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Study on the use of aprotic ionic liquids as potential additives for crude oil upgrading, emulsion inhibition, and demulsification
Fluid Phase Equilibria 489 (2019) 8e15
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Study on the use of aprotic ionic liquids as potential additives for crude oil upgrading, emulsion inhibition, and demulsification Robson L.M. Santos a, Elvio B.M. Filho a, Raul Silva Dourado a, Alexandre Ferreira Santos b, udio Dariva a, Ce sar C. Santana a, Elton Franceschi a, Gustavo R. Borges a, Cla a, * Denisson Santos a Núcleo de Estudos em Sistemas Coloidais (NUESC)/Instituto de Tecnologia e Pesquisa (ITP), PEP/PBI/Universidade Tiradentes (UNIT), Av. Murilo Dantas, ^ndia, Aracaju, Sergipe, 49032-490, Brazil 300, Farola b , Brazil Department of Chemical Engineering - Federal University of Parana (UFPR), Polytechnic Center, 82530-990, Curitiba, Parana
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2018 Received in revised form 23 December 2018 Accepted 1 February 2019 Available online 8 February 2019
Aprotic ionic liquids ([C4pyr]þ[NTf2]-, [C8mim]þ[NTf2]-, [C8mim]þ[OTf]-, [C8mim]þ[PF6]-, [C8mim]þ [BF4]-, [C12mim]þ[NTf2]-, and [C12mim]þ[Cl]-) were successfully evaluated as viscosity modifiers, emulsion inhibitors, and demulsifiers. They are prone to be applied for the heavy oil production since such crude oil type potentializes some issues that are typically observed in the petroleum industry. Among them, it is worth highlighting the low flowability due to the oil's high viscosity. Also, high-viscous crude oil usually generates stable emulsions since they have high resins and asphaltene contents, which are interfacial-active. Thus, for operational and economic reasons there is a need for technologies that facilitate the improvement of the heavy-oil flowability as well as the demulsification process. Then, this work aimed to analyze the influence of the ionic liquid chemical structure (cationic alkyl length, cationic functional group, and anion type) upon the oil viscosity, emulsion inhibition or demulsification, at concentrations of 5 and 100 ppm (m/m). Two Brazilian heavy-oils were used for this study. Viscosity measurements of these oils at room temperature (25 C) were made by an oscillatory rheometer before and after the addition of ionic liquid. Also, the emulsification inhibition and demulsification efficiency were analyzed by centrifugation. The results showed that the 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide promoted a demulsification efficiency higher than 99%. In turn, the 1-butylpyridinium bis(trifluoromethylsulfonyl)-imide was the one that worked as an emulsion inhibitor. On the other hand, the interaction between the studied oils and some of the used ionic liquids enhanced the oil viscosity. All the evidence points out to the fact that it was caused by the asphaltene aggregation due to the formation of an ionic liquid-asphaltene molecular structure. The observed viscosity increase was of 11% for oil A and 7% for the oil B. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ionic liquid Heavy oil Viscosity Emulsion Chemical additive
1. Introduction The heavy oil production plays a vital role in the maintenance of the world recoverable petroleum reserves. Also, the increase in oil consumption and the depletion of light oil reserves has directed technical attention to the operational and economic issues related to the recovery of heavy oil fields [1e3]. Due to its chemical characteristics, such as high asphaltene and resin contents, there are still technical challenges to be overcome in order to enhance the
* Corresponding author. E-mail address: [email protected] (D. Santos). https://doi.org/10.1016/j.fluid.2019.02.001 0378-3812/© 2019 Elsevier B.V. All rights reserved.
exploitation, transportation, and refining of those oils. Among them, it is worth highlighting the propensity for formation of highly-stable emulsions and the flowability limitations due to high viscosity. In the first place, the heavy oil emulsification results in corrosion and incrustation in production and refining facilities but, above all, it increases the native crude oil viscosity [4]. Secondly, the viscosity range of heavy oils typically varies from 0.1 to 1000 Pa.s. However, the usually recommended fluids’ viscosity for the transportation pipelines is around 0.4 Pa. s [5]. Consequently, the heavy oil viscosity reduction is mandatory as a way to avoid pressure drop and flow resistance during production and transportation. The petroleum industry has applied several strategies to promote viscosity reduction: oil blends, heating, inverse emulsion, diluents,
R.L.M. Santos et al. / Fluid Phase Equilibria 489 (2019) 8e15
and chemical additives. Among the additives, the ionic liquids stand as the most recent promising one [3,6e9]. Ionic liquids (ILs) are known as organic salts that generally melt at temperatures lower than 100 C. They have some properties that are operationally desirable such as low density and viscosity, high thermal stability, and low pressure-vapour [10]. Beyond that, any of these properties can be tuned by modifying the cationic or anionic part of the ionic liquid [11]. For instance, the miscibility and the thermal stability are mainly related to the anion's characteristics. Further, density and viscosity are changed by the cation's chemical properties. Also, by combining specific anions and cations, one can fit activity coefficient of the ionic liquid in specific hydrocarbons to make it able or not the be used in the petroleum industry [12e15]. For instance, the ability of imidazolium-based ILs to reduce the surface tension between water and oil as well as its chemical stability in high salinity environment has positioned them as potential additives for enhanced oil recovery [16]. In some applications, the ionic liquid effect doubled the recovery factor in comparison with conventional techniques [17]. Further, the amphiphilic characteristics allow the ionic liquids to be successfully applied as demulsifiers for highly-stable water-in-heavy crude oil emulsions as well as petroleum spill dispersant [18,19]. Also, deacidification of crude oils has been promoted by both reacting the acidic oil with IL or adsorbing the naphthenic acids in supported ionic liquid phases. A regeneration rate of 70% was registered by Anderson et al. (2015) [20e22]. In addition, the strong interaction between asphaltenes and some ionic liquids lead to the formation of aggregates with distinct geometries and complexation energies [23]. Thus, they can be used to control the size of asphaltene aggregates and, consequently, to avoid its deposition. Once ionic liquids act at the molecular level, while the common asphaltene dispersants work at the surface of big aggregates, the ILs has shown better efficiency. In this aspect, they have also been successfully used for the upgrade of heavy oils by reducing their viscosity and modifying their SARA composition [9,24]. In this scenario, this work aims to evaluate the application of aprotic ionic liquids ([C4pyr]þ[NTf2]-, [C8mim]þ[NTf2]-, [C8mim]þ [OTf]-, [C8mim]þ[PF6]-, [C8mim]þ[BF4]-, [C12mim]þ[NTf2]-, and [C12mim]þ[Cl]-) as an additive for viscosity reduction of two heavy crude oils as well as their use for inhibition and remediation of water-in-oil emulsion. Also, it was analyzed the effect of cation alkyl length, the anion charge density, and the cation functional group on their application efficiency.
2. Experimental 2.1. Materials Two Brazilian heavy-crude oils were selected for this study. They had initial water content up to 1% and scarce droplets with size varying from 27 to 80 mm. The density and asphaltene content of the so-called Oil A and Oil B are presented in Table 1. They were determined through the standard methods ASTM D5002-99 [25] and ASTM D6560 [26]. The TAN e total acid number were provided by ANP (2018) [27]. The used toluene (99.8%) was supplied by Sigma-Aldrich®. Distilled water was used in the synthesis of waterin-oil emulsions.
Table 1 Crude oil properties. Crude oil
ºAPI
Asphaltene content (% m/m)
TAN (mgKOH/g)
A B
13.2 12.7
7.3 2.7
1.2 1.3
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The ionic liquids that were used are named as: 1-methyl-3octylimidazolium bis(trifluoromethylsulfonyl)imide [C8mim]þ[NTf2]-; 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide e [C12mim]þ[NTf2]-; 1butylpyridinium bis(trifluoromethylsulfonyl)imide e [C4pyr]þ[NTf2]-; 1-methyl-3-octylimidazolium triflate - [C8mim]þ [OTf]-; 1-methyl-3-octylimidazolium hexafluorophosphate [C8mim]þ[PF6]-; 1-methyl-3-octylimidazolium tetrafluoroborate [C8mim]þ[BF4]-; and, 1-dodecyl-3-methylimidazolium chloride e [C12mim]þ[Cl]-. They were all purchased from Iolitec GmbH with a purity higher than 99%. Their chemical structures are shown in Table 2. 2.2. Experimental procedure 2.2.1. Viscosity measurements The crude oils were pre-homogenized before any analysis. Then, they were doped with each of the selected ionic liquids in concentrations of 5 and 100 ppm (m/m). Such concentrations were defined in a way to simulate the additives concentration typically used in the crude oil chemical treatment [28]. Once the high viscosity of the oils would difficult the LI diffusion, toluene was used as a carrier fluid. Thus, instead of being added directly into the crude oil, the ionic liquids were diluted in the toluene. Firstly, solutions of crude oil þ toluene were prepared with concentrations varying from 1 to 10% (m/m) and analyzed. After that, solutions of ionic liquids in toluene were prepared and added into the oil at the aforementioned concentrations, then analyzed. The solutions were homogenized by ultrasonication at room temperature for 20 min and 40 C for more 40 min. A ultrasound bath (Ultronique) operating at frequency of 40 kHz and power of 200 W was used. After that, the samples were cooled down to 25 C for the viscosity measurements. These measurements were carried out by using an oscillatory rheometer (Physica MCR 301 e Anton Paar) equipped with coaxial cylinders. A shear rate scanning from 0.1 to 100 s1 was performed in triplicate for each experimental condition. 2.2.2. Demulsification efficiency Water-in-oil emulsions with a water content of 30% were synthesized with distilled water as aqueous phase and two oily phases: crude oil þ toluene and crude oil þ toluene þ ionic liquid. The mixture was hand-shaken in order to pre-emulsify the separated phases. Then, the droplet size was reduced through shear application by using a dispersion tool (IKA, Ultra Turrax T25 Basic). The homogenization conditions (stirring frequency of 6,500 rpm and stirring time of 2,5 min) were suitable to generate a bimodal droplet size distribution (DSD) with mean values varying from 710 mm to 8e12 mm. After synthesis, the emulsions were then characterized regarding DSD and water content. The procedures of such analyses can be seen elsewhere [29]. The demulsification itself was promoted by a centrifuge at 2000 rpm and room temperature (25 C). The total centrifugation time was 30 min and it was broken into steps of 5 min each to verify the free-water volume over time. After the demulsification, the free water-oil phase was recovered, and its viscosity was analyzed through the methodology aforementioned. 3. Results and discussions The chemical treatment of petroleum usually applies light fractions as an additive to reduce oil viscosity and/or favours other chemicals diffusion in the so-treated crude oil [30]. In this work, toluene was used as a carrier fluid for the studied additives. It was chosen due to its potential to solubilize both the used ionic liquids and the asphaltenes present in the heavy crude oil. Fig. 1 shows the
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R.L.M. Santos et al. / Fluid Phase Equilibria 489 (2019) 8e15 Table 2 Chemical structures of the studied ionic liquids. Nomenclature
Chemical structure
1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide
1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1-butylpyridinium bis(trifluoromethylsulfonyl)imide
1-methyl-3-octylimidazolium triflate
1-methyl-3-octylimidazolium hexafluorophosphate
1-methyl-3-octylimidazolium tetrafluoroborate
1-dodecyl-3-methylimidazolium chloride
Fig. 1. Cumulative viscosity reduction of heavy-oil in function of toluene concentration at room temperature (25 C). Oil A (yellow columns); Oil B (blue columns).
viscosity reduction of the crude oils A and B caused by toluene addition at concentrations varying from 1 to 10% (m/m). One can note that toluene promoted an exponential viscosity reduction mainly due to dilution of organic particles suspended in the
petroleum. Such reduction was of greater extent for the Oil A, which has a higher asphaltene content. Also, it is worth highlighting that there is a difference in the cumulative reduction as the toluene concentration was increased in each oil. The effect of
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toluene addition at the lowest concentration (1%) for Oil A was almost the double of the effect on Oil B. However, the toluene concentration of 10% produced a similar reduction of >90% for both the oils. Fig. 2 shows the influence of the ionic liquids upon the crude oils viscosity. As already mentioned, the chosen carrier fluid (toluene) promotes a viscosity reduction. However, when the additives were solubilized in the oils, a viscosity increase was noticed for all the ionic liquids. Such increases were detected at different extents for both the studied oils. Although a viscosity reduction was aimed, the opposite was verified. At the first glance, it seems to be a disadvantage. However, it confirms the strong interaction between the ILs and the crude oil components. For the Oil A, one can note that C4pyr-NtF2 and C8mim-BF4 promoted a viscosity increase higher than 8%. On the other hand, C8min-NtF2 and C12mim-Cl were the
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ILs that promoted the most significative change in the viscosity for Oil B. As can be seen, each of these ionic liquids has a peculiarity regarding its cation type, anion type, and alkyl chain length. Once the molecular structural differences result on modifications of chemical properties, it will reflect on the interaction crude oil-ionic liquid as analyzed hereinafter. 3.1. Effect of anion type In order to evaluate the effect of the ionic liquids’ anion type on the viscosity modification, the same imidazolium cation with a branched alkyl length of 8 carbon atoms was selected. Then, the anions NtF2, OTF, BF4 and PF6 were investigated. Fig. 3 shows that for Oil B, the anion NtF2 showed a stronger interaction with the organic compounds that typically rules the crude oil viscosity, such
Fig. 2. Viscosity enhancement for Oil A (yellow columns) and Oil B (blue columns) after addition of ionic liquids (5 ppm) into the heavy-oil doped with toluene (1% m/m).
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Fig. 3. Viscosity enhancement for Oil A (yellow columns) and Oil B (blue columns) doped with toluene (1% m/m). After addition of ionic liquid (5 ppm) with the same cation and different anions.
as resins and asphaltenes [31]. On the other hand, such interaction strength followed the order BF4 > OTF > PF6 > NtF2 for Oil A, which is the one that has the higher asphaltene content. It is worth highlighting that ionic liquids do not fit into the standard behaviour of the additives that are typically used for petroleum upgrade. The theoretical and empirical basis for predicting their behaviour when mixed with such a complex fluid as crude oil is still being developed. In this scenario, the proper ionic liquid to be used as an additive must be the one that strongly interacts with asphaltenes or the resin-asphaltenes complex, thus isolating it and avoiding the aggregation process as well as its correlated problems. Also, such optimal IL must have as less affinity as possible with its own molecules [32,33]. In this scenario, the observed effect of the anion types upon the viscosity of both the studied oils can be explained by the interaction distinctions between IL, asphaltenes and the whole crude oil. Among the studied anions, NtF2 has the greater hydrogen bond acceptor potential due to its molecular structure. As reported by Subramanian et al. (2015) [28], the asphaltene aggregation phenomena is ruled by several types of molecular interaction, such as p-p interaction between the aromatic cores, aliphatic interactions, acid-base, charge-transfer, and hydrogen-bonding. Thus, anions with higher potential to promote hydrogen bond with asphaltene molecules may favor the asphaltene-IL interaction. Also, it will result in a stronger IL-IL hydrogen bonding [14,34]. If the crude oil whole composition does not overcome on such equilibrium forces (asphaltene-IL and IL-IL), it will be verified a greater asphaltene aggregation and a consequent viscosity increase. At the same time, if the crude oil whole composition does surpass the asphaltene-IL interaction forces, the activity coefficient of the ionic liquids will be a relevant information. The so far published data report the activity coefficient of ILs in pure solvents. However, it can be inferred for crude oil by extrapolating the data from representative simple hydrocarbon molecules, as it is usually done. Thus, the lack of solubility of the anions in typical crude oil model fractions (alkanes, cyclo-alkanes, and aromatics), for instance, will be reflected
in a higher aggregation of the complex asphaltene-ionic liquid. It is confirmed by the relationship between the anion's activity coefficients (BF4 > OTF > NtF2) [35] and the viscosity enhancement for Oil A (BF4 > OTF > PF6 > NtF2), the most asphaltenic one. 3.2. Effect of the alkyl chain Fig. 4 shows the viscosity enhancement for the studied oils when both anion and cation were the same and only the alkyl chain length was changed. Again, different effects were noticed. For Oil B, the viscosity enhancement was lower as the alkane chain was lengthened. The longer alkyl length reduces the self-aggregation of the complex asphaltene-ionic liquid, once such complex is more prone to interact with the whole crude oil. As a consequence, the anion's effect is reduced. On the other hand, the opposite phenomenon was observed for the Oil A. In this case, the longer alkyl length increased the viscosity enhancement. It is believed that for this asphaltenic and significantly more viscous oil, the ionic liquid diffusion, which is lower as longer the alkyl chain [34], rules the viscosity enhancement. 3.3. Emulsification inhibition and demulsification efficiency The surface-active ionic liquids have been studied as demulsifiers for water-in-crude oil emulsions due to their amphiphilic property [36]. Further, such characteristics highlight the IL's potential to be applied as an emulsion inhibitor. Thus, the ionic liquids that do not lose the chemical stability when in contact with water and, at the same time, showed greater viscosity enhancement caused by the asphaltene-ionic liquid interaction were selected. The less viscous oil (Oil B) was selected for the demulsification studies in order to avoid that petroleum properties would mask the ionic liquid action. Also, the IL concentration was increased in order to emulate demulsifier concentrations typically used in the petroleum industry [37]. Fig. 5 shows the results of demulsification efficiency by the centrifugation method with and without the crude
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Fig. 4. Viscosity enhancement for Oil A (yellow columns) and Oil B (blue columns) doped with toluene (1% m/m). After addition of ionic liquid (5 ppm) with the same anion and different cationic alkyl chain.
oil dopage by IL. As one can note, the C4pyr-NtF2 showed a good efficiency as an emulsion inhibitor since it was not possible to produce a stable water-in-crude oil emulsion when Oil B was doped with 100 ppm of such ionic liquid. Once the ionic liquid-free emulsion was stable even after 30 min, the demulsifier action of the studied ionic liquids could be analyzed. Thus, Fig. 5 shows that the C8mim-NtF2 at concentration of 100 ppm promoted the same demulsification efficiency as C4pyr-NtF2, but a higher centrifugation time was demanded as concentration increased. It denotes that the optimum
dosage was overreached and instead of working as a demulsifier, the C8mim-NtF2 acted as an emulsifier compound even stronger than the petroleum natural interfacial-active ones. On the other hand, the C4pyr-NtF2 concentration of 50 ppm was not enough to break the emulsion efficiently. Hazrati et al. (2018) as well as Silva et al. (2013) also used ionic liquids to break water-in-crude oil emulsions successfully [38,39]. Their results highlight the influence of the IL's molecular structures on the demulsification efficiency. However, they cannot be compared with this study since heating techniques (conventional and dielectric heating) were needed to
Fig. 5. Demulsification efficiency in function of centrifugation time for the Oil B doped with toluene (10%) with and without the addition of ionic liquids at concentrations of 50 ppm and 100 ppm.
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aid the demulsification promoted by the used ionic liquids. A relevant phenomenon was noticed in crude oil after the demulsification process. The recuperated oil (water content < 5%) showed to have a viscosity increase, as can be seen in Fig. 6. Indeed, once the oil viscosity alteration promoted by addition of ionic liquids is caused by the aggregates formation between asphaltenes and ILs, it must be enhanced by the presence of water. The probability of both asphaltene and ionic liquid molecules to interact is higher at the oil-water interface due to their amphiphilic nature. Thus, when the water-in-oil emulsion is formed, those molecules will compose the interfacial film. Hence, if any of the studied ionic liquids have been used as an emulsion inhibitor and/or demulsifier at the desalter process, for instance, the once demulsified petroleum will probably have a higher viscosity after the demulsification step.
4. Conclusions Aprotic ionic liquids were successfully used as emulsion inhibitors and demulsifiers for two Brazilian heavy-oils. The use of toluene as a carrier fluid aided in the viscosity reduction but above all, it promoted the ionic liquid diffusivity in the crude oil. Also, all the ionic liquids caused a viscosity increase when solubilized in the oils, but in different extent for each oil due to both IL and oil chemical characteristics. Among the studied ILs, the 1-butylpyridinium bis(trifluoromethylsulfonyl)imide e [C4pyr]þ[NTf2]- was the one capable of inhibiting the emulsion formation. The 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide - [C8mim]þ[NTf2]- promoted the highest demulsification efficiency (>99%). However, the demulsified oil showed to have a higher viscosity than it had before the emulsification. Although the studied ionic liquids did not
Fig. 6. Viscosity enhancement for Oil B doped with toluene and tolueneþionic liquid before emulsification (yellow columns) and after demulsification process (blue-dashed columns).
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show effectiveness to be used as an efficient additive for crude oil upgrade by reducing viscosity, the strong interaction between asphaltenes and ionic liquids put them as a potential asphaltene precipitation inhibitor as well as emulsion inhibitor. Thus, it will also improve the oil flowability as long as the ionic liquid is capable of avoiding asphaltene deposition and/or emulsion formation. Acknowledgements ~o de Apoio a Pesquisa e a The authors thank Fapitec-SE (Fundaça ~o Tecnolo gica do Estado de Sergipe), ANP, CNPq, and CAPES Inovaça - Finance Code 001 - for financial support.
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amphiphilic ionic liquid based on ethoxylated octadecylammonium tosylate as demulsifier and petroleum crude oil spill dispersant, J. Ind. Eng. Chem. 33 (2016) 122e130, https://doi.org/10.1016/j.jiec.2015.09.028. K. Anderson, M.P. Atkins, P. Goodrich, C. Hardacre, A.S. Hussain, R. Pilus, D.W. Rooney, Naphthenic acid extraction and speciation from Doba crude oil using carbonate-based ionic liquids, Fuel 146 (2015) 60e68, https://doi.org/ 10.1016/j.fuel.2015.01.015. S.N. Shah, M. Pranesh, J.J. Raj, M.I.A. Mutalib, K.C. Lethesh, O. Ben Ghanem, Z. Ullah, De-acidification of crude oil using supported ionic liquids phases, Separ. Purif. Technol. 196 (2018) 96e105, https://doi.org/10.1016/ j.seppur.2017.08.056. C. Wu, A. De Visscher, I.D. Gates, Interactions of biodegradable ionic liquids with a model naphthenic acid, Sci. Rep. 8 (2018) 176, https://doi.org/10.1038/ s41598-017-18587-1. M. Boukherissa, F. Mutelet, A. Modarressi, A. Dicko, D. Dafri, M. Rogalski, Ionic liquids as dispersants of petroleum asphaltenes, Energy Fuels 23 (2009) 2557e2564, https://doi.org/10.1021/ef800629k. H. Fan, Z. Li, T. Liang, Experimental study on using ionic liquids to upgrade heavy oil, J. Fuel Chem. Technol. 35 (2007) 32e35, https://doi.org/10.1016/ S1872-5813(07)60009-7. Astm, Standard Test Method for Density and Relative Density of Crude Oils by Digital Density Analyser, 2010, p. 5, https://doi.org/10.1520/D5002-99R10.2. American Society For Testing And Materials, Standard test method for determination of asphaltenes (heptane insolubles) in crude and petroleum Products ASTM International, Pet. Prod. Lubr. Foss. Fuels (2013) 1e6, https:// doi.org/10.1520/D6560-12.2. ^ncia, Nacional de Petro leo Ga s Natural e Biocombustíveis, Relato rio de A. Age leos brasileiros, 2018. http://www.anp.gov.br/images/ valoraç~ ao de petro Royalties-e-outras-participacoes/Preco_Referencia_Petroleo/Memoria_de_ calculo/2018/Memoria_de_Calculo_03.pdf. D. Subramanian, K. Wu, A. Firoozabadi, Ionic liquids as viscosity modifiers for heavy and extra-heavy crude oils, Fuel 143 (2015) 519e526, https://doi.org/ 10.1016/j.fuel.2014.11.051. ^ngela, D. Santos, M. Paes, D. Brito, R.C.L. Guimar~ B. Elisa aes, B.M.S. Ferreira, L.S. Freitas, M.C.V. De Campos, E. Franceschi, C. Dariva, A.F. Santos, Microwave demulsification of heavy crude oil emulsions : analysis of acid species recovered in the aqueous phase, Fuel 128 (2014) 141e147. A.H.S. Dehaghani, M.H. Badizad, Experimental study of Iranian heavy crude oil viscosity reduction by diluting with heptane, methanol, toluene, gas condensate and naphtha, Petroleum 2 (2016) 415e424, https://doi.org/ 10.1016/j.petlm.2016.08.012. €blom, Asphaltene fractionation based S. Subramanian, S. Simon, B. Gao, J. Sjo on adsorption onto calcium carbonate: Part 1. Characterization of subfractions and QCM-D measurements, Colloids Surface. A Physicochem. Eng. Asp. 495 (2016) 136e148, https://doi.org/10.1016/j.colsurfa.2016.02.011. z, J. Aburto, Current knowledge and potential applicaJ.-A. Murillo-Hernande tions of ionic liquids in the petroleum industry, in: A. Kokorin (Ed.), Ion. Liq. Appl. Perspect., InTech, Rijeka, Croatia, 2011, pp. 439e458 (Org.), http://www. intechopen.com/books/ionic-liquidsapplications-and-perspectives/currentknowledge-and-potential-applications-of-ionic-liquids-in-thepetroleumindustry. F. Van Rantwijk, R. a. Sheldon, Biocatalysis in Ionic Liquids 107 (2007) 2757e2785, https://doi.org/10.1021/cr050946x. F. van Rantwijk, R.A. Sheldon, Biocatalysis in ionic liquids, Chem. Rev. 107 (2007) 2757e2785, https://doi.org/10.1021/cr050946x. Z.-C. Tan, U. Welz-Biermann, P.-F. Yan, Q.-S. Liu, F. Da-Wei, Thermodynamic properties of ionic liquids - measurements and predictions, in: A. Kokorin (Ed.), Ion. Liq. Theory, Prop. New Approaches, InTech, Rijeka, Croatia, 2011, pp. 3e16 (Org.). E.B. Silva, D. Santos, D.R.M. Alves, M.S. Barbosa, R.C.L. Guimar~ aes, B.M.S. Ferreira, R.A. Guarnieri, E. Franceschi, C. Dariva, A.F. Santos, M. Fortuny, Demulsification of heavy crude oil emulsions using ionic liquids, Energy Fuels 27 (2013) 6311e6315, https://doi.org/10.1021/ef302008d. A.M. Atta, H.A. Allohedan, G.A. El-Mahdy, Dewatering of petroleum crude oil emulsions using modified Schiff base polymeric surfactants, J. Petrol. Sci. Eng. 122 (2014) 719e728, https://doi.org/10.1016/j.petrol.2014.09.017. ~es, B.M.S. Ferreira, R.C.B. Lemos, E.B. Da Silva, A. Dos Santos, R.C.L. Guimara R.A. Guarnieri, C. Dariva, E. Franceschi, A.F. Santos, M. Fortuny, Demulsification of water-in-crude oil emulsions using ionic liquids and microwave irradiation, Energy Fuels 24 (2010) 4439e4444, https://doi.org/10.1021/ ef100425v. N. Hazrati, A.A. Miran Beigi, M. Abdouss, Demulsification of water in crude oil emulsion using long chain imidazolium ionic liquids and optimization of parameters, Fuel 229 (2018) 126e134, https://doi.org/10.1016/ j.fuel.2018.05.010.
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Study on the use of aprotic ionic liquids as potential additives for crude oil upgrading, emulsion inhibition, and demulsification Robson L.M. Santos a, Elvio B.M. Filho a, Raul Silva Dourado a, Alexandre Ferreira Santos b, udio Dariva a, Ce sar C. Santana a, Elton Franceschi a, Gustavo R. Borges a, Cla a, * Denisson Santos a Núcleo de Estudos em Sistemas Coloidais (NUESC)/Instituto de Tecnologia e Pesquisa (ITP), PEP/PBI/Universidade Tiradentes (UNIT), Av. Murilo Dantas, ^ndia, Aracaju, Sergipe, 49032-490, Brazil 300, Farola b , Brazil Department of Chemical Engineering - Federal University of Parana (UFPR), Polytechnic Center, 82530-990, Curitiba, Parana
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2018 Received in revised form 23 December 2018 Accepted 1 February 2019 Available online 8 February 2019
Aprotic ionic liquids ([C4pyr]þ[NTf2]-, [C8mim]þ[NTf2]-, [C8mim]þ[OTf]-, [C8mim]þ[PF6]-, [C8mim]þ [BF4]-, [C12mim]þ[NTf2]-, and [C12mim]þ[Cl]-) were successfully evaluated as viscosity modifiers, emulsion inhibitors, and demulsifiers. They are prone to be applied for the heavy oil production since such crude oil type potentializes some issues that are typically observed in the petroleum industry. Among them, it is worth highlighting the low flowability due to the oil's high viscosity. Also, high-viscous crude oil usually generates stable emulsions since they have high resins and asphaltene contents, which are interfacial-active. Thus, for operational and economic reasons there is a need for technologies that facilitate the improvement of the heavy-oil flowability as well as the demulsification process. Then, this work aimed to analyze the influence of the ionic liquid chemical structure (cationic alkyl length, cationic functional group, and anion type) upon the oil viscosity, emulsion inhibition or demulsification, at concentrations of 5 and 100 ppm (m/m). Two Brazilian heavy-oils were used for this study. Viscosity measurements of these oils at room temperature (25 C) were made by an oscillatory rheometer before and after the addition of ionic liquid. Also, the emulsification inhibition and demulsification efficiency were analyzed by centrifugation. The results showed that the 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide promoted a demulsification efficiency higher than 99%. In turn, the 1-butylpyridinium bis(trifluoromethylsulfonyl)-imide was the one that worked as an emulsion inhibitor. On the other hand, the interaction between the studied oils and some of the used ionic liquids enhanced the oil viscosity. All the evidence points out to the fact that it was caused by the asphaltene aggregation due to the formation of an ionic liquid-asphaltene molecular structure. The observed viscosity increase was of 11% for oil A and 7% for the oil B. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ionic liquid Heavy oil Viscosity Emulsion Chemical additive
1. Introduction The heavy oil production plays a vital role in the maintenance of the world recoverable petroleum reserves. Also, the increase in oil consumption and the depletion of light oil reserves has directed technical attention to the operational and economic issues related to the recovery of heavy oil fields [1e3]. Due to its chemical characteristics, such as high asphaltene and resin contents, there are still technical challenges to be overcome in order to enhance the
* Corresponding author. E-mail address: [email protected] (D. Santos). https://doi.org/10.1016/j.fluid.2019.02.001 0378-3812/© 2019 Elsevier B.V. All rights reserved.
exploitation, transportation, and refining of those oils. Among them, it is worth highlighting the propensity for formation of highly-stable emulsions and the flowability limitations due to high viscosity. In the first place, the heavy oil emulsification results in corrosion and incrustation in production and refining facilities but, above all, it increases the native crude oil viscosity [4]. Secondly, the viscosity range of heavy oils typically varies from 0.1 to 1000 Pa.s. However, the usually recommended fluids’ viscosity for the transportation pipelines is around 0.4 Pa. s [5]. Consequently, the heavy oil viscosity reduction is mandatory as a way to avoid pressure drop and flow resistance during production and transportation. The petroleum industry has applied several strategies to promote viscosity reduction: oil blends, heating, inverse emulsion, diluents,
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and chemical additives. Among the additives, the ionic liquids stand as the most recent promising one [3,6e9]. Ionic liquids (ILs) are known as organic salts that generally melt at temperatures lower than 100 C. They have some properties that are operationally desirable such as low density and viscosity, high thermal stability, and low pressure-vapour [10]. Beyond that, any of these properties can be tuned by modifying the cationic or anionic part of the ionic liquid [11]. For instance, the miscibility and the thermal stability are mainly related to the anion's characteristics. Further, density and viscosity are changed by the cation's chemical properties. Also, by combining specific anions and cations, one can fit activity coefficient of the ionic liquid in specific hydrocarbons to make it able or not the be used in the petroleum industry [12e15]. For instance, the ability of imidazolium-based ILs to reduce the surface tension between water and oil as well as its chemical stability in high salinity environment has positioned them as potential additives for enhanced oil recovery [16]. In some applications, the ionic liquid effect doubled the recovery factor in comparison with conventional techniques [17]. Further, the amphiphilic characteristics allow the ionic liquids to be successfully applied as demulsifiers for highly-stable water-in-heavy crude oil emulsions as well as petroleum spill dispersant [18,19]. Also, deacidification of crude oils has been promoted by both reacting the acidic oil with IL or adsorbing the naphthenic acids in supported ionic liquid phases. A regeneration rate of 70% was registered by Anderson et al. (2015) [20e22]. In addition, the strong interaction between asphaltenes and some ionic liquids lead to the formation of aggregates with distinct geometries and complexation energies [23]. Thus, they can be used to control the size of asphaltene aggregates and, consequently, to avoid its deposition. Once ionic liquids act at the molecular level, while the common asphaltene dispersants work at the surface of big aggregates, the ILs has shown better efficiency. In this aspect, they have also been successfully used for the upgrade of heavy oils by reducing their viscosity and modifying their SARA composition [9,24]. In this scenario, this work aims to evaluate the application of aprotic ionic liquids ([C4pyr]þ[NTf2]-, [C8mim]þ[NTf2]-, [C8mim]þ [OTf]-, [C8mim]þ[PF6]-, [C8mim]þ[BF4]-, [C12mim]þ[NTf2]-, and [C12mim]þ[Cl]-) as an additive for viscosity reduction of two heavy crude oils as well as their use for inhibition and remediation of water-in-oil emulsion. Also, it was analyzed the effect of cation alkyl length, the anion charge density, and the cation functional group on their application efficiency.
2. Experimental 2.1. Materials Two Brazilian heavy-crude oils were selected for this study. They had initial water content up to 1% and scarce droplets with size varying from 27 to 80 mm. The density and asphaltene content of the so-called Oil A and Oil B are presented in Table 1. They were determined through the standard methods ASTM D5002-99 [25] and ASTM D6560 [26]. The TAN e total acid number were provided by ANP (2018) [27]. The used toluene (99.8%) was supplied by Sigma-Aldrich®. Distilled water was used in the synthesis of waterin-oil emulsions.
Table 1 Crude oil properties. Crude oil
ºAPI
Asphaltene content (% m/m)
TAN (mgKOH/g)
A B
13.2 12.7
7.3 2.7
1.2 1.3
9
The ionic liquids that were used are named as: 1-methyl-3octylimidazolium bis(trifluoromethylsulfonyl)imide [C8mim]þ[NTf2]-; 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide e [C12mim]þ[NTf2]-; 1butylpyridinium bis(trifluoromethylsulfonyl)imide e [C4pyr]þ[NTf2]-; 1-methyl-3-octylimidazolium triflate - [C8mim]þ [OTf]-; 1-methyl-3-octylimidazolium hexafluorophosphate [C8mim]þ[PF6]-; 1-methyl-3-octylimidazolium tetrafluoroborate [C8mim]þ[BF4]-; and, 1-dodecyl-3-methylimidazolium chloride e [C12mim]þ[Cl]-. They were all purchased from Iolitec GmbH with a purity higher than 99%. Their chemical structures are shown in Table 2. 2.2. Experimental procedure 2.2.1. Viscosity measurements The crude oils were pre-homogenized before any analysis. Then, they were doped with each of the selected ionic liquids in concentrations of 5 and 100 ppm (m/m). Such concentrations were defined in a way to simulate the additives concentration typically used in the crude oil chemical treatment [28]. Once the high viscosity of the oils would difficult the LI diffusion, toluene was used as a carrier fluid. Thus, instead of being added directly into the crude oil, the ionic liquids were diluted in the toluene. Firstly, solutions of crude oil þ toluene were prepared with concentrations varying from 1 to 10% (m/m) and analyzed. After that, solutions of ionic liquids in toluene were prepared and added into the oil at the aforementioned concentrations, then analyzed. The solutions were homogenized by ultrasonication at room temperature for 20 min and 40 C for more 40 min. A ultrasound bath (Ultronique) operating at frequency of 40 kHz and power of 200 W was used. After that, the samples were cooled down to 25 C for the viscosity measurements. These measurements were carried out by using an oscillatory rheometer (Physica MCR 301 e Anton Paar) equipped with coaxial cylinders. A shear rate scanning from 0.1 to 100 s1 was performed in triplicate for each experimental condition. 2.2.2. Demulsification efficiency Water-in-oil emulsions with a water content of 30% were synthesized with distilled water as aqueous phase and two oily phases: crude oil þ toluene and crude oil þ toluene þ ionic liquid. The mixture was hand-shaken in order to pre-emulsify the separated phases. Then, the droplet size was reduced through shear application by using a dispersion tool (IKA, Ultra Turrax T25 Basic). The homogenization conditions (stirring frequency of 6,500 rpm and stirring time of 2,5 min) were suitable to generate a bimodal droplet size distribution (DSD) with mean values varying from 710 mm to 8e12 mm. After synthesis, the emulsions were then characterized regarding DSD and water content. The procedures of such analyses can be seen elsewhere [29]. The demulsification itself was promoted by a centrifuge at 2000 rpm and room temperature (25 C). The total centrifugation time was 30 min and it was broken into steps of 5 min each to verify the free-water volume over time. After the demulsification, the free water-oil phase was recovered, and its viscosity was analyzed through the methodology aforementioned. 3. Results and discussions The chemical treatment of petroleum usually applies light fractions as an additive to reduce oil viscosity and/or favours other chemicals diffusion in the so-treated crude oil [30]. In this work, toluene was used as a carrier fluid for the studied additives. It was chosen due to its potential to solubilize both the used ionic liquids and the asphaltenes present in the heavy crude oil. Fig. 1 shows the
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R.L.M. Santos et al. / Fluid Phase Equilibria 489 (2019) 8e15 Table 2 Chemical structures of the studied ionic liquids. Nomenclature
Chemical structure
1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide
1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1-butylpyridinium bis(trifluoromethylsulfonyl)imide
1-methyl-3-octylimidazolium triflate
1-methyl-3-octylimidazolium hexafluorophosphate
1-methyl-3-octylimidazolium tetrafluoroborate
1-dodecyl-3-methylimidazolium chloride
Fig. 1. Cumulative viscosity reduction of heavy-oil in function of toluene concentration at room temperature (25 C). Oil A (yellow columns); Oil B (blue columns).
viscosity reduction of the crude oils A and B caused by toluene addition at concentrations varying from 1 to 10% (m/m). One can note that toluene promoted an exponential viscosity reduction mainly due to dilution of organic particles suspended in the
petroleum. Such reduction was of greater extent for the Oil A, which has a higher asphaltene content. Also, it is worth highlighting that there is a difference in the cumulative reduction as the toluene concentration was increased in each oil. The effect of
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toluene addition at the lowest concentration (1%) for Oil A was almost the double of the effect on Oil B. However, the toluene concentration of 10% produced a similar reduction of >90% for both the oils. Fig. 2 shows the influence of the ionic liquids upon the crude oils viscosity. As already mentioned, the chosen carrier fluid (toluene) promotes a viscosity reduction. However, when the additives were solubilized in the oils, a viscosity increase was noticed for all the ionic liquids. Such increases were detected at different extents for both the studied oils. Although a viscosity reduction was aimed, the opposite was verified. At the first glance, it seems to be a disadvantage. However, it confirms the strong interaction between the ILs and the crude oil components. For the Oil A, one can note that C4pyr-NtF2 and C8mim-BF4 promoted a viscosity increase higher than 8%. On the other hand, C8min-NtF2 and C12mim-Cl were the
11
ILs that promoted the most significative change in the viscosity for Oil B. As can be seen, each of these ionic liquids has a peculiarity regarding its cation type, anion type, and alkyl chain length. Once the molecular structural differences result on modifications of chemical properties, it will reflect on the interaction crude oil-ionic liquid as analyzed hereinafter. 3.1. Effect of anion type In order to evaluate the effect of the ionic liquids’ anion type on the viscosity modification, the same imidazolium cation with a branched alkyl length of 8 carbon atoms was selected. Then, the anions NtF2, OTF, BF4 and PF6 were investigated. Fig. 3 shows that for Oil B, the anion NtF2 showed a stronger interaction with the organic compounds that typically rules the crude oil viscosity, such
Fig. 2. Viscosity enhancement for Oil A (yellow columns) and Oil B (blue columns) after addition of ionic liquids (5 ppm) into the heavy-oil doped with toluene (1% m/m).
12
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Fig. 3. Viscosity enhancement for Oil A (yellow columns) and Oil B (blue columns) doped with toluene (1% m/m). After addition of ionic liquid (5 ppm) with the same cation and different anions.
as resins and asphaltenes [31]. On the other hand, such interaction strength followed the order BF4 > OTF > PF6 > NtF2 for Oil A, which is the one that has the higher asphaltene content. It is worth highlighting that ionic liquids do not fit into the standard behaviour of the additives that are typically used for petroleum upgrade. The theoretical and empirical basis for predicting their behaviour when mixed with such a complex fluid as crude oil is still being developed. In this scenario, the proper ionic liquid to be used as an additive must be the one that strongly interacts with asphaltenes or the resin-asphaltenes complex, thus isolating it and avoiding the aggregation process as well as its correlated problems. Also, such optimal IL must have as less affinity as possible with its own molecules [32,33]. In this scenario, the observed effect of the anion types upon the viscosity of both the studied oils can be explained by the interaction distinctions between IL, asphaltenes and the whole crude oil. Among the studied anions, NtF2 has the greater hydrogen bond acceptor potential due to its molecular structure. As reported by Subramanian et al. (2015) [28], the asphaltene aggregation phenomena is ruled by several types of molecular interaction, such as p-p interaction between the aromatic cores, aliphatic interactions, acid-base, charge-transfer, and hydrogen-bonding. Thus, anions with higher potential to promote hydrogen bond with asphaltene molecules may favor the asphaltene-IL interaction. Also, it will result in a stronger IL-IL hydrogen bonding [14,34]. If the crude oil whole composition does not overcome on such equilibrium forces (asphaltene-IL and IL-IL), it will be verified a greater asphaltene aggregation and a consequent viscosity increase. At the same time, if the crude oil whole composition does surpass the asphaltene-IL interaction forces, the activity coefficient of the ionic liquids will be a relevant information. The so far published data report the activity coefficient of ILs in pure solvents. However, it can be inferred for crude oil by extrapolating the data from representative simple hydrocarbon molecules, as it is usually done. Thus, the lack of solubility of the anions in typical crude oil model fractions (alkanes, cyclo-alkanes, and aromatics), for instance, will be reflected
in a higher aggregation of the complex asphaltene-ionic liquid. It is confirmed by the relationship between the anion's activity coefficients (BF4 > OTF > NtF2) [35] and the viscosity enhancement for Oil A (BF4 > OTF > PF6 > NtF2), the most asphaltenic one. 3.2. Effect of the alkyl chain Fig. 4 shows the viscosity enhancement for the studied oils when both anion and cation were the same and only the alkyl chain length was changed. Again, different effects were noticed. For Oil B, the viscosity enhancement was lower as the alkane chain was lengthened. The longer alkyl length reduces the self-aggregation of the complex asphaltene-ionic liquid, once such complex is more prone to interact with the whole crude oil. As a consequence, the anion's effect is reduced. On the other hand, the opposite phenomenon was observed for the Oil A. In this case, the longer alkyl length increased the viscosity enhancement. It is believed that for this asphaltenic and significantly more viscous oil, the ionic liquid diffusion, which is lower as longer the alkyl chain [34], rules the viscosity enhancement. 3.3. Emulsification inhibition and demulsification efficiency The surface-active ionic liquids have been studied as demulsifiers for water-in-crude oil emulsions due to their amphiphilic property [36]. Further, such characteristics highlight the IL's potential to be applied as an emulsion inhibitor. Thus, the ionic liquids that do not lose the chemical stability when in contact with water and, at the same time, showed greater viscosity enhancement caused by the asphaltene-ionic liquid interaction were selected. The less viscous oil (Oil B) was selected for the demulsification studies in order to avoid that petroleum properties would mask the ionic liquid action. Also, the IL concentration was increased in order to emulate demulsifier concentrations typically used in the petroleum industry [37]. Fig. 5 shows the results of demulsification efficiency by the centrifugation method with and without the crude
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13
Fig. 4. Viscosity enhancement for Oil A (yellow columns) and Oil B (blue columns) doped with toluene (1% m/m). After addition of ionic liquid (5 ppm) with the same anion and different cationic alkyl chain.
oil dopage by IL. As one can note, the C4pyr-NtF2 showed a good efficiency as an emulsion inhibitor since it was not possible to produce a stable water-in-crude oil emulsion when Oil B was doped with 100 ppm of such ionic liquid. Once the ionic liquid-free emulsion was stable even after 30 min, the demulsifier action of the studied ionic liquids could be analyzed. Thus, Fig. 5 shows that the C8mim-NtF2 at concentration of 100 ppm promoted the same demulsification efficiency as C4pyr-NtF2, but a higher centrifugation time was demanded as concentration increased. It denotes that the optimum
dosage was overreached and instead of working as a demulsifier, the C8mim-NtF2 acted as an emulsifier compound even stronger than the petroleum natural interfacial-active ones. On the other hand, the C4pyr-NtF2 concentration of 50 ppm was not enough to break the emulsion efficiently. Hazrati et al. (2018) as well as Silva et al. (2013) also used ionic liquids to break water-in-crude oil emulsions successfully [38,39]. Their results highlight the influence of the IL's molecular structures on the demulsification efficiency. However, they cannot be compared with this study since heating techniques (conventional and dielectric heating) were needed to
Fig. 5. Demulsification efficiency in function of centrifugation time for the Oil B doped with toluene (10%) with and without the addition of ionic liquids at concentrations of 50 ppm and 100 ppm.
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aid the demulsification promoted by the used ionic liquids. A relevant phenomenon was noticed in crude oil after the demulsification process. The recuperated oil (water content < 5%) showed to have a viscosity increase, as can be seen in Fig. 6. Indeed, once the oil viscosity alteration promoted by addition of ionic liquids is caused by the aggregates formation between asphaltenes and ILs, it must be enhanced by the presence of water. The probability of both asphaltene and ionic liquid molecules to interact is higher at the oil-water interface due to their amphiphilic nature. Thus, when the water-in-oil emulsion is formed, those molecules will compose the interfacial film. Hence, if any of the studied ionic liquids have been used as an emulsion inhibitor and/or demulsifier at the desalter process, for instance, the once demulsified petroleum will probably have a higher viscosity after the demulsification step.
4. Conclusions Aprotic ionic liquids were successfully used as emulsion inhibitors and demulsifiers for two Brazilian heavy-oils. The use of toluene as a carrier fluid aided in the viscosity reduction but above all, it promoted the ionic liquid diffusivity in the crude oil. Also, all the ionic liquids caused a viscosity increase when solubilized in the oils, but in different extent for each oil due to both IL and oil chemical characteristics. Among the studied ILs, the 1-butylpyridinium bis(trifluoromethylsulfonyl)imide e [C4pyr]þ[NTf2]- was the one capable of inhibiting the emulsion formation. The 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide - [C8mim]þ[NTf2]- promoted the highest demulsification efficiency (>99%). However, the demulsified oil showed to have a higher viscosity than it had before the emulsification. Although the studied ionic liquids did not
Fig. 6. Viscosity enhancement for Oil B doped with toluene and tolueneþionic liquid before emulsification (yellow columns) and after demulsification process (blue-dashed columns).
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show effectiveness to be used as an efficient additive for crude oil upgrade by reducing viscosity, the strong interaction between asphaltenes and ionic liquids put them as a potential asphaltene precipitation inhibitor as well as emulsion inhibitor. Thus, it will also improve the oil flowability as long as the ionic liquid is capable of avoiding asphaltene deposition and/or emulsion formation. Acknowledgements ~o de Apoio a Pesquisa e a The authors thank Fapitec-SE (Fundaça ~o Tecnolo gica do Estado de Sergipe), ANP, CNPq, and CAPES Inovaça - Finance Code 001 - for financial support.
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