Choline Chloride-Based Deep Eutectic Solvents in the

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Choline Chloride-based Deep Eutectic Solvents in the Dearomatization of Gasolines Marcos Larriba, Miguel Ayuso, Pablo Navarro, Noemi DelgadoMellado, Maria Gonzalez-Miquel, Julian Garcia, and Francisco Rodríguez ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03362 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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ACS Sustainable Chemistry & Engineering

Choline Chloride-based Deep Eutectic Solvents in the Dearomatization of Gasolines Marcos Larriba,a Miguel Ayuso,b Pablo Navarro,c Noemí Delgado-Mellado,b Maria GonzalezMiquel,d Julián García,b,* and Francisco Rodríguezb a

Sección de Ingeniería Química, Universidad Autónoma de Madrid, c/ Francisco Tomás y Valiente,7. E–28049 Madrid, Spain.

b

Department of Chemical Engineering, Avda. Complutense s/n, Complutense University of Madrid, E–28040 Madrid, Spain.

c

CICECO – Aveiro Institute of Materials, Department of Chemistry, Campus Universitário de Santiago, University of Aveiro, Aveiro, Portugal d

School of Chemical Engineering and Analytical Science, The Mill, Sackville St., The University of Manchester, Manchester M13 9PL, UK.

Abstract The extraction of aromatic hydrocarbons from reformer and pyrolysis gasolines is currently performed by liquid-liquid extraction using organic solvents. Deep eutectic solvents (DES) are being widely studied as environmentally benign alternatives to conventional solvents since DES can be prepared using nontoxic and renewable chemicals. In this work, we have studied for the first time the application of DES in the extraction of aromatic hydrocarbons from reformer and pyrolysis gasolines. We have tested six choline chloride-based DES formed by ethylene glycol, glycerol, levulinic acid, phenylacetic acid, malonic acid, and urea as hydrogen bond donors. COSMO-RS method was employed to predict the performance of the DES in the extraction of aromatics, whereas experimental results indicate that DES formed by choline chloride and levulinic acid has exhibited the most adequate extractive and physical properties. Afterward, the simulation and optimization of the whole process for extraction of aromatics, recovery of extracted hydrocarbons, and regeneration of the solvent have been performed. The proposed process of dearomatization could work at moderate temperatures using a cheap, sustainable, and nontoxic solvent.

Keywords: Extraction of aromatic hydrocarbons, Reformer and pyrolysis gasolines, Deep eutectic solvents, COSMO-RS, Process simulation.

* Corresponding author. Tel.: +34 91 394 51 19; Fax: +34 91 394 42 43. E–mail address: [email protected] (Julián García).

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Introduction Pyrolysis and reformer gasolines are the main sources of benzene, toluene, ethylbenzene, and xylenes (BTEX).1 The separation of these aromatic hydrocarbons from alkanes in these refinery streams is not performed by conventional distillation because of the proximity between the boiling points of the hydrocarbons and the formation of azeotropes.2 For that reason, the dearomatization of pyrolysis and reformer gasolines is currently performed by liquid-liquid extraction using organic solvents such as sulfolane or diethylene glycol.1,3 However, the industrial processes of liquid-liquid extraction of aromatics have several drawbacks related to the solvent volatility and its partial solubility in hydrocarbons.4 Because of this, ionic liquids and more recently deep eutectic solvents (DES) are being tested as potential replacements of organic solvents.5-12 These new solvents have negligible vapor pressure and their solubilities in alkanes are under the detection limit of the most-advanced analytical techniques and, therefore, their use could considerably simplify the extraction process of aromatic hydrocarbons.4 DES are liquid solvents formed by an eutectic mixture of Brønsted or Lewis acids and usually are composed by a quaternary ammonium salt and an hydrogen bond donor (HBD).13 DES have similarities with ionic liquids, because their physicochemical properties can be modified by changing the structure of the hydrogen bond donor or acceptor and they have a very low vapor pressure and are non-flammable.13,14 However, DES have several advantages with respect to ionic liquids, because the eutectic solvents can be easily prepared from cheap, natural, renewable, and nontoxic chemicals.15 In the last few years, several papers on the application of DES in the extraction of aromatic hydrocarbons from alkanes have been published.6-9,16,17 These studies have been focused on the extraction of an aromatic hydrocarbon from an alkane. In this paper, we have studied the simultaneous extraction of benzene, toluene, ethylbenzene, and p-xylene from reformer and pyrolysis gasoline models to evaluate the real applicability of the DES in an industrial aromatic extraction process. We have selected choline chloride as hydrogen bond acceptor (HBA) in the DES used in this work because this chemical is nontoxic and tons of choline chloride are annually produced to be used as animal feed supplement and, therefore, its cost is moderate.13 As HBD, we have chosen ethylene glycol, glycerol, levulinic acid, phenylacetic acid, malonic acid, and urea since these HBD form DES with freezing temperatures lower than room temperature and could be used in liquid-liquid extraction process at moderate temperatures.18 The structure of


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HBD and HBA and the molar ratios of the seven DES tested are showed Table 1. The selected HBA:HBD molar ratio are those of the eutectic points reported by Abbott et al. (2014).13 However, we have also decided to use the DES formed by choline chloride and levulinic acid with a HBA:HBD molar ratio of 1:3 because Hou et al. found better extractive properties as the levulinic acid content increased in the DES formed by tetrabutylphosphonium bromide and levulinic acid in the separation of toluene from n-hexane.9 In fact, the HBA:HBD molar ratio has been reported to have a substantial effect on the energy required for DES formation due to the intermolecular interaction competition between the like molecules of the HBD neat solvent and the unlike HBA-HBD molecules for DES complexation.19 An initial screening of the performance of the seven DES in the extraction of aromatics was made by measuring the solubility of toluene in the DES with the estimations provided by the quantum chemistry-based Conductor-like Screening Model for Real Solvents (COSMORS) method.20 Subsequently, the extractive properties of the DES were determined in the liquid-liquid extraction of BTEX from reformer and pyrolysis gasolines. From the experimental results, a novel process to perform the dearomatization of gasolines using the most promising DES as solvents was simulated and optimized. The scheme flow of this process is graphically showed in Figure 1, being formed by an extraction column and a flash distillation unit. The liquid-liquid extraction and vapor-liquid separation were experimentally studied to simulate the whole process. Experimental Section Chemicals. Preparation of deep eutectic solvents. In Table S1 in Supplementary Information, suppliers, purities, and analysis methods of the chemicals used in this work are listed. To perform the preparation of the DES, choline chloride was firstly dried under vacuum in a Büchi Glass Oven B-585 connected to a Büchi Vacuum Pump V-700 for 24 h at (323 ± 1) K, 10 kPa, and 50 rpm. Then, the HBD was gravimetrically added using a Mettler Toledo XS 205 balance with a precision of ± 1·10−5 g to the Büchi Glass Oven B-585 in a glove box under a dry nitrogen atmosphere to avoid water absorption. The mixture was heated and stirred until a clear and homogenous liquid was formed. The following DES were prepared at 333 K: ChCl : EG (1:2), ChCl : Glyce (1:2), ChCl : Lev (1:2), ChCl : Lev (1:3), ChCl : Mal (1:1), and ChCl : Urea (1:2), whereas the DES ChCl : Phenylac (1:2) was prepared at 363 K. Once the DES were formed, they were dried in situ in the Büchi Glass Oven B-585 for 24 h at (323 ± 1) K, 10 kPa, and 50 rpm. The water content of DES was measured in a Mettler Toledo DL31 Karl Fischer Titrator, being lower than 0.5 %, just before 3 ACS Paragon Plus Environment


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their use. In addition, handling of DES was also performed in a glove box under dry nitrogen to maintain constant their water content. COSMO-RS calculations of toluene solubility in DES. COSMO-RS was used as a priori tool to select DES in the extraction of aromatics. COSMO-RS calculations were performed using the software COSMOtherm, version C3.0, release 14.01,21,22 following the standard procedure detailed in previous works.23,24 In all cases, the quantum chemical calculations were performed using the continuum solvation COSMO model at the BVP86/TZVP/DGA1 computational level of theory and the corresponding parametrization (BP_TZVP_ C30_1401) was chosen to perform the COSMO-RS calculations. All DES have been implemented in COSMOthermX following an electro-neutral approach, where DES-forming compounds (i.e. HBA and HBD) have been treated as two different compounds in an stoichiometric mixture, as suggested in previous works for related solvents.8,25-27 Toluene solubilities in the DES employed in this work were calculated at 313.2 K and compared with experimentally determined solubilities. Extraction of BTEX from reformer and pyrolysis gasolines using DES. To evaluate the performance of the DES in the dearomatization of reformer and pyrolysis gasolines, models of both gasolines were prepared considering the typical compositions of the streams.1 The reformer gasoline model was formed by: benzene (5 wt.%), toluene (24 wt.%), ethylbenzene (4 wt.%), p-xylene (22 wt.%), n-hexane (15 wt.%), n-heptane (15 wt.%), and n-octane (15 wt.%). On the other hand, the pyrolysis gasoline model had an aromatic content of 66.1 wt % and the following composition: benzene (33.8 wt.%), toluene (19.3 wt.%), p-xylene (13.0 wt.%), n-hexane (11.3 wt.%), n-heptane (11.3 wt.%), and n-octane (11.3 wt.%). To select the DES with better extractive properties in the dearomatization of the gasolines, liquid-liquid extraction experiments using DES as solvents were made at 313.2 K and a solvent to feed ratio (S/F) in mass of 1.0. Gasoline model and DES were added to 8 mL vials using a Mettler Toledo XS 205 balance. The vials were placed in a Labnet Vortemp 1550 shaking incubator for 5 h at (313.2 ± 0.5) K to reach the liquid-liquid equilibrium. The separation of the phases was achieved in a Labnet Accublock dry bath for 12 h at (313.2 ± 0.3) K. To determine the toluene mole fraction in the extract phase, a multiple headspace extraction (MHE) method was employed. Triplicate samples of 100 µL from extract were added to 20 mL closed vials. An Agilent 7890A GC coupled with a Headspace Sampler Agilent 7697A was used to analyze the samples from the extract phases. A more detailed description of this analytical method can be found in our previous publication.28 Triplicate samples from raffinate and extract phases were taken and measured to determine the 4 ACS Paragon Plus Environment

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experimental uncertainties. Mole fraction uncertainty of the raffinate phases analyzed by GC was estimated to be lower than ± 0.0003 whereas the uncertainty of the mole fraction in the extract phase determined by HS-GC technique was lower than ± 0.0004. Samples from the raffinate phases were also analyzed by Fourier Transform Infrared Spectroscopy (FTIR) to ensure that the DES solubility in the raffinate phases was undetectable. A Jasco FT/IR-4700 with a wavenumber measurement range from 7800 to 350 cm-1 was employed and signals corresponding to DES were not found in this analysis, so the DES mole fraction in the raffinate was assumed as zero. The experimental limit of detection of DES composition in the raffinate phase using the FTIR technique was 0.0005 in mole fraction. This limit of detection was determined analyzing by FITR synthetic binary mixtures of choline chloride and water. Rodríguez et al. (2015) and Gouveia et al. (2016) also found undetectable the solubility of DES in the raffinate phases obtained in the liquid-liquid extraction of toluene from n-heptane and Mulyono et al. in the extraction of toluene from noctane.6-8 The negligible solubility of the choline chloride-based DES in the raffinate is an important advantage with respect to conventional solvents such as sulfolane. Thermophysical characterization of the DES. Densities and viscosities of the choline chloride-based DES were measured to consider these physical properties in the selection of the most adequate DES. Densities were measured at temperatures from 293.15 to 353.15 ± 0.01 K using an Anton Paar DMA-5000 oscillating U-tube. Dynamic viscosities were measured in an Anton Paar Automated Micro Viscometer (AMVn) at the same temperatures. A comparison between experimental and literature densities and viscosities of the DES is made in the Results section. Optimization and simulation of the liquid-liquid extraction column. To optimize the extractor, extraction experiments for the best DES were made using both gasoline models at temperatures between 303.2 K and 323.2 K and S/F ratios from 1.0 to 9.0 in mass. The extraction column was simulated using the Kremser method with an iterative method implemented in Microsoft Excel.29 This method was successfully employed in our previous publications to simulate the extractor in the dearomatization of reformer and pyrolysis gasolines using ionic liquids.30,31 To simulate the columns, the extraction factor (Ei) and the reciprocal of E (Ui) of the Kremser method were calculated from the experimental hydrocarbon distribution ratios, whereas 1000 t/h of the reformer or pyrolysis gasoline models were used as basis of calculation. A number of equilibrium stages (Ns) of 20 was fixed in all simulations.



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Thermal stability. The thermal stability of the solvent must be studied to avoid the thermal decomposition of the solvent in the separation of the extracted hydrocarbons. For that reason, the thermal decomposition of the selected DES was determined by dynamic thermogravimetrical analysis (TGA) employing a Mettler Toledo TGA/DSC 1 at 10 K·min-1 under nitrogen atmosphere, the most widely used conditions in thermal stability evaluations.32 Vapor-liquid separation of the extracted hydrocarbons and solvent recovery. Considering the results obtained in the TGA analysis, a temperature range from 343.2 K to 363.2 K was set in the vapor-liquid separation of the extracted BTEX. The Headspace Gas Chromatography (HS-GC) technique was employed to study the effect of temperature on the vapor-liquid separation of the hydrocarbons and the DES. This method has been widely employed to determine the vapor-liquid equilibrium between hydrocarbons and ionic liquids in our previous publications.33,34 From the best of our knowledge, this paper is the first study on the vapor-liquid separation of aliphatic and aromatic hydrocarbons and a DES. Mixtures of aliphatic and aromatic hydrocarbons and the best DES were gravimetrically prepared with the composition of the extract streams obtained in the simulation of the extractors. An Agilent Headspace 7697A injector coupled to an Agilent GC 7890A with a flame ionization detector was employed to determine the vapor-liquid equilibrium. From the experimental K-values for each hydrocarbon, the flash distillation used to recover the hydrocarbons and the DES was simulated and designed to select the optimal values of temperature and pressure. To perform the simulation of the flash, we used an algorithm developed for mixtures with a high concentration of non-volatile compounds recently described in our last publication.34 Results and Discussion Selection of choline chloride-based deep eutectic solvents. Experimental results and predictions using COSMO-RS. As a first approach to evaluate the potential use of the seven choline chloride-based DES in the liquid-liquid extraction of aromatic hydrocarbons, the solubility of toluene in the DES was experimentally determined and predicted by COSMO-RS at 313.2 K. In Figure 2, experimental and predicted toluene solubilities in DES are plotted. As seen, COSMO-RS predicted the order of toluene solubilities in DES although the estimations were higher than the experimental values. Therefore, COSMO-RS seems to be a useful a priori tool to select DES with good extractive capacity but the COSMO-RS predictions of toluene solubility in DES have a significant uncertainty and experimental validation is required. As can be observed, the ChCl : Phenylac (1:2) showed the highest value of toluene 6 ACS Paragon Plus Environment

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experimental solubility followed by the ChCl : Lev (1:3) and ChCl : Lev (1:2). It is important to highlight that a higher content of levulinic acid in the DES caused an increase in toluene solubility. The DES formed by ethylene glycol, glycerol, malonic acid, and urea exhibited toluene solubilities considerably lower than those of the three DES previously cited. The toluene solubility in the ChCl : Lev (1:2) at 313.2 K was 0.0766, being very close to the maximum solubility of toluene in the same DES at 298.15 K (0.0746) published by Gouveia et al. (2016).8 From the experimental and predicted values by COSMO-RS of toluene solubility, the ChCl : Phenylac (1:2), ChCl : Lev (1:3), and ChCl : Lev (1:2) are the DES with highest potentials to be employed in the extraction of BTEX from petroleum streams. Experimental toluene solubilities are listed in Table S2 in Supporting Information. To determine the effect of adding ChCl to the HBD, mutual solubilities of toluene with levulinic acid, ethylene glycol, and glycerol at 313.2 K were also measured, since these HBD are liquid at the experimental temperature used in this work for the liquid-liquid extraction. The experimental results are also listed in Table S2 in the Supporting Information. As seen, toluene solubility in glycerol and ethylene glycol was slightly lower than those for the corresponding ChCl : HBD DES. In addition, glycerol and ethylene glycol were detected in the hydrocarbon-rich phase with a mole fraction of 0.009 and 0.013, respectively. It is important to underline that the DES were not detected in the hydrocarbon-rich phase. Therefore, the addition of ChCl has increased the toluene solubility in the DES and has reduced the solubility of the HBD in the raffinate phase. In the case of the levulinic acid, toluene and this HBD were completely miscible and only one liquid phase was obtained. The dearomatization of reformer and pyrolysis gasoline models was also evaluated using the DES with a solvent to feed ratio in mass of 1.0 at 313.2 K. From the experimental compositions of the extract and raffinate phases and considering the mass added to the vial for each hydrocarbon, yields of extraction of aromatics (Yldarom) and relative purities of extracted aromatic hydrocarbons in the extract phase (Parom) were calculated with the following expressions:

Yldarom (%) = 100

Parom (%) = 100

extract extract extract extract mbenz + mtol + metbenz + mp-xyl feed feed feed feed mbenz + mtol + metbenz + mp-xyl

extract extract extract extract wbenz + wtol + wetbenz + wp-xyl

(w

extract hexa

extract extract extract extract extract extract + whepta + wocta + wtol + wetbenz + wp-xyl ) + ( wbenz )

(1)

(2)

where mi indicates the mass flow of the aromatic in the feed or in the extract phase, whereas wi are the mass fraction of each hydrocarbon in the extract phase. Calculated values of Yldarom 7 ACS Paragon Plus Environment


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and Parom obtained in the dearomatization of reformer and pyrolysis gasolines are listed in Table S3 in the Supporting Information. Extractive properties were higher in the dearomatization of pyrolysis gasoline because this stream has a higher aromatic content with respect to the reformer gasoline model and also a higher benzene concentration. In Figure 3, a linear correlation between the predicted toluene solubilities in DES by COSMO-RS and the experimental yields of extraction of aromatic hydrocarbon from the pyrolysis gasoline is plotted. The highest values of experimental yields of extraction were obtained for the ChCl : Phenylac (1:2), ChCl : Lev (1:3), and ChCl : Lev (1:2). As seen, the order of yields of extraction of aromatics exhibited by the DES is the same that was predicted by COSMO-RS for toluene solubility. Therefore, to perform an initial screening predicting the solubility of toluene in a wide number of DES seems to be a good approach to select adequate DES to be applied in the extraction of aromatic hydrocarbons from reformer and pyrolysis gasolines. The yields of extraction of aromatics exhibited by the DES were lower than 10 % because the S/F ratio of 1.0 is considerably lower than the usual S/F ratios employed at industrial scale for the extraction of aromatics, which is usually between 4 and 10.1 Because of this, in the optimization of the extraction column made in the section 3.5 values of S/F higher than 1.0 were used. In Figure 4, the experimental values of Parom are also depicted. The ChCl : Phenylac (1:2) showed the highest Yldarom but the purity of the extracted BTEX was considerably lower than those for the other DES, which showed purities higher than 94 %. The purities of the BTEX achieved by the ChCl : Lev (1:2) and ChCl : Lev (1:3) DES were comparable to those for the other DES and the yields of extraction of aromatics were considerably higher than the values for the ChCl : EG (1:2), ChCl : Glyce (1:2), ChCl : Mal (1:1), and ChCl : Urea (1:2). Therefore, the DES formed by the levulinic acid seems to be the most adequate considering their extractive properties. Thermophysical characterization of choline chloride-based deep eutectic solvents. Additionally to the extractive properties, the densities and viscosities of the DES were also studied to select the most suitable DES to be employed in the dearomatization of reformer and pyrolysis gasolines. Experimental densities and viscosities as a function of temperature from 293.2 K to 353.2 K are listed in Table S4 in the Supplementary Information. The physical properties of the ChCl : Phenylac (1:2) were measured from 303.2 to 353.2 K because the freezing temperature of this DES is 298 K.13 A comparison with the literature values of the physical properties of the DES has been made. In the densities, the following absolute mean percent deviations were obtained: 0.07 % for ChCl : EG (1:2),35 0.29 % for ChCl : Glyce 8 ACS Paragon Plus Environment

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(1:2),36 0.12 % for ChCl : Lev (1:2),36 0.19 % for ChCl : Mal (1:1),36 and 0.05 % for ChCl : Urea (1:2).37 On the other hand, the percent deviations of dynamic viscosities were 0.36 % for ChCl : EG (1:2),35 1.74 % for ChCl : Glyce (1:2),36 2.12 % for ChCl : Lev (1:2),36 4.00 % for ChCl : Mal (1:1),36 and 4.97 % for ChCl : Urea (1:2).37 Higher deviations were found in the viscosity because the effect of impurities and measurement techniques on this property is greater than on density.38 Densities and viscosities are also plotted as a function of temperature in Figure S1 in Supporting Information. The densities were between 1.08 and 1.23 g·cm-3 and, therefore, the structure of the HBD had a slight influence on the density of the DES. Densities for the pyrolysis and reformer gasoline models used in this work were experimentally determined at 293 K obtaining a value of 0.767 g·cm-3 for the reformer gasoline and 0.795 g·cm-3 for the pyrolysis gasoline. Hence, the values of densities are adequate to be applied in an industrial process of extraction of aromatics because the difference of densities between the DES and the hydrocarbons forming the reformer and pyrolysis gasolines is enough to allow a fast phase separation in the extractor. Analyzing the dynamic viscosities, a strong effect of the HBD on this physical property was observed. The viscosities exhibited by the DES formed by malonic acid, urea, and glycerol were significantly higher than those of the commercial solvents used in aromatic extraction: diethylene glycol (30.2 mPa·s) or sulfolane (10.4 mPa·s) at 303.2 K.39 Thus, the potential use of these three DES at industrial scale would not be possible due to the high cost of mixing and pumping. The ChCl : Phenylac (1:2) previously showed good extractive properties in the dearomatization of reformer and pyrolysis gasolines but its viscosity (140.4 mPa·s at 313.2 K) could limit its use. Finally, the viscosity of ChCl : Lev (1:3) (65.5 mPa·s at 313.2 K) was substantially lower than that of ChCl : Lev (1:2) (91.3 mPa·s). Therefore, considering the good extractive and physical properties exhibited by the ChCl : Lev (1:3), this DES was selected as the best option to further optimize the liquid-liquid extraction of BTEX from reformer and pyrolysis gasolines. In addition to the extractive and physical properties of the selected DES, it is also important to underline that the levulinic acid is considered as a green chemical building block and this chemical has high availability, low price, and renewable origin.40 Optimization and simulation of the dearomatization of reformer and pyrolysis gasolines using Choline Chloride : Levulinic Acid (1:3) as solvent. To optimize the extraction of aromatic hydrocarbons from reformer and pyrolysis gasolines using ChCl : Lev (1:3), liquid-liquid extraction experiments were made at S/F ratios from 1.0 to 9.0 and at 9 ACS Paragon Plus Environment


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temperatures between 303.2 K and 323.2 K. Experimental values of extraction yield of aromatics and relative purity of the extracted BTEX are listed in Table S5 in Supporting Information. To analyze the effect of temperature and S/F ratio on the yield of extraction and purities of extracted aromatics from pyrolysis gasoline, the experimental values are plotted in Figure 5. As seen, the values of Yldarom significantly increased with the S/F ratio and a slight rise in the yield of extraction was observed as the temperature increased. The temperature of 323.2 K seems to be the optimal, because the Yldarom could be maximized and the viscosity of the DES would be reduced from 65.5 mPa·s at 313.2 K to 41.5 mPa·s at 323.2 K. A very slight decrease in the Parom was obtained with the increase of S/F ratio, whereas the temperature had a more significant negative effect on the purity of the extracted BTEX. This result can be due to the higher solubility of n-alkanes in the DES as the temperature rises. Results obtained for the reformer gasoline are also depicted Figure S2 in Supporting Information. The effect of temperature and S/F were analogous to those described for the pyrolysis gasoline. Values of Yldarom and Parom obtained in the extraction of BTEX from reformed gasoline were slightly lower than those using pyrolysis gasoline under the same conditions, as a result of higher aromatic and benzene contents in the pyrolysis gasoline. To select the most adequate value of S/F in the liquid-liquid extraction using ChCl : Lev (1:3), an extractor with 20 equilibrium stages was simulated by the Kremser method at the optimal temperature of 323.2 K and S/F ratios of 5.0, 7.0, and 9.0 using the experimental hydrocarbon distribution ratios at these temperature and S/F ratios. The Kremser method is valid to simulate a liquid-liquid extraction column if the phases are immiscible, for low solute content, and if the distribution coefficient is constant.41 In this case, the phases are immiscible and the solute content in the solvent is low as a result of the high S/F ratio employed in the simulations. In addition, the distribution coefficients were assumed as constant because the experimental aromatic and aliphatic distribution ratios were almost constant in the interval of temperatures and S/F ratios used in this work, as seen in Table S6 in Supporting Information. Flows and compositions of the raffinate and extract streams obtained in the simulations are listed in Tables S7 and S8 in the Supporting Information. Yields of extraction of benzene, toluene, p-xylene, and ethylbenzene were calculated along with the relative purity of the BTEX in the extract stream and plotted in Figure 6 for the pyrolysis gasoline. As seen, the extraction yields significantly increased with S/F ratio, whereas the purity of the BTEX was almost constant. Therefore, a S/F ratio of 9.0 seems to be the most adequate to obtain yields of extraction of benzene close to 100 % and maximize the extraction of toluene, p-xylene, and ethylbenzene. In an extraction column of 20 equilibrium stages and using the ChCl : Lev (1:3) 10 ACS Paragon Plus Environment

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as solvent at 323.2 K, the 99.6 wt. % of the benzene and the 96.4 wt. % of toluene could be extracted from the pyrolysis gasoline, obtaining an extract stream with a relative purity of BTEX of 97.4 wt. %. The almost complete recovery of benzene from the gasoline could imply an important application of this DES at industrial scale, since the most restrictive environmental regulations are focused on the benzene content in commercial gasolines.42 However, the recovery of the extracted BTEX and the regeneration of the DES must be also studied to evaluate the potential use of the ChCl : Lev (1:3). In Figure S3 in Supporting Information, results for the reformer gasoline are also plotted. As expected, better results of extraction yields and purity of the extracted hydrocarbons were obtained in the dearomatization of pyrolysis gasoline due to its higher aromatic content as well as the higher benzene concentration. Thermogravimetrical analysis of the Choline Chloride : Levulinic Acid (1:3). Before studying the recovery of the extracted BTEX, it is essential to determine the thermal stability of the solvent to avoid its thermal decomposition. In Figure S4 in Supporting Information, dynamic TGA-DTG curves obtained in the TGA analysis of the ChCl : Lev (1:3) at 10 Kmin1

are plotted. The onset temperature (Tonset) was 451.8 K, in agreement with the value

published by Florindo et al. (2014) of 449.7 K for the decomposition of ChCl : Lev (1:2).36 Taking into account that the dynamic TGA overestimates the real thermal stability of the solvents at long terms,32 we have decided to select a temperature of 363.2 K as the maximum temperature in the recovery of the extracted hydrocarbons to ensure the thermal stability of the DES in the whole process. Optimization and simulation of the recovery of the extracted hydrocarbons and solvent regeneration. The recovery of the extracted hydrocarbons was studied determining the vapor-liquid equilibrium between the hydrocarbons and the ChCl : Lev (1:3) at 343.2 K, 353.2 K, and 363.2 K with the composition of the extract stream obtained in the simulation of the extractors with a S/F ratio of 9.0. Experimental compositions of liquid and vapor phases in equilibrium are listed in Table S9 in Supporting Information. Vapor-liquid separation at temperatures up to 363.2 K was selected as an appropriate option to remove hydrocarbons from the solvent. In fact, although neat levulinic acid is volatile, it shows vapor pressures lower than 0.1 kPa at selected temperatures and hydrogen bonding with choline chloride clearly impacts on reducing even more its volatility.43 We have experimentally confirmed the negligible volatile character of the solvent up to 363.2 K because no peak areas of levulinic acid were found in the VLE experiments.



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From the experimental K-values for each hydrocarbon and employing the algorithm described in our previous publication,34 the simulation of the flash distillation was performed at experimental temperatures and pressures of 5 kPa, 10 kPa, and 15 kPa. The simulation of the flash distillation was done to calculate the hydrocarbon recovery from the solvent as a function of temperature and pressure. The experimental values of activity coefficient at each temperature were employed assuming that the effect of pressure or composition on the activity coefficient is low in the studied interval. This procedure is not completely rigorous from a thermodynamic perspective but, from an engineering point of view, this focus allows to calculate the hydrocarbon recovery from the DES with moderate deviations as is shown below. From the flows and compositions of the vapor and liquid streams obtained in the simulations, the recovery of aliphatics (Raliph) and aromatics (Rarom) in the vapor stream of the flash and the purity of the DES in the liquid stream (PuChCl:Lev (1:3)) were calculated as follows: Raliph (%) = 100

(m + m (m + m hexa

hepta

hexa

Rarom (%) = 100

+ mocta )

hepta

(m + m + m (m + m + m benz

tol

benz

PuChCl : Lev (1:3) (%) = 100

p -xyl

tol

vapor stream

+ mocta )

(3)

feed

+ metbenz )

vapor stream

p -xyl + metbenz )

(4)

feed

( wDES )liquid stream

(w

DES + whexa + whepta + wocta + wbenz + wtol + w p -xyl + wetbenz )

(5) liquid stream

In Figure S5 and S6 in Supporting Information, values of recovery of aromatic and aliphatic hydrocarbons and purity of the DES obtained in the flash corresponding to the dearomatization of the reformer and pyrolysis gasolines are plotted as a function of pressure and temperature. These simulations were done considering that hydrocarbon K-values are independent of liquid composition. This assumption was checked by comparing the activity coefficients observed for both extract streams studied and evaluating the impact of their differences on the hydrocarbon recoveries as the error bars depicted in Figures S5 and S6. The three parameters increased as the pressure decreased, since the vacuum facilitates the evaporation of the hydrocarbons from the solvent. The same explanation can be used to analyze the results with temperature, higher values of hydrocarbon recovery and purity of the solvent were obtained at the highest temperature (363.2 K), since the evaporation of the hydrocarbons are improved. Therefore, the optimal conditions to perform the recovery of the extracted hydrocarbons and the regeneration of the solvent were 363.2 K and 5 kPa, obtaining a value of Raliph of 98.0 %, a Rarom of 78.9 %, and a purity of the regenerated ChCl : Lev (1:3)


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of 99.2 wt. % for the reformer gasoline. In the case of pyrolysis gasoline, the recovery of the aliphatics was 98.9 %, the recovery of the aromatic hydrocarbons was 88.4 %, whereas the DES would be regenerated with a purity of 99.2 % at 363.2 K and 5 kPa. Therefore, the vapor-liquid separation of the extracted hydrocarbons was achieved at moderate temperatures and under vacuum, obtaining a liquid stream with the regenerated DES that could be recycled to the extraction column.

Conclusions In this work, we have tested the potential use of choline chloride-based DES in an industrial process of extraction of aromatic hydrocarbons from reformer and pyrolysis gasolines. The COSMO-RS methodology was successfully employed as a priori tool to evaluate the performance of the DES in the extraction of aromatics from reformer and pyrolysis gasolines. From the experimental extractive and physical properties of the DES, the ChCl : Lev (1:3) was selected as the most promising DES to perform the dearomatization of pyrolysis and reformer gasolines. The extraction column was simulated and optimized by the Kremser method using the experimental distribution ratios. At the optimal values of temperature and solvent to feed ratio of the extractor, the benzene could be almost completely separated from the gasoline and significant amounts of toluene, p-xylene, and ethylbenzene could be also selectively extracted. The ChCl : Lev (1:3) DES could be used as solvent to comply with the most restrictive regulations about the benzene content in commercial gasoline, since an almost complete extraction of benzene would be achieved. The vapor-liquid separation of the extracted hydrocarbons from the DES was also experimentally determined and simulated ensuring the thermal stability of the solvent and its regeneration. Considering the results obtained, the DES formed by the choline chloride and the levulinic acid with a molar ratio of 1:3 has been revealed as a sustainable, cheap, and nontoxic potential replacement of organic solvents currently used in the extraction of aromatic hydrocarbons.

Supporting Information. Description of chemicals, experimental toluene solubilities, experimental yield of extraction of aromatics and relative purities of the extracted aromatics using DES, densities and viscosities of the DES, experimental aliphatic and aromatic distribution ratios in the dearomatization of reformer and pyrolysis gasolines using ChCl : Lev (1:3), flows and compositions obtained in the simulation of the extractor by the Kremser method, vapor-liquid equilibrium data for the flash distillation unit design, dynamic TGA for the ChCl : Lev (1:3),



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recovery of aromatic and aliphatic hydrocarbons in the vapor phase in the simulation of the flash distillation.

Acknowledgments The authors are grateful to Ministerio de Economía y Competitividad (MINECO) of Spain and Comunidad Autónoma de Madrid for financial support of Projects CTQ2014–53655-R and S2013/MAE-2800, respectively. N.D.M. thanks MINECO for her FPI grant (Reference BES–2015–072855) and M.L. also thanks MINECO for his Juan de la Cierva-Formación Contract (Reference FJCI-2015-25343). P.N. thanks Fundação para a Ciência e a Tecnologia for awarding him a postdoctoral grant (Reference SFRH/BPD/117084/2016).


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Table 1. Hydrogen Bond Acceptor and Hydrogen Bond Donors Employed in the Preparation of Deep Eutectic Solvents: Abbreviations and Molar Ratios Hydrogen bond acceptor (HBA)

Hydrogen bond donors (HBD)

Abbreviations and (HBA : HBD) molar ratios

Ethylene glycol

ChCl : EG (1:2)

Glycerol

ChCl : Glyce (1:2) ChCl : Lev (1:2)

Levulinic acid

ChCl : Lev (1:3)

Phenylacetic acid

ChCl : Phenylac (1:2)

Malonic acid

ChCl : Mal (1:1)

Urea

ChCl : Urea (1:2)

Choline Chloride



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Figure 1. Flow diagram of the dearomatization process of reformer or pyrolysis gasolines using deep eutectic solvents.


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Figure 2. Toluene mole-based solubilities in choline chloride-based DES at 313.2 K.



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Figure 3. Experimental yield of extraction of aromatics from pyrolysis gasoline at 313.2 K and a S/F ratio in mass of 1.0 against toluene solubilities in DES predicted by COSMO-RS at the same temperature using several choline chloride-based DES. Dashed line is a linear correlation.


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Figure 4. Relative purities of the extracted aromatics in the dearomatization of reformer gasoline and pyrolysis gasoline at 313.2 K and a S/F ratio in mass of 1.0 using several deep eutectic solvents.



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Figure 5. Experimental yields of extraction of aromatics and relative purities of the extracted aromatics in the dearomatization of pyrolysis gasoline as a function of S/F ratio in mass using ChCl : Lev (1:3) as solvent.


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Figure 6. Yields of extraction of aromatic hydrocarbons and relative purity of the extracted aromatics obtained in the simulation of the countercurrent liquid-liquid extractor for the dearomatization of pyrolysis gasoline as a function of S/F ratio in mass using ChCl : Lev (1:3) as solvent at 323.2 K and Ns = 20.



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TOC/Abstract Graphic

Synopsis The deep eutectic solvent formed by choline chloride and levulinic acid has been revealed as a sustainable, cheap, and nontoxic replacement of organic solvents used in the extraction of aromatic hydrocarbons


Choline Chloride-Based Deep Eutectic Solvents in the

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