AbstractThe suitability of any item of equipment for a given thermal separation process cannot always be judged solely in the light of efficient and loading range data. In vacuum rectification, involving a large number of theoretical stages, the vapour pressure drop per transfer unit exerts a considerable effect on the energy consumption and thus largely governs the choice of column fittings. Furthermore, the pressure drop per transfer unit determines the temperature at the bottom of a rectification column and, hence, the suitability of the equipment for the separation of heat‐sensitive products. It is also an important criterion for compressor requirements in absorption processes. Another factor of considerable importance in separating heat‐sensitive mixtures by distillation is the time during which the product, particularly the liquid phase, is exposed to a given temperature in the column. Therefore, a standardized liquid residence time has been adopted to evaluate the systems in their entirely. This residence time depends on the hydrodynamic conditions in the column and is thus closely related to pressure drop and efficiency. Further important factors, which may decide the selection of column internals, are the specific volume and mass of the column. Based on theoretical considerations, a flow model is derived for describing the main loading range of countercurrent packed columns; the relationships between the above mentioned parameters are presented.