Isopropyl alcohol (IPOH) is an important organic substance that can be used as a solvent, extractant or chemical intermediate. Isopropyl acetate (IPAC) is an esterification product of isopropyl alcohol with good solubility and miscibility with most organic solvents such as ethanol, ethers and esters. These two compounds are widely used in medicine, coatings, printing inks, fragrances, electronic products and other fields. Due to the limitation of reversible reaction, IPOH is not completely consumed in the process of esterification into IPAC, so the reaction is usually accompanied by the formation of IPAC/IPOH/water ternary azeotrope. It is of great significance to efficiently recover IPOH and IPAC from wastewater.
Extractive distillation (ED) is widely used for the separation of azeotropics in chemical and pharmaceutical processes due to its simple operation and high separation efficiency. High performance solvents are the key to high efficiency and energy saving for the separation of multi-component azeotropics in ED. Traditional organic solvents have the advantages of low cost, low viscosity and easy industrial implementation, but these solvents are volatile and high energy consumption. Ionic liquids are used in ED due to their low saturation vapor pressure, high selectivity, low toxicity and non-flammability. However, high viscosity and high cost limit its industrial application. Mixed extractant consisting of organic solvents and liquefied gas can reduce the viscosity of liquefied gas and has stronger selectivity than pure organic solvents.
Based on this, Yang Jingwei's research team at Qingdao University of Science and Technology proposed an efficient method for separating IPAC/IPOH/water mixtures by ionic liquid mixed solvent extractive distillation (MSED). The structure-activity relationship between the separated azeotrope and the ionic liquid was studied by MD simulation.
Firstly, the selectivity of 169 ionic liquids was calculated using the COSMO-segmental activity coefficient-based pre-screening model. Ionic liquids with AC- have the highest selectivity and can achieve effective separation of IPAC/IPOH/water azeotropic systems. Compared with pyridyl, quaternary ammonium, pyrrolidine and piperidine ionic liquids, the synthesis of imidazolyl ionic liquids is relatively simple and inexpensive. However, alkyl chain length and substituents on imidazole rings have a significant impact on the properties and price of ionic liquids. Therefore, the researchers selected [EMIM] [AC], [EMMIM] [AC], [BMIM] [AC], [BMMIM] [AC], [HMIM] [AC] and [HMMIM] [AC] with high selectivity for further analysis. By further exploring the effect of ionic liquid structure on separation performance through molecular dynamics, the researchers determined that 1-ethyl-3-methylimidazole acetate ([EMIM] [AC]) was the best solvent.
By measuring the vapor-liquid equilibrium of the system under the influence of [EMIM] [AC], the researchers validated the computational model used. The process parameters were optimized using multi-objective genetic algo with the annual total cost and gas emissions as objective functions. They propose an energy-saving heat pump-assisted integrated heat extraction distillation (HPHIED) process to achieve heat recovery in MSED.
Finally, the researchers compared the TAC, TCC, TOC and gas emissions of several processes. The results showed that compared with the single organic solvent extraction distillation process, the TAC and gas emissions of the MSED process were 26.83% and 25.32%, respectively. The TAC and gas emissions of the HPHIED process were 11.02% and 35.83% lower than those of the MSED process, respectively, indicating that the use of mixed extractants significantly reduced the TAC and gas emissions of traditional ED. The combination of thermal integration and heat pump technology further improved the autothermal recovery capacity of MSED and significantly reduced energy consumption. In addition, the researchers obtained the optimal mixing ratio (0.334 [EMIM] [AC] + 0.666 DMSO) and related process parameters through MOGA optimization.
Overall, this work provides theoretical guidance for the conceptual design, solvent screening, and process optimization of ionic liquids in ED, with the potential to promote environmental protection and sustainable resource utilization.
