Non-invasive determination of gas phase dispersion coefficients in bubble columns using periodic gas flow modulation

•A method for non-invasive determination of gas dispersion coefficients is introduced.•Frequency response of a periodic modulated gas flow rate is analyzed.•Holdup measurement by gamma-ray densitometry synchronized with gas flow modulation.•Modulation of small magnitude and densitometry measurement ensure non-invasiveness.•Versatile applicability of the method due to variation of measurement parameters.

Non-uniform bubble size and liquid velocity distribution in bubble columns lead to gas phase dispersion. This gas phase backmixing is quantitatively modelled in the axial gas dispersion model by the axial gas dispersion coefficient. However, only few gas phase dispersion data are currently available since experimental investigations are expensive and require the application of suitable gas tracers and their reliable detection. In this study a new approach is introduced, which is based on a lock-in measurement of gas fraction modulation. Experiments were carried out in a bubble column of 100 mm diameter operated with air/water and air/glycol-water, respectively. Gas holdup was measured via gamma-ray densitometry in synchronization with the modulated inlet flow. The axial dispersion model was adopted to determine the gas phase dispersion coefficient from phase shift and amplitude damping of the gas holdup frequency response. A sensitivity analysis was performed to derive a proper modulation scheme. The calculated gas phase dispersion coefficients show excellent agreement with data from literature.

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