Abstract
A complete knowledge of the bubble column fluid dynamics relies on understanding the global and the local fluid dynamic properties. Unfortunately, most of the previous literature focused on the “global-scale” fluid dynamics, whereas a limited attention was devoted to the “local-scale”. We contribute to present-day discussion by proposing an experimental study concerning the local liquid velocity profiles within the pseudo-homogeneous flow regime. The experimental study, based on a particle-identification and particle-tracking algorithm, was conducted in a large-diameter and large-scale bubble column (height equal to 5.3 m; inner diameter equal to 0.24 m) operated in the counter-current mode. We considered gas superficial velocities in the range of 0.37-1.88 cm/s and liquid superficial velocities up to −9 cm/s. Time-averaged and transient liquid velocity fields were obtained for five superficial gas velocities and four superficial liquid velocities at two measuring heights. Subsequently, the liquid velocity observations were coupled with previously measured bubble size distributions and local void fractions, to provide a complete description of the “local-scale” fluid dynamics. These data would help in the validation procedure of numerical codes, to support the prediction of industrial-scale relevant conditions.
Highlights
In the broader framework of multiphase reactors, bubble columns are widely used as contacting devices in industrial applications, owing to their many advantage in both design and operation
Most of the previous literature focused on the “global-scale” fluid dynamics, whereas a limited attention was devoted to the “local-scale”
Single phase flow conditions Measurements of the single phase flow situation were performed, to provide preliminary observations concerning the influence of the inlet and outlet boundary conditions on the flow field: the averaged liquid velocity profile for the single-phase flow is flat in the center with a steep slope towards the wall (Figure 6), owing to the boundary conditions imposed by the wall itself
Summary
In the broader framework of multiphase reactors, bubble columns are widely used as contacting devices in industrial applications, owing to their many advantage in both design and operation (i.e., low initial cost, low maintenance, high contact area between the phases, as discussed in ref. [1]). On the practical point of view, batch-mode bubble columns are applied in hydrogenation and fermentation processes; waste-water treatment, water ozonation, and three-phase inverse fluidized bed involve a continuous flow of the liquid phase [3]. In these cases, counter-current bubble columns are employed thanks to the higher interfacial mass transfer compared with batch-mode bubble columns [4]. The gas phase, introduced through the gas sparger, evolves in the axial direction of the bubble column either in the form of "dispersed bubbles” or in the form of “coalescence-induced structures”, depending on the prevailing flow regime (please refer to the discussion proposed by Besagni et al [2]). The complete understanding of the flow regime characteristics, which is essential to correctly design and operate bubble columns, depends on the precise knowledge of the multi-scale fluid
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