Investigations on axial gas dispersion coefficients in bubble columns using gas flow modulation

Gas-liquid contactors, such as bubble columns, are subject to dispersion phenomena in both gas and liquid phase. The Axial Dispersion Model (ADM) is the most widely used theoretical approach to account for the effects of dispersion [1].
A reliable quantification of the axial dispersion coefficients is crucial for process performance assessment as well as design and optimization of such contactors. Conventional approaches for determining axial dispersion coefficients rely on tracer substances. However, such methods are hardly universally applicable, may cause detrimental impurities or process downtimes and can even alter the physical properties of the system.
To overcome these issues, Döß et al. [2] introduced a novel non-invasive approach for determining the axial gas dispersion coefficient in bubble columns. Instead of a tracer substance, a marginal sinusoidal modulation is superimposed to the gas inlet flow rate and used as a virtual tracer. This modulation introduces a sinusoidal variation of the gas holdup in time, called gas density wave. Along the column, the gas density wave is damped in amplitude and is shifted in phase, due to gas dispersion. Amplitude damping and phase shift can be measured and related to the value of the axial dispersion coefficient via a dispersion model. A schematic sketch of the working principle is provided in Figure 1.
Döß et al. [2] successfully used sinusoidal-resolved gamma-ray densitometry to investigate the amplitude damping and phase shift. The deviation caused by the statistical behaviour of the gamma-ray photons was reduced by increasing the measurement time.As the operation of gamma-ray sources may be challenging for industrial applicability, this study assesses the possibility of using alternative non-radiative techniques to measure the gas density wave. Several measurement techniques and different gas modulation schemes in terms of initial modulation amplitude and frequency have been studied to ensure detectable amplitude and phase changes at chosen axial positions, while not altering the hydrodynamic behaviour. Uncertainties associated with the axial dispersion coefficient have been evaluated in comparison to gamma-ray densitometry.