Liquid-Liquid extraction of aromatics and sulfur compounds from base oil using ionic liquids

Liquid-Liquid extraction of aromatics and sulfur compounds from base oil using ionic liquids

G Model JECE 1324 No. of Pages 8 Journal of Environmental Chemical Engineering xxx (2016) xxx–xxx Contents lists available at ScienceDirect Journal...

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G Model JECE 1324 No. of Pages 8

Journal of Environmental Chemical Engineering xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Liquid-Liquid extraction of aromatics and sulfur compounds from base oil using ionic liquids Ghassan M.J. Al Kaisya,* , Mohamad Ibrahim Abdul Mutaliba , Mohamad Azmi Bustama , Jean-Marc Levequeb , Nawshad Muhammadc,* a b c

Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia Department of Fundamental & Applied Sciences, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information Technology, Lahore, Pakistan

A R T I C L E I N F O

Article history: Received 21 September 2016 Received in revised form 4 November 2016 Accepted 7 November 2016 Available online xxx Keywords: Base oil Ionic liquids Extraction Polyalphaolefin Naphthalene Dibenzothiophene

A B S T R A C T

The viability of using ionic liquids (ILs) as extractive solvents to remove naphthalene (aromatics) and dibenzothiophene (DBT; Sulfur) from base oil by liquid-liquid extraction was investigated. The experiments were designed using Response Surface Methodology (RSM) with 1-butyl-3-methylpyridinium dicyanamide [BMPY][DCA], 1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA], 1-butyl3-methylimidazolium thiocyanate [BMIM][SCN] and 1-butyl-3-methylimidazolium dimethylphosphate [BMIM][DMP] ILs. The sulfur compound and aromatics were analyzed using Total Sulphur analyzer and High Performance Liquid Chromatography (HPLC) with high coefficient of determination i.e. R2 values of 0.964 and 0.997 respectively. The effects of different ILs, temperatures, and ILs to oil mass ratio (IL:Oil) were optimized.[BMPY][DCA] appeared as the most promising medium with 94.3% of dibenzothiophene and 83.1% of naphthalene removal after a single extraction step. The aromatics and sulfur removal efficiency of [BMPY][DCA] IL was 54.3%, 78.3% and 82.9%, 93.8% at IL:Oil ratios of 0.4 and 1.8, respectively. An increase in temperature did not improve the extraction efficiency, but a slight decrease was noted. Results emphasized that extraction of aromatics and sulfur compoundsfrom base oil can be achieved successfully using selected ionic liquids. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Production of high-class lubricating oil base stock (base oil) has received considerable attention in recent yearsbecause of more stringent environmental regulations and higher product specifications to meet higher performance required by the end users. Group I class base oil is conventionally produced through solvent treatment, whereasproduction of higher base oil classes (Group II and III)requires hydroprocessing.Hydroprocessing includeshydrotreating (catalytic hydrogen treating) and hydrocracking processes, which have increasing importance in the petroleum refining industry in the desulfurizationof light products (e.g., fuels)andin base oil production. [1–9]. Hydrotreating processes in lubricating oil production help remove impurities such as sulfur, nitrogen, oxygen, and saturating olefins and some of the aromatics to meet final product specifications of high quality base oil. Hydrotreating processes

operate under the following conditions:H2 circulation at 50– 675 Nm3/m3, operating pressure at 14–138 bar, and operating temperature of 290–370  C [4–9]. Hydrocracking is a catalytic hydrogenation process that convert andhydrogenates high molecular weight feedstocks into lower molecular weight products. Hydrocracking processes remove wax and aromatic during productionof base oil via catalytic dewaxing and hydrogen saturation, respectively. These techniques are gradually replacing the existing solvent dewaxing and aromatic solvent extraction to produce base oil with higher specifications.Hydrocracking operating conditions are as follows:H2 circulation at 850–1,700Nm3/m3, operating pressure at 103–138 bar, and operating temperature of 357–385  C [4–6,8–10]. However, the significant investment of the hydroprocessing coupled with the severe operating conditions, such as high temperature and pressure, high hydrogen demand, expensive catalyst and environmental impact, led to propose an alternative process involving mild operating conditions with low

* Corresponding authors. E-mail addresses: [email protected] (G.M.J. Al Kaisy), [email protected] (N. Muhammad). http://dx.doi.org/10.1016/j.jece.2016.11.011 2213-3437/ã 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: G.M.J. Al Kaisy, et al., Liquid-Liquid extraction of aromatics and sulfur compounds from base oil using ionic liquids, J. Environ. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.jece.2016.11.011

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investment cost and environmental impact. Liquid-liquid extraction using non-conventional solvent, such as ionic liquids (ILs), is regarded as a potential option. Room Temperature Ionic Liquids (RTILs) have been extensively studied over the two last decades.RTILs have a wide range of potential applications in the industry as green solvents because of their remarkable physical-chemical properties, such asnegligible vapor pressure, good thermal stability, wide liquid range and ability to dissolve polar, non-polar, organic and inorganic compounds on a selective basis [11–13]. In the domain of petroleum refining, ionic liquids were already reported as highly promising extraction media for the separation of aromatic from aliphatic hydrocarbons in various systems containing different types of aromatics (e.g., ethyl benzene, p-xylene, toluene, benzene, propylbenzene) and aliphatic alkanes (e.g., hexane, heptane, dodecane, tetradecane, methylcyclohexane) [14–24]. Separation of sulfur compounds from model diesel or model gasoline in different systems containing sulfur derivatives (e.g., thiophene, benzothiophene, dibenzothiophene, 4,6-dimethyldibenzothiophene) and aliphatic alkanes (e.g., hexane, heptane, dodecane) was investigated. Some systems with extra aromatics, including toluene, benzene, xylol, pyridine, naphthalene, were also reported [23,25–34]. However, allcited works focused onthe removal of aromatics and sulfur compoundsfrom either light fuel (gasoline and diesel) or from their model oils containing aliphatic hydrocarbon such as hexane, heptane, dodecane, or tetradecane. To our knowledge, no previous studywas conductedon longer chain hydrocarbons such as those relevant to the base oil withtypical carbons from C20 to C50. The present work aimed to study the viability of using four ionic liquids, namely,1-Butyl-3-methylpyridinium dicyanamide [BMPY] [DCA] 1-Butyl-3-methylimidazolium dicyanamide [BMIM][DCA], 1-Butyl-3-methylimidazolium thiocyanate [BMIM][SCN], and 1Butyl-3-methylimidazolium dimethylphosphate [BMIM][DMP],to remove naphthalene and dibenzothiophene (DBT) from its mixture with Polyalphaolefin base oil via liquid-liquid extraction, which could lead to an extended application for ionic liquids as a green solvent for the treatment of refinery products.For this purpose Response Surface Methodology (RSM) was used to optimize the extraction process with respect of type of ionic liquids, Oil;ILs ratio and temperature etc. The naphthalene and dibenzothiophene were analyzed withHPLC and Total Sulphur Analyzer respectively.

2. Experimental 2.1. Materials The ionic liquids [BMPY][DCA] > 98%, [BMIM][DCA] > 98%, [BMIM][SCN] > 98% and [BMIM][DMP] >98%were purchased from IoLiTec (Germany). Polyalphaolefin (Synfluid1PAO 6) was from Chevron Phillips Chemicals International N.V. (Belgium). Naphthalene (99%) was from Aldrich and dibenzothiophene (99%) was from Acros Organics. All chemicals were used as received from suppliers without further treatment. Polyalphaolefins are synthetic isoparaffinic hydrocarbons base oil with the general formula of CnHn+2. They are composed of C20, C30, C40 and, C50+ fractions. Polyalphaolefinswere developed as high performance base stock oils for automotive and industrial applications [35–37]. The Polyalphaolefin used in this work is Synfluid1PAO 6, which has a kinematic viscosity of 5.8 cSt at 100  C (CAS No. 68037-01-4) [38] and a molecular formula categorized as UVCB (Substances of Unknown or Variable composition, Complex reaction products or Biological material) [39]. 2.2. Preparation of polyalphaolefin, naphthalene and DBT mixture (Module oil) Specified amounts of naphthalene and dibenzothiophene were dissolved in Polyalphaolefin using ultrasonic bath (Elma Transsonic Digital S) to form 12 wt.% aromatics solution and 1133 ppm (wt. ratio) sulfur content in Polyalphaolefin. Weight measurementswere conducted using an analytical balance Mettler Toledo AB304S with accuracy of 0.0001 g. 2.3. Extraction experiments The extraction experiments were designed according to response surface methodology (RSM) using design Expert 8.0.7.1 software. For each IL, twenty three different runs were conducted under different operating parameters, including temperature (40, 58, 75, 93, and 110  C) and IL:Oil (0.4, 0.75, 1.1, 1.45, and 1.8). All experiments were conducted in 30 mL glass vials with screw caps, which provide hermetic sealing. The vials were placed in an oil bath with a temperature controller, and then magnetically stirred at a fixed speed of 1000 rpm for a fixed time period 1 hto reach

Table 1 Results of extraction experiments responses based on experimental design. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Extraction variables

Response (% Removed)

Temp.  C

[BMPY][DCA]

110 93 58 93 93 75 75 58 40 110 110 40 75 75 75 40 58

IL:Oil Ratio (mass) 1.1 0.75 0.75 1.1 1.45 1.45 0.75 1.45 1.8 1.8 0.4 1.1 1.8 0.4 1.1 0.4 1.1

[BMIM][DCA]

[BMIM][SCN]

[BMIM][DMP]

Sulfur

Aromatics

Sulfur

Aromatics

Sulfur

Aromatics

Sulfur

Aromatics

90.3 87.7 87.8 89.8 92.1 93 86.8 92.1 94.3 93.5 80.2 91.3 93.8 78.3 90.4 78.7 90.5

74.7 66.8 67.4 74.7 79.4 79.8 67.3 79.6 83.1 82.3 53.8 75.3 82.9 54.3 74.6 54 75.4

81.9 76.3 75.5 83 85.5 86 78.1 85.6 88.7 88.8 68.3 85.8 90.3 74 85.8 75 85.5

65 57.5 56 63.8 71.3 70.9 57.1 70.5 78.1 75 41.2 66.3 74.9 39 63.2 40.1 64.3

84.6 80.4 81.1 83.9 88.3 88.2 80.7 88.9 89.2 92 72.8 87.6 87.3 79.6 87.3 77.7 80.1

60.2 50 51.2 58 65.1 66.4 51.4 67.9 73.2 70.5 38.7 62 72 35.9 60.3 38.1 60.9

86 65.1 82.4 84.7 82.9 86.8 75.2 90.5 92.8 88.7 63.6 83.9 91 78.5 81.1 78.7 87.8

49.7 40.4 41.4 50.1 56.7 58.1 40.3 60.2 67 61.6 27.4 55.1 63.6 26.5 50.8 27.9 52.5

Please cite this article in press as: G.M.J. Al Kaisy, et al., Liquid-Liquid extraction of aromatics and sulfur compounds from base oil using ionic liquids, J. Environ. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.jece.2016.11.011

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Table 2 The maximum extraction of sulfur and aromatics during ILs treatment. Ionic Liquid

% Sulfur removed

% Aromatics removed

Temperature/ C

IL:Oil mass ratio

[BMPY][DCA] [BMIM][DCA] [BMIM][SCN] [BMIM][DMP]

94.3 90.3 92.0 92.8

83.1 74.9 70.5 67.0

40 75 110 40

1.8 1.8 1.8 1.8

2.4. Analysis

Fig. 1. Aromatics removal efficiency vs. IL:Oil mass ratio at 75  C.

equilibrium for extraction. The mixtures were then left to settle 12 h at room temperature for decantation. Biphasic systems comprising IL-rich and oil-rich phases were formed. Ionic liquid was simply recycled by adding water into IL-rich phase which solubilize the ionic liquid only, thereby the resultant two layers were separated to obtain the water/IL layer. The water/IL layer was subjected to rotary evaporation to obtain the ionic liquid.

The two immiscible phases were separated after equilibrium was reached. Samples were obtained from the oil-rich phase for sulfur content and aromatics content analysis. Sulfur content was determined usingTotal Sulfur Analyzer (TS 3000,Thermo Fisher Scientific) according to ASTM standard test method ASTM D 5453–06 [40]. Typically, the sample is injected into a high temperature combustion tube where the sulfur based moiety is oxidized to sulfur dioxide (SO2) in an oxygen-rich atmosphere to ensure complete combustion of the sample. The water produced during the sample combustion is removed using a special mechanism for water vapor removal. Sample combustion gases are next exposed to ultraviolet (UV) light where SO2 is excited. The fluorescence emitted when SO2 relaxes is detected and quantified by a photomultiplier tube, leading to the measure of the sulfur content in the sample. Aromatics content was analyzed using High Performance Liquid Chromatography (HPLC; 1200 Infinity Series, Agilent Technologies) equipped with diode array detector (DAD) and refractive index detector (RID) according to the ASTM standard test method ASTM D 7419–07 [41]. A three columns system wasused. Two column packed with silica gel stationary phase (9.4  250; 5 mm), and one column packed with cyano (CN) stationary phase (9.4  250; 5 mm). The mobile phase was heptane with a flow rate of 3.0 mL/ min. A UV-detector set to a wavelength of 254 nm was used in series with the refractive index detector. The results obtained both for sulfur contents using sulfur analyzer and aromatics using HPLC shows high coefficient of determination i.e. R2 values of 0.964 and 0.997 respectively. The aromatics removal efficiency and sulfur removal efficiency were calculated from the analyses result of aromatics content and sulfur content using the following equations: Aromatics Removal Efficiency Aromatics initial  Aromatics f inal Aromatics initial  100

ð% Aromatics RemovedÞ ¼

Sulfur Removal Efficiency ð% Sulfur RemovedÞ ¼

Sulf ur initial  Sulf ur f inal  100 Sulf ur initial

3. Results and discussion 3.1. Effect of different ionic liquids

Fig. 2. Sulfur removal efficiency vs. IL:Oil mass ratio at 75  C.

Table 1 shows the details on the designof all experiments conducted during this study with temperature  C and IL:Oil mass ratios as variables.Results of sulfur and aromatics removal efficiency as two responses are also provided in Table 1. Overall, good results were obtained but noticeable differences suggested that the selection of temperature and IL:Oilmass ratios as variables were of good choice in terms of extraction response. Moreover, the nature of the engaged IL is of crucial importance. For example, for the same temperature and IL:Oil mass ratio, the extraction response of sulfur and aromatics removal reaches up to

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86.8% and 67.3% respectively for [BMPY][DCA] IL, whereas, it is only 75.2% and 40.3% for [BMIM][DCA] IL. Table 2 summarizes the best results displayed in Table 1. Remarkably, the best results for all four ILs displayed the same IL:Oil mass ratio, whereas operating temperatures are different. All used ILsshowed sulfur removal efficiency of more than 90% with the decreasing trend as follows: [BMPY][DCA] > [BMIM][DMP] > [ BMIM][SCN] > [BMIM][DCA]. The extraction efficiency of aromatics removal,which was lesser than that of sulfur removal, still reached a reasonable 70%-85% range with the following decreasing trend: [BMPY][DCA] > [BMIM] [DCA] > [BMIM][SCN] > [BMIM][DMP].This trend was slightly different from the previous trend for sulfur removal. The p-p interactions between the aromatic ring of the solute and the cationic aromatic ring of the ILs are strongly believed to be the driving force of the extraction of naphthalene and dibenzothiophene from base oil. Moreover, the higher removal efficiency of DBT molecule compared with naphthalene molecule might be caused by its higher aromatic p-electrons density [31,32,42,43]. The aromatic solute molecule has a negatively charged p-electron cloud above and below the plane of the ring,as well asa positively charged area around the equator. The IL polarizes the p-electron cloud of the aromatic solute molecule in the extraction process.This phenomenon enables the p-p interaction between the cationic aromatic ring of the ILs and the negatively charged

aromatic ring of the solute to take effect. Thus, the anions locate and orient themselves around the ring of the aromatic solute molecule [44,45]. This behavior might explain why the pyridinium-based ionic liquid [BMPY] displayed overall better extraction efficiency than the imidazolium analogue [BMIM] with the same dicyanamide anion [DCA]. Indeed, the aromatic character of the pyridinium cation is more pronounced than that of the imidazolium cation. Similar results were also observed by Meindersma et al. [20] observed similar results for the removal of toluene from heptane with pyridinium- and imidazolium-based ILs. Pereiro et al. [46] also reported better extraction efficiency of pyridinium-based IL over imidazolium-based IL with [DCA] anion for the removal of benzene from hexane. All investigated ILs showed good extraction response for both DBT and naphthalene from Polyalphaolefin base oil. The ILs also showed a higher selectivity for the removal of DBT over naphthalene. Imidazolium based ILs [BMIM][DCA], [BMIM][SCN] and [BMIM][DMP] displayed different extraction efficiencies, suggesting that the type of the anion significantly influenced the extraction performance. This characteristic was mentioned by Zhang et al. [47], who reported that the extraction performance of ILis mainly determined by the type of bothcation andanion. Notably, anions bearing delocalized charges interact on a lesser extent with the cation, subsequently favoring cation-solute interactions [44,46].

Fig. 3. Response surface plot of the effect of interaction between temperature and IL:Oil mass ratio on aromatics removal efficiency by extraction with ILs.

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Fig. 4. Response surface plot of the effect of interaction between temperature and IL:Oil mass ratio, onsulfur removal efficiency by extraction with ILs.

3.2. Effect of ionic liquid to oil mass ratio (IL:Oil) Figs. 2 and 3 plot the aromatics and sulfur removal efficiencies of the four tested ILs at different IL:Oil mass ratios(0.4, 0.75, 1.1, 1.45 and 1.8) at a fixed temperature of 75  C, (Table 1, entries 6, 7, 14, 15 and 16), respectively. The aromatics removal efficiency of [BMPY][DCA] IL was 54.3% and 82.9% at IL:Oil ratios of 0.4 and 1.8, respectively. The same effect, that is, a high IL:Oil ratio resulted in better extraction efficiency, was observed for the other ILs (Fig. 1 ). The sulfur removal efficiency of the four ILs for different IL: Oil ratios at a fixed temperature of 75  C is plotted in Fig. 2. The lowest sulfur removal efficiency was observedat IL:Oil of 0.4.The performance increased as the IL:Oil ratio increased to 1.8. The same as aromatics extraction trend was found for sulfur removal,in which a high IL:Oil ratio resulted in better extraction efficiency. For example, the sulfur removal efficiency for [BMPY][DCA] IL was 78.3% and 93.8% at IL:Oil ratiosof 0.4 and 1.8, respectively. For [BMIM][DCA], [BMIM][SCN], and [BMIM][DMP] ILs, the sulfur removal efficiencies at IL:Oil ratios of 0.4 and 1.8 were 74% and 90.3%, 79.6% and 87.3%, and 78.5% and 91%, respectively (Fig. 2). All plotted graphs followed the same trend regardless of the engaged IL. This behavior clearly emphasized that higher IL:Oil mass ratio led to better extractive performance of the IL. Likewise, [BMPY][DCA] IL always showed better extraction efficiency compared with other tested ILs.

This finding concurredwith those reported by several earlier studies conducted on different systems, such as dibenzothiophene + dodecane + ILs [25], thiophene + heptane + xylol + ILs [32], real diesel fuel + ILs [48] and DBT + n-tetradecane + ILs [49].

Fig. 5. Aromatics removal efficiency vs. temperature at IL: Oil mass ratio of 1.1.

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Three-dimensional response profiles were plotted to explore the interactive effects of the variables, such as temperature and IL: Oil mass ratios, as well as their mutual interactions on extraction efficiency, Figs. 3 and 4 illustrate aromatics removal and sulfur removal, respectively. Both series of graphs emphasized that the aromatics and sulfur removal efficiencies increased with increasingIL:Oil ratio within the range of 0.4–1.8, at all temperatures within the range of 40– 110  C. This finding demonstrated that the IL:Oil mass ratio was an important variable highly influenced extraction. 3.3. Effect of temperature

Fig. 6. Sulfur removal efficiency vs. temperature at IL:Oil mass ratio of 1.1.

The number of IL molecules in the locality of the aromatic solute molecule increased when the IL:Oil ratio increased, which then enhanced the interaction between the IL and aromatic solute [44]. This process could explain the increasing trend of the aromatic removal efficiency with the increment in the IL:Oil ratio. Moreover, [BMPY][DCA] IL displayedthe best extraction efficiency for both types of pollutant. This finding was consistent with the issued hypothesis stating that having a more pronounced aromatic character that the imidazolium ring, cationic pyridinium moiety highly could interact through favored p-p interactions with the aromatics solutes.

The aromatics and sulfur removal efficiencies of the four tried ILs for different extraction temperatures (40  C, 58  C, 75  C, 93  C and 110  C) at a fixed IL:Oil mass ratio of 1.1 (Table 1, entries 1, 4, 12, 15 and 17) are plotted in Figs. 5 and 6. Contrary to the IL:Oil ratio, temperature did not significantly influence extraction efficiency. For example, the aromatic removal efficiency for [BMPY][DCA] IL, [BMIM][DCA] IL, [BMIM][SCN] IL, and [BMIM][DMP] IL at 40  C and 110  C were 75.3% and 74.7%, 66.3% and 65%, 62% and 60.2%, and 55.1% and 49.7%, respectively. An increase in temperature did not improve the extraction efficiency, but a slight decrease was noted (Fig 5). Sulfur removal efficienciesof the four ILs at different temperatures(40  C, 58  C, 75  C, 93  C, and 110  C) at fixed IL:Oil mass ratio of 1.1, were also plotted (Fig. 6). Typically, the sulfur removal efficienciesfor[BMPY][DCA] IL, [BMIM][DCA] IL, [BMIM][SCN] IL, and [BMIM][DMP] IL at 40  C and 110  C were 91.3% and 90.3%, 85.8% and 81.9%, 87.6% and 84.6%, and 83.9% and 86%, respectively (Fig. 6). Overall, results showed a slight decreasing trend consistent with increasing temperature, except for sulfur removal by [BMIM] [DMP] IL which slightly increased. This slight increase

Table 3 Analysis of the regression models and final model equations for aromatic and sulfur removal. [BMPY][DCA] Aromatics removal model Final equation Sulfur removal model Final equation

[BMIM][DCA] Aromatics removal model Final equation Sulfur removal model Final equation [BMIM][SCN] Aromatics removal model Final equation Sulfur removal model Final equation [BMIM][DMP] Aromatics removal model Final equation Sulfur removal model Final equation

Model Sum of squares d.f. Mean squares Cubic 1.41E-004 9 1.613E-005 2 2 Y A = 0.026931–0.024319 B + 4.42728E-007A + 0.0151 B  3.35047E-003 B3 Model Sum of squares d.f. Mean squares Cubic 750.47 9 83.39 Y S = 62.41538 + 68.88579 B + 2.99828E-003 AB  47.86731 B2  7.95580E-004 A2B + 0.042586 AB2 + 10.90240 B3

F-Value 5190.78

Prob > F <0.0001

Remarks Significant

F-Value 410.44

Prob > F <0.0001

Remarks Significant

Model Sum of squares d.f. Mean squares Cubic 3894.70 9 432.74 Y A = 20.01285 + 111.38299 B  0.10832 AB + 9.10814E-003A2  64.91516 B2 + 14.85345 Model Sum of squares d.f. Mean squares Quadratic 993.28 5 198.66 Y S = 69.85405  0.052293 A + 18.75943 B + 0.065761 AB  5.16236 B2

F-Value 560.46 B3 F-Value 90.94

Prob > F <0.0001

Remarks Significant

Prob > F <0.0001

Remarks Significant

Model Sum of squares d.f. Mean squares Cubic 3838.90 9 426.54 Y A = 6.29094 + 0.39084 A +76.57231 B + 0.14591 AB  8.04267E-003 A2  41.89273 A2 – 1.51548E-003 A2B + 4.71908E-005A3 + 8.59848 B3 Model Sum of squares d.f. Mean squares Cubic 657.29 9 73.03 2 Y S = 80.94761  2.18971 B  0.67399 AB + 36.89548 B + 4.60486E-003 A2B

F-Value 2168.08

Prob > F <0.0001

Remarks Significant

F-Value 27.07

Prob > F <0.0001

Remarks Significant

Model Sum of squares d.f. Mean squares Cubic 4535.35 9 503.93 Y A = 8.03365  0.21472 A + 79.11813 B  0.18893 AB + 2.67605E-003 A2  28.53418 B2 2 3 + 0.074763 AB + 3.13199 B Model Sum of squares d.f. Mean squares 2 Factorial 1315.83 3 438.61 Y S = 86.87796  0.25201 A + 3.86881 B + 0.12111 AB

F-Value 7680.85

Prob >F <0.0001

Remarks Significant

F-Value 27.35

Prob > F <0.0001

Remarks Significant

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Table 4 Statistical parameters obtained from ANOVA for aromatics and sulfur removal responses. Standard deviation

Mean

R2

Adjusted R2

Predicted R2

Coefficient of variance%

PRESS

Adequate precision

Aromatics removal [BMPY][DCA] [BMIM][DCA] [BMIM][SCN] [BMIM][DMP]

5.574E-005 0.88 0.44 0.26

0.015 62.12 56.45 47.60

0.9997 0.9974 0.9993 0.9998

0.9995 0.9956 0.9989 0.9997

0.9995 0.9930 0.9985 0.9991

0.38 1.41 0.79 0.54

7.906E-008 27.49 5.6 4.09

178.289 66.169 127.363 241.406

Sulfur removal [BMPY][DCA] [BMIM][DCA] [BMIM][SCN] [BMIM][DMP]

0.45 1.48 1.64 4.00

88.08 82.05 83.72 82.00

0.9965 0.9640 0.9493 0.8120

0.9941 0.9534 0.9143 0.7823

0.9887 0.9468 0.8516 0.7538

0.51 1.8 1.96 4.88

8.49 54.80 102.74 399.05

53.11 27.708 17.603 15.837

indicatedthat aromatics and sulfur removal are not sensitive to the extraction temperature,so temperature did not significantly affect the extraction process. Thus, the selection of temperature for aromatics and sulfur removal using the ILs would not be as critical as the earlier parameters,such as IL type and IL:Oil mass ratio. Other researchers reported a similar trend for the effect of temperature on the removal efficiency of different systems, such as dibenzothiophene + dodecane + ILs [25], n-Hexane + Toluene + ILs [26], and n-hexane + Toluene + TS + ILs [29]. At first, the temperature seems to exert a contradicting effect on the extraction process.Indeed,temperature should have a positive effect on the removal efficiency in the extraction process by decreasing the viscosity of both the ionic liquid and the oil, leading to better mass transfer performance. On the other hand, increasing temperature means increasing molecular motion, thus potential disruption of the interaction between the ionic liquid and the solute, which could decrease the removal efficiency of the extraction process [50]. The outcome of the two effects of the temperature determines the effect of temperature on removal efficiency. In this work, the Polyalphaolefin base oil had a viscosity of 4.8 cP at 100  C and 25.16 cP at 40  C, and a high viscosity index of 137.This indicated that the viscosity of the base oil did not vary much over the temperature range of the extraction experiments, which was between 40  C and 110  C [37]. [BMPY][DCA], [BMIM][DCA] and [BMIM][SCN] ILs have low viscosities of 32.1 cP [51], 29.3 cP [26] and 51.7 cP [52] at 25  C respectively.Likewise, their viscosity will not vary considerably over the explored temperature range.This behavior would reduce the consequences of the impact of viscosity changes with temperature on removal efficiency. Their removal efficiency slightly decreased with increasing temperature. [BMIM][DMP] IL displayed a much higher viscosity compared to other ILsand base oil. In this case, temperature significantly influenced its viscosity [11,53]. Gong et al. [54] reported that its viscosity dropped from 584.740 cP to 77.252 cP as the temperature changed from 25  C to 60  C. This might explain the slight increment in sulfur removal performance with temperature increase for that IL. Indeed, the dramatic decrease of viscosity as the temperature increased within the studied range 40  C–110  C, might lead to enhanced mass transfer and removal efficiency. The removal performance would be particularly sensitive to changes in temperature when the solute-solvent interactions are specifically ion-pair bonds or hydrogen bonds interactions, because these interactions are strongly temperature-dependent [50]. The interaction in betweenpyridinium and imidazolium ionic liquids with the aromatics solutes in this work is most probably of p-ionic nature, which is weakly dependent on temperature as demonstrated by Meindersma et al. [20] for the extraction of aromatics from its mixture with aliphatic hydrocarbons by ILs.

The response surface plots in Figs. 3 and 4 confirmed that the aromatics and sulfur removal efficiencies did not change much within the temperature range of 40  C–110  C, over the IL:Oil ratio range of 0.4–1.8. This observation suggested that the extraction temperature was not a key parameter affecting the aromatics and sulfur extraction efficiency. According to the experiments conducted, the regression models (that incorporate the two independent factors, i.e. A = Temperature and B = IL:Oil mass ratio as the process variables, and relating them to the performance of the extraction, i.e. Y A = % aromatics removed and Y S = % sulfur removed by extraction with ionic liquid [BMPY] [DCA], [BMIM][DCA], [BMIM][SCN], and [BMIM][DMP]) in terms of the actual factors are expressed by the equations listed in Table 3, along with the analysis of the regression models (Table 3). The results of the statistical analysis conducted on the regression models are given in Table 4, for the four ILs. The determination coefficient R2 values are 0.9997, 0.9974, 0.9993 and 0.9998 for aromatics removal, and 0.9965, 0.9640,0.9493 and 0.8120 for sulfur removal. The value of R2 closer to 1 indicates higher significance of the model and the better prediction of the response; R2 = 0.9979 indicates that only 0.0021% of the total variable is not explained by the model (Table 3). 4. Conclusions TheILs[BMPY][DCA], [BMIM][DCA], [BMIM][SCN] and [BMIM] [DMP] were identified as potential solvents for extraction of naphthalene and dibenzothiophene from base oil.This finding provides opportunities for novel applications in the lubricating oil industry involving ILs as a green solvent.Among tried ILs, [BMPY] [DCA] was the most promising with aromatics and sulfur removal of 83.1% and 94.3%, respectively. This enhanced performance compared to imidazolium analogues could be due to favored and enhanced p-p interactions in between the aromatics rings of the solutes and ofpyridinium. Finally, results clearly indicated that removal of both aromatics and sulfur was barely sensitive to temperature, which may allow industrial applications to conduct extraction within an ambient range.. Acknowledgement This work supported by center of research in ionic liquids (Universiti Teknologi PETRONAS). References [1] A. Gruia, Distillate hydrocracking, in: D.J.S. Jones, P. Pujadó (Eds.), Handbook of Petroleum Processing, Springer, Netherlands, 2006, pp. 287–320. [2] A. Gruia, Hydrotreating, in: D.J.S. Jones, P. Pujadó (Eds.), Handbook of Petroleum Processing, Springer, Netherlands, 2006, pp. 321–354.

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Please cite this article in press as: G.M.J. Al Kaisy, et al., Liquid-Liquid extraction of aromatics and sulfur compounds from base oil using ionic liquids, J. Environ. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.jece.2016.11.011