Energy Fuels 2010, 24, 1094–1098 Published on Web 10/07/2009
: DOI:10.1021/ef9009586
Recovery of Bitumen from Oil or Tar Sands Using Ionic Liquids Paul Painter,* Phillip Williams, and Ehren Mannebach Department of Materials Science and Engineering, Penn State University, University Park, Pennsylvania 16802 Received September 1, 2009. Revised Manuscript Received September 22, 2009
The extraction and separation of bitumen from oil sands for the purpose of processing fuels is relatively expensive and poses several environmental challenges. Roughly two tons of oil sands are required to produce a barrel of oil, and the separation of the bitumen from sand and clay requires significant amounts of energy and the use of large quantities of water. It is shown here that bitumen in a sample of Canadian tar sands can be recovered using ionic liquids (ILs) and organic solvents. 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 at somewhat elevated (∼55 °C) or ambient temperatures (∼25 °C). Essentially all of the bitumen is released from the sand. 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. Because both the IL and water can be recycled through the system and used repeatedly, this process has the potential for ameliorating many of the environmental problems associated with current extraction methods.
and other materials used in processing. It is highly toxic to aquatic life.1-4 As mentioned above, large quantities of tailing pond water are now recycled through the process, but this can lead to scaling and corrosion problems and can adversely affect bitumen recovery. In addition, very fine mineral particles and emulsified salty water are coextracted with the bitumen, and these can also lead to problems in subsequent processing.5-7 A number of different extraction processes have been evaluated over the years, but these can also introduce significant problems (see refs 7 and 8 and citations therein). For example, solvents such as benzene or toluene can dissolve and extract bitumen from tar sands, at least those containing more than 7% bitumen,9 thus reducing water consumption and energy use in an extraction process. However, such solvents introduce problems associated with handling volatile, flammable, and toxic materials and are not always easy to remove from the spent sand.8 In this communication, we present preliminary results demonstrating that the bitumen in a medium grade Canadian tar sand can be readily recovered using ionic liquids and organic solvents at ambient (∼25 °C) or somewhat elevated (∼55 °C) temperatures. Ionic liquids (ILs) consist entirely of ionic species. Although they are not new, interest in these materials has increased dramatically in the past few years, focusing especially on those ILs that are liquids at temperatures below 100 °C. This is a consequence of their unusual range of properties and their potential for use in a number of industrial processes. ILs have outstanding chemical and thermal stability; they are nonflammable (at temperatures below the degradation point) and have a negligible vapor
Introduction Oil or tar sands compose a significant proportion of the world’s known oil reserves. The largest deposits are found in Canada and Venezuela, which have combined oil sand reserves estimated to be equal to the world’s total reserves of conventional crude oil.1 Significant quantities (estimated to be 32 billion barrels of oil) can also be found in Eastern Utah in the US.1 Oil or tar sands are complex mixtures of sand, clays, water, and bitumen, a “heavy” or highly viscous oil. Extraction and separation of bitumen from surface-mined oil sands for the purpose of processing fuels is much more expensive than extracting conventional oil by drilling and involves the use of significant amounts of energy and water.2-4 Although a large proportion of the water used in the process is now recycled from tailing ponds, the production of each barrel of oil still requires importing additional barrels of “fresh” water.3,4 The most commonly used process for the recovery of bitumen involves the addition of hot or warm water and processing aids (historically NaOH) to the tar sands to form a slurry, which is then “conditioned” in processes involving shearing the tar sand particles so that the bitumen is detached from the minerals, forming suspended droplets. The density of bitumen and water are closely matched, so separation is achieved by aeration to form an oil-containing froth that can be skimmed off the surface. Additional processing of the water is performed to remove residual bitumen. This process water is ultimately stored in vast tailing ponds. It is a complex mixture of water, dissolved salts, minerals, residual bitumen, surfactants released from the bitumen, *To whom correspondence should be addressed. E-mail: painter@ matse.psu.edu. (1) Canada0 s Oil Sands - Opportunities and Challenges to 2015: An Update - June 2006; National Energy Board: 2006. http://www.neb.gc.ca/ clf-nsi/rnrgynfmtn/nrgyrprt/lsnd/lsnd-eng.html (2) Heinberg, R. The Party0 s Over: Oil, War and the Fate of Industrial Societies; New Society Publishers: Gabriola Island, BC, Canada, 2005. (3) Allen, E. W. J. Environ. Eng. Sci. 2008, 7, 123–138. (4) Allen, E. W. J. Environ. Eng. Sci. 2008, 7, 499–524. r 2009 American Chemical Society
(5) Masliyah, J.; Zhou, Z.; Xu, Z.; Czarnecki, J.; Hamza, H. Can. J. Chem. Eng. 2004, 82, 628–654. (6) Trong, D-V; Jha, R.; Wu, S.-Y.; Tannant, D. D.; Masliyah, J.; Xu, Z. Energy Fuels 2009, 23, 2628–2636. (7) Budziak, C. J.; Vargha-Butler, E. I.; Hancock, R. G. V.; Neumann, A. W. Fuel 1988, 67, 1633–1638. (8) Houlihan, R.; Williams, K. H. J. Can. Pet. Technol. 1987, 26, 91–96. (9) Jacobs, F. S.; Filby, R. H. Anal. Chem. 1983, 55, 74–77.
1094
pubs.acs.org/EF
Energy Fuels 2010, 24, 1094–1098
: DOI:10.1021/ef9009586
Painter et al.
10,11
pressure (i.e., they are nonvolatile). We will show that the bitumen obtained in this process is essentially free from mineral fines and that residual sand and clays can be obtained in what appears to be a relatively clean and uncontaminated form. Materials and Methods The sample used in this study was a medium grade Canadian tar sand, with a bitumen content of approximately 10%. It was obtained from the Alberta Research Council. The ionic liquids were all purchased from Sigma-Aldrich. Separations were performed by placing a sample of tar sands, toluene, and an IL in the proportions 1:2:3 by weight either in glass vials or centrifuge tubes. Weighed quantities of tar sand of close to 1 g were used in all separations. The IL was added to the tar sand first, followed by the toluene. Initial experiments were conducted by stirring this mixture at 55 °C overnight using a laboratory magnetic stirrer bar. The temperature control of the oil bath in which the vials were placed was somewhat erratic and oscillated between 50 and 55 °C. We chose this temperature on the basis of a review of the literature, where it has been reported that bitumen yields are greatly reduced in the water extraction process at temperatures less than 50 °C.5 In subsequent work, however, we found that simply shaking a vial containing the tar sand/toluene/IL mixture at room temperature also achieved a degree of immediate but incomplete separation, as we will describe in the following section. Stirring for two hours increased the yield to values comparable to those obtained at higher temperatures. FTIR spectra were obtained using a Thermo Scientific Nicolet 6700 Fourier transform infrared (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.
Figure 1. The structures of the ionic liquids used in this study; from left-to right 1-butyl-3-methyl-imidazolium trifluoromethanesulfonate, [bmim][CF3SO3]; 1-butyl-2,3-dimethyl-imidazolium tetrafluoroborate, [bmmim][BF4]; and 1-butyl-2,3-dimethyl-imidazolium trifluoromethanesulfonate, [bmmim][CF3SO3].
Figure 2. The agglomerates formed by mixing tar sands with [bmim][CF3SO3] after removal of the IL with water.
brought into the same phase or multiphase processes can be designed. However, it is not obvious that ILs would be good solvents for removing bitumen from tar sands, in the sense that their highly polar nature makes most of them immiscible with nonpolar hydrocarbon solvents such as toluene and naphtha, not to mention bitumen. It is their ability to engage in electrostatic interactions with the surface of sand and clay particles, thus releasing bitumen, that appears to be crucial. Nevertheless, one of our first attempts to determine if ILs could be used in the separation process ended in dismal failure. Figure 2 shows the results of mixing the ionic liquid 1-butyl-3methyl imidazolium trifluoromethanesulfonate, [bmim][CF3SO3], with a sample of Canadian tar sands. This IL did not separate bitumen from the sand, but instead resulted in the formation of agglomerated, spherical, black balls of bitumenencrusted minerals. Other ILs appeared to at least disperse the tar sands. Upon mixing tar sands with the IL 1-butyl-2,3-dimethyl-imidazolium tetrafluoroborate, [bmmim][BF4], at 55 °C, three phases were formed. Because the layers observed in this multiphase system are not easy to distinguish in photographs, we only describe our observations here. There is an IL/sand slurry at the bottom, a middle dark layer, and a layer of viscous bitumen at the top. We found that a much cleaner and easier way to visualize separation was achieved by mixing tar sands with an IL and toluene at temperatures somewhat above ambient, about 55 °C in some separations, and at room temperature (∼25 °C) in others. A three-phase system is formed and can be clearly observed. We tested three different ILs, [bmmim][BF4], [bmmim][CF3SO3], and [bmim][CF3SO3] (see Figure 1). The proportions used in these particular separations were 3 parts IL to 2 parts toluene to 1 part oil sand (by weight). Other
Results The apparent complexity of tar sand separation processes and the various types of processing aids that have been used are a consequence of the range of interactions that occur between the various organic and inorganic components of these materials. There is a wealth of knowledge that has been obtained over the years on the nature of these interactions (see refs 12-14 and citations therein), and the work of Liu et al.12 strongly indicates that electrostatic forces play a dominant role. This suggested to us that ILs might prove useful in separating bitumen from sand and clay. A particularly versatile group of ILs is based on imidazolium cations, and the structures of the ILs used in this study are illustrated in Figure 1. The properties of these solvents can be “tuned” by varying the imidazolium substituent groups and the nature of the anion, so that unusual combinations of reagents can be (10) Weingartner, W. Angew. Chem., Int. Ed. 2008, 47, 654–670. (11) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123–150. (12) Liu, J.; Xu, Z.; Masliyah, J. Langmuir 2003, 19, 3911–3920. (13) Wang, S.; Liu, J.; Zhang, L.; Xu, Z.; Masliyah, J. Energy Fuels 2009, 23, 863–869. (14) Dai, Q.; Chung, K. H. Fuel 1995, 74, 1858–1864.
1095
Energy Fuels 2010, 24, 1094–1098
: DOI:10.1021/ef9009586
Painter et al.
Figure 3. The phases formed by mixing tar sands with [bmmim][BF4] and toluene in the proportions 1:3:2 at room temperature.
Figure 4. The infrared spectra of the original tar sand, the bitumen obtained by extraction, and the clay/sand mixture obtained after removing residual IL ([bmmim][BF4]) with water.
proportions can be used, but these were the most useful for visualizing phase separation. The cleanest separation between the phases was given by [bmmim][BF4] and this is shown in Figure 3. This particular separation was obtained by simply stirring the components at room temperature, about 25 °C. The bottom layer consists of sand and clays suspended in the IL, the middle layer contains the ionic liquid with a small amount of dissolved or suspended bitumen particles and some mineral fines, and the top layer is a toluene/bitumen mixture. This was easily removed from the other layers using a pipet. Upon evaporation of the toluene from this mixture, we noticed some residual ionic liquid in the bottom of the vial, beneath an oil/bitumen layer, presumably because we had entrained some IL in our crude laboratory separation. We added a small amount of fresh toluene to the vial, swirled it around, let the mixture settle, and then decanted the toluene/bitumen phase. The ionic liquid stayed in the bottom of the vial due to its high viscosity-it did not move when we poured the toluene/ bitumen mixture into a new vial, giving us a clean separation. In our initial experiments, involving separation at about 55 °C, we obtained bitumen yields of 12.6% in our first extraction and 14.7% in a repeat experiment. Shaking a vial containing the oil sand, IL, and toluene at room temperature gave a yield of 7.5% after 10 min. Stirring at room temperature for 2 h gave a yield of 12.7%. This bitumen appears to be free of mineral fines, as we will show below. Toluene extractions of the tar sand gave yields of 14.3 and 14.5%, although the extracts also contained some clay fines, as we will also show below. ILs are expensive materials and as such will need to be reused numerous times in any viable process. Accordingly, we also conducted a set of experiments where we reused the middle IL layer (see Figure 3) five times. IL is also part of the slurry at the bottom of the vial and can readily be separated from the sand using water. However, in these initial, fairly crude experiments we simply made up the missing IL with fresh liquid to restore the initial 1:2:3 proportions by weight of tar sand, toluene, and IL. The yield of bitumen obtained in each extraction (at 55 °C) is listed in Table 1 and varies between 12.7 and 15.1%. This variation is not surprising given the crude nature of these experiments and the fact that we were using only 1 g of tar sand in each experiment. We
Table 1. Yields of Bitumen Obtained by Extraction of Oil Sands Using Recycled Ionic Liquid extraction number 1 2 3 4 5
extraction yield 13.5% 12.7% 14.5% 15.1% 13.4%
assumed there would be some variation in the bitumen content of such small samples. We now address the questions concerning the presence of any mineral fines in the bitumen extracts and the degree to which the IL can be separated from the residual minerals. In the simple experiments described here, the bitumen/toluene layer was simply removed from the phase-separated mixture using a pipet. Any small amounts of entrained IL were easily separated by a second treatment of the bitumen with toluene, as described above. 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 (i.e., room temperature) water, the insoluble residues washed with a second batch of water and then dried under vacuum. The IL layer contained small amounts (∼0.4% by weight of the original oil sand sample) of predominantly minerals. However, there were no mineral fines in the toluene/bitumen layer that could be detected using infrared spectroscopy. This is shown in Figure 4, which compares infrared spectra (in the 2000-500 cm-1 region) of the original tar sands sample. The bitumen obtained after evaporation of the toluene and the sand/clay layer after the ionic liquid was removed using cold water. These samples were from a 55 °C extraction. Bands due to methylene and methyl groups near 1450 and 1370 cm-1 are prominent in the spectrum of the bitumen and appear with very weak intensity in the spectrum of the tar sand. They can be more clearly seen in scale-expanded plots (not shown), but the mineral bands (predominantly quartz and clay) near 1100, 800, and 500 cm-1 absorb very strongly in the infrared and tend to mask bands due to organic groups. However, these hydrocarbon absorption modes are essentially undetectable in the spectrum of the sand/clay mixture 1096
Energy Fuels 2010, 24, 1094–1098
: DOI:10.1021/ef9009586
Painter et al.
Figure 5. The infrared spectrum of the toluene extract of the oil sand.
Figure 6. A comparison of the infrared spectra of the residual sand and clays and the IL [bmmim][BF4].
recovered from the bottom of the tube, even in scale-expanded spectra. Similarly, the mineral bands are absent from the spectrum of the bitumen. This is most easily seen through an examination of the right-hand end of the plots in Figure 4, near 500 cm-1. These results show that we have separated the bitumen from the sands without carrying over detectable amounts of fine particles, unlike the hot or warm water processes presently used. Conversely, the spectrum of the toluene extract of the tar sands clearly displays bands due to clays, principally kaolinite, as shown in Figure 5. The spectrum of the residual sands and clays that remained in the form of a slurry showed no evidence for the presence of residual IL after they had been washed with water. This is more clearly seen in Figure 6, which compares the infrared spectra of these minerals to that of [bmmim][BF4]. Both the washed minerals and this polar IL absorb very strongly in the infrared, and although the most intense bands of each occur in the same region of the spectrum, they do not coincide. Also shown in this figure as an insert is the 1750-1250 cm-1 region of the spectrum on an absorbance expanded scale. Here there are several bands that do not overlap, and it can be seen that in terms of limits of infrared spectroscopy there is no detectable IL left in the washed minerals.
processes presently in use, naphtha or paraffinic solvents are added to the separated water/bitumen froth (and some residual solvent ends up in the tailings). The product is centrifuged and the solvent removed before the bitumen is refined. Naphtha or other nonpolar hydrocarbon solvents could readily be used in a process involving ILs. Accordingly, we have not introduced the use of additional solvents, but simply removed the huge problems associated with the use of water in the bitumen/sand separation process and lowered the temperature at which separation can be achieved. It will still be necessary to use some water to remove the IL from the separated sand and clay layer (see below), but far less than is now required. Furthermore, essentially all of this water can be recycled through the process. Because ILs have essentially zero vapor pressure at temperatures below 300 °C, water can easily be separated from the IL by heating to 100 °C. There are an enormous number of ionic liquids that are now becoming available. Some, like the ones used here, are based on imidazolium ions. Others are based on cations such as tetraalkyl ammonia and pyrrolidinium. At this stage we cannot predict which are capable of giving good separations in a bitumen extraction process. This is a consequence of two factors. First, in tar sands there are a range of interactions that occur between the various organic and inorganic components, and we have yet to identify exactly how ILs interact with the surfaces of sand and clay particles and their associated ions. Second, conventional theories of solubility usually deal with nonelectrolytes. Ionic liquids are very different and conventional measurements, such as heat of vaporization to determine solubility parameters cannot be made, because these materials have extraordinary low vapor pressures at room temperature. These liquids obviously engage in electrostatic interactions, but that does not necessarily mean that all ILs will facilitate a separation of bitumen from sand, as shown here. We presently have a working hypothesis involving the acidity of ILs. Bitumen becomes strongly attached to sand at pH values less than 6.14 Although ILs are salts, they have a surprising range of pH values in aqueous solutions, between
Discussion Although it is scientifically interesting that certain ILs will promote the separation of bitumen from sand in such a facile manner, the results presented here do not provide the basis for a commercial process, but they do suggest an approach that may be worth exploring. There are a number of issues that need to be addressed, such as the choice of IL, the kinetics of the separation process, costs, and environmental concerns. There are a range of ILs and organic solvents that could be used in a process of this type. Any organic solvent that dissolves bitumen but is immiscible with the IL will accomplish a separation. Toluene happened to be readily available and we have determined that most nonpolar hydrocarbon solvents (such as hexane) do not mix with the ILs we have used so far. Note that in the hot or warm water separation 1097
Energy Fuels 2010, 24, 1094–1098
: DOI:10.1021/ef9009586
Painter et al.
Figure 7. Schematic plot of a possible separation process.
about 1.6 and 8.3 for the materials studied by Toma et al.15 Accordingly, we found that [bmim][CF3SO3], estimated to have a pH of about 2.6 in aqueous solutions, did not separate bitumen from sand and clay in the absence of an organic solvent such as toluene. Furthermore, the H2 hydrogen of the imidazolium cation is weakly acidic, with pKa values of the order of 21-23.16 Accordingly, we found that a methyl substituted IL gave the best results in terms of separation of the ILs we have studied so far. More work clearly remains to be done to elucidate the mechanism of separation, modes of interaction and the phase behavior of these mixtures. Although ILs have often been called “green solvents”, this appears to be largely a reflection of their extremely low vapor pressure and imidazolium ILs, particularly those with halogencontaining anions, are toxic to many forms of aquatic life.17 Although biodegradable ILs have now been synthesized,18 the ILs used in this study would have to be cleanly removed from the minerals and recycled through a closed system for a process to be acceptable. The preliminary results presented here indicate that this is feasible. Another major barrier to a commercial process is economic. At the present time, ILs are very expensive materials. Even though they are now being widely used in a large number of research projects, getting even ballpark figures for the cost of large quantities of ILs is not straightforward, as Plechkova and Seddon recently pointed out in a comprehensive review of the use of ILs in the chemical industry.11 However, these authors estimated that in general ionic liquids are about 5-10 times more expensive than conventional solvents (depending on the specific IL), so that 10-20 recycles would give the same cost, while a larger number of recycles would make them cheaper. Accordingly, even at today’s prices an economically viable industrial scale process would be feasible if the IL can be
recycled through the system a large number of times. The ILs used here apparently can be, because extraction conditions are very mild and unlikely to lead to degradation. Also, the products appear to be cleanly separable with no contamination of the bitumen and residual minerals, either with each other or with IL, at least in terms of the limits of detection of infrared spectroscopy. The bitumen/solvent mixture that forms on the surface can be separated by decantation and any entrained IL removed in a further processing step. The IL phase appears to contain a small amount of bitumen and some mineral fines, but these can be simply recycled through the system, as the amount of bitumen would be expected to reach equilibrium and not accumulate. In experiments performed so far we have reused the middle layer five times in successive extractions with no decrease in yield. The sand/clay layer has entrained IL, but this can be removed by using relatively small amounts of water. Because the IL has essentially zero vapor pressure, water is readily distilled from the IL and both can then be recycled. A schematic diagram illustrating one possible process is shown in Figure 7. Conclusions Bitumen can be separated from oil or tar sands using an ionic liquid. This separation is enhanced by the presence of a nonpolar solvent such as toluene. To a first approximation, given the simple nature of the experiments reported here, it appears that yields in excess of 90% are obtained. Additional work is required to provide a precise figure. No water at all is used in this 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 and the bitumen produced in this process was free of both clay fines and residual IL, in terms of the detection limits of the analytical technique used here, infrared spectroscopy.
(15) Toma, S.; Meciarova, M.; Sebesta, R. Eur. J. Org. Chem. 2009, 3, 321–327. (16) Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126, 4366–4374. (17) Latala, A.; Nedzi, M.; Stepnowski, P. Green Chem. 2009, 11, 1371–1376. (18) Harjani, J. R.; Farrel, J.; Garcia, M. T.; Singer, R. D.; Scammells, P. J. Green Chem. 2009, 11, 821–829.
Acknowledgment. The authors gratefully acknowledge the support of the National Science Foundation, Polymers Program, under grant DMR-0901180 and DMR-0851897. 1098
