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.     

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.     

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.     

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.     

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.     

Shape oscillations and path transition of bubbles rising in a model bubble column

We investigate experimentally the occurrence of shape oscillations accompanied by path transition of periodically produced air bubbles rising in water. Within the period of bubble formation, the induced velocity is measured to examine bubble-liquid and bubble-bubble interactions. The flow is produced in a small-scale bubble column with square-shaped cross section. A capillary aerator produces bubbles of size 3.4 mm at a frequency of 5 Hz. Measuring techniques employed are high-speed imaging to capture bubble shape oscillations and path geometry, and laser-Doppler anemometry (LDA) to measure the velocity in the liquid near the rising bubbles. The experimentally obtained bubble shape data are expanded in Legendre polynomials. The results show the occurrence of oscillations by the periodicity of the expansion coefficients in space. Significant shape oscillations accompanied by path transition are observed as the second-mode oscillation frequency converges to the frequency of the initial shape oscillations. The mean velocity field in the water obtained by LDA agrees well with potential theory. An analysis of the decay of the induced flow shows that there is no interaction between the flow fields of two succeeding 3.4 mm bubbles in the rectilinear path when the bubble production frequency is lower than 7.4 Hz.     

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.     

Effects of sparger geometry on the mechanism of flow pattern transition in a bubble column

Electrical resistance tomography (ERT) is used to measure void fraction wave characteristics and to identify flow pattern in a bubble column reactor (0.24 m diameter, 2.75 m height). The effects of column pressure and superficial gas velocities for different sparger geometry and for different flow pattern have been investigated. The ERT sensor can distinguish the void fraction disturbances in different flow regimes with a good clarity. The holdup derived from ERT is in good agreement with the hold-up values measured by pressure transmitters. Different flow regimes have been identified based on void fraction properties and wall pressure fluctuations. The spectral analysis of ERT measurements yields quantitative information, such as a characteristic time and a characteristic frequency of void fraction waves, which are closely related to flow structure in the prevailing regime. The experimental observations are compared with the literature.     

On the development of flow pattern in a bubble column reactor: Experiments and CFD

A comprehensive analysis of the development of flow pattern in a bubble column reactor is presented here through extensive LDA measurements and CFD predictions. In the LDA measurements, the simultaneous measurements of 2D velocity-time data were carried out at several radial locations and many axial cross-sections of the column for two different spargers. The profiles of mean axial liquid velocity, fractional gas hold-up and bubble slip velocity showed excellent agreement between the predictions and the experimentally measured values. The experimental results showed that the mean tangential velocity varies systematically in the radial as well as along the axial co-ordinates. The turbulence parameters viz. turbulent kinetic energy, energy dissipation rate and eddy diffusivity were also analysed. The estimated values of local energy dissipation rate obtained using eddy isolation model were used for establishing the energy balance in the column. The experimental data were used for the estimation of normal and shear stress profiles. For the case of single point sparger, just above the sparger region, the bubble plume was seen to have a strong tangential component of motion thereby yielding higher gas hold-up slightly away from the centre. This visual observation was well captured in profiles of all the hydrodynamic parameters obtained from the experimental data. CFD simulations of the mean velocities, gas hold-up and turbulent kinetic energy compared well with the experimental results.     

Behavior of a single coherent gas bubble chain and surrounding liquid jet flow structure

The motion of a single nitrogen gas bubble chain and the structure of water flow field surrounding the chain were experimentally studied. We developed a bubble generator that can control both the bubble diameter and the generation frequency independently. Experimental conditions of bubble Reynolds number and bubble distance divided by bubble diameter were from 300 to 650 and from 6.5 to 300, respectively. We discuss the interaction effects on the motion of each bubble rising in a chain, as compared to the effects of a single rising bubble. The bubble trajectories and the surrounding water flow fields in the state of bubbles rising in a chain were investigated using a high-speed digital video camera and an analog single-lens-reflex camera. We observed two important physical phenomena. First, bubbles passed through a nearly identical path in the case of low frequency of bubble production. On the contrary, at a height of approximately 50 mm from the nozzle, the bubbles in the case of high frequency deviated and scattered from this path due to bubble-bubble interaction. Second, with higher bubble production frequency, coherent bubble chain and the characteristic structure of the surrounding water flow called “liquid jet” were observed near the nozzle. The direction of liquid jet flow differed from the bubble trajectory. We theoretically investigated the relation of coherent bubble chain and liquid jet by applying the conservation of liquid momentum.     

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.     

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.     

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.     

CFD modeling of slurry bubble column reactors for Fisher-Tropsch synthesis

Industrial bubble column reactors for Fischer-Tropsch (FT) synthesis include complex hydrodynamic, chemical and thermal interaction of three material phases: a population of gas bubbles of different sizes, a liquid phase and solid catalyst particles suspended in the liquid. In this paper, a CFD model of FT reactors has been developed, including variable gas bubble size, effects of the catalyst present in the liquid phase and chemical reactions, with the objective of predicting quantitative reactor performance information useful for design purposes. The model is based on a Eulerian multifluid formulation and includes two phases: liquid-catalyst slurry and syngas bubbles. The bubble size distribution is predicted using a Population Balance (PB) model. Experimentally observed strong influence of the catalyst particles concentration on the bubble size distribution is taken into account by including a catalyst particle induced modification of the turbulent dissipation rate in the liquid. A simple scaling modification to the dissipation rate is proposed to model this influence in the PB model. Additional mass conservation equations are introduced for chemical species associated with the gas and liquid phases. Heterogeneous and homogeneous reaction rates representing simplified FT synthesis are taken from the literature and incorporated in the model.Hydrodynamic effects have been validated against experimental results for laboratory scale bubble columns, including the influence of catalyst particles. Good agreement was observed on bubble size distribution and gas holdup for bubble columns operating in the bubble and churn turbulence regimes. Finally, the complete model including chemical species transport was applied to an industrial scale bubble column. Resulting hydrocarbon production rates were compared to predictions made by previously published one-dimensional semi-empirical models. As confirmed by the comparisons with available data, the modeling methodology proposed in this work represents the physics of FT reactors consistently, since the influence of chemical reactions, catalyst particles, bubble coalescence and breakup on the key bubble-fluid drag force and interfacial area effects are accounted for. However, heat transfer effects have not yet been considered. Inclusion of heat transfer should be the final step in the creation of a comprehensive FT CFD simulation methodology. A significant conclusion from the modeling results is that a highly localized FT reaction rate appears next to the gas injection region when the syngas flow rate is low. As the FT reaction is exothermal, it may lead to a highly concentrated heat release in the liquid. From the design perspective, the introduction of appropriate heat removal devices may be required.     

On the second-order moment turbulence model for simulating a bubble column

Two versions of the second-order moment two-phase turbulence model are proposed in this study for simulating bubble-liquid two-phase turbulent velocity fluctuations and their interactions in bubble-liquid flows under the dispersed bubble regime. One of them is a full transport equation model; the other is an algebraic stresses model. The proposed model is used to simulate liquid and gas mean velocities, gas volume fraction, liquid and gas Reynolds stresses and turbulent kinetic energy in a 2-D bubble column. Furthermore, the bubble and liquid velocities, Reynolds stresses and gas volume fraction are measured using the PIV. The simulation results are in good agreement with the PIV results and experimental data in the literature. The studies reveal the liquid recirculation and bubble up-rising flow patterns, and anisotropic liquid and bubble normal Reynolds stresses. Bubble fluctuation is observed to be stronger than liquid fluctuation. Moreover, both the liquid velocity gradient and bubble-liquid interaction are important for the generation of liquid turbulence.     

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.     

Study of bubble induced flow structure using PIV

An experimental investigation of the flow structure induced by a chain of gas bubbles was carried out in a rectangular bubble column using particle image velocimetry (PIV). It is observed that the bubble rising trajectory changes from one dimension to three dimension as liquid viscosity reduces. The variation of bubble rising trajectory associates with the alternation of bubble motions—with or without oscillatory and rotational motion depending the bubble rising trajectory is 3-D or 1-D. The different behaviors of gas bubbles introduce various instantaneous and averaged liquid flow structures. In general, complex fluid velocity fields present in liquid system of low viscosity where free vortex, cross flow, and irregular circular flow can be observed. The liquid pseudo-turbulence measured in terms of turbulence intensity and Reynolds stress is more intense in liquid of low viscosity. The turbulence is also enhanced by the frequency of bubble formation.     

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.     

Unified study of flow regimes and gas holdup in the presence of positive and negative surfactants in a non-uniformly aerated bubble column

In this paper, a study on the global gas holdup and hydrodynamic flow regimes developed in a partially aerated bubble column at variable air superficial velocities (U_G) in the presence of positive and negative surfactants is presented. According to the results obtained, despite the different liquid phase properties variation caused by the presence of positive (alcohols) and negative (electrolytes) surfactants, both reduce coalescence and the effect in the gas holdup (ε_G) is equivalent: it increases with the surfactant concentration (C) but only when the (C/C_t) ratio is clearly above 1, being C_t the transition concentration. Contrary to the results obtained for totally aerated bubble columns, for lower values of the (C/C_t) ratio, the holdup remains practically invariable. Considering the crucial role that C and C_t play in the resulting ε_G, a new prediction equation for ε_G accounting for the ratio (C/C_t) and U_G is presented and its performance for both types of surfactants validated. Additionally, visual and wall pressure fluctuations studies reveal that the vortical flow (VF), characterized by an oscillating bubble plume, prevails in ultrapure water (UPW) but results destabilized in the presence of surfactants. This destabilization results in an evolution to a pseudo-steady flow regime, the double cell turbulent flow regime (DCTF), characterized by a quasi-static bubble jet, located at the column centerline that determines the appearance of two static symmetrical vortices