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Evaluation of hybrid solvents featuring choline chloride-based deep eutectic solvents and ethanol as extractants for the liquid–liquid extraction of benzene from n-hexane: towards a green and sustainable paradigm

Evaluation of hybrid solvents featuring choline chloride-based deep eutectic solvents and ethanol... Deep eutectic solvents (DESs) have high viscosities, but known to be mitigated by addition of suitable co-solvent. The effect of such co-solvent on the extraction efficiency of the hybrid solvent is hardly known. This study examined the effect of ethanol on three choline chloride-based DESs (glyceline, reline, and ethaline) by mixing each in turn with ethanol in various volume proportions. The hybrid solvents were evaluated for the extraction of benzene from n-hexane. Pseudo-ternary liquid–liquid equilibrium data were obtained using the refractive index method at 303 K and 1 atm for the systems, n-hexane (1) + benzene (2) + hybrid solvent (glyceline/ethanol, ethaline/ethanol, reline/ethanol) (3), and used to evaluate distribution coefficient (D ) and selectivity (S). Furthermore, the physicochemical properties of the hybrid solvents were also determined. The results indicate increase in selectivity with increasing ethanol addition up to 50% and decrease with further addition. All hybrid solvents with 50% ethanol outperform sulfolane and are suitable replacement for same as green and sustainable extractant for aromatics from aliphatics. The glyceline + 50% ethanol emerged the overall best with 49.73% elevation in selectivity and 41.15% reduction in viscosity relative to the neat glyceline. The finding of this study is expected to fillip the drive for paradigm shift in petrochemical industries. Keywords Glyceline · Ethaline · Reline · Ethanol · Distribution coefficient · Separation factor · Viscosity Introduction such as equilibrium data and derived parameters like selec- tivity (S) and distribution coefficient (D ) enable determina- Thermodynamic and transport properties are needed for tion of equipment size and solvent consumption rate. On the design and operation of process and products in diverse the other hand, transport property such as viscosity fixes the areas, more so in Chemical and Petroleum Engineering. A hydrodynamics, mixing and flow issues. In the petrochemi- triangular collaboration between the academia, industries cal industries, aromatic production from various sources and software providers, to address challenges in this regard like pyrolysis gasoline, reformate or naphtha accounts for has been re-emphasized by a recent survey conducted on 40 million metric tons of benzene, 40 million metric tons industry practitioners [1]. A key finding of the survey is the of xylenes and 20 million metric tons of toluene per annum complementary roles of experimental data and model devel- globally [2]. A critical stage of this production process is opment which should not be jettisoned. For example, in a the liquid–liquid extraction of the aromatics from the non- liquid–liquid extraction process, thermodynamic properties aromatic (or aliphatic) media. The choice of solvent with suitable values of thermodynamic and transport properties is crucial and bears overarching influence on the economic * Mohammed Awwalu Usman viability and sustainability of the process. musman@unilag.edu.ng; mawwal04@yahoo.com Deep eutectic solvents (DESs) are mixtures characterized 1 by remarkable depressions in melting point relative to the Sustainable Process Technology Group, Process Systems Engineering Cluster, Department of Chemical and Petroleum constituents and having tunable physicochemical properties. Engineering, University of Lagos, Akoka, Yaba 101017, It has continued to attract research attention in diverse area Lagos, Nigeria Vol.:(0123456789) 1 3 336 Applied Petrochemical Research (2021) 11:335–351 of application. A comprehensive review of DESs, funda- in the evaluation of ionic liquids/deep eutectic solvents as mentals and applications can be found in the literature [3, extractants for separating aromatics from aliphatic hydrocar- 4]. More recent applications include biomass pre-treatment bons, with results that are in good agreement with experi- [5, 6], media for enzymatic hydrolysis [7, 8], platform for mental findings [22– 24]. The green credentials of DESs, lipase extraction [9], biodiesel production and purification particularly glyceline, ethaline and reline, have been well [10, 11], inhibiting shale hydration [12] and COVID-19 established by various studies [25, 26]. intervention [13]. In the specific area of solvent extraction, However, the undesirable transport property (high viscos- a few recent studies are worthy of review. Wojeicchowski ity) of DESs is a huge challenge to its industrial application. et al. [14] explored the capacity of deep eutectic solvents to For example, the viscosities of glyceline, ethaline, reline and extract phenolics from rosemary leaves. The results indicate sulfolane are 342.12 cP, 38.52 cP, 667.28 cP [21], and 10.35 that the DES, choline chloride: 1,2-propanediol, at the opti- cP [27] at 303.15 K, respectively. Zheng et al. [28] pos- mal conditions (65 °C, liquid:solid ratio of 40:1 and 5.0% ited that mixing of DESs with molecular solvents (volatile wt of water), achieved a 39–51% inhibition of antimicrobial organic solvents) can help overcome the drawbacks of neat activity of extract to all tested bacteria. Petracic et al. [15] DESs and thus extend the practical or industrial application investigated deep eutectic solvents as extractants to reduce of DESs. In this regard, several studies have been conducted the free fatty acid content of feedstocks for biodiesel pro- to mitigate the viscosity of DESs by blending with organic duction in a liquid–liquid extraction process. The results solvents. Some of the organic solvents explored include show that the acidity of waste animal fat was significantly methanol [29], ethanol [30, 31], dimethyl sulfoxide [32]. reduced. Lemaoui et al. [16] studied the application of deep The considerably lower viscosities of the organic solvents eutectic solvents as extractants in the simultaneous de-aro- in comparison with DESs or ionic liquids (ILs) suggest that matization, desulfurization and denitrogenation of diesel in the former should have a thinning effect on the latter. This a liquid–liquid extraction process. The results showed that was corroborated in the aforementioned studies as viscosi- 100% removal of pyrrole and pyridine can be achieved in 2 ties of the mixed solvents plummet further with increase in stages. Rezaee et al. [17] investigated the use of deep eutec- the proportion of organic solvent. Ethanol is expected to tic solvents as extractants in the liquid–liquid extraction to exert viscosity reduction on DESs giving its much lower remove dibenzothiophene from model fuel (n-octane). The viscosity of 0.983 cP. result indicates significant removal of the sulfur-containing Traditionally, ethanol is produced from biomass in a pro- compound. duction chain that encompasses some or all of the following The superior performance of DESs as extractants for aro- steps: pre-treatment, hydrolysis, enzymatic fermentation and matics from aliphatic media over the conventional organic purification, depending on the feedstock. The purification solvent (sulfolane) has been well reported in the literature. stage, conventionally done by distillation, is characterized For example, Shekaari et al. [18] reported a maximum selec- by high-energy consumption and inefficiency. Azeotropy of tivity (S ) of 52.4197 for DES (choline chloride: diglycola- the ethanol–water mixture is an additional contributory fac- max mine, 1:5 molar ratio) as against 47.7704 for sulfolane in tor. These tend to detract from the green and sustainable the extraction of benzene from n-hexane at 303.15 K. Simi- indices of the process and that of ethanol. Thankfully, there larly, Usman et al. [19] reported a high value for selectivity are emerging technologies that ensure drastic knockdown on (S = 462.00) using glyceline (choline chloride:glycerol, energy consumption and guarantee improved efficiency of max 1:2 molar ratio) as extractant for separating benzene from the distillation process [33]. Other interventions focused on n-hexane. In a related study using ASPEN simulation, alternative separation methods, such as liquid–liquid extrac- Usman et al. [20] reported higher values of selectivity for tion [34, 35] and adsorption [36, 37]. All these restore con- glyceline (S = 378.283) and ethaline (S = 77.364) com- fidence in the green and sustainable credentials of ethanol. max max pared to sulfolane (S = 55.371) in the extraction of aro- Ethanol can therefore be rightly classified as green and max matics (benzene–toluene–xylene) from waste tire pyrolysis sustainable co-solvent to add to DESs for viscosity reduc- gasoline at 303.15 K and 1 atm. Further, using a mixture tion. However, the effect of such addition on the thermo- of glyceline and ethaline in the volume ratio 80:20, respec- dynamic properties (extraction performance for aromat- tively, as extractant for separating benzene from n-hexane, ics) of the resulting hybrid solvent has not been previously Usman et al. [21] reported the selectivity value of 422.485. explored, to the best of our knowledge. This study therefore These studies eloquently speak to the superiority of DESs to seeks to bridge the gap by mixing glyceline, ethaline and sulfolane in terms of thermodynamic properties. In addition reline in all volume proportions with ethanol to form three to the aforementioned experimental works, several molecu- categories of hybrid solvents (glyceline + ethanol, etha- lar dynamic simulation studies have also been carried out line + ethanol, and reline + ethanol). These hybrid solvents 1 3 Applied Petrochemical Research (2021) 11:335–351 337 were then evaluated as extractants for the extraction of ben- and pure ethanol (Et100). Overall, there were sixty-one (61) zene from n-hexane. The extraction efficiency is assessed solvents or extractants used for this study. The water con- using the thermodynamic parameters, benzene distribution tents in each DESs and hybrid solvents were determined by coefficient (D) and selectivity factor (S). The physicochemi- the method described in our previous article [19], the mass cal properties (density, viscosity and refractive index) of the fraction was ≤ 0.0003 for all studied solvents. extractants were also measured. Extraction and determination of LLE data Experimental The 61 solvents were each investigated for their extraction capacity; each was used as an extractant in the separation Materials of n-hexane + benzene mixture. The extraction process was done on a bench scale as described in our articles [19, 21]. Benzene, n-hexane, Choline chloride, ethylene glycol, urea, Measured volume of n-hexane + benzene mixture (feed) was glycerol and ethanol were purchased from Sigma-Aldrich contacted with hybrid solvent or solvent in 250 ml beaker. (Germany) with a mass fraction purity higher than 0.98. All The extraction runs were carried out in a vessel, the tempera- chemicals were used as received without further puric fi ation ture was controlled by a water bath at 303 K. After bringing and they were stored in a desiccator in their original tightly the feed to extraction temperature, the solvent was added at closed bottles. Table 1 shows the chemicals, CAS number the same temperature (according to the predetermined ratio). and purity. A rotating stainless steel shaft was used for mixing the feed and solvent at a controlled degree of mixing of 500 rpm. The Preparation of deep eutectic solvents and hybrid extraction was carried out for a predetermined mixing time solvents of 1 h and the mixture was left to separate into a raffinate phase (n-hexane-rich phase) at the top and an extract phase Three deep eutectic solvents were prepared in this study, (solvent-rich phase) at the bottom for a predetermined set- namely ethaline (choline chloride and ethylene glycol), tling time of 4 h. The extract was then separated and the glyceline (choline chloride and glycerol) and reline (choline equilibrium compositions of the phases were determined via chloride and urea). The quaternary ammonium salt (choline refractive index measurement. All experiments were dupli- chloride) was used as the hydrogen bond acceptor while cated and average values reported. urea, glycerol and ethylene glycol serve as the hydrogen bond donors in the molar ratio 1:2, respectively. The detailed protocol for preparing these DESs is explained in our previ- Determination of physicochemical properties ous articles [19, 21]. The hybrid solvents of these DESs were then prepared by mixing each DES with ethanol in varying Densities were measured using a density tube meter, the volume proportion and named appropriately. For example, viscosities were measured with a Brooksfield DV2T vis- G95Et5 means 95% glyceline and 5% ethanol; E80Et20 cometer. This viscometer was calibrated with distilled water. means 80% ethaline and 20% ethanol; R60Et40 means 60% Viscosity of the samples (η) was obtained under the follow- reline and 40% ethanol. A total of fifty-seven (57) DES–eth- ing conditions; the flow time of 60 s was used to measure anol hybrid solvents were so prepared and used for this study the flow time in the hybrid solvents or solvent, T = 303  K in addition to the three neat DESs (E100, G100, and R100) and a speed of 50 rpm. The estimated uncertainty of the experimental viscosity was ± 0.02 cP. The refractive index was measured with a digital refractometer (ATAGO DRA1, Table 1 Chemicals used in this study Japan) with an uncertainty of ± 0.001. The analytical balance (AND, GR202, Japan) with the Component Supplier CAS reg. no. Mass frac- tion purity precision of ± 0.0001 g was used for the preparation of mix- (%) tures in molar basis. The studied hybrid solvents were pre- pared in well-sealed glass vials to avoid contamination or Benzene Sigma-Aldrich 71-43-2 ≥ 99.5 mixture evaporation. Measurements were done continually n-Hexane Sigma-Aldrich 110-54-3 ≥ 99 after the mixtures preparation. The standard uncertainty of Ethanol Sigma-Aldrich 64-17-5 ≥ 99 solubility is 0.0014 and density is 0.001 g/cm . Choline chloride Sigma-Aldrich 67-48-1 ≥ 98 Ethylene glycol Sigma-Aldrich 107-21-1 ≥ 99 Glycerol Sigma-Aldrich 56-81-5 ≥ 99 Urea Sigma-Aldrich 57-13-6 ≥ 99 1 3 338 Applied Petrochemical Research (2021) 11:335–351 ethanol, and reline/ethanol). The physicochemical prop- Determination of performance parameters erties of the sixty-one (61) solvents or extractants are then presented in the third sub-section. The section is The performances of the extractants were evaluated based on two metrics, namely benzene distribution coefficient (D ) concluded with a general discussion and comparison of the studied extractants in the fourth subsection. and selectivity (S) as defined mathematically in Eqs. (1 –2). D = , Performance of neat ethanol (Et100, EtOH) (1) This sub-section presents the liquid–liquid equilibria data/ x ⋅ x 23 11 ternary diagram, distribution coefficients and selectivities S = , (2) x ⋅ x 21 13 for the extraction of benzene from n-hexane using neat etha- nol as the extractant. where x is the mole fraction of benzene in the extract (hybrid solvent) phase, x is the mole fraction of benzene LLE data and tie lines in the raffinate (n-hexane) phase, x is the mole fraction of n-hexane in the extract (hybrid solvent) phase and x Table  S1 (Supplementary information) shows the liq- is the mole fraction of n-hexane in the raffinate (hexane) uid–liquid equilibrium data for the ternary system n-hexane phase. (1) + benzene (2) + ethanol (3) at 303 K and 1 atm. These data are plotted in a ternary diagram as shown in Fig.  1. The biphasic region is clearly narrow, indicating a limited operation window for liquid–liquid extraction. The pair of Results and discussion n-hexane and ethanol shows partial miscibility to warrant recovery of ethanol from the raffinate with the attendant In this section, the results of the experimental studies energy expenditure and cost implication. It is instructive to and subsequent analyses are presented and thoroughly mention that this ternary diagram provides justification for discussed. The first sub-section presents and explains the blending of gasoline with ethanol (10% EtOH or 15% the performance of the neat ethanol (Et100, EtOH) while EtOH) as currently practiced in some countries of the world the second subsection presents and discusses the perfor- as such blend lie within the single-phase region. It also pro- mances of the hybrid solvents of DESs and ethanol under vides limit for such blending, as any blend that falls within the banner of the three pairs (ethaline/ethanol, glyceline/ Fig. 1 Experimental tie lines for the system n-hexane + ben- zene + ethanol (Et100, EtOH) at temperature 303 K and atmos- pheric pressure 1 3 Applied Petrochemical Research (2021) 11:335–351 339 the two-phase region would not be acceptable since it com- Ethaline + ethanol (E100–E5Et95) promises fuel function. There were twenty (20) extractants investigated in this study Distribution coefficients and selectivities involving ethaline, including the neat-ethaline (ChCl:EG 100%; E100) and nineteen (19) hybrid solvents of ethaline The benzene distribution coefficients (D) and selectivities and ethanol in volume proportions of ethanol ranging from (S) for the ternary system n-hexane (1) + benzene (2) + etha- 5 to 95% in incremental steps of 5% (E95Et5–E5Et95). This nol (3) at 303 K are presented in Table S1 and plotted in sub-section presents the liquid–liquid equilibria data, ternary Fig. 2 as a function of benzene composition in the extract diagrams, distribution coefficients and selectivities for the phase. The values of D vary from 0.593 to 0.769 while those extraction of benzene from n-hexane using these extractants. of S vary from 1.238 to 2.261 as the benzene composition in the extract phase increases from 0.0215 to 0.2497. In a LLE data and tie lines Table S2 presents the comprehensive related study, Gramajo et  al. [38] reported liquid–liquid experimental LLE data for the pseudo-ternary systems of equilibrium data for n-hexane + benzene + methanol sys- n-hexane (1) + benzene (2) + solvent (E100–E5Et95) (3). tem at 278.15 K in mass fractions from which evaluated For the sake of brevity, the ternary diagrams/tie lines for values of D range from 0.26 to 0.74 while S ranges from seven (7) of the twenty (20) systems are shown in Fig. 3a–g, 1.31 to 4.44. Thus, ethanol is a better extractant compared while the remaining ones are presented in Figure S1 (a–m). with methanol. However, it is clear that ethanol is a poor These seven are considered typical of behavior pattern of extractant for benzene when its S values are compared with the lot. At 0% EtOH, as shown in Fig. 3a, there is no mis- those of other organic solvents: sulfolane (2.7963–47.7704), cibility between ethaline and n-hexane, thus no DES in the N-formylmorpholine (2.8551–21.8382), and diglycolamine raffinate phase thereby obviating the need for solvent recov - (2.1985–47.1515) as reported by Shekaari et  al. [18] at ery from this phase. As the % EtOH addition increases in 303.15 K. the hybrid solvent, the miscibility improves (Fig. 3b–g). The two-phase region also decreases as the volume proportion of Performance of the hybrid solvents: DESs EtOH increases from 0% in Fig. 3a to 95% in Fig. 3g. Type and ethanol 1 behavior according to Treybal’s classification was exhib- ited by all the hybrid solvents [39]. The highly polar nature The extraction performance of the three hybrid solvent cat- of the studied DES may be responsible for its immiscibility egories: ethaline/ethanol, glyceline/ethanol and reline/etha- with n-hexane. Finally, the positive slopes of tie lines show nol is hereby presented. that benzene solubility in n-hexane is higher than its solubil- ity in the solvents. 0.8 2.4 Distribution coefficients and selectivities The benzene dis- tribution coefficients (D) obtained for the twenty pseudo- 0.8 2.2 ternary systems of n-hexane (1) + benzene (2) + hybrid sol- vent (E100–E5Et95) (3) are presented in Table S2. Figure 4 0.7 2.0 shows a plot of D versus benzene composition in the extract phase for the seven chosen systems. The D values increase 0.7 1.8 as % EtOH increase from 0 to 20%, but drop subsequently, 0.6 though still higher than the values at 0% EtOH, up to 60% EtOH. The values of D beyond 60% EtOH are less than the 1.6 0.6 values at 0% EtOH. The selectivity values for the twenty pseudo-ternary sys- 1.4 0.5 tems are shown in Table S2 and those of the chosen seven are plotted in Fig. 5. Generally, the S values decrease with 1.2 0.5 increasing composition of benzene in the extract phase for the seven extractants. Addition of EtOH significantly 0.4 1.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 improved the selectivity of the hybrid solvents relative to neat-ethaline up to 50% and thereafter it decreased very sharply particularly beyond 60% EtOH. The maximum value of S is in the following decreasing order: E50Et50 (50% Fig. 2 Benzene distribution coefficients and selectivities as a function EtOH, S = 48.421) ≻ E60Et40 (40% EtOH, S = 45.097) ≻ of benzene composition in the extract phase for the system n-hex- ane + benzene + EtOH at temperature 303 K and atmospheric pressure E80Et20 (20% EtOH, S = 36.539) ≻ Et100 (0% EtOH, S 1 3 340 Applied Petrochemical Research (2021) 11:335–351 1 3 Applied Petrochemical Research (2021) 11:335–351 341 ◂Fig. 3 a Experimental ternary diagram/tie lines for the n-hex- lines for seven (7) of the twenty (20) systems are shown in ane + benzene + E100 (ChCl:EG + 0%EtOH) at temperature 303  K Fig. 6a–g, while the remaining ones are presented in Fig- and atmospheric pressure. b Experimental ternary diagram/tie lines ure S2a–m. These seven are considered typical of behav- for the n-hexane + benzene + E80Et20 (ChCl:EG + 20%EtOH) ior pattern of the lot. At 0% EtOH, as shown in Fig.  6a, at temperature 303  K and atmospheric pressure. c: Experimen- tal ternary diagram/tie lines for the n-hexane + benzene + E60Et40 there is no miscibility between glyceline and n-hexane, (ChCl:EG + 40%EtOH) at temperature 303  K and atmospheric thus no DES in the raffinate phase, thereby obviating the pressure. d Experimental ternary diagram/tie lines for the n-hex- need for solvent recovery from this phase. As the % EtOH ane + benzene + E50Et50 (ChCl:EG + 50%EtOH) at temperature increases in the hybrid solvent, the miscibility improves 303 K and atmospheric pressure. e Experimental ternary diagram/tie lines for the n-hexane + benzene + E40Et60 (ChCl:EG + 60%EtOH) (Fig.  6b–g). The two-phase region also decreases as the at temperature 303  K and atmospheric pressure. f Experimen- volume proportion of EtOH increases from 0% in Fig. 6a tal ternary diagram/tie lines for the n-hexane + benzene + E20Et80 to 95% in Fig. 6g. Type 1 behavior according to Treybal’s (ChCl:EG + 80%EtOH) at temperature 303  K and atmospheric pres- classification was exhibited by all the extractants [39]. The sure. g Experimental ternary diagram/tie lines for the n-hexane + ben- zene + E5Et95 (ChCl:EG + 95%EtOH) at temperature 303  K and highly polar nature of the studied DES may be responsible atmospheric pressure for its immiscibility with n-hexane. The biphasic region decreases with increase in % MeOH. Finally, the positive = 31.440) ≻ E40Et60 (60% EtOH, S = 28.834) ≻ E20Et80 slopes of tie lines show that benzene solubility in n-hex- (80% EtOH, S = 13.323) ≻ E5Et95 (95% EtOH, S = 6.901). ane is higher than its solubility in the solvent. There is a 54.01% increase in the maximum value of S at 50% MeOH relative to the neat-ethaline. Thus, EtOH impart Distribution coefficients and  selectivities The ben- elevation in selectivity to ethaline up to 50% addition but a zene distribution coefficients (D ) obtained for the sharp attenuation beyond. Also worthy of note is the conver- twenty pseudo-ternary systems of n-hexane (1) + ben- gence of S values beyond 10% mole fraction of benzene in zene (2) + solvent (G100–G5Et95) (3) are presented in the extract irrespective of % EtOH. This signifies selectivity Table S3. Figure 7 shows a plot of D versus benzene com- becomes insensitive to EtOH addition when the composition position in the extract phase for the seven chosen systems. of benzene in the extract phase goes above 10%. Similar The D values of the hybrid solvents from 0 to 60% EtOH trend occurred in the extraction of benzene from n-hexane are generally ≥ 0.6 at all compositions of benzene in the using a binary mixed DES (glyceline/ethaline) as extractant extract phase. Attenuation of D values only becomes sig- [21]. Generally, selectivity tends to unity as the tie line tends nificant with EtOH content beyond 60%. toward the plait point, where the distribution coefficient of The selectivity values for the twenty pseudo-ternary solute and other components of the ternary mixture becomes systems are shown in Table  S3 and those of the cho- unity. Thus, the observed convergence of S values can be sen seven are plotted in Fig.  8. Generally, the S values attributed to the inability of the hybrid solvents to discrimi- decrease with increasing composition of benzene in the nate beyond 10% mole fraction of benzene as the S value extract phase for the seven extractants. Addition of EtOH has become very low. significantly improved the selectivity of the hybrid sol- vent relative to neat glyceline up to 60% and thereafter it Glyceline + ethanol (G100–G5Et95) decreased very sharply. There is a clear enhancement in the S up to 60%, even though the highest value occurs max There were twenty (20) extractants investigated in this study at 50% EtOH. The maximum value of S is in the follow- involving glyceline, including the neat glyceline (ChCl:Gly ing decreasing order: G50Et50 (50% EtOH, S = 692.070) 100%; G100) and nineteen (19) hybrid solvents of glyceline ≻ G60Et40 (40% EtOH, S = 662.413) ≻ G40Et60 (60% and ethanol in volume proportions of ethanol ranging from EtOH, S = 568.774) ≻ G80Et20 (20% EtOH, S = 526.833) 5 to 95% in incremental steps of 5% (G95M5–G5M95). This ≻ G100 (0% EtOH, S = 462.219) ≻ G20Et80 (80% EtOH, sub-section presents the liquid–liquid equilibria data, ternary S = 82.165) ≻ G5Et95 (95% EtOH, S = 9.787). There is a diagrams, distribution coefficients and selectivities for the 49.73% increase in the maximum value of S at 50% EtOH extraction of benzene from n-hexane using these extractants. relative to the neat glyceline. The decrease in S from 60 max to 80% is profoundly sharp, a drop of 85.55%, signifying LLE data and  tie lines Table  S3 presents the compre- sharp attenuation beyond 60% EtOH. Also worthy of note hensive experimental LLE data for the pseudo-ternary is the convergence of S values beyond 20% mole fraction systems of n-hexane (1) + benzene (2) + solvent (G100– of benzene in the extract irrespective of % EtOH. This G5Et95) (3). As earlier explained, the ternary diagrams/tie signifies selectivity becomes insensitive to EtOH addition 1 3 342 Applied Petrochemical Research (2021) 11:335–351 0% EtOH the distribution coefficient of solute and other components 20% EtOH of the ternary mixture becomes unity. Thus, the observed 0.9 40% EtOH convergence of S values can be attributed to the inability 50% EtOH of the hybrid solvents to discriminate beyond 20% mole 0.8 60% EtOH fraction of benzene as the S value has become very low. 80% EtOH 0.7 95% EtOH Reline + ethanol (R100–R5Et95) 0.6 There were twenty (20) extractants investigated in this cat- egory, including the neat-reline (ChCl:Ur 100%; R100) 0.5 and nineteen (19) hybrid solvents of reline and ethanol in volume proportions of ethanol ranging from 5 to 95% 0.4 in incremental steps of 5% (R95Et5–R5Et95). This sub- section presents the liquid–liquid equilibria data, ter- 0.3 nary diagrams, distribution coefficients and selectivities 0.0000 0.1000 0.2000 0.3000 0.4000 for the extraction of benzene from n-hexane using these extractants. Fig. 4 Benzene distribution coefficients as a function of benzene LLE data and tie line Table S4 presents the comprehensive composition in the extract phase for the system n-hexane + ben- experimental LLE data for the pseudo-ternary systems of zene + ethaline/EtOH at temperature 303 K and atmospheric pressure n-hexane (1) + benzene (2) + hybrid solvent (R100–R5Et95) (3). As earlier explained, the ternary diagrams/tie lines for 60.3 seven (7) of the twenty (20) systems are shown in Fig. 9a–g, while the remaining ones are presented in Figure S3a–m. 0% EtOH These seven are considered typical of behavior pattern of 50.3 20% EtOH the lot. At 0% EtOH, as shown in Fig. 9a, there is no misci- 40% EtOH bility between glyceline and n-hexane, thus no DES in the 40.3 50% EtOH raffinate phase, thereby obviating the need for solvent recov - 60% EtOH ery from this phase. As the % EtOH addition increases in 30.3 80% EtOH the hybrid solvent, the miscibility improves (Fig. 9b–g). The 95% EtOH two-phase region also decreases as the volume proportion of 20.3 EtOH increases from 0% in Fig. 9a to 95% in Fig. 9g. Type 1 behavior according to Treybal’s classification was exhibited 10.3 by all the mixed extractants [39]. The highly polar nature of the studied DES may be responsible for its immiscibility with n-hexane. The biphasic region decreases with increase 0.3 0.0000 0.1000 0.2000 0.3000 0.4000 in % EtOH. Finally, the positive slopes of tie lines show that x benzene solubility in n-hexane is higher than its solubility in the solvent. Fig. 5 Selectivities as a function of benzene composition in the Distribution coefficients and selectivities The benzene dis- extract phase for the system n-hexane + benzene + ethaline/EtOH at temperature 303 K and atmospheric pressure tribution coefficients (D) obtained for the twenty pseudo- ternary systems of n-hexane (1) + benzene (2) + mixed solvent (R100–R5Et95) (3) are presented in Table S4. Fig- when the composition of benzene in the extract phase goes ure 10 shows a plot of D versus benzene composition in the above 20%. Similar trend occurred in the extraction of extract phase for the seven chosen systems. The D values benzene from n-hexane using a binary mixed DES (glyce- of the hybrid solvents generally show a gradual increase line/reline) as extractant [21]. Generally, selectivity tends from % EtOH content of 0–80%, values lie between 0.25 to unity as the tie line tends toward the plait point, where and 0.45, with few outliers. There is a sharp increase in the 1 3 Applied Petrochemical Research (2021) 11:335–351 343 D values for 95% EtOH relative to others. This is largely the hybrid solvent is noticed as volume % of EtOH increases. because of the much higher values of D for ethanol com- This is evident from the sharp steep in negative slope of pared to neat-reline. viscosity profile in Fig.  12 and it is in agreement with the The selectivity values for the twenty pseudo-ternary sys- findings of similar work in this regard [29, 32]. For example, tems are shown in Table S4 and those of the chosen seven a decrease of 20.7% in viscosity was achieved with 20% are plotted in Fig. 11. Generally, the S values decrease with EtOH addition while the drop in viscosity at 50% EtOH is increasing composition of benzene in the extract phase 30.17%. The profile for density also shows decrease with for the seven extractants. Addition of EtOH significantly increasing volume % EtOH in the hybrid solvent but not as improved the selectivity of the hybrid solvents relative sharp as viscosity. On the other hand, the refractive index to neat-reline up to 60% and thereafter it decreased very shows almost constant values with increasing volume % of sharply. There is a clear enhancement in the S up to 60%, EtOH in the mixed solvent. max even though the highest value occurs at 50% EtOH. The maximum value of S is in the following decreasing order: Glyceline + ethanol R50Et50 (50% EtOH, S = 45.097) ≻ R60Et40 (40% EtOH, S = 41.450) ≻ R80Et20 (20% EtOH, S = 32.042) ≻ R40Et60 Table S6 (supplementary information) shows the density, (60% EtOH, S = 28.301) ≻ R100 (0% EtOH, S = 15.240) ≻ viscosity and refractive index of the hybrid solvents (glyce- R20Et80 (80% EtOH, S = 9.071) ≻ R5Et95 (95% EtOH, S = line/EtOH) as a function of volume % of EtOH. These values 5.096). There is a staggering 195.91% increase in the maxi- are plotted in Fig. 13. A significant decrease in viscosity of mum value of S at 50% EtOH relative to the neat-reline. The the hybrid solvent is noticed as volume % of EtOH increases. decrease in S from 60 to 80% is profoundly sharp, 67.95% This is evident from the sharp steep in negative slope of vis- max drop, signifying sharp attenuation beyond 60% EtOH. It is cosity profile in Fig.  13. For example, a decrease of 11.76% pertinent to note that at 10% benzene composition in the in viscosity was achieved with 20% EtOH addition while extract phase, the S value for 0% EtOH (neat-reline) is higher the drop in viscosity at 50% EtOH is 41.15%. The profile for than all hybrid solvents. Also worthy of note is the conver- density also shows decrease with increasing volume % EtOH gence of S values beyond 20% mole fraction of benzene in in the hybrid solvent but not as sharp as viscosity. On the the extract irrespective of % EtOH. This signifies selectivity other hand, the refractive index shows almost constant val- becomes insensitive to EtOH addition when the composition ues with increasing volume % of EtOH in the hybrid solvent. of benzene in the extract phase goes above 20%. Similar Similar findings have been reported in the literature [29, 40]. trend occurred in the extraction of benzene from n-hexane using a binary mixed DES (glyceline/reline) as extractant Reline + ethanol [21]. Generally, selectivity tends to unity as the tie line tends toward the plait point, where the distribution coefficient of Table S7 (supplementary information) shows the density, solute and other components of the ternary mixture becomes viscosity and refractive index of the hybrid solvents (reline/ unity. Thus, the observed convergence of S values can be EtOH) as a function of volume % of EtOH. These values are attributed to the inability of the hybrid solvents to discrimi- plotted in Fig. 14. A significant decrease in viscosity of the nate beyond 20% mole fraction of benzene as the S value mixed solvent is noticed as volume % of EtOH increases. has become very low. This is evident from the sharp steep in negative slope of vis- cosity profile in Fig.  14. For example, a decrease of 19.98% Physicochemical properties of hybrid solvents in viscosity was achieved with 20% EtOH addition while the drop in viscosity at 50% EtOH is 63.38%. The profile The physicochemical properties (density, viscosity and for density also shows decrease with increasing volume % refractive index) of the studied hybrid solvents as measured EtOH in the hybrid solvent but not as sharp as viscosity. On experimentally are presented and discussed in this section. the other hand, the refractive index shows almost constant The three pairs of hybrid solvents are analyzed in the fol- values with increasing volume % of EtOH in the hybrid sol- lowing sub-sections. vent. Similar findings have been reported in the literature [31, 41, 42]. Ethaline + ethanol General discussion and comparative analysis Table S5 (supplementary information) shows the density, viscosity and refractive index of the hybrid solvents (etha- The ternary diagram for the system n - h exa n e + b e n- line/EtOH) as a function of volume % of EtOH. These values zene + ethanol provides insight and beneficial guideline are plotted in Fig. 12. A significant decrease in viscosity of for blending gasoline with ethanol in what is now known 1 3 344 Applied Petrochemical Research (2021) 11:335–351 1 3 Applied Petrochemical Research (2021) 11:335–351 345 ◂Fig. 6 a Experimental ternary diagram/tie lines for the n-hex- and urea). For example, Naik et al. [44] reported a higher ane + benzene + G100 (ChCl:Gly + 0%EtOH) at temperature 303  K value of selectivity for ethylene glycol-based DES relative and atmospheric pressure. b Experimental ternary diagram/tie lines to the glycerol-based DES in the extraction of toluene from for the n-hexane + benzene + G80Et20 (ChCl:Gly + 20%EtOH) n-heptane. Similar finding was reported by Park [45] in the at temperature 303  K and atmospheric pressure. c Experimental ternary diagram/tie lines for the n-hexane + benzene + G60Et40 extraction of toluene from n-heptane using a ternary mixed (ChCl:Gly + 40%EtOH) at temperature 303  K and atmospheric solvent of choline chloride: urea: ethylene glycol/glycerol. pressure. d Experimental ternary diagram/tie lines for the n-hex- The sharp disparity or contrast between their findings and ane + benzene + G50Et50 (ChCl:Gly + 50%EtOH) at temperature ours may be attributed to the difference in the polarity of 303 K and atmospheric pressure. e Experimental ternary diagram/tie lines for the n-hexane + benzene + G40Et60 (ChCl:Gly + 60%EtOH) the aromatic (toluene/benzene), the hydrogen bond acceptor at temperature 303  K and atmospheric pressure. f Experimental (choline chloride/methyltriphenylphosphonium bromide), ternary diagram/tie lines for the n-hexane + benzene + G20Et80 and the hydrogen bond network between HBA and HBD (ChCl:Gly + 80%EtOH) at temperature 303  K and atmospheric pres- in each case. sure. g Experimental ternary diagram/tie lines for the n-hexane + ben- zene + G5Et95 (ChCl:Gly + 95%EtOH) at temperature 303  K and The performances of the solvents under study are best atmospheric pressure benchmarked against the conventional organic solvent commonly used in the industries for separating aromatics as gasohol. Such blending must lie within the single-phase from non-aromatics—sulfolane. It is however pertinent to region to remain uniform and deliver the requisite fuel func- make clarification on the seeming discrepancy between the tion. The two-phase region should be avoided; this is a key values of selectivity reported for sulfolane in two different contribution of this study. For the hybrid solvents, the expec- articles for the separation of benzene from n-hexane. Shek- tation is that the properties should lie between those of their aari et al. [18] reported S value of 47.7704 at 303.15 K max constituents. In consonance with this expectation, addition and 0.0865 MPa and another value of 36.3735 at 313.15 K of EtOH imparts viscosity reduction on DES since the vis- and 0.0865 MPa. This suggests that selectivity is sensitive cosity of the former is profoundly lower than that of the to both temperature and pressure. On the other hand, Guo latter. By the same reasoning, the selectivities of the hybrid et al. [46] reported S value of 42.38 at 303.15 K and 1 atm max solvents are expected to lie between those of the neat-DES (0.101325 MPa). Considering the pressure difference, the and ethanol. Since the selectivities of neat-DES (ethaline, values reported by the two contributions are in good agree- glyceline and reline) are much higher than that of ethanol, ment. Consequently, the comparative analysis is here based addition of EtOH to these DESs should lower their selec- on the same temperature (T = 303.15 K) and same pressure tivities. Figure 15 shows the maximum selectivities (S ) (P = 1 atm) as shown in Table  2. It is instructive to note max of the hybrid solvents as a function of volume % EtOH. A that both ethaline- and glyceline-based hybrid solvents have profound increase in S occurs as % EtOH increases up to benzene distribution coec ffi ient higher than that of sulfolane max 50% and decreases with further increase in EtOH content at 50% EtOH. The selectivities of all hybrid solvents at 50% for all hybrid solvents. This occurrence suggests a strong EtOH are higher than the values for sulfolane. These are intermolecular interaction between these DESs and EtOH remarkable enhancement for both ethaline- and reline-based as % EtOH increases to 50%, which dwindles with further hybrid solvents whose neat-DES underperforms sulfolane. addition of EtOH. The underpinning phenomenological The best hybrid solvent is glyceline + 50% EtOH (G50Et50), context for this novel performance enhancement should be having the highest selectivity value of 692.070, representing unraveled by spectroscopic studies. In a related investiga- 49.73% increase in S and with 41.15% reduction in viscosity tion involving two choline chloride-based DESs, Hadj-Kali relative to the neat glyceline. et al. [43] reported that addition of 50 wt% water breaks Table  2 also shows the performance of other solvents, the hydrogen bonding between the HBA (choline chloride) DESs and ionic liquids, used by other workers in the extrac- and HBD (urea and glycerol). This may well explain the tion of benzene from n-hexane. It is clear that the mixed finding of this study. It is also striking to note the remark - solvent G50Et50 outperformed the ionic liquids, 1-ethyl- ably superior performance of glyceline-based solvent rela- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide tive to the ethaline- and reline-based solvent in this study. ([EMIM][NTf ]), 1-ethyl-3-methylimidazolium ethylsulfate This trend was consistently demonstrated in all our previ- ([EMIM][EtSO ]) and their mixtures, with higher values of ous contributions [19–21]. It is however in sharp contrast D and S. In our previous contribution, the binary mixed to the findings in some related studies in terms of the role DES, 80% glyceline + 20% ethaline (G80E20), was consid- of the hydrogen bond donors (glycerol, ethylene glycol, ered the best in comparison to other mixed DESs evaluated in that study, with a decrease of 8.55% in selectivity and 1 3 346 Applied Petrochemical Research (2021) 11:335–351 1.0 Fig. 9 a Experimental ternary diagram/tie lines for the n-hex- ane + benzene + R100 (ChCl:Ur + 0%EtOH) at temperature 303  K and atmospheric pressure. b Experimental ternary diagram/tie 0.9 lines for the n-hexane + benzene + R80Et20 (ChCl:Ur + 20%EtOH) at temperature 303  K and atmospheric pressure. c Experimen- tal ternary diagram/tie lines for the n-hexane + benzene + R60Et40 0.8 (ChCl:Ur + 40%EtOH) at temperature 303  K and atmospheric 0% EtOH pressure. d Experimental ternary diagram/tie lines for the n-hex- ane + benzene + R50Et50 (ChCl:Ur + 50%EtOH) at temperature D 0.7 20% EtOH 303 K and atmospheric pressure. e Experimental ternary diagram/tie 40% EtOH lines for the n-hexane + benzene + R40Et60 (ChCl:Ur + 60%EtOH) at temperature 303  K and atmospheric pressure. f Experimental 0.6 50% EtOH ternary diagram/tie lines for the n-hexane + benzene + R20Et80 60% EtOH (ChCl:Ur + 80%EtOH) at temperature 303  K and atmospheric pres- 0.5 sure. g Experimental ternary diagram/tie lines for the n-hexane + ben- 80% EtOH zene + R5Et95 (ChCl:Ur + 95%EtOH) at temperature 303  K and 95% EtOH atmospheric pressure 0.4 0.0000 0.2000 0.4000 0.6000 0.8000 Viscosity was observed to generally decrease with increasing proportion of EtOH to the neat DESs (glyceline, ethaline and reline) in the hybrid solvent. The molecular Fig. 7 Benzene distribution coefficients as a function of benzene weight of HBD in the hybrid solvent seems to play signifi- composition in the extract phase for the system n-hexane + ben- cant role. The molecular weight of EtOH is 46, which is zene + glyceline/EtOH at temperature 303  K and atmospheric pres- sure lower compared to the molecular weights of the primary HBD in the hybrid solvents (glycerol: 92.09; urea: 60.06; and ethylene glycol: 62.07). Thus, increasing content of 800 EtOH, which results in lowering of the average molecular weight of the HBD in the hybrid solvent, causes decrease in viscosity. This is in contradiction to the findings of Al-Daw - 0% EtOH sari et al. [47], as they observed that the viscosities of DESs 20% EtOH increased as the molecular weight of the HBDs increased for 40% EtOH the same HBA. This disparity may be attributed to the altera- 50% EtOH tion in the strength and nature of hydrogen bond occasioned 60% EtOH by the secondary HBD (EtOH) in the current study. 80% EtOH On the overall basis, using D, S, and viscosity values, 95% EtOH G50Et50 is the best of the mixed solvents under study. Industrial replacement of sulfolane with G50Et50 for liq- uid–liquid extraction of aromatics from aliphatics will imply lower solvent requirement for the extraction and smaller equipment diameter due to higher D, fewer stages and reduced equipment height due to higher S, and higher cost of mixing and transport due to higher viscosity. It must be stated that all hybrid solvents with 50% EtOH can conveni- 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 ently replace sulfolane. Fig. 8 Selectivities as a function of benzene composition in the extract phase for the system n-hexane + benzene + glyceline/EtOH at Consistency of LLE data and thermodynamic temperature 303 K and atmospheric pressure modeling reduction of 9.41% in viscosity relative to neat glyceline The consistency and reliability of the LLE data were tested using both Othmer-Tobias [48] and Hand [49] correlations [21]. The performance of G50Et50 is superior to G80E20 not only in its higher values of D and S, but also in much as detailed in the supplementary information. As shown in Tables S9 and S10, the coefficient of determination, lower viscosity. 1 3 ◂ Applied Petrochemical Research (2021) 11:335–351 347 1 3 348 Applied Petrochemical Research (2021) 11:335–351 0.850 40 1.6 35 1.4 0.750 30 1.2 0.650 25 1 0% EtOH 0.550 20% EtOH 20 0.8 40% EtOH 0.450 15 0.6 Viscosity 50% EtOH 10 Density 0.4 60% EtOH 0.350 80% EtOH Refracve Index 5 0.2 0.250 95% EtOH 0 0 0.150 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 % EtOH Fig. 12 Viscosity, density and refractive index of ethaline/EtOH mixed solvent as a function of volume percent EtOH Fig. 10 Benzene distribution coefficients as a function of benzene composition in the extract phase for the system n-hexane + ben- zene + reline/EtOH at temperature 303 K and atmospheric pressure 350 1.6 1.4 0% EtOH 20% EtOH 1.2 40% EtOH 50% EtOH 1 60% EtOH 0.8 80% EtOH Viscosity 95% EtOH 150 0.6 Density Refracve Index 0.4 0.2 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 x 0 0 % EtOH Fig. 11 Selectivities as a function of benzene composition in the extract phase for the system n-hexane + benzene + reline/EtOH at temperature 303 K and atmospheric pressure Fig. 13 Viscosity, density and refractive index of glyceline/EtOH mixed solvent as a function of volume percent EtOH R > 0.99 for all mixing proportions of DES/EtOH. This clearly validates the consistency and reliability of the experimentally obtained LLE data. Also, thermodynamic information. The binary interaction parameters and root modeling of the LLE data was done using both NRTL [50] mean square deviation (RSMD) values obtained for both and UNIQUAC [51] model as detailed in the supplementary models are presented in Table S12. The highest value of 1 3 Viscosity, cP Viscosity, cP Density, g/cm Density, g/cm Refrac ve Index Refrac ve Index Applied Petrochemical Research (2021) 11:335–351 349 700 1.6 RSMD is 0.0306 for UNIQUAC and 0.0303 for NRTL. Thus, both models adequately describe the experimental 1.4 LLE data. 1.2 Viscosity Density 0.8 Conclusion Refracve Index 0.6 In this study, three categories of hybrid solvents were pre- 0.4 pared by mixing choline chloride-based deep eutectic sol- 0.2 vents (ethaline, glyceline and reline) with ethanol in vari- 0 0 ous volume proportions to reduce the high viscosity of the neat DESs and enhance their industrial appeal. The hybrid % EtOH solvents were explored as extractants for the extraction of aromatic (benzene) from aliphatic (n-hexane). Results show Fig. 14 Viscosity, density and refractive index of reline/EtOH mixed that addition of ethanol has a novel enhancing influence on solvent as a function of volume percent EtOH the extraction capacity of the DESs up to a point but plum- meted same thereafter. The best hybrid solvent was obtained at 50% ethanol content, giving 54.01%, 49.73% and 195.91% increase in the maximum selectivities of ethaline, glyceline and reline, respectively. Thus, ethanol imparts a positive influence on the DESs both in terms of enhanced extrac- tion efficiency and viscosity reduction. The hybrid solvent, glyceline + 50% ethanol (G50Et50), emerged overall best in this regard. Petrochemical industries can therefore embrace Smax this hybrid solvent in place of sulfolane in a drive for green 400 and sustainable paradigm shift. The reliability of the LLE data was confirmed by both Othmer-Tobias and Hand equa- Ethaline tions. The thermodynamic activity coefficient models of Glyceline both NRTL and UNIQUAC adequately represent the experi- Reline mental LLE data. % EtOH Fig. 15 Maximum selectivities of mixed solvents (ethaline/EtOH, glyceline/EtOH, reline/EtOH) as a function of volume percent EtOH Table 2 Comparison of Solvent D S η References distribution coefficient, selectivity and viscosity for Sulfolane 0.0171–0.5014 1.87–42.38 10.35 [46] n-hexane + benzene + solvent at E50Et50 0.621–0.740 1.036–48.421 24.79 This study 303.15 K and 1 atm G50Et50 0.710–0.770 1.480–692.070 194.45 This study R50Et50 0.251–0.422 2.347–45.097 238.33 This study G80E20 0.651–0.750 1.077–422.485 309.92 [21] [EMIM][EtSO ] 0.0093–0.0265 16.58–67.72 ND [46] [EMIM][NTf ] 0.0335–0.1331 5.92–40.61 ND [46] [EMIM][EtSO ]:[EMIM][NTf ], 1:9 0.0273–0.0684 11.56–35.91 ND [46] 4 2 [EMIM][EtSO ]:[EMIM][NTf ], 9:1 0.0114–0.0431 10.58–52.03 ND [46] 4 2 ND not determined a—[24] η: dynamic viscosity in cP 1 3 Viscosity, cP Density, g/cm Refrac ve Index 350 Applied Petrochemical Research (2021) 11:335–351 Supplementary Information The online version contains supplemen- 12. 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Othmer D, Tobias P (1942) Liquid–liquid extraction data the line Alrashdi AA, Bouksaim M, Lgaz H, Essamri A (2021) Dehydra- correlation. Ind Eng Chem 34(6):693–696 tion of bioethanol produced from argane pulp using pervapora- 49. Hand DB (1930) The distribution of consulate liquid between two tion membrane process. Experimental, molecular dynamics and immiscible liquids. J Phys Chem 34:1961–2000 GCMC simulation. J Mol Liq 329:115441 50. Renon H, Prausnitz JM (1968) Local compositions in thermody- 38. Gramajo MB, Veliz JH, Lucena MC, Gonzalez DA (2013) Liquid- namic excess functions for liquid mixtures. AIChEJ 14:135–144 liquid equilibria of the methanol + toluene + methylcyclohexane 51. Abrams DS, Prausnitz JM (1975) Statistical thermodynamics of ternary system at 278.15, 283.15, 288.15, 293.15, 298.15 and liquid mixtures: a new expression for the excess Gibbs energy of 303.15 K. J Solut Chem 42:2025–2033 partly or completely miscible systems. AIChEJ 21:116–128 39. Treybal RE (1963) Liquid extraction. McGraw-Hill, New York 40. Kim K, Park BH (2018) Volumetric properties of solutions of Publisher's Note Springer Nature remains neutral with regard to choline chloride + glycerol deep eutectic solvent with water, jurisdictional claims in published maps and institutional affiliations. methanol, ethanol, or iso-propanol. J Mol Liq 254:272–279 41. Agieienko V, Buchner R (2019) Densities, viscosities, and electri- cal conductivities of pure anhydrous reline and its mixtures with 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Petrochemical Research Springer Journals

Evaluation of hybrid solvents featuring choline chloride-based deep eutectic solvents and ethanol as extractants for the liquid–liquid extraction of benzene from n-hexane: towards a green and sustainable paradigm

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Springer Journals
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Copyright © The Author(s) 2021
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2190-5525
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2190-5533
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10.1007/s13203-021-00282-y
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Abstract

Deep eutectic solvents (DESs) have high viscosities, but known to be mitigated by addition of suitable co-solvent. The effect of such co-solvent on the extraction efficiency of the hybrid solvent is hardly known. This study examined the effect of ethanol on three choline chloride-based DESs (glyceline, reline, and ethaline) by mixing each in turn with ethanol in various volume proportions. The hybrid solvents were evaluated for the extraction of benzene from n-hexane. Pseudo-ternary liquid–liquid equilibrium data were obtained using the refractive index method at 303 K and 1 atm for the systems, n-hexane (1) + benzene (2) + hybrid solvent (glyceline/ethanol, ethaline/ethanol, reline/ethanol) (3), and used to evaluate distribution coefficient (D ) and selectivity (S). Furthermore, the physicochemical properties of the hybrid solvents were also determined. The results indicate increase in selectivity with increasing ethanol addition up to 50% and decrease with further addition. All hybrid solvents with 50% ethanol outperform sulfolane and are suitable replacement for same as green and sustainable extractant for aromatics from aliphatics. The glyceline + 50% ethanol emerged the overall best with 49.73% elevation in selectivity and 41.15% reduction in viscosity relative to the neat glyceline. The finding of this study is expected to fillip the drive for paradigm shift in petrochemical industries. Keywords Glyceline · Ethaline · Reline · Ethanol · Distribution coefficient · Separation factor · Viscosity Introduction such as equilibrium data and derived parameters like selec- tivity (S) and distribution coefficient (D ) enable determina- Thermodynamic and transport properties are needed for tion of equipment size and solvent consumption rate. On the design and operation of process and products in diverse the other hand, transport property such as viscosity fixes the areas, more so in Chemical and Petroleum Engineering. A hydrodynamics, mixing and flow issues. In the petrochemi- triangular collaboration between the academia, industries cal industries, aromatic production from various sources and software providers, to address challenges in this regard like pyrolysis gasoline, reformate or naphtha accounts for has been re-emphasized by a recent survey conducted on 40 million metric tons of benzene, 40 million metric tons industry practitioners [1]. A key finding of the survey is the of xylenes and 20 million metric tons of toluene per annum complementary roles of experimental data and model devel- globally [2]. A critical stage of this production process is opment which should not be jettisoned. For example, in a the liquid–liquid extraction of the aromatics from the non- liquid–liquid extraction process, thermodynamic properties aromatic (or aliphatic) media. The choice of solvent with suitable values of thermodynamic and transport properties is crucial and bears overarching influence on the economic * Mohammed Awwalu Usman viability and sustainability of the process. musman@unilag.edu.ng; mawwal04@yahoo.com Deep eutectic solvents (DESs) are mixtures characterized 1 by remarkable depressions in melting point relative to the Sustainable Process Technology Group, Process Systems Engineering Cluster, Department of Chemical and Petroleum constituents and having tunable physicochemical properties. Engineering, University of Lagos, Akoka, Yaba 101017, It has continued to attract research attention in diverse area Lagos, Nigeria Vol.:(0123456789) 1 3 336 Applied Petrochemical Research (2021) 11:335–351 of application. A comprehensive review of DESs, funda- in the evaluation of ionic liquids/deep eutectic solvents as mentals and applications can be found in the literature [3, extractants for separating aromatics from aliphatic hydrocar- 4]. More recent applications include biomass pre-treatment bons, with results that are in good agreement with experi- [5, 6], media for enzymatic hydrolysis [7, 8], platform for mental findings [22– 24]. The green credentials of DESs, lipase extraction [9], biodiesel production and purification particularly glyceline, ethaline and reline, have been well [10, 11], inhibiting shale hydration [12] and COVID-19 established by various studies [25, 26]. intervention [13]. In the specific area of solvent extraction, However, the undesirable transport property (high viscos- a few recent studies are worthy of review. Wojeicchowski ity) of DESs is a huge challenge to its industrial application. et al. [14] explored the capacity of deep eutectic solvents to For example, the viscosities of glyceline, ethaline, reline and extract phenolics from rosemary leaves. The results indicate sulfolane are 342.12 cP, 38.52 cP, 667.28 cP [21], and 10.35 that the DES, choline chloride: 1,2-propanediol, at the opti- cP [27] at 303.15 K, respectively. Zheng et al. [28] pos- mal conditions (65 °C, liquid:solid ratio of 40:1 and 5.0% ited that mixing of DESs with molecular solvents (volatile wt of water), achieved a 39–51% inhibition of antimicrobial organic solvents) can help overcome the drawbacks of neat activity of extract to all tested bacteria. Petracic et al. [15] DESs and thus extend the practical or industrial application investigated deep eutectic solvents as extractants to reduce of DESs. In this regard, several studies have been conducted the free fatty acid content of feedstocks for biodiesel pro- to mitigate the viscosity of DESs by blending with organic duction in a liquid–liquid extraction process. The results solvents. Some of the organic solvents explored include show that the acidity of waste animal fat was significantly methanol [29], ethanol [30, 31], dimethyl sulfoxide [32]. reduced. Lemaoui et al. [16] studied the application of deep The considerably lower viscosities of the organic solvents eutectic solvents as extractants in the simultaneous de-aro- in comparison with DESs or ionic liquids (ILs) suggest that matization, desulfurization and denitrogenation of diesel in the former should have a thinning effect on the latter. This a liquid–liquid extraction process. The results showed that was corroborated in the aforementioned studies as viscosi- 100% removal of pyrrole and pyridine can be achieved in 2 ties of the mixed solvents plummet further with increase in stages. Rezaee et al. [17] investigated the use of deep eutec- the proportion of organic solvent. Ethanol is expected to tic solvents as extractants in the liquid–liquid extraction to exert viscosity reduction on DESs giving its much lower remove dibenzothiophene from model fuel (n-octane). The viscosity of 0.983 cP. result indicates significant removal of the sulfur-containing Traditionally, ethanol is produced from biomass in a pro- compound. duction chain that encompasses some or all of the following The superior performance of DESs as extractants for aro- steps: pre-treatment, hydrolysis, enzymatic fermentation and matics from aliphatic media over the conventional organic purification, depending on the feedstock. The purification solvent (sulfolane) has been well reported in the literature. stage, conventionally done by distillation, is characterized For example, Shekaari et al. [18] reported a maximum selec- by high-energy consumption and inefficiency. Azeotropy of tivity (S ) of 52.4197 for DES (choline chloride: diglycola- the ethanol–water mixture is an additional contributory fac- max mine, 1:5 molar ratio) as against 47.7704 for sulfolane in tor. These tend to detract from the green and sustainable the extraction of benzene from n-hexane at 303.15 K. Simi- indices of the process and that of ethanol. Thankfully, there larly, Usman et al. [19] reported a high value for selectivity are emerging technologies that ensure drastic knockdown on (S = 462.00) using glyceline (choline chloride:glycerol, energy consumption and guarantee improved efficiency of max 1:2 molar ratio) as extractant for separating benzene from the distillation process [33]. Other interventions focused on n-hexane. In a related study using ASPEN simulation, alternative separation methods, such as liquid–liquid extrac- Usman et al. [20] reported higher values of selectivity for tion [34, 35] and adsorption [36, 37]. All these restore con- glyceline (S = 378.283) and ethaline (S = 77.364) com- fidence in the green and sustainable credentials of ethanol. max max pared to sulfolane (S = 55.371) in the extraction of aro- Ethanol can therefore be rightly classified as green and max matics (benzene–toluene–xylene) from waste tire pyrolysis sustainable co-solvent to add to DESs for viscosity reduc- gasoline at 303.15 K and 1 atm. Further, using a mixture tion. However, the effect of such addition on the thermo- of glyceline and ethaline in the volume ratio 80:20, respec- dynamic properties (extraction performance for aromat- tively, as extractant for separating benzene from n-hexane, ics) of the resulting hybrid solvent has not been previously Usman et al. [21] reported the selectivity value of 422.485. explored, to the best of our knowledge. This study therefore These studies eloquently speak to the superiority of DESs to seeks to bridge the gap by mixing glyceline, ethaline and sulfolane in terms of thermodynamic properties. In addition reline in all volume proportions with ethanol to form three to the aforementioned experimental works, several molecu- categories of hybrid solvents (glyceline + ethanol, etha- lar dynamic simulation studies have also been carried out line + ethanol, and reline + ethanol). These hybrid solvents 1 3 Applied Petrochemical Research (2021) 11:335–351 337 were then evaluated as extractants for the extraction of ben- and pure ethanol (Et100). Overall, there were sixty-one (61) zene from n-hexane. The extraction efficiency is assessed solvents or extractants used for this study. The water con- using the thermodynamic parameters, benzene distribution tents in each DESs and hybrid solvents were determined by coefficient (D) and selectivity factor (S). The physicochemi- the method described in our previous article [19], the mass cal properties (density, viscosity and refractive index) of the fraction was ≤ 0.0003 for all studied solvents. extractants were also measured. Extraction and determination of LLE data Experimental The 61 solvents were each investigated for their extraction capacity; each was used as an extractant in the separation Materials of n-hexane + benzene mixture. The extraction process was done on a bench scale as described in our articles [19, 21]. Benzene, n-hexane, Choline chloride, ethylene glycol, urea, Measured volume of n-hexane + benzene mixture (feed) was glycerol and ethanol were purchased from Sigma-Aldrich contacted with hybrid solvent or solvent in 250 ml beaker. (Germany) with a mass fraction purity higher than 0.98. All The extraction runs were carried out in a vessel, the tempera- chemicals were used as received without further puric fi ation ture was controlled by a water bath at 303 K. After bringing and they were stored in a desiccator in their original tightly the feed to extraction temperature, the solvent was added at closed bottles. Table 1 shows the chemicals, CAS number the same temperature (according to the predetermined ratio). and purity. A rotating stainless steel shaft was used for mixing the feed and solvent at a controlled degree of mixing of 500 rpm. The Preparation of deep eutectic solvents and hybrid extraction was carried out for a predetermined mixing time solvents of 1 h and the mixture was left to separate into a raffinate phase (n-hexane-rich phase) at the top and an extract phase Three deep eutectic solvents were prepared in this study, (solvent-rich phase) at the bottom for a predetermined set- namely ethaline (choline chloride and ethylene glycol), tling time of 4 h. The extract was then separated and the glyceline (choline chloride and glycerol) and reline (choline equilibrium compositions of the phases were determined via chloride and urea). The quaternary ammonium salt (choline refractive index measurement. All experiments were dupli- chloride) was used as the hydrogen bond acceptor while cated and average values reported. urea, glycerol and ethylene glycol serve as the hydrogen bond donors in the molar ratio 1:2, respectively. The detailed protocol for preparing these DESs is explained in our previ- Determination of physicochemical properties ous articles [19, 21]. The hybrid solvents of these DESs were then prepared by mixing each DES with ethanol in varying Densities were measured using a density tube meter, the volume proportion and named appropriately. For example, viscosities were measured with a Brooksfield DV2T vis- G95Et5 means 95% glyceline and 5% ethanol; E80Et20 cometer. This viscometer was calibrated with distilled water. means 80% ethaline and 20% ethanol; R60Et40 means 60% Viscosity of the samples (η) was obtained under the follow- reline and 40% ethanol. A total of fifty-seven (57) DES–eth- ing conditions; the flow time of 60 s was used to measure anol hybrid solvents were so prepared and used for this study the flow time in the hybrid solvents or solvent, T = 303  K in addition to the three neat DESs (E100, G100, and R100) and a speed of 50 rpm. The estimated uncertainty of the experimental viscosity was ± 0.02 cP. The refractive index was measured with a digital refractometer (ATAGO DRA1, Table 1 Chemicals used in this study Japan) with an uncertainty of ± 0.001. The analytical balance (AND, GR202, Japan) with the Component Supplier CAS reg. no. Mass frac- tion purity precision of ± 0.0001 g was used for the preparation of mix- (%) tures in molar basis. The studied hybrid solvents were pre- pared in well-sealed glass vials to avoid contamination or Benzene Sigma-Aldrich 71-43-2 ≥ 99.5 mixture evaporation. Measurements were done continually n-Hexane Sigma-Aldrich 110-54-3 ≥ 99 after the mixtures preparation. The standard uncertainty of Ethanol Sigma-Aldrich 64-17-5 ≥ 99 solubility is 0.0014 and density is 0.001 g/cm . Choline chloride Sigma-Aldrich 67-48-1 ≥ 98 Ethylene glycol Sigma-Aldrich 107-21-1 ≥ 99 Glycerol Sigma-Aldrich 56-81-5 ≥ 99 Urea Sigma-Aldrich 57-13-6 ≥ 99 1 3 338 Applied Petrochemical Research (2021) 11:335–351 ethanol, and reline/ethanol). The physicochemical prop- Determination of performance parameters erties of the sixty-one (61) solvents or extractants are then presented in the third sub-section. The section is The performances of the extractants were evaluated based on two metrics, namely benzene distribution coefficient (D ) concluded with a general discussion and comparison of the studied extractants in the fourth subsection. and selectivity (S) as defined mathematically in Eqs. (1 –2). D = , Performance of neat ethanol (Et100, EtOH) (1) This sub-section presents the liquid–liquid equilibria data/ x ⋅ x 23 11 ternary diagram, distribution coefficients and selectivities S = , (2) x ⋅ x 21 13 for the extraction of benzene from n-hexane using neat etha- nol as the extractant. where x is the mole fraction of benzene in the extract (hybrid solvent) phase, x is the mole fraction of benzene LLE data and tie lines in the raffinate (n-hexane) phase, x is the mole fraction of n-hexane in the extract (hybrid solvent) phase and x Table  S1 (Supplementary information) shows the liq- is the mole fraction of n-hexane in the raffinate (hexane) uid–liquid equilibrium data for the ternary system n-hexane phase. (1) + benzene (2) + ethanol (3) at 303 K and 1 atm. These data are plotted in a ternary diagram as shown in Fig.  1. The biphasic region is clearly narrow, indicating a limited operation window for liquid–liquid extraction. The pair of Results and discussion n-hexane and ethanol shows partial miscibility to warrant recovery of ethanol from the raffinate with the attendant In this section, the results of the experimental studies energy expenditure and cost implication. It is instructive to and subsequent analyses are presented and thoroughly mention that this ternary diagram provides justification for discussed. The first sub-section presents and explains the blending of gasoline with ethanol (10% EtOH or 15% the performance of the neat ethanol (Et100, EtOH) while EtOH) as currently practiced in some countries of the world the second subsection presents and discusses the perfor- as such blend lie within the single-phase region. It also pro- mances of the hybrid solvents of DESs and ethanol under vides limit for such blending, as any blend that falls within the banner of the three pairs (ethaline/ethanol, glyceline/ Fig. 1 Experimental tie lines for the system n-hexane + ben- zene + ethanol (Et100, EtOH) at temperature 303 K and atmos- pheric pressure 1 3 Applied Petrochemical Research (2021) 11:335–351 339 the two-phase region would not be acceptable since it com- Ethaline + ethanol (E100–E5Et95) promises fuel function. There were twenty (20) extractants investigated in this study Distribution coefficients and selectivities involving ethaline, including the neat-ethaline (ChCl:EG 100%; E100) and nineteen (19) hybrid solvents of ethaline The benzene distribution coefficients (D) and selectivities and ethanol in volume proportions of ethanol ranging from (S) for the ternary system n-hexane (1) + benzene (2) + etha- 5 to 95% in incremental steps of 5% (E95Et5–E5Et95). This nol (3) at 303 K are presented in Table S1 and plotted in sub-section presents the liquid–liquid equilibria data, ternary Fig. 2 as a function of benzene composition in the extract diagrams, distribution coefficients and selectivities for the phase. The values of D vary from 0.593 to 0.769 while those extraction of benzene from n-hexane using these extractants. of S vary from 1.238 to 2.261 as the benzene composition in the extract phase increases from 0.0215 to 0.2497. In a LLE data and tie lines Table S2 presents the comprehensive related study, Gramajo et  al. [38] reported liquid–liquid experimental LLE data for the pseudo-ternary systems of equilibrium data for n-hexane + benzene + methanol sys- n-hexane (1) + benzene (2) + solvent (E100–E5Et95) (3). tem at 278.15 K in mass fractions from which evaluated For the sake of brevity, the ternary diagrams/tie lines for values of D range from 0.26 to 0.74 while S ranges from seven (7) of the twenty (20) systems are shown in Fig. 3a–g, 1.31 to 4.44. Thus, ethanol is a better extractant compared while the remaining ones are presented in Figure S1 (a–m). with methanol. However, it is clear that ethanol is a poor These seven are considered typical of behavior pattern of extractant for benzene when its S values are compared with the lot. At 0% EtOH, as shown in Fig. 3a, there is no mis- those of other organic solvents: sulfolane (2.7963–47.7704), cibility between ethaline and n-hexane, thus no DES in the N-formylmorpholine (2.8551–21.8382), and diglycolamine raffinate phase thereby obviating the need for solvent recov - (2.1985–47.1515) as reported by Shekaari et  al. [18] at ery from this phase. As the % EtOH addition increases in 303.15 K. the hybrid solvent, the miscibility improves (Fig. 3b–g). The two-phase region also decreases as the volume proportion of Performance of the hybrid solvents: DESs EtOH increases from 0% in Fig. 3a to 95% in Fig. 3g. Type and ethanol 1 behavior according to Treybal’s classification was exhib- ited by all the hybrid solvents [39]. The highly polar nature The extraction performance of the three hybrid solvent cat- of the studied DES may be responsible for its immiscibility egories: ethaline/ethanol, glyceline/ethanol and reline/etha- with n-hexane. Finally, the positive slopes of tie lines show nol is hereby presented. that benzene solubility in n-hexane is higher than its solubil- ity in the solvents. 0.8 2.4 Distribution coefficients and selectivities The benzene dis- tribution coefficients (D) obtained for the twenty pseudo- 0.8 2.2 ternary systems of n-hexane (1) + benzene (2) + hybrid sol- vent (E100–E5Et95) (3) are presented in Table S2. Figure 4 0.7 2.0 shows a plot of D versus benzene composition in the extract phase for the seven chosen systems. The D values increase 0.7 1.8 as % EtOH increase from 0 to 20%, but drop subsequently, 0.6 though still higher than the values at 0% EtOH, up to 60% EtOH. The values of D beyond 60% EtOH are less than the 1.6 0.6 values at 0% EtOH. The selectivity values for the twenty pseudo-ternary sys- 1.4 0.5 tems are shown in Table S2 and those of the chosen seven are plotted in Fig. 5. Generally, the S values decrease with 1.2 0.5 increasing composition of benzene in the extract phase for the seven extractants. Addition of EtOH significantly 0.4 1.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 improved the selectivity of the hybrid solvents relative to neat-ethaline up to 50% and thereafter it decreased very sharply particularly beyond 60% EtOH. The maximum value of S is in the following decreasing order: E50Et50 (50% Fig. 2 Benzene distribution coefficients and selectivities as a function EtOH, S = 48.421) ≻ E60Et40 (40% EtOH, S = 45.097) ≻ of benzene composition in the extract phase for the system n-hex- ane + benzene + EtOH at temperature 303 K and atmospheric pressure E80Et20 (20% EtOH, S = 36.539) ≻ Et100 (0% EtOH, S 1 3 340 Applied Petrochemical Research (2021) 11:335–351 1 3 Applied Petrochemical Research (2021) 11:335–351 341 ◂Fig. 3 a Experimental ternary diagram/tie lines for the n-hex- lines for seven (7) of the twenty (20) systems are shown in ane + benzene + E100 (ChCl:EG + 0%EtOH) at temperature 303  K Fig. 6a–g, while the remaining ones are presented in Fig- and atmospheric pressure. b Experimental ternary diagram/tie lines ure S2a–m. These seven are considered typical of behav- for the n-hexane + benzene + E80Et20 (ChCl:EG + 20%EtOH) ior pattern of the lot. At 0% EtOH, as shown in Fig.  6a, at temperature 303  K and atmospheric pressure. c: Experimen- tal ternary diagram/tie lines for the n-hexane + benzene + E60Et40 there is no miscibility between glyceline and n-hexane, (ChCl:EG + 40%EtOH) at temperature 303  K and atmospheric thus no DES in the raffinate phase, thereby obviating the pressure. d Experimental ternary diagram/tie lines for the n-hex- need for solvent recovery from this phase. As the % EtOH ane + benzene + E50Et50 (ChCl:EG + 50%EtOH) at temperature increases in the hybrid solvent, the miscibility improves 303 K and atmospheric pressure. e Experimental ternary diagram/tie lines for the n-hexane + benzene + E40Et60 (ChCl:EG + 60%EtOH) (Fig.  6b–g). The two-phase region also decreases as the at temperature 303  K and atmospheric pressure. f Experimen- volume proportion of EtOH increases from 0% in Fig. 6a tal ternary diagram/tie lines for the n-hexane + benzene + E20Et80 to 95% in Fig. 6g. Type 1 behavior according to Treybal’s (ChCl:EG + 80%EtOH) at temperature 303  K and atmospheric pres- classification was exhibited by all the extractants [39]. The sure. g Experimental ternary diagram/tie lines for the n-hexane + ben- zene + E5Et95 (ChCl:EG + 95%EtOH) at temperature 303  K and highly polar nature of the studied DES may be responsible atmospheric pressure for its immiscibility with n-hexane. The biphasic region decreases with increase in % MeOH. Finally, the positive = 31.440) ≻ E40Et60 (60% EtOH, S = 28.834) ≻ E20Et80 slopes of tie lines show that benzene solubility in n-hex- (80% EtOH, S = 13.323) ≻ E5Et95 (95% EtOH, S = 6.901). ane is higher than its solubility in the solvent. There is a 54.01% increase in the maximum value of S at 50% MeOH relative to the neat-ethaline. Thus, EtOH impart Distribution coefficients and  selectivities The ben- elevation in selectivity to ethaline up to 50% addition but a zene distribution coefficients (D ) obtained for the sharp attenuation beyond. Also worthy of note is the conver- twenty pseudo-ternary systems of n-hexane (1) + ben- gence of S values beyond 10% mole fraction of benzene in zene (2) + solvent (G100–G5Et95) (3) are presented in the extract irrespective of % EtOH. This signifies selectivity Table S3. Figure 7 shows a plot of D versus benzene com- becomes insensitive to EtOH addition when the composition position in the extract phase for the seven chosen systems. of benzene in the extract phase goes above 10%. Similar The D values of the hybrid solvents from 0 to 60% EtOH trend occurred in the extraction of benzene from n-hexane are generally ≥ 0.6 at all compositions of benzene in the using a binary mixed DES (glyceline/ethaline) as extractant extract phase. Attenuation of D values only becomes sig- [21]. Generally, selectivity tends to unity as the tie line tends nificant with EtOH content beyond 60%. toward the plait point, where the distribution coefficient of The selectivity values for the twenty pseudo-ternary solute and other components of the ternary mixture becomes systems are shown in Table  S3 and those of the cho- unity. Thus, the observed convergence of S values can be sen seven are plotted in Fig.  8. Generally, the S values attributed to the inability of the hybrid solvents to discrimi- decrease with increasing composition of benzene in the nate beyond 10% mole fraction of benzene as the S value extract phase for the seven extractants. Addition of EtOH has become very low. significantly improved the selectivity of the hybrid sol- vent relative to neat glyceline up to 60% and thereafter it Glyceline + ethanol (G100–G5Et95) decreased very sharply. There is a clear enhancement in the S up to 60%, even though the highest value occurs max There were twenty (20) extractants investigated in this study at 50% EtOH. The maximum value of S is in the follow- involving glyceline, including the neat glyceline (ChCl:Gly ing decreasing order: G50Et50 (50% EtOH, S = 692.070) 100%; G100) and nineteen (19) hybrid solvents of glyceline ≻ G60Et40 (40% EtOH, S = 662.413) ≻ G40Et60 (60% and ethanol in volume proportions of ethanol ranging from EtOH, S = 568.774) ≻ G80Et20 (20% EtOH, S = 526.833) 5 to 95% in incremental steps of 5% (G95M5–G5M95). This ≻ G100 (0% EtOH, S = 462.219) ≻ G20Et80 (80% EtOH, sub-section presents the liquid–liquid equilibria data, ternary S = 82.165) ≻ G5Et95 (95% EtOH, S = 9.787). There is a diagrams, distribution coefficients and selectivities for the 49.73% increase in the maximum value of S at 50% EtOH extraction of benzene from n-hexane using these extractants. relative to the neat glyceline. The decrease in S from 60 max to 80% is profoundly sharp, a drop of 85.55%, signifying LLE data and  tie lines Table  S3 presents the compre- sharp attenuation beyond 60% EtOH. Also worthy of note hensive experimental LLE data for the pseudo-ternary is the convergence of S values beyond 20% mole fraction systems of n-hexane (1) + benzene (2) + solvent (G100– of benzene in the extract irrespective of % EtOH. This G5Et95) (3). As earlier explained, the ternary diagrams/tie signifies selectivity becomes insensitive to EtOH addition 1 3 342 Applied Petrochemical Research (2021) 11:335–351 0% EtOH the distribution coefficient of solute and other components 20% EtOH of the ternary mixture becomes unity. Thus, the observed 0.9 40% EtOH convergence of S values can be attributed to the inability 50% EtOH of the hybrid solvents to discriminate beyond 20% mole 0.8 60% EtOH fraction of benzene as the S value has become very low. 80% EtOH 0.7 95% EtOH Reline + ethanol (R100–R5Et95) 0.6 There were twenty (20) extractants investigated in this cat- egory, including the neat-reline (ChCl:Ur 100%; R100) 0.5 and nineteen (19) hybrid solvents of reline and ethanol in volume proportions of ethanol ranging from 5 to 95% 0.4 in incremental steps of 5% (R95Et5–R5Et95). This sub- section presents the liquid–liquid equilibria data, ter- 0.3 nary diagrams, distribution coefficients and selectivities 0.0000 0.1000 0.2000 0.3000 0.4000 for the extraction of benzene from n-hexane using these extractants. Fig. 4 Benzene distribution coefficients as a function of benzene LLE data and tie line Table S4 presents the comprehensive composition in the extract phase for the system n-hexane + ben- experimental LLE data for the pseudo-ternary systems of zene + ethaline/EtOH at temperature 303 K and atmospheric pressure n-hexane (1) + benzene (2) + hybrid solvent (R100–R5Et95) (3). As earlier explained, the ternary diagrams/tie lines for 60.3 seven (7) of the twenty (20) systems are shown in Fig. 9a–g, while the remaining ones are presented in Figure S3a–m. 0% EtOH These seven are considered typical of behavior pattern of 50.3 20% EtOH the lot. At 0% EtOH, as shown in Fig. 9a, there is no misci- 40% EtOH bility between glyceline and n-hexane, thus no DES in the 40.3 50% EtOH raffinate phase, thereby obviating the need for solvent recov - 60% EtOH ery from this phase. As the % EtOH addition increases in 30.3 80% EtOH the hybrid solvent, the miscibility improves (Fig. 9b–g). The 95% EtOH two-phase region also decreases as the volume proportion of 20.3 EtOH increases from 0% in Fig. 9a to 95% in Fig. 9g. Type 1 behavior according to Treybal’s classification was exhibited 10.3 by all the mixed extractants [39]. The highly polar nature of the studied DES may be responsible for its immiscibility with n-hexane. The biphasic region decreases with increase 0.3 0.0000 0.1000 0.2000 0.3000 0.4000 in % EtOH. Finally, the positive slopes of tie lines show that x benzene solubility in n-hexane is higher than its solubility in the solvent. Fig. 5 Selectivities as a function of benzene composition in the Distribution coefficients and selectivities The benzene dis- extract phase for the system n-hexane + benzene + ethaline/EtOH at temperature 303 K and atmospheric pressure tribution coefficients (D) obtained for the twenty pseudo- ternary systems of n-hexane (1) + benzene (2) + mixed solvent (R100–R5Et95) (3) are presented in Table S4. Fig- when the composition of benzene in the extract phase goes ure 10 shows a plot of D versus benzene composition in the above 20%. Similar trend occurred in the extraction of extract phase for the seven chosen systems. The D values benzene from n-hexane using a binary mixed DES (glyce- of the hybrid solvents generally show a gradual increase line/reline) as extractant [21]. Generally, selectivity tends from % EtOH content of 0–80%, values lie between 0.25 to unity as the tie line tends toward the plait point, where and 0.45, with few outliers. There is a sharp increase in the 1 3 Applied Petrochemical Research (2021) 11:335–351 343 D values for 95% EtOH relative to others. This is largely the hybrid solvent is noticed as volume % of EtOH increases. because of the much higher values of D for ethanol com- This is evident from the sharp steep in negative slope of pared to neat-reline. viscosity profile in Fig.  12 and it is in agreement with the The selectivity values for the twenty pseudo-ternary sys- findings of similar work in this regard [29, 32]. For example, tems are shown in Table S4 and those of the chosen seven a decrease of 20.7% in viscosity was achieved with 20% are plotted in Fig. 11. Generally, the S values decrease with EtOH addition while the drop in viscosity at 50% EtOH is increasing composition of benzene in the extract phase 30.17%. The profile for density also shows decrease with for the seven extractants. Addition of EtOH significantly increasing volume % EtOH in the hybrid solvent but not as improved the selectivity of the hybrid solvents relative sharp as viscosity. On the other hand, the refractive index to neat-reline up to 60% and thereafter it decreased very shows almost constant values with increasing volume % of sharply. There is a clear enhancement in the S up to 60%, EtOH in the mixed solvent. max even though the highest value occurs at 50% EtOH. The maximum value of S is in the following decreasing order: Glyceline + ethanol R50Et50 (50% EtOH, S = 45.097) ≻ R60Et40 (40% EtOH, S = 41.450) ≻ R80Et20 (20% EtOH, S = 32.042) ≻ R40Et60 Table S6 (supplementary information) shows the density, (60% EtOH, S = 28.301) ≻ R100 (0% EtOH, S = 15.240) ≻ viscosity and refractive index of the hybrid solvents (glyce- R20Et80 (80% EtOH, S = 9.071) ≻ R5Et95 (95% EtOH, S = line/EtOH) as a function of volume % of EtOH. These values 5.096). There is a staggering 195.91% increase in the maxi- are plotted in Fig. 13. A significant decrease in viscosity of mum value of S at 50% EtOH relative to the neat-reline. The the hybrid solvent is noticed as volume % of EtOH increases. decrease in S from 60 to 80% is profoundly sharp, 67.95% This is evident from the sharp steep in negative slope of vis- max drop, signifying sharp attenuation beyond 60% EtOH. It is cosity profile in Fig.  13. For example, a decrease of 11.76% pertinent to note that at 10% benzene composition in the in viscosity was achieved with 20% EtOH addition while extract phase, the S value for 0% EtOH (neat-reline) is higher the drop in viscosity at 50% EtOH is 41.15%. The profile for than all hybrid solvents. Also worthy of note is the conver- density also shows decrease with increasing volume % EtOH gence of S values beyond 20% mole fraction of benzene in in the hybrid solvent but not as sharp as viscosity. On the the extract irrespective of % EtOH. This signifies selectivity other hand, the refractive index shows almost constant val- becomes insensitive to EtOH addition when the composition ues with increasing volume % of EtOH in the hybrid solvent. of benzene in the extract phase goes above 20%. Similar Similar findings have been reported in the literature [29, 40]. trend occurred in the extraction of benzene from n-hexane using a binary mixed DES (glyceline/reline) as extractant Reline + ethanol [21]. Generally, selectivity tends to unity as the tie line tends toward the plait point, where the distribution coefficient of Table S7 (supplementary information) shows the density, solute and other components of the ternary mixture becomes viscosity and refractive index of the hybrid solvents (reline/ unity. Thus, the observed convergence of S values can be EtOH) as a function of volume % of EtOH. These values are attributed to the inability of the hybrid solvents to discrimi- plotted in Fig. 14. A significant decrease in viscosity of the nate beyond 20% mole fraction of benzene as the S value mixed solvent is noticed as volume % of EtOH increases. has become very low. This is evident from the sharp steep in negative slope of vis- cosity profile in Fig.  14. For example, a decrease of 19.98% Physicochemical properties of hybrid solvents in viscosity was achieved with 20% EtOH addition while the drop in viscosity at 50% EtOH is 63.38%. The profile The physicochemical properties (density, viscosity and for density also shows decrease with increasing volume % refractive index) of the studied hybrid solvents as measured EtOH in the hybrid solvent but not as sharp as viscosity. On experimentally are presented and discussed in this section. the other hand, the refractive index shows almost constant The three pairs of hybrid solvents are analyzed in the fol- values with increasing volume % of EtOH in the hybrid sol- lowing sub-sections. vent. Similar findings have been reported in the literature [31, 41, 42]. Ethaline + ethanol General discussion and comparative analysis Table S5 (supplementary information) shows the density, viscosity and refractive index of the hybrid solvents (etha- The ternary diagram for the system n - h exa n e + b e n- line/EtOH) as a function of volume % of EtOH. These values zene + ethanol provides insight and beneficial guideline are plotted in Fig. 12. A significant decrease in viscosity of for blending gasoline with ethanol in what is now known 1 3 344 Applied Petrochemical Research (2021) 11:335–351 1 3 Applied Petrochemical Research (2021) 11:335–351 345 ◂Fig. 6 a Experimental ternary diagram/tie lines for the n-hex- and urea). For example, Naik et al. [44] reported a higher ane + benzene + G100 (ChCl:Gly + 0%EtOH) at temperature 303  K value of selectivity for ethylene glycol-based DES relative and atmospheric pressure. b Experimental ternary diagram/tie lines to the glycerol-based DES in the extraction of toluene from for the n-hexane + benzene + G80Et20 (ChCl:Gly + 20%EtOH) n-heptane. Similar finding was reported by Park [45] in the at temperature 303  K and atmospheric pressure. c Experimental ternary diagram/tie lines for the n-hexane + benzene + G60Et40 extraction of toluene from n-heptane using a ternary mixed (ChCl:Gly + 40%EtOH) at temperature 303  K and atmospheric solvent of choline chloride: urea: ethylene glycol/glycerol. pressure. d Experimental ternary diagram/tie lines for the n-hex- The sharp disparity or contrast between their findings and ane + benzene + G50Et50 (ChCl:Gly + 50%EtOH) at temperature ours may be attributed to the difference in the polarity of 303 K and atmospheric pressure. e Experimental ternary diagram/tie lines for the n-hexane + benzene + G40Et60 (ChCl:Gly + 60%EtOH) the aromatic (toluene/benzene), the hydrogen bond acceptor at temperature 303  K and atmospheric pressure. f Experimental (choline chloride/methyltriphenylphosphonium bromide), ternary diagram/tie lines for the n-hexane + benzene + G20Et80 and the hydrogen bond network between HBA and HBD (ChCl:Gly + 80%EtOH) at temperature 303  K and atmospheric pres- in each case. sure. g Experimental ternary diagram/tie lines for the n-hexane + ben- zene + G5Et95 (ChCl:Gly + 95%EtOH) at temperature 303  K and The performances of the solvents under study are best atmospheric pressure benchmarked against the conventional organic solvent commonly used in the industries for separating aromatics as gasohol. Such blending must lie within the single-phase from non-aromatics—sulfolane. It is however pertinent to region to remain uniform and deliver the requisite fuel func- make clarification on the seeming discrepancy between the tion. The two-phase region should be avoided; this is a key values of selectivity reported for sulfolane in two different contribution of this study. For the hybrid solvents, the expec- articles for the separation of benzene from n-hexane. Shek- tation is that the properties should lie between those of their aari et al. [18] reported S value of 47.7704 at 303.15 K max constituents. In consonance with this expectation, addition and 0.0865 MPa and another value of 36.3735 at 313.15 K of EtOH imparts viscosity reduction on DES since the vis- and 0.0865 MPa. This suggests that selectivity is sensitive cosity of the former is profoundly lower than that of the to both temperature and pressure. On the other hand, Guo latter. By the same reasoning, the selectivities of the hybrid et al. [46] reported S value of 42.38 at 303.15 K and 1 atm max solvents are expected to lie between those of the neat-DES (0.101325 MPa). Considering the pressure difference, the and ethanol. Since the selectivities of neat-DES (ethaline, values reported by the two contributions are in good agree- glyceline and reline) are much higher than that of ethanol, ment. Consequently, the comparative analysis is here based addition of EtOH to these DESs should lower their selec- on the same temperature (T = 303.15 K) and same pressure tivities. Figure 15 shows the maximum selectivities (S ) (P = 1 atm) as shown in Table  2. It is instructive to note max of the hybrid solvents as a function of volume % EtOH. A that both ethaline- and glyceline-based hybrid solvents have profound increase in S occurs as % EtOH increases up to benzene distribution coec ffi ient higher than that of sulfolane max 50% and decreases with further increase in EtOH content at 50% EtOH. The selectivities of all hybrid solvents at 50% for all hybrid solvents. This occurrence suggests a strong EtOH are higher than the values for sulfolane. These are intermolecular interaction between these DESs and EtOH remarkable enhancement for both ethaline- and reline-based as % EtOH increases to 50%, which dwindles with further hybrid solvents whose neat-DES underperforms sulfolane. addition of EtOH. The underpinning phenomenological The best hybrid solvent is glyceline + 50% EtOH (G50Et50), context for this novel performance enhancement should be having the highest selectivity value of 692.070, representing unraveled by spectroscopic studies. In a related investiga- 49.73% increase in S and with 41.15% reduction in viscosity tion involving two choline chloride-based DESs, Hadj-Kali relative to the neat glyceline. et al. [43] reported that addition of 50 wt% water breaks Table  2 also shows the performance of other solvents, the hydrogen bonding between the HBA (choline chloride) DESs and ionic liquids, used by other workers in the extrac- and HBD (urea and glycerol). This may well explain the tion of benzene from n-hexane. It is clear that the mixed finding of this study. It is also striking to note the remark - solvent G50Et50 outperformed the ionic liquids, 1-ethyl- ably superior performance of glyceline-based solvent rela- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide tive to the ethaline- and reline-based solvent in this study. ([EMIM][NTf ]), 1-ethyl-3-methylimidazolium ethylsulfate This trend was consistently demonstrated in all our previ- ([EMIM][EtSO ]) and their mixtures, with higher values of ous contributions [19–21]. It is however in sharp contrast D and S. In our previous contribution, the binary mixed to the findings in some related studies in terms of the role DES, 80% glyceline + 20% ethaline (G80E20), was consid- of the hydrogen bond donors (glycerol, ethylene glycol, ered the best in comparison to other mixed DESs evaluated in that study, with a decrease of 8.55% in selectivity and 1 3 346 Applied Petrochemical Research (2021) 11:335–351 1.0 Fig. 9 a Experimental ternary diagram/tie lines for the n-hex- ane + benzene + R100 (ChCl:Ur + 0%EtOH) at temperature 303  K and atmospheric pressure. b Experimental ternary diagram/tie 0.9 lines for the n-hexane + benzene + R80Et20 (ChCl:Ur + 20%EtOH) at temperature 303  K and atmospheric pressure. c Experimen- tal ternary diagram/tie lines for the n-hexane + benzene + R60Et40 0.8 (ChCl:Ur + 40%EtOH) at temperature 303  K and atmospheric 0% EtOH pressure. d Experimental ternary diagram/tie lines for the n-hex- ane + benzene + R50Et50 (ChCl:Ur + 50%EtOH) at temperature D 0.7 20% EtOH 303 K and atmospheric pressure. e Experimental ternary diagram/tie 40% EtOH lines for the n-hexane + benzene + R40Et60 (ChCl:Ur + 60%EtOH) at temperature 303  K and atmospheric pressure. f Experimental 0.6 50% EtOH ternary diagram/tie lines for the n-hexane + benzene + R20Et80 60% EtOH (ChCl:Ur + 80%EtOH) at temperature 303  K and atmospheric pres- 0.5 sure. g Experimental ternary diagram/tie lines for the n-hexane + ben- 80% EtOH zene + R5Et95 (ChCl:Ur + 95%EtOH) at temperature 303  K and 95% EtOH atmospheric pressure 0.4 0.0000 0.2000 0.4000 0.6000 0.8000 Viscosity was observed to generally decrease with increasing proportion of EtOH to the neat DESs (glyceline, ethaline and reline) in the hybrid solvent. The molecular Fig. 7 Benzene distribution coefficients as a function of benzene weight of HBD in the hybrid solvent seems to play signifi- composition in the extract phase for the system n-hexane + ben- cant role. The molecular weight of EtOH is 46, which is zene + glyceline/EtOH at temperature 303  K and atmospheric pres- sure lower compared to the molecular weights of the primary HBD in the hybrid solvents (glycerol: 92.09; urea: 60.06; and ethylene glycol: 62.07). Thus, increasing content of 800 EtOH, which results in lowering of the average molecular weight of the HBD in the hybrid solvent, causes decrease in viscosity. This is in contradiction to the findings of Al-Daw - 0% EtOH sari et al. [47], as they observed that the viscosities of DESs 20% EtOH increased as the molecular weight of the HBDs increased for 40% EtOH the same HBA. This disparity may be attributed to the altera- 50% EtOH tion in the strength and nature of hydrogen bond occasioned 60% EtOH by the secondary HBD (EtOH) in the current study. 80% EtOH On the overall basis, using D, S, and viscosity values, 95% EtOH G50Et50 is the best of the mixed solvents under study. Industrial replacement of sulfolane with G50Et50 for liq- uid–liquid extraction of aromatics from aliphatics will imply lower solvent requirement for the extraction and smaller equipment diameter due to higher D, fewer stages and reduced equipment height due to higher S, and higher cost of mixing and transport due to higher viscosity. It must be stated that all hybrid solvents with 50% EtOH can conveni- 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 ently replace sulfolane. Fig. 8 Selectivities as a function of benzene composition in the extract phase for the system n-hexane + benzene + glyceline/EtOH at Consistency of LLE data and thermodynamic temperature 303 K and atmospheric pressure modeling reduction of 9.41% in viscosity relative to neat glyceline The consistency and reliability of the LLE data were tested using both Othmer-Tobias [48] and Hand [49] correlations [21]. The performance of G50Et50 is superior to G80E20 not only in its higher values of D and S, but also in much as detailed in the supplementary information. As shown in Tables S9 and S10, the coefficient of determination, lower viscosity. 1 3 ◂ Applied Petrochemical Research (2021) 11:335–351 347 1 3 348 Applied Petrochemical Research (2021) 11:335–351 0.850 40 1.6 35 1.4 0.750 30 1.2 0.650 25 1 0% EtOH 0.550 20% EtOH 20 0.8 40% EtOH 0.450 15 0.6 Viscosity 50% EtOH 10 Density 0.4 60% EtOH 0.350 80% EtOH Refracve Index 5 0.2 0.250 95% EtOH 0 0 0.150 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 % EtOH Fig. 12 Viscosity, density and refractive index of ethaline/EtOH mixed solvent as a function of volume percent EtOH Fig. 10 Benzene distribution coefficients as a function of benzene composition in the extract phase for the system n-hexane + ben- zene + reline/EtOH at temperature 303 K and atmospheric pressure 350 1.6 1.4 0% EtOH 20% EtOH 1.2 40% EtOH 50% EtOH 1 60% EtOH 0.8 80% EtOH Viscosity 95% EtOH 150 0.6 Density Refracve Index 0.4 0.2 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 x 0 0 % EtOH Fig. 11 Selectivities as a function of benzene composition in the extract phase for the system n-hexane + benzene + reline/EtOH at temperature 303 K and atmospheric pressure Fig. 13 Viscosity, density and refractive index of glyceline/EtOH mixed solvent as a function of volume percent EtOH R > 0.99 for all mixing proportions of DES/EtOH. This clearly validates the consistency and reliability of the experimentally obtained LLE data. Also, thermodynamic information. The binary interaction parameters and root modeling of the LLE data was done using both NRTL [50] mean square deviation (RSMD) values obtained for both and UNIQUAC [51] model as detailed in the supplementary models are presented in Table S12. The highest value of 1 3 Viscosity, cP Viscosity, cP Density, g/cm Density, g/cm Refrac ve Index Refrac ve Index Applied Petrochemical Research (2021) 11:335–351 349 700 1.6 RSMD is 0.0306 for UNIQUAC and 0.0303 for NRTL. Thus, both models adequately describe the experimental 1.4 LLE data. 1.2 Viscosity Density 0.8 Conclusion Refracve Index 0.6 In this study, three categories of hybrid solvents were pre- 0.4 pared by mixing choline chloride-based deep eutectic sol- 0.2 vents (ethaline, glyceline and reline) with ethanol in vari- 0 0 ous volume proportions to reduce the high viscosity of the neat DESs and enhance their industrial appeal. The hybrid % EtOH solvents were explored as extractants for the extraction of aromatic (benzene) from aliphatic (n-hexane). Results show Fig. 14 Viscosity, density and refractive index of reline/EtOH mixed that addition of ethanol has a novel enhancing influence on solvent as a function of volume percent EtOH the extraction capacity of the DESs up to a point but plum- meted same thereafter. The best hybrid solvent was obtained at 50% ethanol content, giving 54.01%, 49.73% and 195.91% increase in the maximum selectivities of ethaline, glyceline and reline, respectively. Thus, ethanol imparts a positive influence on the DESs both in terms of enhanced extrac- tion efficiency and viscosity reduction. The hybrid solvent, glyceline + 50% ethanol (G50Et50), emerged overall best in this regard. Petrochemical industries can therefore embrace Smax this hybrid solvent in place of sulfolane in a drive for green 400 and sustainable paradigm shift. The reliability of the LLE data was confirmed by both Othmer-Tobias and Hand equa- Ethaline tions. The thermodynamic activity coefficient models of Glyceline both NRTL and UNIQUAC adequately represent the experi- Reline mental LLE data. % EtOH Fig. 15 Maximum selectivities of mixed solvents (ethaline/EtOH, glyceline/EtOH, reline/EtOH) as a function of volume percent EtOH Table 2 Comparison of Solvent D S η References distribution coefficient, selectivity and viscosity for Sulfolane 0.0171–0.5014 1.87–42.38 10.35 [46] n-hexane + benzene + solvent at E50Et50 0.621–0.740 1.036–48.421 24.79 This study 303.15 K and 1 atm G50Et50 0.710–0.770 1.480–692.070 194.45 This study R50Et50 0.251–0.422 2.347–45.097 238.33 This study G80E20 0.651–0.750 1.077–422.485 309.92 [21] [EMIM][EtSO ] 0.0093–0.0265 16.58–67.72 ND [46] [EMIM][NTf ] 0.0335–0.1331 5.92–40.61 ND [46] [EMIM][EtSO ]:[EMIM][NTf ], 1:9 0.0273–0.0684 11.56–35.91 ND [46] 4 2 [EMIM][EtSO ]:[EMIM][NTf ], 9:1 0.0114–0.0431 10.58–52.03 ND [46] 4 2 ND not determined a—[24] η: dynamic viscosity in cP 1 3 Viscosity, cP Density, g/cm Refrac ve Index 350 Applied Petrochemical Research (2021) 11:335–351 Supplementary Information The online version contains supplemen- 12. 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Journal

Applied Petrochemical ResearchSpringer Journals

Published: Dec 1, 2021

Keywords: Glyceline; Ethaline; Reline; Ethanol; Distribution coefficient; Separation factor; Viscosity

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