Analysis of sparger effects on axial gas dispersion in bubble columns

The uniformity of the gas distribution in bubble columns strongly depends on the gas sparger design. For example, not all holes of a perforated plate sparger with relatively high fractional free area are active at the same time. This is true especially at low gas flow rates. Consequently, the random activation of the holes causes gas maldistribution in the radial as well as in the axial directions. Contrary, needle spargers provide rather uniform distribution at the same operating conditions.
Since dispersion phenomena depend on the gas holdup gradient, the gas maldistribution is expected to affect the dispersion. Surprisingly, only few publications address the possible influence of the sparger on gas dispersion. An exception is the study of Kölbel [1]. They measured the axial gas dispersion coefficient in a bubble column using a monolithic type of sparger. Here, the obtained gas dispersion coefficients run through a minimum while increasing the gas superficial velocity. Contrary, literature frequently reports axial dispersion coefficients that increase monotonously with the gas superficial velocity (e.g., [2], [3]).
The objective of the present study was to resolve this discrepancy. Experiments were performed in a 100 mm ID column using three different spargers, namely: a perforated plate sparger with (a) 50 holes of 0.6 mm diameter corresponding to a fractional free area of 0.18%, (b) with 38 holes of 1.0 mm and 12 holes of 1.5 mm corresponding to a fractional free area of 0.66%, and (c) a needle sparger made of 31 needles with 0.8 mm inner diameter each.
Axial dispersion was experimentally measured applying the gas flow modulation technique, which is a tracer-free approach explained by Marchini et al. [4, 5].
The results showed that the above mentioned minimum found by Kölbel [1] is reproducible for spargers with high fractional free area. Figure 1 reports the determined axial dispersion coefficients as a function of the gas superficial velocity for all considered spargers. The minimum is less evident for the considered sparger with lower fractional free area and, finally, it is not identified for the needle sparger.
Considering that dispersion causes back-mixing, which often has a detrimental effect on process yield, the presence of such a dispersion minimum can be considered as an additional option for process optimization in the future. However, more studies are required to reliably predict the occurrence of such dispersion minimum depending on sparger design and operating conditions.