Bubble Diffusion Research Articles

Vesiculation and crystallization under instantaneous decompression: Numerical study and comparison with laboratory experiments

Vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior, and they interplay each other through the water exsolution process. We conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions (decompression history is unknown) and in laboratory experiments (the amount of decompression is constant with time). The numerical results, which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water, viscosity, and the amount of decompression, provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression. The bubble nucleation in the homogeneous nucleation can be divided into two regimes – the diffusion control regime and viscosity control regime – depending on the modified Peclet number and the effective supersaturation. In the cases of both homogeneous and heterogeneous nucleations, the bubble growth is controlled by diffusion or viscosity, depending on the modified Peclet number and bubble number density. The water exsolution rate, which is controlled by the modified Peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime, acts as an effective cooling rate in a decompression-induced crystallization process. A comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion-limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles, and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion-limited growth of crystal. The several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate.

An advanced model for intragranular bubble diffusivity in irradiated UO2 fuel

An advanced model for intragranular bubble diffusivity in irradiated UO2 fuel is developed. Three various (surface, volume and gas-phase) mechanisms for the gas-filled bubbles diffusivity are reconsidered. It is shown that the bubble mobility by the volume diffusion mechanism can be strongly enhanced under irradiation conditions. Influence of the two-phase interface kinetics at a bubble surface on the volume diffusion, evaporation/condensation and surface migration mechanisms that can strongly suppress diffusivity of small nanometre bubbles, is additionally studied using a non-linear adsorption law derived for the van-der-Waals gas in the bubbles. The improved model is implemented in the MFPR code and validated against measurements of the small bubbles mobility.

Aeration Activations of a Water Chamber Type Breakwater with an Inclined Plate Array as a Front Wall

In order to improve the seawater quality in a harbor, a new type of pile supported breakwater with a water chamber was examined experimentally. The new breakwater is comprised of an inclined plate array as a front wall and a curtain wall as a rear wall. A horizontal plate is also attached to the rear wall at the lower end. Especially, for activating the aeration of basin water, the wave interaction process with the breakwater near the water surface was enhanced. The piston mode wave resonance in the water chamber of the dissipater is a main driving force to activate the entrapment and diffusion of air bubbles to the water. Both the functions of wave energy dissipation and activation of the aeration have been examined. It is seen that the front wall of down fin type is most effective to increase dissolved oxygen rate in the water.

Evaluation of Plastic Carrier Media Impact on Oxygen Transfer Efficiency with Coarse and Fine Bubble Diffusers

Evaluation of Plastic Carrier Media Impact on Oxygen Transfer Efficiency with Coarse and Fine Bubble DiffusersA study was conducted using an 840 gallon test tank to evaluate the impact of carrier media on standard oxygen transfer efficiency in clean water (SOTEcw), using coarse and fine bubble air diffusers used in Integrated Fixed-Film Activated Sludge and Mixed Bed Biofilm Reactor systems. The objectives of this study were to evaluate the impact of various fill volumes of carrier media on SOTEcw, to...Author(s)Hao PhamSudhakar ViswanathanRobert F. KellySourceProceedings of the Water Environment FederationSubjectSession 66: How to Optimize Your MBBR-IFAS DesignDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2008ISSN1938-6478SICI1938-6478(20080101)2008:11L.5069;1-DOI10.2175/193864708788805378Volume / Issue2008 / 11Content sourceWEFTECFirst / last page(s)5069 - 5079Copyright2008Word count166

Design and start-up of a high rate anaerobic membrane bioreactor for the treatment of a low pH, high strength, dissolved organic waste water

A Submerged Membrane Anaerobic Reactor (SMAR) is being developed for the treatment of waste water originating in Sasol's coal to fuel synthesis process. The laboratory-scale SMAR uses A4-size submerged flat panel ultrafiltration membranes to induce a 100% solids-liquid separation. Biogas gets extracted from the headspace above the anaerobic mixed liquor and reintroduced through a coarse bubble diffuser below the membranes. This induces a gas scour on the membranes that avoids biomass immobilization and membrane fouling. The substrate is a high strength (18 gCOD/l) petrochemical effluent consisting mostly of C2 to C6 short chain fatty acids with a low pH. Because of this, the pH of the reactor has to be controlled to a pH of 7.1. Organic Loading Rates of up to 25 kgCOD/m3 reactor volume/d has been observed with effluent COD normally <500 mgCOD/l and FSA <50 mgN/l with no particulates >0.45 microm at hydraulic retention times of 17 hours. 98% of the COD is converted to methane and the remainder to biomass. Mixed Liquor (MLSS) concentrations >30 gTSS/l can be maintained without deterioration of membrane fluxes, even though the Diluted Sludge Volume Index (DSVI) indicates that the sludge cannot be settled. No noteworthy deterioration in membrane performance has been observed over the 320 day operational period.

Biological Coating of Membranes of Fine Bubble Diffusers – Impacts and Countermeasures

Biological Coating of Membranes of Fine Bubble Diffusers – Impacts and Countermeasures

Oxygen transfer prediction in aeration tanks using CFD

In order to optimize aeration in the activated sludge processes, an experimentally validated numerical tool, based on computational fluid dynamics and able to predict flow and oxygen transfer characteristics in aeration tanks equipped with fine bubble diffusers and axial slow speed mixers, is proposed. For four different aeration tanks (1;1493;8191 and 29 , 313 m 3 ), this tool allows to precisely reproduce experimental results in terms of axial liquid velocities, local gas hold-ups. Predicted oxygen transfer coefficients are within ± 5% of experimental results for different operating conditions (varying pumping flow rates of the mixers and air flow rates). The actual bubble size must be known with precision in order to have a reliable estimation of the oxygen transfer coefficients.

Stamford Scientific introduces ultra-fine bubble diffuser

Stamford Scientific Incorporated (SSI) of Poughkeepsie, New York, USA, has introduced an ultra-fine bubble diffuser for the membrane market.

After a Decade of Development: A Profile of Municipal Drinking Water Plants Utilizing a Sidestream Injection Process

The technology of rapid ozone mass transfer followed by degasification, the GDT™ Process, was first introduced in 1995 by Mazzei et al. At the time of introduction, municipal ozone installations utilized a low concentration, air-fed ozone gas as a disinfectant in atmospheric contact basins fitted with fine bubble diffusers (FBD). Over the past decade, air- fed ozone has given way to highly concentrated, oxygen-fed ozone. The change to concentrated oxygen feed gas has increased concerns about the corrosive effects of high finished water dissolved oxygen (DO). Water treatment plants using oxygen fed ozone have reported finished water DO levels in excess of 20 mg/L, with some plants resorting to air sparging at the back end of the contact basin to restore finished water to atmospheric gas levels. However, the evolution to oxygen feed gas has also produced significant cost benefits. Operating an ozone generator on oxygen increases its ozone production; reducing the size and capital cost of the generator needed to meet ozone output requirements. The use of a concentrated gas stream has also led to the development of side stream injection systems, which move the gas mixing out of the atmospheric basin and into the upstream pipeline (Neemann, 2002), resulting in a more compact contact basin design. Municipal water plants not having a CT requirement have streamlined one step further, by eliminating the ozone contact basin in favor of a sidestream injection Process. This paper reviews the technology of the GDT™ sidestream injection process and introduces 2 municipal water treatments plant (WTP) installations utilizing this process to remediate taste and odor compounds and as a method to reduce finished water dissolved oxygen concentrations.

Dynamical interaction of helium bubbles with grain boundaries in Fe and Fe–9Cr ferritic alloy

Dynamical interaction of helium bubbles with grain boundaries in Fe and Fe–9Cr has been studied by in situ electron microscopy and thermal desorption spectroscopy (TDS). It is clearly demonstrated that most small helium bubbles are swept to the specimen surface by dynamical interaction with the moving grain boundaries during re-crystallization, which induces easy motion along the boundary. At small angle grain boundaries, helium bubbles are frequently trapped at grain boundary dislocations and move along the dislocation line. The bubble diffusivity along the grain boundary dislocation is statistically estimated from the moving path and the value is similar to or a little higher than that in the matrix. Well correlated and enhanced thermal desorption spectra with the sweeping of helium bubbles by moving grain boundaries in Fe and Fe–9Cr are revealed.

Free-surface aeration and momentum exchange at a bottom outlet

This study aims to provide some new understanding of the air–water flow properties in highvelocity water jets discharging past an abrupt drop. Such a setup has been little studied to date despite the relevance to bottom outlets. Downstream of the step brink, the free-jet entrains air at both upper and lower air–water interfaces, as well as along the sides. An air–water shear layer develops at the lower nappe interface. At the lower nappe, the velocity redistribution was successfully modelled and the velocity field was found to be similar to that in two-dimensional wake flow. The results highlighted further two distinct flow regions. Close to the brink (Wex < 5000), the flow was dominated by momentum transfer. Further downstream (Wex > 5000), a strong competition between air bubble diffusion and momentum exchanges took place

Surfactant effects on alpha factors in full-scale wastewater aeration systems

Aeration is an essential process in the majority of wastewater treatment processes, and accounts for the largest fraction of plant energy costs. Aeration systems can achieve wastewater oxygenation by shearing the surface (surface aerators) or releasing bubbles at the bottom of the tank (coarse- or fine-bubble aerators). Surfactants accumulate on gas-liquid interfaces and reduce mass transfer rates. This reduction in general is larger for fine-bubble aerators. This study was conducted to evaluate mass transfer effects on the characterization and specification of aeration systems in clean and process water conditions. Tests at different interfacial turbulence regimes were analysed, showing higher gas transfer depression for lower turbulence regimes. Higher turbulence regimes can offset contamination effects, at the expense of operating efficiency. This phenomenon is characteristic of surface aerators and coarse bubble diffusers and is here discussed. The results explain the variability of alpha factors measured at small scale, due to uncontrolled energy density. Results are also reported in dimensionless empirical correlations that describe mass transfer as a function of physiochemical and geometrical characteristics of the aeration process.

Dynamics of diffusive bubble growth and pressure recovery in a bubbly rhyolitic melt embedded in an elastic solid

We present a model of gas exsolution and bubble expansion in a melt supersaturated in response to a sudden pressure drop. In our model, the melt contains a suspension of gas bubbles of identical sizes and is encased in a penny‐shaped crack embedded in an elastic solid. The suspension is modeled as a three‐dimensional lattice of spherical cells with slight overlap, where each elementary cell consists of a gas bubble surrounded by a shell of volatile‐rich melt. The melt is then subjected to a step drop in pressure, which induces gas exsolution and bubble expansion, resulting in the compression of the melt and volumetric expansion of the crack. The dynamics of diffusion‐driven bubble growth and volumetric crack expansion span 9 decades in time. The model demonstrates that the speed of the crack response depends strongly on volatile diffusivity in the melt and bubble number density and is markedly sensitive to the ratio of crack thickness to crack radius and initial bubble radius but is relatively insensitive to melt viscosity. The net drop in gas concentration in the melt after pressure recovery represents only a small fraction of the initial concentration prior to the drop, suggesting the melt may undergo numerous pressure transients before becoming significantly depleted of gases. The magnitude of pressure and volume recovery in the crack depends sensitively on the size of the input‐pressure transient, becoming relatively larger for smaller‐size transients in a melt containing bubbles with initial radii less than 10−5 m. Amplification of the input transient may be large enough to disrupt the crack wall and induce brittle failure in the rock matrix surrounding the crack. Our results provide additional basis for the interpretation of volume changes in the magma conduit under Popocatépetl Volcano during Vulcanian degassing bursts in its eruptive activity in April–May 2000.

Bubble growth processes in magma surrounded by an elastic medium

Gas bubble growth in magma plays an important role in volcanic explosivity, and many previous studies have undertaken theoretical and experimental analyses of gas bubble expansion in melt. These previous studies commonly assume constant ambient pressure, but it is more natural to assume that the magma is stressed by the crust or volcanic edifices as the volume of gas bubbles increases. Here we present a bubble growth model that takes into account the elasticity of the surrounding medium; we use this model to examine temporal changes in the bubble growth process. Our model consists of a two-dimensional dike filled with compressible viscous melt and numerous tiny gas bubbles. We use the cell model proposed by Proussevitch et al. [Proussevitch, A., Sahagian, D. L., Anderson, A. T., 1993. Dynamics of diffusive bubble growth in magmas: isothermal case. J. Geophys. Res. 98, 22283–22307] to describe the interaction between the numerous gas bubbles and the melt. The bubble growth process is formulated by the diffusion equation of volatiles in the melt, the mass balance between bubbles and melt, and the momentum equation of the melt. We also introduce pressure balance equations between the melt and the surrounding elastic medium [Nishimura, T., 2004. Pressure recovery in magma due to bubble growth. Geophys. Res. Lett. 31]. Using the finite difference scheme, we numerically calculate temporal changes in bubble radius and melt pressure within magma subjected to sudden depressurization. Simulation results show that the elasticity of the surrounding medium strongly controls the bubble growth process. Under high effective rigidity, the final bubble radius is several times smaller than for zero rigidity, and the time required for bubble growth is an order of magnitude quicker. Temporal changes in the melt pressure are particularly dependent on the elasticity of the surrounding medium. Melt pressure effectively recovers and even exceeds the given pressure drop for conditions of high rigidity and/or small initial bubble radius. Although bubble growth in magma has previously been investigated mainly from geological samples and theoretical perspectives, our model can quantitatively evaluate pressure changes in magma that can also be detected by seismic and geodetic measurements.

Dynamical modelling of an activated sludge system of a petrochemical plant operating at high temperatures

The Mexican petrochemical industry, Morelos S.A. de C.V., is one of the biggest and more important petroleum industries in Mexico and Latin America. It has an activated sludge system to treat its wastewater flow, which is approximately 7,000 m3/d. The wastewater contains volatile organic carbon substances classified as toxics. The old surface aeration system was changed for fine bubble diffusers; however, one major drawback of the new aeration system is that the temperature in the bioreactor has increased due to the compression of the air, which at the compressor exit reaches 85 degrees C. This effect results in the temperature in the bioreactor attaining 32 degrees C during the fall, whereas in the spring and summer, the bioreactor temperature reaches higher values than 40 degrees C. The high temperatures reduce the microorganism activity and cause a higher volatilisation rate of volatile compounds, among other effects, which affect the performance of the biological treatment. This work was performed to obtain a better modelling of the wastewater treatment from the petrochemical industry. The model describes the effect of the temperature on the performance of the biological treatment. The model was obtained from tests that were carried out in laboratory reactors with 14 L capacity, which were operated at different temperatures (from 30 to 45 degrees C), with the same wastewater and conditions as the actual system.

First assessment of methane and carbon dioxide emissions from shallow and deep zones of boreal reservoirs upon ice break‐up

AbstractMost studies dealing with greenhouse gas (GHG) emissions from large boreal reservoirs were conducted during the ice‐free period. In this paper, the potential methane (CH4) and carbon dioxide emissions are estimated for two hydroelectric reservoirs, as well as for a small experimental reservoir from boreal latitudes (northern Quebec, Canada) at the ice break‐up event through diffusion (diffusive fluxes) and release of bubbles (bubbling fluxes). The results of this preliminary study suggest that the winter diffusive fluxes at the air–water interface of the sampled reservoirs represent &lt; 7% of their cumulative carbon emissions during the ice‐free period. Furthermore, the release upon ice‐break of CH4 bubbles accumulated under the ice cover during the winter could represent 2% of the summer carbon emissions from hydroelectric reservoirs in northern Quebec. The results presented herein suggest that the GHG emissions upon ice break‐up from the boreal reservoirs investigated are a small, but non‐negligible, component of their annual GHG emissions.

Gas transfer from air diffusers

The bubble and surface volumetric mass transfer coefficients for oxygen, k L a b and k L a s, are separately determined for 179 aeration tests, with diffuser depths ranging from 2.25 to 32 m, using the DeMoyer et al. [2003. Impact of bubble and free surface oxygen transfer on diffused aeration systems. Water Res 37, 1890–1904] mass transfer model. Two empirical characterization equations are developed for k L a b and k L a s, correlating the coefficients to air flow, Q a, diffuser depth, h d, cross-sectional area, A cs, and volume, V. The characterization equations indicate that the bubble transfer coefficient, k L a b, increases with increasing gas flow rate and depth, and decreases with increasing water volume. For fine bubble diffusers, k L a b is approximately six times greater than k L a b for coarse bubble diffusers. The surface transfer coefficient, k L A s, increases with increasing gas flow rate and diffuser depth. The characterization equations make it possible to predict the gas transfer that will occur across bubble interfaces and across the free surface with a bubble plume at depths up to 32 m and with variable air discharge in deep tanks and reservoirs.

Stability Analysis of an Activated Sludge Bioreactor at a Petrochemical Plant at Different Temperatures

The Mexican petrochemical industry, Morelos S.A. de C.V. is one of the biggest and most important petroleum industries in Mexico and in Latin America. It has an activated sludge system to treat its wastewater flow, which is about 7000m3/d. The wastewater contains volatile organic carbon substances classified as toxic. The old surface aeration system was changed for fine bubble diffusers; however, one major drawback of the new aeration system is that the temperature in the bioreactor has increased due to the compression of the air, which at the compressor exit reaches up to 85oC. This effect produces a temperature in the aerated bioreactor of 32oC during the fall, whereas in the spring and summer, the bioreactor temperature reaches higher values than 40oC. The high temperatures reduce the microorganisms’ activity and cause higher volatilization rates of volatile compounds, among other effects, which affect the performance of the biological treatment. In this work we performed a stability analysis at different equilibrium points in different scenarios of temperatures, recycle flowrate and wastage flowrate. The eigenvalues of the state matrix of dynamical model of the petrochemical wastewater treatment were evaluated. The model describes the effect of the temperature on the soluble and volatile pollutants removal rate, heterotrophic organism growth rate, oxygen uptake rate and biomass concentration in the settler. The model was obtained from tests that were done in laboratory reactors with 14 L of capacity, which were operated at different temperatures (from 30oC to 45oC), with the same conditions as the actual system. Simulations were done based on the model obtained manipulating the operation variables Qw and Qr. The stability analysis was also done to the reactor without recycle (Qr = 0 m3/d). In all the cases the stability analyses was performed. The stripping of VOC´s is an important process of COD removal at high operation temperatures.

The Heat Balance of the Western Hemisphere Warm Pool

Abstract The thermodynamic development of the Western Hemisphere warm pool and its four geographic subregions are analyzed. The subregional warm pools of the eastern North Pacific and equatorial Atlantic are best developed in the boreal spring, while in the Gulf of Mexico and Caribbean, the highest temperatures prevail during the early and late summer, respectively. For the defining isotherms chosen (≥27.5°, ≥28.0°, ≥28.5°C) the warm pool depths are similar to the mixed-layer depth (20–40 m) but are considerably less than the Indo–Pacific warm pool depth (50–60 m). The heat balance of the WHWP subregions is examined through two successive types of analysis: first by considering a changing volume (“bubble”) bounded by constant temperature wherein advective fluxes disappear and diffusive fluxes can be estimated as a residual, and second by considering a slab layer of constant dimensions with the bubble diffusion estimates as an additional input and the advective heat flux divergence as a residual output. From this sequential procedure it is possible to disqualify as being physically inconsistent four of seven surface heat flux climatologies: the NCEP–NCAR reanalysis (NCEP1) and the ECMWF 15-yr global reanalysis (ERA-15) because they yield a nonphysical diffusion of heat into the warm pools from their cooler surroundings, and the unconstrained da Silva and Southampton datasets because their estimated diffusion rates are inconsistent with the smaller rates of the better understood Indo–Pacific warm pool when the bubble analysis is applied to both regions. The remaining surface flux datasets of da Silva and Southampton (constrained) and Oberhuber have a much narrower range of slab surface warming (+25 ± 5 W m−2) associated with bubble residual estimates of total diffusion of –5 to –20 W m−2 (±5 W m−2) and total advective heat flux divergence of –2 to –14 W m−2 (±5 W m−2). The latter are independently confirmed by direct estimates using wind stress data and drifters for the Gulf of Mexico and eastern North Pacific subregions.

Fifteen Years of Offgas Transfer Efficiency Measurements on Fine‐Pore Aerators: Key Role of Sludge Age and Normalized Air Flux

Fine-pore diffusers, often called fine-bubble diffusers, have nearly replaced coarse bubble diffusers in municipal wastewater treatment over the past 20 years. The rapid increases in energy costs, which began in the 1970s, created financial incentives to upgrade to this more expensive and maintenance-intensive method of aeration. Fine-pore diffusers have the added benefit of reducing volatile organic compound stripping and reduced aeration heat loss. This paper summarizes 15 years of oxygen transfer efficiency measurements using the offgas technique. Efficiencies are shown for different types of diffusers at various tank geometries (depth, diffuser size, and number), airflow rates, and mean cell retention times (MCRT or sludge age). By normalizing the airflow rates per unit of depth and diffusing area, efficiencies measured in different plants can be compared. The results show that aeration efficiencies are logarithmically related to the ratio between MCRT and the normalized air flux, with transfer rates suppressed by low MCRT or high normalized air flux systems. There is no evidence for different alpha factors among the different types of fine-bubble diffuser types.