Systematic investigation of distillation efficiency in aqueous systems

The reduction of the distillation efficiency on the water-rich side has been reported in all types of internals: trays, random and structured packings. In order to find an explanation of this, several authors have presented diverse approaches. However, a direct comparison of the results is extremely difficult and the studies are limited in scope because of the specific setup employed and the distillation conditions chosen. In addition, the test systems used by the researchers are very heterogeneous and allow only a reduced range of application. For these reasons, the Thermal Process Engineering group at AVT (Aachener Verfahrenstechnik) of RWTH Aachen University and the industrial partners BASF AG, Bayer AG, Evonik AG, and DSM launched a cooperation project in order to develop a better prediction model for the distillation efficiency of aqueous systems. This work presents the methodology and results obtained in this project. The focus of this research was the investigation of the parameters responsible for the pronounced drop in the distillation efficiency at the water-rich side in aqueous systems. For the identification and quantification of parameters affecting the efficiency, an extensive experimental procedure was carried out on the miniplant scale and its results were compared to a rate-based modelling of the distillation efficiency. Five test systems were measured in four different types of distillation internals: three tray columns and one packed column. These choices allow a systematic evaluation of the various proposed key variables. Thus, on the whole, by careful measurements in systematically chosen column types comparing several well-selected material systems and modelling, it has been possible to elucidate the major contributions to the decrease in efficiency.