Understanding the role of perforations on the local hydrodynamics of gas–liquid flows through structured packings

Structured packings are widely used to perform gas–liquid separation processes as they provide a large mass-transfer area and low-pressure drop. The overall performance of these packings is governed by the local liquid distribution/gas–liquid interfacial area. The local liquid distribution is substantially influenced by the arrangement and geometrical features of the structured packing. While the influence of arrangement and few geometrical features on the local liquid distribution has recently been investigated using the structure-resolved simulations, the perforations on the structured packings are usually ignored. In the present work, we have performed structure-resolved gas–liquid flow simulations, using the Volume-of-Fluid (VOF) method implemented in the open-source C++ library OpenFOAM, in a periodic domain of Mellapak.250 with and without resolving the perforations. We show that the presence of perforations results in flow separation as well as droplet formation leading to an increase in the liquid holdup, interfacial and wetted areas, irrespective of the fluid properties and wetting conditions (specified using static contact angle ‘θw’) considered in this work. We also show that at an inclination angle ‘β’ of 45° from horizontal, the location of the perforations governs the local liquid distribution and the resulting flow metrics. However, at a β of 90°, the number of perforations governs the local liquid distribution, irrespective of their location. Further, we also show that the predictions of structure-resolved VOF simulations, with perforations resolved, are in a relatively better agreement with the correlations in the literature at small values of ‘θw’) in terms of liquid holdup and interfacial area.<br/

Mixing in bubble column reactors: role of unsteady flow structures

Gas-liquid flows in bubble column reactors are inherently unsteady. In the present work, we have characterized dynamics of gas-liquid flow in rectangular bubble columns and studied the role of unsteady flow structures in liquid phase mixing using experiments and CFD simulations. The need for considering the unsteady nature of gas-liquid flows was demonstrated by using mixing simulations carried out with transient and time-averaged flow. A new methodology of using a computationally amenable 'multiple snapshots' approach, which can adequately account for dynamics of underlying flow, is proposed. CFD simulations were carried out to study the influence of superficial gas velocity and height-to-width ratio on mixing time. The predictions were compared with mixing time measured using conductivity probes. The present work gives useful insights into the mixing process in inherently unsteady flows

Characterization of gas-liquid flows in rectangular bubble columns using conductivity probes

Unsteady gas-liquid flows in bubble columns are comprised of various flow processes occurring with varying length and time scales and govern mixing and transport processes. In the present work, we have characterized dynamic and time-averaged properties of gas-liquid flows in rectangular bubble columns using conductivity probes. The development of a single-tip conductivity probe, data processing methodology, and photographic validation procedure is discussed in detail. The effect of superficial gas velocity and aerated liquid height-to-width (H/W) ratio on voidage fluctuations and time-averaged gas holdup was investigated. The experimental data presented here can help in understanding the dynamics of various flow processes and validating computational fluid dynamics based models

Characterization of dynamics of gas-liquid flows in rectangular bubble columns

Gas-liquid flow in bubble columns is inherently unsteady. The unsteady fluid dynamics influences mixing and other transport processes occurring in bubble column reactors. In this work, we have characterized dynamics of gas-liquid flows in rectangular bubble columns using wall pressure and voidage fluctuations. The low-frequency oscillations caused by meandering bubble plume were characterized using the plume oscillation period. Experiments were carried out to study the influence of superficial gas velocity, sparger configuration, and liquid height-to-width (H/W) ratio on the low-frequency oscillations. The dimensional analysis based on analogy between buoyancy-driven thermal and bubbly flows was carried out to relate low-frequency oscillations to various design and operating parameters. This analysis was able to correlate present as well as previously published experimental data. We have further demonstrated the importance of establishing such a quantitative relationship for validation of computational fluid dynamics (CFD)-based models by carrying out CFD simulations of gas-liquid flow in rectangular bubble columns. The importance of bubble-scale information obtained using conductivity probes in validating Eulerian-Lagrangian models is also demonstrated

Dynamics of gas-liquid flow in a rectangular bubble column: experiments and single/multi-group CFD simulations

Several flow processes influence overall dynamics of gas-liquid flow and hence mixing and transport processes in bubble columns. In the present work, we have experimentally as well as computationally studied the effect of gas velocity, sparger design and coalescence suppressing additives on dynamics of gas-liquid flow in a rectangular bubble column. Wall pressure fluctuations were measured to characterize the low frequency oscillations of the meandering bubble plume. Bubble size distribution measurements were carried out using high-speed digital camera. Dispersed gas-liquid flow in bubble column was modelled using Eulerian-Eulerian approach. Bubble population was represented in the model with a single group or multiple groups. Bubble coalescence and break-up processes were included in the multi-group simulations via a suitable population balance framework. Effect of superficial gas velocity and sparger configurations was studied using single-group simulations. Model predictions were verified by comparison with the experimental data. Role of bubble size in determining plume oscillation period was studied. Multi-group simulations were carried out to examine evolution of bubble size distribution. An attempt is made to understand the relationship between local and global (over all the dispersion volume) bubble size distribution. The models and results reported here would be useful to develop and to extend the applications of multi-group CFD models

Modelling of gas-liquid/gas-liquid-solid flows in bubble columns: Experiments and CFD simulations

Gas-liquid/gas-liquid-solid systems are widely used in the chemical process industry for a variety of applications. In the present work, we have characterized the dynamics of gas-liquid/gas-liquid-solid flows in cylindrical bubble columns using experiments and CFD simulations. The low frequency oscillations corresponding to the local re-circulatory flow were characterized using wall pressure fluctuation measurements. Eulerian-Eulerian two-/three-phase simulations were carried out with a focus on characterizing the dynamic properties of gas-liquid/gas-liquid-solid flows. The effects of superficial gas velocity, H/D ratio and solid loading on the dynamic and time-averaged flow behavior were studied experimentally and computationally. The simulated results were compared with the experimental measurements. The results presented are useful for understanding the dynamics of gas-liquid/gas-liquid-solid flows in bubble columns and provide a basis for further development of CFD models for three-phase systems

Eulerian-Lagrangian simulations of unsteady gas-liquid flows in bubble columns

We studied the dynamics of gas-liquid flows in a rectangular bubble column using Eulerian-Lagrangian simulations. Three-dimensional, unsteady simulations were performed to simulate the dynamic characteristics of the oscillating bubble plume. The effect of superficial gas velocity and aerated liquid height-to-column width (H/W) ratio on the dynamic and time-averaged flow properties was studied and the simulated results were validated using wall pressure and voidage fluctuation measurements. The effect of lift force and numerical diffusion on the dynamic and time-averaged properties is discussed in detail. Further, the results obtained using the Eulerian-Lagrangian simulations were compared with the Eulerian-Eulerian simulations. The bubble scale information, which is otherwise lost in the Eulerian-Eulerian simulations, was validated using the voidage fluctuation measurements. Such experimentally validated Eulerian-Lagrangian models will be useful for the simulation of mass transfer and reactions in bubble columns

Gel-based separation of an o-toluidine-water emulsion

The present work describes an attractive and economically viable novel separation technique using superabsorbing hydrophilic polymer gel for dewatering an industrial o-toluidine-water emulsion. It is demonstrated that the initial water content of 30% in the emulsion is reduced to 3% within 10 min by using a semicontinuous separation strategy, which can have significant energy savings over the conventional distillation process. The mechanism of water removal and the effect of the presence of interfacial agents on the rate of water removal are also studied