Low transition temperature mixtures (LTTMs) as novel entrainers in extractive distillation
Introduction
The separation of azeotropic mixtures is one of the main challenges in the separation technology field. The scientific community is continuously trying to improve this kind of separation, because it is one of the most inefficient steps in process technology. Ethanol dehydration was chosen for this study due to the industrial relevance of this system.
Ethanol is commercially produced by either catalytic hydration of ethylene or fermentation of sugars. Anhydrous ethanol is used as fuel, chemical reagent, organic solvent or raw material for many important chemicals among other applications [1]. Many processes have been proposed to break the water–ethanol azeotrope in order to separate pure ethanol from a dilute aqueous solution: azeotropic distillation, extractive distillation, supercritical fluid extraction, adsorption on molecular sieves, pervaporation, and vacuum distillation [2]. Extractive distillation is a promising option, because a relatively non-volatile liquid solvent can be used (low emissions) and it can be carried out in a continuous mode (no swing operation). The presence of the low volatile solvent alters the volatility of one of the feed components more than the other, so that the separation of the feed components can be achieved in the column [1].
Salts can be used as selective entrainers in extractive distillation, but the main disadvantages are solid handling, corrosion and pollution. Novel solvents could show similar selectivities, but because they are liquid, the handling problem is overcome. For example, in the last two decades, ionic liquids (ILs) have attracted considerable attention because of their potential use as solvents in extractive distillation of alcohol–water systems [3], [4] with advantages, such as low price and easy synthesis with no purification required [6]. LTTMs are commonly defined as mixtures of two or more solid compounds, which have much lower melting point than the initial components due to hydrogen bonding. Originally, LTTMs were called deep eutectic solvents (DESs), but this name does not cover the complete class of solvents, because many mixtures do not show an (eutectic) melting point, but a glass transition instead.
DESs were first reported by Abbott in 2003 [7] and since then, the publications in this field have increased drastically. Different applications for LTTMs have been studied [8]: electrochemistry [9], [10], [11], [12], [13], [14], [15], material preparation [16], [17], catalytic reactions [18], [19], biomass deconstruction [20], biodiesel processing [21], [22] and separation processes such as liquid–liquid extraction [23], [24], [25] and CO2 capture [26] among others. The most studied LTTM is the urea-choline chloride (2:1) mixture. However, there exist many different LTTMs by combining different hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) in different ratios, most of which still need to be studied. Each LTTM has its own properties, but there are some common properties that make them suitable as entrainers. Among these properties the most remarkable for extractive distillation are: low vapour pressure, wide liquid range, water compatibility, biodegradability, non-flammability and easy and cheap preparation by mixing natural and readily starting materials [5].
In this work, LTTMs are for the first time used as entrainers for the ethanol–water extractive distillation. Four different choline–chloride-based LTTMs were selected in order to study the influence of the HBD and the HBD:HBA ratio: (i) lactic acid–choline chloride 2:1 (LC 2:1), (ii) malic acid–choline chloride 1:1 (MC 1:1), (iii) glycolic acid–choline chloride 3:1 (GC 3:1) and (iv) glycolic acid–choline chloride 1:1 (GC 1:1). The LTTMs GC 3:1 and GC 1:1 are reported for the first time, and have also been characterized. VLE data of the pseudo-binary (water–LTTM and ethanol–LTTM) and pseudo-ternary (ethanol–water–LTTM) mixtures have been measured in order to study if the azeotrope can be broken
Section snippets
Chemicals
Table 1 shows the chemicals used in this work, including their supplier and purity. Ethanol (≥ 99.5%) was purchased from TechniSolv. Glycolic acid (≥ 99%) was provided by Sigma–Aldrich. D-L malic acid was purchased from Merck Chemicals (≥ 99%). Choline chloride was suplied by Sigma–Aldrich (≥98%). L-lactic acid was kindly provided by Purac. Deionized MilliQ water was used in all the studied mixtures (≥18.2 MΩ × cm). Choline chloride, malic acid, lactic acid and glycolic acid were dried under vacuum
Results and discussion
The vapour pressure of pure ethanol and water was measured in the range of 50–1000 mbar in order to test de accuracy of the ebulliometer. Antoine coefficients for the experimental data were calculated together with the absolute deviation between the experimental and the calculated data using literature coefficients [27]. All results are shown in Table 4.
In order to study the interactions between the entrainers and the components of the azeotropic system (ethanol/water), VLEs of the pseudo-binary
Conclusions
This is the first time that LTTMs are used as entrainers in extractive distillation. It is shown that the addition of three of the four studied LTTMs are able to break the azeotrope. The LTTMs GC 3:1, GC 1:1 and MC 1:1 were found to be feasible alternatives for the extractive distillation of the ethanol–water system. However, the LTTM LC 2:1 does not break the azeotrope, but a large displacement to the pure ethanol side is achieved. Thus, the ethanol–water system (and probably many others
Acknowledgements
Financial support from the Netherlands Organization for Scientific Research (NWO), ECHO-STIP grant no. 717.012.002, is gratefully acknowledged. The authors would also like to thank Mr. W.M.A. Weggemans for the technical lab support. PURAC Biochem B.V. is acknowledged for providing the lactic acid free of charge.
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