Cylindrical Bubble Column Research Articles

Study of the interfacial forces and turbulence models in a bubble column

Three-dimensional Eulerian simulations of gas–liquid flows in a cylindrical bubble column were performed in order to evaluate the flow pattern in the heterogeneous flow regime. Simulations were conducted numerically by the finite element method using a commercial CFX code (v.12.0) and an analysis of interfacial forces such as drag, lift and turbulent dispersion, as well as a comparison of k–ɛ and Reynolds Stress (RSM) turbulence models were performed. Different bubble sizes and two boundary conditions for the column exit were also verified. Gas velocities and holdup profiles were compared with experimental data. Results have shown that at the sparger region and without any simplification on the boundary condition the RSM model better represents the flow. At the fully developed region k–ɛ also showed good results.

An Assessment on the Performance of Reynolds Stress Model (RSM) in Modeling Churn Fully Turbulent Flow in Bubble Column Reactors

Churn-turbulent flow of gas and liquid phase are simulated in a cylindrical bubble column reactor in laboratory scale. The model is established in a full three dimensional, unsteady state mode in an Euler-Euler multiphase modeling approach. Numerical solution of model equations is carried out by finitevolume tensor-based formulation method using open-source CFD package (OpenFOAM).The attempt is made to assess the performance and applicability of two different turbulence models named RSM and RNG k–e for modeling turbulence in churn fully turbulent flow of bubble column. For this purpose, the predictions are compared with the experimental data from literature. Performance of the turbulence models is assessed on basis of comparison of axial liquid velocity, turbulent kinetic energy and their impact on bubble breakup predictions. It is found that simulated data using RSM model has better agreement with the experimental data in comparison with RNG k–e.

Evaluation of Breakage and Coalescence Kernels for Vertical Bubbly Flows Using a Combined Multifluid-population Balance Model Solved by Least Squares Method

Abstract In two-phase gas-liquid flows, the properties of the interface between the phases are of particular interest. Unfortunately the complexity of the interface dynamics due to breakage and coalescence represents a considerable difficulty in the study of bubbly flows. In recent years population balance modeling has been used increasingly to account for size distribution of bubbles in the dispersed phase. In this work we attempt to evaluate various breakage and coalescence kernels developed in literature. The implementation uses a multifluid-population balance model of a cylindrical bubble column. The simulated results for breakage frequency and breakage daughter size distribution closures are verified by evaluating their bubble number and mass conservative properties.

Hydrodynamic stability of a bubble column with a bottom-mounted point air source

Bubble column is widely used in both industrial and environmental applications. In this study, we examine the flow dynamics and stability of a bubble column driven by a point air source centrally mounted at the bottom using Phase Doppler anemometry (PDA). The model cylindrical bubble column had an inner diameter of 152 mm and was filled with the liquid to about 1 m height, above the point air source, which was made of a 30-mm diameter perforated air stone. The bubble diameters were within the range of 400–1300 μm. A customized setup was developed for accurate PDA measurements of the two phases, and detailed turbulent characteristics of the liquid phase velocity, bubble diameter, bubble velocity and the slip velocity were collected throughout the column. The comprehensiveness of the data set enabled a close examination of the hydrodynamic stability inside the column. Measurements were taken at three different air rates, namely 0.13, 0.25 and 0.38 L/min (corresponding to average gas volume fractions of 0.0065, 0.0138 and 0.0197, respectively). The results illustrated a large-scale coherent liquid circulation pattern inside the column. The circulation pattern in the upper column was relatively steady, while the pattern in the lower column was strongly unsteady with the probability density functions (pdf) for both the liquid and bubble velocities showing distinct twin peaks. An analysis based on the determination of the bubble drag forces and transversal lift forces is performed by decomposing the twin-peaked pdfs into two separated Gaussian distributions, one for the upward flow due to the bubble rises and the other for the downward flow due to circulation. Through the decomposition, a stability criterion can then be established by choosing the local bubble size as the representative length scale for the turbulent eddies inside the column. The analysis with the criterion illustrates why a steady circulation pattern was achieved in the upper column, and at the same time shows that the instability at the bottom column was induced by the low frequency meandering of the bubble swarm. ► Phase Doppler anemometry (PDA) was used to measure the flow field in a bubble column. ► The two-phase flow fields with three different gas fractions were investigated. ► The results revealed the detailed turbulent characteristics. ► A stability criterion was proposed based on the drag and the lift forces for bubble swarm. ► The criterion can predict the transition from an unsteady to a steady flow pattern.

Determination of interfacial area in a tapered bubble column

BACKGROUND: Investigations of hold up (εg) and interfacial area (a) in cylindrical bubble columns have been reported extensively but reported similar investigations in tapered bubble columns are sparse in the existing literature. Thus the current article reports the experimental determination of εg and a using a tapered bubble column. RESULTS: The present system generated εg (0.556 to 0.641) in a CO2NaOH system 20% higher than in an air–water system (0.466 to 0.534) and values were higher than in existing systems. Also, the values of εg in the air–water system were higher than reported for a column with shorter tapered angle. Values of εg fitted very well with the well-known Akita and Yoshida correlation. The observed values of a (235 and 700 m2 m−3) were higher than values obtained (2 to 600 m2 m−3) in existing systems. The energy dissipation was 203 to 335 W m−3, which was lower than that (100 to 1200 W/m3) in existing systems. A correlation developed to predict the pressure drop in terms of Euler number was statistically highly significant. CONCLUSION: The present research achieved higher values of hold up and interfacial area, and lower values of energy dissipation per unit volume of dispersion compared with existing systems. Findings of the present study coupled with previous studies indicate that the tapered bubble column developed could find potential application not only in air pollution control but also in gas-liquid mass transfer operations. Copyright © 2011 Society of Chemical Industry

Experimental and Numerical Analysis of a Gas-liquid Flow Operating in the Heterogeneous Regime

Experimental and CFD studies in a laboratory scale cylindrical bubble column under different operational conditions were performed. Axial liquid velocities were measured with the non-intrusive PIV technique. The effects of breakup and coalescence were applied with population balance method. For the interfacial forces, only drag was considering by the Zhang and Vanderheyden model. The effects of turbulence in the continuous phase were applied for the continuous phase with the k-e model. It was found that the approach used in this work provided physically consistent results, showing good agreement between the experimental and numerical data obtained for a heterogeneous flow regime.

Investigation of flow structures and transport phenomena in bubble columns using particle image velocimetry and miniature pressure sensors

The bubble column reactors are usually operated in a heterogeneous regime where the liquid phase turbulence is generated by the bubble motion and the velocity gradients in the mean motion. The turbulent flow comprises of fluid elements moving in a random fashion with different sizes and energies, called ‘flow structures’. Both the large and small scale flow structures within a reactor play an important role in governing the local momentum, heat and mass transfer. The current work is focused on the estimation of the time averaged flow pattern and flow structures. The experimental data has been collected using miniature pressure sensors, PIV+shadowgraphy and LDA. The data was subjected/analyzed to/with multipoint linear stochastic estimation (MLSE), wavelet transforms, image processing and eddy isolation (EIM) to identify the flow structures. Two bubble columns have been used: a narrow rectangular (2D) column and a cylindrical (3D) column. Wavelet transforms (WT) were applied to isolate individual structures from PIV data to get their shape, size and energy in the 2D column. MLSE has been used to obtain the velocity profiles from pressure fluctuation signals. This data, augmented by PIV and LDA data, is subjected to WT and EIM to get the eddy age and its energy distribution. The data of the eddy shape size and energy was used to predict the mass transfer coefficient in the cylindrical bubble column as a test case. Overall, in this work we present a methodology to utilize the experimental data to get a better insight of the dynamics of flow structures, and propose a path forward for the deeper understanding of transport phenomena in bubble columns.

CFD modelling of a bubble column with an external loop in the heterogeneous regime

AbstractCFD simulations of a cylindrical bubble column with an external loop were carried out. The effects of average bubble size, bubble size distribution, axial liquid velocity, and the two‐inlets configuration were evaluated and compared with experimental and simulation data. The drag force was modelled with the Ishii–Zuber model and the turbulence of the continuous phase with the k − ε model. For breakup the Luo and Svendsen model was chosen and for the coalescence effects the Prince and Blanch model was used. Results show that the external loop influences gas holdup near the entrance and at high gas superficial velocities a radial symmetry is obtained in the fully developed region.

CFD simulation of effects of the configuration of gas distributors on gas–liquid flow and mixing in a bubble column

Abstract Numerical simulations of gas–liquid flow in a cylindrical bubble column of 400 mm in diameter at the superficial gas velocity U g = 0.10 m s - 1 were conducted to investigate effects of the configuration of gas distributors on hydrodynamic behaviour, gas hold-up and mixing characteristics. Eight different gas distributors were adopted in the simulation. The simulation results clearly show that the configuration of gas distributor have an important impact on liquid velocity and local gas hold-up in the vicinity of the gas distributor. Comparisons of the overall gas holdup and mixing time among different gas distributors have demonstrated that none of the adopted gas distributors was able to produce the highest interfacial area and also yield the shortest mixing time. The CFD modelling results reveal that an increase in the number of gas sparging pipes used in gas distributors is beneficial in improving the gas hold-up but is disadvantageous in reducing bubble size due to a decrease in turbulent kinetic dissipation. It has been demonstrated from the simulations that the appearance of asymmetrical flow patterns in the bubble column and the adoption of smaller gas sparging pipes for gas distributors are effective in improving the mixing characteristics.

Model validation for low and high superficial gas velocity bubble column flows

In the present work, gas–liquid flow dynamics in a bubble column are simulated with CFDLib using an Eulerian–Eulerian ensemble-averaging method in a two-dimensional Cartesian system. The two-phase flow simulations are compared to experimental measurements of a rectangular bubble column performed by Mudde et al. [1997. Role of coherent structures on Reynolds stresses in a 2-D bubble column. A.I.Ch.E. Journal 43, 913–926] and a cylindrical bubble column performed by Rampure et al. [2003. Modeling of gas–liquid/gas–liquid–solid flows in bubble columns: experiments and CFD simulations. The Canadian Journal of Chemical Engineering 81, 692–706] for low and high superficial gas velocities, respectively. The objectives are to obtain grid-independent numerical solutions using CFDLib to reconcile unphysical results observed using FLUENT with increasing grid resolutions [Law, D., Battaglia, F., Heindel, T.J., 2006. Numerical simulations of gas–liquid flow dynamics in bubble columns. In: Proceedings of the ASME Fluids Engineering Division, IMECE2006-13544, Chicago, IL], and to validate computational fluid dynamics (CFD) simulations with experimental data to demonstrate the use of numerical simulations as a viable design tool for gas–liquid bubble column flows. Numerical predictions are presented for the local time-averaged liquid velocity and gas fraction at various axial heights as a function of horizontal or radial position. The effects of grid resolution, bubble pressure (BP) model, and drag coefficient models on the numerical predictions are examined. The BP model is hypothesized to account for bubble stability, thus providing physical solutions.

CFD simulation of bubble columns incorporating population balance modeling

A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51, 1311–1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k – ε model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas hold-up, liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485–1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201–9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas–liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071–5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359–6365] also show a good agreement for liquid velocity and gas hold-up profiles.

Lift force on bubbles in a bubble column reactor: Experimental analysis

The lift force acting on bubbles in a swarm has been estimated by analyzing the instantaneous velocity–time data obtained using LDA in a cylindrical bubble column. Phase distinction was achieved through the multiresolution analysis of the velocity–time data. Several important issues related to the transverse motion of bubbles subjected to a shear field have been discussed quantitatively. The actually measured bubble sizes, the respective slip velocity values in transverse and axial directions and the local shear rates ( γ ) enabled the verification of known formulations for the lift coefficient ( C L ) for bubbles. At many locations in the column the radial flux of the gas phase by turbulent dispersion and the radial slip were estimated. The radially inward movement of bubbles from low to high axial velocity (from column wall to center, i.e., C L < 0 ) was observed at most of the measurement locations. The local lift coefficient was estimated using the transverse drag force and the values support the results from the material balance approach. The estimated C L values showed a wide variation over the column cross-section.

Three-Dimensional Gas-Liquid CFD Simulations in Cylindrical Bubble Columns

Results from CFD simulations in a laboratory scale cylindrical bubble column with an internal diameter of 0.44 m under different operation conditions are presented. The effects of the continuous phase viscosity, bubble diameter and drag model were evaluated and the results were compared with experimental data found in the literature in three different gas inlet velocities. It was found that the approach used in this work provided physically consistent results, showing the transient effects in the column and good agreement with experimental data found in the literature for a homogeneous flow regime. Also, a case for a column with internals was simulated and a flow behavior qualitatively different from the column without internals was found.

Dynamic modeling and simulation of absorption of carbon dioxide into seawater

In order to elucidate the dynamic performance of the CO 2 ocean disposal process, effects of operating parameters, such as gas flow rate, salinity and temperature, on the absorption of CO 2 into seawater were examined. The rate-based model consisting of the rates of chemical reaction and gas–liquid mass transfer was developed for simulating dynamic process of CO 2 ocean disposal. In modeling, non-ideal mixing characteristics in the gas and liquid phases are described using a tanks-in-series model with backflow. Experiments were performed to verify dynamic CO 2 absorption prediction capability of the proposed model in a cylindrical bubble column. The operation was batch and continuous with respect to liquid phase and gas phase, respectively. Experimental results indicate that the CO 2 gas injection rate increased the absorption rate but the increase in salinity concentration caused inhibition of the absorption of CO 2 . The proposed model could describe the present experimental results for the dynamic changes and the steady-state values of dissolved CO 2 concentration and hydrogen ion concentration. The proposed model might effectively handle the prediction of the absorption of CO 2 into seawater in the CO 2 ocean disposal.

Continuous removal of carbon dioxide by absorption employing a bubble column

The present work includes studies related to the gas/liquid mass transfer process of carbon dioxide in polluted aqueous solutions with surface active substances. These studies have been carried out employing a thermostated cylindrical bubble column, where the gas phase was fed through different gas spargers based on glass porous plates. The absorption process has been characterized taking into account the influence of the gas and the liquid flow-rate, as well as the surfactant liquid phase composition and bubble size. Two opposite effects have been detected in relation to the influence of the surfactant presence upon the mass transfer rate.

Carbon dioxide absorption in glucosamine aqueous solutions

The present paper includes studies related to gas/liquid mass transfer process of carbon dioxide in aqueous solutions of glucose and glucosamine. These studies have been carried out employing a cylindrical bubble column, where the gas phase was fed through an only orifice. The absorption process has been characterized taken into account the influence of gas flow-rate and the liquid phase composition. The interfacial area was determined using a photographic method to calculate the value of the liquid side mass transfer coefficient. Liquid phase composition influences the value of the mass transfer coefficient due to the variation of solution viscosity. Eddy and slip models have been employed to correlate the experimental data of mass transfer coefficient.

CFD simulations of bubble column reactors: 1D, 2D and 3D approach

CFD simulations have been carried out for the predictions of flow pattern in bubble column reactors using 1D, 2D and 3D k – ε models. An attempt has been made to develop a complete correspondence between the operation of a real column and the simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. All the models showed good agreement with the experimental data for axial liquid velocity and the fractional gas hold-up profiles. However, as regards to eddy diffusivity, only the 3D model predictions agree closely with the experimental data. The CFD model has been extended for the estimation of an axial dispersion coefficient ( D L ) using 1D, 2D and 3D models. Excellent agreement was found only between the experimental values and the 3D predictions. The 1D and 2D simulations, however, yielded D L values, which were lower by 25–50%. For this, a mechanistic explanation has been provided.

Two‐Phase Flow with Mass Transfer in Bubble Columns

AbstractBubble columns are widely used in the chemical and biochemical industries. In these reactors a gaseous phase is dispersed into a continuous liquid phase thus the rising bubble swarm induces a circulating flow field. For the dimension of these reactors the local interfacial area and the residence time of the liquid and the gaseous phase are key parameters. In this paper an Euler‐Euler approach is used to calculate the flow field in bubble columns numerically. Therefore a transport equation for the mean bubble volume based on a population balance equation approach is coupled with the balance equations for mass and momentum. The calculations are performed for three‐dimensional, instationary flow fields in cylindrical bubble columns considering the homogeneous and the heterogeneous flow regime. For the interphase mass transfer the physical absorption of the gaseous phase into the liquid is assumed. The back mixing in the gaseous and liquid phase is calculated from the local and time dependent concentration of a tracer.

Modelling of Gas‐Liquid/Gas‐Liquid‐Solid Flows in Bubble Columns: Experiments and CFD Simulations

AbstractGas‐liquid/gas‐liquid‐solid systems are widely used in the chemical process industry for a variety of applications. In the present work, we have characterized the dynamics of gas‐liquid/gas‐liquid‐solid flows in cylindrical bubble columns using experiments and CFD simulations. The low frequency oscillations corresponding to the local re‐circulatory flow were characterized using wall pressure fluctuation measurements. Eulerian‐Eulerian two‐/three‐phase simulations were carried out with a focus on characterizing the dynamic properties of gas‐liquid/gas‐liquid‐solid flows. The effects of superficial gas velocity,H/Dratio and solid loading on the dynamic and time‐averaged flow behavior were studied experimentally and computationally. The simulated results were compared with the experimental measurements. The results presented are useful for understanding the dynamics of gas‐liquid/gas‐liquid‐solid flows in bubble columns and provide a basis for further development of CFD models for three‐phase systems.

Numerical description of flow regime transitions in bubble column reactors by a multiple gas phase model

The study of the hydrodynamics of bubble column reactors reveals that three main flow regimes can be distinguished. The present study is an original application of standard numerical multiphase models aiming at a better prediction of the transitions and a better description of these regimes. This approach consists of a two-step calculation with, in a first time, the determination of bubble size distributions using coalescence and break-up laws, and in a second time, the implementation of this distribution in a multiple gas phase Euler–Euler model. Calculations are compared with experimental data: liquid velocities and global gas hold-ups. They were obtained in a cylindrical bubble column in which local or uniform aerations were performed. The comparison shows that our approach allows a good prediction of both flow regime transitions and flow characteristics such as hydrodynamics or turbulence. Furthermore, two bubble-induced turbulence models consisting, respectively, of an added viscosity and of two turbulence source terms added in the k– ε transport equations have been compared.