Pneumatic Foam Research Articles

Radial bubble size distributions in a rising foam column

The diameter distribution of bubbles in foam is one of the most important features in foam-based separation processes like foam fractionation and froth flotation.In this study the bubble size at different radial positions of pneumatically produced foams without coalescence and coarsening of bubbles is investigated in a cylindrical column by employing an invasive sampling probe. It is shown that pronounced differences of the local Sauter-mean diameter of the bubbles can appear in radial direction. Oftentimes a parabolic profile with the largest mean bubble diameter in the center of the column is found. The difference of the Sauter-mean diameter between wall- and center region is in the order of up to 60%. Experiments on foams produced with different spargers, gas flow rates and liquid filling levels reveal that the actual degree of the inhomogeniety depends on the specific bubble size distribution that is produced by the sparger, and becomes more pronounced if the range of bubble diameters in the foam increases. As an explanation for the observations, hydrodynamic interactions in the liquid phase, as well as the behavior of different sized bubbles close to the liquid/foam interface are proposed.The observed existence of local differences of the bubble diameters can have a strong influence the dynamic behavior, like liquid drainage, and measurement methods of pneumatic foams. In particular it can limit the applicability of surface-based bubble size measurements.

Foaming and gas holdup of esterified nanoparticle dispersions in the presence of sodium chloride

The topic of particle-stabilised foams is relevant in many industrial applications such as the food industry, cosmetic, and mineral processing. This study is concerned with the influence of solid nanoparticles on the froth stability and gas holdup of aqueous solutions of sodium chloride. Silica nanoparticles with a nominal size of 300 nm were treated by esterification with 1-octanol to achieve intermediate hydrophobicity. Pneumatic foams were generated by passing nitrogen gas through a porous disc into the solutions. The results showed that increases in the concentration of nanoparticles in the colloidal dispersions caused slight increases in the gas holdup and had no significant effect on the foaminess of aqueous dispersions in the concentration range 0–6 wt%. Sodium chloride in sufficient amount to coagulate the particles significantly improved both gas holdup and foaminess. Electrolytes reduce the energy barrier that has to be overcome for the adsorption of particles to an interface and increase the hydrophobicity of the particles. As well, coagulated particulates have greater capture efficiency and hinder drainage in the froth, which could explain the improved foaminess in the present study. Additional experiments were performed to determine the effect of the composition of the continuous phase on the foaming properties. This was essential due to the use of a co-solvent to disperse the nanoparticles, which was shown to impact the foaminess of the dispersion if not removed adequately.

Gas absorption and reaction in a wet pneumatic foam

Results for the rate of absorption of carbon dioxide gas into a pneumatic (i.e. continuously rising) foam formed from a sodium carbonate–bicarbonate buffer solution stabilised by a polyglycol frother are presented. Previous studies upon the use of gas–liquid foam for gas absorption operations have demonstrated limited potential for the technique, mainly because the liquid films rapidly become saturated with the absorbing gas due to the relatively low liquid fraction within the foam. However, by adding washwater to the free surface of the foam we created a wetter and more stable foam that (1) exhibits a high liquid fraction and therefore avoids film saturation, and (2) creates greater liquid advection past the gas-liquid surfaces, thereby enhancing the mass transfer coefficient. The interfacial area achieved within the foam was in the range 2100–3200m2m−3 (i.e. much higher than in conventional gas-liquid contactors and the liquid-side mass transfer coefficient was the same magnitude as that achieved in packed beds (i.e. 4–8×10–5ms−1). Indeed, by modelling the foam as a packed-bed of solid spheres (with no adjustable constants), the mass-transfer coefficient in a bed of closely packed solid spheres was found to be in satisfactory agreement with the mass-transfer measured in the wet gas-liquid foam. The hydrodynamic state and the slip velocity between gas and liquid phases is required for the design of wet foam absorption columns, and this can be estimated using a theory of pneumatic foam (Stevenson, 2007).

Flooding in a Vertically Rising Gas–Liquid Foam

The maximum liquid fraction that stable pneumatic foam can support is studied in this paper in order to illuminate change of flow regime from turbulent foam flow to the “emulsion regime” in which the distinct interface between the foam layer and the bubbly liquid is lost; this regime transition is nominated as “flooding”. With further increases in gas flow rate, large (or “gross”) bubbles can appear and rise in the column, the existence of which depends on the absolute pressure at the column bottom. Experiments are carried out using foam stabilized by sodium dodecyl sulfate, and the liquid fraction of both the foam layer and the bubbly liquid are measured using a pressure gradient method. The maximum liquid fraction is found to be at a critical point when the interface between the foam and the bubbly liquid layer disappears. The predicted maximum liquid fraction in the foam is given by ε* = (n – 1)/(n + 1), where n is an adjustable constant that depends upon the characteristics of the gas–liquid interface, and the predictions have been experimentally verified.

Scaling laws in steady-state aqueous foams including Ostwald ripening

This paper presents scaling laws governing the steady-state behavior of pneumatic foams produced by injecting gas in aqueous surfactant solutions under isothermal conditions. Dimensional analysis of the governing equation for the time rate of change of bubble radius in foams due to Ostwald ripening yielded a dimensionless similarity parameter representing the ratio of the average contact time between bubbles to the characteristic time for gas permeation. This dimensionless number was combined with two other dimensionless numbers previously derived for high viscosity fluids and ignoring foam coarsening. Semi-empirical parameters of a power-law relation between these three dimensionless numbers were determined from experimental data collected in the present study as well as from the literature for various gases and aqueous surfactant solutions. They cover a wide range of physical parameters including foam thickness, superficial gas velocity, solubility, surface tension, and average bubble radius.

On the growth of pneumatic foams

This work reports on the behaviour of tenacious and transient pneumatic foams produced at large range of values of gas delivery rates (or superficial velocities) and surface tensions. Experimental data from the literature and produced in the course of this study were processed and analyzed. The tenacious foams were stabilized via Polyoxiethylene-2 sulfate (SDP(2)S) in presence of 0.024M NaCl and 0.003M AlCl(3) ( CMC = 1.83×10(-2) M) in the concentration range of 3.33×10(-3) M to 3.8×10(-2) M (0.18CMC -2.08CMC corresponding to values of the dynamic surface tension in the range of 42.7mN/m to 37.5mN/m. The range of gas delivery rates was from 20.5ml/min to 482.8ml/min. It was found out that the rate of foam generation coincides with the gas delivery rate until a certain critical value of the latter, beyond which the rate of foam growth exceeds the rate of gas delivery. The level of this exceeding depends on the dynamic surface tension. The lower the value of the dynamic surface tension the larger the level of this exceeding. This rule was found valid until a certain upper limit of the gas delivery rate, at which the dependence on the dynamic surface tension ceases to exist. The second set of experiments was conducted on transient foams. The latter were stabilized by three members of homologue surfactants series: sodium octylsulfate (SOS), sodium decylsulfate (SDeS), and sodium dodecylsulfate (SDS) in the concentration range of 0.01CMC -0.1CMC corresponding to 72.75mN/m to 68.18mN/m. The aqueous solutions of the three surfactant homologues had identical values of static surface tension at the same ratio C/CMC . Bikerman's "unit of foaminess" was measured for each particular case. It was shown that at identical equilibrium surface tensions both the foaminess and the rate of foam decay increase upon lengthening of the surfactant's hydrocarbon chain. It was indicated as well that the foaminess increases linearly upon raising the gas delivery rate until a certain critical value, above which substantial increase is observed. It was finally concluded that both the tenacious and the transient foams have completely different behaviour. For this reason they should be modeled separately but more experimental data are still needed.

Foam fractionation of crystal growth for nanotechnology

The synthesis and fractioning of nanoparticles by pneumatic foam fractionation, using NiO as a demonstration is presented. The foam was allowed to travel to different vertical column heights and was then fractionated. The nickel ions in the cetyltrimethylammonium bromide (CTAB) foam reacts with ammonia vapor to precipitate Ni(OH)2, and the capillary formed within the bubbles provided a unique microenvironment for the formation of nanoparticles. During the travel of the foam, the concentration of CTAB does not remain constant at different column heights, i.e., liquid near an interface with a gaseous phase has a different composition to that of the bulk liquid. Thus the height of the foam column affects the degree of enrichment of the foamate. Various morphologies of Ni(OH)2 were obtained, such as petal/coralloid and thin flake-like structures, owing to the concentration gradient across the column height of the fractions. On thermal decomposition, Ni(OH)2 produced NiO of various morphologies, such as a lotus root-like structure and flat angular plate-like structures. These NiO nanoparticles of different fractions have different BET surface areas and magnetic properties. Our experiments demonstrate that in a single batch one can control and separate the crystal structures with different physical properties using fractionation with various column heights.

Start‐up transients in a pneumatic foam

AbstractIn the current work, transient features of initiation in a gas‐liquid pneumatic foam are investigated by measuring the evolution of volumetric liquid fraction as a function of height within the column. The addition of wash water to a flotation froth is only effective when the foam liquid fraction has reached a steady state. This makes start‐up transients in pneumatic foam worthy of study. For the conditions adopted in the experiments, an approximately steady state was achieved after typically 500s, but there was significant fluctuation in liquid fraction after this time. In general, three possible regimes in the start‐up transient (induction, growth and evolution) have been identified and a tentative mathematical model has been described for the last two. However, because it has been demonstrated that the method of obtaining bubble size distributions by analysing images taken through the column wall is deficient, no comparison of these models with the data has been attempted. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd.

Process intensification of foam fractionation by successive contraction and expansion

Enrichment in foam fractionation is enhanced by reducing the liquid flux while maintaining the flux of bubble surfaces. We show that it is possible to do so in a continuous foam fractionation column by inserting a plate into the foam layer and forcing the foam through a narrower tube (foam riser) mounted at the centre of the plate. A 35% reduction in liquid flux was observed by using a single plate-foam riser assembly in a foam stabilised by 0.52 g/L sodium dodecyl sulphate solution, and the liquid rejected from the foam due to the presence of the foam riser had the same concentration as the feed solution. However, due to the nature of the adsorption isotherm of SDS, the enrichment enhancement measured in the experiments hererin was modest. The mechanism of the foam riser plate was explained by invoking recent theoretical studies on pneumatic foam. It was demonstrated that the reduction in liquid flux was affected by sudden expansion of the flow area causing a rejection of interstitial liquid to the top of the plate and extracted, whence it was recycled back to the feed. Preliminary experiments on the effect of liquid pool depth and foam height on interfacial adsorption were also carried out to demonstrate that the insertion of the plate does not affect the adsorption of the target substance to the bubble surface in the current system. It was found that equilibrium could be achieved within 0.2 m of liquid pool depth or foam height, however the equilibrium surface excess on a rising bubble in the liquid pool is roughly 51% of the equilibrium value on a quiescent interface, while that in the foam layer is roughly 73% of the equilibrium value on a quiescent interface.

Foam flowing vertically upwards in pipes through expansions and contractions

The drift-flux analysis of one-dimensional two-phase flow of Wallis (Wallis, G.B., 1969. One-dimensional two-phase flow. McGraw-Hill Book Company, New York.) is utilised for the first time to model the behaviour of pneumatic foam flowing vertically through an expansion or an contraction. It is demonstrated that, although a sudden contraction of flow area decreases the liquid fraction, it does not affect the volumetric liquid over-flow rate. It is also demonstrated that a sudden expansion of flow area decreases both the liquid fraction and the volumetric liquid over-flow rate. The liquid fraction of a foam stabilised by 2.92 g/L sodium dodecyl sulphate (SDS) solution flowing through a sudden contraction or expansion was measured by an improved pressure gradient method. The results were found to be consistent with the theoretical analysis. This study has implications for foam fractionation device design, optimisation and process intensification.

Measurement of bubble size distribution in a gas–liquid foam using pulsed-field gradient nuclear magnetic resonance

Pulsed-field gradient nuclear magnetic resonance, previously used for measuring droplet size distributions in emulsions, has been used to measure bubble size distributions in a non-overflowing pneumatic gas-liquid foam that has been created by sparging propane into an aqueous solution of 1.5g/l (5.20mM) SDS. The bubble size distributions measured were reproducible and approximated a Weibull distribution. However, the bubble size distributions did not materially change with position at which they were measured within the froth. An analysis of foam coarsening due to Ostwald ripening in a non-overflowing foam indicates that, for the experimental conditions employed, one would not expect this to be a significant effect. It is therefore apparent that the eventual collapse of the foam is due to bubble bursting (or surface coalescence) rather than Ostwald ripening. This surface coalescence occurs because of evaporation from the free surface of the foam. An analytical solution for the liquid fraction profile for a certain class of non-overflowing pneumatic foam is given, and a mean bubble size that is appropriate for drainage calculations is suggested.

Dependency of liquid overflow rate upon humidity of a pneumatic foam

In this work, the liquid flowrate in an overflowing pneumatic foam is studied as a function of environmental humidity. It is found that in general, liquid flowrate is greater when the relative humidity of the air above the foam is high. This observation is attributed to bubble coalescence on the surface of the foam. Because the evaporation rate is diminished when the relative humidity is high, less bubbles collapse on the surface of the foam. Because no experiments have been conducted on mineralised flotation froths, a dependency on the operation of froth flotation devices on environmental humidity cannot be directly asserted; although the effect is so important in demineralised foams that it should not be overlooked in the context of flotation. The authors know of no laboratory or plant studies of froth flotation that have controlled or measured the environmental humidity.

A drainage-enhancing device for foam fractionation of proteins

The foam fractionation of nisin from its fermentation broth was studied. Two types of devices consisting of a rubber piston and a foam riser were developed to enhance foam drainage. The separation performance of these two devices was investigated. Experimental results indicated that the second device could significantly reduce the liquid fraction of the foam leaving the column, ɛout, leading to a higher enrichment of the out-flow stream. As its mounting height increased from 0 to 15 cm, ɛout declined from 7.07‰ to 6.13 ‰ and the maximum nisin activity in the foamate could reach 39.6 IU/μL. The slight increase in nisin inactivation rate indicated the applicability of this method in the recovery and concentration of proteins. Finally, the mechanism of the process was primarily explained by invoking recent work on pneumatic foams. This research provides a basis for the design of multistage draining foam fractionator which could potentially be an effective separation equipment.

Effect of humidity on dynamic foam stability

Abstract It is experimentally shown that the stability of non-overflowing pneumatic foams, such as those used in the Bikerman foam stability test, is strongly dependent upon the gradient of humidity within the freeboard of the column. Growing foams can be prevented from exhibiting collapse, even at surfactant concentrations significantly lower than the critical micelle concentration, by saturating the air above the foam surface thereby preventing liquid evaporation. These observations have important implications for the performance of flotation devices, as well as instruments designed to measure the stability of aqueous foams. Meteorological data for two mining sites in Australia throughout a single year shows the variability of humidity, and therefore stability, of flotation froths.

The Wetness of a Rising Foam

A simple graphical method of calculating the liquid volume fraction, or wetness, of a rising pneumatic foam is presented that is deterministic, given system properties (such as the bubble size distribution and the rate at which gas is sparged to the column) and the kinematic viscosity of the interstitial fluid along with some information about the drainage behavior of the stationary foam analogue. Thus, it is shown that data for the wetness of rising columns of SDS foam (Stevenson, P.; Stevanov, C. Ind. Eng. Chem. Res. 2004, 43, 6187) can be well-predicted using an equation (Neethling, S. J.; Lee, H. T.; Cilliers, J. J. J. Phys.: Condensed Matter 2002, 14, 331) for the drainage of liquid through stationary columns of SDS foam. Further, the liquid overflow rate can be well-predicted once the liquid volume fraction is calculated. The effects of washwater added to the top of the column and bubble bursting on the equilibrium volumetric fraction have been considered theoretically. It is shown that the volumetric liquid fraction of the foam will be enhanced and the enhancement can be determined graphically. However, the rate at which liquid will be recovered from the top of the column is seen to be independent of washwater addition and bubble bursting.

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.

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.

Analysis of transient thickness of pneumatic foams

This paper presents a simple, experimentally validated approach to analyze the transient formation of a foam layer produced by injecting gas bubbles into a foaming solution. Based on experimental observations, three different regimes in the transient growth of the foam have been identified as a function of the superficial gas velocity. A model based on the mass conservation equation for the gas phase in the foam combined with three different models for the average porosity is proposed. It is shown that for practical calculations a constant average porosity equal to 0.82 can be used. The model predictions show very good agreement with experimental data for low superficial gas velocity and provide an upper limit of the foam thickness for intermediate and large superficial gas velocities. The paper discusses the physical mechanisms that may occur during the foam formation and the effects of the superficial gas velocity on the foam dynamics. The present analysis speculates several mechanisms for the bursting of the bubbles at the top of the foams and proposes the framework for more fundamental and detailed studies.

Modeling of the Generation and Collapse of Aqueous Foams

Drainage of the continuous phase liquid from a foam plays a pivotal role in determining its stability to collapse. A theoretical model for the drainage of the liquid during generation of a foam by bubbling and its subsequent collapse is presented. The model accounts for drainage in the films as well as in the plateau border channels. Drainage of the films is modeled using Reynold's equation for the flow between parallel flat circular disks under the influence of van der Waals, electrical double-layer, and plateau border suction forces with film rupture occurring when the film thickness attains a certain critical value. Since this model accounts for collapse at the foam/gas interface during generation, it is able to predict the steady state height attained by pneumatic foams. The model is also able to predict the establishment of a drainage equilibrium when the opposing forces of gravity and plateau border suction gradient balance each other. The effect of various parameters such as superficial gas velocit...

259 Hydrophobisation of virus specific antibodies and antisense oligonucleotite promotes their antiviral actions: Y.G.Suzdaltseva, N.S.Melik-Nubarov, [formula omitted] V.P.Pheoktistov, A.V.Kabanov, O.P.Zhirnov The D.I.Ivanovsky Virology Institute, Moscow 123098, Research Center of Molecular Diagnostics, Moscow 113149, Russia

The topic of particle-stabilised foams is relevant in many industrial applications such as the food industry, cosmetic, and mineral processing. This study is concerned with the influence of solid nanoparticles on the froth stability and gas holdup of aqueous solutions of sodium chloride. Silica nanoparticles with a nominal size of 300 nm were treated by esterification with 1-octanol to achieve intermediate hydrophobicity. Pneumatic foams were generated by passing nitrogen gas through a porous disc into the solutions. The results showed that increases in the concentration of nanoparticles in the colloidal dispersions caused slight increases in the gas holdup and had no significant effect on the foaminess of aqueous dispersions in the concentration range 0–6 wt%. Sodium chloride in sufficient amount to coagulate the particles significantly improved both gas holdup and foaminess. Electrolytes reduce the energy barrier that has to be overcome for the adsorption of particles to an interface and increase the hydrophobicity of the particles. As well, coagulated particulates have greater capture efficiency and hinder drainage in the froth, which could explain the improved foaminess in the present study. Additional experiments were performed to determine the effect of the composition of the continuous phase on the foaming properties. This was essential due to the use of a co-solvent to disperse the nanoparticles, which was shown to impact the foaminess of the dispersion if not removed adequately.