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An advanced recirculation cell model is proposed, which describes fluid dynamics and mass transfer in bubble columns with and without internals. The new model incorporates the cell approach of Shimizu et al. [Chem. Eng. J. 2000, 78, 21-28.] with latest breakup and coalescence kernels. Additionally, the gas flow structure is divided according to the two-bubble class assignment with fast-rising large bubbles in the column center and descending small bubbles near the wall following the liquid circulation pattern within the column’s cross-section. The effect of internals is considered dividing the column further into ‘sub-columns’ derived from the internals’ radial profile, which physically refines the liquid circulation pattern. The model was validated with experimental data of Möller et al. [Chem. Eng. Sci. 2018, 179,265-283; Chem. Eng. Res. Des. 2018, 134; Chem. Eng. Sci. 2019] for narrow (0.1 m diameter) and pilot-scale (0.39 m diameter) columns, respectively, with and without internals operated up to the well-developed churn-turbulent flow regime. Predictions for bubble size distribution, total gas holdup, Sauter mean diameter as well as interfacial area and volumetric mass transfer coefficients agree well with the experiments.

Many correlations have been developed to predict the hydrodynamics of bubble columns. Often, these studies are performed for incomparable systems in terms of column and sparger dimensions as well as physical fluid properties. In this work, a different approach is proposed comprising interrelated correlations for liquid velocity, gas holdup and bubble size. The correlations are developed on the basis of complementary experiments with non-invasive measurement techniques, namely, Ultrafast X-ray Computed Tomography (UXCT) and Radioactive Particle Tracking (RPT). The experimental setup consists of a bubble column equipped with a needle sparger. The developed correlations consider sparger dimensions, operating conditions and bubble size. The bubble size is applied as the characteristic length in the Reynolds and the Eötvös numbers, which are utilized for the gas holdup and liquid velocity correlations. In comparison with previous approaches, the developed correlations show better agreement with experimental data from this study as well as from the literature.

Ultrafast X-ray tomography is a recently developed imaging technique for multiphase flows. As conventional X-ray tomography it involves reconstruction of images from X-ray projection data. If used for multiphase flow measurements it moreover needs to be complemented with automated image processing algorithms for the extraction of flow features, such as gas holdup profiles, bubble/particle size distributions or disperse phase velocities. So far image reconstruction was carried out with the standard filtered backprojection technique, which is fast but may not be optimal in the presence of noisy or corrupted data. As the latter is a frequent issue, search for optimal image reconstruction and data processing algorithms is continuously ongoing. This paper serves as a foundation of the image reconstruction and processing framework for the application to multiphase flow. A description is given of the procedure from reconstruction to thresholding to properties extraction of ultrafast X-ray tomographic images. Two reconstruction techniques, FBP and SART are employed based on phantom measurements. Each technique is evaluated separately, for FBP regarding the choice of filter and for SART regarding the termination criteria. Image reconstruction resolution, computational costs and sensitivity to the threshold value are investigated. Based on the analysis, FBP with the Ram-Lak filter is selected for image processing purposes. Furthermore it is shown that from experiments with moving objects, there is fair agreement between measurements and the phantom dimensions. The described imaging process can be applied to different attenuation materials, simulating gas-solid and gas-liquid properties.

The influence of dense vertical tube bundles in a batch bubble column reactor of 100 mm diameter and 1100 mm clear liquid height on liquid dispersion and gas-liquid mass transfer was studied. In particular, the effects of different tube patterns (triangular and square pitch), tube diameters (8 and 13 mm) and bottom end designs (flat and U-tube) having a tube diameter-to-pitch ratio of approx. 1.3 were investigated. Dispersion coefficients were determined based on conductive tracer experiments recorded via wire-mesh sensors (WMS) with up to 90 measurement points distributed in the column’s cross-section in between the tubes. The gas-liquid mass transfer coefficient was determined via fast-responding oxygen needle probes. Tube pitch and pattern were identified as the most crucial design parameters for the extent of liquid dispersion. We found that particularly the U-tube bottom end design induces large liquid circulation patterns, which enhance dispersion. The presence of internals decreases the k_l a value as a consequence of turbulence damping, which is also confirmed by lower k_l values (e.g. 0.6 × 10-3 m s-1 for the empty BCR and 0.25 × 10-3 m s-1 for the square pitch with 8 mm tubes at 0.05 m s-1 superficial gas velocity), whereas the pitch is the most decisive design parameter. The U-tube bottom end design was identified as the most beneficial configuration with respect to liquid mixing and gas-liquid mass transfer.

In this work, the hydrodynamics of a bubble column with vertical heat exchanger internals in a narrow bubble column of D_i = 0.1 m inner diameter with a clear liquid height of L_c = 1.1 m was comprehensively studied. We applied ultrafast X-ray tomography to obtain hydrodynamic parameters, such as, gas holdup, bubble size distribution, bubble number flux and flow patterns at hitherto inaccessible positions within the sub-channels of the tube bundles. To investigate the influence of the tube bundle patterns, square and triangular pitches were considered. Tubes of d_o = 8 and 13 mm outer diameter were installed to study the effect of tube size, while maintaining approx. A_c = 25 % coverage of the cross-sectional area, which is typical for e.g. Fischer-Tropsch process operated in bubble column reactors. The superficial gas velocity was varied from u_g = 2 to 20 cm s-1 to cover homogeneous and heterogeneous flow regimes. Internals’ type and tube diameter were found to crucially influence the gas holdup distribution across the column diameter, which is known to generate liquid circulation, to shape the gas velocity profile and to cause intensive bubble interactions. The higher flow resistance induced by triangular tube configurations and dense tube patterns, which is attributed to the smaller hydraulic diameter of the respective configurations, forces bubbles to preferably rise near the column wall. Within the tube bundle, the radial holdup profiles show a pronounced non-parabolic trend, indicating zones of reverse liquid flow directions between the internal tubes. Furthermore, the gas-liquid flow morphology within various sub-channels was analyzed revealing a slug-like flow formation at superficial gas velocities larger than 10 cm s-1.

Opaqueness and visual accessibility in turbulent bubbly flows are the main cause of difficulty for conventional experimental techniques to extract properties from the bulk flow. Ultrafast X-ray tomography presents an unique possibility to visualise the dense bubbly flows and to provide the ability to characterise the bubbles. With this technique, temporally resolved measurements can be obtained from a scanning plane. The post-processed images are stacked in time, resulting in a three-dimensional matrix with two spatial and one temporal resolutions. The gas flow structures are given straightforward by the image stack, which provides unique insights of the bubbly flow dynamics. However, sizing the bubbles requires the velocity to be known, which can be achieved by means of measurement or assumption. Typically, a second measurement plane is needed for the velocity estimation. However, the employment of dual plane measurement data is limited to dilute bubbly flows. To characterise bubbles in the dense regime, a method is presented using only single plane data for size extraction without the need of velocity data. The procedure comprises determinating the Sauter diameter (d32), which depicts the ratio between volume and surface area of the bubble. Contrary to the use of dual plane measurements, here, the bubble diameter is firstly determined and from the bubble size with the assumption of the bubble’s shape, the absolute bubble velocity can be roughly estimated. This method is verified with synthetic data of simple ideal-shaped bubbles. Subsequently, real bubbly flow measurements in bubble columns (with and without internals) are applied for the assessment of the resulting bubble sizes and velocity trend.

In this work, the effect of the sparger design on the hydrodynamic performance in a bubble column of 0.1 m ID downstream a single (coarse) and multi-orifice (fine) perforated plate sparger was studied using the ultrafast X-ray tomography. The liquid was kept in semi-batch mode and the superficial gas velocity was varied between 0.011 and 0.025 m s-1 to ensure non-jetting flow through the sparger holes. The effect of the orifice patterns on the hydrodynamic performance was evaluated through bubble size distribution (BSD), radial gas holdup profile and overall gas holdup as well as Sauter mean bubble diameter and magnitude of the interfacial area. To evaluate sparger and bubble column performance, respectively, also the mass transfer was investigated. Due to the high turbulence induced by the large bubbles released from the coarse sparger, the equilibrium BSD was already reached at a dimensionless height of h/D = 1.0. However, average bubble characteristics, such as interfacial area and Sauter mean diameter, were similar for both sparger types at a column height of h/D ≥ 7.0. Based on a comprehensive hydrodynamic analysis, requirements for sparger refinement were derived depending on respective reaction rates, mixing properties, heat production and removal duty. Eventually, adapted correlations are proposed for radial holdup profile and Sauter mean diameter accounting for various plate refinements using liquids which inhibit coalesce of gas bubbles.

Several techniques have been developed in the past to measure gas and liquid phase dynamics; however, reported data were mostly gathered individually for either liquid velocity, or volume fraction (phase holdup), but never when both are measured in the same system. In this work, arguably for the first time, bubble column hydrodynamics have been investigated using two complementary advanced non-invasive measurement techniques, namely Ultrafast X-ray Computed Tomography (UXCT) and Radioactive Particle Tracking (RPT). The UXCT experimental data in terms of gas phase structure is used in a supportive way to explain the liquid velocity profiles of the RPT data. Results of both experimental techniques are verified in a complementary manner using the mass conservation calculation. The results show good agreement. It is envisioned that the presented data would be helpful in the development and validation of numerical models for better predicting the flow profiles in bubble columns.

Although clustering of bubbles plays a signifant role in bubble column reactors regarding the heat and mass transfer due to bubble-bubble and flow field interactions, it has yet to be fully understood. Contrary to flows in bubble columns, most literature studies on clustering report numerical and experimental results on dilute or micro-bubbly flows. In this paper, clustering of bubbles in a cylindrical bubble column of 100 mm diameter is experimentally investigated. Ultra fast X-ray tomographic imaging is used to obtain the bubble positions within a hybrid Eulerian framework. By means of Voronoï analysis, the clustering behaviour of bubbles is investigated. Experiments are performed with different superficial gas velocities, where Voronoï diagrams are constructed at several column heights. From the PDFs of the Voronoï diagrams, it is shown that the bubble structuring in terms of hybrid volume dimensions develops slower than the bubble size distribution. The measured PDFs are compared with the PDF of randomly distributed points, which showed that the amount of bubbles as part of clusters (Voronoï cells < V/V_cluster ) as well as bubbles as part of voids (Voronoï cells > V/V_void) increases with the superficial gas velocity. It is found that all experiments have an approximate cluster limit V/V cluster of 0.63, while the void limit V/V void varies between 1.5 and 3.0.

The ability to accurately predict the bubble size distribution in bubble column reactors is a requirement for any process design as well as for scale-up. The bubble size distribution depends mainly on the magnitude of bubble breakup and coalescence. Several breakup and coalescence models have been developed assuming different driving mechanisms, such as turbulence dissipation and shear rate of the liquid phase. The proposed breakup and coalescence models contain turbulence contributions in breakup and coalescence of the bubbles and also the relative velocity of the bubbles. The realistic expressionfor the mentioned terms is missing for the implementation of the breakup and coalescence models due to poor knowledge on the turbulence behavior of the bubbly flows. Here, bubble-liquid turbulence interactions of the bubbly flows are considered to predictparticipating turbulence energy in breakup and also relative velocity of the bubbles at coalescence of the bubbles.

We present measurements of bubble size distribution in bubble columns using X-ray tomography. The experimental setup is an ultrafast electron beam X-ray tomographic system applied to a cylindrical bubble column with a diameter of 0.07 m. Measurements are taken placed on two planes, which are separated with an axial distance of 10 mm. The obtained reconstructed images are filtered, segmented and stacked in the time-domain to form a three-dimensional matrix of bubble objects. By cross-examining the matrices of both planes, we can identify the corresponding bubble objects and determine bubble velocities and volumes as well.

Bubble columns are widely used in the chemical industry for gas-liquid operations. In-spite of the wide spread use of bubble column reactors, fundamental understanding of interactions between hydrodynamics, mass transfer and chemical reaction in dense systems is still lacking. Several experimental techniques are available to study gas-liquid flows: photographic imaging, particle image velocimetry, X-ray tomography, wire mesh sensors, etc. We developed a digital image analysis technique (DIA) to study the gas-liquid flows in bubble column reactor.