(117d) Comprehensive Study of Axial Dispersion on Several Modern Structured and Random Packings

Axial dispersion is an undesirable phenomenon and it is not the subject of attention in the literature in the last years. Axial dispersion degrades efficiency of the separation equipment through reduction of the mass-transfer driving force, and in the limit, a column could behave like a single equilibrium stage. This work is focused on determination of axial dispersion in the form of Bodenstein number for liquid phase for five types of modern packings in a packed absorption column. Packings used in this work are random packings NeXRing0.7 and NeXRing1.2 and structured packings MellaPak350Y, 452Y and 500Y. The data obtained from dynamic experiments in the form of a response to a step change in tracer concentration, are compared to diffusion-type differential equation, which was solved with boundary conditions for a closed system. By measuring responses at two different packed bed heights and using convolution, the influence of end effects was eliminated.

The lowest axial dispersion was obtained for M452Y, and it is almost twice as lower than for M250Y. Other structured packings have an axial dispersion comparable, especially at low liquid rates. It was also found that the values of the Bodenstein number for M350Y and M452Y are dependent on the gas flow rate which is in contrary to the literature.

Axial dispersion found for random packings is generally higher compared to structured packings. However, compared to other types of random packings (Pall rings 25mm, Intalox saddle 25mm), NXR0.7 and NXR1.2 have an axial dispersion up to twice as lower.

The power function fits the obtained data for M250Y, M350Y and M500Y with an accuracy of ± 20 %. The correlation for random packings fits the data with an accuracy of ± 15 % for NXR0.7 and ± 20 % for NXR1.2.