Higher Gas Holdup Research Articles

Hydrodynamics and model of gas‐liquid flow in microbubble column reactors

AbstractMicrobubble column reactors (MBCRs) have the significant advantages of high gas holdup, large gas‐liquid interfacial area, and excellent mass transfer performance, making them attractive for applications such as hydrogenation and oxidation. However, there is still a lack of comprehensive understanding of the gas‐liquid flow behavior in MBCRs. In this work, the effects of gas and liquid superficial velocities on flow regimes, residence time distribution, and gas holdup are investigated. The gas holdup increases linearly with the increase of gas superficial velocity, even in heterogeneous regimes, ranging from 0% to 60% at gas superficial velocities of 0–25 mm/s. Additionally, the gas‐liquid interfacial areas in MBCRs are 1–2 orders of magnitude larger than those in traditional bubble column reactors. A prediction model for gas holdup is established, and the predicted values are in good agreement with the experimental values.

Measurement of gas holdup characteristics in two-phase column flotation by electrical capacitance volume tomography

Electrical capacitance volume tomography measured the gas holdup in two-phase column flotation. The purpose was to characterize gas holdup and understand its relation to flow patterns as a function of the types and doses of frothers. Experiments were conducted using laboratory-scale column flotation cells with dimensions of 5 ×125 cm. The superficial gas velocities in the experiment were 0.5–4 cm s−1 with an interval of 0.5 cm/s. Methyl isobutyl carbinol (MIBC) and Oreprep F-583-OZ (Oreprep) were used as frothers at 5–40 ppm doses. The results showed that the difference between the lowest and highest gas holdup in the bubbly flow regime was ∼0.07, and the transition from the bubbly to churn-turbulent flow occurred at a superficial gas velocity of 2 c m /s .

Global characterization of hydrodynamics and gas-liquid mass transfer in a thin-gap bubble column in presence of Newtonian or non-Newtonian liquid phase

Bubble columns are commonly used as photobioreactors (PBRs) due to their good mixing and mass transfer capabilities. Modification in design and operating parameters are nevertheless needed to intensify volumetric productivity. Thin-gap bubble column can be used in this context to operate with high biomass concentration, up to conditions where the culture becomes non-Newtonian. In the present study, the global hydrodynamics and gas–liquid mass transfer inside a 4 mm thickness thin-gap bubble column are characterized in presence of low viscosity Newtonian fluid (water) and two aqueous solutions of Carboxymethyl Cellulose and Xanthan Gum. These shear-thinning solutions are used to mimic high concentration cultures of Chlorella Vulgaris at 42 g.L−1. A range of superficial gas velocities and different capillary diameters are explored to investigate the effect of sparging on hydrodynamics and gas–liquid mass transfer. Studied parameters are flow regimes, mixing, and gas–liquid mass transfer. Results are compared with literature results in classical bubble columns to put into evidence the original effects of both confinement and liquid rheology. Among others, these results show that compared to Newtonian media, non-Newtonian media result in higher gas holdup and mixing times and a lower gas–liquid mass transfer coefficient. The findings suggest that the presence of smaller bubbles would improve mass transfer due to the increased interfacial area, while the presence of larger bubbles would improve mixing performance due to increased vorticity in their wake and relatively greater species transport.

Effect of agitation and aeration on gas dispersion efficiency in coaxial mixers containing yield‐pseudoplastic fluids: Experimental and numerical analysis

AbstractAerated stirred vessels are commonly employed to enhance gas dispersion. However, the associated high energy consumption is a challenging feature, particularly when dealing with complex non‐Newtonian fluids. Coaxial mixers comprising a central impeller and a close‐clearance impeller have emerged as an energy‐efficient alternative that effectively intensifies gas dispersion. Hence, the objective of this study is to investigate the effect of aeration and agitation on the gas dispersion effectiveness of a coaxial mixer containing a yield‐pseudoplastic fluid. An anchor‐pitched blade turbine was employed to disperse air into a 1 wt.% xanthan gum solution, and the analysis primarily focused on characterizing the gas holdup and fluid flow behaviour. Gas holdup data were obtained experimentally using electrical resistance tomography (ERT), while computational fluid dynamics (CFD) simulations provided a detailed analysis of fluid flow patterns within the coaxial mixer. The rotational speed of the impeller exhibited a non‐monotonic effect on the gas holdup, and a significant influence of the interaction between variables was identified. For instance, the experimental data showed that the aeration effect varied with the anchor speed. Nevertheless, the variables' interaction effect was explained by the change in flow pattern observed numerically. Furthermore, the CFD results demonstrated that high gas holdup does not necessarily indicate intensified mixing. Therefore, combining experimental data and numerical simulations enables a more accurate characterization of mixing performance. These findings contribute to the understanding and improvement of mixing performance in such a complex system, which is crucial for designing efficient operations.

Proton exchange membrane water electrolysis at high current densities: Response time and gas‐water distribution

AbstractUnderstanding the distribution of oxygen in proton exchange membrane water electrolysis (PEMWE) is crucial for improving electrolysis efficiency and gas removal. In this study, we developed a two‐dimensional (2D) transient model that couples the Euler–Euler multiphase model with electric potential equations to investigate two‐phase flow in PEMWE. Our simulation reveals that the system's response time initially decreases and then increases with current density, indicating longer response times at high current densities. Modifying the wetting properties of the porous transport layer (PTL) affects gas removal at low gas holdup, resulting in a maximum 15% decrease in gas holdup. However, at high gas holdup, the flow field in the channel predominantly governs bubble removal, making changes in PTL wetting properties less influential. With increasing gas production rate, an inverse gradient distribution of gas saturation appears, leading to uneven gas saturation and hindering efficient oxygen removal. This non‐uniform gas saturation adversely affects electrolysis performance.

Gas segregation in a pilot-scale ebullated bed system: Experimental investigation and model validation

Gas hold-up reduction inside hydroprocessors is critical to improving the efficiency of bitumen conversion. The use of internal gas/liquid segregation devices, such as recycle cups, has been the focus of much past research and development over the previous five decades. In this study, the effects of different recycle cup designs and working fluids were evaluated using a pilot-scale cold-flow ebullated bed reactor and compared to multiphase flow simulation predictions. Two recycle cup designs from the literature were fabricated using 3D printing technology and two working fluids were used: air-water and air-5 wt% ethanol aqueous solutions. The addition of ethanol helped create high gas hold-up and foaming conditions frequently observed in commercial hydroprocessors. It was observed that in foaming systems, the performance of the recycle cups deteriorated. Additionally, increasing the liquid flow rate, either through the inlet or increased recycle, resulted in diminished gas/liquid segregation efficiency. The accuracy of simulations using the two-fluid model was evaluated to predict gas hold-up in the pilot scale system as well as inside the recycle line. In the homogeneous flow regime, given the proper bubble size, the simulations predicted the gas hold-up in the column with less than 5% error. However, unlike the experimental observations, simulations did not predict any gas entrained in the recycle line, which is attributed to variation in the average bubble size and limitations of the existing momentum exchange closures.

Microbubbles preparation by partial vaporization of low boiling condensate for crude oil/water separation

The poor adhesion of oil droplets and bubbles in the flotation process leads to inefficient separation of oil and water. In this paper, condensate bubbles were prepared by partial vaporization of low boiling point condensate to increase the oil removal effect. The purpose of partial vaporization was achieved by bringing emulsified condensate droplets into the produced water at different temperatures. The structure of the bubble is composed of condensate film wrapped around the gas core of the vaporized portion. The contact angle between condensate bubble and oil surface is >150°, which show great similarity-intermiscibility. The average bubble size prepared at 303 K - 318 K is 20 - 100 μm. The stability of bubbles decreased with an increase in temperature, the half-decay time decreased from 340 s to 110 s, and the zeta potential decreased from −31 mV to −12 mV. However, the gas hold-up of the bubbles first increased and then decreased with the increase in temperature, while the highest gas holdup was 21 % at 313 K. The rise of the condensate/water ratio (Rcon-wat) did not increase the stability of the bubbles, but the gas holdup increase. The condensate bubble flotation experiment found that at the optimum temperature 313 K, the oil removal efficiency of 1.5 % Rcon-wat can reach 85 %. In contrast, the oil removal efficiency of the dissolved air flotation experiment is <75 %. Otherwise, with a smaller amount of flocculant (ferric chloride), the oil removal efficiency of condensate bubble flotation can reach >95 %.

Revisiting Basics of kLa Dependency on Aeration in Bubble Columns: a Is Surprisingly Stable

AbstractA comprehensive experimental characterization of a small‐scale bubble column bioreactor (60 mL) is presented. Bubble size distribution (BSD), gas holdup, and kLa were determined for different types of liquids, relevant fermentation conditions and superficial gas velocities uG. The specific interfacial area a and liquid mass transfer coefficient kL have been identified independent of each other to unravel their individual impact on kLa. Results show that increasing uG leads to larger bubbles and higher gas holdup. As both parameters influence a in opposite ways, no increase of a with uG is found. Furthermore, kL increases with increasing bubble size outlining that improved oxygen transfer is not the result of higher a but of risen kL instead. The results build the foundation for further simulative investigations.

A deep learning-based image processing method for bubble detection, segmentation, and shape reconstruction in high gas holdup sub-millimeter bubbly flows

The sub-millimeter bubble technique can enhance the gas–liquid inter-phase mass transfer by significantly reducing the bubble size and increasing the gas–liquid interfacial area. To accurately describe the flow and mass transfer characteristics, it is necessary to characterize bubble parameters. High-speed photography followed by image processing is an effective way to characterize the gas bubbles in the multiphase flows. However, the efficient image processing method for the sub-millimeter bubbly flows with high gas holdup and high bubble overlap has not been reported yet. The present work developed a novel deep learning-based image processing method for bubble detection, segmentation, and shape reconstruction in high gas holdup sub-millimeter bubbly flows. In order to segment the highly overlapping sub-millimeter bubbles, our method was built based on Mask R-CNN, with which the pixel-level segmentation masks can be obtained, and the shape of the overlapping bubbles can be accurately described. The feature pyramid architecture was coupled with ResNet101 and Feature Pyramid Network to detect sub-millimeter bubbles with significant size differences. A shape reconstruction module was proposed to restore the real shape of overlapping bubbles and improve prediction accuracy. In order to sufficiently validate the proposed method, adequate images of sub-millimeter bubbly flows were obtained by changing the experimental media (air-tap water, air-sodium dodecyl sulphate aqueous solution, air-diesel, and air-diesel-fine catalyst particles), reactor configurations (3D beds and 2D beds), lenses, and photography (shadowgraphy and front illumination). Our method shows high accuracy under the experimental conditions and can process sub-millimeter bubble images under gas holdup up to 20%.

Different efficiency toward the biomimetic aerobic oxidation of benzyl alcohol in microchannel and bubble column reactors: Hydrodynamic characteristics and gas–liquid mass transfer

The selective aerobic oxidation of benzyl alcohol to benzaldehyde has attracted considerable attention because benzaldehyde is a high value-added product. The rate of this typical gas–liquid reaction is significantly affected by mass transfer. In this study, CoTPP-mediated (CoTPP: cobalt (II) meso-tetraphenylporphyrin) selective benzyl alcohol oxidation with oxygen was conducted in a membrane microchannel (MMC) reactor and a bubble column (BC) reactor, respectively. We observed that 83% benzyl alcohol was converted within 6.5 min in the MMC reactor, but only less than 10% benzyl alcohol was converted in the BC reactor. Hydrodynamic characteristics and gas–liquid mass transfer performances were compared for the MMC and BC reactors. The MMC reactor was assumed to be a plug flow reactor, and the dimensionless variance was 0.29. Compared to the BC reactor, the gas–liquid mass transfer was intensified significantly in MMC reactor. It could be ascribed to the high gas holdup (2.9 times higher than that of BC reactor), liquid film mass transfer coefficient (8.2 times higher than that of BC reactor), and mass transfer coefficient per unit interfacial area (3.8 times higher than that of BC reactor). Moreover, the Hatta number for the MMC reactor reached up to 0.61, which was about 15 times higher than that of the BC reactor. The computational fluid dynamics calculations for mass fractions in both liquid and gas phases were consistent with the experimental data.

Technological assessments on recent developments in fine and coarse particle flotation systems

• Froth flotation of fine and coarse particles is studied, focusing on cell designs. • Generating micro-bubbles, intensive turbulence, and high gas hold-up are advantages of intensified cells. • Scaling up/down procedures for the intensified cells are found unclear. • Positioning the optimum location of discussed cells in flotation circuits is found challenging. After more than a century applying flotation to the mining industry, two completely different strategies have been introduced for processing purposes. One is the classical approach viz. grinding ores to a certain extent (fine particles) and floating them via conventional mechanical and pneumatic cells i.e., Jameson, Imhoflot™ and Reflux™. This strategy continues because mines face declining cut-off grades, complex and poly-mineralized ores, and they are required to achieve an acceptable degree of mineral liberation. The other school of thought deals with coarse particle processes mainly owing to the low energy requirements, that includes SkimAir® flash, fluidized bed and HydroFloat™ cells. There is no study in the literature to comparatively present the recent developments of flotation apparatuses versus the conventional mechanical cells. To cover this knowledge gap in the literature, the present paper endeavors to critically evaluate these concepts from several points of view, including existing technological advancements, water and energy usage, kinetics, and circuit design. A brief introduction of advanced technologies, along with their applications is presented. The data from literature and case studies showed that the Jameson, Imhoflot™ and recently developed Reflux™ flotation cells can be very effective for recovering fine particles owing to their specific hydrodynamic designs, intensive energy dissipation rate and generation of micron-sized bubbles (100–700 µm). Very low (less than a few minutes) mean particle residence time, high gas-hold up (ca. 50–70 %), no agitation and high efficiency of particle-bubble collision were identified as their main advantages compared to traditional mechanical flotation cells. In addition to their common applications in cleaner stage, these cells were used in pre-flotation and scalping (producing final concentrate from the rougher feed) duties. Their main challenges were recognized as relatively unclear procedure on their scale up/down, optimization and simulation. The HydroFloat™ cell was indicated as a promising technology for recovering coarse particle fraction sizes by taking advantage of the fluidized-bed concept with plug-flow dispersion regime, high particle residence time, and limited cell turbulence. We finally concluded that fine particle flotation may remain as the main focus of re-processing tailings dams, while coarse particle treatment should be the focus of this century to reduce total energy consumptions.

Gas phase hydrodynamics in a surface‐aerated tank with a long‐short blades agitator

AbstractThis work aims to study the gas phase hydrodynamics in a stirred tank with a surface‐aerated long‐short blades agitator by the Eulerian–Eulerian approach coupled with population balance model. Predicted local gas holdup and bubble size distribution agree well with those measured by a conductivity probe technique. The predictions demonstrate that the pressure depression in the center is the main driving force for gas suction and the downward flow carries the bubbles down to redistribute in the whole tank. The gas phase has higher gas holdup with large bubble size in the upper part and lower gas holdup but with small bubble size in the lower part of the tank. The predicted liquid‐phase mass transfer coefficients agree well with our previous experimental results and just depend on the power consumption per unit volume when the aspect ratio of the liquid height to the tank diameter varies from 1.1 to 2.0.

Bubble size distribution and gas holdup in bubble columns employing non‐Newtonian liquids: A CFD study

AbstractThe hydrodynamics involved with the rise of air bubbles in shear‐thinning non‐Newtonian liquids in bubble columns was investigated using computational fluid dynamics (CFD). In bubble columns, the bubble size distribution (BSD) and gas holdup significantly affect the mass transfer rates and the reactor design. Consequently, the influence of superficial gas velocity, flow index, consistency index, sparger bubble size, and yield stress were analyzed on a local and global scale. The bubble sizes and the coalescence and breakage phenomena were incorporated using the homogeneous discrete method of the population balance model (PBM). The bubbles underwent coalescence during ascent and exhibited bimodal distribution. The radial homogeneity of the gas phase increased axially. Moreover, the zones of low liquid dynamic viscosity produced zones of a high gas holdup. Though the dual effect of viscosity on gas holdup was non‐existent, the gas holdup surged after 35.1 mPa s at the mid‐zone and remained constant thereafter. The noteworthy differences in simulation results put further emphasis upon the importance of sparger bubble size in CFD modelling. A decrease in the overall gas holdup and the axial air velocity with yield stress was observed. CFD simulation proved capable of providing results in reasonable agreement with experiments.

On Inter-bubble distances and bubble clustering in bubbly Flows: An experimental study

• Positional relationships of bubbles were studied experimentally in the dense swarms. • Dimensionless inter-bubble distances were extracted from X-ray measurement data. • The distance of nearest neighbor bubbles is close to that of FCC structure. • Vertical clustering is observed for Eo ≥ 3.82. • Clustering orientation was found to be independent of the gas holdup in this study. Swarm effects in bubble columns result from the interaction of the bubbles at higher gas holdup. The flow around individual bubbles in a swarm influences the movement of the following bubbles and this has a feedback on the flow regime on a larger scale. That is, hydrodynamics that can no longer be described with models for single rising bubbles. The experimental investigation of the multiscale hydrodynamics in three-dimensional bubble columns was so far limited by the available measurement techniques. In this study, ultrafast X-ray tomography (UFXCT) was applied on bubbly flows in a lab scale bubble column to determine inter-bubble distances and clustering characteristics in the vicinity of individual bubbles for different gas holdups and bubble diameters. Applying the pair correlation function reveals a pronounced vertical clustering for large bubbles with Eo ≥ 3.82, whereas no clustering has been observed for bubbles of lower Eo . It was found that clustering orientation mainly depends on deformability of the bubbles but not on gas holdup.

CO2 sequestration using a novel Belt Conveyor Reactor with rotating sieve trays compared with Airlift Bubble Column as photobioreactors

The present work aims to evaluate the performance of a novel three-phase reactor Belt Conveyor Reactor BCR (with rotating sieve trays patent GB2567340B) as a photobioreactor compared with a traditional Airlift Bubble Column ALR, both influenced by the flow of the gas. Chlorella vulgaris was cultivated in these two photobioreactors using ambient air has a CO2 concentration of 0.038% with different aeration flowrates 0.145, 0.195, 0.24, and 0.29 vvm (gas volume per minute/liquid volume in the reactor).The maximum growth rate achieved on the 14th day of culture was 2.120 and 1.420 g/L for BCR, ALR respectively, with initial biomass concentrations of 0.2 g/L and aeration flow of 0.29 vvm. Moreover, the removal efficiency of carbon dioxide sequestration by the two photobioreactors is 40% for BCR and 25% for ALR. The innovative design succeeded in operational quality of agitation with high gas holdup inside the sieve trays to increase the biomass growth up to 50% higher than in the ALR. The maximum CO2 fixation for ALR was at about 18% at a specific aeration rate of 0.145 vvm in the bubbly regime. Whereas in the BCR can be reached about 38% with wide range operation condition of airflow rate 0.145–0.24 vvm due to well-distributed liquid due to rotating trays, and good gas–liquid mass transfer surface area.

Feasibility Study on Downhole Gas–Liquid Separator Design and Experiment Based on the Phase Isolation Method

As oil exploitation enters its middle and late stages, formation pressure drops, and crude oil degases. In production profile logging, the presence of the gas phase will affect the initial oil–water two-phase flowmeter’s flow measurement results. In order to eliminate gas-phase interference and reduce measurement costs, we designed a downhole gas–liquid separator (DGLS) suitable for low flow, high water holdup, and high gas holdup. We based it on the phase isolation method. Using a combination of numerical simulation and fluid dynamic measurement experiments, we studied DGLS separation efficiency separately in the two cases of gas–water two-phase flow and oil–gas–water three-phase flow. Comparative analysis of the numerical simulation calculation and dynamic test results showed that: the VOF model constructed based on k−ε the equation is nearly identical to the dynamic test, and can be used to analyze DGLS separation efficiency; the numerical simulation results of the gas–water two-phase flow show that when the total flow rate is below 20 m3/d, the separation efficiency surpasses 90%. The oil–gas–water three-phase’s numerical simulation results show that the oil phase influences separation efficiency. When the total flow rate is 20 m3/d–50 m3/d and gas holdup is low, the DGLS’s separation efficiency can exceed 90%. Our experimental study on fluid dynamics measurement shows that the DGLS’s applicable range is when the gas mass is 0 m3/d~15 m3/d, and the water holdup range is 50%~100%. The research presented in this article can provide a theoretical basis for the development and design of DGLSs.

Towards a CFD-PBE simulation of aerated stirred tanks at high gas hold ups and different flow regimes

In this work we analyze the challenges of the adoption of a population balance approach in aerated stirred tanks operating at high gas volume fractions, in different flow regimes and when segregation of the phases is present, i.e. due to the formation of aerated cavities behind flat blades. A decoupled solution of the equations governing the fluid flow and of the population balance equation (PBE) is adopted to obtain preliminary information on the bubble size distribution and to identify the specific issues that need to be addressed to extend the applicability of the population balance model to different regimes. A workflow is proposed for the simulation of aerated stirred tanks with geometries for which established correlations for the estimation of bubble size are not available and a simplified approach to determine whether a coupled CFD PBE solution may be required is presented.

Characterization of the gas dispersion behavior of multiple impeller stages by flow regime analysis and CFD simulations.

Multiple impeller reactors are widely used due to their advanced gas utilization and an increased volumetric mass transfer coefficient. However, with the application of Rushton impellers, gas dispersion efficiency varies between the bottom and the upper impeller levels. The present study analyzes the individual flow regime, power input, and gas hold-up in each compartment of a reactor equipped with four Rushton impellers. The results indicate that the pre-dispersion of the air introduced by the bottom impeller (up to 80%) plays a key role in a better gas retention efficiency of the upper impellers (>300%) and leads to a shift of the cavity and flooding lines in the flow map (Fr- vs Fl-Number) of the upper impellers. A novel analysis of the bubble flow in the dispersed state via a two-phase LES-based CFD model reveals that a more homogenous distribution of air bubbles in the upper compartments leads to high compartment gas hold-up values, but fewer bubbles in the vicinity of the impellers. The measured and simulated data of this study indicate that the upper impellers' efficiency mostly depends on the flow regime of and the pre-dispersion by the bottom impeller rather than on the upper impellers' flow regimes. These results contribute to the understanding of essential mixing processes and scaling of aerated bioreactors.

CFD-PBM investigation of the hydrodynamics in a slurry bubble column reactor with a circular gas distributor and heat exchanger tube

In this work, the effects of a circular gas distributor with the opening downward and a circular heat exchanger tube on the hydrodynamics of a pilot-scale slurry bubble column reactor were investigated by employing the CFD-PBM modeling. The simulations indicated that the circular structure of heat exchanger tube leads to a significant gas phase dispersion, inducing a stronger circulation flow of slurry. Different from the traditional perforated plates, the gas holdup profile with the circular gas distributor shows a bimodal structure in the radial direction. Moreover, the circular heat exchanger tube intensifies the bimodal distribution, resulting in a higher gas holdup in the reactor. Increasing the superficial gas velocity was beneficial in improving the whole time-averaged gas holdup. While the increase of the initial loading height of slurry has insignificant effect on the momentum transfer. In addition, proper axial height of the gas distributor is beneficial to enhance gas holdup.

Reaction performance of fluidized bed catalytic reactor of Group C+ particles

Group C particles are often regarded as non-fluidizable but have proven to effectively fluidize with nanoparticle addition, which results in small bubbles and a high gas holdup in the dense phase during the experiments. Group C+ particles provide an increased surface area for gas–solid contact and improve the reaction performance, especially for gas-phase catalytic reactions. On the basis of a previous study of the ozone decomposition reaction using Group C+ particles, a two-phase model was used to evaluate the reactor contact efficiency, and was used to compare the partial oxidation performance of the n-butane to maleic anhydride reaction in fluidized-bed catalytic reactors of Group C+ and Group A particles. The reactor with Group C+ particles achieved a higher n-butane conversion and MAN yield compared with that using Group A particles, based on the identical catalyst quantity or on the same gas residence time. Therefore, the reactor with Group C+ particles can achieve the same reaction conversion and yield with fewer catalysts or a smaller reactor size, or both. Therefore, the fluidized bed catalytic reactor of Group C+ particles is expected to be of major significance in industrial processes, especially for gas-phase catalytic reactions.