Hydrodynamic characterization of a needle sparger rectangular bubble column: Homogeneous flow, static bubble plume and oscillating bubble plume

In this work a detailed experimental hydrodynamic characterization of a needle sparger rectangular bubble column has been performed. The liquid velocity profiles and bubble plume oscillation frequency have been measured by means of laser Doppler anemometry (LDA), and the bubble velocity map by particle image velocimetry (PIV). In this way, the influence of the superficial gas velocity, liquid height and aeration pattern on the column flow structure was analysed. A highly uniform upward flow structure with down flow near the walls was obtained by means of a full-length aeration pattern. This flow structure was preserved even for high gas fractions values. The partial-length aeration patterns with the aerated zone (defined as the aerated width divided by the column width) larger than 0.7 provide a bubble plume and two pure liquid vortical structures in the column bottom, although they are static in nature. With aerated zones lower than 0.6, an oscillating bubble plume is obtained. A non-dimensional analysis of bubble plume oscillation frequency shows a dependence of bubble plume behaviour with the aerated zone. In this way, two different types of bubble plume oscillations, namely confined bubble plume oscillation and free bubble plume oscillation, are introduced and analysed.     

Gas–liquid flows in medium and large vertical pipes

Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems.     

LDA measurements of liquid velocities in a refractive index matched packed bubble column

LDA has been used to measure liquid velocities in a small-scale bubble column, internal diameter of 50 mm, packed with glass Raschig rings, 10 and 15 mm. A mixture of benzyl-alcohol and ethyl alcohol was index matched against the packing material. A method to separate the signals from liquid and bubbles was developed. It was found that the axial time-averaged liquid velocity was lower than that obtained in empty bubble columns, and that both the time-averaged liquid velocity and the RMS value of the liquid increased for the larger packing size.     

Development of a 3D Hybrid Intelligent‐Mechanistic Model for Simulation of Multiphase Chemical Reactors

A novel intelligent‐mechanistic model was developed to understand the behavior of multiphase chemical reactors. Computational fluid dynamics (CFD) and an intelligent algorithm were combined to predict different levels of 3D cylindrical bubble‐column reactors. An adaptive neuro‐fuzzy inference system (ANFIS) was used as the intelligence algorithm, and different ANFIS parameters were evaluated. With about one third of the training data the method can predict the overall behavior of the gas fraction in the reactor. A number of rules significantly influence the accuracy of the ANFIS method. After finding appropriate parameters, the method is applied for prediction of points which are not simulated with CFD, representing ANFIS mesh refinement. Also, bubble‐column reactors without training of exact values of measured data or numerical results can be predicted. Main advantages are time savings and reduction of computational expenses.     

Investigation of the unsteady two-phase flow with small bubbles in a model bubble column using phase-Doppler anemometry

The unsteady two-phase flow of water laden with small air bubbles in a model bubble column is investigated experimentally. Phase-Doppler anemometry (PDA) is used for measuring the velocities of water and bubbles. The measured sizes of reflecting tracers in the water and of the air bubbles are used to discriminate between water and bubble data. The investigations are focussed on the unsteady behaviour of the flow and on the interaction between the two phases. The measurement of relative (slip) velocities between bubbles and water reveals information about the dynamic behaviour of the two-phase system under the action of buoyancy on the disperse phase. The evaluation of time series of bubble velocities yields insight into typical frequencies at which the flow fluctuates. It is shown that, at all locations in the flow field, the velocity probability density functions of bubbles and liquid can be described by two superimposed Gaussian functions. The bubbles belonging to the two Gaussians exhibit different slip velocities. The probability for the occurrence of bubble collisions is quantified on the basis of the PDA data.     

Simulation of polydisperse multiphase systems using population balances and example application to bubbly flows

In this work the relationship between multiphase computational fluid dynamics models and population balance models is illustrated by deriving the main governing equations from the generalized population balance equation. The resulting set of equations, consisting of the well known two-fluid model coupled with a bivariate population balance model, is then implemented in the CFD code OpenFOAM. The implementation is used to simulate a particular multiphase problem: bubbly flow in a rectangular column. Results show that, although the different mesoscale models for drag force, coalescence, breakup and mass transfer, can be improved, the agreement with experiments is nevertheless good. Moreover, although the problem investigated is quite complex, as the evolution of bubbles is solved in real-space, time and phase-space (i.e. bubble size and composition) the resulting computational costs are reasonable. This is due to the fact that the bivariate population balance model is solved here with the so-called conditional quadrature method of moments, that very efficiently deals with these problems. The overall approach is demonstrated to be efficient and robust and is therefore suitable for the simulation of many polydisperse multiphase flows.     

X-ray computed tomography in large bubble columns

X-ray computed tomography (CT) is used to explore the differences in a semi-batch bubble column operated at superficial gas velocities of U_g=3, 10, and 18 cm/s. Air-water or air-water-cellulose fiber systems comprise the multiphase flow, and the bubble column has a 32.1 cm internal diameter. A CT image of a phantom object composed of several air-filled tubes immersed in water is used to identify several characteristic features of the X-ray CT system. CT images are then compared between air-water and air-water-cellulose fiber systems. When the fiber mass fraction is 0.1%, gas holdup is slightly higher than that of the air-water system in the column center and near the column wall. In 1.0% cellulose fiber slurries, gas holdup is lower than that of air-water results at all radial positions.     

PIV study of bubble rising behavior

Bubble behaviors are studied in a rectangular bubble column using particle image velocimetry (PIV). Bubbles rise in a chain through a stagnant liquid. As liquid viscosity reduces, bubble rising trajectory changes from one-dimensional to three-dimensional. This transition is due to different bubble wake structures. The bubble shapes also show dissimilar characteristics in liquid of different viscosities. The instantaneous liquid flow fields measured by PIV show the diversities as bubbles rise in different paths. Based on the experimental data obtained in this study, a correlation is proposed to calculate the terminal velocity of bubbles in a chain.     

Time-dependent gas holdup variation in an air-water bubble column

Time-dependent gas holdup variation in a two-phase bubble column is reported with air and tap water as the working fluids. The results indicate that time-dependent gas holdup is closely related to the water, whose quality is unsteady and changes, not only during the two-phase flow, but also during idle periods. The significance and characteristics of the time-dependent gas holdup variation are influenced by the bubble column operation mode (cocurrent or semi-batch), the sparger orientation, the superficial gas velocity, and the superficial liquid velocity. It is proposed that a volatile substance (VS), which exists in the water in very small concentrations and inhibits bubble coalescence, evaporates during column operation and results in a time-dependent gas holdup. The influence of bubble column operation mode, sparger orientation, superficial gas velocity, and superficial liquid velocity on the time-dependent gas holdup variation are explained based on their effects on bubble size, bubble contacting frequency and mixing intensity. This work reveals that regular tap water may cause significant reproducibility problems in experimental studies of air-water two-phase flows.     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

Modelling local bubble size distributions in agitated vessels

Photography and capillary suction probe were used to measure local bubble size distributions (BSDs) from Rushton turbine agitated (14/200 L) air-tap water and CO₂-n-butanol dispersions. A multiblock stirred tank model with population balances (PBs) for bubbles was created to describe local BSDs in agitated vessels. Unknown parameters in breakage and coalescence models were adjusted by comparing the predicted and measured local BSDs. The BSDs from both investigated systems and varying vessel-operating conditions were included simultaneously to the fitting. The adjusted models were incorporated to MUSIG PB model in CFX-5.7 and tested for the laboratory stirred tanks. The multiblock model showed to be an optimal trade-off between the accuracy and CPU time for the investigation of gas-liquid hydrodynamics and validation of closure models. As a result of fitting, the adjusted model seems to describe local BSDs more accurately in agitated vessels than the model of Lehr et al. [2002. Bubble-size distributions and flow fields in bubble columns. A.I.Ch.E. Journal 48, 2426-2443], which has been successful in bubble column studies. This shows that phenomenological breakage and coalescence closures need experimental validation for various flow environments.     

液-液混合澄清萃取器的研究动态与发展方向

介绍了传统萃取设备的主要特点和各种型式。综述了新型结构的塔式混合澄清萃取器的研究与应用状况 ,提出了塔式混合澄清萃取器的不足及发展方向     

Population balance modeling of bubble size distributions in a direct-contact evaporator using a sparger model

The study of bubble size distributions in direct-contact evaporators was addressed both theoretically and experimentally. Recently developed models for calculating bubble coalescence and breakage frequencies in isothermal bubble columns were adapted to the population balance equation using the bubble mass as the internal coordinate which was discretized using an expansion of the number density function by impulse functions. A sparger model was developed based on experimental data for a non-coalescing system and using bubble formation models for isothermal and non-isothermal conditions. Bubble size distributions in a direct-contact evaporator operating in the quasi-steady-state regime for four different gas superficial velocities, including the homogeneous and heterogeneous regimes, together with the sparger model, were used for estimating the three empirical parameters from the population balance model, which were observed to be functions of the gas superficial velocity. In all cases considered, the population balance model fitted the experimental data rather well and the regressed parameters exhibit the physically expected behavior with changes in the gas superficial velocity.     

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.     

Multiphase reacting flow studies of bubble column ozonation reactor for water remediation

A multiphase Volume‐of‐fluid (VOF) model was developed to gain further insights into the reactive flow parameters and electrical capacitance tomography (ECT) measurements on the remediation of hazardous organic pollutants. Low ozone bubble frequencies were obtained for high surface tension fluids, and the liquid viscosity affected the ozone bubbling frequency. The VOF model indicated that the increase of inlet gas velocity enriched the ozone bubble detachment and concomitantly generated larger ozone bubbles, decreasing the detoxification rates. VOF mappings and ECT visualizations of gas‐liquid unveiled preferential routes and highlighted the attenuation of the axisymmetric behavior of the ozonation bubble column under high‐interaction regimes.     

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 O₂ methods for mass transfer investigation were not convenient in this work (cyclohexane, liquid circulation). Steady-state mass transfer of CO₂ in an absorption-desorption loop has been quantified by IR spectrometry. Using a simple RTD characterization, mass transfer efficiency and k_La 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_La values stand between 0.015 and 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_La/ε_G and k_La/a.     

Effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column

An experimental investigation is reported on the effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column. Different combinations of 1 and 3 mm Rayon fibers are used to simulate different fiber length distributions. At a constant total fiber mass fraction, gas holdup generally decreases with increasing mass fraction of the 3 mm Rayon fiber while other conditions remain constant. Crowding factors estimated using four different methods (N_c=N_c,A, , N_c,L, and N_c,M) and the parameters and are tested on their performance to quantify the overall effects of fiber mass fraction and fiber length and its distribution on gas holdup. and provide the best characterization of the fiber effects on gas holdup in the cocurrent air-water-fiber bubble column. The crowding factor estimated using the model-based average fiber length (N_c,M) also provides a good characterization and is better than the other crowding factor definitions.     

Dynamic simulation of a 2D bubble column

The present paper demonstrates how 2D, dynamic simulations of a flat bubble column are feasible, applying state-of-the-art dynamic turbulence models, when an appropriate turbulent dispersion term is applied in the conservation equation for the gas volume fraction. The kω turbulence model yielded a better qualitative prediction of the bubble plume than the kε model, due to the low-Reynolds number treatment of the former model. The simple mixing length turbulence model gave the best prediction of the meandering plume, without any dispersion term. The mixing length model is, however, almost identical to a Large-Eddy simulation when run time-dependent on a fine mesh, and should be applied with care due to the use of a constant turbulence length scale and the inherent 3D nature of turbulence. By refining the mesh to the extreme end, it was shown that an apparently grid independent numerical solution was really grid-dependent, even when dynamic turbulence models were applied. The apparently grid independent solution was computed with an increment in the computational mesh that was of the same size as an equilibrium Kolmogorov length scale.     

Use of models for lift, wall and turbulent dispersion forces acting on bubbles for poly-disperse flows

Closure laws are needed for the qualification of CFD codes for two-phase flows. In case of bubbly and slug flow, forces acting on the bubbles usually model the momentum transfer between the phases. Several models for such forces can be found in Literature. They show, that these forces depend on the liquid flow field as well as on the size and the shape of the bubbles. A validation of consistent sets of bubble force models for poly-disperse flows is given, basing on a detailed experimental database for vertical pipe flows, which contains data on the radial distribution of bubbles of different size as well as local bubble size distributions. A one-dimensional (1D) solver provides velocity profiles and bubble distributions in radial direction. It considers a large number of bubble size classes and is used for the comparison with the experiments. The simplified model was checked against the results of full 3D simulations done by the commercial code CFX-5.7 for simplified monodisperse cases. The effects of the number of bubbles classes as well as the effect of the lateral extension of the bubbles were analyzed. For the validation of bubble force models measured bubble size distributions were taken as an input for the calculation. On basis of the assumption of an equilibrium of the lateral bubble forces, radial volume fraction profiles were calculated separately for each bubble class. In the result of the validation of different models for the bubble forces, a set of Tomiyama lift and wall force, deformation force and Favre averaged turbulent dispersion force was found to provide the best agreement with the experimental data. Some discrepancies remain at high liquid superficial velocities.