Investigation Of Ionic Liquids As Solvent For Extractive Separation Of 1-Methylimidazole And Water

1-Methylimidazole is an important fine chemical raw material and intermediate,which has a wide range of uses.It can be synthesized by one-step Debus-Radziszewski reaction of direct cyclization of glyoxal,formaldehyde,methylamine and ammonia to obtain crude products with high water content.In the traditional separation process,1-methylimidazole is obtained by direct distillation,but due to the extremely low concentration of 1-methylimidazole,a lot of energy is consumed.Based on this situation,the recycling ionic liquid is used as extractant to enrich 1-methylimidazole in aqueous solution,and then separate the extractant and 1-methylimidazole product to save energy consumption.In this study,the liquid-liquid equilibrium data of the ternary system of water+1-methylimidazole+1-butyl-3-methylimidazolium hexafluorophosphate（[Bmim][PF₆]）were determined at the temperatures of 303.15-323.15 K.The highest solute distribution coefficient（D）and the selectivity（S）were 1.39 and 41.7,indicating that[Bmim][PF₆]could effectively extract 1-methylimidazole.The experimental results also revealed that[Bmim][PF₆]was more suitable for the extraction of low concentration 1-methylimidazole aqueous solution at lower temperature.At the same time,the NRTL model was used to correlate the liquid-liquid equilibrium data,and the root mean square deviation（RMSD）between the experimental value and the calculated value was less than 0.0218,which indicated that the model could well correlate the experimental data and be used for the practical process design.In order to optimize the designed ionic liquids,the separation properties of several common hydrophobic ionic liquids were investigated.The liquid-liquid equilibrium data of water+1-methylimidazole+different ionic liquids（i.e.,[Hmim][PF₆],[Omim][PF₆],[Omim][BF₄]）systems at 303.15 K were determined.The results showed that the increase of cation chain length of ionic liquids would reduce the solubility of ionic liquids in water,while the selectivity coefficient of the system was improved.The results also indicated that the anion of[PF₆]^-was more favorable for the extraction of 1-methylimidazole.[Omim][PF₆]was believed as the preferred extractant for system,whose solute distribution coefficient and the selectivity could be as high as 1.71 and 111.92,respectively.The NRTL model was used to correlate the liquid-liquid equilibrium data of the systems.The RMSD between the experimental value and the calculated value was less than 0.0188,indicating NRTL model had a good calculation accuracy for the ternary LLE of water+1-methylimidazole+ionic liquid.According to the above experimental results,the process of extracting and separating 1-methylimidazole aqueous solution was designed by using ionic liquid[Omim][PF₆]as the extractant.The compound in 1-methylimidazole aqueous solution was extracted by ionic liquid in the extraction tower,[Omim][PF₆]was separated and recovered by flash,and 1-methylimidazole was purified by a distillation column.The process was simulated by Aspen Plus.The influence of theoretical plate,ratio of ionic liquid extractant to feed amount on the separation effect of the extraction tower were systematically investigated.The optimized operation conditions of 1-methylimidazole extraction by ionic liquid were obtained and the total flow was optimized.Under the optimized conditions,the recovery and the purity of 1-methylimidazole could both reach 99.9%.The energy consumption of the extraction process was compared with the traditional separation of 1-methylimidazole aqueous solution.When the same separation effect was achieved,the heat load of direct distillation separation was 270.6kW.Compared with the thermal load of extraction process of 61.11 kW,it revealed that the energy consumption of the process of extracting 1-methylimidazole aqueous solution by ionic liquid was far lower than that of the traditional separation process.The results proved that the process designed by this study was a promising method for the separation of 1-methylimidazole solution at low concentration in the practical applications.