Elsevier

Fluid Phase Equilibria

Volume 473, 15 October 2018, Pages 98-105
Fluid Phase Equilibria

Tetramethylammonium chloride + glycerol deep eutectic solvent as separation agent for organic liquid mixtures: Assessment from experimental limiting activity coefficients

https://doi.org/10.1016/j.fluid.2018.06.003Get rights and content

Abstract

Solute retention data acquired by gas-liquid chromatography were used to determine the activity coefficients at infinite dilution for 24 organic solutes in the deep eutectic solvent (DES) consisting of tetramethylammonium chloride and glycerol in a molar ratio of 1.001:2. The organic solutes included alk-1-anes, alk-1-enes, alk-1-ynes, cycloalkanes, alkanols, alkylbenzenes, ketones, esters and heterocyclics. The measurements were undertaken at four different temperatures, viz T = (313.15, 323.15, 333.15 and 343.15) K with an estimated uncertainty of ±3.8%. From the experimental infinite dilution activity coefficient data, the values of partial molar excess enthalpy at infinite dilution were calculated using Gibbs Helmholtz relationship. Limiting selectivity and capacity values were also calculated from experimental limiting activity coefficients and compared to those of other separation agents, including other deep eutectic solvents, ionic liquids and industrial molecular agents. Results obtained in this study indicate that tetramethylammonium chloride + glycerol DES can potentially be used as an alternative solvent for nitrogen and sulphur removal from transportation fuels as well as the separation of cycloalkanes, aromatics and esters from light alkanols.

Introduction

In chemical and petrochemical industries, many separation processes are energy intensive while some use solvents which raise safety and environmental concerns. In general, liquid mixtures encountered in these industries are separated by distillation. When components exhibit close-boiling points or form azeotropes, separation by conventional distillation is not practical. Mixtures containing such components may necessitate the use of advanced units that use solvents to enhance separation. Such separation units include extractive distillation or liquid-liquid extraction. Activity coefficients at infinite dilution data can be exploited to preselect suitable solvents to separate mixtures [1]. They can also be useful for the design and optimisation of separation processes, and the development of predictive thermodynamic models [2,3].

Activity coefficients at infinite dilution can give some indications on separation performance through selectivity (Sij) and capacity (kj) at infinite dilution. These two parameters can be used for the preliminary assessment of the feasability of a solvent–enhanced separation process. Selectivity indicates the ability of a solvent to separate the components of a mixture. A high selectivity value leads to a low number of equilibrium stages in the separation unit (i.e., low investments cost). Capacity is associated with the solubility of a given substance in a solvent. It is related to the amount of solvent to be used in a separation process as well as the operating cost. Low capacity values (i.e kj < 1) indicate that large amounts of extractor agent would be required. In most cases, when selectivity increases, capacity decreases. When both parameters are low, the separation process is inefficient.

Common industrial molecular solvents such as N-methyl-pyrrolidone (NMP) [[4], [5], [6]], N-formylmorpholine (NFM) [4,5] and sulfolane [7,8] provide good separation for various industrial separation problems. However, these solvents are toxic organics. Ionic liquids (ILs) are currently under investigation as potential alternative solvents due to their unique properties such as negligible vapour pressure, wide liquid range, non-toxicity, stability at high temperatures and biodegradability [9]. Due to these favourable properties, ILs have been more appealing for several applications. However, the use of ILs in industrial processes has been hindered by their expensive synthesis and difficult purification processes. This has disadvantaged ILs as potential alternative solvents to industrially used solvents [10] and has triggered the extended quest for more alternative solvents.

The present work is a contribution to the investigation of solvents that can be used to replace industrial molecular solvents for the separation of organic mixtures. The solvent of interest in this work is the deep eutectic solvent (DES) consisting of tetramethylammonium chloride and glycerol mixed in a molar ratio of 1.001:2. A deep eutectic solvent is a mixture generally comprised of a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA), having a melting temperature considerably lower than that of its individual components [9]. DESs are attractive environmentally friendly separation solvents. They are generally cheaper than ionic liquids [11] and are sometimes derived from renewable and non-toxic bioresources while sharing similar properties with ILs [9,12].

Experimental infinite dilution activity coefficient (IDAC) data for systems involving DESs are very scarce. Verevkin et al. [13] reported activity coefficients of selected solutes at infinite dilution in the DES consisting of choline chloride and glycerol in a 1:1 and 1:2 M ratios. A similar study undertaken by our group revealed, from experimental IDAC data, that tetramethylammonium chloride + ethylene glycol DES would be an effective separation solvent for alkanes-pyridine, alkanes-thiophene, aromatics-alcohols, ketones-alcohols and cycloalkanes-alcohols mixtures [13]. The promising results from these two studies have highlighted the need for further investigations into DES-based processes for various separation problems relevant to Chemical Engineering, motivating the present study. Its aim was to assess tetramethylammonium chloride + glycerol deep eutectic solvent as a separation agent for organic liquid mixtures to gain insights into the effect of the HBD and HBA on the separation performance of deep eutectic solvents.

In this study, gas-liquid chromatography (GLC) was used to explore interactions between 24 selected solutes (alk-1-anes, alk-1-enes, alk-1-ynes, cycloalkanes, alkanols, alkylbenzenes, ketones, esters and heterocyclics) and the DES (methylammonium chloride + glycerol) in a 1.001:2 M ratio. Limiting activity coefficient measurements were undertaken at T = (313.15, 323.15, 333.15 and 343.15) K. Furthermore, selectivities and capacities for different separation problems calculated from activity coefficients in this DES were compared with values reported in the literature for some commonly used industrial solvents as well as ionic liquids and deep eutectic solvents.

Section snippets

Materials

Chemicals used in this study were purchased from various suppliers and are provided in Table 1. The suppliers' certified minimum purity of solutes, hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) was better than 98% by mass. Solutes purification was not necessary since the gas-liquid chromatograph (GLC) could separate any impurities during measurements. The deep eutectic solvent was synthesised and purified as part of this work. The structure of its components is shown in Fig. 1,

Equations for limiting activity coefficient calculations

In this work, experimental activity coefficients at infinite dilution were obtained using the following equation [22,23]:lnγ13=ln(n3RTVNP1)(B11v1)P1RT+(2B12v1)J23PoRTwhere n3 is the number of moles of the solvent in the liquid phase; R is the gas constant; VN denotes the net retention volume; Po is the column outlet pressure and is the same as the atmospheric pressure; P1 is the saturated vapour pressure of the solute (1) at temperature T, obtained from the Antoine equation [24]; B11

Results and discussion

Experimental activity coefficients values at infinite dilution for two different amounts of solvent loading on the column packing, 30.21% (7.1213 mmol) and 33.95% (8.0029 mmol) were used to obtain average values reported in Table 3. The importance of using two different column loadings for the measurements of IDACs has been explained in previous studies [[28], [29], [30]]. Fig. 2, Fig. 3, Fig. 4 illustrate the temperature dependence of limiting activity coefficient of all investigated solutes

Conclusion

In this study, activity coefficients at infinite dilution of 24 polar and non-polar organic solutes were determined in tetramethylammonium chloride + glycerol (1.001:2 M ratio) DES through gas-liquid chromatography at four different temperatures. It was found that for polar compounds, solute-solvent interactions are highly dependent on alkyl chain length. Polar organic solutes and hydrogen bond donor compounds strongly interacted with the DES.

Selectivity and capacity values at infinite dilution

Acknowledgement

Funding for this research was provided by the National Research Foundation (NRF).

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