Bubbles growth and their stability in reactive flotation process

Reactive flotation is a process originally used to separate dolomite from phosphate. It depends mainly on the formation of CO₂ bubbles as a product of acid reaction with dolomite surface in presence of a polyvinyl alcohol (PVA) polymeric membrane. Elasticity of PVA membrane is one of the critical factors that affect the bubble stability at the interface. The dynamic surface tension (DST) at different CO₂ rates was used as a measure for the membrane elasticity. The DST, at different CO₂ rates, was used to simulate the bubbles formation at the interface due to the surface reaction of dolomite with acidic media. The results indicated that the high evolution of CO₂, due to vigorous reaction, negatively affects the membrane elasticity and leads to fast rupture of the formed bubbles.     

Blistering of Glass-Epoxy Amine Adhesive Joints in Water Vapour at High Pressure. An Indication of Interfacial Crumpling

When joints are made between solids, the surfaces of which are soluble in water, with an epoxy-amine adhesive, blistering is observed in water vapour at high temperature and pressure. The occurrence of these blisters follows the cavitation theory in elastomers. Microscopic observation of the blisters suggest that they come from initial smaller ones which are homogeneously spread along the interface (not air bubbles or defects) and grow under the osmotic pressure developed by water condensation in the initial “sucker” and by surface dissolution. These observations led us to suggest that the solid/adhesive interface crumples during hardening of the adhesive and that many small suckers exist along the interface. This leads to a new model for the loss of adherence of epoxy-metal joints kept in high humidities.     

Blistering of Glass-Epoxy Amine Adhesive Joints in Water Vapour at High Pressure. An Indication of Interfacial Crumpling

When joints are made between solids, the surfaces of which are soluble in water, with an epoxy-amine adhesive, blistering is observed in water vapour at high temperature and pressure. The occurrence of these blisters follows the cavitation theory in elastomers. Microscopic observation of the blisters suggest that they come from initial smaller ones which are homogeneously spread along the interface (not air bubbles or defects) and grow under the osmotic pressure developed by water condensation in the initial “sucker” and by surface dissolution. These observations led us to suggest that the solid/adhesive interface crumples during hardening of the adhesive and that many small suckers exist along the interface. This leads to a new model for the loss of adherence of epoxy-metal joints kept in high humidities.     

Numerical and experimental investigation of the lift force on single bubbles

In recent years CFD has proven itself as a valuable tool for gaining insight in flow phenomena in general and complex multiphase flows arising in process equipment in particular. However for (dispersed) multiphase flows, the reliability of the outcome of these computations depends in a sensitive way on the correctness of the representation of the phase interactions (for instance due to drag and lift forces) which leads to the well-known and difficult closure problem. In this paper we report results of direct numerical simulations supplemented with dedicated experiments to obtain quantitative data for the representation of the lift force. This force is known to be responsible for the segregation of small and large (deformed) bubbles in bubbly flows through pipes and bubble columns.Both numerical simulations using an improved front tracking (FT) model and experiments under well-defined conditions have been performed for air bubbles rising through water/glycerine mixtures, where the bubble diameter, liquid viscosity and linear shear rate were varied. The numerical simulations show a good agreement with the correlation presented by Legendre and Magnaudet (1998) for spherical bubbles at sufficiently high Reynolds numbers. For large deformed bubbles a good agreement with the correlation by Tomiyama et al. (2002) was found over a wide range of liquid viscosities, although the computed lift force was always slightly lower. Therefore a new correlation has been proposed, which combines a fit of the numerical data for deformed bubbles with the correlation by Legendre and Magnaudet (1998) for small bubbles. Finally, it was shown that the shear rate has no significant influence on the drag and lift coefficient.An experimental set-up (similar to the one used by Tomiyama) was constructed using a running belt submerged in a liquid, consisting of a glycerine–water mixture of varying viscosity. PIV measurements have been used to calibrate the linear shear field and to obtain the flow profile around the bubbles. Contrary to the numerical simulations, the experimental data show a very strong influence of the shear rate on the lift force coefficient. This may be attributed to the rigid behaviour of the contaminated bubble surface, which changes the shear stress at the bubble interface.     

On the theory of the bulk boiling of a liquid with transition into a metastable state

The equilibrium adiabatic curve describing the behavior of liquid-gas vapor bubbles upon reduction of pressure lower than equilibrium for the initial temperature is constructed under the assumption that the liquid contains nuclei of the insoluble gas. It was found that, for sufficient small radii of nuclei, the dependence of pressure on the specific volume of the liquid boiling on nuclei can be nonmonotonic, i.e., has a form similar to the Van der Waals isotherm. Based on such a feature of the adiabatic curve, related to the appearance of capillary forces at the interface, an explanation is proposed of a phenomenon of transition into a metastable region upon the abrupt reduction in pressure observed in experiments. A theory and a principle of formation of solutions, which describes the transition from the metastable state, are proposed.     

析气界面上气泡滑移促进的传质模型探索

本文在充分研究析气界面上气泡行为的基础上，根据界面上析出气泡的动力学行为，结合界面附近流体及相间传质理论。导出了气泡滑移促进的传质模型     

The behaviour of large gas bubbles at a liquid-liquid interface. Part 2: Liquid entrainment

The roles played by large gas bubbles in the generation and coalescence of liquid drops at a liquid-liquid interface are elucidated. The amount of lower liquid entrained by individual bubbles and the resulting drop size distributions in the upper liquid phase are quantified for the three phase system: sunflower oil + 50 wt % decane, water + 50 wt % sugar, air, and qualitative theoretical models are presented. Drops settling to the interface were found to coalesce rapidly and bubble flux had no apparent effect on the rate of drop coalescence at the liquid-liquid interface.     

Improvement of the surface quality of foam injection molded products from a material property perspective

Microcellular injection molding is an attractive method. However, their surface imperfections have been a major problem hindering wide industrial applications. Several methods have been proposed to improve the surface appearance of foams. In this study, we proposed a method to improve the surface appearance of polypropylene (PP) foams from the material property perspective, especially with regard to crystallization and viscosity. The basic idea of the surface improvement is to reduce the size of bubbles generated at the flow front, delay the solidification behavior of the polymer at the mold interface, squeeze the bubbles existing at the mold–polymer interface, and redissolve the bubbles into the polymer by holding pressure. Blending a low-modulus PP delays the crystallization of the polymers at the skin layer and solidification, taking enough time to squeeze the bubbles smaller. A sorbitol-based gelling agent, bis-O-([4 methylphenyl]methylene)-D-Glucitol, was used to increase the viscosity at a low strain rate to reduce the size of the bubbles generated at the flow front during the filling stage. The foam injection molding experiments demonstrated that the proposed method effectively improved the surface appearance of the foams. In particular, the surface appearance of the foams became almost equivalent to that of solid samples using low-modulus PP.     

Nucleation of microcellular foam: Theory and practice

Microcellular polymer foams exhibit greatly improved mechanical properties as compared to standard foams due to the formers' small bubble size. Microcellular foams have bubbles with diameters on the order of 10 microns, volume reductions of 30 to 40 percent, and six or seven times the impact strength of solid parts. They are produced through the use of thermodynamic instabilities without the use of foaming agents. This method leads to a very uniform cell size throughout a part's cross section. A theoretical model for the nucleation of microcellular foams in thermoplastic polymers has been developed and experimentally confirmed. This model explains the effect of various additives and processing conditions on the number of bubbles nucleated. At levels of secondary constituents below their solubility limits, an increase in the concentration of the additive or the concentration of gas in solution with the polymer increases the number of bubbles nucleated. Nucleation in this region is homogeneous. Above the solubility limit of additives, nucleation is heterogeneous and takes place at the interface between second phase inclusions and the polymer. The number of bubbles nucleated is dependent on the concentration of heterogeneous nucleation sites and their relative effect on the activation energy barrier to nucleation. In the vicinity of the solubility limit, the two mechanisms compete.     

微小空间内光合细菌与气-液相界面作用的可视化实验

实验研究了光合细菌在微槽道光生化反应器内的产氢过程以及光合细菌与气-液相界面之间的相互作用。实验发现微槽道内气泡的初始生长点多位于粗糙的侧壁面,气-液相界面内外气体分子的浓差扩散导致气泡不断长大,液体流速增大加快了气泡的生长速率。在某些区域还观察到气泡逐渐缩小直至溶解于液相中的现象;光合细菌会在气-液相界面既存在不可逆吸附现象又存在排斥现象,吸附在气-液相界面的光合细菌会围绕气泡表面运动。同时吸附在气-液相界面的光合细菌增加了气泡的机械强度和稳定性并阻碍气泡的聚合。     

Gas–liquid mass transfer in rotating solid foam reactors

Three-phase reactor designs based on rotating solid foams for the application in the fine chemical industry are developed. The aim is to use solid foams both as a catalyst support and stirrer in order to mix the gas and liquid phases and create fine gas bubbles. Gas–liquid mass transfer data are presented for different solid foam stirrer configurations and compared to an optimized Rushton stirrer. Solid foam stirrers were developed in a blade and a block design. Both foam reactor designs work at stirring rates below 600 rpm. Using the foam blade design, gas bubbles are mainly created by the turbulence at the gas–liquid interface. Large bubbles are broken up by the foam blades. Using a foam block design, rotation leads to the structurization of the reactor volume into sections strongly differing in gas holdup, flow behavior and bubble size distribution. This results in a gas–liquid mass transfer, which is 50% higher than the Rushton stirrer used as comparison. The foam stirrer designs can be easily used in ordinary three-phase reactors and show a high potential for further optimization of the gas–liquid flow pattern and therefore for further increase of the rate of mass transfer.     

INTERACTION BETWEEN ADSORPTION AND CONDENSATION PROCESSES IN A PORE AND ITS EFFECT ON HYSTERESIS

Capillary condensation is treated on the basis of its relation to the properties of the interface between the pore walls and the condensed sorbate. Effects resulting from the interaction between adsorption and condensation processes at the pore are examined. The approach allows a more accurate assessment of the sorption isotherm, and leads to a simple explanation of hysteresis.     

Interaction between adsorption and condensation processes in a pore and its effect on hysteresis

Capillary condensation is treated on the basis of its relation to the properties of the interface between the pore walls and the condensed sorbate. Effects resulting from the interaction between adsorption and condensation processes at the pore are examined. The approach allows a more accurate assessment of the sorption isotherm, and leads to a simple explanation of hysteresis.     

A quantitative assessment of bubble-curtain effects upon interelectrode resistance of a conductimetric cell

Adhesion of gas bubbles upon electrode surfaces of a conductimetric cell causes a reduction in the effective surface area of the electrodes and hence leads to false estimations of measured conductivities. By considering the random dispersal of bubbles on the electrode surfaces, the increase of interelectrode resistance due to bubble adhesion is determined as a function of the fractional surface area covered by the gas bubbles. An experimental study using an electrolytic tank with electrodes partially covered by spherical Styrofoam (insulating) spheres (which simulate the gas bubbles) is carried out. Computed results due to the present model and those of other existing models (based on Maxwell's and Bruggeman's formulations) are compared with the measured data.     

Mechanism of bubble formation in step-emulsification devices

Step-emulsification device is an emerging technology for bubble generation, which is convenient for numbering-up. However, the mechanism of bubble formation in such devices remains unclear. In this article, the mechanism of bubble formation in step-emulsification devices is investigated by using a high-speed camera. First, the evolution of gas–liquid interface with time during bubble formation is observed. Second, the variation of characteristic parameters of bubbles, such as bubble's volume V_B and generation frequency f, with gas and liquid flow rates is described under various liquid viscosities and step widths. Meanwhile, the coupling law of interface evolution with characteristic parameters of bubbles is explored. Finally, the basic guideline for generation of monodispersed bubbles in step-emulsification is put forward: low liquid viscosity. Connecting the interface evolution with the characteristic parameters, predictive models for bubble's volume, and generation frequency in step-emulsification are proposed, respectively.     

Coalescence of bubbles and stability of foams in aqueous solutions of Tween surfactants

Coalescence of air bubbles and stability of foams in aqueous solutions of Tween 20, 40, 60 and 80 are reported in this work. Adsorption of the surfactants at air–water interface was studied by measuring the surface tension of the surfactant solutions. The surface tension profiles were fitted using a surface equation of state derived from the Gibbs and Langmuir adsorption equations. The critical micelle concentration and surface tension at this concentration were determined. The effect of surfactant concentration on coalescence of air bubbles at flat air–water interface was studied. The role of steric force on coalescence time was investigated. The coalescence time distributions were fitted by the stochastic model. The mean values of the distributions were compared with the predictions of seven film-drainage models. Stability of foams was analyzed by the Ross–Miles test. The initial and residual foam heights were measured at different surfactant concentrations. The stability of foams was compared with the coalescence time of the bubbles.     

Effects of uniform magnetic field on the interaction of side-by-side rising bubbles in a viscous liquid

Effects of uniform magnetic fields on the interaction and coalescence of side-by-side rising bubbles of dielectric fluids were not studied; so in the present research, effects of different strengths of uniform magnetic field on the interaction of two bubbles rising side by side in a viscous and initially stagnant liquid are studied, numerically. For numerical modeling of the problem, a full computer code was developed to solve the governing equations which are continuity, Navier-Stokes, magnetic field and interface capturing equations which are level set and re-initialization equations. The finite volume method is used for the discretization of the hydrodynamic equations where the finite difference method is used to discretization of the magnetic field equations. The results are compared with available numerical and experimental results which show a good agreement. It is found that the uniform magnetic field can be used for contactless control of side-by-side coalescence of bubbles.     

Breaking bubbles and the water-to-air transport of particulate matter

Bubbles collapsing at an air-liquid interface eject small, high-speed droplets into the adjacent gas phase. The composition of these aersol-sized droplets may be markedly different from the bulk solution in which the bubbles originate. This bubble-breaking/fractionation phenomenon has major implications for a variety of environmental and industrial processes. In preliminary work reported here we have measured the transfer of micron-size latex particles to jet drops produced by the bursting of one-millimeter diameter bubbles. Jet drop concentration was measured as a function of: bulk particle concentration, bulk ionic strength, and depth of bubble release. Results show the droplet concentration to be relatively insensitive to bulk concentration but strongly dependent on ionic strength and trace contaminants.     

单孔及微孔曝气低气速鼓泡床内气泡行为比较

引言鼓泡床反应器被广泛应用于吸收、液相氧化、好氧生化等气液反应过程,气体在液相中的分散情况对鼓泡床的反应和传质特性都有很大影响.为了提高气液传质效率,增加生产强度,工业反应器很多都是在高气速下操作(Ug>0·05m·s-1),很多研究都集中在高气速湍动鼓泡区[1~3].但对有机     

Effect of bubble interaction on interphase mass transfer in gas fluidized beds

A non-interfering technique has been used to measure the concentration of ozone in pairs of bubbles injected into a bed of inactive 390 μm glass beads fluidized by ozone-free air. The transfer of the ozone tracer from the bubble phase to the dense phase is enhanced when compared to the transfer from isolated bubbles in the same particles and the same column. Bubble growth is also greater for the case where pairs of bubbles are introduced than when bubbles are present in isolation. Enhancement of interphase mass transfer for interacting bubbles in the present work and in previous studies incr with particle size and can be explained in terms of enhancement of the throughflow (or convective) component of transfer while the diffusive component unaltered. This mechanism leads to new equations for estimating interphase mass transfer in freely bubbling fluidized beds.