Article pubs.acs.org/EF
Demulsification of Heavy Crude Oil Emulsions Using Ionic Liquids Elisângela B. Silva,† Denisson Santos,† Douglas R. M. Alves,† Milson S. Barbosa,† Regina C. L. Guimaraẽ s,‡ Bianca M. S. Ferreira,‡ Ricardo A. Guarnieri,‡ Elton Franceschi,† Cláudio Dariva,† Alexandre F. Santos,† and Montserrat Fortuny*,† †
Núcleo de Estudos em Sistemas Coloidais, ITP, PEP, Universidade Tiradentes, Avenida Murilo Dantas 300, Aracaju, 49032-490 Sergipe (SE), Brazil ‡ CENPES/PDP/TPAP, Petrobras, Avenida Horacio de Macedo 950, Cidade Universitária, Rio de Janeiro, 21941-915 Rio de Janeiro (RJ), Brazil ABSTRACT: The use of ionic liquids (ILs) as demulsifiers of water-in-crude oil emulsions represents a new field of study. The main purpose of this work is to investigate the effect of five ILs, [C4mim]+[NTf2]−, [C8mim]+[NTf2]−, [C12mim]+[NTf2]−, [C4py]+[NTf2]−, and [C8mim]+[ [OTf]−, and a set of operation parameters on the demulsification process, including the heating type (conventional and microwave), IL concentration (0.74−8.9 μmol/g), effect of alkyl chain length, and effect of cation and anion type on demulsification efficiency. The results indicated that the demulsification was favored when more hydrophobic ILs and longer cation alkyl chains were employed, such as [C12mim]+[NTf2]−, reaching values close to 92% of water removal. Moreover, the joint use of microwaves and hydrophobic ILs allowed us to maximize the demulsification efficiency.
1. INTRODUCTION About 80% of crude oil produced in the world is recovered in emulsified form. For economic and operational reasons, it is necessary to separate water from oil prior to transportation and refining.1 The presence of water during production together with high shear stress induced by valves and pipes favors the generation of highly stable emulsions.2 This stability is attributed especially to surfactant species existing in the oil, such as resins, asphaltenes, naphthenic acids, and fine solids.3,4 When the aqueous and oily phases are submitted to the intimate contact, these surfactants will accumulate in the oil− water interface. This phenomenon will lead to a strong, rigid, and viscoelastic film that resists droplet coalescence and consequent destabilization of the emulsified system. The use of microwave radiation in the destabilization of water-in-oil (W/O) emulsions is considered a promising alternative to conventional heating in terms of ensuring production processing and productivity gains.5 The dielectric heating generated by microwaves is based on the interaction of radiation with matter. The friction between molecules and the increase of the ionic collision rate generated by alignment and relaxation of dipoles under the electromagnetic field will result in heating.6,7 The microwave irradiation can improve the separation efficiency of W/O emulsion without the use of chemical additives.8,9 Nevertheless, recent works also indicated that the addition of some ionic liquids (ILs) at the crude W/O emulsions can help the demulsification process.10,11 ILs are part of a specific class of molten salts based on organic cations associated with anions of inorganic or organic nature. They have low vapor pressure and thermal stability and possibility to manipulate their properties by the adequate selection of their cations and anions.12 Some ILs reduce the interfacial tension (IFT) of water/oil systems and, consequently, may contribute to the system destabilization. Moreover, the ability of these compounds to absorb microwaves and © 2013 American Chemical Society
to position themselves on the droplet interface may increase the demulsification efficiency. These findings suggest that the combination of microwaves and ILs is a promising technique for the separation of heavy stable emulsions.11 The objective of the current work is to go beyond what was performed previously10,11 and to use a set of novel ILs to understand how these species can be used to produce efficient demulsification of water-in-heavy crude oil emulsions.
2. EXPERIMENTAL SECTION 2.1. Crude Oil Characterization. The crude oil used in this study was a heavy oil commonly found on the Brazilian coast. The experimental techniques applied in the characterization of oil properties and emulsions as well as the characterization and investigation of properties of ILs used are described in the following sections. Table 1 presents the oil features and main characterization techniques used. 2.2. ILs Studied. The ILs 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim]+[NTf2]−, 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [C8mim]+[NTf2]−, 1dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C12mim]+[NTf2]−, 1-butylpyridinium bis(trifluoromethylsulfonyl)imide [C4py]+[NTf2]−, and 1-methyl-3-octylimidazolium triflate bis(trifluoromethylsulfonil) [C8mim]+[OTf]− were chosen for this study as a function of their physicochemical properties that might fit for the purpose of this study. All ILs were purchased from Ionic Liquids Technologies (Iolitec) with a purity of 99%. The ILs were characterized in terms of thermal stability by thermogravimetric analysis (TGA). This analysis has fundamental importance, once the emulsion separation process proposed in this work involves high temperatures. The equipment used in TGA consists basically of a microbalance, an oven, temperature sensors, a gas flow system, and an apparatus for simultaneously recording data (Shimadzu, DTG 60H). Received: December 7, 2012 Revised: September 2, 2013 Published: September 4, 2013 6311
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g/L) and the Brazilian crude oil. The water content of these emulsions was set at 40 wt %. The preparation procedure was carried out in accordance with the following steps: (i) oil heating in a water bath at 70 °C to promote viscosity reduction, (ii) water heating at 70 °C to facilitate its incorporation into the oily phase, (iii) incorporation of the aqueous phase into the heated oil phase to generate a pre-emulsion by hand agitation, and (iv) emulsification using an appropriate homogenizer system (IKA, Ultra Turrax T25 Basic). The homogenization conditions, such as the stirring frequency and the time of homogenization, were kept constant at 6500 rpm and 2.5 min, respectively, yielding emulsions with monodisperse droplet size distributions (DSDs) with mean values D(0.5) in the range of 8−13 μm, as measured by the laser diffraction technique (Malvern instrument model 2000 mastersizer).11 The emulsions obtained with these procedures showed relatively high stability, so that the formation of free water during the demulsification process using microwaves was just possible with the introduction of additives (ILs). The emulsions used in demulsification tests were characterized in terms of water content (WC) before and after each test, using the Karl Fischer reagent method in conformity with ASTM D1744.15 2.5. Microwave Demulsification Tests. Demulsification tests employing microwave heating were carried out using a commercial microwave reactor system (Anton Paar, Synthos 3000). This equipment has two magnetrons generating radiation at a frequency of 2.45 GHz, with a maximum irradiation power of 1400 W. The reactor is equipped with pressure and temperature sensors and enables establishing different heating programs, in which power/temperature profiles can be set. The temperature is measured at the bottom of each tube by an infrared device, while one tube has a temperature gas sensor within the fluid. The device can perform reaction at temperatures up to 300 °C and pressures up to 80 bar. Demulsification tests were conducted at a constant temperature set at 120 °C and process time of 15 min. In all tests, four quartz vessels located at positions 1, 3, 5, and 7 of the reactor were used. All vials were filled with 20 g of emulsion. Prior to the test, known amounts of IL were added to the vials followed by homogenization for 2 min at 6500 rpm. The IL concentration in these tests was varied in the range
Table 1. Main Characterization Parameters for the Heavy Crude Oil Used and the Analytical Standards Followed density (°API) relative density (20/4 °C) viscosity at 120 °C (cSt)a water content (wt %) total acidity (mg of KOH/g of oil) hydrocarbon content (wt %)
ASTM D500213 ASTM D500213
16.8 0.9506
ASTM D704214
15.2
ASTM D174415 ASTM D66416
0.1450 3.35
salinity (NaCl) (wt %)
ASTM D323017
a
chromatographic procedure developed by Petrobras
saturates aromatics resins asphaltenes
44.8 31.3 21.6 2.3 0.24
Curve extrapolation at 120 °C.
Nowadays, the most studied ILs are those based on the imidazolium cation. In our line of research, only ILs with imidazolium or pyridinium in its structure will be studied, although there are many other types (pyrrolidinyl, phosphonium, ammonium, and guanidinium). The ILs studied in this work are summarized in Table 2. 2.3. IFT Analyses. To investigate the surface activity of ILs, the IFT between crude oil samples (without or with known amounts of ILs) and the aqueous phase was obtained with a dynamic drop tensiometer (Tracker H/Teclis/IT Concept). The analysis consisted of measuring the IFT of a crude oil droplet formed in the water phase. For these analyses, ultrapure water (Milli-Q grade) was used. The experimental conditions of the temperature and contact time were set to 40 °C and 60 min, respectively. The molar concentration of ILs used in these tests was 4.18 mM, which represents approximately the average concentration of ILs used for demulsification tests. These conditions provided equilibrium times of 40 min, which correspond to stable IFT values associated with the interface formation. 2.4. Emulsion Preparation and Characterization. Stable waterin-heavy crude oil emulsions were prepared using a brine solution (50
Table 2. ILs Studied
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from 0.74 to 8.9 μmol/g of emulsion. After tests, the samples were cooled by air convection (using the cooling system available in the reactor) to reach a temperature of 60 °C. For those tests in which free water formation was observed, the oil phase and/or unresolved emulsion were completely recovered and characterized in terms of the water content. The separation efficiency (EF) was calculated in accordance with eq 1, considering the water content of the initial emulsion (WC0) and the residual final emulsion (WCf). All tests were duplicated, and the mean of the demulsification efficiency was used as the final result. The experimental error of the demulsification efficiency was 2.4%.
EF =
WC0 − WCf × 100% WC0
3.3. Effects of the Alkyl Chain Length and Cation Type. The effect of the alkyl chain length and type of cation was investigated by microwave demulsification experiments. Figure 1 shows the efficiency obtained during different tests
(1)
2.6. Conventional Demulsification Tests. The tests using conventional heating were performed on a water bath using silicone oil as a heat-transfer fluid and quartz vials typically used in a microwave reactor to keep the geometry of the vials between both heating systems (microwaves and conventional system). To install the quartz vial in the conventional heating system, a rack was constructed to support the flask inside the water bath, ensuring the safety of experiments performed under high temperature (120 °C). The experimental conditions of the temperature and process time were set to 120 °C and 25 min, respectively. This process time is beyond the value adopted in tests using microwaves (15 min), because the conventional heating is slower than the dielectric process. The emulsion samples in quartz vials reached the temperature of 120 °C after 10 min from the beginning of thermal exchange. Thus, the time of 25 min was established in the conventional process comprising 10 min to heating and 15 min during which the temperature would remain constant at the process set point. After the processing step, the samples were cooled for 5 min in an ice/water bath and characterized (water content) following the same procedure mentioned in section 2.5. As in the demulsification procedure assisted by microwaves, the conventional demulsification tests were performed without agitation.
Figure 1. Effect of the alkyl chain length and type of cation in the efficiency of microwave demulsification.
conducted under the same experimental conditions as a function of the IL concentration. It can be observed that the larger alkyl chain, [C8mim]+[NTf2]− and [C12mim]+[NTf2]−, favored higher demulsification efficiencies, reaching values around 74 and 90%, respectively. It can be argued that the larger alkyl chain has a greater tendency to migrate to the interface, displacing the natural surfactants. In some cases, longchain ILs resemble themselves to conventional surfactants and can form aggregates.21 Thus, a greater concentration of these surfactants at the interface reduces the IFT. Even though a direct correlation between IFT reduction and interfacial film destabilization is not obvious, these results could indicate the formation of a new interfacial film, less rigid and with a shorter lifetime. To compare the ability of these ILs to position themselves at the emulsion interface, IFT measurements for crude oil exempt of additives and in the presence of [Cnmim]+[NTf2]− were performed (Table 4). According to these data, the addition of
3. RESULTS AND DISCUSSION 3.1. TGA. Table 3 shows a comparison of the decomposition temperatures of ILs studied in this work and those reported in Table 3. Decomposition Temperatures of ILs Studied IL
Tdecomp (°C) in this work
Tdecomp (°C) in the literature
[C4mim]+[NTf2]−
456
[C8mim]+[NTf2]− [C12mim]+[NTf2]− [C4py]+[NTf2]− [C8mim]+[OTf]−
455 453 425 418
42718 43919 42518
Table 4. IFT Data after 60 min of Contact Time: Crude Oil/ Aqueous Phase
42420
the literature.18−20 As seen, the values are very close to each other. Among the parameters that may explain the difference observed between temperature values is the purity of each IL. It should be emphasized however that the decomposition temperatures of ILs studied here are well above the demulsification temperature adopted on this work (120 °C). 3.2. Emulsion Breaking Test. The ILs [C4mim]+[NTf2]−, [C8mim]+[NTf2]−, [C12mim]+[NTf2]−, [C4py]+[NTf2]−, and [C8mim]+[OTf]− were employed as chemical additives during demulsification tests using microwaves and conventional heating under distinct experimental conditions. In such tests, the concentration of IL was varied in the range from 0.74 to 8.9 μmol/g of emulsion. In addition to this parameter, the effects of alkyl chain length and type of cation and anion on process efficiency were also investigated.
sample
IL
1 2 3 4
[C4mim]+[NTf2]− [C8mim]+[NTf2]− [C12mim]+[NTf2]−
IFT at 40 °C (mN/m) 25.2 24.9 24.1 16.6
± ± ± ±
0.3 0.2 0.3 0.4
[C12mim]+[NTf2]− into the oil phase produces a deep reduction of the IFT, which indicates its higher affinity for the interface when compared to the other ILs. Conversely, the use of shorter chain species (C4 and C8) yielded low IFT reduction. Indeed, the use of the [C4mim]+ species produced a negligible change of IFT when compared to the crude oil exempt of additives, corroborating the idea that short-chain species have little ability to displace the natural surfactants of the crude oil and ultimately promote the system destabilization. Concerning the change in the cation type (comparing [C4mim]+ and [C4py]+), Figure 1 shows that no significant 6313
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change in the separation efficiency was observed. Studies conducted by Kato and Gmehling22 observed the effect of imidazolium and pyrrolidinium cations (this compound with properties similar to pyridinium, object of the present study) on IL solubility in alkanes. The modification of these functional groups had no significant effect on solubility. This observation might be the reason that the separation efficiency of the emulsified system was unchanged. Finally, these issues have encouraged the study of ILs with different alkyl chains. 3.4. Effect of the Anion Type. When analyzing the effect of the anion type on demulsification, the cation [C8mim]+ was selected and two types of anions [OTf]− and [NTf2]− were investigated. As seen in Figure 2, there was a greater separation
Figure 3. Effect of the heating type on the demulsification by conventional heating and microwave radiation using the IL [C8mim]+[NTf2]−.
Figure 2. Effect of the anion type on the efficiency of microwave demulsification.
efficiency of emulsions using anion [NTf2]− when compared to [OTf]−, especially for concentrations beyond 7 μmol/g. The higher hydrophobicity of [NTf2]− has been reported in the literature.23−25 These findings support the observations made before,11 for which the use of an hydrophilic anion, such as Cl (as in the case of [C8mim]+[Cl]−) could not yield adequate separation results. In contrast, the use of [C8mim]+[PF6]−, which is more hydrophobic, provided high separation efficiency of water-in-medium crude oil emulsions. According to these results, the key point concerning the use of ILs in the demulsification of crude oils is their hydrophobic nature. 3.5. Effect of the Heating Type. Demulsification tests using microwave heating (MW) and conventional heating (CN) were compared to estimate the effect of the heating type on the demulsification process. The ILs that have shown higher separation efficiency in tests by MW were used in tests by CN. Figures 3 and 4 present a comparison of separation efficiency between the distinct processes. Note that, only at high concentrations, the ILs promote the demulsification through conventional heating. From the results shown in the figures, it can be noted that, for all tests and both ILs, the use of microwaves in the demulsification process enhanced the separation efficiencies. These results clearly indicate that the combined use of microwave radiation and ILs can help the demulsification of crude in oil emulsions. From the results, it could be stressed that, besides the demulsifier/surfactant character of ILs, these compounds could be acting as a guide to direct the microwave radiation to the W/O interface,
Figure 4. Effect of the heating type on the demulsification by conventional heating and microwave radiation using the IL [C12mim]+[NTf2]−.
decreasing the interfacial viscosity by enhancing the local temperature and helping the rupture of the interfacial film. To compare the demulsification performance of [C 12 mim] + [NTf 2 ] − (the best IL) with a commercial demulsifier, tests conducted using ILs were reproduced using a copolymer of poly(ethylene oxide-b-propylene oxide) (PEO− PPO), described elsewhere.26 Table 5 summarizes the demulsification efficiencies obtained in tests conducted under the same experimental conditions and different types of heating. According to data presented in Table 5, lower concentrations of PEO−PPO are required to achieve efficiencies of 90%, when compared to pure Table 5. Separation Efficiency for Tests under Conventional (CN) and Microwave (MW) Heating Using [C12mim]+[NTf2]− and a Commercial Demulsifier26 additive +
−
[C12mim] [NTf2] [C12mim]+[NTf2]−/EtOH PEO−PPO PEO−PPO 6314
μmol/g
EF (%) MW
EF (%) CN
6.02 0.50 0.21 0.43
91.4 79.0 90.0 92.0
6.2 43.0 82.3 90.1
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[C12mim]+[NTf2]−. However, when this IL is mixed with ethanol (50%, v/v) before introduction into the emulsion, improved demulsification results are obtained, in such a way that the required concentration of IL can be reduced appreciably. As in the case of the commercial demulsifier, the use of solvent provides better mixing of the additive into the heavy crude oil, making the diffusion of the IL into the interface easier, yielding improved separation results. Finally, the use of microwaves yielded improved demulsification efficiencies in all tests and for both additives, when compared to the conventional heating tests. For tests carried out with the commercial demulsifier, the gains of efficiency when changing from CN to MW heating are not so prominent as in the case of ILs. Conversely, the use of microwaves allowed for very significant increases of efficiency when combined with pure or a mixture of IL and solvent.
(6) Kappe, C. O.; Dallinger, D. The impact of microwave synthesis on drug discovery. Nat. Rev. 2006, 5, 51−63. (7) Lidström, P.; Tierney, J.; Wathey, B.; Westman, J. Microwave assisted organic synthesisA review. Tetrahedron 2001, 57, 9225− 9283. (8) Fang, C. S.; Lai, P. M. C. Microwave heating and separation of water-in-oil emulsions. J. Microwave Power Electromagn. Energy 1995, 30, 46−57. (9) Fortuny, M.; Oliveira, C. B. Z.; Melo, R. L. F. V.; Nele, M.; Coutinho, R. C. C.; Santos, A. F. Effect of salinity, temperature, water content, and pH on the microwave demulsification of crude oil emulsions. Energy Fuels 2007, 21, 1358−1364. (10) Guzmán-Lucero, D.; Flores, P.; Rojo, T.; Martínez-Palou, R. Ionic liquids as demulsifiers of water-in-crude oil emulsions: Study of the microwave effect. Energy Fuels 2010, 24, 3610−3615. (11) Lemos, R. C. B.; da Silva, E. B.; Santos, A.; Guimarães, R. C. L.; Ferreira, B. M. S.; Guarnieri, R. A.; Dariva, C.; Franceschi, E.; Santos, A. F.; Fortuny, M. Demulsification of water-in-crude oil emulsions using ionic liquids and microwave irradiation. Energy Fuels 2010, 24, 439−4444. (12) Van Rantwijk, F.; Sheldon, R. A. Biocatalysis in ionic liquids. Chem. Rev. 2007, 107, 2757−2785. (13) ASTM International. ASTM D-5002. Standard Test Method for Density and Relative Density of Crude Oils by Digital Density Analyzer; ASTM International: West Conshohocken, PA, 2010. (14) ASTM International. ASTM D-7042. Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer (and the Calculation of Kinematic Viscosity); ASTM International: West Conshohocken, PA, 2004. (15) ASTM International. ASTM D-1744. Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent; ASTM International: West Conshohocken, PA, 1992. (16) ASTM International. ASTM D-664. Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration; ASTM International: West Conshohocken, PA, 2004. (17) ASTM International. ASTM D-3230. Standard Test Method for Salts in Crude Oil (Electrometric Method); ASTM International: West Conshohocken, PA, 2000. (18) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical properties and structures of room temperature ionic liquids. 2. Variation of alkyl chain length in imidazolium cation. J. Phys. Chem. B 2005, 109, 6103−6110. (19) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation. Green Chem. 2001, 3, 156−164. (20) Crosthwaite, J. M.; Muldoon, M. J.; Dixon, J. K.; Anderson, J. L.; Brennecke, J. F. Phase transition and decomposition temperatures, heat capacities and viscosities of pyridinium ionic liquids. J. Chem. Thermodyn. 2005, 37, 559−568. (21) Geng, F.; Liu, J.; Zheng, L.; Yu, L.; Li, Z.; Li, G.; Tung, C. Micelle formation of long-chain imidazolium ionic liquids in aqueous solution measured by isothermal titration microcalorimetry. J. Chem. Eng. Data 2010, 55, 147−151. (22) Kato, R.; Gmehling, J. Systems with ionic liquids: Measurements of VLE and data and prediction of their thermodynamic behavior using original UNIFAC, mod. J. Chem. Thermodyn. 2005, 37, 603−619. (23) Hallett, J. P.; Welton, T. Room-temperature ionic liquids: Solvents for synthesis and catalysis. 2. Chem. Rev. 2011, 111, 3508− 3576. (24) Mallakpour, S.; Rafiee, Z. New developments in polymer science and technology using combination of ionic liquids and microwave irradiation. Prog. Polym. Sci. 2011, 36, 1754−1765. (25) Endres, F.; El Abedin, S. Z. Air and water stable ionic liquids in physical chemistry. Phys. Chem. Chem. Phys. 2006, 8, 2101−2116. (26) Ferreira, B. M. S.; Ramalho, J. B. S.; Lucas, E. F. Demulsification of water-in-crude oil emulsions by microwave radiation: Effect of aging, demulsifier addition, and selective heating. Energy Fuels 2013, 27, 615−621.
4. CONCLUSION The use of ILs as demulsifiers of heavy crude oil emulsions is an original area of study. Results obtained on the current work show that ILs based on imidazolium cations and the NTf2 anion can be successfully used during the demulsification of water-in-heavy crude oil emulsions. Their effects are greatly accelerated by action of microwaves, which can be explained by a strong interaction of radiation with these ionic species. For some tests, high demulsification efficiencies (beyond 90%) were obtained in a short time. Concerning the structure of ILs, the results indicated that the demulsification is favored when hydrophobic ILs containing cations with long alkyl chains are involved, such as [C12mim]+[NTf2]−. When the demulsification performance of these pure ILs is compared to a commercial demulsifier, higher IL concentrations are still necessary. However, the selection of a proper solvent can improve the mixing of the IL into the heavy crude oil, yielding better demulsification results at lower concentrations.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank CNPq, CAPES, and FAPITEC/SE (Brazilian funding agencies) and Petrobras (Petróleo Brasileiro S.A., Brazil) for supporting this work.
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REFERENCES
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