Mass Transfer In Bubble Column Research Articles

Computational fluid dynamics (CFD)- deep neural network (DNN) model to predict hydrodynamic parameters in rectangular and cylindrical bubble columns

Bubble columns are omnipresent in the chemical, bio-chemical, petrochemicals, petroleum industries, but their design and scale-up is complex owing to its complex hydrodynamics. Liquid velocity and gas holdup is one of the critical hydrodynamic parameters which effects the mixing, heat and mass transfer in bubble columns. CFD is widely recognized as a powerful tool for estimating critical hydrodynamic parameters but requires significant computational resources, time and expertise. These limitations restrict its practical use in hydrodynamic simulations that need real-time processing involving large-scale simulations of bubble columns. To overcome these limitations, CFD-DNN model is developed to predict the time averaged gas holdup and axial liquid velocity at various operating conditions. The DNN model was trained using CFD data that was produced for rectangular (with dimensions L=0.2 m, W=0.05 m, H=1.2 m) and cylindrical (with a diameter of 0.19 m) bubble columns. The data covers a range of operating conditions and various flow regimes. The superficial gas velocity for the rectangle column was selected at 1.33 and 7.3 mm/s, whereas for the cylindrical bubble column, it was fixed at 0.02 and 0.12 m/s. The CFD-DNN model was validated against the experimental and the CFD data from the literature. Further, the model was tested for new data that the CFD-DNN model has not seen with existing literature and showed good agreement with their data and it reflects the excellent generalization ability of the model. The proposed CFD-DNN approach improves current CFD models by providing shorter computing time, decreasing computational expenses, and reducing the expertise in CFD simulations. The accuracy of the developed CFD-DNN model was evaluated using different metrics for gas holdup and axial liquid velocity. For rectangular bubble columns, the model achieved MSE of 0.0001 for gas holdup and 0.0007 for axial liquid velocity. Similarly, for cylindrical bubble columns, the MSE values were 0.0009 for gas holdup and 0.0006 for axial liquid velocity.

Numerical Simulation of Gas–Liquid Mass Transfer in Bubble Column Using the Wray–Agarwal Turbulence Model Coupled with PBM

Numerical Simulation of Gas–Liquid Mass Transfer in Bubble Column Using the Wray–Agarwal Turbulence Model Coupled with PBM

Effect of Swirl Gas Injection on Bubble Characteristics in a Bubble Column

Swirling gas injection is a well-known technique to improve mass transfer in bubble columns. It can be used to create small bubbles with a high surface area-to-volume ratio, which is beneficial for mass transfer. Swirl gas injection can also be used to create a more uniform bubble size distribution and improve the mixing of gas and liquid in the column. This study aims to determine the impact of swirl gas injection on bubble properties, including bubble shape, size, and velocity. A bubble detection approach has been developed for quick and precise determination of bubble size distributions in gas-liquid systems. Advanced digital image processing, including edge detection and bubble edge recognition, is used in this method. The experiment is conducted in a bubble column at a height of 57 cm and 61 cm. The column had a ring sparger and was made of Plexiglas. Tap water was used as the liquid, while air from an air compressor was utilized as the gas phase. The shape, size, population, and velocity of the bubble are measured using a high-speed digital camera. According to this study, the average bubble size reduced as the impeller speed increased, while the population of bubbles increased when the sparger rotation speed increased from 30 to 150 rpm.

CFD simulation of mass transfer in bubble columns: Detailed study of mass transfer models

The accuracy of the computational fluid dynamics simulations of mass transfer process in bubble columns is significantly influenced by the mass transfer models. Various mass transfer models have been developed based on different theories. However, it is still confusing on the choice of mass transfer models when performing the simulations of mass transfer in bubble columns. In the present work, the applicability and accuracy of three types of widely-used mass transfer models, including the single bubble-based models, the slip velocity model, and the surface renewal stretch model, were assessed with the spatially resolved data of gas holdup and species concentration in the literature. By comparing simulation results with experimental data, the present work provided suggestions on choosing a proper mass transfer model when performing simulations of the mass transfer process in bubble columns. The influence of bubble-induced turbulence and its different modeling approaches on different mass transfer models were investigated.

Euler/Euler large eddy simulation of bubbly flow in bubble columns under CO2 chemisorption conditions

It has been generally recognised that the turbulent dispersion force plays an important role in interphase momentum transfer. However, the effect of the added mass stress on the momentum and mass transfer in bubble column bubbly flow has not been addressed appropriately. As the turbulent eddies in the surroundings of bubbles interact strongly with the rising bubbles in bubble column bubbly flow, such interactions will bring out the change of interfacial areas between the bubbles and carrier fluid, consequently leading to changes in the interfacial mass transfer. When employing large eddy simulation for modelling bubbly flow coupled with the chemisorption process, the SGS filtered velocity fluctuations of liquid phase can be interpreted as the turbulent eddies that continuously hit the surfaces of bubbles, causing bubble deformation and the variation of bubble interfacial areas, which give rise to the turbulent dispersion and added mass stress forces. The present study will demonstrate through Euler/Euler large-eddy simulations (LES) modelling that by considering the turbulent dispersion force (SGS-TDF) and added mass stress (SGS-AMS), the bubble dynamics and mass transfer under the chemisorption conditions can be better indicated, which leads to remarkable improvements in the prediction of bubble lateral dispersion and the interfacial mass transfer. The turbulent dispersion and added mass stress related to the spatially filtering were proposed with a modification on SGS eddy viscosity to reflect turbulent dispersion due to bubble-induced turbulence. A comprehensive assessment of the effects of these additional filtered stress terms on the time-averaged velocity and bubble volume fraction profiles, flow patterns, mass transfer and the pH variation during CO2 chemisorption and the turbulent kinetic energy and species concentration spectra was conducted.

Influence of Fresh Water on Microbubble Generation in an Airlift Column Applied to Aquaculture: Extraction Capacity

Bubble flows consist a liquid phase and a gaseous phase dispersed as bubbles. They occur in nature and in many industrial applications, such as oil transportation in pipelines and steam generators for power generation. Due to large difference in density between gas and liquid, the flottability force causes bubbles to rise, which in turn can generate overall motion and agitation in liquid. This use of gravity as a flow driver, which is specific to disperse phase systems, is used in process engineering (bubble columns and gasosiphon) to sparingly promote mixing and exchange between gas and liquid. In many applications, bubbles are used to agitate a liquid in order to promote mixing and transfers. This work is devoted to study of hydrodynamics of a bubble column. Experimentally, we have determined properties fluctuations of velocities inside the aquarium of rising homogeneous bubbles for different bubble sizes and vacuum rates. The interfacial area between gas and liquid phase is a crucial factor for mass transfer in bubble columns. The molecular exchange between a given volume of gas and water can be enhanced by formation of smaller bubbles, leading to a larger gas-liquid interface. This work presents the various physical phenomena that apply to bubbles, as well as associated dimensionless numbers. A state art of Micro-Bubble Generators (MBG) is then presented, presenting systems using various phenomena such as cavitation, electrolysis, or shear.

Bubble Column CFD Model with Effects of Forced Oscillations on Bubble Dynamics

AbstractBubble size distribution has a significant effect on gas holdup, residence time distribution, and mass transfer in bubble column reactors (BCRs). The occurrence of small gas bubbles has potential advantages such as a limited residence time distribution and perfect gas‐liquid phase contact. Gas‐liquid mass transfer in bubble columns can be significantly enhanced by subjecting the liquid phase to low‐frequency vibrations. However, the effect of vibration frequency on bubble formation and hydrodynamic characteristics in BCRs is still not well understood. Herein, a 3D numerical model of a BCR was developed to study the effect of vibration frequency on bubble dynamics under different operating conditions. The Sauter mean bubble diameter was found to be approximately inversely proportional to the vibration frequency.

Numerical investigation and modelling of the venous injection of sclerosant foam

Sclerosant foam, a mixture of a surfactant liquid and air, is injected directly into varicose veins as a treatment that causes the vein to collapse. This investigation develops a model that will allow the medical specialist to visualise how the sclerosant foam will interact with the blood and behave within the vein. The process is simulated using a multiphase computational fluid dynamics model with the sclerosant foam considered as a two-phase non-Newtonian power law viscosity liquid. The governing multiphase equations are solved using an Eulerian⁠–⁠Eulerian approach coupled with a population balance model to predict the bubble size distribution within the flow field. The computational results demonstrate similar flow characteristics and flow features to an available set of experimental results. The model predicts the mixing layers between the sclerosant foam and the ambient fluid, and the sclerosant liquid and the ambient fluid, as well as the sclerosant liquid coverage on the vein wall and the bubble size distribution within the vein. These quantities are of interest to medical specialists allowing them to assess the treatment feasibility and safety before treating the patients. 
 
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A recirculation cell approach for hydrodynamic and mass transfer modeling in bubble columns with and without internals

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.

Investigation of Mass Transfer and Hydrodynamics in a Model Bubble Column

AbstractGas‐liquid mass transfer in bubble columns is strongly influenced by the flow field in the column. The underlying transport processes are often the limiting factor. Since bubble columns are widely used in the chemical process industry, it is essential to quantify how the dispersed gas can come into reaction with the liquid phase through gas‐liquid mass transfer. CO2 bubble swarms in a cylindrical model bubble column are considered. Optical measurement methods are used for the non‐intrusive measurement of velocity and pH change, resulting from the dissolution of CO2 in the liquid phase. The mean and fluctuating velocity components in the center plane of the column as well as the time‐dependent pH fields in this plane are determined. From these quantities, derived values characterizing the mass transfer from gaseous CO2 to the liquid phase can be calculated.

Hydrodynamics and mass transfer in bubble column, conventional airlift, stirred airlift and stirred tank bioreactors, using viscous fluid: A comparative study

The performance of four bioreactors (bubble column, concentric tube airlift, concentric tube stirred airlift, and mechanically stirred tank) were evaluated in this study in terms of the hydrodynamics and mass transfer, using viscous a Newtonian fluid (glycerol 65%) and a non-Newtonian fluid (xanthan 0.25%). The experimental results showed that the gas holdup and mass transfer coefficient were higher in the stirred airlift and stirred tank, on the other hand these reactors had high shear rates. In relation to power consumption, lower values were obtained in the bubble column and airlift bioreactors. In a viscous medium in which microorganisms or shear-sensitive cells are used, the use of airlift bioreactors may be the best choice for presenting a low shear environment and a reasonable oxygen transfer rate, in addition to the low power consumption. On the other hand, if the process involves microorganisms that require high oxygen rates, a stirred airlift bioreactor may be the best choice.

Simulation of Coalescence, Breakup, and Mass Transfer in Polydisperse Multiphase Flows

AbstractA two‐fluid model implemented in OpenFOAM is extended to account for the polydispersity of the disperse phase by coupling CFD with population balance models. The method is applied to predict the fluid dynamics and mass transfer in bubble columns. Here, the conditional quadrature method of moments is implemented in the open‐source code OpenFOAM for describing bubble coalescence, breakage, and mass transfer in two reference cases of rectangular bubble columns. The overall approach is accurate, efficient and robust, and therefore, suitable for the simulation of multiphase flows with industrial relevance.

Co- and counter-current mass transfer in bubble column

Design and scale-up has gained considerable attention in recent years because of complex hydrodynamics and its influence on bubble column reactor performance. The concentration difference is important variable while characterizing bubble column reactor efficiency and this is a function of hydrodynamics prevailing inside the column. The efficiency of bubble column reactor is a function of physical properties of phases, geometry of column and operating conditions. Literature lacks on the simulation work on variation of concentration of solute for mass transfer rate and mass transfer efficiency in the complex system of bubble column device. In the present work a mechanistic model is formulated to predict the mass transfer efficiency of column and its dependency on various physical parameters, operating condition and column geometry. In this work the mass transfer efficiency has been analyzed based on a mechanistic model in the case of both co-current and counter current operations in bubble column reactor. The concentration variation of the phases obtained by simulation of model may be useful for further understanding the mass transfer phenomena in bubble column reactor.

Modeling of Regime Transition in Bubble Columns with Stability Condition

Understanding the physical essence of regime transition is of crucial importance for the modeling and simulation of hydrodynamics and heat and mass transfer in bubble columns. The regime transition of the air−water system has been captured by the dual-bubble-size (DBS) model in our previous work [Yang et al. Chem. Eng. Sci. 2007, 62, 6978−6991] as a jump change from one minimum point of the stability criterion to the other. The DBS model features the incorporation of a stability condition with hydrodynamic conservation equations through the analysis of compromise between dominant mechanisms. This work reiterates our previous work and further explores some remaining issues and underlying physics related to the jump change by analyzing the trajectory of global minimum points in the three-dimensional space of structural parameters. The influence of drag coefficient correlations on the model prediction is investigated. The effect of liquid viscosity on regime transition is evaluated using the DBS model for sa...

Hydrodynamics and mass transfer in bubble column: Influence of liquid phase surface tension

According to literature, few experiments are performed in organic solvents which are mostly used in commercial gas–liquid reactors. However, it is commonly accepted that data obtained in aqueous solution allow to predict the surface tension effects, and to model the behaviour of organic solvents. In this work, we examine the validity of this approximation. In this objective, the flows observed in two pure media having similar viscosity but different surface tension—respectively, water (reference) and cyclohexane (solvent)—are successively compared at two scales: in a bubble column and in bubble plumes. In bubble plumes, as expected, the mean bubble size is smaller in the medium having the smallest surface tension (cyclohexane), but for this medium the destabilisation of flow is observed to occur at smaller gas velocity, due to break-up and coalescence phenomena. In bubble column, these phenomena induce the bubbling transition regime at lower gas velocity, whatever the operating conditions for liquid phase: batch or continuous. Consequently, when the two media are used at similar gas superficial velocity, but in different hydrodynamic regimes, greater gas hold-up and smaller bubble diameter can be observed in water; the interfacial area is then not always higher in cyclohexane. This result differs from the behaviour observed in the literature for aqueous solutions. The analysis of bubble plumes in aqueous solutions of butanol shows that this difference is due to a fundamental difference in coalescent behaviour between pure solvents and aqueous mixtures: the surface tension effect is less important in pure liquid than in aqueous solutions, because of the specific behaviour of surfactants. It is then still difficult to predict a priori the bubbling regime or the flow characteristics for a given medium, and all the more to choose an appropriate liquid as a model for industrial solvents.

Numerical simulations of gas–liquid mass transfer in bubble columns with a CFD–PBM coupled model

Gas–liquid mass transfer in a bubble column in both the homogeneous and heterogeneous flow regimes was studied by numerical simulations with a CFD–PBM (computation fluid dynamics–population balance model) coupled model and a gas–liquid mass transfer model. In the CFD–PBM coupled model, the gas–liquid interfacial area a is calculated from the gas holdup and bubble size distribution. In this work, multiple mechanisms for bubble coalescence, including coalescence due to turbulent eddies, different bubble rise velocities and bubble wake entrainment, and for bubble breakup due to eddy collision and instability of large bubbles were considered. Previous studies show that these considerations are crucial for proper predictions of both the homogenous and the heterogeneous flow regimes. Many parameters may affect the mass transfer coefficient, including the bubble size distribution, bubble slip velocity, turbulent energy dissipation rate and bubble coalescence and breakup. These complex factors were quantitatively counted in the CFD–PBM coupled model. For the mass transfer coefficient k l , two typical models were compared, namely the eddy cell model in which k l depends on the turbulent energy dissipation rate, and the slip penetration model in which k l depends on the bubble size and bubble slip velocity. Reasonable predictions of k l a were obtained with both models in a wide range of superficial gas velocity, with only a slight modification of the model constants. The simulation results show that CFD–PBM coupled model is an efficient method for predicting the hydrodynamics, bubble size distribution, interfacial area and gas–liquid mass transfer rate in a bubble column.

Mass transfer to the wall of a packed and unpacked bubble column operating with Newtonian and non-Newtonian liquids

Mass transfer to the wall of a bubble column is examined, focusing on the influence of the liquid viscosity. The effect of introducing structured packing in the column is analysed by comparing results for the empty column with those of packed column at similar operating conditions. Visual observation allowed detecting of flow transition in the empty column with non-Newtonian fluids; hence, data obtained in a different flow regime are correlated separately. Results from the empty and packed columns could be correlated by the same expression, indicating that the presence of the structured packing inside the bubble column has no remarkable influence on the liquid to wall mass transfer. Expressions of the form St = f(Re, Fr, Sc) or Sh = f(Sc, Ga, E), mostly used in investigations on heat or mass transfer in bubble columns, are proposed.

Mass transfer in bubble column for industrial conditions—effects of organic medium, gas and liquid flow rates and column design

Most of available gas–liquid mass transfer data in bubble column have been obtained in aqueous media and in liquid batch conditions, contrary to industrial chemical reactor conditions. This work provides new data more relevant for industrial conditions, including comparison of water and organic media, effects of large liquid and gas velocities, perforated plates and sparger hole diameter. The usual dynamic O 2 methods for mass transfer investigation were not convenient in this work (cyclohexane, liquid circulation). Steady-state mass transfer of CO 2 in an absorption–desorption loop has been quantified by IR spectrometry. Using a simple RTD characterization, mass transfer efficiency and k L a have been calculated in a wide range of experimental conditions. Due to large column height and gas velocity, mass transfer efficiency is high, ranging between 40% and 90%. k L a values stand between 0.015 and 0.050 s - 1 and depend mainly on superficial gas velocity. No significant effects of column design and media have been shown. At last, using both global and local hydrodynamics data, mass transfer connection with hydrodynamics has been investigated through k L a / ε G and k L a / a .

Mathematical modeling of gas–liquid mass transfer rate in bubble columns operated in the heterogeneous regime

In this paper, a multi-scale approach is followed to study gas–liquid mass transfer in bubble columns. First, a single bubble of equivalent diameter d is considered. Its morphology and its gas to liquid relative velocity are related to the bubble diameter through the use of known correlations. Then, the gas–liquid mass transfer between the bubble and the surrounding liquid is studied theoretically. An equation describing the transport of the transferred species in the viscous boundary layer around the bubble is solved. In a second step, a bubble column of 6–10 m height is studied experimentally. The gas phase in the column is characterized experimentally by means of a gammametric technique. Finally, the two studies are linked, yielding a 1D mathematical model able to predict the gas–liquid mass transfer rate in a bubble column operated in the heterogeneous regime.

Prediction of Backmixing and Mass Transfer in Bubble Columns Using a Multifluid Model

Three-dimensional, instationary flow fields in bubble columns are numerically calculated using a Euler multifluid model. The local bubble size is calculated depending on the flow field using a transport equation for the mean bubble volume originating from a population balance equation. The resulting set of equations enables the simultaneous calculation of mass transfer and backmixing in the gas and liquid phases. Therefore, a perfect tracer is added to both the gaseous and liquid phases. From the calculations, the time-dependent concentration fields of the tracer are obtained. Assuming the one-dimensional dispersion model, the axial dispersion coefficients are calculated for comparison with experimental results. The obtained dispersion coefficients for the gas and liquid phases are in good agreement with the experimental investigations of several authors. On the basis of the reasonable prediction of the interfacial area and backmixing, the multifluid model is extended to consider mass transfer. The absorp...