Gas Holdup Distribution Research Articles

Effect of solid loading and particle size on bubble behavior and flow field structure in slurry bubble column

The complexity of fluid dynamics in a slurry bubble column reactor introduces significant uncertainty in reactor design and scale-up. This paper investigates the hydrodynamic performance of the gas–liquid–solid system within the reactor by employing computational fluid dynamics-population balance modeling numerical simulations alongside particle image velocimetry (PIV) experiments. The effect of superficial gas velocity and particle conditions on the overall gas holdup were analyzed, focusing on the effects of particle size and solid concentration on bubble size, bubble behavior, flow field structure, and local gas holdup distribution at high superficial gas velocities. Bubble size was evaluated using calibrated image measurements, and the impact of varying solid conditions was thoroughly explored. The results revealed that an increase in solid size correlated with higher gas holdup and smaller bubble sizes, whereas a greater solid concentration resulted in decreased gas holdup and larger bubble sizes. PIV experiments indicated that bubbles exhibited a tendency to migrate toward the central region of the reactor, leading to the formation of larger bubbles that accelerated the rise of surrounding bubbles, while smaller bubbles near the wall moved downward. As the slurry bed height increased, the range of local gas holdup distribution expanded, resulting in a symmetrical distribution of radial local gas holdup in the fully developed stage at a height of 0.16 m.

Bubble characteristics in a novel microbubble gas-liquid-solid fluidized bed

The bubble characteristics are critical in gas-liquid-solid fluidized beds and microbubbles significantly enhance mass transfer in multiphase systems. In this work, a novel concept of microbubble gas-liquid-solid fluidized bed is proposed. Telecentric camera is employed to measure and analyze the microbubble characteristics including bubble size and gas holdup in this three-phase fluidized bed. Additionally, solid holdups in the fluidized bed are also examined. The results demonstrate that the bubble size adheres a lognormal distribution and is significantly influenced by superficial gas velocity. Bubble diameter exhibit a relatively uniform distribution in the radial direction. Bubbles exceeding than 1 mm substantially affect local gas holdup, and the microbubble flow promotes a more uniform distribution of local gas holdup in radial position. The average gas holdup deviation is less than 15 % compared to overall gas holdup obtained from pressure drop. Although microbubbles are more abundant, larger bubbles (> 1 mm) contribute more to solid particle fluidization. This paper introduces a methodology for assessing smaller-sized microbubbles in three-phase flow. The hydrodynamic analysis of the microbubble gas-liquid-solid fluidized bed establishes a foundational framework for enhancing gas-liquid mass transfer in fluidized bed.

Characteristics of gas–liquid two-phase flow in a stirred tank with double-layer punched impeller

This study investigates the gas–liquid two-phase flow characteristics in a stirred tank equipped with a double-layer punched impeller. Numerical simulations are conducted to analyze flow dynamics, gas holdup, bubble sizes, and distributions under various operational conditions. The results show a high degree of agreement between experimental and simulated power values and gas holdup distributions, validating the reliability of the computational fluid dynamics–population balance model coupling approach. The combination of the punched four-inclined-blade up-pumping turbine and the punched Rushton impeller exhibits excellent bubble dispersion characteristics, with overall small bubble sizes. Increasing the rotational speed can enhance turbulence within the flow field and accelerate the liquid phase velocity, which facilitates gas diffusion and improves gas–liquid mixing efficiency. Additionally, higher rotational speed further intensifies the shear effect of the punched impeller, resulting in a reduction in average bubble size.

Modeling of Dual‐Factor Drag Correction Model for Bubbly Flow under Elevated Pressure

AbstractA pressure correction method is proposed considering the influence of a dual factor. The applicability of a pressure correction method coupled with a drag model is discussed along with the accuracy of the simulation results obtained by such a pressure correction method. It is found that the present pressure correction method combined with the DBS (dual bubble size) drag model can accurately reflect the changing trend of gas holdup distribution with pressure. It is also established that results from this model applied to a bubble column match well with the experimental data. Finally, when compared with other pressure correction models, the proposed model shows better robustness in three‐dimensional simulations and can predict radial gas holdup distributions with better accuracy.

Numerical investigation on mixing behaviour in the contact zone of an industrial‐scale dissolved air flotation tank

AbstractThe mixing behaviour of the wastewater and the recycle bubbly water in the contact zone could affect the separation performance of dissolved air flotation systems. In this study, the mixing characteristics in the contact zone of an industrial‐scale dissolved air flotation tank were investigated using the computational fluid dynamics method. At first, the influence of the turbulence model on the simulation results was clarified. It is demonstrated that the realizable k‐ε model could predict the gas holdup distribution with better accuracy, and the maximum relative deviation between the predicted gas holdup and the experimental results is only 10%. The influence of the geometry and operating parameters on the mixing process was then studied numerically. It is found that the mixing performance in the contact zone could be improved by decreasing the height position of the nozzle inlet in a certain range. The reason is that microbubbles disperse better in the bottom region and a double circulation flow state forms in the contact zone when the nozzle inlet is installed at a low height. Additionally, the mixing process could be further enhanced by decreasing the diameter of the nozzle inlet. As to the operating conditions, increasing the recycle ratio could not only increase the gas holdup but also improve the mixing performance in the contact zone. When the recycle flow rate remains constant, increasing the wastewater flow rate will lead to a decrease in gas holdup and deterioration of the mixing effect.

Influence of internal components on the hydrodynamic properties of the scrubbing-cooling chamber

The bubble-breaking plate is a crucial internal component in the scrubbing-cooling chamber. The study of hydrodynamics under its influence is the basis for designing and scaling up industrial installations. Experimental measurements of characteristic bubble parameters and liquid phase structure parameters at local locations were carried out using a conductivity probe and a pitot tube, respectively, to investigate the effects of five types of bubble-breaking plate installation on the hydrodynamic properties of the annular gap bubble bed. The experimental results show that the staggered placement of two plates is the most favorable for increasing the gas holdup, reducing the bubble size, increasing the gas–liquid interface area, improving the liquid velocity distribution and suppressing the turbulence fluctuations. In addition, a dimensionless mathematical model predicted the radial distribution of local gas holdup and axial time-averaged liquid velocity, and the expected results agreed with the experimental values.

Simulation of Multi-Phase Flow in Autoclaves Using a Coupled CFD-DPM Approach

In this work, a numerical simulation study on the mixing characteristics of multiphase flow in an autoclave was carried out using CFD technology. The Eulerian–Eulerian model and discrete phase model (DPM) were employed to investigate the solid holdup, critical suspension speed, nonuniformity of solid suspension, gas holdup distribution, bubble tracks, and residence time during stirring leaching in the autoclave. Experiments validate the accuracy of the numerical model, and the experimental values correspond well with the simulation results. The numerical simulation results show that the solid–liquid mixing is mainly affected by the axial flow, the best agitation speed is 400 rpm, and increasing the speed further cannot make the mixture more homogenous and buildup occurred above the autoclave. The calculated critical suspension speed is 406 rpm, which is slightly lower than that obtained from the empirical formula. The gas phase is mainly concentrated in the vortex area above the blade. When the gas phase is in a completely dispersed state (N = 300 rpm), the average residence time of the bubbles is 5.66 s.

Impact of Curved‐Blade Impellers on Gas Holdup and Liquid Homogenization Dynamics in Stirred Tanks

AbstractDesign and optimization of aerobic fermentation processes in stirred tanks must factor in specific features, such as the formation of gas cavities on the rear of impeller blades and the need for high power input. Here, the ability of a curved‐blade impeller to reduce these drawbacks, which are typical of flat‐blade impellers, is investigated. The analysis is based on gas holdup distributions and liquid homogenization dynamics collected by electrical resistance tomography in a pilot‐scale stirred tank of geometry similar to typical industrial aerated fermenters. A wide range of gas flow rates and impeller speeds in single‐impeller and multiple‐impeller configurations are considered and the differences arising when a Rushton turbine is replaced with a Bakker turbine are discussed.

Study on hydrodynamics characteristics in a gas-liquid stirred tank with a self-similarity impeller based on CFD-PBM coupled model

BackgroundIn general, radial impeller has been the most widely employed for the gas-liquid mixing process in the stirred tank. However, gas cavities are easy to form behind the impeller blades, which results in the waste of power consumption, the limited gas handling capacity of impeller and the poor dispersion of gas. Based on self-similarity property of nonlinear theory, a type of self-similarity impeller was employed to strengthen gas-liquid dispersion process in this work. MethodsThe hydrodynamics of gas-liquid dispersion process in a stirred tank with Rushton turbine and self-similarity impeller were comparatively investigated by computational fluid dynamics (CFD) simulation combined with a population balance model (PBM). The gas holdup distribution, gas cavities, relative power demand (RPD), bubble size distribution and turbulent intensity analysis in the gas-liquid dispersion process were predicted. Significant findingsResults showed that self-similarity impeller could destroy the gas cavity structure, reduce the negative pressure zone and gas accumulation, enhance the relative power demand (RPD) and improve the uniformity of gas holdup distribution compared with Rushton turbine, and the gas-liquid mixing efficiency can be further improved with the increase of the self-similar iteration number of self-similarity impeller. The proper increase of impeller speed was beneficial to overcome gas flooding and improve the gas-liquid two phase dispersion uniformity. The increase of gas flow rate obviously increased the gas holdup in liquid phase. Meanwhile, self-similarity impeller could increase the uniformity of bubble size compared with Rushton turbine, and increasingly so with the increase of self-similar iteration number of self-similarity impeller. The uniformity of bubble size was improved with the increase of impeller speed and was decreased with the increase of gas flow rate. In addition, self-similarity impeller could significantly enhance the turbulent intensity and turbulent kinetic energy dissipation rate by the jet flows through the concave-convex edges, which was beneficial to the gas dispersion process.

Drag models coupled with CFD–PBM method for simulation in bubble columns

AbstractIn this study, three‐dimensional numerical simulation of gas–liquid flow in bubble columns was realized by using the computation fluid dynamics (CFD)–population balance model (PBM). The new drag model improves the stability‐constrained multi‐fluid (SCMF‐C) model because of the consideration of the wake accelerating and the hindering effects for calculating the drag correction factor. The gas holdup, axial liquid velocity, and bubble size distribution (BSD) predicted by four drag models at 0.02 and 0.1 m/s were compared. The results revealed that the proposed drag model can provide excellent predictions for both bubbly and heterogeneous flows. Because the wake accelerating and the hindering effects were considered, reliable predictions were achieved for the gas holdup, and the problem of uniform gas holdup distribution was mitigated. Therefore, the SCMF‐C model can be extended for nonuniform BSD. The gas holdup and liquid velocity increased, and the nonuniformity of radial results became pronounced at 0.1 m/s. The profiles of four drag models were similar at a low height, whereas the difference between the simulations of the four models became obvious with the variation of heights. The results of the four models were accurate, and the BSD was wide at 0.1 m/s. Subsequently, the feasibility of the four drag models was evaluated at 0.2 and 0.4 m/s. The results of the comparison revealed that the proposed drag model exhibited excellent feasibility at higher gas velocities and was powerful for the simulation of bubble columns.

Gas–liquid mixing in the stirred tank equipped with semi-circular tube baffles

Abstract Despite extensive contributions have been made over the past several decades, there are still ongoing challenges in improving gas dispersion performance in stirred tanks. To that end, a stirred tank equipped with four semi-circular (SC) tube baffles was designed. Hydrodynamics in the stirred tank before and after aeration were studied. The turbulent fluid flow was simulated using the standard k-ε turbulence model. The gas–liquid two-phase flow was simulated by the Eulerian–Eulerian multiphase model and the k-ε dispersed turbulence model. The impeller rotation was modeled with the multiple reference frame (MRF) approach. Firstly, the grid independence test was made. By comparing the distributions of gas holdup in the flat-plate (FP) baffled stirred tank with literature results, the reliability of the numerical model and simulation method was verified. Subsequently, the flow field, gas holdup and power consumption of the SC and FP configurations were studied, respectively. Results show that the former can increase the fluid velocities and promote the gas holdup dispersion. Besides, it is energy-saving and has a higher relative power demand (RPD). The findings obtained here lay a preliminary foundation for the potential application of the SC configuration in the process industry.

Study of Gas Holdup Distribution in Cylindrical Split Airlift Reactor by Using Gamma-Ray Densitometry (GRD)

The local gas holdup details and behaviors in the cylindrical split airlift column by using an unconventional gamma-ray densitometry (GRD) measurement in non-invasive manner technique was investigated for the first time in this work for such kind of airlift column. With different gas velocities, 1, 2, and 3 cm/s, at three various axial planes (different levels) in z = 3, 60, and 110 cm were studied for local distribution in radial gas holdup profiles. The distribution in gas–liquid phases (air-water system) in the entire split reactor column, in the rising and descending sides, including their behavior in the upper and lower zones of the split plate, were investigated as well. The results of this study showed that approximately all reactor zones had exemplary gas–liquid phases and that there was a large magnitude over both the dividing ring and the top sections. The results further indicated that the distribution of which flow variable in the implementation of the cylindrical split reactor can have an important impact on its behavior, especially for cultivating applications of microorganisms. These data can be used as benchmarks results for CFD simulations and validation.

Study on flow characteristics of bidirectional sinusoidal liquid pulsed gas-liquid-solid multiphase fluidized bed

Gas-liquid-solid three-phase fluidized bed has been widely used in chemical, biotechnology and environmental industries for its advantages of large inter-phase contact area, high heat/mass transfer efficiency and intense reaction process. In this work, the bidirectional sinusoidal liquid pulsed GLS fluidized bed was proposed to improve the interphase relative velocity and reaction efficiency. The effects of particle properties and pulsed liquid amplitude/frequency on the flow characteristics were studied by numerical simulations and experiments. In a complete circulation period, the average axial solids holdup is larger near the middle region of the fluidized bed and gradually decreases along the upper and lower inlets. With the increase of pulsation amplitude or period, the axial average gas holdup distribution uniformity and the liquid-solid relative velocity increase significantly. Compared with the conventional upward gas-liquid-solid fluidized bed, the bidirectional fluidized bed can further improve the liquid-solid relative velocity and reaction efficiency. The obtained studies have important implications for better design, scale-up and operation of gas-liquid-solid multiphase fluidization systems.

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.

Gas Dispersion Characteristics in a Novel Jet-Stirring Coupling Flotation Device

A jet-stirring coupling flotation device (JSCFD) was proposed to analyze the distribution characteristics of gas holdup and bubble Sauter mean diameter (D32) in a gas–liquid system under various parameters. Results of studies suggested that the gas holdup increased with methyl isobutyl carbinol concentration, feeding pressure, and gas flow rate. The maximal gas holdup in the absence of the stirring impeller was ∼23.29% for the bubble size of 0.59 mm, which was considerably lower than the maximum gas holdup of 66.27% for the bubble size of 0.31 mm in the presence of the stirring impeller; the gas holdup was raised by ∼43% due to the bubbles torn by the stirring impeller to generate extensive smaller size bubbles and increase the content of small bubbles, and increasing the stirring impeller speed was conducive to reduce the bubble size and increase the gas holdup in JSCFD. Compared to traditional flotation machines, the size of bubbles generated by JSCFD was smaller, and the gas holdup distribution conforms to the following order: JSCFD > mechanical flotation machine > column flotation, which demonstrated that the JSCFD had a noticeable effect on increasing the gas holdup and reducing the bubble size.

Insight into the effect of particle density and size on the hydrodynamics of a particular slurry bubble column reactor by CFD-PBM approach

The present work numerically investigated the effect of catalyst properties on the hydrodynamics of a pilot-scale slurry bubble column reactor with circular sparger and helical heat exchangers by employing the CFD-PBM modeling. Strong dependency on particle size (dp) were observed when dp>75 μm. With the particle size increased, the axial concentration gradient of gas holdup and solid holdup gradually increased, which were not conducive to heat and mass transfer. When dp was in the range of 10–75 μm, the gas holdup increased with increasing the particle size, which is beneficial to the momentum transfer in the bed. The variation of particle density (ρp) had insignificant effect on the overall gas holdup distribution, though the solid holdup increased obviously in the sparger region with ρp increased. This work provides reminding to search for the optimal operating conditions and gives reliable guidelines to prepare high-efficiency catalysts in the synthesis process.

Comparison of Different Impellers for Gas Dispersion in Power-Law Fluids

The 3D multiphase computational fluid dynamic (CFD) models have been developed for gas–liquid systems in a standard baffled stirred tank reactor (diameter 0.3 m) for non-Newtonian power-law fluids. The standard k-ε model has been used to model the turbulence, and drag has been modeled with the Grace drag model with Brucato modifications. CFD simulations have been carried out for three impellers, namely, the Rushton turbine, pitched blade turbine, and hydrofoil impeller. The effects of impeller design, power input per unit volume of the fluid (P/V) (1200–5200 W/m3), fluid rheology (K = 0–0.1561 Pa·sn, n = 0.687–1.000), and superficial gas velocity (7.1–28.3 mm/s) have been analyzed. The CFD model has been validated with the local distribution of axial, radial, and tangential velocities and overall gas holdup data reported in the literature. The CFD model has been used to predict the regime transition from flooding to loading and complete dispersion. A good agreement was observed for the prediction of regime transition for the Rushton turbine when compared with the experimental data. The average value of absolute error for the prediction of the gas holdup is 20%, which indicates that the developed CFD model can be successfully used to predict the gas holdup and regime transition in stirred tank reactors. Among the impellers studied, the hydrofoil impeller shows the highest gas holdup and uniform gas distribution in the vessel at low power consumption.

Investigating the gas holdup in a gas-liquid cyclonic flotation column through the electrical resistance tomography

ABSTRACT Gas dispersion characteristics are of significant importance in froth flotation. Studies show that cyclonic static microbubble flotation column (FCSMC) is an efficient device to recover fine minerals. In the present study, the gas holdup distribution and the mean gas holdup in a laboratory gas-liquid FCSMC are first quantitatively measured by electrical resistance tomography (ERT). Different flowrates of gas and liquid are considered. Performed experiments show that when the air flowrate increases from 1.1 L/min to 2.3 L/min, the mean gas holdups on measured planes increase to about 4 ~ 5 times the original value. The gas dispersion along the radial direction is poor, which is dense in the center and rare in the periphery. This uneven bubble distribution aggravates as the amount of the supplying air increases. As the circulating liquid flux increases, the corresponding measured gas holdup gradually decreases and the bubble shrinks to the center along the radial direction. When the liquid flux increases from 1.5 m3/h to 1.8 m3/h, the averaged decline of the mean gas holdup on measured planes reduces to 54 ~ 65% of the original value. Finally, it is validated that the sieve plate packing effectively reduces non-uniform radial and axial distributions of the gas holdup. The obtained results might lead to more effective operating of FCSMC, provide a reference for further numerical study and ERT application in other complicated devices.

Size effect of γ-MnO2 precoated anode on lead-containing pollutant reduction and its controllable fabrication in industrial-scale for zinc electrowinning

Lead (Pb) is the most widely used anode in zinc (Zn) electrowinning and other metallurgical industries. The resource loss and environmental pollution caused by Pb anode corrosion are urgent problems to be solved. A γ-MnO2 precoated anode was prepared successfully to reduce the Pb-containing pollutant. The size effects with its controllable preparation on an industrial scale were studied. Severe nonuniform distribution of γ-MnO2 film was observed with curbing the reduction of anode slime only 68%, when anode size increased from lab to industry. Nonuniform rate (R) and average thickness (d) were found to be the key indicators to determine the film structure distribution and their performance differences, which were random and difficult to be controlled in scale-up size. However, a controllable industrial γ-MnO2 precoated anodes (IMPA) fabricated through optimized current density (J0) and electrodeposition time (t) in our developed film-forming system. Then, the long-term performances of two IMPA with different indicators (IMPA-1: R = 34%, d = 108 μm, IMPA-2: R = 23%, d = 55 μm) were compared with the industrial typical Pb-based anode (ITPA). Of the three different anodes, the optimized IMPA-2 displayed the best performance. Within 24 d of electrowinning cycle, the corrosion inhibition effect and the anode slime reduction rate for IMPA-2 improved by 56% and 30% than IMPA-1, and improved by 100% and 91% than ITPA. Furthermore, the mechanism analysis of size effect change showed that R of IMPA was contributed to the local gas holdup distribution along the anode. Controlled size effect of uniform oxide film will have a future application prospect for the sustainability of industry, which provides an important cleaner production of Zn electrowinning and related hydrometallurgy industries.

Local Distribution of Oxygen Mass Transfer Coefficient in CMC Solutions in Bioreactors Furnished with Different Types of Coaxial Mixers

Aerated stirred tank bioreactors are one of the most commonly used systems in various processes such as biofermentation where ensuring adequate mass transfer rate is the sole purpose. In this study, the crucial role of the central impeller type (including Rushton turbine, pitched blade turbine, elephant ear impeller and their combinations) on the performance of an aerated bioreactor equipped with two central impellers and an anchor on the local and overall volumetric mass transfer coefficient was investigated at a variety of operating conditions such as non-Newtonian solution concentrations, and central and anchor impeller speeds. Carboxymethyl cellulose (CMC) solution was used as a power-law fluid. Through the use of electrical resistance tomography, simplified dynamic pressure method, and computational fluid dynamics, several aspects of the aerated coaxial mixer namely local and overall gas holdup and mass transfer were studied. The local mass transfer coefficients were measured using three dissolved oxygen sensors installed at three various heights of the bioreactor. The local gas holdup was also measured using tomography. The results showed the positive effect of the anchor impeller on the mass transfer coefficient up to 10 rpm for different coaxial mixing configurations with various impeller type combinations and CMC concentrations. Among different impeller types, the system with two Rushton turbines resulted in the highest volumetric mass transfer coefficient, which was attributed to the higher gas holdup provided by this configuration as well as the flow pattern generated by the coaxial mixer. Moreover, the local distribution of gas holdup and mass transfer coefficient demonstrated a reduction in both parameters from the bottom to the top of the bioreactor.