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Gas Bubbles (gas + bubble)
Selected AbstractsIntensification of Rate of Diffusion Controlled Reactions in a Parallel Plate Electrochemical Reactor Stirred by a Curtain of Electrochemically Generated Gas BubblesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 10 2007G. H. Sedahmed Abstract Rates of mass transfer were measured at a vertical plate stirred by a rising curtain of oxygen bubbles generated electrochemically at an upstream plate by measuring the limiting current of the cathodic reduction of K3Fe(CN)6 in alkaline solution. The rate of mass transfer was found to increase over the natural convection value by a factor ranging from 2.4 to 25 compared to the previously reported range of 2,5 in the case of copper deposition from acidified solutions. The high tendency of oxygen bubbles to coalesce in alkaline solutions is believed to be responsible for the high rates of mass transfer in alkaline solutions. The rate of mass transfer at a plate stirred by a curtain of oxygen bubbles was found to decrease with increasing plate height and electrolyte concentration. Curtains of H2 bubbles were found to be less effective in enhancing the rate of mass transfer compared to that of oxygen. Practical application of the results in designing a modified parallel plate electrochemical reactor stirred by a counterelectrode gas curtain was highlighted. The suggested design has the potential of saving part or all of the mechanical stirring energy as well as floor space since it extends vertically. [source] Numerical simulation of a single bubble by compressible two-phase fluidsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6 2010Siegfried Müller Abstract The present work deals with the numerical investigation of a collapsing bubble in a liquid,gas fluid, which is modeled as a single compressible medium. The medium is characterized by the stiffened gas law using different material parameters for the two phases. For the discretization of the stiffened gas model, the approach of Saurel and Abgrall is employed where the flow equations, here the Euler equations, for the conserved quantities are approximated by a finite volume scheme, and an upwind discretization is used for the non-conservative transport equations of the pressure law coefficients. The original first-order discretization is extended to higher order applying second-order ENO reconstruction to the primitive variables. The derivation of the non-conservative upwind discretization for the phase indicator, here the gas fraction, is presented for arbitrary unstructured grids. The efficiency of the numerical scheme is significantly improved by employing local grid adaptation. For this purpose, multiscale-based grid adaptation is used in combination with a multilevel time stepping strategy to avoid small time steps for coarse cells. The resulting numerical scheme is then applied to the numerical investigation of the 2-D axisymmetric collapse of a gas bubble in a free flow field and near to a rigid wall. The numerical investigation predicts physical features such as bubble collapse, bubble splitting and the formation of a liquid jet that can be observed in experiments with laser-induced cavitation bubbles. Opposite to the experiments, the computations reveal insight to the state inside the bubble clearly indicating that these features are caused by the acceleration of the gas due to shock wave focusing and reflection as well as wave interaction processes. While incompressible models have been used to provide useful predictions on the change of the bubble shape of a collapsing bubble near a solid boundary, we wish to study the effects of shock wave emissions into the ambient liquid on the bubble collapse, a phenomenon that may not be captured using an incompressible fluid model. Copyright © 2009 John Wiley & Sons, Ltd. [source] The , -ray burst phenomenon treated as the collapse of a QED magnetized vacuum bubble: analogy with sonoluminescenceMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2000YU. N. Gnedin We treat the phenomenon of a , -ray burst as the non-linear collapse of a magnetic cavity surrounding a neutron star with extremely large magnetic field B,1015,1016 G due to the process of bubble shape instability in the resonant MHD field of an accreting plasma or on a neutron star surface. The QED effect of vacuum polarizability by a strong magnetic field is taken into a consideration. We develop an analogy with the phenomenon of sonoluminescence in which the gas bubble is located in a surrounding liquid with a driven sound intensity. [source] Measurement of sliding velocity and induction time of a single micro-bubble under an inclined collector surfaceTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2008Aref Seyyed Najafi Abstract In this study, interactions between a gas bubble and a flat solid surface were investigated by determining two dynamic parameters, bubble sliding velocity underneath an inclined solid surface and induction time of the gas bubble attaching to the solid surface in aqueous solutions. A single micro-bubble was allowed to move vertically toward an inclined solid surface. After reaching its terminal velocity, the bubble approaches the inclined solid surface and slides underneath it. Complete trajectory of the bubble movement was monitored and recorded by a high-speed CCD video imaging system. Various types of gas bubbles (CO2, air, H2, and O2) and solid surfaces such as bitumen-coated Teflon, hydrophobized and hydrophilic silica were used in sliding velocity and induction time measurements. The effect of water chemistry (industrial process water and de-ionized water) and surface heterogeneity on bubble sliding velocity and induction time was investigated. The results showed that the sliding velocity of micro-bubbles under an inclined solid surface is a strong function of water chemistry, gas type, temperature and hydrophobicity of the solid surface. This study provides relevant information on bubble,solid interactions that would assist in the understanding of bubble,solid attachment under diverse conditions. Dans cette étude, on a étudié les interactions entre une bulle de gaz et une surface solide plane en déterminant deux paramètres dynamiques, la vitesse de glissement de bulle sous une surface de solides inclinée ainsi que le temps d'induction de l'attachement d'une bulle de gaz à une surface solide en solution aqueuse. On a fait se déplacer une micro-bulle unique verticalement vers une surface solide inclinée. Après avoir atteint sa vitesse terminale, la bulle s'approche de la surface solide inclinée et glisse sous elle. La trajectoire complète de la bulle a été enregistrée par un système d'imagerie vidéo CCD à haute vitesse. Différents types de bulles de gaz (CO2, air, H2, et O2) et de surfaces solides telles que du téflon enduit de bitume, de la silice hydrophobe et hydrophile, ont été utilisés dans les mesures de vitesse de glissement et de temps d'induction. L'effet de la chimie de l'eau (eau de procédés industriels et eau déminéralisée) et de l'hétérogénéité de la surface sur la vitesse de glissement et le temps d'induction a été étudié. Les résultats montrent que la vitesse de glissement des micro-bulles sous une surface solide inclinée dépend fortement de la chimie de l'eau, du type de gaz, de la température et de l'hydrophobicité de la surface solide. Cette étude fournit des informations pertinentes sur les interactions bulles-solides pouvant permettre de comprendre l'attachement bulles-solides dans des conditions diverses. [source] Face down posturing for macular hole surgery.ACTA OPHTHALMOLOGICA, Issue 2009Is it really required? Purpose Background: In macular hole surgery pars plana vitrectomy and intravitreal gas injection with or without inner limiting membrane peel, is considered the mainstay of treatment. The requirement for face down posturing is generally regarded as part of the traditional postoperative routine. Several mechanisms have been postulated to explain the action of the gas bubble including exertion of a large floatation force on the macula and prevention of the macular hole exposure to vitreous fluid. Recently the need to face down has been chalenged since this regime compromises patients' postoperative quality of life and it makes macular hole surgery almost impossible for individuals with mental or physical limitations. Methods Review of personal data and systermatic literature review of studies investigating macular hole surgery with shortened or eliminated face down posturing. Results There is considerable body of evidence suggesting successful anatomical and functional outcome in patients with shorter duration of posturing or no posturing at all following macular hole surgery. The pros and cons of each technique will be presented in detail. Conclusion Prone posturing following macular hole surgery provides no functional or anatomic benefit but it is associated with slower progression of cataract. Combined phacovitrectomy without face down positioning may be considered for phakic patients undergoing macular hole surgery. [source] Fixation of heavy contaminants of a dirty bomb attack: Studies with uranium and metal simulantsENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 1 2007Thomas L. McGehee Abstract Asphalt emulsions were evaluated as a means to immobilize radiological contaminants deposited on urban surfaces after a dirty bomb attack. Contaminated surfaces would be sprayed with thin coatings of asphalt emulsion to encapsulate the radioactive particles until the site can be safely remediated. This research investigated applications of an asphalt emulsion (Topein C, Encapco Technologies, LLC, Napa, CA) to treat (zero-valent) iron, lead, and uranium powders on various building material surfaces. Initial studies found that some of the building materials (limestone, concrete, and metal) reacted with the emulsion producing gas bubbles, which formed 0.001 to 1 cm vesicles in the cured asphalt emulsion. These vesicles, however, did not expose the building material surface, and the reaction appeared to aid in the setting of the emulsion. Powdered lead did not react with the asphalt emulsion, but iron powder and uranium did. Iron powder and the emulsion formed vesicles up to 0.5 mm (but not exposing the building material surface), while the uranium (U3O8) had a moderate reaction when compared with to the lead and iron powders. Scanning electron micrographs showed that the lead powder formed nonreactive layers adjacent to the concrete surface while iron particles were evenly distributed in the asphalt matrix due to the reaction with the asphalt, indicating that the physical and chemical reactions between the iron metal particles, asphalt, and concrete affected particle distribution in the asphalt matrix. A vertical operation sediment tube was used to determine the flowing shear stress durability of the asphalt/metal/substrate complex. The asphalt treatment with iron had no loss at the shear range tested (0.1,2.5 Pa), while the asphalt stabilized powdered lead lost 8% asphalt and lead at 2.5 Pa mean shear stress applied for 5 h. The chemical reaction between asphalt emulsion and iron increased the resistance of the asphalt/metal/substrate complex to shear when compared with lead. Some hydrogen was formed in reactions with iron, but the amount formed was well below the lower flammability limit. Treatment of uranium indicated that the emulsion was effective at reducing leaching of the uranium 10 fold. These experiments indicate that asphalt emulsions may be a viable means for containing metallic or dense radiological contaminants on common building materials. © 2007 American Institute of Chemical Engineers Environ Prog 26:94,103, 2007 [source] Performance Tests and Pressure Drop Measurements in the Anode Flowfield of a ,DMFCFUEL CELLS, Issue 4 2010S.-S. Hsieh Abstract Cell performance tests and measurements of the pressure drops in the anode flow channels of a custom-made microdirect methanol fuel cell (,DMFC) are conducted and studied for different methanol concentrations (0.5,2,M), flow rates (10,20,sccm) and operating temperatures (40,80,°C). The anode flowfields consist of three channel/four pass flow channels with widths of 500,2000,,m and a total length of 300,400,mm. Moreover, flow characteristics of the CO2 gas bubbles and methanol solution in the anode flow channels are identified and analysed for CO2 fraction through visualisation. Finally, an optimal channel size for the present ,DMFC is obtained. [source] Transfer of hydrocarbons from natural seeps to the water column and atmosphereGEOFLUIDS (ELECTRONIC), Issue 2 2002I. R. MacDonald Abstract Results from surface geochemical prospecting, seismic exploration and satellite remote sensing have documented oil and gas seeps in marine basins around the world. Seeps are a dynamic component of the carbon cycle and can be important indicators for economically significant hydrocarbon deposits. The northern Gulf of Mexico contains hundreds of active seeps that can be studied experimentally with the use of submarines and Remotely Operated Vehicles (ROV). Hydrocarbon flux through surface sediments profoundly alters benthic ecology and seafloor geology at seeps. In water depths of 500,2000 m, rapid gas flux results in shallow, metastable deposits of gas hydrate, which reduce sediment porosity and affect seepage rates. This paper details the processes that occur during the final, brief transition , as oil and gas escape from the seafloor, rise through the water and dissolve, are consumed by microbial processes, or disperse into the atmosphere. The geology of the upper sediment column determines whether discharge is rapid and episodic, as occurs in mud volcanoes, or more gradual and steady, as occurs where the seep orifice is plugged with gas hydrate. In both cases, seep oil and gas appear to rise through the water in close proximity instead of separating. Chemical alteration of the oil is relatively minor during transit through the water column, but once at the sea surface its more volatile components rapidly evaporate. Gas bubbles rapidly dissolve as they rise, although observations suggest that oil coatings on the bubbles inhibit dissolution. At the sea surface, the floating oil forms slicks, detectable by remote sensing, whose origins are laterally within ,1000 m of the seafloor vent. This contradicts the much larger distance predicted if oil drops rise through a 500 m water column at an expected rate of ,0.01 m s,1 while subjected to lateral currents of ,0.2 m s,1 or greater. It indicates that oil rises with the gas bubbles at speeds of ,0.15 m s,1 all the way to the surface. [source] Mesoporous Hydrous Manganese Dioxide Nanowall Arrays with Large Lithium Ion Energy Storage CapacitiesADVANCED FUNCTIONAL MATERIALS, Issue 7 2009Dawei Liu Abstract Novel nanowall arrays of hydrous manganese dioxide MnO2,·,0.5H2O are deposited onto cathodic substrates by the potentiostatic method from a mixed aqueous solution of manganese acetate and sodium sulfate. The deposition is induced by a change of local pH resulting from electrolysis of H2O, and hierarchical mesoporous nanowall arrays are formed as a result of simultaneous precipitation of manganese hydroxide and release of hydrogen gas bubbles from the cathode. The morphology and lithium ion intercalation properties are found to change appreciably with the concentration of the precursor electrolyte, with a significant reduction in specific surface area with an increased precursor concentration. For example, mesoporous nanowall arrays deposited from 0.1,M solution possess a surface area of ,96,m2 g,1 and exhibit a stable high intercalation capacity of 256,mA hg,1 with a film of 0.5,µm in thickness, far exceeding the theoretical limit of 150,mA hg,1 for manganese dioxide bulk film. Such mesoporous nanowall arrays offer much greater energy storage capacity (e.g., ,230,mA hg,1 for films of ,2.5,µm) than that of anodic deposited films of the same thickness (,80,mA hg,1). Such high lithium ion intercalation capacity and excellent cyclic stability of the mesoporous nanowall arrays, especially for thicker films, are ascribed to the hierarchically structured macro- and mesoporosity of the MnO2,·,0.5H2O nanowall arrays, which offer large surface to volume ratio favoring interface Faradaic reactions, short solid-state diffusion paths, and freedom to permit volume change during lithium ion intercalation and de-intercalation. [source] X-ray computed tomography of peat soils: measuring gas content and peat structureHYDROLOGICAL PROCESSES, Issue 25 2008Nicholas Kettridge Abstract The potential of using X-ray computed tomography (CT) to (i) analyse individual biogenic gas bubbles entrapped within peats and (ii) produce reliable descriptors of peat structure is examined. Existing approaches used to study biogenic gas bubbles measure the gas content of volumes of peat many orders of magnitude larger than most bubbles, and are, therefore, of little use in helping to understand bubble dynamics. In many peatland studies, the description of peat structures is derived from only a few relatively basic metrics; principally the porosity, the bulk density, and the von Post humification scale. CT is applied to identify and quantitatively analyse the size, location and shape of individual gas bubbles entrapped during the saturation of a 200 cm3 sample of S. fuscum. 3421 gas bubbles were identified, ranging in size from 0·1 mm3 to 99·9 mm3. These gas bubbles were non-randomly distributed, clustered predominantly in the vertical plane. When analysing the peat structure, Sphagnum peat and water are shown to be indistinguishable within CT scans. Peat samples were therefore prepared prior to scanning by flushing the peat with lead (II) nitrate solution to increase the linear attenuation of the Sphagnum. Sphagnum stems and branches were analysed, producing metrics of the peat structure; including stem and branch lengths, radii and orientation. In a 100 cm3 sample of S. magellanicum, the length of all Sphagnum stems totalled 1·82 m, with an average radius of 0·65 mm. The Sphagnum stems and branches were both preferentially orientated in the horizontal direction. Copyright © 2008 John Wiley & Sons, Ltd. [source] Geochemical weathering at the bed of Haut Glacier d'Arolla, Switzerland,a new modelHYDROLOGICAL PROCESSES, Issue 5 2002M. Tranter Waters were sampled from 17 boreholes at Haut Glacier d'Arolla during the 1993 and 1994 ablation seasons. Three types of concentrated subglacial water were identified, based on the relative proportions of Ca2+, HCO3, and SO42, to Si. Type A waters are the most solute rich and have the lowest relative proportion of Si. They are believed to form in hydrologically inefficient areas of a distributed drainage system. Most solute is obtained from coupled sulphide oxidation and carbonate dissolution (SO,CD). It is possible that there is a subglacial source of O2, perhaps from gas bubbles released during regelation, because the high SO42, levels found (up to 1200 µeq/L) are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 °C (c.414 µeq/L). A more likely alternative is that sulphide is oxidized by Fe3+ in anoxic environments. If this is the case, exchange reactions involving FeIII and FeII from silicates are possible. These have the potential to generate relatively high concentrations of HCO3, with respect to SO42,. Formation of secondary weathering products, such as clays, may explain the low Si concentrations of Type A waters. Type B waters were the most frequently sampled subglacial water. They are believed to be representative of waters flowing in more efficient parts of a distributed drainage system. Residence time and reaction kinetics help determine the solute composition of these waters. The initial water,rock reactions are carbonate and silicate hydrolysis, and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites. Hydrolysis is followed by SO,CD. The SO42, concentrations usually are <414 µeq/L, although some range up to 580 µeq/L, which suggests that elements of the distributed drainage system may become anoxic. Type C waters were the most dilute, yet they were very turbid. Their chemical composition is characterized by low SO42, : HCO3, ratios and high pH. Type C waters were usually artefacts of the borehole chemical weathering environment. True Type C waters are believed to flow through sulphide-poor basal debris, particularly in the channel marginal zone. The composition of bulk runoff was most similar to diluted Type B waters at high discharge, and was similar to a mixture of Type B and C waters at lower discharge. These observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow. Little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric CO2. The progression of reactions is from carbonate and silicate hydrolysis, through sulphide oxidation by first oxygen and then FeIII, which drives further carbonate and silicate weathering. A crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 : 1. We speculate that microbial oxidation of organic carbon also may occur. Both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions. Hence, we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric CO2 and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity, which catalyses the reduction of FeIII and the oxidation of FeS2. Copyright © 2002 John Wiley & Sons, Ltd. [source] Polymerization at the gas/solution interface: Preparation of polymer microstructures with gas bubbles as templatesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2007Maciej Mazur Abstract We report on the chemical polymerization of 2-methoxyaniline at the interface between an aqueous solution and air. The polymer is formed in the interfacial region, whereas the soluble trimer is yielded in the bulk of the polymerization solution. The preferential polymerization of 2-methoxyaniline is discussed in terms of monomer and oligomer accumulation at the interface, which influences the reactivity of these species and allows further polymerization. The phenomenon of polymer growth is employed to selectively deposit polymeric material onto glass slides decorated with gas microbubbles. Because of preferential polymerization at the bubble/solution interface, hemispherical features are produced on the surface of glass. When some polymeric material is mechanically removed, microrings or microholes are obtained. The anomalous polymerization of 2-methoxyaniline is compared to that of 2-methylaniline. This monomer polymerizes uniformly within the entire volume of the reaction mixture; thus, no preferential polymer formation at the gas/solution interface is observed. As a result, deposition on microbubble-decorated glass slides produces polymeric films containing a number of microholes. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source] Enhancement of ultrafiltration using gas sparging: a comparison of different membrane modules,JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2-3 2003Zhanfeng Cui Abstract Ultrafiltration is widely used in the chemical, pharmaceutical, food and water industries. Practical difficulties arise in designing and operating the process due to concentration polarisation and membrane fouling. Enhancement of ultrafiltration is highly desirable to achieve a higher permeate flux at a fixed energy input, or a reduced energy input whilst maintaining the level of permeate flux, or an improved selectivity of the membrane. One effective, simple, and economic technique used to enhance ultrafiltration is the use of gas bubbles, ie injecting gas into the feed stream to create a gas,liquid two-phase cross-flow operation. In this paper, an attempt is made to compare the effect of ,bubbling' on the ultrafiltration performance, using different membrane modules (in particular, tubular and hollow fibre membrane modules). The difference in performance can be related to the feature of two-phase flow hydrodynamics and its respective effect on mass transfer. The advantages and drawbacks of using this technique to enhance ultrafiltration are discussed. © 2003 Society of Chemical Industry [source] Ultrasonic Investigation of the Effect of Vegetable Shortening and Mixing Time on the Mechanical Properties of Bread DoughJOURNAL OF FOOD SCIENCE, Issue 9 2009K.L. Mehta ABSTRACT:, Mixing is a critical stage in breadmaking since it controls gluten development and nucleation of gas bubbles in the dough. Bubbles affect the rheology of the dough and largely govern the quality of the final product. This study used ultrasound (at a frequency where it is sensitive to the presence of bubbles) to nondestructively examine dough properties as a function of mixing time in doughs prepared from strong red spring wheat flour with various amounts of shortening (0%, 2%, 4%, 8% flour weight basis). The doughs were mixed for various times at atmospheric pressure or under vacuum (to minimize bubble nucleation). Ultrasonic velocity and attenuation (nominally at 50 kHz) were measured in the dough, and dough density was measured independently from specific gravity determinations. Ultrasonic velocity decreased substantially as mixing time increased (and more bubbles were entrained) for all doughs mixed in air; for example, in doughs made without shortening, velocity decreased from 165 to 105 ms,1, although superimposed on this overall decrease was a peak in velocity at optimum mixing time. Changes in attenuation coefficient due to the addition of shortening were evident in both air-mixed and vacuum-mixed doughs, suggesting that ultrasound was sensitive to changes in the properties of the dough matrix during dough development and to plasticization of the gluten polymers by the shortening. Due to its ability to probe the effect of mixing times and ingredients on dough properties, ultrasound has the potential to be deployed as an online quality control tool in the baking industry. [source] Inactivation of Food Spoilage Microorganisms by Hydrodynamic Cavitation to Achieve Pasteurization and Sterilization of Fluid FoodsJOURNAL OF FOOD SCIENCE, Issue 9 2007P.J. Milly ABSTRACT:, Hydrodynamic cavitation is the formation of gas bubbles in a fluid due to pressure fluctuations induced by mechanical means. Various high-acid (pH , 4.6) fluid foods were processed in a hydrodynamic cavitation reactor to determine if commercial sterility can be achieved at reduced processing temperatures. Sporicidal properties of the process were also tested on a low-acid (pH < 4.6) fluid food. Fluid foods were pumped under pressure into a hydrodynamic cavitation reactor and subjected to 2 rotor speeds and flow rates to achieve 2 designated exit temperatures. Thermal inactivation kinetics were used to determine heat-induced lethality for all organisms. Calcium-fortified apple juice processed at 3000 and 3600 rpm rotor speeds on the reactor went through a transient temperature change from 20 to 65.6 or 76.7 °C and the total process lethality exceeded 5-log reduction of Lactobacillus plantarum and Lactobacillus sakei cells, and Zygosaccharomyces bailii cells and ascospores. Tomato juice inoculated with Bacillus coagulans spores and processed at 3000 and 3600 rpm rotor speeds endured a transient temperature from 37.8 to 93.3 or 104.4 °C with viable CFU reductions of 0.88 and 3.10 log cycles, respectively. Skim milk inoculated with Clostridium sporogenes putrefactive anaerobe 3679 spores and processed at 3000 or 3600 rpm rotor speeds endured a transient temperature from 48.9 to 104.4 or 115.6 °C with CFU reductions of 0.69 and 2.84 log cycles, respectively. Utilizing hydrodynamic cavitation to obtain minimally processed pasteurized low-acid and commercially sterilized high-acid fluid foods is possible with appropriate process considerations for different products. [source] Viscous co-current downward Taylor flow in a square mini-channelAICHE JOURNAL, Issue 7 2010Özge Keskin Abstract This article presents a computational study of the co-current downward Taylor flow of gas bubbles in a viscous liquid within a square channel of 1 mm hydraulic diameter. The three-dimensional numerical simulations are performed with an in-house computer code, which is based on the volume-of-fluid method with interface reconstruction. The computed (always axi-symmetric) bubble shapes are validated by experimental flow visualizations for varying capillary number. The evaluation of the numerical results for a series of simulations reveals the dependence of the bubble diameter and the interfacial area per unit volume on the capillary number. Correlations between bubble velocity and total superficial velocity are also provided. The present results are useful to estimate the values of the bubble diameter, the liquid film thickness and the interfacial area per unit volume from given values of the gas and liquid superficial velocities. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Dispersed oil,water,gas flow through a horizontal pipeAICHE JOURNAL, Issue 5 2009K. Piela Abstract An experimental study of three-phase dispersed flow in a horizontal pipe has been carried out. The pressure drop over the pipe strongly increases with increasing bubble and drop volume fraction. Because of the presence of drops the transition from dispersed bubble flow to elongated bubble flow occurs at a lower gas volume fraction. The gas bubbles have no significant influence on the phase inversion process. However, phase inversion has a strong effect on the gas bubbles. Just before inversion large bubbles are present and the flow pattern is elongated bubble flow. During the inversion process the bubbles break-up quickly and as the dispersed drop volume fraction after inversion is much lower than before inversion, a dispersed bubble flow is present after inversion. (When inversion is postponed to high dispersed phase fractions, the volume fraction of the dispersed phase can be as high as 0.9 before inversion and as low as 0.1 after inversion.) © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Hydrogenation of 2-ethylanthraquinone under Taylor flow in single square channel monolith reactorsAICHE JOURNAL, Issue 3 2009Dingsheng Liu Abstract The hydrogenation of 2-ethylanthraquinone (EAQ) to 2-ethylanthrahydroquinone (EAHQ) was carried out under Taylor flow in single square channel monolith reactors. The two opening ends of opaque reaction channel were connected with two circular transparent quartz-glass capillaries, where Taylor flow hydrodynamics parameters were measured and further used to obtain practical flow state of reactants in square reaction channels. A carefully designed gas-liquid inlet mixer was used to supply steady gas bubbles and liquid slugs with desired length. The effects of various operating parameters, involving superficial gas velocity, superficial liquid velocity, gas bubble length, liquid slug length, two-phase velocity and temperature, on EAQ conversion were systematically researched. Based on EAQ conversion, experimental overall volumetric mass transfer coefficients were calculated, and also studied as functions of various parameters as mentioned earlier. The film model, penetration model, and existing semi-empirical formula were used to predict gas-solid, gas-liquid, and liquid-solid volumetric mass transfer coefficients in Taylor flow, respectively. The predicted overall volumetric mass transfer coefficients agreed well with the experimental ones. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Measuring bubble, drop and particle sizes in multiphase systems with ultrasoundAICHE JOURNAL, Issue 11 2004A. H. G. Cents Abstract A technique is developed for measurement of bubble, droplet and particle-size distributions in multiphase systems, based on the propagation speed and attenuation of ultrasound. The measurement of the size distribution of the dispersed phase in multiphase systems was desired to analyze the mass-transfer mechanism in gas-liquid-liquid and gas-liquid-solid systems. To obtain this information, both the ultrasonic velocity and the attenuation coefficient of tone-burst signals are determined for a large frequency range (typically 100 kHz , 100 MHz). From these parameters, the size distributions and the volume fraction of the different dispersed phases can be determined using a scattering model. It was shown that the interfacial area can be determined very accurately, however, for the exact size distribution of the gas bubbles in the used size range (1,3 mm) an independent gas holdup determination is required. Experiments were performed in gas-liquid, solid-liquid, and gas-liquid-solid systems. The results showed good agreement of the particle-size distribution compared to a commercial laser-scattering analyzer, both with and without gas bubbles present. Furthermore, a good agreement between the scattering model and the experiments was found in the systems that contained gas bubbles, but these results should be validated using for instance, a digital camera technique. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2750,2762, 2004 [source] Extractive bioconversion in a four-phase external-loop airlift bioreactorAICHE JOURNAL, Issue 7 2000Lidija Sajc The integration of biosynthesis and product separation can increase the productivity of immobilized plant cells in airlift bioreactors. Extractive bioconversion of anthraquinones was studied in an external-loop airlift bioreactor consisting of a riser, a downcomer, and two horizontal sections, while containing alginate-immobilized Frangula alnus cells, a continuous aqueous phase (nutrient solution), dispersed solvent phase (n-hexadecane or silicone oil), and gas bubbles. A simple mathematical model was developed to describe the cocurrent liquid-liquid extraction in the riser section of the bioreactor and to rationalize the measured product concentrations in the aqueous and solvent phase. The model equations were solved analytically in a dimensionless form and used to study the effects of flow conditions, solvent properties, product formation rate, droplet size, and contactor length on the extraction efficiency and product concentration profiles in the continuous and dispersed phase. [source] RHEOLOGICAL CHARACTERIZATION OF WET FOOD FOAMSJOURNAL OF TEXTURE STUDIES, Issue 2 2007B. EDGAR CHÁVEZ-MONTES ABSTRACT Incorporating air bubbles into foods is a technical challenge, and in all cases, the fabrication of a foam goes through a "wet foam" stage, where bubbles are diluted in the food matrix and require stabilization. Sometimes, the end product is itself a wet foam, and a popular example is ice cream. This article describes a study of structural aspects of wet foams, where the continuous phase is a fluid, by means of dynamic rheological tests. The effects of formulation and processing conditions on aerated food foams are studied, and an example is presented for ice cream mix. The incorporation of gas bubbles at volume fractions of 50% or less modifies moderately the bulk rheological properties, and their effect can be predicted by the foam limit case. The continuous phase dominates to a great part the bulk's rheological behavior, and in the case of food systems, it stems from the presence of polysaccharide thickeners. PRACTICAL APPLICATIONS This work presents an alternative approach to study the rheological properties of short life and difficult-to-sample products, such as wet food foams, in a rheo-reactor. Through the analysis of mechanical properties in oscillatory regime, the structure and stability of wet food foams can be characterized immediately after being fabricated in situ. This work presents new insights on the foaming step of ice cream mix (decoupled from the freezing step), and shows how the mechanical properties are affected by the incorporation ofbubbles, by process conditions and by the presence of thickeners and emulsifiers. This work can be a valuable guide to decide on optimal process and formulation to fabricate wet food foams (e.g., ice cream, aerated desserts) with specific mechanical properties and stability. [source] Freeze,thaw-induced embolism in Pinus contorta: centrifuge experiments validate the ,thaw-expansion hypothesis' but conflict with ultrasonic emission dataNEW PHYTOLOGIST, Issue 4 2010Stefan Mayr Summary ,The ,thaw-expansion hypothesis' postulates that xylem embolism is caused by the formation of gas bubbles on freezing and their expansion on thawing. We evaluated the hypothesis using centrifuge experiments and ultrasonic emission monitoring in Pinus contorta. ,Stem samples were exposed to freeze,thaw cycles at varying xylem pressure (P) in a centrifuge before the percentage loss of hydraulic conductivity (PLC) was measured. Ultrasonic acoustic emissions were registered on samples exposed to freeze,thaw cycles in a temperature chamber. ,Freeze,thaw exposure of samples spun at ,3 MPa induced a PLC of 32% (one frost cycle) and 50% (two cycles). An increase in P to ,0.5 MPa during freezing had no PLC effect, whereas increased P during thaw lowered PLC to 7%. Ultrasonic acoustic emissions were observed during freezing and thawing at ,3 MPa, but not in air-dried or water-saturated samples. A decrease in minimum temperature caused additional ultrasonic acoustic emissions, but had no effect on PLC. ,The centrifuge experiments indicate that the ,thaw-expansion hypothesis' correctly describes the embolization process. Possible explanations for the increase in PLC on repeated frost cycles and for the ultrasonic acoustic emissions observed during freezing and with decreasing ice temperature are discussed. [source] Effect of molecular weight and end capping on poly(lactic- co -glycolic acid) ultrasound contrast agentsPOLYMER ENGINEERING & SCIENCE, Issue 9 2008J.R. Eisenbrey Ultrasound contrast agents (CA) consist of stabilized gas bubbles that, when injected intravenously, provide an acoustic impedance mismatch, producing additional contrast to a diagnostic ultrasound scan. These agents must be smaller than 8 ,m in order to pass safely through the capillaries, contain gas for an impedance mismatch and should be stable enough to survive the duration of the imaging session. A double emulsion technique has previously been optimized within our laboratory to create CA with 50:50 poly (lactic- co -glycolic acid) (PLGA). Although a great deal of research has focused on the effects of molecular weight and end capping on solid PLGA particles, very little has been done to examine the effects of these parameters on hollow CAs formed in a double emulsion. Non-end capped PLGA was found to provide maximum enhancement at a molecular weight of 66.0 kDa, giving an ultrasound enhancement of roughly 18.5 dB. The enhancement demonstrated by CA formed using the end-capped PLGA rose to a maximum enhancement of 19 dB at the highest commercially available molecular weight of 82.4 kDa. A strong correlation was seen between ultrasound enhancement, stability under ultrasonic conditions, surface morphology and zeta potential. This study shows the influence of polymer characteristics on the resulting properties of CA and the ability to tailor CAs to particular applications by varying the polymer choice. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers [source] Expansion mechanisms of plastic/wood-flour composite foams with moisture, dissolved gaseous volatiles, and undissolved gas bubbles,POLYMER ENGINEERING & SCIENCE, Issue 7 2003G. M. Rizvi The large quantity of moisture in wood-flour may lead to the deterioration of the cell structure of foamed plastic wood-flour composites in terms of cell size, non-uniformity, and poor surface quality. Since these anomalies can cause poor mechanical properties of the foamed composites, the removal of the moisture from wood-flour becomes a critical issue with respect to the improvement of these properties. The wood-flour in this experimental study was first oven-dried at different temperatures and then subjected to acetone extraction and thermogravimetric analysis (TGA). The oven-dried wood-flour was blended with plastic and then subjected to extrusion foaming. The results obtained from the TGA studies indicate that most volatiles were released from the extractives. Conversely, a comparative experimental study of the foaming behavior of these plastic/wood-flour composites versus that of undried wood-flour composites confirms that removal of the adsorbed moisture from wood-flour results in a better cell morphology. However, it seems that some gaseous emissions released from wood-flour are soluble in plastic and thereby favorably contribute to the development of the cell morphology. This paper describes the expansion mechanisms of wood-flour composite foams resulting from the adsorbed moisture and dissolved gaseous emissions as well as resulting from the finely dispersed undissolved gas bubbles released from a chemical blowing agent. [source] Measurement of sliding velocity and induction time of a single micro-bubble under an inclined collector surfaceTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2008Aref Seyyed Najafi Abstract In this study, interactions between a gas bubble and a flat solid surface were investigated by determining two dynamic parameters, bubble sliding velocity underneath an inclined solid surface and induction time of the gas bubble attaching to the solid surface in aqueous solutions. A single micro-bubble was allowed to move vertically toward an inclined solid surface. After reaching its terminal velocity, the bubble approaches the inclined solid surface and slides underneath it. Complete trajectory of the bubble movement was monitored and recorded by a high-speed CCD video imaging system. Various types of gas bubbles (CO2, air, H2, and O2) and solid surfaces such as bitumen-coated Teflon, hydrophobized and hydrophilic silica were used in sliding velocity and induction time measurements. The effect of water chemistry (industrial process water and de-ionized water) and surface heterogeneity on bubble sliding velocity and induction time was investigated. The results showed that the sliding velocity of micro-bubbles under an inclined solid surface is a strong function of water chemistry, gas type, temperature and hydrophobicity of the solid surface. This study provides relevant information on bubble,solid interactions that would assist in the understanding of bubble,solid attachment under diverse conditions. Dans cette étude, on a étudié les interactions entre une bulle de gaz et une surface solide plane en déterminant deux paramètres dynamiques, la vitesse de glissement de bulle sous une surface de solides inclinée ainsi que le temps d'induction de l'attachement d'une bulle de gaz à une surface solide en solution aqueuse. On a fait se déplacer une micro-bulle unique verticalement vers une surface solide inclinée. Après avoir atteint sa vitesse terminale, la bulle s'approche de la surface solide inclinée et glisse sous elle. La trajectoire complète de la bulle a été enregistrée par un système d'imagerie vidéo CCD à haute vitesse. Différents types de bulles de gaz (CO2, air, H2, et O2) et de surfaces solides telles que du téflon enduit de bitume, de la silice hydrophobe et hydrophile, ont été utilisés dans les mesures de vitesse de glissement et de temps d'induction. L'effet de la chimie de l'eau (eau de procédés industriels et eau déminéralisée) et de l'hétérogénéité de la surface sur la vitesse de glissement et le temps d'induction a été étudié. Les résultats montrent que la vitesse de glissement des micro-bulles sous une surface solide inclinée dépend fortement de la chimie de l'eau, du type de gaz, de la température et de l'hydrophobicité de la surface solide. Cette étude fournit des informations pertinentes sur les interactions bulles-solides pouvant permettre de comprendre l'attachement bulles-solides dans des conditions diverses. [source] Direct simulation of the buoyant rise of bubbles in infinite liquid using level set methodTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2008Zhao Yu Abstract In this study, 3-D level set method is applied to investigate the rise of gas bubbles in infinite liquid domain due to the buoyancy force. A number of typical regimes for single bubble rising are studied, including the ellipsoidal, ellipsoidal cap, spherical cap, and skirted bubbles. The bubble shape and rise velocity predicted by the simulation are compared with the graphical correlations of Grace, Trans. Inst. Chem. Eng., 51, 116,120, (1973) and Bhaga and Weber, J. Fluid Mech., 105, 61,85, (1981). Good agreement is found between the simulation results and the correlations. These simulations cover a wide range of the parameters, including Eo, Mo, and Re, and demonstrate the capability and accuracy of level set method for simulation of bubbles under various conditions with considerable deformation. Finally, simulation results for the coalescence of two bubbles are also presented. Dans cette étude, une méthode de level set en 3-D est utilisée pour examiner la montée des bulles de gaz due à la force de flottabilité dans un domaine liquide infini. Plusieurs régimes typiques de montée d'une bulle sont étudiés, dont le régime ellipsoïdal, le chapeau ellipsoïdal, le chapeau sphérique et les ceintures de bulles. La forme des bulles et la vitesse de montée prédites par la simulation sont comparées aux corrélations graphiques de Grace, Trans. Inst. Chem. Eng., 51, 116,120, (1973), et Bhaga et Weber, J. Fluid Mech., 105, 61,85, (1981). Un bon accord est trouvé entre les résultats des simulations et les corrélations. Ces simulations couvrent un large éventail de paramètres, notamment Eo, Mo, et Re, et montrent la capacité et la précision de la méthode level set pour la simulation des bulles dans des conditions diverses avec une déformation considérable. Enfin, les résultats des simulations sont également présentés pour la coalescence de deux bulles. [source] Using non-invasive magnetic resonance imaging (MRI) to assess the reduction of Cr(VI) using a biofilm,palladium catalystBIOTECHNOLOGY & BIOENGINEERING, Issue 1 2010D.A. Beauregard Abstract Industrial waste streams may contain contaminants that are valuable like Pd(II) and/or toxic and mutagenic like Cr(VI). Using Serratia sp. biofilm the former was biomineralized to produce a supported nanocrystalline Pd(0) catalyst, and this biofilm,Pd heterogeneous catalyst was then used to reduce Cr(VI) to less dangerous Cr(III) at room temperature, with formate as the electron donor. Cr(VI)(aq) is non-paramagnetic while Cr(III)(aq) is paramagnetic, which enabled spatial mapping of Cr species concentrations within the reactor cell using non-invasive magnetic resonance (MR) imaging experiments. Spatial reactivity heterogeneities were thus examined. In batch reactions, these could be attributed primarily to heterogeneity of Pd(0) distribution and to the development of gas bubbles within the reactor. In continuous flow reactions, spatial reactivity heterogeneities resulted primarily from heterogeneity of Cr(VI) delivery. Biotechnol. Bioeng. 2010;107: 11,20. © 2010 Wiley Periodicals, Inc. [source] Hydrodynamics and Mass Transfer in Gas-Liquid-Solid Circulating Fluidized BedsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 12 2003Z. Liu Abstract Although extensive work has been performed on the hydrodynamics and gas-liquid mass transfer in conventional three-phase fluidized beds, relevant documented reports on gas-liquid-solid circulating fluidized beds (GLSCFBs) are scarce. In this work, the radial distribution of gas and solid holdups were investigated at two axial positions in a GLSCFB. The results show that gas bubbles and solid particles distribute uniformly in the axial direction but non-uniformly in the radial direction. The radial non-uniformity demonstrates a strong factor on the gas-liquid mass transfer coefficients. A local mass transfer model is proposed to describe the gas-liquid mass transfer at various radial positions. The local mass transfer coefficients appear to be symmetric about the central line of the riser with a lower value in the wall region. The effects of gas flow rates, particle circulating rates and liquid velocities on gas-liquid mass transfer have also been investigated. [source] Frontal Polymerization Synthesis of Starch-Grafted Hydrogels: Effect of Temperature and Tube Size on Propagating Front and Properties of HydrogelsCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2006Qing-Zhi Yan Prof. Dr. Abstract The frontal polymerization process was used to produce superabsorbent hydrogels based on acrylic acid monomers grafted onto starch. Using a simple test tube which was nonadiabatic and permitted contact with air, the effects of initial temperature and tube size on the propagating front of grafting copolymerization and the properties of hydrogels were explored. The unrestricted access of the reaction mixture to oxygen delayed the formation of self-propagating polymerization front. The ignition time was markedly lengthened with the increasing of tube size attributed to the formation of large amounts of peroxy radicals. The front velocity dependence on initial temperature could be fit to an Arrhenius function with the average apparent activation energy of 24 kJ,mol,1, and on tube size to a function of higher order. The increase of the initial temperature increased the front temperature, which lead to more soluble oligomers and higher degree of crosslinking. The interplay of two opposite effects of oligomer and crosslinking determined the sol and gel content. An increase in tube size had two effects on the propagating front. One was to reduce heat loss. The other effect was to increase the number of escaping gas bubbles. The combined action of the two effects resulted in a maximum value of front temperature, an increase in sol content and a reduction in gel content with tube size. The highest swelling capacity of hydrogels was obtained when the initial temperature or tube size favored a formation of porous microstructure of hydrogels. [source] | ||||||||||||||||||||||||||||||||||