Flow mixing of two miscible liquids with the addition of gas bubbles is a process often found in industrial chemical apparatus for the production of primary matter. The ongoing optimization of such ...processes also involves the transformation of batch to continuous mode operation. In that case, the use of helically coiled tubes is an interesting alternative, since those reactors have narrow residence time distributions, very good radial mixing properties and excellent mass transfer can be realized between gases and liquids. For these reasons, in this study the mixing of two miscible liquids with addition of air bubbles in gas–liquid flows has been characterized in a horizontal helically coiled reactor in the laminar flow regime at
Re
total
=
300
…
1088
. Eight different superficial liquid velocities and five superficial gas velocities were investigated. In order to characterize mixing in the liquid plugs between two bubbles, laser-induced fluorescence of resorufin was used and particle image velocimetry has been employed to characterize the flow field. Pseudo-3D-visualizations of the resorufin concentration and the Q-criterion, representing the mixing efficiency and vorticity, respectively, were established for individual liquid plugs from the time-resolved measurement results. A time-resolved mixing coefficient, as well as a mean mixing coefficient obtained from multiple liquid plugs, is calculated from the fluorescence images for all examined flow conditions. The experimental results clearly show an increase in the mixing coefficient compared to single-phase conditions, caused by the bubbles. However, distinct mixing pattern, depending on the flow structure, can be recognized on different locations inside the liquid plug. Compared to a stationary case without air bubbles, mixing is worse behind the bubbles and increases inside the plug, reaching a maximum mixing coefficient in front of the next bubble. Overall the mixing coefficient is always increased by the presence of the bubbles. Pseudo-3D-visualizations of the Q-criterion and the vorticity show the presence of secondary vortices right in front of the bubbles, shifted to the outer tube walls, and in addition to the steady Dean vortices. In small plugs, these secondary vortices appear in the whole plug and increase the mixing coefficient drastically.
Graphical Abstract
Time-resolved tomographic particle tracking velocimetry (TR-3D-PTV), also called 4D-PTV, is used here to obtain the instantaneous 3D liquid flow field information in the wake of a single rising ...bubble in water. Simultaneously, the bubble shape, size and velocity are determined by tomographic reconstruction of the 3D bubble shape. Both, tracer particles for PTV and bubbles, are imaged in a shadow mode with background illumination. The Lagrangian method used in this paper, especially combined with the shake the box algorithm, has big advantages compared to particle image velocimetry, in situations, where only low particle per pixel values can be obtained. In this research, single air bubbles of different sizes, with diameters of around 2.4 mm, 4.0 mm, 6.0 mm and 9.6 mm, were injected into stagnant de-ionized water. Their shape was reconstructed in 3D, and an equivalent bubble diameter was determined from this reconstruction. Compared to conventionally used 2D shadow imaging, this diameter is about 13% smaller. The 3D bubble trajectory can be analysed and decomposed into a sinusoidal function curve lateral projection and an ellipsoidal shape vertical projection. As the bubble diameter increases, the radius of the spiral trajectory is decreasing as well as the amplitude of vertical sinusoidal oscillation. The wake structure in the liquid behind the bubbles is also changing with bubble size: from simple vortex pairs for smaller bubbles to an intertwined structure of several twisted vortices for the bigger ones.
Graphical abstract
Three-dimensional bubble reconstruction (grey surface) and liquid stream lines coloured with velocity magnitude around an ascending air bubble in de-ionized water.
•New experimental data for fluid dynamics and turbulence in bubble columns are presented.•The data feature large spatial and temporal resolution and high accuracy for a comprehensive set of ...observables.•A model validated previously for a range of different conditions is used to simulate the experiments with good agreement.•This highlights the trustworthyness of the model for predictive applications.
New high quality experimental data for the fluid dynamics of a bubble column are used to validate a baseline set of closure relations for bubbly flows. Development and validation of such closure relations is an important and active area of research, since they facilitate CFD simulations on industrial scales by means of the Euler–Euler two-fluid model. The new dataset features in particular large spatial and temporal resolution and high accuracy for a comprehensive set of observables and a range of different conditions. The closure model, which has been validated previously for a range of different conditions, is shown to agree with the new data quite well. In this way, the confidence in the model for predictive applications, such as optimization and scale-up of chemical engineering processes, is further enhanced.
•Liquid mixing in a helical pipe is investigated.•Numerical simulations are done using various mesh types, resolutions, and gradient limiters.•The simulated mixing coefficients are compared with LIF ...experiments.•Unstructured tetrahedral and polyhedral meshes predict an unrealistic fast mixing.•Hexahedral mesh with a proper resolution is recommended for future studies.
Liquid mixing is studied in a helical pipe numerically and experimentally to investigate the influence of computational domain discretization and gradient limiters on the unwanted numerical diffusion. Five mesh types with multiple resolutions are examined, including hexahedral, automated polyhedral, extruded polyhedral, automated tetrahedral, and extruded tetrahedral meshes. The mixing efficiency is calculated using the scalar transport technique and compared with data obtained by Laser-Induced Fluorescence measurements. Two gradient limiters are considered in the analysis, i.e., Venkatakrishnan and Min-Mod limiters, which are typically used to stabilize the simulation while limiting the numerical diffusion. The results reveal that all extruded meshes show generally a very good agreement with the experiments, while automated meshes involve unavoidable numerical diffusion, even for resolutions up to 90 million cells. The reason for this is that the cells of the automated meshes are hardly aligned with the flow direction, resulting in higher truncation errors. Furthermore, the use of MinMod limiter with extruded meshes effectively minimized the numerical diffusion in most cases. Compared to all other types, the hexahedral mesh is found the least diffusive and most accurate. Comparing the pressure drop, an acceptable accuracy on all meshes was found already at low mesh resolutions with slightly increased errors when using the automatic tetrahedral mesh. Finally, the computational cost is compared among all the considered cases. All extruded meshes are found very cost-efficient since they are mostly aligned with the flow, consuming only about 50% CPU time compared to automated unstructured meshes. Therefore, using the extruded meshes, and in particular the hexahedral mesh as far as possible, with the MinMod limiter is strongly recommended for an accurate prediction of the mixing performance at low cost.
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•Two-phase gas/liquid flows are investigated experimentally in a diverging channel.•Involving large recirculation zones results in big gas accumulations.•The accumulated gas size is ...lower for stratified flows.•For very high turbulence levels the accumulated gas size is minimized.
In the present study the flow properties of a turbulent two-phase (air/water) flow through a horizontal, diverging channel are investigated experimentally. The mixture flows from a rectangular channel of 40 × 44 mm through a diffuser to a rectangular channel of 100 × 44 mm, corresponding to a hydraulic diameter ratio of 1.45. The diffuser is designed with an increasing opening angle, ensuring flow separation within the studied range of flow conditions. The main objectives are (1) to investigate the possible gas accumulation due to flow separation, and (2) to deliver a complete experimental database allowing later validation of computational models. The superficial Reynolds number of water and air phases has been varied in the range of ReL = 50,130–87,730, and ReG = 3–18.5, respectively. The results show large gas accumulation even at very low air volume fraction (0.05%), which significantly affects the velocity field and the pressure recovery of the diffuser. When increasing ReG, the accumulated gas always increases. However, when ReL is increased, the accumulated gas first increases up to a certain limit, then decreases again. This can be explained by noticing that, at very high ReL, the turbulence intensity in the liquid phase becomes very high, leading to a break-up of the large gas pocket into smaller bubbles that are transported away with the liquid phase; in this manner, the huge gas accumulation is prevented. The flow in such a divergent channel is somewhat similar to the flow found in a centrifugal pump impeller transporting gas/liquid mixtures. However, since it does not involve any rotating part, the investigated configuration allows a far more accurate and complete experimental characterization. Therefore, this study provides a first step toward understanding the complex flow patterns occurring into centrifugal pumps transporting gas/liquid mixtures, and opens the door for validation studies.
•Excellent mixing and mass transfer properties of the helically coiled reactor.•Yield and selectivity predictions are in agreement to the experimental results.•The novel reactor setup increases the ...yield and selectivity of the target product.
To intensify new or existing chemical processes, novel reactor designs have to be developed. For a homogeneously catalyzed multiphase model reaction, the hydroformylation of 1-dodecene, Kaiser et al. 13 theoretically derived a promising tandem reactor system applying a model-based rector synthesis and dimensioning approach. The reactor tandem, consisting of a helically coiled tubular reactor (HCTR) followed by a continuously stirred tank reactor (CSTR), was constructed and firstly operated within this work. For the validation of the reactor design methodology, a comprehensive hydrodynamic study focusing on the HCTR was performed. Different complementary techniques were used to investigate the geometry influences on mixing and the gas-liquid mass transfer, and liquid phase mixing in smaller model geometries. Since the same Reynolds-number and flow regime in the model geometries and the HCTR were ensured, the transfer of the results between these geometries was possible. The outcome confirms, that the geometrical parameter only minor influence the excellent mixing properties of coiled tubes and the helix geometry has enhanced gas-liquid mass transfer rates. In a next step, the HCTR + CSTR-tandem was operated with closed catalyst recycle and optimized reaction parameters. Thereby, full conversion of 1-dodecene and a selectivity of up to 0.70 to the linear aldehyde, tridecanal, were achieved. These values match the predictions of the numerical calculations and are higher than published results for this process. The outcomes of this study confirm the assumptions made by Kaiser et al. 13 during the reactor dimensioning and validate the reactor network design approach.
Bubble column reactors are multiphase reactors that are used in many process engineering applications. In these reactors a gas phase comes into contact with a fluid phase to initiate or support ...reactions. The transport process from the gas to the liquid phase is often the limiting factor. Characterizing this process is therefore essential for the optimization of multiphase reactors. For a better understanding of the transfer mechanisms and subsequent chemical reactions, a laboratory-scale bubble column reactor was investigated. First, to characterize the flow field in the reactor, two different methods have been applied. The shadowgraphy technique is used for the characterisation of the bubbles (bubble diameter, velocity, shape or position) for various process conditions. This technique is based on particle recognition with backlight illumination, combined with particle tracking velocimetry (PTV). The bubble trajectories in the column can also be obtained in this manner. Secondly, the liquid phase flow has been analysed by particle image velocimetry (PIV). The combination of both methods, delivering relevant information concerning disperse (bubbles) and continuous (liquid) phases, leads to a complete fluid dynamical characterization of the reactor, which is the pre-condition for the analysis of mass transfer between both phases.
Helically coiled tubes are widely used in industry to enhance heat and mass transfer in the laminar flow regime, due to their secondary flow pattern. In this study, tomographic particle image ...velocimetry (tomo-PIV) is used in a horizontally coiled helical tube to systematically acquire 3C-3D velocity fields for Reynolds numbers ranging from 20 to 1400 and Dean numbers from 8 to 567. The velocity field evaluations are performed using two different approaches: time-averaged velocity field calculation from instantaneous velocity fields and velocity field determination by cross-correlation from an ensemble of instantaneous reconstructed volumes. Equivalent velocity field accuracy is achieved in both velocity approaches when the flow can be considered stationary. Moreover, numerical simulations were carried out in the same geometry at the same flow conditions and were validated against the experimental 3C-3D data sets. The simulation results show good agreement with the measured velocities, offering the possibility of parametric studies and design optimization. To the authors’ best knowledge, this is the first systematic experimental investigation of a helical coil flow by means of 3C-3D velocity measurements, which results can now be used for validation of numerical models in computational fluid dynamics.
Graphic abstract
Measured time-averaged velocity visualized by vector-magnitude colour at horizontal and vertical slices (left) and Dean vortices detected by 3D Q-criterion (right) from the time-averaged measurements (purple isosurfaces) and from the simulation (red isosurfaces) inside the helically-coiled tube at Re = 220 and De = 89.
Gas‐liquid mass transfer in bubble columns is strongly influenced by the flow field in the column. The underlying transport processes are often the limiting factor. Since bubble columns are widely ...used in the chemical process industry, it is essential to quantify how the dispersed gas can come into reaction with the liquid phase through gas‐liquid mass transfer. CO2 bubble swarms in a cylindrical model bubble column are considered. Optical measurement methods are used for the non‐intrusive measurement of velocity and pH change, resulting from the dissolution of CO2 in the liquid phase. The mean and fluctuating velocity components in the center plane of the column as well as the time‐dependent pH fields in this plane are determined. From these quantities, derived values characterizing the mass transfer from gaseous CO2 to the liquid phase can be calculated.
In bubble columns, gas‐liquid mass transfer is significantly influenced by the flow field. The mass transfer from a CO2 bubble swarm in a cylindrical model bubble column is analyzed by different optical methods. High‐speed particle image velocimetry is applied to investigate the hydrodynamics in the column, two‐tracer laser‐induced fluorescence to follow the pH change caused by CO2 dissolution.
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