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Local bubble characteristics were measured using a novel monofibre optical probe manufactured for elevated pressures and high gas holdups. The objective of this study is to test the performance of the optical probe, as well as to investigate the impact of operating conditions on bubble characteristics in a bubble column when operating under high gas holdup conditions. Experiments were conducted in a 101.6 mm diameter column operating at pressures up to 6.5 MPa. A 0.5 wt% aqueous ethanol solution was used in this study to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. The probe struggled to detect all the bubbles due to significant bubble size reduction and non-rectilinear bubble rise. However, successfully detected bubbles were deemed representative of the mean bubble rise velocity as indicated by the dynamic gas disengagement technique. Global gas holdup profiles in conjunction with the drift flux analysis showed that pressure does not have a significant impact at elevated liquid flow rates as bubbles tend to follow the convective current of the liquid. Relatively narrow chord length distributions were observed, where 90% of the bubbles were 1.0 mm or less. The energy dissipated through the gas–liquid distributor plate was shown to have a significant impact on the initial bubble size generated and high gas holdups were also achieved at atmospheric pressure by varying the open-surface area of the distributor.

This work investigates the effects of coalescence and breakup models on the radial distribution of gas holdup and local bubble size distribution in a laboratory scale bubble column operated at 0.11 m/s, 0.14 m/s, 0.19 m/s and 0.23 m/s. The results quantitatively show the influences of bubble coalescence due to various mechanisms, coalescence efficiency calculated by different models, modification coefficients for coalescence rates, critical energy required for breakup and bubble breakage caused by large scale eddies and viscous shear on numerical simulations. Furthermore, qualitative analysis is performed to analyze the effects of these factors. The performance of various models is evaluated (e.g. Han’s breakup model and Das’s coalescence model). Some optimized combinations of coalescence and breakup models that can be used to accurately predict the local gas holdup and bubble size distribution in laboratory scale bubble columns operated at heterogeneous regime are proposed.

Computational fluid dynamics coupled with a population balance model (CFD-PBM) combines the use of CFD to calculate the flow field and phase holdup and that of PBM to calculate the local bubble size distribution. However, the generality of the CFD-PBM coupled model for different operating conditions and liquid properties has not been systematically studied. The CFD-PBM coupled model where the PBM used our bubble breakup and coalescence models was shown to give accurate results in wide operating conditions (P = 0.1–3.5 MPa, Ug = 0.02–0.16 m/s), liquid properties (0.02–0.07 N/m, 1–185 mPa·s) with the same set of model parameters. Overall, the coupled model showed good generality when the superficial gas velocity, operating pressure, liquid viscosity and surface tension are the factors in bubble coalescence and breakup and the appropriate mechanism models are used to account for them, and forces at the gas-liquid interface are calculated using the local gas holdup and bubble size distribution.

Local bubble characteristics, including gas holdups, bubble rise velocities, and chord lengths, were measured using a monofibre optical probe manufactured to withstand elevated pressures. Previous studies have validated the use of single tip probes for simultaneous measurement of local bubble properties at atmospheric conditions; however no study has been currently reported for these probes at elevated pressures. Experiments were conducted in a 101.6 mm diameter column operating at pressures up to 9.0 MPa. Surfactant addition and operating pressure were studied to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. Experiments were hence completed using two liquid phases: tap water and a 0.5 wt% aqueous ethanol solution. Liquid and gas superficial velocities were varied between 0–90 mm/s and 0–150 mm/s, respectively. Radial profiles at atmospheric conditions validated the probe measurements in water. Local holdups, rise velocities and chord lengths were adequately measured in water up to 9.0 MPa. The probe struggled in the aqueous ethanol solution due to its physical constraints (i.e., tip diameter and sensing length) when compared to the significant bubble size reduction (chord lengths below 0.5 mm). Comparisons with fluidized bed freeboard measurements demonstrated that flow through the bed enhanced bubble breakup for a coalescing system, but had a negligible impact with the added surfactant.

Flow regime transitions were experimentally characterized using several local bubble properties measured by a double-needle resistivity probe. Experiments were carried out in a bubble column of 0.09 m diameter and 1.8 m height. The superficial gas velocity was changed from 0.005 to 0.070 m/s and the superficial liquid velocity from 0.003 to 0.060 m/s. Average bubble chord length, bubble frequency and sauter mean bubble chord length were obtained at various locations of the column by placing the probe at three axial and six radial positions. Transitions between dispersed bubble flow, discrete bubble flow, coalesced bubble flow and slug flow were detected at these locations. A comparison between transition points at different locations verified that regime transition occurs throughout the bubble column at nearly the same superficial gas and liquid velocities. A flow regime map was also obtained based on the local measurements of the bubble properties at different locations.

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.

Particle image velocimetry (PIV) was used to measure local bubble size distributions (BSD), gas–liquid interfacial areas and gas holdups in an up-flow ejector, based on the water–air system with different liquid and gas flow rates under the presence/absence of the swirl body. The results show that the bubble flow patterns are different whether to add the swirl body into the nozzle, especially at low gas flow rate because the bubbles formed “bubble chain” in the ejector with swirl. The mean bubble sizes D 32 of the two are both related to the pressure drop between import and export, gas ratio and liquid flow. The interfacial area and D 32 are both mainly dependent on the local gas holdups. The mean bubble sizes in the absence of swirl body are smaller than that in the presence of swirl under different operating conditions. The gas holdups and interfacial area are larger with swirl than those without swirl. With the increase of the gas fraction, the differences of D 32 , a t and e G become smaller.

In typical bubbly flow applications, bubbles can break or coalesce due to bubble–bubble and bubble–fluid interactions in presence of turbulence. Under this assumption, a fixed bubble size model might not be suitable for predicting correct multiphase flow behaviour in the gas–liquid system. For example, breakage and coalescence events produce very different bubble size distribution and then affects the interfacial interactions between the phases as heat and mass transfer and exchange forces, for example the drag and lift forces. In the present work, a rectangular bubble column and stirred tank reactor are modelled using an open-source computational fluid dynamics CFD package OpenFOAM. A population balance equation is introduced in the mathematical model to account the effects of bubble size distribution taking account the effect of coalescence and break-up phenomena on the hydrodynamic behaviour of multiphase flow. For solving the population balance equation an efficient numerical technique is integrated to enable the simulation of more complex flows that are encountered in industrial applications. Furthermore, the direct quadrature method of moments (DQMOM) and the method of classes (CM) are implemented and compared using an open source CFD package (OpenFOAM). An Eulerian–Eulerian approach with a standard k – e model of turbulence is used. The momentum exchange between the bubbles and the continuous phase is taken into account with drag, lift and virtual mass forces. The predicted results are compared with measured data available in the scientific literature; they show that the gas volume fraction, velocity profiles and local bubble size are in good agreements.

The complex pressure fluctuation phenomenon in gas–solid fluidized beds is systematically examined in this paper based on a comprehensive review of the literature data. The local pressure fluctuations are composed of multiple sources, including local bubble induced fluctuations, global bed oscillations and propagating pressure waves originating in other locations (e.g. bed surface, distributor and windbox). The interaction and coupling among bubble motion, under-damped oscillations of fluidized particles and bed surface, propagating compressible pressure waves and flow pulsation in gas–solid fluidized beds creates the complexity of local pressure fluctuations, and is likely responsible for the formation of complex but unique flow patterns. A few attempts have been reported in the literature on examining the interaction between bed oscillations, plenum chamber air pulsation and propagating pressure waves in fluidized beds, showing some promises on predicting the local pressure fluctuations. Future work should be focused on predicting local and global pressure fluctuations and the formation of unique surface flow patterns by coupling different contributing mechanisms.

The dimension of interaction dynamics, d 2 int , is used to study the nonlinear interactions in a fluidized bed between the local bubble, “bulk” bed and particle dynamic components. The different timescales are obtained by a multi-resolution analysis based on the Hilbert–Huang Transform Method (HHTM). The state space analysis is used to study the time–space structure of the fluidized bed dynamics. A method for constructing Poincare projections based on the join probability matrix computation is provided. The Kolmogorov entropy, K , and the correlation dimension, D 2 , are used to compute the invariant properties of the corresponding attractor. It is concluded that the local bubble dynamics has chaotic features and is responsible for the determinism found in pressure fluctuation signals. It has been found that, as expected, the bubble component is the driver of the gas–solid fluidized bed hydrodynamics.

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 CO 2 – 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.

The Hilbert–Huang transform method, HHTM, is applied to perform a multi-resolution analysis by means of the empirical mode decomposition, EMD, of noisy pressure signals collected from a gas–solid fluidized bed operating in slugging regime. Several important dynamic components, such as local bubble, “bulk” and particle dynamics, are identified. The bulk dynamic component is used to estimate the bed expansion ratio and bed height. The Kolmogorov entropy, K , and the correlation dimension, D 2 , are used for monitoring slugging dynamics from the resulting bubble, particle and bulk time scales. To study the attractor structure, a method for elaborating return-maps, based on the join probability matrix, is used. It has been found that the bulk dynamics provides information of the global behavior of the dense phase and allows characterizing the slugging dynamics with correlation dimension 2.2 D 2 2.6 . In contrast, the local bubble dynamics is characterized by a correlation dimension that ranges from 1.8 to 2.

In the present paper, gas dispersion in a double turbine baffled stirred tank is modeled using a commercial computational fluid dynamics (CFD) code FLUENT 6.1 (Fluent Inc., USA). A bubble number density equation is implemented in order to account for the combined effect of bubble break-up and coalescence in the tank. In the proposed work, the impellers are explicitly described in three dimensions using multiple reference frame model. Dispersed gas and bubbles dynamics in the turbulent water are modeled using an Eulerian–Eulerian approach with dispersed k – e turbulent model and modified standard drag coefficient for the momentum exchange. The model predicts spatial distribution of gas holdup, average local bubble size and flow structure. The results are compared with experimental and numerical finding reported in the literature and good agreement between the present model and measurements of Alves et al. [Gas liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics. Chemical Engineering Science, 2002, 57, 487–496] is achieved.

Bubble breakage and coalescence phenomena and multicomponent gas–liquid mass transfer were studied in a 194 dm 3 Rushton turbine agitated vessel. Local bubble size distributions (BSD) were measured from air–tap water system at several agitation conditions with capillary suction probe (CSP) technique. The CSP was compared to the digital imaging (DI) and phase Doppler anemometry (PDA) techniques in a 14 dm 3 stirred vessel. The volumetric BSDs between the CSP and DI were in agreement, but number BSDs showed notable deviation. The limitations of measurement techniques seem to be the main reason. A multiblock stirred tank model with discretized population balances for bubbles and two-film Maxwell–Stefan multicomponent mass transfer between gas and liquid was created for the 194 dm 3 agitated vessel. The model considers local mass transfer conditions in the vessel and is simple enough for the mathematical optimization of unknown model parameters. Unknown parameters in the mechanistic bubble breakage and coalescence models were fitted against measured local BSDs. After this, a parameter in the liquid film mass transfer correlation was adjusted against absorption and desorption experiments of oxygen. Local gas–liquid mass transfer areas were calculated from the population balance model. The simulations with the validated models show good agreement against experiments. On the other hand, the fitted parameters deviate from the theoretical values, which emphasizes the need of model validation against accurate experiments. Due to their fundamental character and the validation process, the fitted models seem to be useful tools for the design and scale-up of agitated gas–liquid reactors.

The dimension of bubble column reactors is often based on empirical correlations. Very popular is the axial dispersion model. However, the applicability of these models is limited to the experimental conditions for which the dispersion coefficients are measured, because backmixing depends strongly on the columns dimension and the flow regime. This paper presents a numerical method for the calculation of the three-dimensional flow fields in bubble columns based on a multi-fluid model. Therefore, the local bubble size distribution is considered by a transport equation for the mean bubble volume, which is obtained from the population balance equation. For comparison with experimental results, the axial dispersion coefficients in the liquid and gas phase are calculated from the instationary, three-dimensional concentration fields of a tracer. The model is then extended to include mass transfer between the gas and liquid phase. Increasing mass transfer rates significantly influence the flow pattern. For several applications, a dispersed solid phase is added. For the calculation of three-phase gas–liquid–solid flow, the solid phase is considered numerically by an additional Eulerian phase.

We investigated the effect of bubble size on the drift–flux parameters at low liquid flow conditions by measuring the radial profiles of void fraction and phase velocities in a vertical bubbly pipe flow of diameter and height . To study the effect of the bubble size we used two different types of bubble inlets. We measured the local bubble fraction and velocity Ug by using single and four-point-optical fibre probes, and we used Laser Doppler Anemometry to determine the liquid velocity Ul. The distribution parameter C0 and the weighted mean drift velocity |Udrift| were directly computed from the local measurements at a height on our experimental set-up. Both parameters were influenced by the bubble size. Provided no liquid flow reversal occurred at the near wall region, the distribution parameter reached a below unity minimum plateau value of C0=0.95 for wall peaking void fraction profiles. At low liquid input conditions both the liquid input and bubble size had an influence on the distribution parameter. Extreme values such as C0>2 were measured. From these measurements we developed models for the drift–flux parameters to take into account the effect of bubble size and input-flow conditions for our intermediate pipe diameter value. These models were tested and validated with separately collected experimental data.

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream. A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.

A previously derived concept of multiscale modeling of bubble column reactors has been applied to study the behavior of a nonisothermal parallel/consecutive reaction with and without evaporation of one of the reaction products. The concept is based upon the observation that most gas/liquid synthesis reactions are rather slow compared to the rapid local fluctuations of the gas/liquid hydrodynamics but are sufficiently fast to be considerably affected by the medium- and large-scale mixing behavior in industrial scale bubble columns. A simplified one-dimensional steady-state zone model for the bubble column reactor is parameterized in its flow and mixing parameters by the detailed, multidimensional, unsteady-state hydrodynamics and used to calculate the respective mass, energy and bubble size balance equations for the reactor. Local bubble sizes and total mass fluxes between the gas and liquid phase are fed back from the simplified reactor model into the detailed hydrodynamics until convergence between the two models is obtained. The results show that the important interactions between hydrodynamics and mass transfer with reaction can be well represented by the new concept, which makes it an efficient and comparatively simple candidate for the in detail analysis, design and scale-up of bubble column reactors.

There seems to be no method available to measure all parameters of interests in multiphase flow systems with high solids concentration and with reasonable accuracy. Parameters include, local particle velocity vectors, solids volume fractions and bubble rise velocity, size, frequency and volume fraction. In this study a novel method based on a fiber optical technique and tracer particles has been developed for simultaneous measurements of the mentioned local flow properties in highly concentrated multiphase flow systems such as gas–solid fluidized bed reactors. A particle present in the measuring volume in front of the probe is marked with a fluorescent dye. A light source illuminates the particles and the detecting fibres receive reflected light from uncoated particles and fluorescent light from the tracer particle. Using optical filters, the fluorescent light can be distinguished and together with a small fraction of background light from uncoated particles can be used for the determination of local flow properties. The method gives the possibility for simultaneous measurement of the local movement of a single tracer particle, local bubble properties and the local solids volume fractions in different positions in the bed.

Local bubble characteristics in different flow regimes of three-phase fluidized beds were investigated by means of an electro-conductivity probe. Bubble size, bubble rising velocity, bubble frequency and gas holdup were correlated with the operating parameters, axial and radial positions of the column in different flow regimes. Based on the measured results and the analyses of the forces acting on each phase, a one-dimensional three-phase hydrodynamic model was developed. With the radial distribution of liquid turbulent kinematic viscosity, the hydrodynamic model was solved for different flow regimes with parameters accordingly. The results show that the calculations correspond well to the measured results of local gas and liquid velocities.

Simulated pressure gradient records, based on the theoretical pressure field around a bubble in fluidized bed derived by Davidson, are analysed by statistical methods to determine significant bubble parameters. It is shown that from two records measured by two pairs of pressure differential probes located on common vertical axis the following can be computed: bubble velocity, bubble depth (diameter), vertical spacing of bubbles, bubbling frequency, distributions of bubble sizes and spacings, and local bubble phase fraction. Simulated and actual records of the passage of non-interacting single bubbles over four pairs of probes located in a fluidized bed with internals show good agreement.

The cultivation of Hansenula polymorpha was carried out on ethanol as well as glucose substrates in a bench scale tower bioreactor in batch and “extended culture” (“feed batch”) operations. Following values were measured: The concentrations of cell mass and substrate in the medium as well as the oxygen and CO2 concentrations in exhaust gas, the local bubble size distribution (Sauter bubble diameter), the local gas hold up and the longitudinal concentration profiles of dissolved oxygen in the medium. The cell productivity, the oxygen transfer rate, substrate and O2 utilization and CO2 production rates were calculated and the specific interfacial area, a, and volumetric mass transfer coefficients, kLa, were evaluated as function of the longitudinal distance from the aerator in non limited, substrate limited as well as oxygen transfer limited growth ranges. These systems were simulated by means of a hybride computer and good agreements between calculated and measured data were achieved. The ethanol concentration strongly influences a and kLa. Glucose causes a growth repression already at low substrate concentrations.

Measurements of local bubble properties in a gas—liquid fluidized bed have been made using an electroresistivity probe. The probe data were analyzed with a hybrid computer using the techniques applied by Park et. al. to gas fluidized beds. Results are reported for the variation of bubble frequency, volume fraction, size and size distribution, and rising velocity as a function of fluidization level, particle size and position within the bed. The bubble size and rising velocity size and rising velocity were correlated in terms of the overall bed porosity. Bed viscosity was also measured to assess its effect on bubble velocity. A comparison of the present data has been made with the available literature.