Separation of near-boiling mixtures (acetic acid and water) enhanced by ionic liquid

Abstract

Acetic acid aqueous solution is a common and typical near-boiling mixture in the petroleum, chemical, polymer, pharmaceutical and food industries. Because of the large number of ideal plates for separating acetic acid and water by ordinary distillation, the energy consumption and production cost are high. Designable ionic liquids (ILs) and deep eutectic solvents (DESs) are environmentally-friendly entrainers for extractive distillation. Many azeotropic systems enhanced by adding ILs or DESs have been studied, but the near-boiling water/acetic acid system is rarely discussed.In this study, an IL 1,2,3-trimethylimidazolium dimethylphosphate ([TMIM]+[Me2PO4]−) with easy synthesis, low production cost and good thermal stability was employed as the rectification entrainer for water/acetic acid system, while the DESs were easy to react in acetic acid aqueous solution and was abandoned. First, the relative volatility of water (1) and acetic acid (2) at the more difficult separation point (x1′ = 0.9) was measured using a modified Othmer still at 101.32 kPa. When the mole fraction of entrainers was 0.071, these relative volatilities were 2.80 (n,n-dimethylformamide), 2.88 (1-methyl-2-pyrrolidinone), 4.82 ([TMIM]+[Me2PO4]−) respectively. The separation performance of [TMIM]+[Me2PO4]− was 67% higher than that of 1-methyl-2-pyrrolidinone. Then, the effect of [TMIM]+[Me2PO4]− concentration on the relative volatility was investigated. The relative volatility was dramatically increased from 1.60 to 6.85 with 0–0.103 mol fraction of [TMIM]+[Me2PO4]− and was 4.28 times that in an IL-free system. After that, isobaric vapor–liquid equilibrium (VLE) data of water + acetic acid, water + [TMIM]+[Me2PO4]− and water + acetic acid + [TMIM]+[Me2PO4]− ([TMIM]+[Me2PO4]− at ∼ 5, 15, and 25 mass%) systems were determined at 101.32 kPa. Also, Based on the chemical theory and Hayden–O’Connell method, the binary energy parameters of the NRTL (Non-Random-Two-Liquid) model for these systems were regressed and the predicted values were found to fit the experimental VLE data quite well..For the first time, we have found that [TMIM]+[Me2PO4]− as an entrainer is superior to conventional volatile solvents for water/acetic acid distillation in terms of performance, which provides more choices and imagination for industry. At the same time, it promotes understanding of the influence of IL structure on water/acetic acid separation performance, and is also an important inspiration for the distillation of other near-boiling systems.