Liquid phase axial mixing in distillation column packed with structured packing: Effect on column performance

Abstract

We present values of the Bodenstein number, BoL, characterizing liquid phase axial mixing, for the atmospheric distillation of three binary mixtures of primary alcohols and a mixture of cyclohexane/n-heptane, all of which were distilled in a column packed with Mellapak 250.Y or Mellapak 452.Y. In addition, volumetric mass transfer coefficients (kLa, kGa) were evaluated from the experimental composition profiles along the distillation column. To describe the distillation process, axial mixing in the phases was considered and modelled as axial dispersion. The results of our modelling call for a reassessment of the relative importance of the process-affecting phenomena. In particular, while vapour phase mass transfer resistance was confirmed as most important, liquid phase axial mixing is shown to have a significant impact under all conditions. For the C6/C7 mixture on Mellapak 452.Y, liquid phase mass transfer resistance was negligible when axial mixing was taken into account. Conversely, non-zero liquid phase mass transfer resistance was found for the alcohol mixtures in all cases. However, our investigation revealed that this is most likely a relic of the usage of the simplistic plug flow model.