ILs have extremely low vapor pressures under these conditions, water is easily distilled from the IL, and both can be reused numerous times. Here we will show that the same procedure can be used to separate bitumen from a sample of Utah tar sands, although with much more difficulty. It will also be argued that the interactions between ILs and mineral surfaces drive the separation, but cannot be modeled with the interfacial theories presently used to interpret surface forces in water-based separation processes. Materials and Methods The tar sand sample used in this study was provided by the Utah Geological Survey and obtained from the Asphalt Ridge Area of Uintah County Utah (AF Hole No. 1, 44-54 ft., Box 3). Samples of the IL 1-butyl-2,3-dimethyl-imidazolium tetrafluoroborate [bmmim][BF4] were obtained from Sigma Aldrich and Alfa Aeser. Samples of the ILs 1-butyl-3-methyl-imidazolium trifluoromethanesulfonate [bmim][CF3SO3] and 1-ethyl3-methyl-imidazolium acetate [emim][Ac] were obtained from Sigma Aldrich. All ILs were used as-received without further purification. Toluene was obtained from Sigma Aldrich and was used as-received. An initial separation was performed by mixing the tar sand, an IL, and toluene in the proportions 1:2:3 by weight in glass vials at room temperature (∼25 °C). Other proportions and procedures can be used, as described in the Results section of this paper, but these particular proportions allow an easy visualization of the separation. Weighed quantities of tar sand of close to 1 g were used in initial studies, but 2 and 3 g quantities were used in subsequent separations. The IL was added to lumps of tar sand first, followed by the toluene. The mixture was stirred using a laboratory magnetic stirrer bar at room temperature (∼25 °C). The rate of stirring was approximately 400 rpm. The large pieces of tar sand used in this work broke apart quickly, and this was accompanied by a degree of detachment of the bitumen. Because fairy large, nonground pieces of tar sand were used in most experiments, the extraction was allowed to proceed overnight, about 12 h. Other separation protocols were investigated, and these will be described in the Results section of this paper. FTIR spectra were obtained using a Thermo Scientific Nicolet 6700 Fourier transform infrared (FTIR) spectrometer or a BioRad FTS 3000 FTIR spectrometer. A wavenumber resolution of 2 cm-1 was used, and 200 scans were signal averaged. Spectra of the tar sands and minerals were obtained using a diffuse reflectance accessory. Samples were prepared by grinding with KBr in a Wig-L-Bug, and spectra were referenced against pure KBr. Spectra of bitumen were obtained by casting toluene solutions onto a KBr window and evaporating the solvent in a vacuum oven at 100 °C, forming a thin film for analysis. Care had to be taken that these films were evenly distributed over the window, or poor base lines and misleading relative intensities due to the beam passing through both thick and thin parts of the sample were obtained.
Figure 1. A sample of the Utah tar sand is shown on the right, and on the left is the product obtained by mixing a smaller chunk of the sample with the IL [bmmim][BF4] and toluene in the proportions 1:2:3.
[bmmim][BF4] and toluene in the proportions tar sand/IL/ toluene of 1:2:3, as shown on the left-hand side of Figure 1. The tar sand lumps essentially disintegrated, and a three-phase mixture was formed. The top phase consists of a bitumen/ toluene solution, the middle phase is IL with some entrained bitumen and possibly some mineral fines, while the bottom layer is a sand/IL slurry. In the simple experiments described here, the bitumen/toluene layer was removed from the phaseseparated mixture using a pipet. Any small amounts of entrained IL were easily separated by a second treatment of the bitumen with toluene. The IL middle layer and the IL/mineral slurry at the bottom of the tube were also separated by pipet, and each was poured into cold water (i.e., room temperature, about 25 °C), the insoluble residues were washed with a second batch of water and then dried under vacuum. The IL layer contained small amounts of minerals and entrained bitumen that became mixed with the IL upon the crude separation of the top layer by pipetting. The infrared spectra of the original tar sand and the residual sand obtained in the first extraction experiment are compared in Figure 2. The sloping baseline is a consequence of the difficulty encountered in grinding these samples in order to obtain spectra using diffuse reflectance techniques (agglomeration appears to be the problem). Bands due to minerals, predominantly carbonates, silicates, and clays, observed near 1400, 1100, 800, and 500 cm-1, absorb very strongly in the infrared and dominate the “fingerprint” region between 2000 and 500 cm-1. They largely mask bands due to organic groups in this region, but methylene and methyl CH stretching modes between 2800 and 3000 cm-1 can be clearly seen in the spectrum of the parent tar sand and with much weaker intensity in the spectrum of the residual minerals. This shows that in this initial separation extraction was incomplete, although much of the bitumen appeared to be removed. On the other hand, bands due to clays and other minerals are indetectable in the spectrum of the bitumen, as shown in Figure 3 (also obtained using diffuse reflectance). This is most clearly demonstrated by a close examination of the left and right-hand end of the plots, near 3800 and 500 cm-1, respectively, where there are prominent bands due to clays and there is no overlap with modes due to organic groups. Also shown in this figure is the spectrum of a toluene Soxhlet extract of the tar sand. Bands due to mineral fines, principally kaolinite, can be clearly seen between 3600 and 3800 cm-1 and near 1000 and 500 cm-1.
Results A picture of pieces of the Utah tar sand used in this study is shown on the right-hand side of Figure 1. In an initial experiment, the protocol we had previously applied to Canadian oil sands was applied.6,7 This involved simply mixing a tar sand sample in the form of a lump of material with the IL 5082
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Figure 4. Going clockwise from top left, pictures of a sample of tar sands (1.1 g) mixed with the IL [bmmim][BF4] (about 2 g) for 15 min (A); the same sample 5 min after the addition of a few drops of toluene (about 0.13 g) (B); the sample 5 min after the addition of a few more drops of toluene (about 0.33 g) (C); finally (D) 5 min after the addition of a final few drops of toluene (about 0.33 g).
Figure 2. A comparison of the infrared spectra of the original tar sand and the residual minerals obtained after separating most of the bitumen.
different to Canadian oil sands. The latter could be dispersed in [bmmim][BF4] in the absence of toluene, whereas the IL [bmim][CF3SO3] resulted in the formation of balls of bitumenencrusted sand.6 If [bmmim][BF4] is placed in contact with Utah tar sands, however, nothing apparently happens, as can be seen in the top left-hand picture in Figure 4, where about 1.1 g of tar sands have been stirred with [bmmim][BF4] at 25 °C and the stirring subsequently stopped to allow this picture to be taken. Under these conditions, the oil sand does not break up and the IL liquid layer remains optically clear, indicating that little, if any, bitumen is extracted. This is probably a result of the absence of a water layer between the bitumen and the minerals in tar sands from Utah, inhibiting the degree of contact between IL and mineral surfaces. However, upon addition of even small amounts of toluene, the tar sand starts to break up and bitumen is extracted. Going clockwise around Figure 4, the second picture shows the same sample 5 min after the addition of a few drops of toluene (about 0.13 g) to the IL/tar sand mixture; the next picture shows the sample 5 min after the addition of a few more drops of toluene (about 0.33 g); and then finally the bottom left picture shows the sample 5 min after the addition of a final few drops of toluene (about 0.33 g). It can be seen that the mixture becomes darker as bitumen is released and that the initial large particles of tar sands break up into a number of smaller ones. The residual sand at the bottom of the vial appeared to have attached bitumen, however, so in an attempt to drive this process to completion a further 0.89 g of toluene was added, and the vial was stirred overnight. After stirring was stopped to allow phase separation, the top bitumen/toluene layer was removed by pipet. After evaporating the toluene it was determined that a (cumulative) yield of 10.7% (by weight) bitumen was obtained. However, the residual sand still appeared to be slightly discolored, so a further 5 mL of toluene was added to the IL/residual oil sand mixture, which was then stirred again overnight. A further 0.8% of bitumen (relative to the initial weight of the tar sand) was removed for a total yield of 11.5%. Interestingly, the structure of this more difficult to remove final batch appears to be somewhat different to the material that came off initially. The infrared spectra of cast films of the two bitumen samples
Figure 3. A comparison of the infrared spectra of the bitumen obtained from the tar sand by extraction using [bmmim][BF4]/toluene mixtures and toluene alone.
This shows that in terms of the detection limits of infrared spectroscopy, a bitumen/solvent solution that is free of mineral fines has been obtained using an IL. The yield of bitumen obtained in the IL/toluene extraction was 7.4% (by weight, relative to the initial weight of the tar sand sample), whereas a toluene extraction alone gave a yield of 10.2%. As the spectra in Figure 2 demonstrate, the toluene extract also consists of some mineral fines, whereas the IL/toluene extraction was incomplete. Nevertheless the results are intriguing, indicating that a good but incomplete separation can be achieved without the use of water in the initial stages and at low (∼25 °C) temperatures. Although bitumen was recovered from the Utah tar sands at low temperatures in the absence of added water (although the IL undoubtedly had some water absorbed from the atmosphere), the behavior of this material is in some important ways 5083
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Figure 5. A comparison of the infrared spectra of the bitumen obtained from an initial (first) extraction of the tar sand with [bmmim][BF4]/ toluene mixtures and a second extraction using the same solvents. The letter C designates a carbonate band, and the letter T marks a band due to residual toluene.
Figure 7. A comparison of the infrared spectrum of the original tar sand to that of the residual sand after successive extraction of bitumen with [bmmim][BF4]/toluene mixtures.
due to organic groups, but this is practically in the noise level, indicating that well over 90% of the bitumen has been extracted. Although shoulders near 1730 cm-1 can be clearly seen in the spectra of both the IL/toluene and toluene-extracted bitumen shown in Figure 3, they are much less intense; and other bands, such as those near 1288 and 1273 cm-1 (C-O single bond stretch of esters?), are not as apparent. In addition, although it would not be surprising that some mineral matter becomes entrained as a result of our relatively crude laboratory separations, why would this be limited to carbonates? An examination of the spectrum of the original tar sand (Figure 2) indicates that silicates and clays (intense bands between 1100 and 1000 cm-1, and near 800 and 500 cm-1) comprise a larger proportion of the mineral matter in these samples. In addition, in the extraction with toluene alone, clays, predominantly kaolinite, were extracted with the bitumen (figure 3). We will suggest below that esters are actually being formed in a condensation reaction mediated by carbonates and the IL. Although approximately 75% of the bitumen appears to be released quickly from the tar sands, the extraction of residual bitumen depends on the sequential addition of toluene and occurs more slowly under these conditions. This could be a consequence of phase behavior, how the bitumen is distributed between the phases, or it could be kinetic in origin. Bitumen from Utah oil sands is more viscous than bitumen from Canadian oil sands, and its release from relatively large lumps of samples might just take more time and depend on the diffusion of toluene into a bitumen phase. However, little change appears to occur after stirring for a couple of hours in initial extractions, which were usually continued for 12 h or so, leading us to believe that we are observing some aspect of phase behavior. To test this, we took a lump of the tar sands and used a spatula to scrape off small particles, which were then used in an additional extraction experiment. In order to observe the separation in more detail, we mixed a weighed amount (close to 1 g) of tar sands with 3 g of IL. About 1.5 g of toluene was then added and we let the mixture stir for 4 days, stopping the
Figure 6. A wavenumber scale-expanded comparison of the infrared spectra of the bitumen obtained from an initial (first) extraction of the tar sand with [bmmim][BF4]/toluene mixtures and a second extraction using the same solvents.
are compared in Figures 5. It can be seen that the spectra of both are dominated by aliphatic CH stretching modes near 2900 cm-1, but there are clear differences in the 1700 cm-1 region of the spectra, and the second extract also has a weak band near 878 cm-1 (labeled C in the figure) that can be assigned to carbonates. A scale expanded plot of the region between 1800 and 1200 cm-1 is shown in Figure 6. The bands near 1705 cm-1 can be assigned to carboxylic acid groups, whereas the band near 1730 cm-1 can be assigned to esters. The latter is much more intense in the spectrum of the final, most difficult to remove extract. We will return to a consideration of the origin of this band shortly, but first we consider the spectrum of the residual sand, shown in Figure 7. It can be seen that there is some weak residual absorption near 2900 cm-1 5084
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stirring occasionally and allowing the components to settle and phase separate for observation. The toluene layer was then removed, and the solvent was evaporated. It was determined that the yield of bitumen obtained was only 4.8% by weight, and a visual examination of the residual IL/tar sand mixture indicated that there was still bitumen attached to the sand. This suggests that there is a saturation effect;the thermodynamics of the mixture is such that small amounts of toluene will only help drive the separation so far. An additional 1.5 g of toluene was then added to the IL/sand slurry and stirred, this time for just 1.5 h. Upon removal of the toluene layer it was determined that a further 4.2% (by weight) of material was released, although this contained some mineral matter. The residual sand still appeared slightly discolored, so a final 1.5 g of toluene was added to the residual sand, and the slurry was stirred for 1 h. The yield of material in this final extraction was 1.4% by weight, so that a total of 10.5% of material was extracted. An infrared spectrum of the residual sand (obtained after removal of the IL with water) was similar to Figure 7, showing that most of the bitumen was recovered from this sample of tar sands. The spectrum of the final bitumen extracts also displayed ester and carbonate bands. These results strongly suggest that an excess of toluene is necessary to drive phase separation and that if these are conducted over an extended period of time then esters are formed and carbonates appear in the final extracts. These initial studies used relatively small amounts of tar sands (∼1 g), which would limit the accuracy of the reported yields and also make clean separations of the phase-separated layers more difficult. We therefore increased the amount of tar sands in subsequent extractions to 2 g (and in one case 3 g), keeping the relative proportions of tar sands to IL to toluene constant. The tar sands were in the form of small lumps and were not ground to give fine particles. In addition, two other ILs were studied, [bmim][CF3SO3] and [emim][Ac]. Although ILs are salts, they have a surprising range of pH values in aqueous solutions, between about 1.6 and 8.3 for the materials studied by Toma et al.8 The IL [bmmim][BF4] is neutral; the IL [bmim][CF3SO3] is acidic, with a pH value in aqueous solution of about 2.6; and [emim][Ac] is basic. Using these latter ILs, two successive extractions were performed, and in some cases this was followed by a third extraction. Some of these third extractions appeared to contain mineral matter as well as residual bitumen. As long as the extractions were not performed over a period of days, stronger than usual ester bands were not observed in the spectra of the bitumen residues. Because bitumen is strongly attached to sand at pH values less than 6 and basic conditions favor extraction in water based processes,9 we anticipated that the IL [emim][Ac] might improve initial yields. However, [bmmim][BF4] still gave the best results, as discussed in the following paragraphs. We do not know why at the present time, but interactions in these systems are presently being studied using atomic force microscopy. Figure 8 compares spectra of the bitumen obtained in the first and second extractions of tar sands with [bmmim][BF4] and toluene. Spectra were obtained in the form of thin cast films rather than using diffuse reflectance methods. The spectra of the two extracts are almost identical, indicating that there is little difference in the gross distribution of functional groups
Figure 8. A comparison of the infrared spectra of films of the material obtained in the first and second extractions of tar sands with [bmmim][BF4]/toluene mixtures.
in the material that is eluted in the first extraction (7.4% yield) relative to that which is eluted in the second extraction (2.3% yield). There is no evidence of mineral matter or residual IL in these extracts. A third extraction was also performed, and the spectrum of the material obtained is shown in Figure 9. It appears that the aromatic mode near 1600 cm-1 and the carboxylic acid mode near 1700 cm-1 are more prominent in this spectrum. In addition, the relative intensity of the methyl mode near 2960 cm-1 appears more intense relative to the 2920 cm-1 methylene mode than in extracts 1 and 2. However, these differences are artifacts of sample preparation. Only a 0.4% yield of bitumen by weight was obtained in this extraction and preparing a film that completely and evenly covered the KBr window from small amounts of bitumen was difficult. We have observed that if some regions of a cast film of bitumen are thicker than others we obtain this type of distortion in relative intensities. There are also weak bands near 1020 cm-1 in the spectrum of extract 3 that are not apparent in the spectra of extracts 1 and 2. Absorptions between 1000 and 1100 cm-1 are characteristic of clays, and they may be due to very small amounts of kaolinite and perhaps other clay fines that could be part of this extract. However, the silicon-oxygen bending modes near 520 cm-1 that accompany the Si-O stretching modes near 1020 cm-1 cannot be observed. Low wavenumber modes are significantly enhanced in intensity in the diffuse reflectance spectra of residues shown above, so the fact that any absorption near 520 cm-1 is indetectable in transmission spectra may not be definitive in terms of the absence of small amounts clay fines. In this regard, we observed many years ago that very small amounts of clay, about 0.1 mg, had to be used with 300 mg of KBr in making pellets for infrared absorption studies if the absorption of the strongest modes near 1020 cm-1 were to be within the Beer-Lambert law range (absorptivities less than about 1).10 Nevertheless, the intensity of the kaolinite mode near 520 cm-1 was about 75% of the modes near 1020 cm-1 in these spectra, so the assignment of weak bands near 1020 cm-1 in extract 3 remains uncertain. In one experiment with the IL [bmmim][BF4], the amount of toluene used in the second extraction was doubled. This
(8) Toma, S.; Meciarova, M.; Sebesta, R. Eur. J. Org. Chem. 2009, 3, 321–327. (9) Dai, Q.; Chung, K. H. Fuel 1995, 74, 1858–1864.
(10) Painter, P. C.; Rimmer, S. M.; Snyder, R. W.; Davis, A. Appl. Spectrosc. 1981, 35, 102–106.
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Painter et al. Table 1. Extraction Yields Obtained with IL/Toluene Mixtures ionic liquid [bmmim][BF4] [bmim][CF3 SO3] [bmim][Ac]
yield (wt %) yield (wt %) yield (wt %) total yield first extract second extract third extract (wt %) 7.4 7.2 9.4 10.5b 8.8 7.9
2.3 1.8 1.9a 0.8b 1.3 1.4
0.4 1.4 (0.4)c
10.1 10.4 11.3 11.3 10.2 9.3
a
Amount of toluene doubled on second extraction. b Three grams of tar sand used in extraction. c Predominantly carbonates.
Figure 9. The infrared spectrum of a film of the material obtained in the third extraction of tar sands with [bmmim][BF4]/toluene mixtures. The bands labeled T are residual toluene, and the question mark identifies bands of unknown origin, as discussed in the text.
appeared to remove most of the remaining bitumen, making a third extraction unnecessary, as an examination of the spectra of the residues, considered later, will indicate. Similar extraction experiments were performed using the ILs [bmim][CF3SO3] and [emim][Ac]. The extractions yields obtained using these materials and [bmmim][BF4] are summarized in Table 1. All gave total yields of the order of 10-11% bitumen (by weight, relative to the initial weight of the tar sand). The appearance of the toluene/bitumen top layer was a little different, with what appeared to be a dark interphase between this layer and the middle IL layer. (This does not show up well on photographs.) As a result, the spectra of both first and second extracts of tar sands using these ILs and toluene displayed bands due to small amounts of ILs. These ILs were easily removed from the bitumen using water, as can be seen from the spectra of the first extracts before and after washing with water shown in Figures 10 and 11. ILs absorb very strongly in the infrared, so the amount of IL in each extract is probably not large, but these results stand in contrast to those obtained with [bmmim][BF4], where IL was not usually observed in the bitumen phase unless accidently entrained during separation of the phase separated layers. Finally, the infrared spectra of the mineral residues obtained from some of these extraction experiments are compared in Figure 12. These residues were washed with room temperature water, and there is no evidence of residual IL. The spectra are dominated by bands due to silicates and clays, together with weaker bands due to carbonates. There are very weak bands due to residual bitumen that can be observed in the aliphatic CH stretching region near 2900 cm-1 in the spectra of the samples extracted with [bmim][CF3SO3] and [emim][Ac], which can be more clearly seen in an absorbance scale expanded plot shown as an insert. A sample extracted twice using [bmmim][BF4] had even weaker CH bands, whereas the spectrum of the tar sands where the amount of toluene in the second extraction was doubled showed no evidence for residual bitumen.
Figure 10. The infrared spectrum of the first extract obtained using [bmim][CF3SO3] and toluene compared to the spectrum of the same sample after washing with water. The bands near 1250 and 1050 cm-1 are SO3- modes, and the bands near1150 and 650 cm-1 are CF3 modes.
Figure 11. The infrared spectrum of the first extract obtained using [emim][Ac] and toluene compared to the spectrum of the same sample after washing with water.
Canadian oil sands could be applied to consolidated tar sands from Utah. The results indicate that it can, but there are some interesting differences. First, within the detection limits of infrared spectroscopy, it appeared that we obtained essentially complete recovery of bitumen from both medium and low-grade
Discussion The goal of this work was to determine if the IL/toluene procedure used previously to separate bitumen from unconsolidated 5086
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presence of K2CO3 at 70 °C. We suggest that a condensation reaction between hydroxyl and carboxylic acid groups present in the bitumen could also occurs and is similarly mediated by an IL/carbonate mixture. In this work a condensation appears to have occurred at much lower temperatures (∼ 25 °C), but only after an extended period of time. The separation of bitumen from oil or tar sands depends on the balance of interactions in these complex systems. Masliya, Xu, and co-workers have published an extensive body of work that used ζ potential distribution measurements and atomic force microscopy (AFM) to characterize the interactions between the components of Canadian tar sands in an aqueous environment.13-18 These authors also comprehensively reviewed earlier work.13,16 Some of the most important results to come out of this research involved the measurement of repulsive long-range forces between bitumen and silica particles,14 bitumen and clays, and bitumen and fines.15,16 As might be expected, these forces are electrostatic in origin. They are weakest at low pH but increase significantly at higher pH.14 As with other colloidal systems, the range of these forces decreases with increasing electrolyte concentration. Increasing the temperature also increases the repulsive forces, while adhesive forces decrease significantly as the temperature is raised, becoming very small for clay-bitumen interactions at temperatures above 32-35 °C.17 In addition, the presence of montmorillonite clays and divalent cations such as Ca2þ sharply depresses bitumen recovery as a result of the formation of a “slime coating” of clay particles on bitumen droplets.13,15,16 These authors interpreted their results in terms of DLVO theory, although an empirical term describing hydrophobic forces had to be included to fit some of the data. Although this provides a good tool for rationalizing the results, Bostrom et al.19 have argued that in general DLVO theory fails and agreement with theory is “illusory”, in that a number of fitting parameters involving surface charge and/or potential have to be used. Stability predictions can vary significantly with the choice of such parameters. Furthermore, there are major problems in applying this approach to IL-based systems. First, the theory is not expected to hold at high concentrations of electrolytes, such as those found in IL/tar sand mixtures.19,20 We actually obtain a good separation at high electrolyte concentration and low temperature (∼25 °C), where electrostatic forces should be subject to larger screening effects and adhesive forces should be larger. This is because ILs behave very differently at surfaces and charged interfaces than small ions in dilute solutions, forming alternating, discrete solvation layers of ions.21-23 As a result, oscillatory forces superimposed upon an exponential repulsion are observed at small distances. 12
Figure 12. The infrared spectra of the washed residues obtained after treatment using [emim][Ac]/toluene mixtures and [bmim][CF3SO3]/ toluene mixtures. An insert shows the CH stretching region of these samples plus the same spectra of materials obtained after treatment with [bmmim]BF4]/toluene.
Canadian oil sands and this bitumen was not contaminated with mineral fines.6,7 Separation of bitumen from Utah tar sands was much more difficult, and although most of the organic material appears to have been extracted, infrared spectroscopy indicates the presence of small amounts of residual bitumen in the sands unless a large excess of toluene is used in successive extractions. A quantitative measure of this residual material is difficult to obtain, because we are not confident in the quality of the infrared spectra obtained using diffuse reflectance techniques, which in turn is related to the difficulty in reproducibly grinding the samples without particle agglomeration. However, the relative intensities of the organic and inorganic bands before and after extraction indicate that well in excess of 90% of the bitumen can be extracted. Part of the difficulty in extracting the bitumen appears to be related to either its limited solubility in toluene or some aspect of the phase behavior of IL/toluene/tar sand mixtures that is not understood at this time. A saturation effect was observed, such that fresh quantities of toluene or a significant excess of this solvent was necessary to extract most of the bitumen. Toluene may not be the best solvent to use in the extraction of bitumen from Utah tar sands, and other polar or relatively nonpolar solvents are being investigated. Nevertheless, the IL [bmmim][BF4] and toluene were very effective in breaking up relatively large particles of tar sands and giving a relatively good yield that appeared to be uncontaminated with minerals. Extractions using [bmim][CF3SO3] and [emim][Ac] were less clean, with small amounts of IL being incorporated into the bitumen phase. Extractions conducted over an extended period of time (days) gave bitumen that was contaminated with carbonates, however. This is also interesting, in that extraction with toluene alone resulted in the extraction of clay fines, not carbonates. The kerogen in Green River shale is thought to be bound to carbonate minerals through carboxylic acid groups,11 and an interaction on a smaller scale may be occurring in these tar sands. The presence of carbonates could also account for the presence of esters in these extracts. Lapis et al. showed that transesterification of vegetable oils occurs in an IL in the
(12) Lapis, A. A. M.; de Oliveira, L. F.; Neto, B. A. D.; Dupont, J. Chemsuschem 2008, 1, 759–762. (13) Liu, J.; Zhou, Z.; Xu, Z.; Masliya, J. J. Colloid Interface Sci. 2002, 252, 409–418. (14) Liu, J.; Xu, Z.; Masliya, J. Langmuir 2003, 19, 3911–3920. (15) Liu, J.; Xu, Z.; Masliya, J. Can. J. Chem. Eng. 2004, 82, 655–666. (16) Liu, J.; Xu, Z.; Masliya, J. Mats. J. Colloid Interf. Sci. 2005, 287, 507–520. (17) Long, J.; Xu, Z.; Masliya, J. Energy Fuels 2005, 19, 1440–1446. (18) Zhao, H.; Long, J.; Masliya, J.; Xu, Z. Ind. Eng. Chem. Res. 2006, 45, 7482–7490. (19) Bostrom, M.; Williams, D. R. M.; Ninham, B. W. Phys. Rev. Lett. 2001, 87, 168103-1–168103-4. (20) Kornyshev, A. A. J. Phys. Chem. B 2007, 111, 5545–5557. (21) Atkin, R.; Warr, G. G. J. Phys. Chem. C 2007, 111, 5162–5168. (22) Lauw, Y.; Horne, M. D.; Rodopoulos, T.; Leermakers, F. A. M. Phys. Rev. Lett. 2009, 103, 117801-1–117801-4. (23) Min, Y.; Akbulut, M.; Sangoro, J. R.; Kremer, F.; Prud’homme, R. K.; Israelachvili, J. J. Phys. Chem. C 2009, 113, 16445–16449.
(11) Vandegrift, G. F.; Winans, R. E.; Horwitz, P. Fuel 1980, 59, 634– 636.
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: DOI:10.1021/ef100765u
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In addition, effective Debye screening lengths are much longer than would be expected on the basis of classical theories that use the Poisson-Boltzmann equation and attractive forces are overwhelmed by repulsive electrostatic interactions.23 Accordingly, traditional theories are not applicable to the systems studied here or to a rationalization of our results. Although large repulsive electrostatic forces probably play a key role in driving bitumen extraction, the presence of an IL as an intermediate phase separating the largely mineral components from a bitumen/solvent layer would also appear to be important (and may trap mineral fines). This, coupled with the observation that excess amounts of what is usually regarded as a good solvent for bitumen, toluene, is necessary to drive extractions toward completion, suggests that there is a complexity to the phase behavior of these systems that needs to be studied. Although small amounts of low molecular weight, largely nonpolar hydrocarbons such as benzene, toluene, and paraffins dissolve in various ILs, recent work has shown that certain imidazolium ILs are apparently completely immiscible with these hydrocarbons,24 a factor which also drives phase behavior and is probably one of the reasons we have obtained such clean separations with certain ILs. (High molecular weight hydrocarbons could only be less miscible than their low molecular weight counterparts.) This observation appears to hold for the IL [bmmim][BF4], but not for [bmim][CF3SO3] and [emim][Ac], where small amounts of IL are found in the bitumen phase.
solvent at ambient temperatures (∼25 °C). To a first approximation, given the simple nature of the experiments reported here, it appears that yields in excess of 90% by weight can be obtained. Unlike Canadian oil sands, however, the separation is more difficult and requires multiple applications of an organic solvent, toluene, to IL/bitumen mixtures in order to extract most of the bitumen. This may be a saturation effect, and other organic solvents used in conjunction with an IL might give superior results. No water at all is used in the stage of the separation. Water is used to remove IL from the residual sand and clays, but this is easily removed from the IL by distillation, because the latter has a negligible vapor pressure under these conditions. There was no detectable IL contamination of the residual sand and clays after washing. The bitumen produced in initial extractions appeared to be free of minerals, but as the process was pushed toward maximum extraction at long time periods some carbonates were also extracted. These could be removed with additional treatments, which surprisingly also resulted in the formation of esters, presumably as a result of an IL/carbonate catalyzed condensation reaction. Extractions using [bmmim][BF4] gave bitumen apparently free of IL, but use of the ILs [bmim][CF3SO3] and [emim][Ac] resulted in some incorporation of these materials into the bitumen phase.
Conclusions
Acknowledgment. The authors gratefully acknowledge the support of the National Science Foundation, Polymers Program, under grant DMR-0901180 and the Utah Geological Survey for the sample donation through the effort and good offices of Craig Morgan of the Utah Geological Survey and Michael Laine of the Utah Geological Survey’s Utah Core Research Center.
Bitumen can be separated from consolidated Utah oil or tar sands using an ionic liquid in conjunction with an organic (24) Garcia, J.; Torrecilla, J. S.; Fernadez, A.; Oliet, M.; Rodriguez, F. J. Chem. Thermodyn. 2010, 42, 144–150.
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