Binary Solvents with Ethanol for Effective Bitumen Displacement at

Jun 19, 2015 - Displacements of bitumen from silica surfaces were studied using various solvents and binary mixtures of ethanol. It was found that ...
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Binary Solvents with Ethanol for Effective Bitumen Displacement at Solvent/Mineral Interfaces Zheng Yang,† Hossein Nikakhtari,‡ Sebastian Wolf,‡ Dan Hu,† Murray R. Gray,‡ and Keng C. Chou*,† †

Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada

‡

ABSTRACT: Displacements of bitumen from silica surfaces were studied using various solvents and binary mixtures of ethanol. It was found that single-component solvents, including water, toluene, cyclohexane, and ethanol, were not able to completely remove bitumen on silica. However, adding 5% ethanol to toluene greatly enhanced its ability to displace bitumen from the silica surface. Surface vibrational spectra obtained by sum frequency generation vibrational spectroscopy showed that no trace of bitumen could be detected on silica after treatment with the 5% ethanol−toluene mixture. Similar effects were observed for cyclohexane with 10% ethanol. The results suggested that binary solutions with amphiphilic components, such as ethanol, are effective solvents for release of bitumen from the mineral surface. This expectation was confirmed by batch extraction experiments on Athabasca oil sands samples using cyclohexane−ethanol solutions.

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INTRODUCTION Surface chemistry of bitumen has been gaining a great deal of interest because there are large deposits in oil sands available for further development to supply the future energy needs. Natural bitumen is the remnants of degraded conventional oils and usually mixed with large proportions of sand. In the oil sands, each grain of sand is surrounded by a slick of bitumen, and the bitumen content may vary from 1 to 18 wt %.1 In the Athabasca deposits, more than 90% of the bitumen can be recovered using hot-water extraction techniques, in which water at 40−50 °C is added to the crushed oil sands, allowing the bitumen to reduce its viscosity and separate from the sand.2 However, the water-based extraction process is energy-intensive because of the large volumes of water, generating greenhouse gas emissions. Additionally, the accumulation of wet tailings creates a serious problem of mine reclamation and eventual treatment of the water. A number of oil sands deposits occur in areas with limited water and with physical properties unsuited to the aqueous process, including Utah and Kazakhstan.3 These issues associated with water-based extraction processes may potentially be resolved using a non-aqueous extraction process, which uses solvents to separate bitumen and eliminate or reduce water intake in the extraction process. Because bitumen dissolves in the solvent, non-aqueous extraction can be carried out at ambient temperature and significantly lowers the energy demand.4 Currently, very little is known about the bitumen/ mineral and solvent/mineral interfaces, and an effective solvent for bitumen separation remains unidentified. Bitumen is a complex mixture of hundreds of different types of organic molecules, and its complexity has thus far defied complete analysis of molecular structure and composition.5,6 Studies have suggested that bitumen is composed primarily of polyaromatic hydrocarbons with heteroatoms, such as oxygen, nitrogen, and sulfur. The average molecular weight has been reported to be about 540−800 g/mol.1 Generally, the components of bitumen are classified into two groups according to the solubility in n-pentane. The n-pentane-soluble © 2015 American Chemical Society

components are maltene (74−84 wt %), and the precipitates are asphaltenes (18−25 wt %).7,8 The exact molecular structures are difficult to determine because intramolecular aggregation of some components occurs at a few parts per billion (ppb).9 Studies using nuclear magnetic resonance (NMR) and theoretical calculations proposed that the structure included condensed aromatic rings and aliphatic chains.10,11 Because bitumen consists mainly of alkyl aromatic hydrocarbons, it is soluble in relatively nonpolar solvents, such as toluene and cyclohexane, but insoluble in highly polar solvents, such as water.12 From a microscopic point of view, an effective non-aqueous extraction process requires that the solvent displaces bitumen and adsorbs on the mineral surface. As a main constituent in oil sands, silica has been a representative mineral in many bitumen extraction studies.2,13−15 Silica surfaces, bearing silanol groups (SiOH), interact with hydrophilic molecules, such as water, via hydrogen bonds.16 On the other hand, bitumen, mainly consisting of cyclic-saturated hydrocarbons and aromatics, prefers hydrophobic solvents, such as toluene and cyclohexane.12 Therefore, an ideal solvent for bitumen separation must be able to accommodate hydrophobic bitumen as well as the hydrophilic silica surface. As an amphiphilic molecule, ethanol can potentially satisfy the requirements to interact with both the hydrophobic bitumen and the hydrophilic mineral surfaces. In this paper, we will compare the effectiveness of bitumen displacement from silica surfaces using water, toluene, cyclohexane, ethanol, and binary solutions with ethanol. To study the effectiveness of a solvent for bitumen separation, sum frequency generation (SFG) vibrational spectroscopy was used to detect the residual bitumen on silica substrates after the solvent extraction. SFG has been known to have sub-monolayer sensitivity.17,18 The high sensitivity allowed Received: April 2, 2015 Revised: June 19, 2015 Published: June 19, 2015 4222

DOI: 10.1021/acs.energyfuels.5b00696 Energy Fuels 2015, 29, 4222−4226

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Figure 1. SFG spectra of bitumen films (a) at the air/silica interface before extraction and after 2 min of ultrasonic extraction in (b) water, (c) toluene, (d) cyclohexane, and (e) ethanol. end to end tumbling action at the speed of 60 rpm for 10 min in a rotary mixer (Rotator drive STR4, Stuart). The digested oil sands slurry was transferred into a 250 mL graduated cylinder; the mixture was allowed to stand for 30 min for complete separation of two distinct layers. The “first supernatant” (top phase) was separated by siphoning; a second batch digestion was carried out on the remaining slurry (bottom phase). The slurry (bottom phase) was further mixed with 50 g of solvent and was agitated at 60 rpm using the rotary mixer for 10 min. This mixture slurry was transferred on a sieve of 45 μm aperture (ASTM E-11 Standard test sieve no. 325, Fisher Scientific Co., Pittsburgh, PA), and the solids were separated by vibrating the sieve for 5 min. The solids retained on the sieve were washed with two batches of 50 g of solvent while vibrating the sieve for 5 min. The amount of bitumen extracted was determined by elemental analysis of the oven-dried initial oil sands and the solids remaining after each digestion or wash step. The amount of fine solids in the recovered bitumen was determined by placing the extracted bitumen solution in 250 mL Teflon containers and centrifuged (model Avanti J30I centrifuge, Beckman Coulter, Mississauga, Ontario, Canada, with JA-10 rotor) at 3000 relative centrifugal force (RCF) for 1 h.

us to identify solvents that are capable of displacing bitumen from the mineral surface with sub-monolayer residual. In the current study, we examined silica surfaces by SFG after extraction by single-component solvents, including water, toluene, cyclohexane, and ethanol, and by blends of toluene and cyclohexane with 5−10% ethanol. Toluene and cyclohexane were chosen in this study because of their high bitumen solubility; toluene is the most commonly used solvent for heavy petroleum fractions, while cyclohexane is attractive as a less toxic solvent for industrial use.19 Cyclohexane−ethanol blends were then used to extract oil sands samples for comparison to cyclohexane only.

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EXPERIMENTAL SECTION

Material and Preparation. Silica substrates (25.4 mm in diameter, ISP Optics) used for SFG studies were cleaned by Extran AP12 cleansing reagent for 3 min, followed by soaking in a 98% sulfuric acid and Nochromix (GODAX Laboratories, Inc.) mixture for 12 h. The substrates were rinsed thoroughly with pure water (Millipore; resistivity ∼ 18.2 MΩ cm) and dried with N2 before use. Bitumen films were prepared using bitumen solution in toluene. A total of 1 mL of bitumen (Athabasca, Alberta, Canada) was dissolved in 40 mL of toluene. About 0.015 mL of the bitumen solution was drop-casted on the silica substrate (1 in. diameter, mechanically polished to optical smoothness) to form a bitumen film. On the basis of the volume of the bitumen solution and the surface area of the film, the thickness of the bitumen film was estimated to be about 1 μm. The bitumen on the substrate was then characterized by SFG spectroscopy. Two different spots were measured. The substrate was then immersed in the solvent with a volume of 40 mL (the bitumen-coated side facing up) and ultrasonicated for 2 min. Water was obtained using a Millipore water purification system. Toluene (99.9%), cyclohexane (99%), and ethanol (98%) were purchased from Fischer Scientific. After ultrasonication, the silica substrate was dried with N2 and characterized using SFG vibrational spectroscopy. All experiments were performed at room temperature (20 ± 1 °C) and ambient pressure. SFG Vibrational Spectroscopy. A femtosecond Ti:sapphire laser (800 nm, 1 kHz, and 2 mJ/pulse) was used to pump an optical parametric amplifier and generate an infrared (IR) beam. A narrowband ps 800 nm beam (s-polarized) and the IR beam (p-polarized) were temporally and spatially overlapped on the sample with incident angles of 50° and 60°, respectively. The SFG signal from the sample went through a polarizer (s-polarized), a band-pass filter, and a monochromator and then was recorded by a charge-coupled device (CCD) camera. Bitumen Extraction. Bitumen extraction was carried out using 150 g of oil sands and 250 g of solvent or solvent blend using the method of Nikakhtari et al.19 The oil sands sample contained 13.0 wt % bitumen, 8.6 ± 0.5 wt % fine solids (particles smaller than 44 μm), and 3.4 ± 0.4 wt % moisture. The balance of the sample weight was sand. The first stage of batch digestion of 150 g of oil sand with 100 g of solvent was carried out in a sealed 500 mL Teflon bottle through an

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RESULTS AND DISCUSSION

Figure 1a shows the SFG spectra of a drop-casted bitumen film at the air/silica interface. The bitumen film showed three peaks at 2850, 2870, and 2935 cm−1, which are consistent with the CH2 symmetric stretch, CH3 symmetrical stretch, and CH3 Fermi resonance, respectively.1 The Fermi resonance is a coupling between the CH3 symmetric stretch and the overtone of the CH3 bending mode. The peak intensities are slightly spot-dependent, which is likely the result of variations in the film because of the drop-casting process. The CH2 and CH3 peaks dominate the SFG spectra and are good indicators for identifying the bitumen residual after the extraction process. The CH modes associated with the aromatics were not observable. The reason could be that the aromatics rings were parallel to the surface or not well ordered.20,21 Figure 1b shows the SFG spectra of the bitumen film after 2 min of ultrasonication in water. Overall, the SFG intensity of the CH2 and CH3 symmetric peaks has decreased after the water treatment, but significant bitumen residuals are observable on the silica. It is known that bitumen film in water can be displaced from the glass surface and shrink its footprint on the surface.15 Bitumen has been found to be amphoteric containing anionic surfactants, such as carboxylic acids, and amine-bearing cationic surfactants.22 Additionally, the silica surface bearing silanol is negatively charged. Therefore, bitumen may interact with the silica surface via charge−dipole or charge−charge interactions. Figure 1c shows the SFG spectra of the bitumen film after 2 min of ultrasonication in toluene. Although bitumen is known to be miscible with toluene,23,24 residual bitumen is also 4223

DOI: 10.1021/acs.energyfuels.5b00696 Energy Fuels 2015, 29, 4222−4226

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Energy & Fuels observable on the silica. Toluene has been intensively studied as a candidate solvent for non-aqueous extraction.19,25−27 The recovery yield was reported to be as high as 93−95% in triple extraction.19 At a molecular level, toluene failed to displace all bitumen molecules on the substrate. This result is likely due to low affinity of toluene to the silica surface. Because the interaction between toluene and silica is weaker than the interaction between charged bitumen and silica, a complete displacement of bitumen by toluene is thermodynamically unfavorable. Figure 1d shows the bitumen spectra of the bitumen film after 2 min of ultrasonication in cyclohexane. Cyclohexane is expected to have a good extraction ability because it can completely dissolve bitumen and has also been proposed as a candidate solvent for non-aqueous extraction.19 The SFG spectrum showed that cyclohexane was also unable to produce a microscopically clean surface. The enhancement of the CH3 Fermi stretch suggests that the ordering of bitumen has been altered by cyclohexane because of high solubility of bitumen in cyclohexane. Figure 1e shows the bitumen spectra of the bitumen film after 2 min of ultrasonication in ethanol. It is not surprising to see residual bitumen after ultrasonication in ethanol because bitumen dissolves only slightly in ethanol.12 Liu et al. reported that the extraction using pure ethanol yielded about 68% bitumen recovery at 60 °C.28 Ethanol has a higher propensity to silica surface than toluene and cyclohexane because it can form hydrogen bonds with the SiOH groups on silica. However, the bitumen solubility of ethanol is too low to completely remove bitumen from the silica surface. The current SFG study showed that none of the above single-component solvents was able to completely displace the bitumen film on silica. The poor performance of these singlecomponent solvents can be understood by the very different hydrophilicity of bitumen and silica. Although toluene and cyclohexane exhibit high solubility of bitumen, their affinity to silica is low. On the other hand, water and ethanol can interact with silica through hydrogen bonds but their bitumen solubility is low.29 Ideally, an effective solvent should demonstrate both high bitumen solubility and high silica affinity to promote bitumen detachment from the silica surface. These two properties are generally mutually exclusive in a single-component solvent. However, binary solutions with ethanol are potential candidates to improve the extraction yield because ethanol is completely miscible with nonpolar solvents, such as toluene and cyclohexane, and its OH group interacts with silica via relatively strong hydrogen bonds. To explore the potential of ethanol binary solutions for nonaqueous extraction, experiments were repeated using toluene/ ethanol mixtures. Figure 2 shows bitumen spectra before and after ultrasonication (2 min) in toluene−ethanol mixtures with 2.5 and 5 vol % ethanol. The toluene−ethanol mixture with 2.5 vol % ethanol (Figure 2b) was able to remove significantly more bitumen compared to pure toluene (Figure 1c). As shown in Figure 2c, when the ethanol fraction was increased to 5 vol %, no signal of bitumen could be detected on the silica surface, which suggested a near microscopically clean silica surface. It is known that ethanol, with hydroxyl groups, can form hydrogen bonds with the mineral surface bearing hydroxyl groups, such as silica and illite.30 Previous studies on the competitive adsorption of water−alcohol binary mixtures at liquid/silica interfaces showed that alcohol adsorbs preferentially to water at

Figure 2. SFG spectra of bitumen films on silica (a) before extraction and after 2 min of ultrasonic extraction using (b) toluene (97.5%) and ethanol (2.5%) mixture and (c) toluene (95%) and ethanol (5%) mixture.

the liquid/silica interfaces.16 Therefore, the results shown in Figure 2 are consistent with the expectation that ethanol is more favorable to adsorb onto the silica surface compared to toluene. On the other hand, it is energetically favorable for the displaced bitumen to stay with toluene. Therefore, toluene with a 5 vol % ethanol binary mixture is a highly effective solvent to displace bitumen from the silica surface. Similarly, studies were carried out for cyclohexane−ethanol mixtures, and ethanol was also able to significantly improve the effectiveness of cyclohexane to displace bitumen on silica. Figure 3 shows the bitumen spectra before (Figure 3a) and

Figure 3. SFG spectra of bitumen films on silica (a) before extraction and after 2 min of ultrasonic extraction using (b) cyclohexane (95%) and ethanol (5%) mixture and (c) cyclohexane (90%) and ethanol (10%) mixture.

after ultrasonication in cyclohexane−ethanol mixtures. With 5 vol % ethanol, some bitumen residue was detectable, as shown in Figure 3b. However, the majority of bitumen was removed with 10 vol % ethanol, as shown in Figure 3c. Lab-scale bitumen extraction using 150 g samples of oil sands were carried out using cyclohexane to test the effectiveness of the binary solution for bitumen extraction. Solvent loss is a major issue associated with non-aqueous extraction. On the basis of an evaporation method reported by Wu et al., solvent loss is ∼4 barrels per 1000 barrels of bitumen production,26 which produces a significant environmental impact. Cyclohexane is a preferred solvent for non-aqueous extraction compared to toluene because of its much lower toxicity. The results of lab-scale bitumen extraction are shown in Table 1. The cumulative recovery increased from 96.5% with no ethanol to 99% with 30 vol % ethanol, with the balance of cyclohexane. The recovery in the first stage of extraction was enhanced even more by the addition of ethanol, increasing from 68.7 to 83.8% with 10 vol % ethanol. These experimental results agree with the trend of the SFG data that the release of bitumen from the surface was enhanced by the addition of ethanol. The sand 4224

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vol % ethanol. This enhancement in bitumen recovery was confirmed by extraction experiments on Athabasca oil sands using cyclohexane−ethanol solutions.

Table 1. Bitumen Recovery Using Cyclohexane−Ethanol Binary Solutions

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cumulative bitumen recovery (wt %) ethanol (wt %) 0 10 20 30.5

first digestion 68.7 83.8 83.9 80.5

first digestion + second digestion

first digestion + second digestion + sieve washing

80.5 91.2 93.5 93.4

AUTHOR INFORMATION

Corresponding Author

96.5 97.1 98.7 99.0

*Telephone: 1-604-8225850. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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material in the oil sands would have similar surface properties to the silica in the SFG experiments, but the fine solids were complex, including mainly quartz, kaolinite, and illite, with smaller quantities of carbonate and sulfide minerals.26 The fine solids comprised the majority of the surface area exposed to bitumen, but the release of bitumen was still enhanced by the addition of ethanol. A second important criterion for performance of a solvent is the release of fine solids into the bitumen product, which is very sensitive to the amounts of fine solids and moisture in the oil sands ore.26 The data of Table 2 show the amounts of fine

ACKNOWLEDGMENTS This work was financially supported by the Institute for Oil Sands Innovation at the University of Alberta, the Natural Sciences and Engineering Research Council of Canada, and the Canada Foundation for Innovation.

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(1) Czarnecka, E.; Gillott, J. E. Formation and characterization of clay complexes with bitumen from Athabasca oil sand. Clays Clay Miner. 1980, 28 (3), 197−203. (2) Liu, J. J.; Xu, Z. H.; Masliyah, J. Studies on bitumen−silica interaction in aqueous solutions by atomic force microscopy. Langmuir 2003, 19 (9), 3911−3920. (3) Gray, M. R. Upgrading Oilsands Bitumen and Heavy Oil; The University of Alberta Press: Edmonton, Alberta, 2015. (4) Cormack, D. E.; Kenchington, J. M.; Phillips, C. R.; Leblanc, P. J. Parameters and mechanisms in solvent-extraction of mined Athabasca oil sand. Can. J. Chem. Eng. 1977, 55 (5), 572−580. (5) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. The chemistry of Alberta oil sand bitumen. Proc.Int. Symp. Heavy Oil Residue Upgrading Util. 1992, 29−35. (6) Hansen, J. S.; Lemarchand, C. A.; Nielsen, E.; Dyre, J. C.; Schroder, T. Four-component united-atom model of bitumen. J. Chem. Phys. 2013, 138, 9. (7) Speight, J. G. Petroleum asphaltenesPart 1Asphaltenes, resins and the structure of petroleum. Oil Gas Sci. Technol. 2004, 59 (5), 467−477. (8) Hubbard, R. L.; Stanfield, K. E. Determination of asphaltenes, oils, and resins in asphalt. Anal. Chem. 1948, 20 (5), 460−465. (9) McKenna, A. M.; Donald, L. J.; Fitzsimmons, J. E.; Juyal, P.; Spicer, V.; Standing, K. G.; Marshall, A. G.; Rodgers, R. P. Heavy petroleum composition. 3. Asphaltene aggregation. Energy Fuels 2013, 27 (3), 1246−1256. (10) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Quantitative molecular representation and sequential optimization of Athabasca asphaltenes. Energy Fuels 2004, 18 (5), 1377−1384. (11) Murgich, J.; Abanero, J. A.; Strausz, O. P. Molecular recognition in aggregates formed by asphaltene and resin molecules from the Athabasca oil sand. Energy Fuels 1999, 13 (2), 278−286. (12) Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007; pp 155−157. (13) Zhao, H. Y.; Long, J.; Masliyah, J. H.; Xu, Z. H. Effect of divalent cations and surfactants on silica−bitumen interactions. Ind. Eng. Chem. Res. 2006, 45 (22), 7482−7490. (14) Jin, Y.; Liu, W. K.; Liu, Q.; Yeung, A. Aggregation of silica particles in non-aqueous media. Fuel 2011, 90 (8), 2592−2597. (15) Basu, S.; Kanda, W. C.; Nandakumar, K.; Masliyah, J. H. Effect of hydrophobic and hydrophilic clays on bitumen displacement by water on a glass surface. Ind. Eng. Chem. Res. 1998, 37 (3), 959−965. (16) Zhang, L. N.; Liu, W. T.; Shen, Y. R.; Cahill, D. G. Competitive molecular adsorption at liquid/solid interfaces: A study by sumfrequency vibrational spectroscopy. J. Phys. Chem. C 2007, 111 (5), 2069−2076. (17) Hunt, J. H.; Guyotsionnest, P.; Shen, Y. R. Observation of C−H stretch vibrations of monolayers of molecules optical sum-frequency generation. Chem. Phys. Lett. 1987, 133 (3), 189−192.

Table 2. Solids in Recovered Bitumen from Cyclohexane− Ethanol Extraction solids (wt % of initial oil sands) ethanol in solvent (wt %)

first digestion supernatant

second digestion and sieve wash

total solids

0 10 20 30.5

0.091 0.041 0.002 0.016

0.08 0.86 3.68 5.26

0.17 0.90 3.69 5.27

REFERENCES

solids released during the initial digestion and then in the second digestion and sieve washing. The addition of ethanol decreased the release of the solids in the first digestion, possibly by giving faster sedimentation of the fine solids. The opposite trend was observed in the second digestion and sieve wash, with a dramatic increase in the release of fines with the addition of ethanol. This trend was likely due to the disruption of the moisture layers in the oil by the ethanol. The moisture is important for retention of fine solids by capillary forces,26 so that dissolution of the moisture by a polar solvent component would give weakened binding of the fine solids. The data of Table 2 suggest that the use of ethanol would require careful design of mechanical agitation to avoid the unwanted release of fine solids. A further criterion for solvent selection is recovery from the gangue after the extraction process. While cyclohexane is readily desorbed, we would expect more retention of the more polar ethanol on the mineral surfaces. A further study of this variable is required.

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CONCLUSION Studies using SFG vibrational spectroscopy showed that singlecomponent solvents, including water, toluene, cyclohexane, and ethanol, were not able to completely remove bitumen on silica surfaces. However, 5 vol % ethanol in toluene facilitates near complete displacement of bitumen from silica surfaces. The improved extraction capability of the binary solvents is a result of the amphiphilic nature of ethanol, which can interact with both the hydrophobic compounds and the hydrophilic silica surface. Similar effects were observed for cyclohexane with ∼10 4225

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DOI: 10.1021/acs.energyfuels.5b00696 Energy Fuels 2015, 29, 4222−4226