Heteroazeotropic Distillation as a Method for Separating Thermally Sensitive Substances

Abstract

This work introduces heteroazeotropic distillation as a continuous alternative to vacuum distillation for separating thermally unstable substances. Instead of lowering pressure, the approach exploits that heteroazeotropes boil at temperatures below those of their pure components by adding an entrainer that forms heteroazeotropes with the target compounds. Maintaining two liquid phases and one vapor phase throughout the column keeps temperatures low along the full column height, while continuous operation also reduces high-temperature residence times compared to batch processing. The concept is demonstrated in a 50 mm laboratory column equipped with 2.9 m structured gauze packing using two test systems: ethyl acetate/1-butanol/water and 2-methyl-4-pentanone/1-butyl acetate/water, with water as entrainer. These systems differ mainly in heteroazeotrope boiling-point separations and miscibility-gap width, enabling assessment across different phase behaviors. Key operating variables—reflux ratio, feed composition, heat duty, and entrainer hold-up—are systematically varied, showing that the process is feasible and can achieve efficient separations while providing data for model development and validation. Fluid dynamics of the two liquid phases on the packing are additionally studied to support estimation of separation efficiency using literature-based models. The process is modeled with an equilibrium-stage framework that allows one or two liquid phases; because convergence is challenging, a homotopy continuation method is applied to solve the equations robustly. Model predictions agree well with experimental results. Finally, the work discusses degrees of freedom, identifies mixture types best suited to the method, and provides guidance on entrainer selection and process control, positioning heteroazeotropic distillation as a promising complement or alternative to vacuum distillation.