Separation and Purification Technology 234 (2020) 116128 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Separation of benzene from methylcycloalkanes by extractive distillation T with cyano-based ionic liquids: Experimental and CPA EoS modelling ⁎ Miguel Ayusoa, Pablo Navarrob,c, , André M. Palmab, Marcos Larribaa, Noemí Delgado-Melladoa, Julián Garcíaa, Francisco Rodrígueza, João A.P. Coutinhob, Pedro J. Carvalhob a Department of Chemical Engineering and Materials, Complutense University of Madrid, Madrid, Spain b CICECO – Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, Aveiro, Portugal c Department of Chemical Engineering, Universidad Autónoma de Madrid, Madrid, Spain ARTICLE INFO ABSTRACT Keywords: The aromatic/aliphatic separation using ionic liquids (ILs) has been proposed as an enhanced technology when Ionic liquids compared to conventional liquid-liquid extraction and extractive distillation processes. Some ILs show extractive Aromatic/aliphatic separation properties (distribution ratios and selectivities) greater than those of conventional solvents, like sulfolane and N- Extractive distillation methylpyrrolidone, positioning these solvents to address challenging separations. Methylcycloalkanes and HS-GC benzene are close-boiling mixtures in the petrochemical industry, presented at the hydrogenated pyrolysis CPA EoS naphtha where benzene is at a high concentration (ca. 70 wt%). Aiming to tackle this separation, cyano-based ILs were tested as mass agents in the benzene separation from methylcycloalkanes. A complete phase equilibria characterization with the most promising ILs, [C2C1im][DCA] and [C2C1im][SCN], were done, using a recently developed experimental-modelling strategy. Isothermal vapour-liquid equilibria for binary, ternary and qua- ternary systems was measured by Headspace Gas-Chromatography (HS-GC) and modelled using the Cubic Plus Association (CPA) Equation of State (EoS). The presence of the ILs improved the methylcycloalkane/benzene relative volatilities by one order of magnitude, whereas homogeneous and heterogeneous regions were de- termined by HS-GC, leading to a consistent model. 1. Introduction combination of widely different anions and cations [4–11]. Applica- tions range from the recovery of valued-product from water streams The separation of aromatics hydrocarbons from linear and cyclic with hydrophobic ILs [4,9] to hydrocarbon separations [5,6,10,11] and aliphatic hydrocarbons presents several challenges due to the proximity biological and pharma separations [7,8]. of their boiling points. Concentrations of aromatic hydrocarbons in the Among others, their negligible vapor pressure, high thermal and feedstock determine the technology of separation to be selected. Liquid- chemical stability, and wide liquidus range are their most outstanding liquid extraction is used when aromatic contents are between 20 and properties [12,13]. ILs have been successfully evaluated as solvents to 65 wt%, whereas the extractive distillation is selected for aromatic extract aromatics and olefins from alkanes [14–23]. Aromatic/aliphatic contents between 65 and 90 wt% [1]. The main sources of aromatic separation was studied by experimental [15,16] and computational hydrocarbons are the reformer and pyrolysis gasolines with aromatic approaches [17] in simplified n-heptane/toluene systems to narrow contents of 55 wt% and 66 wt%, respectively [2]. However, other down the most effective solvents, whereas multicomponent streams feedstocks with higher aromatic contents can be found in a refinery, were evaluated by approximate simulations [18,20] and pilot plant such as the hydrogenated pyrolysis naphtha or benzole from coal car- studies [23]. In these studies, ILs allowed to achieve high aromatic/ bonization [3]. Although these industrial streams are not the principal aliphatic selectivity and higher capacities than those reported for con- sources of aromatic hydrocarbons, they constitute a considerable op- ventional solvents, such as sulfolane or N-methylpyrrolidone [24], portunity to explore additional resources. evidencing a real applicability. Scarce explored, the alkane/alkene se- In the last decade, ILs have received great interest as alternatives paration is being developed in the recent years with interesting inter- solvents in a wide variety of separation processes, owing to their tun- actions but more limited extractive properties [19,21,22]. In a previous able physicochemical and extractive properties by the simple work the suitability of the extractive distillation with ILs to separate ⁎ Corresponding author at: Department of Chemical Engineering, Universidad Autónoma de Madrid, Madrid, Spain. E-mail address: [email protected] (P. Navarro). https://doi.org/10.1016/j.seppur.2019.116128 Received 28 May 2019; Received in revised form 13 August 2019; Accepted 23 September 2019 Available online 24 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved. M. Ayuso, et al. Separation and Purification Technology 234 (2020) 116128 Table 1 Compound description, supplier, mass fraction purity and water content. Chemical Supplier Mass fraction purity/wt.% Water content/ppm 1-ethyl-3-methylimidazolium thiocyanate Iolitec GmbH 98 < 300 [C2C1im][SCN] 1-ethyl-3-methylimidazolium dicyanamide Iolitec GmbH 98 < 300 [C2C1im][DCA] 1-ethyl-3-methylimidazolium tricyanomethanide Iolitec GmbH 98 < 300 [C2C1im][TCM] 1-butyl-4-methylpyridinium tricyanomethanide Iolitec GmbH 98 < 300 [4-C4C1py][TCM] bis(1-ethyl-3-methylimidazolium) tetrathiocyanatocobaltate Iolitec GmbH 99 < 300 [C2C1im]2[Co(SCN)4] bis(1-butyl-3-methylimidazolium) tetrathiocyanatocobaltate Iolitec GmbH 99 < 300 [C4C1im]2[Co(SCN)4] Methylcyclopentane Sigma-Aldrich ≥99.0 Anhydrous Methylcyclohexane Sigma-Aldrich ≥99.0 Anhydrous Benzene Sigma-Aldrich 99.8 Anhydrous Table 2 because its relative volatility is close to unity and the formation of an Mass basis composition of the hydrogenated pyr- azeotrope of minimum boiling temperature (T = 333.2 K; olysis naphtha. ymethylcyclopentane = 0.861) [32]. This mixture presents great interest in Chemical wt.% the hydrogenated pyrolysis naphtha where methylcyclohexane also has a non-negligible presence [30,33]. Methylcyclohexane/benzene re- Methylcyclopentane 24.0 lative volatility is near unity at high benzene concentration, which is Methylcyclohexane 4.0 the case of interest, making also difficult the separation of benzene from Benzene 72.0 its binary mixture with methylcyclohexane. Regarding the ternary mixture involving both methylcycloalkanes and benzene, boiling point toluene and n-heptane was evaluated [25,26], highlighting the notably is just in the middle, making a real challenge the benzene separation. grown interest on the extractive distillation to overcome the inherent Therefore, an effective process to separate the benzene from these cy- limitations to liquid-liquid extraction, showing enhanced relative vo- cloalkanes could suppose an additional source of this aromatic com- latility of the key components to be separated; the standards of purity pound to the petrochemical industry. are obtained in only one equipment and further purification steps at The separation of benzene from methylyclopentane or methylcy- high vacuum were not needed as detailed in the literature for liquid- clohexane by extractive distillation with ILs is studied in this work. liquid extraction [27–31]. Firstly, a screening of six cyano-based ILs, namely 1-ethyl-3-methyli- Aiming at evaluating the use of ILs and extractive distillation for midazolium thiocyanate ([C2C1im][SCN]), 1-ethyl-3-methylimidazo- these challenging separations more complex and industrially relevant lium dicyanamide ([C2C1im][DCA]), 1-ethyl-3-methylimidazolium tri- mixtures must be addressed. The benzene/methylcyclopentane mixture cyanomethanide ([C2C1im][TCM]), 1-butyl-4-methylpyridinium is one of the most difficult separations in the oil industry context tricyanomethanide ([4-C4C1py][TCM]), bis(1-ethyl-3-methylimidazo- lium) tetrathiocyanatocobaltate ([C2C1im]2[Co(SCN)4]) and bis(1- Fig. 1. Cyano-based IL screening in the methylcycloalkane separation from benzene (MCP: methylcyclopentane; MCH: methylcyclohexane; B: benzene). Diagonal stripes are the methylcycloalkane/benzene relative volatility used as benchmark, black bars denote S/F = 1, grey bars denote S/F = 5 and white bars denote S/ F = 10. Free solvent content of benzene in the feed was 80 wt%. 2 M. Ayuso, et al. Separation and Purification Technology 234 (2020) 116128 Table 3 butyl-3-methylimidazolium) tetrathiocyanatocobaltate ([C4C1im]2[Co △ CPA EoS molecular parameters, deviations for , CP and critical properties for (SCN)4]), was made. The choice of these ILs is based on the good per- the [C2C1im][DCA] and [C2C1im][SCN] ILs. formance in the toluene/n-heptane separation, with these ILs pre- IL [C2C1im][SCN] [C2C1im][DCA] senting the best combination of toluene/n-heptane selectivity and to- 2- luene distribution ratio. Indeed, the [Co(SCN)4] anion-based ionic Molecular parameters 6 −2 liquids were chosen because the presence of transition salts improves a0/Pa·m ·mol 6.79 9.31 4 3 −1 the aromatic separation from alkanes, as reported in previous works 10 b/m ·mol 1.46 1.56 c1 1.16 0.84 [25,34]. The corresponding isothermal vapor-liquid or vapor-liquid-li- 102 β 2.79 2.79 quid equilibria (VLE/VLLE) was evaluated for the {methylcyclo- 4 −1 10 ε/kJ·mol 1.51 1.51 pentane/methylcyclohexane + benzene