Local Gas Holdup Research Articles

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.

Local Gas Holdup, Bubble Size Distribution, and Bubble Velocity in a Submerged Rotating Packed Bed Reactor

Local Gas Holdup, Bubble Size Distribution, and Bubble Velocity in a Submerged Rotating Packed Bed Reactor

The impact of particle size in fluidized bed on heat transfer behavior: A review

This review paper explores the significance of fluidized bed heat exchangers in various industrial applications. By delving into the operation of fluidized beds as multiphase flow systems, the aim is to enhance their capabilities and efficiency. Key parameters such as minimum fluidization velocity and local gas holdup are crucial for characterizing the hydrodynamic behavior of materials within fluidized beds. Fluidization, achieved by passing atmospheric air through particulate solids, imparts fluid-like properties to the bed. Fluidized beds serve as reactors where this phenomenon takes place, offering several advantages in industrial processes, including high rates of heat and mass transfer, low pressure drops, and uniform temperature distribution. In future work, a focus on understanding and optimizing the fluidization process will contribute to further advancements in the performance of fluidized bed heat exchangers.

3D CFD Simulation of a Bubble Column with Internals: Validation of Interfacial Forces and Internal Effects for Local Gas Holdup Predictions

3D CFD Simulation of a Bubble Column with Internals: Validation of Interfacial Forces and Internal Effects for Local Gas Holdup Predictions

New dual modality technique of gamma-ray densitometry (GRD) and optical fiber probe (OFP) to investigate line-averaged diameter profiles of gas, liquid, and solid holdups along the height of a slurry bubble column

A new hybrid measurement technique was developed and employed to study line-averaged three-phase holdup distribution in a Slurry Bubble Column Reactor (SBCR) using gamma-ray densitometry (GRD) and point optical fiber probe (OFP) techniques. The OFP technique was used to measure the local gas holdup at 45 nodes in the cross-section area of the SBCR at three different axial levels. Thus, the GRD technique measured the spectrum of the SBCR for two different orientations simultaneously with the OFP technique at the same operating conditions to determine the solid holdup distribution radially and axially. The line-averaged solid holdup for 25% solids loading on the higher measurement level at the column’s center decreased compared to the lower level by 45.7%, 43.6%, and 41.7% using superficial gas velocities 0.2, 0.3, and 0.4 m/s, respectively. These results are highly remarkable as it often assumed that the solids exhibit a uniform distribution inside the slurry phase.

Analysis of the local gas hold-up, under constant retrofitted power input in a multiphasic mycelial fermentation model

Multiphase fermentations are still poorly understood since much of the available information focused on global rather than local data and the most used parameter to control the dispersion of gas in the stirred tank is the agitation rate. However, the energy dissipated in the stirred tank is influenced by the properties of the broths, such as viscosity. Therefore, the objective of this work was to use a retrofitted power input strategy in order to maintain constant the total power input per unit volume (Pg/V), studying the local gas hold-up (φ) and interfacial area (a) values in three regions in a pilot-scale stirred tank at different values of Trichoderma harzianum mycelial biomass concentration, airflow rate and constant Pg/V. We found that the constant Pg/V regime was an effective strategy that allows to discern the effects of different parameters in the local values of gas hold-up and interfacial area. Under constant retrofitted total gassed power input, the biomass concentration showed no influence on the local values of interfacial area and gas hold-up. Depending on the position of the probe, φ and a varied up to 60.6% and 63.8%, respectively. The highest Pg/V and airflow rate conditions determined the highest values in φ and a, but not in the same proportion in the three zones tested.

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.

Effect of resistance internal on hydrodynamic behaviours and bubble characteristics in a laboratory‐scale bubble column

AbstractA novel resistance internal is proposed to optimize the flow field and improve the gas–liquid contact in a co‐hydrogenation reactor of coal and vacuum residuum. Local gas holdup, local liquid velocity, and characteristics of the bubble were investigated in a scaled‐down laboratory model. The quantitative results showed that the resistance internals could reduce the thickness of the liquid reflux layer by a percentage up to 32% and reduce the difference in the local gas holdup at cross‐sections of up to 44%. The Sauter mean diameter of the bubble decreased from 20.30 to 16.00 mm, which aroused the increase in bubble surface area by a percentage of up to 71.9%. The resistance internal promoted the breakup of the bubble with multiple mechanisms and provided diversion to fluid. In this work, improvement at multiple scales was realized, and the technical support for industrial application was provided.

Investigation of power consumption, torque fluctuation, and local gas hold-up in coaxial mixers containing a shear-thinning fluid: Experimental and numerical approaches

The performance of a coaxial bioreactor in the intensification of gas dispersion in shear-thinning fluids in terms of the power consumption of the central and anchor impellers, torque fluctuation for the anchor impeller, global and local gas hold-up, and cavity size was assessed. Electrical resistance tomography and computational fluid dynamics methods were used to evaluate the effects of the central impeller type, impeller speed, pumping direction, and rotating mode on the efficacy of the aerated coaxial mixers. Unlike the coaxial mixer equipped with a radial-flow impeller, for the coaxial mixer furnished with an axial-flow impeller, the rotating mode had a pronounced effect on the relative power demand. Surprisingly, it was observed that increasing the central impeller speed increased the contribution of the anchor impeller to the total power consumption of the co-rotating coaxial mixer furnished with a down-pumping impeller. Furthermore, by increasing the central impeller speed the torque fluctuation of the coaxial mixer was diminished due to the uniform distribution of the energy dissipation rate. Unexpectedly, for a coaxial mixer furnished with a down-pumping impeller in both rotating modes and an up-pumping impeller in the counter-rotating mode, the gas hold-up decreased by a further increase in the central impeller speed.

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.

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.

Spatio-temporal 1D gas–liquid model for biological methanation in lab scale and industrial bubble column

Numerical simulation of biological methanation in bubble column reactors is challenging due to strong coupling between hydrodynamics, mass transfer and bioreaction. A satisfactory prediction of the mass transfer coefficient and the gas solubility is crucial to couple hydrodynamics and biokinetics. A comprehensive 1D multispecies multiple-way coupled gas–liquid model is developed. The hydrodynamics of the model is validated using experimental gas loading data at very low gas holdup. The local mass transfer is validated using literature data on CO2 mass transfer in the large-scale. The local gas holdup and interfacial CO2 mass transfer flux are correctly described thanks to a two-way coupling between hydrodynamics and mass transfer, including changes in bubble diameter and pressure effects. When a mixture of H2, CO2 and CH4 is used, highly non-linear mass transfer profiles due to differences in solubilities are observed. An original flux-based metabolic model is proposed to simulate an industrial biological methanation process. This allows smooth transition between biologically and physically controlled regimes as the culture reaches a steady-state. The comprehensive model enlightens that the biological methanation performances are governed by the H2 mass transfer limitation balanced by growth in the transient state and biological maintenance in the steady-state.

Method to concurrently measure local gas and solid holdups visually with higher precision in circulating fluidized bed

AbstractSimultaneous and visual measurements of local gas and solid phase holdups with high‐precision in a gas–liquid–solid circulating fluidized bed is of great importance for the design and scale‐up of such a reactor. However, such measuring techniques are still limited due to the difficulty of discrete phase identification of gas bubble and solid particle phases in the complex three‐phase flow. Immersed telecentric camera method developed based on the digital image processing is such a measuring technique, but its measuring precision is to be improved due to the existing three problems in the depth‐of‐field particle extraction, reconstruction of dim bubbles, and counting of overlapping particle clusters. This work has solved these problems by developing corresponding algorithms and the relative errors of measured solid holdup and gas holdup are as low as 0.3% and 1.0%, respectively.

Impacts of Particle Properties on Gas Holdup under Four Flow Regimes in Three‐Phase Bubble Columns

AbstractThe impacts of the solid volume fraction Vs and particle size dp on the total and local gas holdups in gas‐liquid‐solid bubble columns under different flow regimes were investigated. The flow regimes discussed were the discrete bubble regime (DBR), transition regime (TR), bubble coalescence regime (BCR), and strong turbulent regime (STR). The results demonstrated that the bed average gas holdup εg decreased with higher solid volume fraction in DBR, TR, and BCR. The increment of the solid volume fraction promoted bubble coalescence and also can enhance the particle effect on the average gas holdup. The εg changed slightly when shifting the Vs or dp in STR. A new correlation for εg was developed considering the particle properties and the flow regimes studied.

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.

Evaluation of models for bubble-induced turbulence by DNS and utilization in two-fluid model computations of an industrial pilot-scale bubble column

This paper presents numerical studies on modeling of bubble induced turbulence at two significantly different scales. On the scale of a bubble swarm, direct numerical simulations are performed with a geometric volume-of-fluid method and the budget equation of liquid phase turbulence kinetic energy is analyzed. By a priori testing of models for the interfacial term in this equation, suitable closure relations are identified. On the scale of an industrial bubble column, simulations with a two-fluid model are performed with water and cumene as liquid phases under elevated pressures. Turbulence is taken into account by a mixture k–ε model, considering two closure relations for the interfacial term identified from the DNS. For both liquids, an influence of the model for the interfacial term on turbulence kinetic energy and gas holdup is found, which is, however, small at elevated pressures. Numerical results for local and overall gas holdup are in reasonable agreement with measurements reported for this industrial pilot-scale bubble column. For the overall gas holdup, an empirical correlation from literature is identified which predicts the present numerical results reasonably well.

Electrical Capacitance Volume Tomography (ECVT) for Characterization of Additively Manufactured Lattice Structures (AMLS) in Gas-Liquid Systems

Against the background of current and future global challenges, such as climate change, process engineering requires increasingly specific solutions adapted to the respective problem or application, especially in gas–liquid contact apparatuses. One possibility to adjust the conditions in this kind of apparatuses is an intelligent and customized structuring, which leads to consistent fluid properties and flow characteristics within the reactor. In the course of this, the interfacial area for mass transfer, as well as residence times, have to be adjusted and optimized specifically for the respective application. In order to better understand and advance the research on intelligent customized additively manufactured lattice structures (AMLS), the phase distributions and local gas holdups that are essential for mass transfer are investigated for different structures and flow conditions. For the first time a tomographic measurement technique is used, the Electrical Capacitance Volume Tomography (ECVT), and validated with the volume expansion method and a fiber optical needle probe (A2PS-B-POP) for an air-water system for different modes of operation (with or without co-current liquid flow in empty or packed state). The ECVT proved to be particularly useful for both in the empty tube and the packed state and provided new insights into the phase distributions occurring within structured packings, which would have led to significantly underestimated results based on the visual reference measurements, especially for a densely packed additively manufactured lattice structure (5 mm cubic on the tip). Particularly for the modified structures, which were supposed to show local targeted differences, the ECVT was able to resolve the changes locally. The additional use of a pump for co-current flow operation resulted in slightly higher fluctuations within the ECVT data, although local events could still be resolved sufficiently. The final comparison of the empty tube at rest data with a fiber optical needle probe showed that the results were in good agreement and that the local deviations were due to general differences in the respective measurement techniques.

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.

CFD simulation of bubble column hydrodynamics with a novel drag model based on EMMS approach

Drag model is of critical importance for CFD simulation of flow characteristics in bubble columns. In this study, a novel drag model termed DBS-Local for the ratio of effective drag coefficient to bubble diameter is derived from the Dual-Bubble-Size (DBS) model based on Energy-Minimization Multi-Scale (EMMS). A thorough comparison demonstrates that without adjustable parameters, both the DBS-Local model and the DBS-Global model are capable of improving the prediction of total and local gas holdup as well as the flow characteristics in different flow regimes. By contrast, other drag models either overestimate or underestimate the total or local gas holdup in certain flow regimes. Moreover, the centerline liquid velocity predicted by DBS-Local model shows best agreement with experiments, whereas the other models, especially the Ishii-Zuber model, over-estimate liquid velocity. This study demonstrates the significance of modeling the bubble-swarm effect based on EMMS approach.

Hydrodynamic Characteristics Effect of Foam Control in a Three-Phase Fluidized Bed Column

The present work was devoted to study the effect of operating parameters (e.g., superficial gas and liquid velocities, size of solid particles, volume percentage of particles loaded in column and type of particles) on foams, and to investigate the process of foam suppression.
 The experimental apparatus was operated in continuous mode for the two phases (i.e. air and a solution of aqueous anionic surfactant). Sands which were considered as hydrophilic behavior particles were used as solid phase. A specific surface treatment was performed on hydrophilic sands particles to transfer it into hydrophobic one. These two versions of sands were used in the experimental setup respectively to study their effect on suppression of foams. The average gas holdup for the entire column was measured by means of the local gas holdup which was computed from the pressure difference in each segment of the column. Local solid concentration along the column was measured experimentally by analyzing the samples of mixture drawn from the sampling ports. From the present work it was found that: Flow regimes of multiphase system could be easily determined by utilizing local gas holdup profile measured by pressure drop transducer method. The transition from the homogenous to heterogeneous regime was advanced, from ug=4 to 2cm/s with increasing solid concentration from 10 to 20%v and decreasing average particle diameter from dp=(0.8) to (0.25) mm.
 When a mixture of water/surfactant was employed in the bubble column, foam could be present depending on the input operating parameters. The fluid mechanism of foam suppression with hydrophilic particles was enhanced by a direct attack on the foam by hydrophobic particles (i.e. hydrophobic particles were more effective in retaining liquid–destroying foam than the hydrophilic particles). It was found that hydrophobic particles of 0.25 mm average diameter and 10%v loading in the reactor could reduce foaminess fraction from 0.85 to 0.15 if the liquid velocity was 0.3 cm/s. The foaminess fraction could be reduced to 0.0 if the liquid velocity increased to 0.4 cm/s. The results of this study may have abroad applications in petroleum and petrochemical industries where liquid hydrocarbons are processed.