Applicability of gas flow modulation technique for measuring axial gas dispersion coefficients in bubble columns

Dispersion phenomena significantly influence the residence time of the fluid phases in gas-liquid contactors and, thus, their yield. With the axial dispersion model (ADM), the effect of dispersion is considered during the process design. However, this requires a reliable quantification of the axial dispersion coefficients.
A novel non-intrusive approach to determine the axial gas dispersion coefficient in bubble columns was recently pronounced by Hampel [1], called gas flow modulation technique (GFM). This approach is based on an imposed marginal sinusoidal modulation on the constant gas inlet flow. This leads to a modulation of the gas holdup in the bubble column, called holdup wave. Along the column axial direction, the gas dispersion damps the rising holdup wave in amplitude and shifts its phase. Amplitude damping and phase shift can be non-invasively determined, e.g., using gamma-ray densitometry to relate it to the value of the axial dispersion coefficient using the ADM. Figure 1 shows the principle of GFM and a simplified scheme of the experimental setup.
Döß et al. [2] applied the GFM to bubble columns for the first time, although only for a narrow range of operating conditions in a 100 mm ID bubble column. The present study proves the applicability of the GFM to larger columns and for a wider range of gas superficial velocities and liquid properties. Experiments were performed in columns of 100, 150 and 330 mm ID at gas superficial velocities ranging from 17 to 47 mm/s. Air was used as the gas phase. Water, aqueous solutions with 2% wt. ethanol and 30% wt. glycerin were used as the liquid phase. This study, together with the uncertainty analyses recently performed by Marchini et al. [3, 4], qualifies the GFM as a viable non-invasive alternative to traditional tracer studies, for measuring the axial gas dispersion coefficient in bubble columns.