Influence of liquid viscosity on separation efficiency of structured packings – modelling approach based on X-ray tomography investigations

The objectives of this work are the investigation of liquid flow morphology inside a structured packing using X-ray tomography and the development of a modelling approach based on hydrodynamic analogy between the real complex flow patterns and simplified fluid-dynamic elements. To study the influence of viscosity, water and mixtures of water and glycerine with varying glycerine fraction are used as working liquids.
X-ray tomography is applied to determine the spatial distribution of liquid in the cross-section of a column filled with MellapakPlus 752.Y packing elements. The resulting images are used to evaluate liquid hold-up, gas-liquid interfacial area and to analyse liquid morphology. Liquid flow patterns (film flow, contact-point liquid, flooded regions) are identified, and the fraction of liquid within each flow pattern depending on flow rate and liquid viscosity is determined.
The results of the liquid flow morphology analysis are used to develop a hydrodynamic analogy model. To implement the gas-liquid contact area and the flooded regions into this model, the packing is represented as a bundle of dry, filled and irrigated cylindrical channels, while the ratio between different type channels is determined from the analysis of tomographic images. This simplified hydrodynamic description allows a direct application of rigorous partial differential transport equations, and their solution yields local concentration fields which are used for the evaluation of the separation efficiency.
The new modelling approach is validated by comparison with separation efficiency data obtained from experiments with CO2 desorption from saturated water-glycerine mixtures into air. The presented modelling approach is capable of predicting the influence of viscosity on separation efficiency of columns filled with structured packings.