Cylindrical 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.

Effect of solid loading and particle size on the phase holdup distribution and bubble behaviour in a pilot-scale slurry bubble column

Complexities in the hydrodynamics of slurry bubble columns lead to noticeable uncertainty in the design and scale-up of these multiphase reactors. Knowledge of local hydrodynamics can alleviate this problem. The effect of solids properties on the local hydrodynamic parameters is a critical subject where the lack of knowledge is undeniable. In the present study, optical fiber probes were used to study the effect of particle size and concentration on the solids and gas distribution inside the bubble column. A pilot-scale cylindrical bubble column 270 cm in height and 29.2 cm in diameter was employed. Glass beads with various mean diameters (35, 71, 156 µm) and 1, 3 and 5 vol% were added to tap water in order to make the slurry phase. Air with a wide range of superficial gas velocities (up to 20 cm/s) was injected into the slurry phase. The results demonstrated that the solid particles' radial and axial distributions were not uniform in the column. Solids concentration showed a local maximum in the near-wall region. Gas holdup had a parabolic shape except for large particles at high solids concentration in which the maximum of the gas holdup was observed at the sides rather than the centerline. The variations in the pressure fluctuations revealed that the particle size decrease led to an increase in the bubble size; however, the initial bubble size was independent of the particle size. Moreover, the increase in the solids concentration resulted in an increase in the gas holdup in the homogeneous flow regime; and a decrease in the heterogeneous one. Finally, a predictive model that can estimate the radial and axial solids distributions in the slurry bubble column is presented.

Experiments and CFD simulation of mass transfer and hydrodynamics in a cylindrical bubble column

Flow visualization, Particle Image Velocimetry (PIV) and dissolved oxygen charging experiments are carried out in a small-scale cylindrical air-water bubble column at different superficial gas velocities. Void fraction profile, total gas holdup, liquid phase velocity field and volumetric mass transfer coefficient are obtained. Details of each experiment are given, with a new procedure for obtaining a 2D void fraction profile from flow visualization by image processing and time averaging which could provide useful information for CFD model validation. The experimental results are analyzed and compared with simulations where good agreement is found for both local and global flow patterns. The averaged governing equations and constitutive relations used in the simulation are discussed briefly, with emphasis on the proper comparison between simulations and experiments based on the derivation of the two-fluid model. By conducting three sets of experiments related to phase distribution, velocity field and mass transfer in the same apparatus, a validated CFD model for multiphase mass transfer is established and can be used in the future design of a multi-phase reactor.

Multidimensional machine learning algorithms to learn liquid velocity inside a cylindrical bubble column reactor

For understanding the complex behavior of fluids in a multiphase chemical bubble column reactor, a combination of the computational fluid dynamic (CFD) method and the adaptive network-based fuzzy inference system (ANFIS) method is used to predict bubble flow inside a reactor based on the function of column height. In this study, the Euler–Euler model is employed as a CFD method. In the Eulerian method, continuity and momentum governing equations are mathematically computed for each phase, while the equations are connected together by source terms. After calculating the flow pattern and turbulence flow in the reactor, all data sets are used to prepare a fully artificial method for further prediction. This algorithm contains different learning dimensions such as learning in different directions of reactor or large amount of input parameters and data set representing “big data”. The ANFIS method was evaluated in three steps by using one, two, and three inputs in each one to predict the liquid velocity in the x-direction (Ux). The x, y, and z coordinates of the location of the node of the liquid were considered as the inputs. Different percentages of data and various iterations and membership functions were used for training in the ANFIS method. The ANFIS method showed the best prediction using three inputs. This combination also shows the ability of computer science and computational methods in learning physical and chemical phenomena.

Bubble motion and reaction in different viscous liquids

Reactive bubble columns are omnipresent in the chemical industry. The layout of these columns is still limited by correlations and therefore improved simulation techniques are required to describe the complex hydrodynamics/reaction interaction. In this work, we focus on the numerical and experimental study of the viscosity influence on bubble motion and reaction using an Euler-Lagrange framework with an added oscillation and reaction model to bring the column layout base closer to a predictive level. For comparison and validation, experimental data in various water-glycerol solutions was obtained in a cylindrical bubble column at low gas hold-up, where the main parameters such as bubble size, motion, and velocities were detected. Glycerol leads thereby to a change in viscosity and surface tension. Further, the surface tension was modified by addition of a surfactant. The bubble oscillating motion in low to higher viscosity could be described using an Euler-Lagrange framework and enables a description of industrial bubble flows. In addition, the simulations were in good agreement concerning reactive mass transfer investigations at higher viscosity of the liquid which led to an overall lower mass transfer compared to the cases with lower viscosity.

Large-eddy simulation of gas–liquid two-phase flow in a bubble column reactor using a modified sub-grid scale model with the consideration of bubble-eddy interaction

The Eulerian–Eulerian Large-eddy simulations (LES) of gas–liquid two-phase flow in a cylindrical bubble column reactor have been conducted. When considering the turbulent eddy viscosity in LES, apart from the well-accepted contributions from shear turbulence and bubble induced turbulence (BIT), the effect of the interaction between entrained bubbles and eddies with a similar turbulence length scale to the sub-grid scale (SGS) cannot be neglected. With the consideration of the bubble response to the eddies on the induced sub-grid stresses, a modified SGS model, which incorporates the Stokes number, St, was proposed. The results of LES clearly indicate that the use of the modified SGS model can effectively capture the transient bubbly flow in the cylindrical bubble column. The power turbulent kinetic energy spectrum obtained in LES indicates that a slope similar to Komogorov -5/3 scaling law and the -3 scaling law can still be identified for a critical frequency f=10.70 Hz.

Experimental investigation of hydrodynamic parameters and bubble characteristics in CO2 absorption column using pure ionic liquid and binary mixtures: Effect of porous sparger and operating conditions

For first time, bubble size distribution, bubble shape, bubble orientation, mean bubble size (d32), gas holdup (εg) and interfacial area in pure IL [BMIM][BF4] were compared with those in its binary mixtures with MEA and MeOH. Experiments were performed in small-scale cylindrical bubble column with a porous sparger using a high-speed image capture system. Further, to investigate effects of sparger pore size, column diameter, temperature and gas flow rate, experiments were performed employing gas-pure IL system. Compared with pure IL, d32 in binary mixtures was smaller, which decreased with increasing MEA or MeOH concentration, whereas, gas holdup and interfacial area were increased. MeOH exhibited profound effect as compared to MEA. In gas-pure IL, sparger pore size and temperature significantly influenced on aforementioned parameters. Correlations were proposed for predicting d32 and εg in pure IL for two different column sizes demonstrating their applicability in higher column dimensions and scaling-up.

Multiphase numerical modeling of a pilot-scale bubble column with a fixed poly-dispersity approach

A three-dimensional numerical study of air/water bubbly flow in a cylindrical large-scale bubble column is performed using Euler-Euler approach. The main objective is to investigate the influence of different boundary conditions such as bubble size distribution (BSD), polydisperse effects (mono and bi-dispersed approach), lift force modelling and flow rate distribution at sparger using a computationally affordable approach. In the bi-dispersed approach the population of bubbles are divided into two groups of small and large bubbles and a mean diameter is considered for each group. The division is based on the critical bubble diameter, for which the lift coefficient changes its sign from positive to negative. For air/water system Tomiyama lift coefficient model is widely used and the critical bubble diameter is equal to 5.8 mm. An alternative critical bubble diameter and lift force coefficient correlation is used in this study and compared with the well-known Tomiyama model. The numerical predictions are compared against the experimental data and the effect of different conditions is assessed on basis of comparison of axial gas fraction (local holdup) and global holdup. Better predictions are obtained by taking into account polydisperse flow with new critical bubble diameter and new lift coefficient model. Also, it was found that mass flow-rate distribution at the sparger does not affect numerical results for global and local holdup, however a different flow pattern is observed near the sparger region.

Transient large-scale two-phase flow structures in a 3D bubble column reactor

This paper analyses the local and time-dependent behavior of large-scale structures responsible for liquid circulation in gas-water flow of a 3-D cylindrical bubble column of high aspect ratio with a multiple orifice for uniform aeration. The large-scale flow structures play an important role in the mixing and the mass transfer while coherent structures dominate hydrodynamic characteristics of the turbulent flow field. A three-dimensional Euler-Euler large eddy simulation (LES) model was used to calculate large-scale structures and their interaction with bubbles at inlet superficial gas velocities of UG= 6 and 8.4 cm/s where vortical-spiral and turbulent flow regimes occur. The two-phase model gives good agreements with experimental measurements. We use a conditional sampling procedure of liquid velocity and gas hold-up time series to identify and educe the development of coherent flow structures which consists in a pair of counter-rotating vortices convected in a staggered pattern along the column in both the vortical-spiral and central plume regions. On average, the detected instantaneous events for each template account for about 12–15% of the data recorded and may appear simultaneously. These events produce important fluctuations in the axial liquid velocity and gas void fraction. The sampling procedure yielded the averaged topology of the three-dimensional large-scale structures which was visualized using isosurfaces of the vorticity for different gas flow rates. The structures have spiral tube-shaped topology rotating along the column near the walls with a pair of counter-rotating cells sustained through the flow. This work provides deep insights into turbulent flow field in gas-liquid bubble column by LES and pattern recognition.

A novel sieve plate hole pattern: effect on gas hold-up and flow regimes in a bubble column

The effect of an innovative sieve plate orifice layout on the hydrodynamics of a cylindrical bubble column is investigated. The classical triangular layout is compared with a novel spiraled layout, using the same number and diameter of orifices. The experiments include two different column aspect ratios and two liquid phases of different coalescing nature. According to the holdup data, if the spiraled layout is used, the flow regimes evolve from a pseudo-homogeneous regime to a heterogeneous regime going through a transition regime, as the superficial gas velocity increases. When the triangular layout is used, the homogeneous regime is not detected and pure heterogeneous regime is obtained at all superficial gas velocities. When using the spiraled layout, the gas retention increases, resulting 24% higher in the homogeneous regime and 13% higher in the heterogeneous regime with respect to the gas retention obtained with the triangular layout. If bubble coalescence is modified by the addition of NaCl, the holdup increases for both layouts, but to a lower extent in the spiraled layout (10%) compared to the triangular layout (24%). The reason behind the reduction of coalescence and higher gas holdup values is the ability of the spiraled layout to produce bubbles that do not interact with each other during a longer path along the column.

Effect of drag correlation and bubble‐induced turbulence closure on the gas hold‐up in a bubble column reactor

AbstractBACKGROUNDGas–liquid hydrodynamics play an important role in processes across a wide range of applications in chemical technology and biotechnology. In most of these applications, gas is bubbled in the bottom of the reactor to conduct physical mixing or mixing‐induced chemical reaction. In either case, the gas bubble hydrodynamics play a critical role on the overall performance of these processes. In the present study, the gas–liquid multiphase system is simulated using a Eulerian–Eulerian two‐fluid Computational Fluid Dynamics (CFD) model.RESULTSThe physical model is a cylindrical bubble column contactor wherein gas is sparged into liquid through a perforated sparger with a superficial gas velocity of ≈3–40 cm s−1. The CFD model for the contactor is built using the free and open source tool OpenFOAM v.4.0 to simulate the gas–liquid system. The time‐averaged volume fraction distribution was studied at different heights in the column. The effect of drag force and bubble‐induced turbulence are studied to arrive at appropriate closures for the CFD model. The results obtained from the numerical simulations were validated against available experimental results found in literature.CONCLUSIONThe recommended models, based on the present CFD study of the bubble column reactor, are the Ishii–Zuber drag closure with the modification that the parameter p is set to 2 for superficial gas velocities ≤20 cm s−1 and is set to 4 for 40 cm s−1 superficial gas velocity) and the mixed k–ϵ model to account for drag correlation and bubble‐induced turbulence closure, respectively. © 2019 Society of Chemical Industry

Novel drag coefficient models of ionic liquid – spherical particle system

Ionic liquids (ILs) have been proved to be excellently potential catalysts and solvents in a few chemical processes. In order to provide fundamental data for the reactor design and process optimization, it is essential to investigate the hydrodynamics and the mass transfer phenomena of liquid-solid two-phase systems containing ILs. In this work, we focus on the investigation of the hydrodynamic properties of a spherical particle in ILs, and a series of experiments are implemented in a cylindrical bubble column with three different diameters of spherical particles made of borosilicate glass (2 mm, 3 mm and 4 mm) in three kinds of ILs ([Bmim][BF4], [Bmim][PF6] and [Bmim][Tf2N]). Moreover, Two key physical quantities (terminal velocity of particle and contact angle) are measured with a high-speed camera system and an automatic contact angle meter. Based on dimensional analysis, two novel empirical models of the drag coefficient (CD) between a spherical particle and ILs are developed in the range of particle’s Reynolds number (Re) from 0.1 to 85. The first model is simple and the calculated CD agrees fairly well with the experimental data. Furthermore, in order to evaluate the effects of surface properties on CD, a newly defined dimensionless number (Mo′) is introduced into IL-particle flow studies for the first time, which comes into being a more rigorous model. The average predicted deviations of the two models are ±1.1% and ±1.5% respectively, which is much lower than that of contrastive models in the same situations (about 13.3% or more). These two models are significant for studying the hydrodynamics and mass transfer characteristics of IL-particle systems as well as the fluid dynamics simulation in related reactors.

Study of Axial Solid Concentration Distribution in Slurry Bubble Columns

The axial distribution of solid concentration was studied in semibatch slurry bubble columns. Tapered slurry bubble column (TSBC) and cylindrical slurry bubble column (CSBC) have been used in this study. Air, sodium chloride (NaCl) aqueous solution of concentration 0.15 M, and quartz sand, were represented gas, liquid and solid phases, respectively. The superficial velocity of gas was varied from 2.5 to 10 cm/s. Quartz sands with a mean diameter of 120 µm were used in the concentrations of 3,5,10 & 20 vol. %. Influences of gas velocity and solid loading on longitudinal profile of solid concentration were investigated. It was observed that the longitudinal distribution of solid concentration tends to be more uniform as superficial velocity of gas or solid loading increased. Under the same operation conditions, experimental results showed that TSBC offered more uniform distribution of solids along the column. The observed axial concentration distribution of solid particles in tapered slurry bubble column was analyzed on the basis of precipitation-dispersion model. The model successfully predicts the axial concentration distribution for narrow-sized range of solids systems in TSBC. Empirical correlation for the solid dispersion coefficient for solid particles in tapered slurry bubble column was proposed.

Characterization of bubble column photobioreactors for shear-sensitive microalgae culture

The shear-sensitive marine algal dinoflagellate Karlodinium veneficum was grown in a cylindrical bubble column photobioreactor with an internal diameter of 0.044 m. Initial liquid height varied from 0.5 to 1.75 m, superficial gas velocities from 0.0014 to 0.0057 ms−1, and nozzle diameter from 1 to 2.5 mm. Computational fluid dynamics was used to characterize the flow hydrodynamics and energy dissipation rates. Experimental gas holdup and volumetric mass transfer coefficient strongly depended on the liquid height and correlated well with the Froude number. Energy dissipation near the head space (EDtop) was one order of magnitude higher than the average energy dissipation in the whole reactor (EDwhole), and the value in the sparger zone (EDspar) was one order of magnitude higher than EDtop. Cultures of K. veneficum were limited by CO2 transfer at low EDwhole and severely stressed above a critical value of EDwhole.

Measurement of Gas Holdup and Mass Transfer in a Bubble Column by Using Electrical Resistance Tomography (ERT)

Gas dispersion performance and gas hold-up in a bubble column was investigated by using electrical resistance tomography (ERT). Experiments were carried out to visualize flow pattern and gas hold-up at different gas velocity, type of impeller and speed of impeller. In this study, cylindrical bubble-column with a diameter of 13.5 cm and a height of 97.5 cm, made of glass that was mounted with four planes of 16 stainless steel electrodes are connected to an ITS P2000 ERT system. The column was equipped with perforated plate consisting of five holes and each has internal diameter of 3.0 mm. The effect of four parameters; gas flowrate, speed of impeller, 2 types of impeller; Rushton and Lightnin impeller, and three types of gas; oxygen, hydrogen and nitrogen gas on the gas dispersion performance in the bubble column were investigated. It was observed that the gas dispersion rate is increased with increasing gas density where the oxygen gas has the highest of dispersion gas rate as compared to the others gas. The gas hold-up was determined from the correlation method to estimate the transition velocity and dispersion of gas in a bubble column. The results showed that with increasing of the superficial gas velocity, the total gas hold-up increases. Besides, the gas hold-up increased due to decreasing of gas density. It can be concluding that the optimum mixing condition was obtained by using Lightnin A320 impeller with the speed rate at 250 rpm.

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.

CFD calculations of flow pattern maps and LES of multiphase flows

Boiling crisis and flows occurring in a steam generator or a heat exchanger remain a major limiting phenomenon for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems. Firstly, the choice is made to investigate a hybrid modelling of the flow, considering the gas phase as two separated fields, each one being modelled with different closure laws. In so doing, the small and spherical bubbles are modelled through a dispersed approach within the two-fluid model, and the distorted or large bubbles are simulated with an interface locating method.This kind of approach is requiring a set of mass transfer terms between the continuous and the dispersed fields of the same physicochemical phase.The main outcome is the simulation of three field cases with a complete set of coupling terms between the two gas fields. Firstly, validation results are presented in the Castillejos test case where air is injected at the bottom of a cylindrical bubble column. Next, METERO experiments are simulated with this multifield approach. METERO is an experimental rig dedicated to the study of turbulent mixing of air and water in horizontal flows. The different regimes encountered in the METERO experiments, i.e. slug flows, plug flows and stratified dispersed bubbly flows are simulated and presented in the paper. Quantitative results are also presented for the first time in the paper. Finally, MAXI2 experiment (liquid/vapour flow in oblique tubes bundle geometry) is simulated: the three-field approach gives a reasonable agreement with experimental data.The important topic of turbulence modelling for two-phase flows is also addressed in the paper. The key point is that the ADM model could constitute an interesting improvement in further studies.

Simulation and experimental validation of reactive bubble column reactors

Bubble columns are widespread in chemical process engineering. The overall performance of these reactors relies on the interaction between the hydrodynamics, interfacial mass transfer and chemical reactions. The layout of these columns is commonly based on simplified integral models that are not able to track the complex interactions between the local hydrodynamics and the reactions. Hence, a detailed knowledge about the ongoing interactions is required. The local and temporal evolution of the carbon dioxide (CO2) chemisorption in aqueous NaOH solution in a cylindrical bubble column is investigated. Therefore, local measurements of hydrodynamics, bubble size and velocity, liquid velocity and concentration (pH-value) are done using a high speed PIV/LIF system combined with shadowgraphy. The results are compared to Euler-Euler based computational fluid dynamics simulations that take into account the ongoing reaction.

Effects of column diameter and liquid height on gas holdup in air-water bubble columns

Experiments on the total gas holdup, αG, in air–water cylindrical bubble columns were carried out to investigate effects of the column diameter, DH, and the initial liquid height, H0, on αG. Ranges of DH and H0 were 160⩽DH⩽2000mm and 400⩽H0⩽4000mm, respectively. The superficial gas velocity, JG, was varied from 0.025 to 0.35m/s. The characteristics of gas holdup showed that all the flows in the present experiments were pure heterogeneous. The following conclusions were obtained for αG in air–water bubble columns: (1) the effects of DH and H0 on αG are negligible when scaling up from small to large bubble columns, provided that αG in the small columns are obtained for DH⩾200mm and H0≳2200mm. The height-to-diameter ratio is useless in evaluation of the critical height, above which αG does not depend on H0, (2) for the above ranges of DH and H0, Akita-Yoshida’s and Koide’s correlations can give good evaluations of αG for a wide range of JG by tuning the model constants, (3) for DH<200mm, the decrease in DH increases the population of large bubbles, which results in the decrease in αG, and (4) for H0≲2200mm and DH⩾200mm, αG at a constant JG decreases with increasing H0 and approaches an asymptotic value, and the Froude number using JG and H0 as the characteristic scales well correlates αG in this regime.

Effect of ring sparger diameters on hydrodynamics in bubble column: A numerical investigation

This paper applies the Eulerian–Eulerian approach to study the effect of ring sparger diameter on the gas and liquid dynamics in a cylindrical bubble column reactor. The distribution of the gas hold-up and liquid flow pattern using different ring sparger diameters (i.e., 0.07, 0.14 and 0.20m) are investigated. In addition, the influence of different bubble sizes and interfacial force models on the accuracy of numerical results are examined. The results show that the size of ring sparger affects the amount of gas in the column and the position of the liquid phase recirculation (upward and downward liquid flow direction). Furthermore, the appropriate selection of bubble size, and interfacial force models results in accurate liquid velocity and gas hold-up in the bubble column reactor.