Dehydration of Aqueous Acetonitrile Solutions by Extractive Distillation Using Deep Eutectic Solvents

Abstract

The increasing concern about environmental issues and the requirement to reduce the negative influence of industrial processes has directed the attention of scientific community towards the development of novel “green solvents”. This growing demand for green solvents has spurred the development of environmentally benign solvents. Separation of azeotropic mixtures is of particular interest for scientific community because it represents separation limit by conventional distillation. Extractive distillation is the most applied technique for the separation of azeotropic/close boiling mixtures. Application of non-toxic and biodegradable solvents in extractive distillation is today’s need, due to the environmental concern. From the last two decades, application of ionic liquids (ILs) as an entrainer for azeotropic mixture separation, significantly encouraged the researchers due to their distinctive prominent features. Gradually, the use of ILs for separation applications became restricted due to complex synthesis process, difficulties in purification, high cost, potential toxicity and poor degradability. Recently, Deep Eutectic Solvents (DESs) which are ionic liquids (ILs) analogues have appeared as a promising substitute to conventional volatile organic solvents. Unlike ILs, DESs offer inexpensive and easy synthesis, less toxicity and good biodegradability. Due to the multitasking nature of DESs, these have been applied to many chemical processes. DESs are combination of two (or more) components, commonly a hydrogen-bond acceptor (HBA) and a hydrogen-bond donor (HBD), which forms a eutectic mixture. Because of the strong hydrogenbonding impact, these have much lower melting temperature than those of starting original substances. Due to these unusual chemical and physical properties, DESs are currently getting remarkable attention as “greener” means for a wide variety of applications. This study focuses on the separation of a common industrial waste, acetonitrile (ACN) + water mixture via extractive distillation. Due to the very high market demand of ACN, its separation from water mixture is of special interest for researchers. However, its recovery from waste effluents is often difficult as ACN + water solution forms a minimum-boiling azeotrope of composition of 67.4 mole% ACN at 76.5 ˚C and standard atmospheric pressure. Many conventional solvents like; ethylene glycol, butyl acetate and dimethyl sulfoxide (DMSO) have been used earlier for dehydration of acetonitrile. Although, high entrainer requirement, uneven mixing of ethylene glycol with the mixture and recycling issue of butyl acetate (because of its high volatility), restricts the application of these entrainers. Few imidazolium based ionic liquids (ILs) have been used for separation of this azeotropic mixture. 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), 1-butyl-3- methylimidazolium tetrafluoroborate ([Bmim]-[BF4]), and 1-butyl-3- methylimidazolium dibutyl phosphate ([Bmim][DBP]) have been used for this purpose. In spite of many extraordinary physicochemical properties of these ILs, its toxic nature, non-biodegradability and high cost of synthesis and purification became the main barrier in its extensive utilization in separation processes. The main objective of this thesis was to synthesize different types of DESs and examine its viability as entrainer for separation of ACN + water mixture by extractive distillation. Two different classes of DESs i.e. sugar based DESs and natural DESs were prepared by considering different types of HBDs and HBAs. Glycolic Acid and Malic acid were used as HBD and choline Chloride and Tetramethylammonium chloride were used as HBA in different molar ratios for this purpose. Two sugar based DESs glycolic acid + choline chloride 3:1 (GC 3:1) and glycolic acid + tetramethylammonium chloride 3:1 (GTM 3:1) and two natural DESs malic acid + choline chloride 1:1 (MC 1:1) and malic acid + tetramethylammonium chloride 1:1 (MTM 1:1) were synthesized. These were characterized for relevant chemical, physical and thermal properties like, FT-IR spectroscopy, 1H NMR spectroscopy, Thermogravimetric analysis, viscosity, density and moisture content. These synthesized DESs were utilized as entrainer for vapor–liquid equilibrium (VLE) studies of ACN + water mixture via extractive distillation. A modified Othmer type recirculation still was employed for VLE studies. Isobaric VLE studies for pseudobinary systems (ACN + DES, water + DES) and pseudoternary systems (ACN + water + DES) were performed at atmospheric pressure. Studied pseudobinary systems include, ACN + GC3:1, ACN + GTM 3:1, ACN + MC1:1, ACN + MTM 1:1, water + GC 3:1, water + GTM 3:1, water + MC1:1 and water + MTM 1:1. Pseudoternary VLE data were generated for systems ACN + water + GC 3:1, ACN + water + GTM 3:1, ACN + water + MC1:1 and ACN + water + MTM 1:1 for different DES molar compositions (5%, 10%, 15% mol/mol). From results it was observed that, addition of these DESs (GC 3:1, GTM 3:1, MC1:1 and MTM 1:1) created a significant salting-out effect by increasing the relative volatility of ACN to water, and could eventually remove the azeotrope. However, this effect was better in case of sugar based DESs (GC 3:1, GTM 3:1) as compared to natural DESs (MC 1:1, MTM 1:1). MTM 1:1 could not break the azeotrope at lowest studied dose (5% mol/mol) but at higher dosage it could successfully break the azeotrope. Nonetheless, exceptional solubilizing and stabilizing properties of NADESs (due to uncommon intermolecular adjustment in its matrix), natural & biodegradable starting compounds and non-corrosive nature adds more value to be used as entrainer. Although all four DESs were capable in breaking the ACN + Water azeotrope (at different dosage) but GTM 3:1 presented the highest relative volatility at a given concentration. Comparing the performances of simple sugar based DESs with NADES, the simple DESs were performing better than NADESs following the trend GTM 3:1> GC 3:1 > MC 1:1 > MTM 1:1. The high viscosity of MC1:1 and MTM 1:1 is a limiting factor in separation processes, but by manipulating its properties it can be used as entrainer in future. Further, experimental results (for pseudobinary & pseudoternary systems) were validated employing nonrandom two-liquid (NRTL) model with minor deviations. The average absolute deviation in vapor phase mole fraction of ACN in case of GC 3:1, GTM 3:1, MC 1:1 and MTM 1:1 was 0.009, 0.007, 0.002 and 0.001 respectively. The average absolute difference for the equilibrium temperature of GC 3:1, GTM 3:1, MC 1:1 and MTM 1:1 was 0.42 K, 0.45K, 0.15 K and 0.45 K respectively. Both types of DESs were also retrieved successfully and reused till five cycles with no notable change in chemical properties but slight decrease in performance of NADESs was observed.