Validation of the First Transition Velocities in Various Bubble Columns Based on Comparison of Several Sophisticated Identification Methods

The reliable identification of the first transition velocity U_trans[1] in bubble columns (BCs) operating at various operating conditions is very important since this parameter indicates the onset of bubble coalescence. The main hydrodynamic regimes (homogeneous, transition and heterogeneous) in BCs are characterized with different degrees of mixing, heat and mass transfer. In this work, the U_trans[1] values will be identified at various operating conditions by means of different parameters: degree of randomness (DR) (Nedeltchev et al., 2020), reconstruction entropy (RE) (Nedeltchev, 2015), information entropy (IE) (Nedeltchev and Shaikh, 2013) and Kolmogorov entropy (KE) (Schouten et al., 1994). Each of these parameters will be extracted from different time-dependent signals recorded by means of Computed Tomography (CT), X-ray Tomography, Nuclear Gauge Densitometry (NGD), wire-mesh sensor, gauge and differential pressure fluctuations. The BCs have different column diameters and they are equipped with different perforated plate (PP) gas distributors or cross spargers. The signals have been recorded in an air-distilled water, nitrogen-tap water, nitrogen-ethanol and air-therminol systems. It will be demonstrated that in some cases the different identification methods distinguish the same critical gas velocity (see Figs. 1-2), whereas in other cases there is some difference, which implies the existence of a transition region rather than transition point.

A simple empirical rule-of-thumb has been established: when the open area (OA) of the PP gas distributors is lower than 1.0 %, then the U_trans[1] value occurs at around 0.03 m/s. On the other hand, when the OA is higher than 1.0 %, then the U_trans[1] value is identifiable at 0.04 m/s.

References

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Nedeltchev, S., Top, Y., Hlawitschka, M., Schubert, M., Bart, H.-J., "Identification of the regime boundaries in bubble columns based on the degree of randomness in the signals", The Can. J. Chem. Eng., in press (2020). DOI: 10.1002/cjce.23719

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