Measurement of liquid distributions in separation columns

Design and optimization of separation units, e.g. distillation and absorption columns, both reactive and non-reactive, require detailed knowledge about the underlying phenomena and their impact on the process behaviour. Although the separation efficiency is pre-determined by complex hydrodynamic conditions in the columns, current studies are mainly based on conventional techniques with strongly limited insights, e.g. on using compartment-type liquid collectors below packings. Except for the Chemical Engineering group at the University of Liège (e.g., Aferka et al., 2010) tomographic imaging technique so far has not found a proper place in investigations of separation columns with structured internals.
Compared to catalytic packings that contain active elements for heterogeneous reaction, column internals are mainly designed to provide sufficient mass transfer area and to generate local turbulence for high mass transfer rates. To meet this goal, both structured packings (e.g., corrugated sheet Sulzer KatapakTM and MellapakTM) and random packings (e.g., Pall rings, Raschig rings, Bearl saddles) have been designed. The strong challenge in flow visualization in such packings is attributed to the flow patterns that incorporate thin liquid films on the surface of the structures, and to their dynamics. Thus, high temporal and spatial resolution is required. In this regard, the application of up-to-date measurement techniques and new visualization concepts would make a step forward in evaluating such processes and in re-designing the apparatuses and internals. In the present work, a detailed study was performed focusing on important hydrodynamic aspects in packings, namely:

• liquid distribution in the cross-section in and below the packing
• resolution of the packing structure
• visualization of liquid film structures, dynamics and their detection limit
• classification of by-passing wall flow and utilizable core flow
• disturbance of the disperse liquid flow using new wire-based sensors
• dynamics of the transition from the loading to the flooding point

Ultra-fast X-ray tomography, high resolution gamma-ray tomography and wire-mesh sensor techniques were applied. In our contribution, the potential of the new measurement techniques for separation columns is discussed and new insights on the hydrodynamic behaviour are provided.