Multicomponent extraction of aromatics and heteroaromatics from diesel using acidic eutectic solvents: Experimental and COSMO-RS predictions

https://doi.org/10.1016/j.molliq.2021.116575Get rights and content

Highlights

  • Simultaneous dearomatization of four impurities using a salt-acid-based eutectic solvent.

  • The solvent was characterized by its eutectic point, physical, and critical properties.

  • Phase behavior was determined experimentally, predicted via COSMO-RS, and correlated via NRTL.

  • Molecular-level interactions and mechanisms of extraction were studied.

  • Parametric investigation of the multicomponent extraction process was conducted.

Abstract

Eutectic solvents (ESs) have been extensively studied in the literature for the purification of fuels. Nevertheless, most studies investigated the extraction of a single type of aromatic from n-alkanes. In this work, aiming to provide insights about the performance of ESs in a process that mimics the multicomponent dearomatization used industrially, a salt-acid-based ES, comprised of methyltriphenyl-phosphonium bromide and acetic acid, was applied in simultaneously extracting toluene, thiophene, quinoline, and pyrrole from n-decane. First, the DES was characterized for its eutectic composition, physicochemical, and critical properties. Then, an initial screening to determine the molecular-level interactions and extraction mechanism were studied experimentally and using COSMO-RS screening charge density profiles and potentials. A physical mechanism was confirmed for the extraction of pyrrole, thiophene, and toluene while for quinoline, an acid-base reaction was the predominant extraction mechanism. The phase diagrams of each impurity were also experimentally determined, predicted using the COSMO-RS model, and correlated using the NRTL model in Aspen Plus. Lastly, a parametric investigation studying the impact of key parameters including stirring time, initial concentration, mixing effects, solvent-to-feed ratio, multi-stage extraction, and repetitive usage of solvent was conducted. On multi-stage extraction, full recovery of pyrrole and quinoline (99.9%) was achieved in only 2-stages, whereas for thiophene and toluene efficiencies of 82.2% and 58.4% were reached after the 5th stage, respectively.

Introduction

The combustion of diesel fuel leads to the production of particulate matters and various harmful gaseous pollutants that include carbon oxides (COx), sulfur oxides (SOx), and nitrogen oxides (NOx) as they are rich in aromatics and heteroaromatics (i.e., sulfur-/nitrogen- containing aromatics) [1]. Thus, stringent legislations were introduced worldwide to control the aromatic and heteroaromatic content in diesel [2]. Catalytic hydrotreatment is the conventional process utilized in the industry for the simultaneous dearomatization of diesel [3]. However, this process suffers from numerous drawbacks, most notably, its harsh operating conditions (600–700 K; 20–50 bar) and its production of poisonous by-products such H2S and NH3 [3]. Therefore, the development of novel “greener” purification methods for diesel fuels has been a hot research topic, especially since the demand for diesel fuels as an energy source is increasing with the rapid industrial development and economic growth [4].

Based on the literature [4], [5], [6], [7], liquid-liquid extraction (LLX) has been proposed as a promising alternative for the hydrotreatment process due to its advantageous characteristics, including its mild operating conditions, and simple mechanism of separation. Various solvents such as sulfolane [8], dimethyl sulfoxide [9], N-methyl-2-pyrrolidone [10], and glycols [11] have been utilized successfully in the last few decades. Nonetheless, several drawbacks associated with their usage were reported, such as high cross-contamination, difficult regeneration, losses due to volatility, and environmental emissions [12]. To overcome these disadvantages, “designer solvents” such as ionic liquids (ILs) and, more recently, deep eutectic solvents (DESs) have been proposed [13], [14].

DESs were presented to the literature in 2003 as a new generation of designer solvents that could potentially be used as alternatives to ILs and classical organic solvents [15]. ILs can be defined as organic salts in a liquid state with a melting point lower than 373 K [13]. On the other hand, given that the DES field is still in its infancy, a consensus regarding the definition of DESs is yet to be reached [16], [17], [18], [19], [20]. According to Martins et al. [20], “a ‘deep eutectic solvent’ is a mixture of two or more pure compounds for which the eutectic point temperature is below that of an ideal liquid mixture, presenting significant negative deviations from ideality. Additionally, the temperature depression should be such that the mixture is liquid at operating the temperature for a certain composition range. Otherwise, a simpler term ‘eutectic solvent’ could be used to describe mixtures that do not fulfil these criteria”. Therefore, since the definition of these solvents are still being reviewed [16], [17], [18], [19], [20], in this work, the term “eutectic solvent” (ES) was adopted. It should be mentioned that the ESs term suggested by Martins et al. [20] has already been used by several recent papers [17], [19], [21].

ESs have been commonly characterized by their low volatility, their wide liquidus range, and their simple preparation, which does not require a chemical synthesis [22]. ESs are also considered as “designer solvents” as their physiochemical properties and extractive capabilities can be tuned with ease by varying the ES’s constituents and their mixing ratio [22], [23]. Since their discovery, ESs have been successfully utilized in various fields that include separation, catalysis, electrochemistry, and biochemistry [19], [24], [25]. In terms of fuel purification, ESs have been extensively utilized in the literature for the dearomatization of aromatics, sulfur heteroaromatics, and nitrogen heteroaromatics [4]. However, most research studied the extraction of a single contaminant (“either an aromatic or a heteroaromatic”) from n-alkanes [26], [27]. On the other hand, the utilization of ESs in the combined dearomatization, desulfurization, and denitrification of fuel has been investigated by a few studies only [26], [28], [29], [30].

For instance, Hatab et al. [7] reported that efficiencies of 11%, 27%, and 99% could be obtained for the simultaneous extraction of toluene, thiophene, and pyridine, respectively, using betaine: levulinic acid (1:7) at a 1:1 solvent-to-feed mass ratio (S/F). In another study by Warrag et al. [26] it was reported that efficiencies of 8%, 33%, and 90% could be obtained for the simultaneous extraction of toluene, thiophene, and quinoline, respectively, using methyltriphenyl-phosphonium bromide: triethylene glycol (1:4) at a 1:1 S/F ratio. Kučan et al. [28], [29], [30] investigated the performance of nine ESs based on choline chloride, betaine, and several polyols in the simultaneous extraction of toluene, thiophene, and pyridine. They selected betaine: propane-1,2-diol (1:3.5) as the highest performing ES with extraction efficiencies of 18%, 38%, and 67% for toluene, thiophene, and pyridine, respectively, using a 1:2 S/F ratio. Regarding the application of ILs in this field, it should be noted that the ILs have been extensively reported for the simultaneous dearomatization, desulfurization, and denitrification of fuels [31], [32], [33], [34], [35], [36], [37]. For instance, Larriba et al.[36] proposed utilizing 1-butyl-4-metylpyridinium tricyanomethanide ([4bmpy][TCM]) in the simultaneous extraction of benzene, thiophene, and pyrrole, and their results showed that efficiencies above 98% for all three impurities could be achieved using a 5:1 S/F ratio.

In this work, LLX using an ionic salt-acid-based ES comprised of methyltriphenyl-phosphonium bromide (MTPPBr) as a hydrogen bond acceptor (HBA) and acetic acid (AA) as a hydrogen bond donor (HBD) was investigated for its performance in the simultaneous dearomatization of aromatics and heteroaromatics from diesel. It was previously reported that aromatic-based HBAs can increase the selectivity of the ES towards the aromatic impurities [38]. Hence, MTPPBr was selected as the HBA in the ES. For the HBD, acetic acid was used since its molecule is surrounded by a carbonyl (C = O) functional group. It is expected that the aromatic diesel impurities could easily interact with the molecule’s carbonyl group via the formation of π-π electrostatic interactions [39], [40]. A mixture of impurities consisting of (1) an aromatic “toluene”, (2) a sulfur heteroaromatic “thiophene”, (3) a basic nitrogen heteroaromatic “quinoline”, and (4) a non-basic nitrogen heteroaromatic “pyrrole” were considered to realistically represent the conventional hydrotreatment process, while diesel was represented by n-decane. The composition of the diesel model was arbitrarily selected as {5wt% toluene, 5wt% thiophene, 5wt% quinoline, 5wt% pyrrole, and 80wt% n-decane}.

First, the ES was characterized by its eutectic point and physical properties. Then, the molecular level interactions between the ES and each diesel component were studied using COSMO-RS to gain insights into how the diesel model interacts with the ES. A solubility screening for each diesel component in the ES was then conducted to check the suitability of the ES. The ES was also evaluated by its performance in extracting the impurities from the diesel model using single-stage LLX, and its results were compared to pure acetic acid and benchmark solvents (i.e., sulfolane and dimethyl sulfoxide). Additionally, the LLE data of each diesel impurity with the ES were determined experimentally, predicted using the COSMO-RS quantum-chemical model, and correlated using the non-random two-liquid (NRTL) thermodynamic model. Lastly, a parametric investigation of the influences of several extraction parameters including (1) stirring time, (2) initial concentration, (3) mixing effects, (4) solvent-to-feed ratio, (5) multi-stage extraction, and (6) repetitive usage was conducted.

Section snippets

Chemicals

Table 1 presents the chemical specifications with their corresponding structures, CAS numbers, and mass fraction purities. All chemicals received by Sigma-Aldrich were used without any further treatment.

Solvent preparation and identification of the eutectic point

The ESs were prepared according to the thermal treatment method [15], [41] by mixing a precisely measured amount of MTPPBr with acetic acid at molar ratios of 1:8, 1:6, 1:4, 1:3, and 1:2. The mixtures were then heated to 323.2 K and stirred at 300 rpm using an IKA KS 4000 incubator shaker (±0.1K

Identification of the eutectic point

The solid-liquid phase diagram of the ES was constructed by measuring the melting point of several mixtures of MTPPBr and acetic acid using molar ratios of 1:8, 1:6, 1:4, 1:3, and 1:2 to confirm the eutectic behavior of the MTPPBr and acetic acid mixture. Fig. 1a displays the mixtures 24 h after preparation, and it can be seen that all the mixtures remained liquid at room temperature except the ES at 1:2 M ratio. Following the centrifuge method reported by van den Bruinhorst et al. [53], the

Conclusion

In this work, a salt-acid-based ES comprised of methyltriphenyl-phosphonium bromide (MTPPBr) and acetic acid (AA) was investigated for its performance in the simultaneous dearomatization of aromatics and heteroaromatics from diesel. The diesel model was arbitrarily selected as {5wt% toluene, 5wt% thiophene, 5wt% quinoline, 5wt% pyrrole, and 80wt% n-decane}. First, the ES was characterized by its eutectic point, physical, and critical properties. Second, the molecular-level interactions between

CRediT authorship contribution statement

Ahmad S. Darwish: Conceptualization, Methodology, Investigation, Software, Writing - original draft, Writing - review & editing. Farah Abu Hatab: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Tarek Lemaoui: Methodology, Investigation, Software, Writing - original draft, Writing - review & editing. Omar A. Z. Ibrahim: Methodology, Investigation, Writing - original draft. Ghaiath Almustafa: Methodology, Investigation, Writing - original

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

“The authors would like to express their gratitude to Khalifa University for financially supporting this work through award numbers CIRA-2018-069 and CIRA-2018-023. The authors also acknowledge the support of the Center for Membrane and Advanced Water Technology (CMAT, RC2-2018-009) at Khalifa University in UAE, and the support of Laboratoire des Matériaux Polymères Multiphasiques (LMPMP) and Energetics and Solid-State Electrochemistry Laboratory (LEES) at Université Ferhat ABBAS in Algeria”.

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    Shared first authorship between Ahmad S. Darwish and Farah Abu Hatab.

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