Geometry and viscosity effects on separation efficiency in distillation

Abstract In a distillation process the physical properties of the mixture, operating conditions and the geometry of the packing determine fluid dynamics and mass transfer and thereby the performance of the column. Liquid viscosity has an impact on separation efficiency as well as on capacity. Measurements were carried out with two test mixtures, CB/EB as reference system and one with elevated viscosity, using two different structured packings. Based on experimental data, different predictive models were assessed with respect to their capability to describe the impact of viscosity on column efficiency. Increase of viscosity by a factor up to 6 resulted in a decrease of separation efficiency by 50%, higher pressure drop and lower capacity. The measurement of liquid hold-up revealed no changes due to packing geometry, but a significant increase for higher viscosities. Whilst separation efficiency at low viscosity was sufficiently met by the rate based models, the influence of increased viscosity could not be predicted correctly. The results reveal that even at moderate viscosities column performance is affected significantly. Hence, if rate based models are applied to conditions deviating from those they were validated for, their accuracy has to be considered carefully.