(393b) Extractive Distillation with Ionic Liquids: Continuous Vs. Batch Operation

Distillation is still the most favored separation technique in chemical industries for separating liquid mixtures [1]. However, the formation of azeotropes impedes distillation operations, as they cannot be separated by conventional means. To address this, extractive distillation (ED) has been suggested as a viable technique for separating azeotropic mixtures by utilizing a higher boiling point entrainer. Organic solvents have been widely used over the years as an entrainer [2]. In recent years, there has been a growing interest in using ionic liquids (ILs) as an alternative to organic solvents due to their high separation efficiency, low vapor pressure, relatively low melting point, and recyclability [3, 4]. Continuous mode is the preferred operating method for ED columns in most industrial applications. For small-scale operations such as the production of uncertain or small quantities, seasonal, high-purity chemicals in pharmaceuticals, fine chemicals, and a variety of other industries, batch extractive distillation (BED) has been a widely used separation method over the years [5].

In BED operation, both the solvent and the azeotropic mixture are fed at the still pot (i.e., the reboiler) at the beginning of the operation [6]. Then the still pot is heated to create the vapor-liquid equilibrium. Due to its low vapor pressure, IL solvents may fail to attain the required separation purity in BED operation. To examine the feasibility of ILs in BED, we have investigated two different configurations, batch still pot without any stages and with multiple stages. Additionally, we have also investigated a semi-batch process where the ILs are fed to the column continuously from the top. While separating the azeotropic refrigerant mixture R-410A, both batch configurations fail with IL as the solvent. Conversely, the semi-batch configuration attains more than 99 wt.% product purity. However, it requires a significantly higher amount of IL and heat duty compared to a continuous process. We have also investigated the feasibility of the BED process for the dehydration of ethanol with both organic solvent and IL. Our results show that while the organic solvent, ethylene glycol can produce higher-purity ethanol in a BED process, the IL-based process fails to attain the required purity. The main difficulty of IL-solvents is that they do not vaporize and so the additional stages above the still pot (or reboiler) are not used. This summarizes the fact that ILs are not a feasible entrainer in BED operation for any azeotropic separations. In order to take advantage of ILs, the process must be converted into a scaled-down continuous process. In this work, we have also investigated the economic feasibility of the scaled-down continuous ED process.

References:

[1] Tian, Y., Demirel, S.E., Hasan, M.M.F. and Pistikopoulos, E.N., 2018. An overview of process systems engineering approaches for process intensification: State of the art. Chemical Engineering and Processing-Process Intensification, 133, pp.160-210.

[2] Fadia, G., Hassiba, B. and Weifeng, S., 2022. Separation of ethanol–water mixture by extractive distillation using pyridinium-based ionic liquid 1-ethyl-3-methylpyridinium ethylsulfate. Chemical Engineering and Processing-Process Intensification, 173, p.108815.

[3] Monjur, M.S., Iftakher, A. and Hasan, M. M. F., 2022. Separation Process Synthesis for High-GWP Refrigerant Mixtures: Extractive Distillation using Ionic Liquids. Industrial & Engineering Chemistry Research, 61(12), pp.4390–4406.

[4] Monjur, M.S., Iftakher, A. and Hasan, M.F., 2022. Sustainable Process Intensification of Refrigerant Mixture Separation and Management: A Multiscale Material Screening and Process Design Approach. In Computer Aided Chemical Engineering (Vol. 49, pp. 661-666). Elsevier.

[5] Parhi, S.S., Pramanik, A., Rangaiah, G.P. and Jana, A.K., 2020. Evolutionary algorithm based multiobjective optimization of vapor recompressed batch extractive distillation: assessing economic potential and environmental impact. Industrial & Engineering Chemistry Research, 59(11), pp.5032-5046.

[6] Pacheco-Basulto, J.Á., Hernández-McConville, D., Barroso-Muñoz, F.O., Hernández, S., Segovia-Hernández, J.G., Castro-Montoya, A.J. and Bonilla-Petriciolet, A., 2012. Purification of bioethanol using extractive batch distillation: Simulation and experimental studies. Chemical Engineering and Processing: Process Intensification, 61, pp.30-35.