Ionic Liquid-Assisted Solvent Extraction for Unconventional Oil Recovery

Aug 2, 2016 - solvent extraction for heavy hydrocarbon recovery from unconventional oil ores. In this study, the COSMOtherm software was applied to sc...
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Ionic Liquid-Assisted Solvent Extraction for Unconventional Oil Recovery: Computational Simulation and Experimental Tests Xingang Li,†,‡,§ Junyan Wang,†,§ Lin He,*,†,§ Hong Sui,*,†,‡,§ and Wentao Yin† †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China National Engineering Research Center of Distillation Technology, Tianjin 300072, China § Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

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ABSTRACT: Selection of solvents and process aids (i.e., ionic liquids (ILs)) is considered to be one of the key steps during solvent extraction for heavy hydrocarbon recovery from unconventional oil ores. In this study, the COSMOtherm software was applied to screen highly efficient solvents and ionic liquid systems based on oil fraction solubility and surface free energy calculations. It is found that the dispersion force parameter (δd) of solvents plays the dominant role in dissolving oil fractions compared with the roles of the polar force parameter (δp) and hydrogen bonding force parameter (δh). The simulation results also show that the surface free energy of oil fractions (SARA fractions) at the organic solvent-IL interface is significantly lower than that found at the IL-SARA fraction interface, which is favorable for unconventional oil dissolution in solvents. In addition, further interactive energy calculation shows that the interaction between IL and the silica surface is stronger than that between oil fraction and the silica surface. These results suggest that the presence of IL between the organic solvent and oil fraction is beneficial for the transfer of the oil fraction from the solid surface and bulk oil phase to the bulk organic solvent. Additionally, unconventional oil recovery has been found to be highly influenced by the mutual solubility between the solvent and IL, which increased the entrainments of oil components in the IL phase. Calculation of surface free energy and mutual solubility suggests that increasing the chain length of IL molecules is detrimental for bitumen extraction due to the higher mutual solubility of solvents and entrainments of bitumen in ILs. The above simulation results are confirmed by the bottle extraction tests and instrumental detection in which the oil sands ores are extracted by organic solvents with or without ILs. These findings suggest that the COSMOtherm simulation is a potential way for future solvent and IL screening as well as a way to reveal mechanism during unconventional oil exploitation, which would save time and cost.

1. INTRODUCTION

However, most of the aromatic solvents and halogenated hydrocarbons are highly toxic chemicals which would cause health threats such as sensory irritation symptoms, severe respiratory systems diseases, metabolic disorders,11 and even carcinogenic effects.12 Accordingly, great efforts have been made in searching for the much more relatively environmentally friendly and effective solvents for oil sands extraction. For example, Nikakhtari et al. tested the feasibility of different solvents on bitumen recovery, including aromatics, cylcoalkanes, biologically derived solvents such as limonene (1methyl-4-(1-methylethenyl)-cyclohexene, C10H16), isoprene (2methyl-1,3-butadiene, C5H8), and their mixtures.9 On the basis of the evaluation in maximal bitumen recovery, removal rate of the solvent from tailings, the residual solvent concentration in the dry tailings, and the release of fine solids to bitumen, cyclohexane was found to be the best candidate in their study for extracting the Alberta oil sands.9 After repeatable tests by npentane, n-hexane, n-heptane, cyclohexane, naphtha, o-xylene, ethyl acetate, toluene, chloroform, trichloroethylene, acetone, and ethanol using Xinjiang oil sands, Wang et al. summarized that the selection of solvent for bitumen recovery from oil sands should be based on the solubility parameters of the bitumen and solvents.13 Similar results were obtained by

Unconventional oils, including oil/tar sands, shale oil, heavy oil, etc., have been considered as an alternative to traditional crude oils due to their huge reserves (about two-thirds of the total oils) buried around the world. Taking oil sands as an example, it is estimated that there are about 1700 trillion barrels of bitumen contained in the oil sands ores in Alberta, Canada.1,2 To release this kind of petroleum, many advanced technologies have been proposed or even applied in industry, such as hot water-based extraction (HWBE), steam-assisted gravity-drainage (SAGD) technique, solvent extraction, and pyrolysis of oil sands.3−6 Solvent extraction, different from the commercialized HWBE process, has been attracting increasing attention from industry due to its high extraction efficiency, lower water and energy consumption, and high compatibility for different kinds of ores, including oil-wet, water-wet, and neuter-wet oil sands.7,8 Practical approaches to nonaqueous bitumen extraction have been studied since the 1960s. During the past few decades of research and practice on solvent extraction, many solvents or solvent mixtures have been selected as extraction agents, including toluene, naphtha, heptane, kerosene, ethanol, cyclohexane, Heptol, etc.8−10 A common consensus has been reached that aromatic solvents or halogenated hydrocarbons perform better than alkane hydrocarbons in extracting bitumen from oil/tar sands, even resulting in up to 99% recovery by several extraction steps. © 2016 American Chemical Society

Received: May 28, 2016 Revised: August 1, 2016 Published: August 2, 2016 7074

DOI: 10.1021/acs.energyfuels.6b01291 Energy Fuels 2016, 30, 7074−7081

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Energy & Fuels

thousands of hydrocarbons with or without heteroatoms.31 For simplification, during academic research or industrial applications, one often divides petroleum into four fractions, namely saturates, aromatics, resins, and asphaltenes (SARA fractions), based on the polarity and solubility of the molecules.32 In this way, molecules with similar physical/chemical properties are classified as one group. In some recent studies, some model molecules have been selected on behalf of these fractions to investigate their behaviors in different processes, such as sorption or desorption on a solid surface, precipitation in nonaqueous solution, accumulation at oil−water interfaces, etc.33,34 In this study, four typical molecular structures denoting the SARA fractions have been selected and used in the simulations, as shown in Figure 1.34−37 The initial structures of SARA

Nikakhtari et al. and He et al. when they extracted Athabasca oil sands using solvent extraction, where the polarity was also considered as an important impact factor.9,14 However, due to the complexity of the bitumen (heavy petroleum) and diversity of solvents, it is also an arduous and time-consuming task to screen a proper solvent for potential industry application, especially for some newly discovered unconventional oil ores. On the other hand, to enhance the solvent extraction efficiency, some process aids (e.g., water, ionic liquid, switchable hydrophilicity tertiary amines, etc.) are reported to be added together with the solvent to extract the heavy hydrocarbons from unconventional oil ores.15−19 Paul et al. evidenced a bitumen yield of over 90% without detectable IL in the residual sands or bitumen products after extraction when the ionic liquids (ILs) [Bmmim][BF4], [Bmim][CF3SO3], and [Emim][Ac] were added together with solvents for extraction.15 Our previous work using [Emim][BF4] as a process aid also showed that an added recovery of 10% could be obtained compared with that without IL addition.17 It was also found that less clays or fines were entrained in the recovered bitumen.17 These works present a promising future for IL applications in unconventional oil production. However, due to the relatively high cost and huge number of IL types, it is not easy for one to select a suitable IL candidate for extending its industrial application. In addition, the exact mechanisms of ILassisted solvent extraction are still not clear. Accordingly, the aims of this study are (i) to find a much more feasible and faster way to help screen suitable solvents and process aids (i.e., ILs) for heavy hydrocarbon recovery from unconventional oil ores and (ii) reveal the potential mechanisms of IL-enhanced solvent extraction of heavy oil from unconventional oil ores.

Figure 1. Three-dimensional molecular structures of SARA fractions: (a) aromatics (C46H50S, MW: 635), (b) saturates (C20H42, MW: 282), (c) resins (C50H80S, MW: 713), and (d) asphaltenes (C50H48O4, MW:713) .34−37

2. METHODOLOGY

molecular models were drawn by the Material Studio (MS) software. The molecular geometries were further optimized by the TURBOMOLE software which computes the COSMO file. After reading the COSMOtherm files, the COSMOtherm software obtains the compound information and transforms the screening charge surface into a screening charge distribution (the σ-profile). Other compounds, including both organic solvents and ILs, were selected from the TZVP database. The organic solvents used in this study are toluene, tetrahydrofuran (THF), n-heptane, and ethyl acetate, which include the categories of aromatic, heterocyclic, nonpolar aliphatic, and strong polar simple ester, respectively. For the ILs, the complete dissociation is taken to be equal to the dissociation of the cation and anion.11 The σ-profiles of the ILs are normalized simply by the algebraic σ-profile sum of the cation and anion. Thus, the mixture acts as a single component of an IL. The cations used are 1-ethyl-3-methylimidazoliu ([Emim]+), 1-butyl-3methylimidazolium ([Bmim] + ), 1-pentyl-3-methylimidazolium ([Pmim]+), and 1-hexyl-3-methylimidazolium ([Hmim]+), which have almost the same structure but are different in chain length. To avoid the influence of anion variation, tertrafluorobotate (BF4−) is the only anion used in this study. Solubility Calculation. The solubility (xj) is calculated using eqn 1 in the COSMOtherm software:22,23,38−40

2.1. Computational Simulation. To save time and investigation cost, the simulation method has been widely used in different academic fields by researchers recently, such as molecular dynamics (MD) simulations, computational flow dynamics (CFD), and some process simulations.20,21 Inspired by these simulations, here, a software program named COSMOtherm was selected and applied in the simulation to provide references for the solvent and process aid screening. The attractive feature of this software is the prediction of the nonideal activity coefficient of any component in a liquid mixture with the initial input of only the molecular structure of the component. Working Principles. The COSMOtherm software works based on the COSMO-RS theory. COSMO-RS is a predictive method for thermodynamic equilibrium of fluids and liquid mixtures which uses a statistical thermodynamics approach based on the results of quantum chemical calculations.22,23 In COSMOtherm calculations, the solute molecules are calculated in a virtual conductor environment where the solute molecule induces polarization charge density (σ) on the interface between the molecule and the conductor (i.e., on the molecular surface). The polarization density distribution on the surface of each molecule (i) is converted into a distribution function called σprofile (pi (σ)), which gives the relative amount of surface with polarity σ on the surface of the molecule. The COSMO-RS representations of molecular interactions, namely the σ-profiles and σ-potentials of compounds and mixtures, respectively, provide qualitative and quantitative information. Accordingly, with these merits, COSMO-RS is quite a good potential method for screening the optimal candidates from large number of solvents or solutes by thermodynamic calculations of all compounds involved, including chemical potentials, vapor pressures, solubility, activity coefficients, etc.24−30 Model Molecules. Molecule selection is the key step of the simulation. Petroleum is a well-known complex mixture consisting of

⎛ ⎡ (μ pure − μ solvent − ΔG ⎤⎞ j,fusion) ⎟ j j ⎥ lg(xj) = lg⎜exp⎢⎢ ⎜ ⎥⎟ RT ⎦⎠ ⎝ ⎣

(1)

μjpure

is the chemical potential of pure compound j (kcal/mol), where μjsolvent represents the chemical potential at infinite dilution (kcal/ mol), ΔGj,fusion indicates the free energy of fusion for the liquid (kcal/ mol), R is the gas constant (1.9863 cal/mol·K), and T is absolute temperature (K). 7075

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Energy & Fuels μiS,S ′ ,res (z , Γ) =

The solubility of SARA fractions in different liquid systems (e.g., different organic solvents and IL-solvent mixtures) under ambient conditions was calculated with two steps. First, the geometry and polarization charge densities on the molecular surface for the molecules of both solutes and solvents were calculated. Subsequently, the solubility of each system was obtained using their charge densities. The typical SARA model compounds dissolved in four common solvents were simulated using the COSMOtherm software at 25 °C under ambient pressure and were further confirmed by the experimental extraction described below. To further analyze the dissolution results, the solubility parameters were calculated according to the Hildebrand solution theory. Hansen divided the solubility parameter into three parts: dispersion force parameter (δd), polar force parameter (δp), and hydrogen bonding force parameter (δh).48 The total (or Hildebrand) solubility parameter δ is related to the sum of the squares of the δd, δp, and δh components (eqn 2).48

δ 2 = δd2 + δp2 + δ h2

μiS,res =

wp =

wh =

δp δd + δp + δ h

(3)

× 100%

δh × 100% δd + δp + δ h

(4)

(5)

a v μS ′(σ )

∑ a vμS(σ )

(7) (8)

Eadsorbate/surface = Etotal − (Eadsorbate + Esurface)

The solubility parameter that possesses the highest weight plays the dominant role in the dissolving process. Surface Free Energy Calculation. Surface free energy, also known as specific surface energy, is defined as the increase in free energy when the area of a surface increases by every unit area. The COSMOtherm software was developed to calculate the surface free energy of compounds, which is computed at the interface of the two solvents or solvent mixtures. The position of the solute at the interface is described by the distance z of the solute center from the interface and the immersion angle θ, as shown in Figure 2.

(9)

where Eadsorbate/surface, Esurface and Eadsorbate represent energies of the binary system, solid surface, and adsorbate molecules, respectively (kJ/ mol). The more negative the interaction energy is, the stronger the adsorption is. The detailed procedures of molecular simulation by MS software can be found elsewhere.34,41 2.2. Laboratory Extraction Tests. To confirm the simulation results, a set of solvent extraction experiments using real oil sands ores were designed and conducted accordingly. Chemicals and Samples. The unconventional oil ore samples (i.e., oil sands) were obtained from Alberta, Canada and used in the extraction. The heavy hydrocarbon (bitumen) content in the ores was determined to be 12.88 wt % using the standard Dean−Stark method.42 Organic solvents, including toluene, THF, n-heptane, ethyl acetate, acetone, and ethyl alcohol were purchased and used at their analytical grade from Tianjin Jiangtian Chemical Engineering Technology Co. Ltd. Four types of ILs (1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), 1-pentyl-3-methylimidazolium tetrafluoroborate ([Pmim][BF4]), and 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF4]), were provided by Lanzhou Yulu Fine Chemical Engineering Technology Co. Ltd. The properties of these ILs were simulated and are presented in the Table 1. Bottle Extraction Test. In the bottle test, 5 g of oil sands ores and 15 mL of organic solvent were mixed in a 100 mL conical flask followed by ultrasonication for 10 min at 50 °C under ambient conditions. The mixture after extraction was separated into liquid and solid phases using centrifugation for 5 min at 7000 rpm. The supernatant liquid was transferred into another clean conical flask to determine the heavy oil recovery by distillation and drying steps. When the process aids (e.g., ILs) were involved in the extraction, the experiments were carried out similarly as follows: 5 g of ILs and 5 g of oil sands were put into a 100 mL conical flask and soaked for 30 min followed by the addition of 15 mL of organic solvent (toluene or composite solvents: Vn‑heptane:Vacetone = 2:6). The mixture was agitated by ultrasonication for 10 min and treated by centrifugation, distillation,

Figure 2. Schematic of surface free energy (SFE) calculation by COSMOtherm software. Assuming that the free combinatorial part of the free energy stays essentially unchanged, subtracting the residual chemical potential in the bulk phase S from the total energy will yield the maximum free energy gain of molecule i at the S−S′ interface. In addition to the maximum free energy gain, a total free energy gain from the interface partition sum can be calculated by following equations:22,23,38−40

⎧ ⎡ μ S,S ′ ,res (z , Γ) − μ S,res ⎤ ⎪ i ⎥ GiS,S ′ = − RT ln⎨∑ exp⎢ − i ⎪ ⎢ ⎥⎦ RT ⎣ ⎩ z ,Γ

∑ ν ∈ innS ′

where μiS,S′,res is the minimum of the free energy of the solute i at the interface of S and S′ (kcal/mol), μiS,res represents the residual part of the chemical potential of solute compound i in phase S (kcal/mol), μS(σ) is the σ-potential of phase S (kcal/mol), z is the distance of the solute center from the interface (Å), and Γ is the orientation of a fixed solute axis with respect to the surface normal direction (90° − θ). The total free energy of the solute i at the interface of S and S′, GiS,S′ of eqn 6, can be used as a significant and thermodynamically rooted descriptor for the determination of surface activity in a solution. The calculations of free energy at 25 °C under ambient pressure were performed for SARA at the SARA fraction-IL interface and the organic solvent-IL interface. Because the COSMOtherm software could only calculate thermodynamic behaviors in the liquid phase, the interaction energy between ILs (or bitumen components) and the silica surface was calculated by MS software. With the interaction energies between oil or IL and solid surface, their corresponding adhesion behaviors can be obtained. The adsorption of asphaltenes on a silica surface was conducted at the condition of the ideal adsorption for asphaltene molecules in the vacuum state. The layer of asphaltene molecules was placed into a box with the same length and width to the corresponding solid surface. The interaction energy simulations were carried out using Forcite modules in an NVT ensemble (constant number of atoms (N), volume (V), and temperature (T)) for the established adsorption system at T = 298 K, P = 1 atm, and a simulation time of 200 ps. The interaction energies were calculated by the following equation:

(2)

δd × 100% δd + δp + δ h

a v μS (σ )+

ν∈ i

The weights of each solubility parameter (the weight of dispersion force parameter, wd; the weight of polar force parameter, wp; and the weight of hydrogen bonding force parameter, wh) are calculated by the following equations: wd =

∑ ν ∈ innS

(6) 7076

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Energy & Fuels Table 1. Physical/Chemical Properties of ILs Used in This Study

and drying successively to determine the heavy hydrocarbon recovery. Each test was repeated at least three times to obtain a credible result. The heavy hydrocarbon recovery was calculated by the following equation: R=

mB /ms × 100% RO

of organic solvents (toluene, THF, n-heptane, and ethyl acetate) calculated by the COSMOtherm software. It was found that, although dependent on the solvent type, the solubility of SARA fractions in the solvents shows a descending order of saturates > aromatics > resins > asphaltenes, which is consistent with the classification criteria of SARA.45,46 The saturates possess the highest solubility (x > 0.88) in the solvents involved in this study, leading to a much more complete dissolution into the solvents compared with that of other fractions. Due to the high solubility (x > 0.70), toluene and THF allow most of the fractions to be dissolved. However, only very small amount of aromatics, resins and asphaltenes could be dissolved in n-heptane and ethyl acetate due to their extremely low solubility (x < 0.09) in these two solvents. The high dissolution of SARA fractions in toluene and THF could be attributed to the similarity in molecular structures (i.e., hydrocarbon rings) between the solvents and solutes, while there are only carbon chains or branches embedded in n-heptane and ethyl acetate, which leads to low compatibility between the solvent and bitumen fractions. This can also be understood from the solubility parameters. According to the Hildebrand solution theory, the solvent and solute should have the similar solubility parameters if they are miscible.47 For the SARA molecular models used in this study, the solubility parameter of saturates is about 16.5 MPa1/2, while that for aromatics, resins, and asphaltenes is about 20 MPa1/2.13,48,49 The solubility parameters of solvents, including toluene, THF, n-heptane, and ethyl acetate, are listed in Table 2. Compared with that of n-heptane (15.3 MPa1/2), toluene, THF, and ethyl acetate possess relatively higher solubility parameters (18.2 MPa1/2, 19.5 MPa1/2, and 18.2 MPa1/2, respectively) which are much closer to those of SARA fractions. However, ethyl acetate is experimentally proven to be a poor solvent, even worse than

(10)

where R is oil recovery (wt %), mB presents the mass of extracted bitumen (g), mS is the mass of the oil sands sample (g), and RO is the oil content of the oil sands (12.88 wt %). Colorimetric Detection. The UV−vis spectrophotometer (TU1901, Puxi, China) was used to determine how many bitumen components were dissolved in the ionic liquids by measuring the absorbency. To obtain the calibration curve for quantitative analysis, the IL phase was diluted 1−10 times by ethyl alcohol. The diluted ionic liquids were used in an absorption test in a 1 cm quartz cuvette by the UV−vis spectrophotometer. The ethyl alcohol was also used as the reference solution. Absorbance at 280−320 nm was determined to be the working wavelength based on the full-wave scanning. Finally, the calibration curve was used to determine the concentration of bitumen components in the IL phase.

3. RESULTS AND DISCUSSION 3.1. SARA Fraction Solubility in Organic Solvents. Figure 3 gives the solubility of SARA fractions in different types

Table 2. Solubility Parameters of Organic Solvents48

toluene THF n-heptane ethyl acetate

Figure 3. Calculated solubility (x) of SARA fractions and bitumen recovery from oil sands in different types of organic solvents. 7077

δd

δp

δh

δ

18.0 16.8 15.3 15.8

1.4 5.7 0.0 5.3

2.0 8.0 0.0 7.2

18.2 19.5 15.3 18.2

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Energy & Fuels n-heptane, which is also shown in Figure 3. This is mainly due to the high polarity of ethyl acetate due to the existence of an ester group (i.e., -CO-O-). However, from the thermodynamic point of view, the above results could be understood through the Hansen solubility theory. After careful calculation, it suggests that the dispersion force parameter of the solvent plays the dominant role (weight >55%)13 in dissolving bitumen fractions, followed by hydrogen bonding force parameter (weight at 0−26%) and the polar force parameter (weight at 0−18%). Comparing with that of other solvents such as toluene and THF, ethyl acetate possesses a relatively low dispersion force parameter which plays the dominant role. This finding allows us to consider more detailed interaction forces, such as dispersion force, polar force, and hydrogen bonding force, when selecting potential solvents for the bitumen recovery process. In addition, the solubility calculation simulation based on oil fractions provides a relatively efficient and accurate previous prediction even though there are a range of factors that need to be considered. 3.2. Surface Free Energy of SARA Fractions at Oil-IL Interfaces. Figure 4 shows the SFE of different SARA molecules at the corresponding SARA fraction bulk phase-IL interface or solvent-IL interface simulated by the COSMOtherm software. To make the observation much easier, the SFE (all of the values are negative) of the molecules at the interface is presented as absolute value (ASFE) and shown in Figure 4. Therefore, the higher the ASFE, the smaller the real SFE is. A small real SFE means it is much easier for bitumen components to build a new surface at the interface. It is clear that the ASFE values of heavy fraction molecules (resins and asphaltenes) are higher than those of lighter fractions (saturates and aromatics) at their corresponding fraction-IL interfaces. This result suggests that it is much easier for resin and asphaltene molecules to transfer from bulk resin and bulk asphaltene to the bulk ionic liquid phase, respectively, than it is for saturate and aromatic molecules. From Figures 4a and b, at a given IL, the ASFE values of SARA fraction molecules at toluene-IL interfaces are larger than those at the SARA fraction-IL interface. The visualization of COSMO screening charges on the asphaltene molecular surface proved this more intuitively in Figure 5, which presents the position of asphaltene at the interface. Both the immersion depth and immersion angle in Figure 5a are found to be less than those in Figure 5b, suggesting that the transfer of SARA fractions from bulk SARA to toluene through the IL phase would happen spontaneously. These results deliver the information that the presence of ILs could help the SARA fractions to transfer from the bitumen phase to solvent phase much easier, finally leading to the dissolution of SARA fractions in solvent. During the IL-assisted solvent extraction, the IL phase is added between the solvent phase and the bitumen (or oil sands). Previous published work shows that the presence of IL not only improves the bitumen recovery, but also improves the bitumen quality and residual solid quality, making the residual solids much more clear than those without IL addition.17,50 Therefore, the successful application of ILs in enhancing bitumen recovery from oil sands is dependent on two steps: (i) the detachment of bitumen components from the solid surface and (ii) dissolution of the detached components into the solvents. The above results show us that the presence of ILs is beneficial for the detached bitumen fractions to transfer. To demonstrate how ILs help bitumen detach from the solid

Figure 4. Surface free energy of different SARA molecules at specific interfaces with the corresponding bitumen extraction recovery: (a) ILs-SARA fraction interface, (b) toluene-IL interface, and (c) CS-IL interface.

surface, the interaction energy calculation was conducted by MS simulation. Results show that the interaction energy between [Emim][BF4] and the silica crystal (−528.088 kJ/mol) is higher than that between asphaltene and the silica crystal (−712.977 kJ/mol), which means IL has stronger interactive forces with the silica surface than with asphaltene. Accordingly, the ILs play a “carrier” role in the IL-assisted solvent extraction of oil sands, carrying the bitumen fractions from the solids surface to the solvent phase for dissolution, as shown in Figure 6. In the real extraction process, under the condition of experimental stirring, the IL has abundant opportunities to 7078

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Figure 5. Visualization of COSMO screening charges on the asphaltene molecular surface of (a) asphaltene at the [Emim][BF4]asphaltene interface and (b) asphaltene at toluene-[Emim][BF4] interface.

contact oil sands and then transfer the bitumen into toluene due to the low energy state of ILs and high solubility of toluene. Not only dependent on the bitumen fraction type, the ASFE of SARA fraction molecules at the oil-IL interface is also found to be influenced by the IL type (Figure 4). An increase in the side chain of IL cations from C2 to C6 decreases the ASFE of SARA fraction molecules at the oil-IL interface. This decrease indicates that long side chains of IL cations are detrimental to the transfer of SARA fraction molecules from oil sands to the liquid phase. This result has been confirmed by an observed decreased bitumen recovery during the corresponding ILassisted solvent extraction, as shown in Figure 4b. Similar results were obtained using a composite solvent (CS, a mixture of acetone and n-heptane at volume ratio of 2:6) and ILs by computational simulation and experimental extraction, as shown in Figure 4c. Therefore, the value of ASFE could be one of the screening criteria to select the IL and reveal the mechanism. 3.3. Organic Solvent Solubility in ILs. In addition to the effects of solvent and IL types, solvent extraction efficiency is also observed to be influenced by the mutual solubility between the solvent and ionic liquid. Figure 7 shows the effect of IL type on the mutual solubility of solvent in ILs and the solvent extraction efficiency. The toluene solubility in ILs increases from 0.04 to 0.24 along with the increase of the side chains of ILs from C2 to C6, suggesting that more toluene may be dissolved into the IL with higher molecular weight. Although the SARA fractions are hardly dissolved in the ILs (the solubility is almost 0, as shown in Table 3), the dissolution of toluene in ILs will reduce the amount of bitumen dissolution in the toluene and increase the entrainments of bitumen components in the ionic liquid phase. This could be confirmed by the results of bitumen recovery shown in Figure 7a and the increased absorbance (from 0.41 ± 0.15 to 6.47 ± 0.39) of the IL phase after extraction, as shown in Figure 8. A similar

Figure 7. Solubility of solvents in ILs and the corresponding bitumen extraction recovery: (a) the solubility of toluene and (b) the partition coefficient of acetone.

phenomenon was observed when the composite solvent is used instead of toluene, as shown in Figure 7b. The partition coefficient of acetone in n-heptane and the IL phase is used as the evaluation indicator because acetone has high solubility in both n-heptane and ILs. A high partition coefficient means that less acetone is dissolved in ILs. With the side chains of ILs increasing from C2 to C6, the partition coefficient decreases from 0.72 to 0.40, leading to a decrease of bitumen recovery from 72.13 to 65.61%. Therefore, the solubility of organic solvent in ILs should also be taken into consideration when selecting the solvent system of organic solvents and ILs for oil

Figure 6. Proposed role of ionic liquids ([Emim][BF4]) in assisting solvents to recover unconventional oils from their host rocks. The ionic liquids play a carrier role in detaching and transferring the oil fractions from their host rock surfaces and bulk heavy oil phase to the bulk solvent phase. 7079

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Energy & Fuels Table 3. Calculated Solubilities (x) of SARA Fractions from Bitumen in Different Types of ILs [EMIM][BF4] as re aro sat

1.6400 3.3145 3.1192 1.2236

× × × ×

10−6 10−12 10−7 10−7

[BMIM][BF4] 3.4730 3.5840 5.9045 1.7328

× × × ×

10−5 10−10 10−6 10−6

[PMIM][BF4]

■

1.3104 2.7428 2.1471 5.4557

× × × ×

10−4 10−9 10−5 10−6

[HMIM][BF4] 4.3230 1.7864 7.0362 1.5664

× × × ×

10−4 10−8 10−5 10−5

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Grants 41471258, 21306129, and 21506155), the China Postdoctoral Science Foundation (Grant 2016T90207), and The National Key Basic R&D Program (“973” Program, Grant 2015CB251403)

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Figure 8. Phases formed by mixing oil sands with IL and toluene at room temperature and the absorbency (A) of IL phases: (a) [Emim][BF4], A = 0.41 ± 0.15; (b) [Bmim][BF4], A = 1.87 ± 0.33; (c) [Pmim][BF4], A = 4.16 ± 0.28; and (d) [Hmim][BF4], A = 6.47 ± 0.39. The IL phases were diluted 10 times and contrasted by ethyl alcohol using a UV−vis spectrophotometer.

sands processing because lower solubility means less wastage of bitumen and solvent, which is beneficial for recycling and reuse of ILs.

4. CONCLUSIONS A computational simulation method has been successfully applied to screen organic solvents and ILs for the solvent extraction of heavy hydrocarbons from unconventional oils through oil fraction solubility and surface free energy calculations. It was found that the dominant role of the solubility parameter in dissolving heavy oil fractions is the dispersion force parameter (δd). In addition, the surface free energy was found to be a potential evaluator for the selection of IL and to reveal the mechanism. ILs play a role in decreasing the surface free energy of bitumen fraction molecules when in contact with the oil fraction molecules, resulting in the transfer of bitumen fraction molecules from the solid surface and bulk bitumen phase to the IL phase and finally being dissolved into the organic solvent phase. Consequently, bitumen recovery through solvent extraction is enhanced by the ILs. Additionally, increasing the chain length of the IL molecules is found to be detrimental to oil recovery due to the mutual solubility of the solvent and oil components in IL phases. The consistency in the simulation results and experiment data suggests the feasibility of the application of COSMOtherm software to screen solvents and ILs as well as the reveal the mechanism of IL-assisted solvent extraction.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7080

DOI: 10.1021/acs.energyfuels.6b01291 Energy Fuels 2016, 30, 7074−7081

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DOI: 10.1021/acs.energyfuels.6b01291 Energy Fuels 2016, 30, 7074−7081