Mean Bubble Size Research Articles

Laser Doppler Anemometer Measurements in Bubble Column: Effect of Sparger

Measurements of flow pattern in a 150 mm i.d. bubble column were carried out using a laser doppler anemometer (LDA) in a forward scatter mode. A superficial gas velocity of 20 mm/s was used in all the experiments. Two different spargers (perforated plate and porous plate) were employed. The liquid flow starts developing from the sparger where non-uniformities in the gas sparging exist. From z/D ≥ 2 (z = axial location; D = column diameter), the fully developed axial liquid velocity profiles are seen. The radial variation of mean axial velocity shows gross liquid circulation in the column. Circulation velocities are smaller for the bubble column with porous plate sparger mainly due to the fact that the mean bubble size is small and the gas distribution is relatively uniform over the column cross-section. The difference in the hydrodynamic behavior with two different spargers is also apparent from the profiles of the turbulent kinetic energy and Reynolds stress measured from the velocity−time series.

Analysis of bubble behaviors in bubble columns using electrical resistance tomography

The distribution of gas holdup, the rise velocity of gas bubble swarm and the Sauter mean bubble size are estimated with a small diameter laboratory scale bubble column using electrical resistance tomography (ERT). The theory of gas disengagement based on ERT methods has been developed for estimations of bubble size and bubble rise velocity. The gas holdups of large bubble swarm and small bubble swarm, the distribution of both bubble size are derived through the analysis of gas disengagement based on the differences of the rise velocity of bubble swarm at the cross-section imaged by electrical resistance tomography. Experimental results are in very good agreement with correlations and conventional estimation obtained using pressure transmitter methods. The proposed methodology can be also used as an analysis tool for quantifying and optimizing the performance of other types of complex reaction systems.

Development of an on-line optical method for assessment of the bubble size and morphology in aerated food products

The quality of whipped products of the food industry is closely linked to the characteristics of the dispersed gas phase, such as the bubble morphology, the mean bubble size and the uniformity of the bubble size distribution. Here, an on-line method based on imaging techniques was developed for measuring these quantities in food foams manufactured in a continuous foaming device. On-line image acquisition was based on a quartz visualization cell that ensured a continuous renewal of the samples. The mean bubble size and the bubble size distribution were obtained using a semi-automated image analysis procedure. The method was validated on four kinds of aerated food emulsions (ice cream, whipped cream, aerated fresh cheese and a foamed sauce), corresponding to a large range of overrun (30–180%). The σ/ d 32 ratio of bubble size distributions of dairy foamed emulsions was shown to be nearly constant, regardless of the recipe and of operating conditions.

Impact of static pressure and volumetric energy input on the microstructure of food foam whipped in a rotor–stator device

The variety in foamed products found in the food, cosmetic and pharmaceutical domain has been steadily growing for years. Related research in the field of process engineering is directed at optimizing foam microstructure since mean bubble size and size distribution strongly influence the texture and stability of the product. Within this study, a model system containing milk protein as foaming agent and guar gum as thickener was foamed on a rotor–stator gas dispersing device. A wide range in gas volume fraction was achieved (0.2 ⩽ ϕ v ⩽ 0.9) and the resulting foam microstructure analyzed. The work focuses on the impact of static pressure, volumetric energy input and gas volume fraction on the resulting foam microstructure. The static pressure applied during the whipping process was shown to have a governing impact on foam microstructure. The dispersing characteristics revealed a minimum mean bubble size found for increasing volumetric input. The gas volume fraction had no impact on the mean bubble size in a lower range (0 < ϕ ⩽ 0.65), while a steady, linear increase in mean bubble diameter with increasing gas volume fraction was found in a higher range ( ϕ v > 0.65).

Age dependence of the drag force in an aqueous foam

The drag force on a sphere moving through an aqueous foam is measured as the foam ages. After an initial period, the steady-state drag decreases with age T as T−0.54±0.14. As the mean bubble size R in the foam coarsens as T0.5, this implies that the drag force scales as \(1/\overline{R} .\) The transient buildup of the force when the sphere starts to move is described by a single exponential approach to the steady-state drag while its relaxation when the motion stops is described by the sum of three exponential relaxations. This is as for fresh foam, but the coefficients and time constants vary systematically with age. For the most part, these quantities also show a power law scaling with T. The age dependence of the quantities determined in this study is discussed in terms of the mean bubble size.

The Effect of Sparger Geometry on Gas Bubble Flow Behaviors Using Electrical Resistance Tomography

Abstract By electrical resistance tomography (ERT) the cross sectional profiles of gas hold-up in a ø56mm bubble column are obtained with four designs of gas sparger. The effect of sparger geometry on the bubble distribution is revealed by applying a sensitivity conjugated gradients reconstruction method (SCG). Experimental results show that overall hold-up obtained by ERT is generally in good agreement with those measured with the pressure transducer and the ERT system produces informative evidence that the radial profiles of hold-up is very similar to the sparger design in the lower section of bubble column. Meanwhile, the rise velocity of bubble swarm and the Sauter mean bubble size are evaluated using ERT based on dynamic gas disengagement theory. The experimental results are in good agreement with correlations and conventional estimation obtained using pressure transmitter methods.

Local gas holdup measurement of a bubble column using SONIA-ultrasonic non-invasive method

A non-invasive technique for local gas holdup measurement of a bubble column using Self-Organized Network inspired by Immune Algorithm (SONIA) neural network and ultrasonic method is investigated. The energy attenuation and the transmission time difference of ultrasound are used as measurement parameters to obtain the local gas holdup in an air–water dispersion system using SONIA neural network reconstruction. Bubble size distributions in the bubble column are obtained by using a photographic method. The experimental results and simulations on three different mean bubble size condition show that the errors of SONIA neural network method is 1/9 times lower than those of the conventional back-propagation neural network. The results show a good agreement with measured data.

Characterization of Homogeneity of Bubble Flows in Bubble Columns Using RPT and Fibre Optics

Homogeneous flows of air-water in two bubble columns, 0.130 m and 0.292 m in diameter, were investigated using RPT (radioactive particle tracking) and fibre optics. The homogeneity of bubble flows at U g &lt; 0.04 m/s was revealed not only in the hydrodynamics of the gas/liquid phase but also in the transport phenomena (turbulence intensity ). RPT Lagrangian data derived that turbulence was about 0.15 m/s for the radial direction and 0.26~0.35 m/s for axial direction in the 0.292 m column, 0.05~0.16 m/s for the radial direction and 0.25~0.35 m/s for the axial direction in the 0.13 column. Average bubble diameter was 4~5.3 mm and was almost constant for the radial locations. The result was twice the sizes of the predicted bubble sizes in the CFD modeling. The slip velocity was 0.3 m/s at U g=0.015~0.033m/s as it was assumed in many reactor designs, and only a small variation was found at the radial locations. In flow patterns, stable circulation cells were revealed using velocity vector plots if a single nozzle sparger was used in the 0.13m column, which confirmed Joshi’s assumptions (Joshi and Sharma, 1979) and Lin’s assumption (Lin et al., 1996) based on 2-D columns. Using a perforated sparger was helpful to form a stable mean velocity field. Mean bubble diameters and bubble size distribution varied only slightly with radial/axial locations, which was consistent with the homogeneity of radial/axial turbulence. Bubble-size evolution from their formation to equilibrium mean diameter was discussed. Mean bubble size correlation was approached. The homogeneity of investigated flows was also substantiated by the criteria of stability for bubble flows.

Some gas dispersion characteristics of mechanical flotation machines

The gas dispersion properties that are the subjects of this communication include three measured parameters, gas superficial velocity (gas rate), gas holdup and bubble size, plus the derived parameter, bubble surface area flux. Over the past 10 years sensors to measure these parameters have been introduced and those originating from McGill University are briefly described. The sensors have seen extensive use in plant campaigns and experiences on four cell types at three sites are discussed. The impact of frother in controlling bubble size by retarding coalescence is illustrated, indicating the need to understand chemistry/machine interactions in interpreting gas dispersion results. An increase in Sauter mean bubble size ( D 32) with superficial gas velocity (gas rate, J g) was found, which was fitted empirically by D 32 = D 0 + C, J g n where D 0 ∼ 0.5–0.6 mm and C and n are fitted parameters dependent on the system (machine and chemistry). Following this, a consistent trend in bubble surface area flux ( S b) vs. J g was also observed. The relationship between Sauter mean and number mean ( D 10) is suggested to interpret the nature of the distribution. Analysing gas holdup vs. gas rate shows both a dependence on location in the cell as well as cell position in a bank, factors which must be considered when comparing data from different plants and circuits. The instrumentation and methodologies being developed are presenting new opportunities for incorporating cell hydrodynamics into metallurgical diagnostics. A case study at one plant serves as illustration.

Bubble size distribution in laboratory scale flotation cells

Bubble size was measured in two lab scale cells using the HUT bubble size analyzer. The influence of operating conditions on bubble size was investigated. The Sauter mean bubble size was found to vary over the range of 1.2–2.9 mm. The experimental bubble size distributions were satisfactorily represented by the upper-limit distributions. The frothers were found to have a profound impact on bubble size. It seems that frothers not only prevent bubble coalescence, but they also, affect the bubble break-up process. The air flow rate was also found to have a strong influence on bubble size. The Sauter mean bubble diameter increased as the air flow rate increased. The Sauter mean bubble diameter was found to decrease with increasing impeller speed. The addition of quartz on the other hand was found to increase the size of bubbles, this effect was more evident at solid concentration exceeding 20% (kg/kg). The size of bubbles generated with the multi-mix and free-flow mechanisms were different at similar operating conditions, in a non-coalescing environment. The free-flow design generated broader bubble size distributions than the multi-mix mechanism.

Bubble sizes in agitated air–alcohol systems with and without particles: Turbulent and transitional flow

In heterogeneous catalytic processes, the relationship between bubble size and various process parameters is important in determining interfacial area, hence mass transfer rate. This paper presents the results of an experimental study of bubble sizes in air–alcohol dispersions with and without the presence of catalyst particles in a batch stirred reactor at different agitation speeds and mean specific energy dissipation rates. Factors investigated include catalyst type, particle size and concentration. It has been found that: • Since there are large differences in the viscosity of the various alcohols used, by dividing them into two groups based on the flow type (turbulent or transitional), a consistent correlation has been found for mean bubble size as a function of Weber number, We, for each region, though neither correlation agrees with the precise relationship suggested by Kolmogoroff's theory of locally, homogeneous, isotropic turbulence. • At the same We number, the size is bigger in transitional flow than in turbulent. The functionality of the correlation of d 32 / D versus We is similar whether particles are present or not. • In fully developed turbulent flow, the bubble size is slightly reduced when catalyst particles are present, independent of catalyst size or type. • In transitional flow, the effect of catalyst particles on bubble size is much stronger, the size being reduced by approximately 30%.

Ultrasound-Assisted Generation of Foam

Foams represent an important class of structured liquids of relevance to a wide range of industries. Their quality is mostly determined by their complex cellular structure, which defines primarily their texture and to a large extent their rheology and stability. The foam structure is influenced not only by the formulation used but also crucially by the production process adopted, hence the need for production techniques that can assist in the generation of foams with controlled structures. The ability of high-intensity ultrasound waves to influence the mean bubble size and texture homogeneity of foam generated pneumatically has been investigated. An ultrasound probe located near the pneumatic foam generator and operating at a fixed frequency produced significant changes in the foam bubble size and foam homogeneity. Under certain conditions, foam exhibited a smaller bubble size and a narrower bubble-size distribution. Such enhanced homogeneity in texture is highly desirable to reduce the presence of aesthetically unattractive large cavities and to reduce the destabilizing effects of coarsening or Ostwald ripening, i.e., the growth of large gas bubbles at the expense of smaller ones due to gas diffusion driven by the higher Laplace pressure of the gas in the smaller cells. This enhanced stability was reflected in a slower rate of foam collapse and, hence, a longer foam lifetime. These effects are demonstrated for foams with a wide range of formulations.

Local bubble size distributions in agitated vessels

Local bubble size distributions (BSD) were measured from 12, 14 and 194 dm3 agitated vessels with capillary suction probe (CSP) technique. The four investigated systems were air–water, air–aqueous NaCl, air–aqueous starch and CO2–n-butanol. Several gassing rates and stirring speeds were investigated to find their effect on local BSDs. The reproducibility tests and the comparison against photography showed that Sauter mean bubble sizes and volumetric BSDs can be measured accurately from the air–water system with the capillary probe. In the CO2–n-butanol experiments the variation of BSDs with the agitation conditions was reasonable, although the majority of bubbles was likely smaller than the capillary diameter. In the air–water system, bubble size varied more in the Rushton turbine than in the flat-blade turbine (FBT) agitated vessel because of differences in flow fields. The addition of a small amount of NaCl caused a sharp decrease of bubble size in the vicinity of liquid surface. In the impeller discharge flow, bubble size decreased less. The lower coalescence rates due to higher apparent viscosity likely explain the smaller bubbles in the air–aqueous starch system than in the air–water system. In the CO2–n-butanol system, bubbles were notably smaller compared to other systems and the bubble size decreased unexpectedly in the flow from the impeller discharge to the liquid surface. The measured local BSDs are useful for the validation of population balance models that are, as connected to a computational fluid dynamic code, useful tools for the simulation of local mass transfer areas in agitated gas–liquid reactors.

Local Bubble Size Distributions in Agitated Vessel: Comparison of Three Experimental Techniques

Local bubble size distributions (BSD) were measured at multiple locations and agitation conditions in a Rushton turbine agitated 14 dm3 vessel. Experiments were made with air–deionized water and CO2–n-butanol systems. Three commonly used measurement techniques, capillary suction probe (CSP), phase Doppler anemometry (PDA) and digital imaging (DI), were tested, allowing critical evaluation of measurement techniques. All techniques revealed qualitatively reasonable variation of BSDs with the measurement point and agitation conditions. The quantitative comparison of techniques, instead, revealed a notable deviation. The main reason was related to the limited detectable bubble size range of each technique. The DI detected 0.1–8.0 mm bubbles, PDA 0.005–1.4 mm bubbles and CSP 0.4–8.0 mm bubbles. The sampling errors in the CSP, the identification errors of bubbles from the images in the DI and the optical distortions in the PDA were other reasons for the deviations. It appeared that the shape of number BSDs and arithmetic mean bubble sizes were captured better with the PDA than with DI or CSP. The DI and CSP techniques instead detected the volumetric BSDs and Sauter mean bubble sizes better than PDA. The comparison indicates that the limitations of measurement techniques are a major reason for the spread between bubble size studies for stirred tanks.

Aeration During Bread Dough Mixing: III. Effect of Scale-up

This paper presents the results of an investigation into the scale-up of bread dough mixers. Previous research reports into dough mixer scale-up only consider the dough development aspect of mixing and neglect aeration, despite over-aeration being a known problem that occurs with scale-up. A series of experiments was performed on three scales of the Tweedy-type high-intensity mixer. A pressure step-change was introduced during mixing, and dimensionless aeration parameters for each mixer were found through measurement of the dough voidage response, using a previously reported aeration model. The degree of steady state aeration, the turnover rate and the volumetric entrainment rate were all found to increase with mixer scale, or alternatively with Reynolds number. The number density of bubbles in the dough and the mean bubble size could not be seen to vary significantly with scale, but both increased with mixer headspace pressure.

Determination of bubble size distributions in bubble columns using LDA

AbstractA new method is developed for the determination of bubble size distributions from experimental laser Doppler anemometry (LDA) data through simultaneous measurement of only two orthogonal velocity components and the intermittent time gaps arising out of bubbles passing through the measurement point. An experimentally verifiable assumption of axisymmetry allowed the calculation of the three‐dimensional velocity distributions from the two‐dimensional measurement through the elegant use of a mathematical transform called Abel's transform. In conjunction with the use of probability theory, it allows calculation of the chord length distribution. The bubble size distribution is obtained by an inversion procedure, which facilitates the transformation of chord lengths to bubble sizes. The inversion procedure is successfully tested with Monte Carlo simulations, in which bubbles of prescribed size distribution were generated for obtaining synthetic data on subtended chord lengths at any given point in the column. It is unambiguously demonstrated that assumptions that do not account for correlation between bubble size and direction of motion can yield highly erroneous size distributions from chord length data. The procedure was then used on experimental chord length distributions obtained using LDA to estimate local bubble size distributions in our bubble column. It is particularly interesting to observe that the mean bubble size estimated from the calculated bubble size distributions were consistent with an estimation based on slip velocity information in accord with the Zuber–Findlay drift flux model. © 2004 American Institute of Chemical Engineers AIChE J, 50: 3068–3084, 2004

A study of non-symmetric air water flow using wire mesh sensor

This study investigates the characteristics of upward air–water flow for non-symmetric initial condition in the 50 mm vertical tube. Wire mesh sensor is utilized to obtain the local properties of the gas phase i.e. void fraction, gas velocity and bubble size. To generate the non-symmetric initial flow condition, air is supplied from a single injecting hole and mixed into the liquid flow inside the pipe through a narrow-ring shaped exit. In this case, large bubble can be generated at the inlet. In this experiment, the characteristics of bubble formation at the inlet and flow evolution along the pipe are emphasized. The results show the capability of the measurement technique to analyze the non-symmetric distribution of the gas phase. The effects of superficial gas and liquid velocity on the bubble distributions and on the local void fraction spreading at the inlet are clarified. The flow evolution dominated by bubble coalescence and break up is discussed in term of the development of Sauter mean diameter over the cross-section of the pipe. The results also show an inconsistency with uniform flow conditions at low gas flux. Hence, the effect of phase distribution spreading is proposed. In addition, the development of rising gas velocity in term of mean bubble size during coalescence and break up is revealed.

Pneumatic foam generation in the presence of a high-intensity ultrasound field

Designer foams find applications in a wide range of industries. Foam quality is mostly determined by its complex cellular structure which defines its texture, rheology and stability. In addition to formulation design, the formation process is crucial to the development of a foam with an optimum structure. There is, therefore, a need for techniques that can assist in the generation of controlled foam structures. The work described in this paper demonstrates the potential of using high-intensity ultrasound to control foam structure during production. Foam generated in the presence of ultrasound usually exhibits a narrower bubble size distribution, i.e. a more uniform texture. Such enhanced homogeneity in texture is desirable to reduce the presence of aesthetically unattractive large cavities, and to reduce the destabilising effects of foam coarsening. In addition, a smaller mean bubble size and a slower rate of foam collapse usually result when ultrasound is applied. The work shows the effects on foams stabilised with different surfactants.

Structural and Rheological Properties of Aerated High Sugar Systems Containing Egg Albumen

ABSTRACT: Structural and rheological properties have been investigated for an aerated system containing a high content of invert sugar and 2 to 10 wt% egg albumen. The specific gravity and the overrun were found to be influenced by the protein concentration and the whipping time. The systems exhibited rheopexy and shear‐thinning at high shear rate, with flow data fitted to a power‐law model. At small deformations, viscous character was found to dominate over elastic character over the frequency range 10−2 to 10 Hz. Images taken from confocal microscopy indicated that mean bubble size and concentration varied systematically with protein concentration and whipping time, and that more protein accumulated on the bubble surfaces as whipping proceeded. The study of this model system illustrates how the texture of a high sugar content aerated food depends on protein content and whipping conditions.

Kinetics of bubble nucleation in a rhyolitic melt: an experimental study of the effect of ascent rate

In order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma, we performed three series of experiments decompressed at rates of either 1000, 167, or 27.8 kPa/s. The experiments were carried out in an externally heated pressure vessel at 800°C and in the pressure range 260–59 MPa; the starting material was a crystal-free and bubble-free rhyolitic glass containing 7.0 wt% dissolved H 2O. In all the decompression experiments, homogeneous bubble nucleation began at 90±2 MPa, that is, ≈150 MPa below the water saturation pressure of the silicate liquid, 240 MPa. The degree of supersaturation Δ P HoN required to trigger homogeneous bubble nucleation was almost independent of decompression rate (Δ P HoN is the difference between the saturation pressure and the nucleation pressure): nucleation pressure decreased by ≤3 MPa for a 36-fold increase in decompression rate. These results are in good agreement with the classical theory of nucleation assuming a rhyolite–H 2O surface tension of 0.106 N m −1. Our major experimental finding is that, after a short nucleation event, the nucleation rate dropped and the bubble number density N reached a stationary value that was strongly sensitive to decompression rate: 6.8 mm −3 at 27.8 kPa/s, 470 mm −3 at 167 kPa/s, and 5800 mm −3 at 1000 kPa/s. The smaller value of N at low decompression rate was compensated by a larger mean bubble size, so that, at a given pressure, vesicularity was almost independent of decompression rate. The experimental values of N can be reproduced within a factor 0.3–1.4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas. The nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth, which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation. Once a critical value of N is attained, diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure. The strong correlation between bubble number density and decompression rate has fundamental volcanological implications. If we extrapolate the experimental data to the typical ascent rates of silicic magmas, we obtain bubble number densities (10 −3–10 1 mm −3 for homogeneous nucleation; ≈10 −1–10 3 mm −3 for heterogeneous nucleation) that are orders of magnitude smaller than those measured in most natural pumices. We therefore propose that the large values of N in silicic pumices may be due to two successive nucleation events: (1) a first event, which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles; and (2) a second nucleation event, yielding a very large number of small bubbles, and presumably related to the dramatic increase of decompression rate that precedes fragmentation. The small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates.